Watersheds In Iowa Research Articles

Understanding the Impact of Precipitation Bias‐Correction and Statistical Downscaling Methods on Projected Changes in Flood Extremes

AbstractThis study evaluates five bias correction and statistical downscaling (BCSD) techniques for daily precipitation and examines their impacts on the projected changes in flood extremes (i.e., 1%, 0.5%, and 0.2% floods). We use climate model outputs from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to conduct hydrologic simulations across watersheds in Iowa and determine historical and future flood extreme estimates based on generalized extreme value distribution fitting. Projected changes in these extremes are examined with respect to four Shared Socioeconomic Pathways (SSPs) alongside five BCSD techniques. We find the magnitude of the estimates of future annual exceedance probabilities (AEPs) are expected to increase under all SSPs, especially for the emission scenarios with higher greenhouse gases concentrations (i.e., SSP370 and SSP585). Our results also suggest the choice of BCSD impacts the magnitude of the projected changes, with the SSPs that play a more limited role compared to the choice of downscaling method. The variability in projected flood changes across Iowa is similar across the downscaling technique but increases as the AEP increases. Our findings provide insights into the impact of downscaling techniques on flood extremes' projections and useful information for climate planning across the state.

Evaluating the tradeoff between cost effectiveness and participation in agricultural conservation programs

AbstractUsing a survey of 430 farmer respondents in the Boone and North Raccoon River watersheds in Iowa, we examine the impacts of three program innovations—reverse auctions, spatially targeted payments, and higher offered payments—on agricultural conservation program cost effectiveness and participation by farmers. We combine farmer responses to a discrete choice experiment offering voluntary conservation contracts with township‐level estimates of per‐acre nitrogen reductions from each practice derived from the process‐based ecohydrological Soil and Water Assessment Tool model. Using a random‐parameters logit model, we show that both cost‐reducing and benefit‐boosting interventions reduce budgetary costs per projected pound of nitrogen removed from the watershed for each practice and thus are more cost effective than the prevailing current cost‐share programs. However, we find that these interventions can reduce participation by 30%–70%. Our policy simulations show that even with large budgets, the watershed‐level nitrogen reduction from all policy interventions remains far below the policy targets set by the Iowa Nutrient Reduction Strategy. Furthermore, we find cover crop contracts are far more cost effective than no‐till/strip‐till split nitrogen application contracts.

An Approach for Prioritizing Natural Infrastructure Practices to Mitigate Flood and Nitrate Risks in the Mississippi-Atchafalaya River Basin

Risks from flooding and poor water quality are evident at a range of spatial scales and climate change will exacerbate these risks in the future. Natural infrastructure (NI), consisting of structural or perennial vegetation, measures that provide multiple ecosystem benefits have the potential to reduce flood and water quality risks. In this study, we intersected watershed-scale risks to flooding and nitrate export in the Mississippi-Atchafalaya River Basin (MARB) of the central U.S. with potential locations of seven NI practices (row crop conversion, water, and sediment control basins, depressional wetlands, nitrate-removal wetlands, riparian buffers, and floodplain levees and row crop change) to prioritize where NI can be most effective for combined risk reduction at watershed scales. Spatial data from a variety of publicly-available databases were analyzed at a 10 m grid cell to locate NI practices using a geographic information system (GIS). NI practices were presented at the regional basin scale and local Iowa-Cedar watershed in eastern Iowa to show individual practice locations. A prioritization scheme was developed to show the optimal watersheds for deploying NI practices to minimize flooding and water quality risks in the MARB. Among the 84 HUC4 basins in the MARB, 28 are located in the Upper Mississippi and Ohio Rivers basins. The Wabash and Iowa-Cedar basins (HUCs 0512 and 0708, respectively) within these basins were found to rank among the uppermost quintile for nearly all practices evaluated, indicating widespread opportunities for NI implementation. Study results are a launching point from which to improve the connections between watershed scale risks and the potential use of NI practices to reduce these risks.

Determination of accurate baseline representation for three Central Iowa watersheds within a HAWQS-based SWAT analyses

Improving food systems to address food insecurity and minimize environmental impacts is still a challenge in the 21st century. Ecohydrological models are a key tool for accurate system representation and impact measurement. We used a multi-phase testing approach to represent baseline hydrologic conditions across three agricultural basins that drain parts of north central and central Iowa, U.S.: the Des Moines River Basin (DMRB), the South Skunk River Basin (SSRB), and the North Skunk River Basin (NSRB). The Soil and Water Assessment Tool (SWAT) ecohydrological model was applied using a framework consisting of the Hydrologic and Water Quality System (HAWQS) online platform, 40 streamflow gauges, the alternative runoff curve number method, additional tile drainage and fertilizer application. In addition, ten SWAT baselines were created to analyze both the HAWQS parameters (baseline 1) and nine alternative baseline configurations (considering the framework). Most of the models achieved acceptable statistical replication of measured (close to the outlet) streamflows, with Nash-Sutcliffe (NS) values ranging up to 0.80 for baseline 9 in the DMRB and SSRB, and 0.78 for baseline 7 in the NSRB. However, water balance and other hydrologic indicators revealed that careful selection of management data and other inputs are essential for obtaining the most accurate representation of baseline conditions for the simulated stream systems. Using cumulative distribution curves as a criterion, baselines 7 to 10 showed the best fit for the SSRB and NSRB, but none of the baselines accurately represented 20% of low flows for the DMRB. Analysis of snowmelt and growing season periods showed that baselines 3 and 4 resulted in poor simulations across all three basins using four common statistical measures (NS, KGE, Pbias, and R2), and that baseline 9 was characterized by the most satisfactory statistical results, followed by baselines 5, 7 and 1.

Nitrate losses across 29 Iowa watersheds: Measuring long-term trends in the context of interannual variability.

In the U.S. Corn Belt, annual croplands are the primary source of nitrate loading to waterways. Long periods of fallow cause most nitrate loss, but there is extreme interannual variability in the magnitude of nitrate loss due to weather. Using mean annual (2001-2018) flow-weighted nitrate-N concentration (FWNC; mg NO3 - -N L-1 ), load (kg NO3 - -N), and yield (kg NO3 - -N ha-1 cropland) for 29 watersheds, our objectives were (a) to quantify the magnitude and interannual variability of 5-yr moving average FWNC, load, and yield; (2) to estimate the probability of measuring 41% reductions in nitrate loss after isolating the effect of weather on nitrate loss by quantifying the interannual variability of nitrate loss in watersheds where there was no trend in 5-yr moving average nitrate loss (Iowa targets a 41% nitrate loss reduction from croplands); and (c) to identify factors that, in the absence of long-term trends in nitrate loss, best explain the interannual variability in nitrate loss. Averaged across all watersheds, the mean probability of measuring a statistically significant 41% reduction in FWNC across 15 yr, should it occur, was 96%. However, the probabilities of measuring 41% reductions in nitrate load and yield were only 44 and 32%. Across watersheds, soil organic matter, tile drainage, interannual variability of precipitation, and watershed area accounted for interannual variability in these nitrate loss indices. Our results have important implications for setting realistic timelines to measure nitrate loss reductions against the background of interannual weather variation and can help to target monitoring intensity across diverse watersheds.

Determination of Accurate Baseline Representation for Three Central Iowa Watersheds within a Hawqs-Based Swat Analyses

Determination of Accurate Baseline Representation for Three Central Iowa Watersheds within a Hawqs-Based Swat Analyses

Environmental and cost benefits of multi‐purpose buffers in an agricultural watershed for biomass production

AbstractThe cost‐benefit of riparian buffers to landowners has not been fully understood despite its ecosystem benefits. This study investigates the environmental and economic value of multi‐purpose buffers for biomass production under alternative land‐management scenarios at the watershed scale. The work incorporates the Soil and Water Assessment Tool with a field‐scale tool, the Agriculture Conservation Practice Framework, and cost‐benefit analysis. Our work took place in the agriculturally dominant Raccoon River watershed (RRW) in Iowa. The cost analysis includes implementing and harvesting the switchgrass buffers, the value of nitrogen and phosphorus fertilizer, and the forgone income of cropland to determine the net return under 12 alternative feedstock scenarios. We estimated that 14 603 Mg of nitrogen and 2563 Mg of phosphorus runoffs can be intercepted by riparian buffers in the RRW, representing a nutrient loss reduction of 11% of nitrogen and 26% of phosphorus. These nutrients are valued at $1.31 million (M) for phosphorus and $1.24 M for nitrogen as fertilizers. The trapped nutrients account for 12% of the riparian buffers’ total cost. We found that net economic returns increase with biomass market price, increase with maintenance fertilizer application, and decrease with the cropland areas that are converted to buffers. Under current land management, a biomass market price at $60 per dry ton of biomass would be economically viable at the sub‐basin level when maintenance fertilizers are applied. At $80 per dry ton, all 108 sub‐basins in the RRW would be profitable up to $517 a year. The results highlight the trade‐offs between water quality and profitability. Published 2021. This article is a U.S. Government work and is inthe public domain in the USA. Biofuels, Bioproducts and Biorefining published by Society of Industrial Chemistry and John Wiley & Sons Ltd.

Location Matters: A Framework to Investigate the Spatial Characteristics of Distributed Flood Attenuation

Distributed attenuation in flood management relies on small and low-impact runoff attenuating features variously distributed within a catchment. Distributed systems of reservoirs, natural flood management, and green infrastructure are practical examples of distributed attenuation. The effectiveness of attenuating features lies in their ability to work in concert, by reducing and slowing runoff in strategic parts of the catchment, and desynchronizing flows. The spatial distribution of attenuating features plays an essential role in the process. This article proposes a framework to place features in a hydrologic network, group them into spatially distributed systems, and analyze their flood attenuation effects. The framework is applied to study distributed systems of reservoirs in a rural watershed in Iowa, USA. The results show that distributed attenuation can be an effective alternative to a single centralized flood mitigation approach. The different flow peak attenuation of considered distributed systems suggest that the spatial distribution of features significantly influences flood magnitude at the catchment scale. The proposed framework can be applied to examine the effectiveness of distributed attenuation, and its viability as a widespread flood attenuation strategy in different landscapes and at multiple scales.

Development of statistical models for estimating daily nitrate load in Iowa

There is an ongoing need to increase our understanding of the sources and timing of stream nitrate loads across agricultural watersheds in Iowa as water quality improvement strategies are implemented. The goal of this study was to model the relationship between nitrate load and the two components of streamflow (i.e., baseflow and stormflow) to quantify in-stream nitrate patterns and develop a new method for estimating loads on days when monitoring data are not available. We analyzed eight watersheds in Iowa that had long-term water quality data where grab samples have been collected from 1987 to 2019. Four regression models were developed that related daily nitrate load to daily baseflow, stormflow, and streamflow discharge. The first model considered baseflow as a predictor, the second model used stormflow, the third model included both baseflow and stormflow as two different covariates, and the final model used total streamflow (unseparated). For all eight watersheds, the baseflowstormflow models had the highest correlation coefficients, which indicates that both components are necessary and together improve nitrate load estimates. While baseflow models estimated lower nitrate loads better, stormflow models captured the variability associated with larger loads. In addition, streamflow models tended to overestimate large nitrate loads. This simple modeling framework can be used to calculate daily, monthly and annual nitrate loads. Delineating nitrate loads between stormflow and baseflow can help identify differences in nitrate sources for nutrient reduction and remediation.

An Integrated Flood Risk Assessment and Mitigation Framework: A Case Study for Middle Cedar River Basin, Iowa, US

Property buyout is one of the most frequently preferred flood mitigation applications by decision-makers for long-term risk reduction. Due to its high-level funding requirements as a mitigation solution, it requires extensive benefits and costs analysis for the selected region. Many communities in the State of Iowa experienced extreme flood events (i.e., 1993, 2008, 2014, 2019), which resulted in a heavy economic impact over the last couple of decades. Nearly 3000 property acquisitions have been made between 2007 and 2017 using federal programs. This study presents a web-based Flood Risk Assessment and Mitigation Environment (FRAME), which provides visual data analytics capabilities to analyze property and community level benefit-cost analysis for property acquisitions. The FRAME allows users to explore and visualize historical mitigation projects and buyouts, and evaluate avoided damages for their communities. As a case study, a detailed benefit-cost analysis of historical property buyouts and direct losses of existing properties in the Middle Cedar watershed in Iowa is studied using stream gauge data from the United States Geological Survey (USGS). Projected stream gauge datasets, which are outputs of two climate scenarios (A1FI-fossil intensive and A2-low emission), are also utilized to assess future avoided losses for acquisitions and possible direct economic losses for existing properties. Case study results indicate that the average benefit-cost ratio (BCR) for buyouts in the studied region is around 0.86. Nearly half of the buyouts reached 4.72 BCR in low emission and 6.3 BCR in fossil intensive climate projections if future floods are considered.

Return of Topeka Shiner (Notropis topeka) to Restored Oxbows in the White Fox Creek Watershed, Iowa, USA

The Topeka shiner (Notropis topeka), a federally endangered species of minnow endemic to the Great Plains region of the central United States, has experienced widespread population declines resulting from loss of habitat. N. topeka habitat in Iowa, most notably oxbow wetlands, was mostly eliminated from the landscape during European settlement and agriculturalization of the region. Over the last two decades, restoration of oxbow habitats in Iowa has been increasing. Restorations provide critical habitat for N. topeka and regenerate a variety of ecosystem services that benefit many species of flora and fauna. There are signs that restoration of oxbow ecosystems is generating positive impacts on N. topeka recovery. Recent studies revealed that N. topeka populations are recovering within a few Iowa watersheds. In the investigation described here, we report the 2020 discovery of two N. topeka specimens in different restored oxbows within the White Fox Creek HUC10 of north central Iowa. Prior to these collections, the species had been undetected within this basin for 36 years. (It is possible that N. topeka persisted within the basin but remained undetected during sampling efforts.) Multiple oxbows have been restored within the White Fox Creek basin in recent years, and significant source populations of N. topeka can be found in nearby watersheds. These collections suggest that as oxbow restorations continue and land stewardship practices improve, N. topeka populations might recover and become reestablished within the historical range.

Erosion and sediment delivery in southern Iowa watersheds: Implications for conservation planning

Soil sediment export from agricultural watersheds is a major environmental concern and is a primary contributor to nonpoint source sediments in streams. The purpose of this study was to quantify total suspended solid (TSS) export and current sediment delivery ratios (SDRs) in four southern Iowa watersheds and evaluate how existing and potential best management practices (BMPs) have affected SDRs. Our study updated estimates of SDRs that were previously developed using mid-20^th century data and largely unknown methods. We estimated TSS export using continuous turbidity measurements and total phosphorus (P) data and measured discharge data to calculate TSS loads. By comparing annual TSS export to watershed-scale soil erosion estimated with the Revised Universal Soil Loss Equation (RUSLE) model, we calculated annual SDRs for the study watersheds and found that current SDRs were significantly lower than previous estimates. This new analysis provides an exceptional story of conservation progress in our study watersheds over the past four decades. Further, they are likely a worst-case scenario for sheet and rill eroded sediment export since TSS export does not distinguish among other sediment sources, such as stream bank and gully erosion. Based on the extent of BMP implementation in the watersheds and the potential for future BMPs determined using the Agricultural Conservation Planning Framework (ACPF) toolbox, we found that there is only limited potential for further reducing TSS export using additional in-field practices. Hence, we believe that further work toward reducing TSS export in these Iowa watersheds should be shifted to reducing contributions from other TSS sources including from streambed, bank, and gully erosion.

Crops as sensors: Using crop yield data to increase the robustness of hydrologic and biogeochemical models

Hydrologic models are most commonly calibrated only for streamflow at one or multiple stations within the watershed. However, it has been argued that in distributed hydrologic models with large numbers of parameters, it is possible to get good streamflow statistics with a completely incorrect representation of the internal watershed processes. Consequently, these models fail when they are used for other purposes, for example, biogeochemical prediction. Specifically, in watersheds dominated by agricultural land use, there is information on crop yield that is rarely used for constraining hydrologic fluxes. However, crops can potentially act as distributed sensors in the landscape and help in constraining evapotranspiration fluxes and improving model consistency. Our main goal in this paper is to address this important process understanding and show how using the information on crop yield helps to identify the proper model structure, thereby improving the robustness of hydrologic models.Using a 32,660 km2 agricultural watershed in Iowa as a case study, we used stepwise model refinement to show how the consideration of additional data sources can increase model consistency. We first developed a hydrologic model using the Soil and Water Assessment Tool that provided excellent monthly streamflow statistics at eight stations within the watershed. However, comparing crop yield measurements with modeled results revealed a strong underestimation in model estimates, and it was apparent that the model was getting the “right” streamflow by lowering the evapotranspiration flux and allowing the crops to die. Adding crop yield as an additional calibration target allowed us to identify the models’ structural deficiency and alter the potential evapotranspiration estimation method, thereby considerably improved model predictions of crop yield, while not altering streamflow predictions considerably. The modified model was able to (i) better capture variations in nitrate loads at the catchment outlet with no calibration and (ii) reduce parameter uncertainty, model prediction uncertainty, and equifinality. These findings highlight that using additional data sources to improve the hydrological consistency of distributed models increases their robustness and predictive ability.

Biomass Production with Conservation Practices for Two Iowa Watersheds

AbstractHydrologic modeling was used to estimate potential changes in nutrients, suspended sediment, and streamflow in various biomass production scenarios with conservation practices under different landscape designs. Two major corn and soybean croplands were selected for study: the South Fork of the Iowa River watershed and the headwater of the Raccoon River watershed. A physically based model, the Soil and Water Assessment Tool, was used to simulate hydrology and water quality under different scenarios with conservation practices and biomass production. Scenarios are based on conservation practices and biomass production; riparian buffer (RB), saturated buffer, and grassed waterways; various stover harvest rates of 30%, 45%, and 70% with and without winter cover crops; and conversion of marginal land to switchgrass. Conservation practices and landscape design with different biomass feedstocks were shown to significantly improve water quality while supporting sustainable biomass production. Model results for nitrogen, phosphorus, and suspended sediments were analyzed temporally at spatial scales that varied from hydrologic response units to the entire watershed. With conservation practices, water quality could potentially improve by reducing nitrogen loads by up to 20%–30% (stover harvest with cover crop), phosphorus loads by 20%–40% (RB), and sediment loads by 30%–70% (stover harvest with cover crop and RB).

Seasonal variations in export of antibiotic resistance genes and bacteria in runoff from an agricultural watershed in Iowa

Seasonal variations of antimicrobial resistance (AMR) indicators in runoff water can help improve our understanding of AMR sources and transport within an agricultural watershed. This study aimed to monitor multiple areas throughout the Black Hawk Lake (BHL) watershed (5324 ha) in central Iowa during 2017 and 2018 that consists of both swine and cattle feeding operations as well as known areas with manure application. The measured indicators included plate counts for fecal indicator bacteria (FIB) E. coli, Enterococcus, antibiotic resistant fecal indicator bacteria (ARBs) tylosin resistant Enterococcus, tetracycline resistant Enterococcus, and antibiotic resistance genes (ARGs): ermB, ermF (macrolide), tetA, tetM, tetO, tetW (tetracycline), sul1, sul2 (sulfonamide), aadA2 (aminoglycoside), vgaA, and vgaB (pleuromutilin). Both the plate count and the ARG analyses showed seasonal trends. Plate counts were significantly greater during the growing season, while the ARGs were greater in the pre-planting and post-harvest seasons (Wilcoxon Rank-Sum Test p < 0.05). The ermB gene concentration was significantly correlated (p < 0.05) with E. coli and Enterococcus concentrations in 2017, suggesting a potential use of this ARG as an indicator of environmental AMR and human health risk. Flow rate was not a significant contributor to annual variations in bacteria and AMR indicators. Based on observed seasonal patterns, we concluded that manure application was the likely contributor to elevated ARG indicators observed in the BHL watershed, while the driver of elevated ARB indictors in the growing season can only be speculated. Understanding AMR export patterns in agricultural watersheds provides public health officials knowledge of seasonal periods of higher AMR load to recreational waters.

Local Government Perspectives on Collaborative Governance: A Comparative Analysis of Iowa's Watershed Management Authorities

Collaborative governance has been promoted for decades as a means to tackle complex water management problems worldwide. Yet, watershed based efforts often lack interdependent consequences that can motivate participation, given upstream–downstream asymmetries. Additionally, watershed collaborations often have limited legal authority to take action, which can be due to political conflicts. While local governments often participate in collaborations, few studies have examined their motivations or how local governments could use existing legal authority to enact projects or change policies. This paper focuses on four cases in Iowa, USA, where local governments self‐organized to form watershed management authorities and undertake collaborative planning and management. We conduct a qualitative study to examine why local governments participate in collaborative governance and how they use their existing legal authorities. We found that local governments participated primarily to leverage external funding opportunities, while the advantage of multijurisdictional collaboration to reduce flooding and water quality was important but secondary. Using legal authorities to form agreements occurred in two cases to address flooding, but in all four cases collaboratives largely avoided water quality because of political tensions. We discuss the implications for how local governments might address the challenges of generating commitments and issues of legitimacy to act.

Catchment-scale export of antibiotic resistance genes and bacteria from an agricultural watershed in central Iowa.

Antibiotics are administered to livestock in animal feeding operations (AFOs) for the control, prevention, and treatment of disease. Manure from antibiotic treated livestock contains unmetabolized antibiotics that provide selective pressure on bacteria, facilitating the expression of anti-microbial resistance (AMR). Manure application on row crops is an agronomic practice used by growers to meet crop nutrient needs; however, it can be a source of AMR to the soil and water environment. This study in central Iowa aims to directly compare AMR indicators in outlet runoff from two adjacent (221 to 229 ha) manured and non-manured catchments (manure comparison), and among three catchments (600 to 804 ha) with manure influence, no known manure application (control), and urban influences (mixed land use comparison). Monitored AMR indicators included antibiotic resistance genes (ARGs) ermB, ermF (macrolide), tetA, tetM, tetO, tetW (tetracycline), sul1, sul2 (sulfonamide), aadA2 (aminoglycoside), vgaA, and vgaB (pleuromutilin), and tylosin and tetracycline resistant enterococci bacteria. Results of the manure comparison showed significantly higher (p<0.05) tetracycline and tylosin resistant bacteria from the catchment with manure application in 2017, but no differences in 2018, possibly due to changes in antibiotic use resulting from the Veterinary Feed Directive. Moreover, the ARG analysis indicated a larger diversity of ARGs at the manure amended catchment. The mixed land use comparison showed the manure amended catchment had significantly higher (p<0.05) tetracycline resistant bacteria in 2017 and significantly higher tylosin resistant bacteria in 2017 and 2018 than the urban influenced catchment. The urban influenced catchment had significantly higher ermB concentrations in both sampling years, however the manure applied catchment runoff consisted of higher relative abundance of total ARGs. Additionally, both catchments showed higher AMR indicators compared to the control catchment. This study identifies four ARGs that might be specific to AMR as a result of agricultural sources (tetM, tetW, sul1, sul2) and optimal for use in watershed scale monitoring studies for tracking resistance in the environment.

The Iowa Watershed Approach

The Iowa Watershed Approach

Assessment of Spatial Nitrate Patterns in An Eastern Iowa Watershed Using Boat-Deployed Sensors

Water quality sensors deployed on boats, buoys, and fixed monitoring stations along rivers allow high frequency monitoring at dense spatial and temporal resolutions. Research characterizing nitrate (NO3–N) delivery along extended reaches of navigable rivers, however, is sparse. Since land use and stream biogeochemistry can vary within agricultural watersheds, identifying detailed spatial patterns of stream NO3–N can help identify source area contributions that can be used to develop strategies for water quality improvement. Identifying spatial patterns is especially critical in agricultural watersheds that span multiple landscapes and have dynamic hydrological regimes. We developed and tested a new method that quantifies NO3–N delivery to streams at a high spatial resolution by continuously measuring stream NO3–N using a boat-deployed sensor. Traveling up the Iowa and Cedar Rivers (located within agricultural Upper Mississippi River Basin) and their major tributaries with the system, we automatically measured NO3–N concentrations every 15 s during four excursions spanning the months of May to August, 2018, and characterized stream NO3–N both laterally and longitudinally in river flow. Iowa River NO3–N concentrations were highest nearest the headwaters and gradually declined as the river flowed toward the Mississippi River. Conversely, Cedar River NO3–N concentrations increased from the headwaters toward the mid-watershed areas due to elevated NO3–N delivery from tributaries of the Middle Cedar River; NO3–N concentrations declined in the lower reaches. Our results confirm that NO3–N mitigation efforts should focus on level and intensely-farmed subwatersheds. Data collected with our sensor system compliments permanently deployed sensors and provides an option to support NO3–N removal efforts.

Potential for saturated riparian buffers to treat tile drainage among 32 watersheds representing Iowa landscapes

The saturated riparian buffer (SRB) is a new and cost-effective conservation practice that diverts agricultural tile drainage toward subsurface discharge within riparian buffers to achieve nitrate (NO3-N) removal. Conservation planners want to understand the potential role of the SRB practice for reducing NO3-N loads from tile-drained agricultural watersheds. The Agricultural Conservation Planning Framework (ACPF) includes a tool for identifying riparian zones where the SRB practice can be installed with minimal risks of unintended consequences (i.e., crop inundation and streambank failure). Watershed assessment of the potential role for SRBs, however, must identify where SRB-suited sites can actually receive drainage from tile-drained fields. This study compared the extent of SRB-suited riparian sites among 32 Iowa watersheds, and estimated the proportion of each watershed that was tile drained and located above SRB-suited riparian zones. Results showed the extent of sites suited for SRBs did not significantly differ among three Major Land Resource Areas (MLRAs) in Iowa, from which the selected watersheds were randomly chosen. Most watersheds had suitable sites along 30% to 70% of streambank lengths, where tile drainage from 15% to 40% of the watershed areas could be diverted, based on estimated extents of tile drainage above suitable sites. Therefore, the SRB has an important potential role for water quality improvement in many tile-drained watersheds in Iowa. However, the SRB practice is not readily designed for treating drainage from headwater catchments, which frequently comprised more than 30% of watershed areas in headwater streams of north central Iowa (MLRA 103), where tile drainage is extensive.