Accessibility Version: Tracking the Iowa Nutrient Reduction Strategy v5

Version 5.0 | Data updated June 2026

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The Iowa Nutrient Reduction Strategy (INRS) is a science- and technology-based approach to assess and reduce nutrients delivered to Iowa waterways and the Gulf of America (Gulf of Mexico). The Strategy outlines opportunities for reducing nutrients in surface water from both point sources, such as municipal wastewater treatment plants and industrial facilities, and nonpoint sources, including agricultural operations and urban areas, in a scientific, reasonable, and cost-effective manner. The Iowa Nutrient Reduction Strategy was developed in response to recommendations provided by the United States Environmental Protection Agency (EPA) in its March 16, 2011 memo, “Working in Partnership with States to Address Phosphorus and Nitrogen Pollution through Use of a Framework for State Nutrient Reduction.” Ongoing action for nutrient load reductions is further supported by the recent EPA recommendations, “Renewed Call to Action to Reduce Nutrient Pollution and Support for Incremental Actions to Protect Water Quality and Public Health,” released September 22, 2016, and “Accelerating Nutrient Pollution Reductions in the Nation’s Waters,” released April 5, 2022.

This page presents an analysis of changes for each indicator of the Iowa Nutrient Reduction Strategy Logic Model to facilitate reporting.

From 2014 to 2020, a comprehensive progress report was released annually by the Iowa Department of Agriculture and Land Stewardship, the Iowa Department of Natural Resources, and Iowa State University. The process of reporting nutrient reduction efforts transitioned in 2021 to a revised approach by publishing data and findings in a set of web-based dashboards. 

This page presents screen reader-friendly versions of the text and tables in the web-based dashboards. Version 5.0 summarizes all updates that have been published so far for the 2024 INRS tracking period (Human, Nonpoint Source, Point Source, and Water); data for the Inputs indicator are from the 2023 INRS tracking period and will be updated once the 2024 dashboard is published.

Tracking Inputs for the Iowa Nutrient Reduction Strategy

Note that this section will be updated when the 2024 Tracking Funding and Resources dashboard is published.

Introduction to INRS Inputs

Tracking efforts of the Iowa Nutrient Reduction Strategy (INRS) – the amount of funding, outreach, practice implementation, and changes in water quality – are completed to summarize changes made to advance the INRS goal of reducing nitrogen and phosphorus loading by 45% from the 1980-96 baseline period. The dashboards serve as a comprehensive reporting tool to report ongoing efforts from multiple entities collectively working to advance the adoption of conservation practices to improve water quality. The INRS Logic Model provides a structure to evaluate the progression of changes in resources, practice adoption, and impacts of practice adoption over time. Materials for reporting on INRS efforts – funding, outreach, practice implementation, and changes in water quality – are collected annually, and dashboards are updated after data is collected, aggregated, and processed. 

Each of the four indicators of the INRS logic model is described below:

• Inputs indicator summarizes staff, funding, agency resources, and NGO sector resources;
• Human indicator summarizes partner organizations, farmer knowledge and attitude, point source communities, and management knowledge and attitude;
• Land indicator summarizes land use changes, practice adoption, and point source implementation; and
• Water indicator summarizes annual statewide nutrient loads by year and modeled load reduction

Inputs are necessary to expand Iowa's capacity for encouraging and realizing changes in human behavior and for promoting and incentivizing conservation practice implementation to improve water quality. Targeting inputs toward specific INRS facets may be required to support the goals set forth by the INRS. Due to data availability, this dashboard aims to provide an overview of reported statewide funding and staff resources supporting or complementary to the INRS.

Estimates of investment encompass public and non-governmental organizations' (NGO) funding summarized through reports voluntarily submitted by Water Resources Coordinating Council (WRCC) and (historically) Watershed Planning Advisory Council (WPAC) member organizations, and other partner organizations. Most public programs described in this dashboard are considered base programs (described briefly below) and have generally existed for decades. In addition, these estimates include the farmer and landowner contribution to the implementation of cover crops, terraces, water and sediment control basins (WASCOBs), and grade stabilization structures based on landowner costs to install the practice(s). These estimates do not account for the investments made by private entities, farmers, or landowners for practices financed independently of public sector programs.

 A growing number of private sector funds are available to facilitate conservation and best management practice adoption on private lands. Many of these efforts operate independently of the INRS and cannot be reliably quantified through existing reporting mechanisms. However, the lack of trackability does not diminish their importance.

Continued Research on Nutrient Reduction

Continuation of research in the physical and social sciences is necessary to better understand the processes driving nutrient losses in Iowa and how conservation measures can alleviate nutrient losses. The Iowa Nutrient Research Center (INRC) has continued to be a dedicated source of research funding for nutrients since its founding in 2013. The INRC fosters innovative research, led by Iowa researchers at Regents institutions, on land management, edge-of-field practices, nutrient management, or multi-objective research. 

More information regarding projects funded at Regents Institutions through the INRC may be found on the INRC website.

INRC Projects relate to Nutrient Management, Land Use, Edge-of-Field Practices, and Multi-Objective. These project themes include:

• Nutrient Management: Agronomic activities related to the timing, source, rate, and placement of fertilizers based on crop and replacement needs depending on the cropping rotation.
• Land Use: How cropping, livestock management, and wildlife habitat intersect for a farm operation and environmental benefits.
• Edge-of-Field Practices: Best Management Practices designed with water quality benefits as a primary benefit.
• Multi-Objective: Research into Iowa's agroeconomic system and components that influence water quality.

INRS Priority Watersheds

Nine priority watersheds (hydrologic unit code 8 (HUC8) basins) across Iowa were identified by the Water Resources Coordinating Council in 2013 as areas in which to conduct outreach and focus targeted conservation and water quality efforts. Current demonstration project information for nonpoint source efforts is available on the Clean Water Iowa website.

Data Sources for INRS Inputs

Funding and staffing levels have been voluntarily reported since 2015 by members of the Water Resources Coordinating Council (WRCC) and, historically, by the Watershed Planning Advisory Council (WPAC). Respondents provide information about the number of full-time employee equivalents, by job category, dedicated to implementation of the Iowa Nutrient Reduction Strategy. Information about funding sources is also submitted. A common template is used for reporting to standardize responses across organizations. Where data are unavailable, public records are utilized for public investments for appropriations and expenditures.

Information is collated for all submitted reports to summarize funding, staff, outreach efforts, practice implementation, and monitoring efforts, and reported efforts are reviewed to minimize duplication. For example, a grant disbursed by one organization and awarded to another may be reported by both organizations, but double-counting was minimized by obtaining specific information about different funding sources.

Reports submitted by partners may be downloaded as supplemental materials of the INRS web page. The 2023 report is available as an Excel (.xlsx) file.

INRS Funding by Partner Organizations from 2012 to 2023

Investments by the four primary investment categories - public cost-share programs, estimated farmer and landowner investment, non-governmental organizations (NGOs), and land rental as Conservation Reserve Program (CRP) payments - are summarized in the table below. 

Partner funds became available for reporting in the current Iowa Nutrient Reduction Strategy methodology in 2016 for reporting purposes. Non-governmental organization investments occurred prior to this time, but data are not available. Estimated farmer and landowner investment includes implementation of cover crops, terraces, water and sediment control basins, ponds, grade stabilization structures, and sediment basins that utilized a public sector program.

State and Federal Funding in Support of INRS by Program

Program funding is reported as state appropriations by fiscal year, with the exception of Clean Water State Revolving Funds, which are reported as loans originating per state fiscal year. Farm Service Agency (FSA) expenditures are obtained from the CRP Enrollment and Rental Payments by State, 1986-2023 report. Natural Resources Conservation Service (NRCS) expenditures come from annual At-A-Glance reports by federal fiscal year. Detailed funding information can be found in Appendix A (available at the end of this document).

Full-Time Employees (FTEs) Reported for Iowa Nutrient Reduction Strategy Inputs

The table below summarizes FTEs by category, as reported by partners. Changes in tracking staff have been reported by several organizations since 2019 and are known to impact FTEs reported, both as total FTEs and within the reporting categories "On-the-ground implementation Staff" and "Infrastructure Staff."

Changes in Funding

State conservation programs have evolved from 2012 to 2023, with funding for longstanding conservation programs independent of the Iowa Nutrient Reduction Strategy (INRS) continuing to receive increased funding. The Water Quality Initiative was first established in 2014 to directly support the INRS. Additional funding became available in 2018 with the establishment of the Water Quality Infrastructure Fund, funded by Senate File 512. 

Changes in federal conservation program expenditures have largely increased from the beginning of the INRS. These programs provide financial support for implementation of nonpoint source conservation practices or, through the Conservation Reserve Program (CRP), for reservation of sensitive land. Expenditures through CRP in Iowa have increased, driven by an average rental rate per acre increase from $132 to $238 from 2012 to 2023. CRP enrollment has also prioritized continuous sign-up acres that are often sited to buffer runoff from adjacent cropped land. Conservation practice implementation programs administered by the Natural Resources Conservation Service are funded through the Farm Bill. Program offerings and funding for each program have evolved over the past decade. 

Conventional programs have provided strong funding support for projects in Iowa; Iowa entities have also been competitive and demonstrated innovative conservation delivery concepts to receive grant funding through the Regional Conservation Partnership Program (RCPP).

Tracking the Human Dimension

Overview of the Human Indicator

The Human indicator summarizes knowledge, attitudes, and behavior related to water quality and nutrient reduction. Changes in management and conservation practices reflect the outreach, training, and educational events aimed at increasing knowledge among communities, farmers, landowners, the public, and conservation professionals. The outreach impacts have been assessed using farmer surveys to gauge farmers’ knowledge, attitudes and awareness related to water quality and nutrient reduction.

Outreach activities regarding water quality and management practices are summarized in the following panels of this dashboard. These efforts primarily capture outreach regarding nonpoint sources and the extensive network that supports educational opportunities across the state.

While point source activities directly engage fewer people, outreach amongst publicly owned treatment works (POTWs) continues through education for plant operators by organizations such as the Iowa Water Environment Association and direct contact with permitting agencies. Individual contacts amongst DNR, municipalities, and consultants who assist with plant operations and ensure regulatory compliance standards are not tracked; however, the development of phase one and phase two assessments for major POTWs as required by the INRS requires significant time and dedicated funding to ensure community and commercial needs are economically met. More importantly, urban and rural partnerships continue to be explored across the state through the creation of Nutrient Reduction Exchanges (NREs) by pilot cities in consultation with the DNR. Nonpoint conservation practice benefits are assessed by the Nutrient Tracking Tool, validated, and then registered via the Regulatory In-Lieu Fee and Bank Information Tracking System (RIBITS) database to foster nutrient trading within a watershed.

Changes in Public Education and Outreach

INRS partners remain engaged in outreach, with 542 events reported for the 2024 reporting period that reached an estimated 28,511 attendees*. These events ranged from general public programs such as fairs and educational program visits to schools to educational opportunities for professionals such as workshops and conferences. Program delivery modes have evolved over the past several years with the widespread adoption of virtual programming. The extent of virtual program development by INRS partners varies and includes webinars for general audiences to virtual field days that facilitate virtual attendance of programming conducted in the field.

The geographic distribution of events continues to be evaluated with county-level data summarized for the number of events and attendance. For example, counties with an event center will likely host larger programs that draw attendees from multiple counties or statewide. In contrast, rural counties may host more programs focused on local issues for farmers and landowner audiences. The total number and attendance of partner-reported events are summarized by county and event type.

These events, which provide information to make informed decisions about conservation practices and educate attendees about water quality issues, were self-reported by INRS partner organizations, and include five general categories:

• Conferences – Multi-session events to facilitate knowledge-sharing, networking, and partnering.
• Community Outreach – Includes fairs, tours, and other community events.
• Field days – Often serve to educate farmers, landowners, and agribusiness representatives through direct demonstration.
• Workshops – Entail training in a particular skill or topic area related to nutrients and water quality.
• Youth – Focuses on spreading understanding about natural resources and watershed issues through K-12 educational programming.
• Supplemental – A training that included content related to water quality, but water quality was not the primary focus of the event.

*Because nutrient management and water quality were not the primary focus of events or activities classified as "Supplemental", supplemental events are excluded from the statewide total reported above.

A summary of programs by county can be found in Appendix B (available at the end of this document).

The Nutrient Reduction Strategy Farmer Survey

Completed over five years, the NRS Farmer Survey was designed to assess farmer knowledge, attitude, and behavior related to water quality and to gain insight into practices that are favorably received or barriers to BMP adoption. With surveys completed by respondents over multiple years, results from five of the six HUC6 watersheds in Iowa have been published and are summarized in this report. Surveys were completed within the larger HUC6 watersheds and priority HUC8 watersheds across the state. 

Reports may be found in the INRS webpage supplemental documents or through the Iowa State University Extension Outreach Store (search for Iowa Farmers and the Iowa Nutrient Reduction Strategy).

Each watershed was surveyed in the years summarized in the table below. Data summarized in the tables below reflect the respondents' answer in the first year of the survey.

Selected results from the Iowa Nutrient Reduction Strategy Farmer Survey are summarized below. The INRS Farmer Survey tracked farmers’ knowledge, attitudes, and behavior related to nutrient reduction beginning in 2015 with the final survey completed in 2019. Responses are aggregated in the tables below by topic area from the survey and by the basin in which the farmer operates.

The source(s) from which farmers who completed the Iowa Nutrient Reduction Strategy Farmer Survey learned about the strategy is summarized in the table below by the basin in which the farmer operates.

The source(s) from which farmers who completed the Iowa Nutrient Reduction Strategy Farmer Survey learned about nutrient management is summarized in the table below by the basin in which the farmer operates.

Tracking Nonpoint Source Nutrient Reduction Practices - Agricultural Conservation Practices

Agricultural Land Use in Iowa Over Time

Iowa’s total land area is 35.7 million acres. Based on data from the United States Department of Agriculture (USDA) Census of Agriculture, nearly 90% of Iowa’s total area is dedicated to agricultural uses, with total annual agricultural land use averaging over 31 million acres since 1982. Land area dedicated to field crops — corn, soybeans, and other annual and perennial crops — has remained relatively steady since the 1980s, averaging just below 27 million acres annually. Total acres enrolled in the USDA Conservation Reserve Program (CRP), which aims primarily to convert environmentally sensitive land from crops to perennial cover, has fluctuated between approximately 1.5 and two million acres in Iowa since the start of the program in 1986.

Records from the USDA Census of Agriculture, the USDA National Agriculture Statistics Service (NASS), and the USDA Farm Service Agency (FSA) were compiled to estimate historical and recent crop acreages from 1992 to the current reporting period. Acreages prior to 1992 were tabulated from digitized documents in the USDA Census of Agriculture Historical Archive. Crop acres from the Census of Agriculture and National Agriculture Statistics Service were used for annual values from 1993 to 2010. For both periods, harvested acres were used when available; planted acres were used as an alternative value when harvested acres were not available. NASS survey values were used for years when the Census did not occur. For annual crop acres since 2011, planted crop acres were aggregated from Farm Service Agency crop acreage reports and reflect the annual crop acreage values provided in NASS (in lieu of combined records from archival, NASS, and FSA databases).

Acres enrolled in the Conservation Reserve Program in Iowa were obtained from the FSA crop acreage reports and aggregated by year.

Iowa Cover Crops

During the baseline and benchmark time periods —1980-96 and 2006-10 — there were no or very few acres of cover crops in Iowa. The USDA Census of Agriculture reported that 1.28 million acres of cover crops were planted in Iowa in the fall of 2022, and the Survey of Agricultural Retailers estimated 3.7 million acres. Based on county-level data from the 2022 USDA Census of Agriculture, counties in the eastern region of the state tend to have higher rates of cover crop use than those in other parts of Iowa.

Of these statewide estimates, public conservation programs accounted for more than 1.1 million acres in 2022. It should be noted that these publicly funded cover crop acres – including state and federal cost share programs as well as the crop insurance discount program – represent only a portion of Iowa’s total cover crop acres; annual publicly funded acres do not represent a total statewide estimate of Iowa cover crops.

Cover crop acres are reported by the crop year with which they are associated (i.e., cover crop acres reported as "2024" were planted in 2023, benefitting the 2024 cash crop).

A summary of cover crop distribution in Iowa, summarized in Appendix C, was created using the USDA Census of Agriculture county-level acres planted in the fall of 2022. County values were assigned proportionally to Iowa HUC8 watersheds based on the percentage of county land area that intersects each watershed.

The types of cover crop species planted from 2017-2024 are summarized in the table below. Annual distribution of cover crop species was determined using data from the Iowa Nutrient Research and Education Council Survey of Agricultural Retailers.

The use of cover crop mixes was not included in the INREC Ag Retailer Survey in 2017 and 2018.

Data Sources - Iowa Cover Crops

There are currently three data sources utilized for tracking the rate of cover crop adoption in Iowa. First, the Survey of Agricultural Retailers, conducted by the Iowa Nutrient Research and Education Council, has estimated annual statewide cover crop acres since 2017 (capturing the cover crops planted in the fall of the prior year). Second, the USDA Census of Agriculture, which is conducted every five years, provides county-level cover crop acres for fall 2012, 2017, and 2022, allowing for aggregated statewide totals for those years (corresponding to the 2013, 2018, and 2023 crop years). Third, state and federal conservation programs (whereby government cost-share is given to farmers and landowners) provide spatially explicit records of publicly funded cover crop acres. All state programs recorded by the Iowa Department of Agriculture and Land Stewardship were included in this analysis of cost-share acres, as well as acres under the federal Environmental Quality Incentive Program and Conservation Stewardship Program.

Iowa Tillage Practices

In the last few decades, the use of no-till and conservation tillage in Iowa has increased dramatically. Conservation tillage represents a range of reduced tillage practices that leave at least 30% of crop residue on the soil surface following harvest and planting. No-till further minimizes soil disturbance by leaving most of the crop residue on the surface.

During the INRS baseline period from 1980-1996, no-till was used on an average of two million acres. In 2012, the United States Department of Agriculture (USDA) Census of Agriculture estimated 6.9 million acres of no-till. Since 2012, no-till acres have increased to a peak of 8-10 million acres, as estimated by the Iowa Nutrient Research and Education Council (INREC) Survey of Agricultural Retailers and the Census of Agriculture. No-till practices account for a higher portion of row crop acres in the rolling landscapes of western Iowa, for example, the Loess Hills region and some southern and northeastern watersheds.

Conservation tillage was practiced on 5.2 million acres during the baseline period, on average, and on an estimated 8.8 million acres in 2012. Since then, conservation tillage has generally increased, with estimates from the Survey of Agricultural Retailers and Census of Agriculture data suggesting use of conservation tillage on anywhere from 5 to 11 million acres annually. 

The increased use of no-till and conservation tillage in row crop operations since the 1980s is paired with a decrease in the use of conventional tillage. Conventional tillage was used on an estimated 12 million acres during the baseline period, and this practice has been used on fewer acres since that period, although there are annual fluctuations.

A summary of tillage practice distribution in Iowa, summarized in Appendix C, was created using the USDA Census of Agriculture county-level acres planted in the fall of 2022. County values were assigned proportionally to Iowa HUC8 watersheds based on the percentage of county land area that intersects each watershed.

Data Sources - Iowa Tillage Practices

Tillage acres were estimated using three data sources. First, the 1980-96 baseline period (displayed here as 1996) and the 2006-10 benchmark period (displayed here as 2010) are derived from the Crop Management Residue Survey, conducted by the Conservation Technology Information Center for Iowa from 1982 to 2011. Methods for using these findings to determine average annual acreages are described in the Iowa Nutrient Reduction Strategy Nonpoint Source Science Assessment and the corresponding Iowa Nutrient Reduction Strategy baseline study, both of which can be found at the Iowa Nutrient Reduction Strategy website.

Statewide acreages for the 2012, 2017, and 2022 crop years were estimated using the USDA Census of Agriculture, which provides county-level data for no-till, conservation tillage, and conventional tillage practice implementation.

Annual statewide acreages of tillage practices in corn and soybean fields are estimated by the INREC Survey of Agricultural Retailers for the 2017 to 2024 crop years.

Nutrient Management in Iowa - Nitrogen Rates and Phosphorus Application

During the 1980-96 baseline period, corn-soybean rotations received an estimated average of 149 pounds of commercial and manure nitrogen; continuous corn rotations received 199 pounds per acre. This figure was estimated using a similar methodology for the 2006-10 benchmark period at 151 pounds per acre for corn-soybean rotations and 201 pounds for continuous corn. These estimates were derived from the state fertilizer sales data, which is publicly available from the Iowa Department of Agriculture and Land Stewardship (IDALS), and the USDA Census of Agriculture’s reported animal units in Iowa.

The Iowa Nutrient Research and Education Council designed a Survey of Agricultural Retailers to estimate the extent of in-field practice use, including commercial fertilizer application practices, and has been completing the survey since 2017. The annual survey has found that in corn-soybean rotations, corn acres received, on average, between 167 and 183 pounds per acre during the 2017-24 period. On average, continuous corn rotations received between 186 and 209 pounds per acre during that time.

These annual nitrogen fertilizer rates represent statewide averages; however, nitrogen application rates to corn vary across agricultural fields and, in some cases, vary by acre within a field. The percent of total acres that received various levels of commercial nitrogen rates varies by crop rotation. In 2024, for example, 40 percent of corn-soybean acres received 176-200 pounds of commercial nitrogen on their most recent corn year, and 29 percent received 151-175 pounds. Some fields lay at the ends of this distribution, with 11 percent of acres receiving 150 pounds of nitrogen per acre or less and 19 percent receiving 201 pounds per acre or more. Commercial nitrogen application rates trended higher for continuous corn rotations, with 45 percent of acres receiving 176-200 pounds of nitrogen per acre, 41 percent receiving at least 201 pounds per acre and 14 percent receiving 175 pounds per acre or less. 

These estimates of annual nitrogen applications from 2017-24 represent an increase in acres in row crop and fertilizer use per acre since the 1980-96 baseline period. While the 2017-24 estimates of commercial fertilizer rates were obtained via a different data collection process than for the baseline and benchmark time periods, complementary evidence from recent fertilizer sales data shows that commercial fertilizer application rates for corn-soybean operations have increased gradually since before 1990. Increases in Iowa’s corn acres since that time have not increased at the same rate as the increase in commercial nitrogen fertilizer sales (disproportionate ratio of fertilizer sales to corn acres in the state), supporting the finding that average commercial nitrogen application rates (in pounds per acre) have increased over time as crop production has increased.

Research into nitrogen application rates lies at the forefront of the Iowa Nitrogen Initiative, a new program started in 2022 that incorporates soil, weather, and management data through on-farm trials throughout the state to develop the probability-based decision system N-FACT. The initiative will encourage fertilizer application decisions to be made based on more site-specific conditions to give farmers, agronomists, and landowners more confidence in determining the optimum rate at which to apply nitrogen.

Phosphorus fertilizer application methods that inject or incorporate it into the soil, compared with broadcasting across the soil surface, reduce potential nutrient losses from the field. The Survey of Agricultural Retailers estimated that 14.4 million acres received phosphorus fertilizer was incorporated, injected, or knifed into the soil for the 2017 crop year, a figure which decreased to 9.5 million acres for the 2024 crop year. These estimates account mostly for commercial fertilizer. For the 2024 crop year, approximately 77% of fields (~17.9 million acres) utilized soil testing for phosphorus, which can guide fertilizer application decisions.

Data Sources - Nitrogen Rates and Phosphorus Application

Commercial nitrogen application rates were obtained from the Iowa Nutrient Research and Education Council's Survey of Agricultural Retailers, which has been conducted annually since 2017. The statewide average annual rates of commercial nitrogen fertilizer application were calculated using a stratified, weighted average approach, based on each field's size and the number of observations within each major land resource area in Iowa.

The distributions of varying application rates for continuous corn and corn-soybean rotations were determined using the survey's records for agricultural fields that received only commercial nitrogen fertilizer in 2024.

The total application of commercial nitrogen fertilizer, in tons per year, was estimated from Iowa’s fertilizer sales data and the USDA Census of Agriculture, using the methods described in the Iowa Nutrient Reduction Strategy Nonpoint Source Science Assessment, which can be accessed on the INRS website.

To estimate the total plant-available nitrogen from manure applied to crops since the 1980-96 baseline period, researchers evaluated livestock animal unit data, USDA Census of Agriculture data, and published studies on manure nutrient availability. The methodology is described in the NRS Nonpoint Source Science Assessment, which can be accessed on the INRS website.

Acres of various timing methods for commercial phosphorus application were obtained from the Iowa Nutrient Research and Education Council's Survey of Agricultural Retailers, which has been conducted annually since 2017. The statewide acreages of each phosphorus application method were calculated using a stratified, weighted approach, based on each field's size and the number of observations within each major land resource area in Iowa.

Nutrient Management in Iowa - Nitrogen Application Timing

Nitrogen (N) application timing also affects nitrogen loss, as described in the Iowa Nutrient Reduction Strategy (INRS) Science Assessment of Nonpoint Source Practices (revised in 2024). The Iowa Nutrient Research and Education Council's (INREC) Survey of Agricultural Retailers tracks and provides annual estimates of when nitrogen is most commonly applied. In 2024, commercial N fertilizer was applied via spring, split, and/or in-season application on approximately 9.96 million corn acres (~76% of Iowa's 13.1 million corn acres), with fall application occurring on approximately 5.76 million corn acres (~44% of corn acres). While the amount of fall-applied anhydrous fertilizer varies from year to year, shifting all fall-applied anhydrous fertilizer to spring application has the potential to reduce nitrate-N loads. For example, the INRS Science Assessment modeled the effectiveness of changes in fertilizer timing at reducing nitrate-N loads; this assessment found that shifting benchmark period (2006-10) anhydrous application from fall to spring could have resulted in a nitrate-N load reduction of 200 tons/year.

According to the INRS Nonpoint Source Science Assessment, fall-applied anhydrous with EPA-approved nitrification inhibitors (e.g., nitrapyrin) could reduce nitrate-N concentrations in tile drainage water by an estimated 5% when compared to fall anhydrous applications without a nitrification inhibitor. Based on the Science Assessment, researchers estimated that during the 2006-10 benchmark period, fall anhydrous was applied annually to 5.7 million acres of corn-soybean and continuous corn acres. Of these acres, nitrification inhibitor was applied to 3.5 million acres. According to the Survey of Agricultural Retailers, farmers’ nitrification inhibitor use has increased since the benchmark period. As a comparison to the 1980-96 baseline period, researchers associated with the INRS Nonpoint Source Science Assessment suggest, based on professional knowledge, that nitrification inhibitor was used on a negligible number of acres due to the recent development of the technology.

Data - Nitrogen Application Timing

The data showing the timing of commercial N applications were obtained from the INREC Survey of Agricultural Retailers, which has been conducted annually since 2017. The statewide proportions of the data were calculated using a stratified, weighted approach, based on each field's size and the number of observations within each major land resource area in Iowa.

Bioreactors, Saturated Buffers, and Multi-Purpose Oxbows

Denitrifying bioreactors and saturated buffers are edge-of-field practices that are constructed by routing agricultural drainage water through a woodchip trench or vegetated buffer, respectively, to remove nitrate before the water enters an adjacent stream, ditch, or tile main. These practices are highly effective at reducing annual nitrate loads to streams. According to the Iowa Nutrient Reduction Strategy Science Assessment of Nonpoint Source Practices, bioreactors reduce nitrate load at an estimated average rate of 5 lb N per acre treated per year, and saturated buffers remove nitrate at an estimated average rate of 10 lb N per acre treated per year. Estimated reduction in flow-weighted nitrate concentration is 24% for water treated by bioreactors and 45% for water treated by saturated buffers. The suitability of bioreactors and saturated buffers for a farm field is highly dependent upon the presence of tile drainage, topography, and soil types.

Oxbows are old stream channels that have been cut off. They fill in with sediment over time and can be restored by excavating the soil down to the original channel level. Outletting tile to an oxbow provides both wildlife habitat and water quality benefits; this practice is known as a "multi-purpose" oxbow. With an estimated 63% reduction in flow-weighted nitrate concentration according to the Iowa Nutrient Reduction Strategy Science Assessment of Nonpoint Source Practices, or an estimated average annual nitrate load reduction of 6 lb N per acre treated, multi-purpose oxbows are effective at reducing nitrate delivery to streams.

A summary of edge-of-field practice distribution in Iowa by HUC8 watershed is summarized in Appendix C.

Data Sources - Bioreactors, Saturated Buffers, and Multi-Purpose Oxbows

Acres protected by bioreactors, saturated buffers, and multi-purpose oxbows were summarized using state and federal conservation program data as well as practices known to be installed by conservation staff, which provide detailed, spatial records of publicly funded practices. All state programs recorded by the Iowa Department of Agriculture and Land Stewardship were included in this analysis of cost-share practices, as well as practices under the federal Environmental Quality Incentive Program and Conservation Stewardship Program. Practices installed without financial assistance were primarily designed by Iowa State University. All of these practices have undergone design standard revisions over the past decade, and it is currently estimated that 50 acres are protected by each practice.

Water Quality Wetlands

Wetlands that are designed for water quality improvement (water quality wetlands) are estimated to reduce flow-weighted nitrate concentration by 30%, according to the Iowa Nutrient Reduction Strategy Nonpoint Source Science Assessment.  Put another way, it is estimated that WQ wetlands remove nitrogen at an average rate of 12 lb N per acre treated per year. In designing these types of wetlands, agricultural drainage is routed through the wetland for nitrate removal. Currently, water quality (WQ) wetlands require higher financial investment and development time than many other best management practices but have a lifespan of multiple decades or more, and have a lower cost per pound of nitrogen removed. Most of Iowa’s WQ wetlands, to date, have been constructed under the Conservation Reserve Enhancement Program (CREP), but novel wetland siting standards have been implemented over the past decade to expand WQ wetlands from the traditional CREP breakpoint wetland design. Novel wetland positions on the landscape include full and fractional flow designs, both intentionally designed to protect agricultural drainage water similar to the classic breakpoint design used for CREP-designed wetlands. Programs and individuals other than the Iowa Department of Agriculture and Land Stewardship and Farm Service Agency have installed wetlands in Iowa that are similarly sited and constructed similar to WQ wetland design guidelines, but data currently are not available to assess the full extent of this non-CREP implementation.

Currently, Iowa has at least 145 WQ wetlands, which have all been constructed since the 1980-96 baseline period of the Iowa Nutrient Reduction Strategy. These wetlands have a cumulative drainage area of over 165,000 acres. Iowa experienced its highest rate of installations in 2020, with 14 new wetlands treating nearly 18,800 acres. Program implementation continues, with wetland design types developed in recent years expanding landscape positions on which WQ wetlands can be sited. WQ wetlands that were constructed from 2011-2024 (i.e., since the 2006-10 benchmark period of the Iowa Nutrient Reduction Strategy) protect more than 107,000 acres of agricultural land.

A summary of water quality wetland distribution in Iowa by HUC8 watershed is summarized in Appendix C.

Data Sources - Water Quality Wetlands in Iowa

Acres protected by WQ wetlands were estimated using data from the Iowa Department of Agriculture and Land Stewardship. A majority of these wetlands were installed under the Conservation Reserve Enhancement Program, but some were funded through other programs and partnerships.

Structural Erosion Control Practices

The Iowa Nutrient Reduction Strategy Nonpoint Source Science Assessment identified a set of structural practices that capture sediment or reduce erosion within or at the edge of an agricultural field, leading to a reduction in soil-bound phosphorus loss. These practices include terraces, water and sediment control basins (WASCOBs), farm ponds, and grade stabilization structures. The Science Assessment estimates the effectiveness of these practices at reducing phosphorus loads to range from 77% to 85%.

Currently, it is assumed a significant portion of erosion control practices are constructed through the financial assistance of state and federal government cost-share programs and this report presents data from those sources. It is estimated that over 315,000 acres are protected by terraces, WASCOBs, ponds, and grade stabilization structures that have been installed under cost-share programs since 2011. 

An additional source of information on structural erosion control practices is the Iowa BMP Mapping Project, an ongoing effort that will estimate the extent of all erosion control installations, not just those funded by state or federal cost-share programs. Practices visible in aerial imagery and high-resolution topography data (statewide LiDAR data) during several time periods have been reviewed and practices have been mapped to track best management practice adoption over time. The project’s data collection is complete for three time periods: the 1980s, 2007-10, and 2016-17. Practices depicted by the Iowa BMP Mapping Project include both practices implemented with and without cost-share assistance; however, INRS reporting efforts only include practices that receive state or federal cost-share as self-funded practices are not reported annually.

A summary of structural erosion practice distribution in Iowa by HUC8 watershed is summarized in Appendix C.

Data Sources - Structural Erosion Control Practices in Iowa

Structural erosion control practices were summarized using state and federal conservation program data, which provide detailed, spatial records of publicly funded acres. This report accounts for practices installed between 2011 and 2024. All state programs recorded by the Iowa Department of Agriculture and Land Stewardship were included in this analysis of cost-share practices and practices under the federal Environmental Quality Incentive Program and Conservation Stewardship Program. Structural erosion control practices reported include terraces, sediment basins, grade stabilization structures, ponds, and water and sediment control basins (NRCS practice codes 600, 350, 410, 378, and 638). The database for state programs provides an estimate of the acres protected by each erosion control practice. For terraces, water and sediment control basins, and grade stabilization structures, and within each HUC8 watershed, the state database’s mean acres protected per foot installed was applied to federally-funded cost-share practices to obtain an estimate of total acres protected. For ponds, there were no federal practices to extrapolate, so only state data were used.

Tracking Point Source Nutrient Reduction Effort - Wastewater Treatment and Industrial Facilities

Understanding Point Source Efforts Associated with the Iowa Nutrient Reduction Strategy

The Iowa Nutrient Reduction Strategy identifies 160 industrial (54 permits) and municipal wastewater treatment point source facilities (106 permits) that are required to evaluate the amounts of nutrients in their discharges in order to meet the goals of the strategy. Upon receiving a National Pollutant Discharge Elimination System (NPDES) permit under the Strategy, each facility works to develop a feasibility study, which outlines the resources required to achieve nutrient reduction goals. The permits also incorporate requirements for measuring nutrient concentrations in influent and effluent to determine current nutrient removals and provide an empirical basis for feasibility studies.

As of January 7, 2025, municipal and industrial permits that have been amended with construction schedules to meet INRS goals are summarized in the table below.

Point source facilities listed in the strategy are required to monitor raw waste and final effluent for total nitrogen (TN) and total phosphorus (TP). However, some industries (e.g., power plants) that do not have a treatment plant are required to monitor only the final effluent as water is only to cool equipment. This extensive monitoring effort has generated one of the country’s most complete sets of point source nutrient data, and the extent of this data collection will continue to increase as the remaining permits are issued. This data has enabled the facilities and the Iowa Department of Natural Resources to determine current TN and TP loads associated with these point sources, even before additional nutrient reduction technologies are installed.

A facility uses the data collected during the two-year period after permit issuance to evaluate the feasibility and reasonableness of reducing the amounts of nutrients discharged into surface water. The Iowa Nutrient Reduction Strategy establishes a target of reducing TN and TP from point sources by 66% and 75%, respectively. A facility’s feasibility study must include an evaluation of operational changes that could be implemented to reduce the amounts of TN and TP discharged. If the implementation of operational changes alone cannot achieve the targets, the facility must evaluate new or additional treatment technologies that could achieve reductions in the nutrient amounts discharged. At the end of 2024, 153 feasibility studies had been submitted.

For INRS priority watersheds, two major POTWs are in the process of revising their NPDES permits. Note that changes in the number of facilities by year reflect changes in facility flow resulting in the change of the POTW classification, demonstrating that a source is not a nutrient source (e.g., industrial cooling purposes), or the combination of facilities (e.g., industrial waste treated by a municipal plant).

As these feasibility studies are reviewed and approved by the Iowa Department of Natural Resources, the schedules these contain for installing nutrient reduction technologies or optimizing existing treatment are added to the facilities’ NPDES permits by amendment. Once the construction or optimization outlined by the schedules is complete and treatment processes are optimized, facilities will submit twelve months of effluent TN and TP sampling results. Effluent limits based on those sampling results will then be added to facilities’ permits and become enforceable.

Point source facility permits with Nitrogen and/or Phosphorus limits as of the end of 2024 are summarized in the table below.

Of the permitted point source facilities, the number achieving INRS N and P load reduction goals since 2013 are summarized in the table below.

Reported N and P loads for major public and industrial facilities since the INRS was adopted in 2013 are summarized in the table below. The point source N and P load goals are 7,556 and 1,303 tons, respectively.

Water

An Overview of the Water Measurement Indicator

The INRS Logic Model and reporting on efforts via the dashboards enable the documenting of key metrics that inform feedback to identify and focus inputs, activities, and outputs to advance the goals of the INRS. Reporting water quality changes includes monitored nutrient loads from rivers and modeled changes in nonpoint source nutrient export (point source tracking reported in the Land dashboard) based on agricultural management and BMPs adopted or installed.

This dashboard summarizes each tracked component of the Water indicator:

• Annual N and P export from the state of Iowa based on measured loads from rivers
• Modeled impacts of INRS-related BMP adoption and construction, agronomic practices, and land use on water quality

Monitoring results demonstrate that water quality varies from year to year as a result of the interactions between and influences of weather, land management practices, conservation practice adoption, and point source nutrient loading.

The scales, spatially and temporally, at which it is anticipated that water quality benefits may be detectable from monitoring data are reviewed in the next tab. A 2020 nitrogen-focused report prepared for the Iowa DNR, How Long Will it Take to Measure an Improvement in Iowa's Water Quality?, explored this topic in greater detail.

Changes from 1980-1996, the "baseline period", to the initial INRS assessment from 2006-2010, the "benchmark period", are summarized in the table below for nonpoint (NPS) and point (PS) loads. Nonpoint and point source changes between the two periods are also summarized in a short publication. Practice effects on nutrient loads are compared to the baseline period.

*The methods used to derive the total nitrogen estimate of 292,022 tons indirectly reflected the point source contributions.

Measuring N and P Load at Scale and Time

Measuring changes in nutrient loads is challenging, as flow, the amount of runoff that leaves fields, is the strongest predictor of nutrient loss and varies annually. Detecting change at any scale over time is further complicated by the presence of "legacy nutrients" which have accumulated in soil and groundwater over time and are slowly lost to waterways, contributing to persistent water quality impairment even in areas with improved management practices. Understanding water quality improvement over time is important to evaluating progress towards INRS goals, and it is important to note that longer-term trends are better at illustrating progress than values associated with individual years.

Ongoing research continues to examine the spatial and temporal scales at which N and P load reductions can be quantified. The scales at which changes in water quality are likely detectable are summarized in the table below.

Water Quality Monitoring Infrastructure in Iowa

Statewide Monitoring
Water quality monitoring for statewide reporting in 2024 was conducted by the United States Geological Survey (USGS), the Iowa Department of Natural Resources (DNR), the Iowa Institute of Hydraulic Research (IIHR), and the Iowa Geological Survey (IGS). For years, the USGS has monitored a large number of sites, providing a long-term historical flow record. This allows for estimation of nutrient loads for prior years based on monitoring efforts by state agencies. 

The Ambient Water Quality Monitoring Network was comprised of 52 monitoring sites to measure N and P in 2024. Of these 52 sites, statewide loads were estimated from monitoring sites near the boundaries of Iowa at 18 and 16 sites for N and P, respectively. Nutrient loads reported are the total for each river at the monitoring site, including loads that originated from outside of Iowa.  These sites utilize DNR monthly sampling data, and USGS sensor data or IIHR-operated probes that offer “real-time” data as available, for each monitoring site to estimate load based on USGS flow data for each site.

More information about USGS, DNR, or IIHR-administered monitoring sites can be found on the following websites:

• USGS resources: See the USGS National Water Dashboard for nationwide gages
• DNR resources: DNR’s Water Monitoring summary (see Ambient Monitoring Programs)
• IIHR resources: IIHR’s Iowa Water Quality Information System provides real-time data

Note that real-time data from sensors is made publicly available upon collection, but records may not be certified until several months after data collection by the agency operating the gage or sensor.

Local Monitoring

Local monitoring efforts have been implemented by INRS reporting partners to monitor baseline conditions or to measure the effect of implementing a BMP(s). These efforts include DNR programs (other than the Ambient Stream Network) that monitor smaller rivers, streams, and lakes, and research programs conducted by Regents institutions, Iowa Soybean Association, and Agriculture’s Clean Water Alliance. The scope of monitoring includes general monitoring of surface waters, small watersheds (to assess the impact of a BMP at the field or small catchment scale), or tile drain outlet monitoring.

Monitoring efforts submitted by reporting partners are summarized at the HUC-12 watershed scale in Appendix D.

Surface Water Monitoring Sites in Iowa

Statewide N and P loads are measured at monitoring sites on major rivers (18 N sites and 16 P sites) near the boundary of Iowa before flowing into the Mississippi or Missouri Rivers. These monitoring sites are able to cover the majority of the surface area of the state of Iowa. In addition, areas of river basins originating in Minnesota are included in the Iowa statewide load as loads are reported at the basin outlet. 

The major rivers and contributing areas (upstream areas draining to each monitoring point), as well as monitoring efforts by organizations submitting monitoring efforts to INRS tracking, are summarized by HUC-12 watershed size in Appendix D.  

Iowa Precipitation Summary (2023)

The INRS reports on nutrient loading and water yields at the state scale. However, the amount of water each region receives, which varies regionally and temporally each year, drives the amount of flow. In 2023, the average precipitation for Iowa was 26.82 inches, 8.73 inches less than the long-term average. The water yield during the INRS baseline period was approximately one-third of precipitation, while in 2023, the water yield was about 16% of precipitation.

More information about Iowa's climate - monthly or annual climate summaries, maps, current conditions, and drought reports - can be found on the Iowa Climate Bureau's webpage on the Iowa Department of Agriculture and Land Stewardship website.

Iowa Precipitation Summary (2024)

The INRS reports on nutrient loading and water yields at the state scale. However, the amount of water each region receives, which varies regionally and temporally each year, drives the amount of flow. In 2024, the 29th wettest year on record for Iowa, the state's average precipitation was 36.95 inches, which is 1.4 inches greater than the long-term average precipitation depth. The water yield during the INRS baseline period was approximately one-third of precipitation, while in 2024, the water yield was about one-fourth of precipitation.

More information about Iowa's climate - monthly or annual climate summaries, maps, current conditions, and drought reports - can be found on the Iowa Climate Bureau's webpage on the Iowa Department of Agriculture and Land Stewardship website.

Water Yield for Iowa

The net amount of water generated on the basis of stream flow versus precipitation regressions for watersheds across Iowa. Flow is summarized below and used to assess nutrient load relative to flow in subsequent sections.

Measured Changes in N Export Based on River Monitoring (2023)

The nitrate-N load from Iowa for 2023 was about four times lower than the 24-year average load, while the 2023 flow-weighted nitrate-N load (FWNL) was about 60% of the 24-year average load.

The statewide water yield in 2023 was less than half of the average water yield for the 2000-2023 period. Periods of low statewide flow tend to correspond to low statewide N loads, as less N is lost through runoff or drainage from fields. However, N losses per water yield (FWNL) during wet periods following a drought year are typically larger as increased flow leads to leaching of unutilized nutrients from soils. 

Measured Changes in N Export Based on River Monitoring (2024)

The nitrate-N load from Iowa for 2024 was about 1.2 times greater than the 25-year average load, while the 2024 flow-weighted nitrate-N load (FWNL) was the greatest recorded since 2000 (approximately 135% of the 25-year average load).

The statewide water yield in 2024 was about 1.25 inches less than the average water yield for the 2000-2024 period. While years with lower flows (drier years) often see low statewide N loads, N losses per water yield (FWNL) in a year following a drought year (e.g., 2024) are typically larger, as unutilized nutrients that have been able to accumulate in soils are "flushed out" with increased flow.

Annual data is driven primarily by flow within each monitoring year. Applying a five-year moving average assists in characterizing statewide loads and variability anticipated with changes in inter-annual precipitation. These trends are cyclical and are observed in the nearly twenty years of available data. Statewide N loads reflect agronomic practices and adoption of N management BMPs in addition to hydrologic trends.

Water Quality - Nitrogen Monitoring

Nutrient loading from Iowa is reported as the monitored load in rivers (modeled for each river based on collected monitoring data) and modeled impact of land use practices on nutrient load. Information on how nonpoint and point source loads were determined for the INRS baseline can be found in the reports titled:

• Assessment of the Estimated Non-Point Source Nitrogen and Phosphorus Loading from Agricultural Sources from Iowa During the 1980-96 Hypoxia Task Force Baseline Period; and
• Nitrogen and Phosphorus Load Estimates from Iowa Point Sources During the 1980-96 Hypoxia Task Force Baseline Period

These resources established the baseline from which the nutrient load reduction goals for nonpoint and point sources are established. Both N and P loads during the baseline and benchmark periods (INRS Science Assessment period of 2006-2010) are summarized in the reports above.

State agencies, universities, and the United States Geological Survey have expanded river monitoring over the past decades to improve understanding of flow and monitor nutrients. Monitoring infrastructure was not available to measure N loads during the INRS baseline period of 1980-1996, so nonpoint and point source loads were interpolated from wastewater and population data, land use, agronomic management, practice adoption, and precipitation during the time period. The infrastructure that has been available since at least 2000 now provides a means to quantify statewide nutrient losses. Nitrogen losses can be represented as concentrations, annual loads, and flow-normalized loads.

In 2017, the INRS science team evaluated and recommended the Linear Interpolation method be used to model N load for river monitoring data (see Schilling et al. 2017 and the NRS 2017 Supplemental report titled “Assessment of the Estimated Non-Point Source Nitrogen and Phosphorus Loading from Agricultural Sources from Iowa During the 1980-96 Hypoxia Task Force Baseline Period"). This method fills in data gaps between sampling events for each monitoring site by drawing a straight line and provides a robust measure of N load. Frequent sampling provides the highest quality data with a longer time between sampling periods increasing the potential uncertainty in the modeled load.

Precipitation is the primary driver of stream flow in Iowa's rivers, and flow from rivers is monitored by the USGS (see more about gaging locations in the previous panel). Annual average flow depths by watershed are calculated using streamflow and watershed area. Statewide flow is then calculated as an area-weighted average of the flow calculated for each watershed.

Measured Changes in P Export Based on River Monitoring

Total phosphorus (P) loads are strongly correlated with the amount of flow, and P losses associated with just a few large storm events can comprise the majority of a year's P load. 

Corresponding to lower flows over many of the past several years, annual P loads and flow-weighted P loads (FWPL) have generally been lower than the target loads set by the INRS (i.e., 45% reduction in total P loads from those of the 1980-1996 baseline period).

However, a 5-year moving average is a better metric of patterns over time as it reduces the influence of year-to-year streamflow variation on P loads. For nearly every year since 2002 (the period where calculations begin), the 5-year moving average P load has been greater than the INRS P load goal, driven by P losses during years of average or above-average flow despite the lower losses from occasional periods of drought.

The FWPL is largely below the INRS baseline for the available historical record annually, and the 5-year moving average FWPL is continuously below the baseline period. This demonstrates the impact of high flow years compared to the 5-year moving average. However, no consistent trend in the annual or 5-year moving average FWPL is observed.

Annual P loads are calculated using the WRTDS-K model. Streamflow, season, measured P, and turbidity data from monitoring stations are used to calibrate the model. Values are updated each year as inclusion of additional data results in improved P load predictions. Annual flow-weighted P loads are calculated by dividing annual P loads (tons) by the amount of streamflow occurring that year (inches of flow/year). This reduces the influence of annual streamflow variation on nutrient loads when looking at trends over time.

Between the baseline period and the historical record for which statewide P loads have become available, there were significant changes in farm operations. Farm tillage practices rapidly transitioned in the 1980s to comply with soil conservation provisions established in the 1985 Farm Bill, the development of farm implement tools to manage higher residue systems, P application management in conjunction with soil testing, and crop protection practices. These phases of soil erosion conservation and P application management had significant impacts on statewide P losses during the INRS baseline period and before the release of the INRS in 2013.

Water Quality - P Monitoring

The river monitoring network for P is comparable to N, and infrastructure was described previously in the N section.

Iowa’s annual P loads are modeled as two distinct chemical forms, orthophosphate (OP) and particulate phosphorus (Part P). Part P is the P bound to particulate matter, such as sediment, while OP represents the dissolved form of P. Summing these two P sources, OP and Part P, produce Iowa’s overall P load. 

While they are used for estimating N loads, linear interpolation methods are not appropriate for estimating P. P loads in rivers are strongly influenced by storms, and high flow events, although infrequent, can strongly impact the P load; hence, other estimation methods are needed.

Using data from monitoring stations, P loads are estimated for each of 16 rivers near the border of Iowa using a combination of two modeling techniques: 1) the Weighted Regression on Time, Discharge and Season-Kalman Filter (WRTDS-K; Hirsch et al., 2010; Zhang & Hirsch, 2019) framework developed by the United States Geological Survey and 2) turbidity-based surrogacy models. A unique relationship between streamflow, measured P, and turbidity is used for each monitoring station to estimate P loads for that river. Data are made available for the state of Iowa. Additional research is ongoing to improve estimates of P loads during high-flow events in several watersheds in the western half of the state.

The WRTDS-K models used for measuring P use streamflow, time of year, water quality trends, and observed P data to fill gaps between measured P concentrations. The surrogacy models use power regression to establish a relationship between turbidity, a measure of the water’s cloudiness, and Part P. These surrogacy models are more accurate than their WRTDS-K counterparts in estimating Part P and are used whenever on-site turbidity data are available.

More information about the WRTDS-K methodology may be found in the following references:

Hirsch, R. M., Moyer, D. L., & Archfield, S. A. (2010). Weighted regressions on time, discharge, and season (WRTDS), with an application to Chesapeake Bay river inputs. Journal of the American Water Resources Association, 46(5), 857-880. doi:10.1111/j.1752-1688.2010.00482.x

Zhang, Q., & Hirsch, R. M. (2019). River Water-Quality Concentration and Flux Estimation Can be Improved by Accounting for Serial Correlation Through an Autoregressive Model. Water Resources Research, 55(11), 9705-9723. doi:10.1029/2019WR025338

Changes in N from Nonpoint Sources

Modeled N load impacts of INRS practices for which reported practice adoption data is currently available compared to the baseline period. Negative values indicate a modeled N load reduction – practices are advancing statewide INRS N goals - and positive values indicate a modeled increase in N load for management or practice compared with the 1980-1996 INRS baseline period (292,022 tons N). The percent change for each practice is calculated independently of other practices and represents the benefits of the stand-alone practice without interaction with any other practice or management effect (values aren’t additive). The potential interaction of practices remains an active area to be assessed and will be integrated into reporting as methods become available.

Adoption of BMPs that can be broadly adopted across Iowa increased in number, and the acreage benefitted from these practices. Cover crop adoption increased to an estimated 3.7 million acres in 2022 (INREC Ag Retailer Survey) – up from 2.8 million acres in 2021 – and is estimated to reduce N losses from the baseline period by 5.3%, or more than 15,356 tons. A rye cover crop reduces N load by an estimated 28% per the INRS (majority of cover crops planted are cereal rye or include it in the mix), with cereal rye accounting for more than 85% of cover crops planted in each of the past five years per the INREC Survey.

Similarly, practices that can be implemented at the edge-of-field (bioreactors, saturated buffers, and multi-purpose oxbows) and water quality wetlands, have seen an increase in practice adoption in recent years. These practices benefitted at least 12,600 and 147,000 for EOF practices and wetlands, respectively. These practices reduced N loads by 46.8 and 863 tons for EOF practices and nitrate removal wetlands in 2022. The adoption of these practices has increased primarily due to more recent developments of these practices and increasing prioritization in the state to scale up installations.

The effect of in-field management and land use on modeled N load indicated an increase in statewide N load with changes in agronomic practices and land use. Agronomic management and land uses are cyclical and reflect market conditions and field access. The N application rate to corn in a corn-soybean rotation had the greatest estimated increase in statewide N load, with an increase of approximately 13.3% (38,900 tons) when compared to the estimated rate during the baseline period. These rates are stand-alone and compare rates from 2022 to the 1980-1996 baseline period. The adoption of other agronomic practices or BMPs (application timing, method, inhibitor use, etc.) that have been adopted at the same time N rates have increased are also depicted in the estimated N load.

For some nutrient reduction practices, insufficient data are available to complete a statewide assessment. The evaluation of other data sources for tracking these practices over time is ongoing.

Data Analysis - N Modelling of Nonpoint Sources

Consistent with modeling approaches as in the original INRS Science Assessment, load reduction estimates were calculated for a selection of INRS practices for which practice adoption data is available. The acreages and extent of these practices were determined using various data sources, including public conservation program databases, the Cropland Data Layer, the USDA Census of Agriculture, and the Iowa Nutrient Research Education Council Agricultural Retailer Survey. For more information on the approximation of BMP use in Iowa, refer to the "Tracking Nonpoint Source Nutrient Reduction Practices - Agricultural Conservation Practices" section (farther above). These assessments are on a per-practice basis and don’t factor in the additive or in-series effects of multiple or layered practices. Modeled load changes for the INRS are based on changes by Major Land Resource Area aggregated to the state level for individual practices.

Changes in P from Nonpoint Sources

Modeled P load impacts of INRS practices for which reported practice adoption data is currently available compared to the baseline period, except for terraces and basins that are compared to changes since 2010 (contingent on records availability). Negative values indicate a modeled P load reduction – practices are advancing statewide INRS P goals - and positive values indicate a modeled increase in P load for management or practice compared with the 1980-1996 INRS baseline period (23,822 tons P). The percent change for each practice is calculated independently of other practices and represents the benefits of the stand-alone practice without interaction with any other practice or management effect (values aren’t additive). The potential interaction of practices remains an active area to be assessed and will be integrated into reporting as methods become available.

In contrast to N, nonpoint P losses in Iowa from fields are dominated by erosion of sediment and phosphorus bound to it. Adopting tillage practices that decrease soil disturbance or leave residue to protect soil is critical to reducing P loads. Tillage practices such as no-till and conservation tillage (leaving at least 30% residue) are estimated to reduce P loads by 90% and 33%, respectively, per the INRS. The adoption of these practices has grown to an estimated 8,467,057 and 6,550,844 acres, reflecting a reduction of 17.8% (4,230 tons) and 0.1% (31.6 tons) in statewide P load as of 2022, respectively.

Structural BMPs such as terraces and basins that can reduce or trap sediment offer similar benefits by preventing soil and, thereby, the attached phosphorus from leaving a field. Since 2010, practices that protect approximately 310,000 acres have been built using public financial assistance programs and are estimated to reduce P loads by 191 tons, or 0.8% of the baseline P load. The modeled benefits of practices are independent of other practices and the interaction of adopted BMPs, agronomic practices, and land use continue to be explored.

An analysis by Geosyntec consultants, Quantification of Phosphorus Loss due to Structural Agricultural BMP Implementation – Final Report, supported by the Iowa Nutrient Research Education Council, leveraged the Iowa BMP Mapping project to assess the benefits of practice adoption from the 1980s (the leading period of the INRS baseline) to 2016-2018 (the last data collection period of the BMP mapping project). This project assessed practice adoption in approximately 20% of HUC-12 watersheds across Iowa that were determined to be statistically representative of the Iowa Major Land Resource Areas for which nutrient losses were determined for the INRS. Using INRS practice efficiencies, these practices were estimated to reduce phosphorus losses from fields by 5.2% in the 1980s and increase to 9.5% by 2016-2018. Researchers continue to develop models to assess the benefits of structural BMPs at the state level and build upon this first effort to assess structural BMPs that provide year-over-year benefits in reducing phosphorus runoff from fields.

For some nutrient reduction practices, insufficient data are available to complete a statewide assessment. The evaluation of other data sources for tracking these practices over time is ongoing. 

Data Analysis - P Modelling of Nonpoint Sources

The same sources for P modeling were used as N (see Data Analysis - N Modelling of Nonpoint Sources above).

Recommended citation: Iowa Department of Agriculture and Land Stewardship, Iowa Department of Natural Resources, and Iowa State University. (June 2026). Tracking the Iowa Nutrient Reduction Strategy. Version 5.0.

Appendix A

Note that this table will be updated when the 2024 Tracking Funding and Resources dashboard is published.

Appendix B