Watersheds In Iowa Research Articles

Stream bank erosion as a source of sediment and phosphorus in grazed pastures of the Rathbun Lake Watershed in southern Iowa, United States

Livestock grazing of riparian areas can have a major impact on stream banks and stream integrity if improperly managed. The goals of this study were to determine the sediment and phosphorus (P) losses from stream bank soils under varying cattle stocking rates and to identify additional factors that impact stream bank erosion in the southern Iowa Drift Plain. The research was conducted on 13 cooperating beef cow-calf farms within the Rathbun Lake Watershed in south central Iowa. Over three years, stream bank erosion rates were estimated by using an erosion pin method. Eroded stream bank lengths and area, soil bulk density, and total P (TP) content in stream bank soil were measured to calculate soil and TP losses via stream bank erosion. The length of severely eroded stream banks and soil compaction of the riparian areas of the pastures were positively related to stocking rates. There was no direct relationship between bank erosion and stocking rate. These results suggest that use of riparian areas as pasture can negatively impact the integrity of the major source areas and that the impact could be reduced through management of livestock stocking densities within these riparian areas.

Evaluation of Variation in Nitrate Concentration Levels in the Raccoon River Watershed in Iowa

The Raccoon River Watershed in Iowa has received considerable attention in the recent past due to frequent detections of nitrate concentrations above the federal drinking water standard. This paper econometrically investigates the determinants of variation of nitrate concentrations in the Raccoon River. The analysis relies on a generalized autoregressive conditional heteroscedastic process to model the serial dependence of volatility of the monthly nitrate concentrations in the Raccoon River. Monthly nitrate concentration data from Des Moines Water Works at Van Meter from 1992 to 2008 are used in the study. We found no statistically significant increasing trend in nitrate concentrations over the study period. There are substantial intra-annual variations in nitrate concentrations, and we noted a very strong seasonal pattern. Variations in rainfall and temperature contribute more to the monthly variation in nitrate concentration than do the changes in nitrogen application rates.

Sediment Removal by Prairie Filter Strips in Row‐Cropped Ephemeral Watersheds

Twelve small watersheds in central Iowa were used to evaluate the effectiveness of prairie filter strips (PFS) in trapping sediment from agricultural runoff. Four treatments with PFS of different size and location (100% rowcrop, 10% PFS of total watershed area at footslope, 10% PFS at footslope and in contour strips, 20% PFS at footslope and in contour strips) arranged in a balanced incomplete block design were seeded in July 2007. All watersheds were in bromegrass ( L.) for at least 10 yr before treatment establishment. Cropped areas were managed under a no-till, 2-yr corn ( L.)-soybean [ (L.) Merr.] rotation beginning in 2007. About 38 to 85% of the total sediment export from cropland occurred during the early growth stage of rowcrop due to wet field conditions and poor ground cover. The greatest sediment load was observed in 2008 due to the initial soil disturbance and gradually decreased thereafter. The mean annual sediment yield through 2010 was 0.36 and 8.30 Mg ha for the watersheds with and without PFS, respectively, a 96% sediment trapping efficiency for the 4-yr study period. The amount and distribution of PFS had no significant impact on runoff and sediment yield, probably due to the relatively large width (37-78 m) of footslope PFS. The findings suggest that incorporation of PFS at the footslope position of annual rowcrop systems provides an effective approach to reducing sediment loss in runoff from agricultural watersheds under a no-till system.

Late 20th‐century trends in Iowa watersheds: an investigation of observed and modelled hydrologic storages and fluxes in heavily managed landscapes

AbstractMany climate change studies have shown that streamflow has been changing across the US over the last century. However, there are fewer studies examining trends in hydrologic variables such as soil moisture and snow, which tend to be poorly observed in some regions. Hydrologic models can be applied to understand changes in processes with limited spatial or temporal observational records. However, the application of hydrologic models to long time periods can be complicated by human influences on the hydrologic cycle. In this study, we explore hydrologic changes and the applicability of a commonly used hydrologic modelling system for climate change analysis in a heavily managed landscape. Historical streamflow observations and model‐derived time series of subsurface storage, frozen ground, and snow cover were analysed for 10 Iowa watersheds. The modelled data were generated using precipitation and temperature data spanning 1948–2003 as inputs to common hydrologic prediction models. Time series were tested for trends and significance using the Mann‐Kendall (MK) test with the Trend‐Free Pre‐Whitening procedure at a p = 0.1 significance level. Plots of 10‐year running averages were also examined. Significantly increasing trends in mean daily discharge and the number of high‐flow days were found for several watersheds. Increasing monthly subsurface water content was observed throughout the year in most basins, and was greatest during the winter. The number of modelled frozen‐ground days declined at all sites. Three different model calibration periods were tested and found to reproduce the general upward trend observed in the daily mean discharge. However, the model results varied from the observed in both magnitude and the number of sites identified to have MK trends. Additional functionality that considers land management and land cover change may make the models applied here more accurate for studying climate change impacts in regions such as Iowa. Copyright © 2011 Royal Meteorological Society

Land Use, Stream Habitat and Benthic Invertebrate Assemblages in a Highly Altered Iowa Watershed

Abstract Knowledge of relationships between land cover (i.e., land use) and abiotic and biotic features of headwater streams enhances our ability to predict and effectively assess conditions in a variety of aquatic ecosystems. We evaluated land use effects on stream condition in an Iowa watershed dominated by intensive row crop agriculture and low- intensity urban development by quantifying relationships among land cover, stream invertebrate assemblages and other stream biophysical characteristics (i.e., invertebrate habitat) at 29 sites. On average, 81% of subbasin land cover was agricultural and 6% of land cover was urban across study sites. High nitrate concentrations (range = 5.6–29.0 mg/L) and high relative abundance of oligochaetes and chironomid midges reflected degraded conditions at all sites. However, agriculture and urban land use appeared to have different effects on stream features. Nitrate concentrations were positively related to agricultural land cover, and turbidity and nitrate concentr...

Impacts of Land-Cover Change on Suspended Sediment Transport in Two Agricultural Watersheds1

Schilling, Keith E., Thomas M. Isenhart, Jason A. Palmer, Calvin F. Wolter, and Jean Spooner, 2011. Impacts of Land-Cover Change on Suspended Sediment Transport in Two Agricultural Watersheds. Journal of the American Water Resources Association (JAWRA) 47(4):672-686. DOI: 10.1111/j.1752-1688.2011.00533.x Abstract: Suspended sediment is a major water quality problem, yet few monitoring studies have been of sufficient scale and duration to assess the effectiveness of land-use change or conservation practice implementation at a watershed scale. Daily discharge and suspended sediment export from two 5,000-ha watersheds in central Iowa were monitored over a 10-year period (water years 1996-2005). In Walnut Creek watershed, a large portion of land was converted from row crop to native prairie, whereas in Squaw Creek land use remained predominantly row crop agriculture. Suspended sediment loads were similar in both watersheds, exhibiting flashy behavior typical of incised channels. Modeling suggested that expected total soil erosion in Walnut Creek should have been reduced 46% relative to Squaw Creek due to changes in land use, yet measured suspended sediment loads showed no significant differences. Stream mapping indicated that Walnut Creek had three times more eroding streambank lengths than did Squaw Creek suggesting that streambank erosion dominated sediment sources in Walnut Creek and sheet and rill sources dominated sediment sources in Squaw Creek. Our results demonstrate that an accounting of all sources of sediment erosion and delivery is needed to characterize sediment reductions in watershed projects combined with long-term, intensive monitoring and modeling to account for possible lag times in the manifestation of the benefits of conservation practices on water quality.

Effects of Watershed Configuration and Composition on Downstream Lake Water Quality

We evaluated the relationships between landscape characteristics and lake water quality in receiving waters by regressing four water quality responses on landscape variables that were measured for whole watersheds and three different buffer distances (30, 60, and 120 m). Classical percolation theory was used to conceptualize nutrient pathways and to explain nonlinear responses. The response variables were total nitrogen (TN), total phosphorus (TP), chlorophyll-a (Chl-a), and Secchi transparency (SD). Landscape data were obtained from satellite image-derived maps of 130 watersheds in Iowa using geographic information systems software. We developed regression models with a stepwise protocol selecting the optimal number of significant explanatory variables. Configuration variables such as contagion, the cohesion of cropland and urban land, and the aggregation index of forest were very important and more important than variables assessing landscape composition (e.g., percentage farmland). Whole watershed models predicted between 15 and 67% of the variability in TN, TP, Chl-a, and SD. Proximity-explicit data offered only slightly improved statistical power over land cover data derived from the entire watershed for variables TN, Chl-a. and SD, but not for TP.

Nonpoint‐Source Pollution Reduction for an Iowa Watershed: An Application of Evolutionary Algorithms

We apply an integrated simulation‐optimization framework to search for cost‐efficient mix and location of agricultural conservation practices in a typical agricultural watershed for two types of nitrogen reduction targets: control of mean annual nitrogen loadings, and a “safety‐first” type constraint, insisting that nitrogen targets be met in every weather realization (weather‐resilient solutions). Evolutionary algorithms are developed for each of the appropriate water quality targets. Our approach allows for the derivation of a watershed‐level total and marginal nitrogen abatement cost curve. Controlling for the probability of meeting water quality targets (looking for weather‐resilient solutions) is found to be significantly more costly than controlling the average nitrogen loadings. Both types of solutions are assessed for robustness with respect to weather uncertainty: solutions selected to reduce average loadings do well under weather uncertainty, while the robustness of solutions selected to be resilient decreases with the stringency of the water quality goal. Nous avons appliqué un modèle intégré de simulation‐optimisation pour trouver des combinaisons de pratiques agricoles de conservation efficaces en terme de coûts et les endroits où elles devraient être adoptées dans un bassin versant agricole typique pour deux types de cibles de réduction des charges d’azote : la surveillance des charges moyennes annuelles d’azote et une contrainte du type « sécurité d’abord », selon laquelle les cibles d’azote doivent être respectées peu importe les conditions météorologiques (solutions robustes aux changements météorologiques). Nous avons élaboré des algorithmes évolutifs pour chaque cible relative à la qualité de l’eau. Notre approche a permis de dériver la courbe de coût liéà la réduction marginale et totale de l’azote dans un bassin versant. L’ajustement de la probabilité que les cibles relatives à la qualité de l’eau soient respectées (solutions robustes aux changements météorologiques) s’est révélé significativement plus coûteux que de contrôler les charges moyennes d’azote. Nous avons évalué les deux types de solutions afin de vérifier leur performance dans des conditions météorologiques incertaines : les solutions retenues pour réduire les charges moyennes fonctionnent bien dans des conditions météorologiques incertaines, tandis que la performance des solutions robustes aux conditions météorologiques diminue avec le niveau visé de qualité de l’eau.

Conservation practice establishment in two northeast Iowa watersheds: Strategies, water quality implications, and lessons learned

Coldwater trout streams are important natural resources in northeast Iowa. Extensive efforts have been made by state and federal agencies to protect and improve water quality in northeast Iowa streams that include Sny Magill Creek and Bloody Run Creek, which are located in Clayton County. A series of three water quality projects were implemented in Sny Magill Creek watershed during 1988 to 1999, which were supported by multiple agencies and focused on best management practice (BMP) adoption. Water quality monitoring was performed during 1992 to 2001 to assess the impact of these installed BMPs in the Sny Magill Creek watershed using a paired watershed approach, where the Bloody Run Creek watershed served as the control. Conservation practice adoption still occurred in the Bloody Run Creek watershed during the 10-year monitoring project and accelerated after the project ended, when a multiagency supported water quality project was implemented during 2002 to 2007. Statistical analysis of the paired watershed results using a pre/post model indicated that discharge increased 8% in Sny Magill Creek watershed relative to the Bloody Run Creek watershed, turbidity declined 41%, total suspended sediment declined 7%, and NOx-N (nitrate-nitrogen plus nitrite-nitrogen) increased 15%. Similar results were obtained with a gradual change statistical model. The weak sediment reductions and increased NOx-N levels were both unexpected and indicate that dynamics between adopted BMPs and stream systems need to be better understood. Fish surveys indicate that conditions for supporting trout fisheries have improved in both streams. Important lessons to be taken from the overall study include (1) committed project coordinators, agency collaborators, and landowners/producers are all needed for successful water quality projects; (2) smaller watershed areas should be used in paired studies; (3) reductions in stream discharge may be required in these systems in order for significant sediment load decreases to occur; (4) long-term monitoring on the order of decades can be required to detect meaningful changes in water quality in response to BMP implementation; and (5) all consequences of specific BMPs need to be considered when considering strategies for watershed protection.

Effect of Historic Land Cover Change on Runoff Curve Number Estimation in Iowa

Within three decades of European-descended settlers arriving in Iowa, much of the land cover across the state was transformed from prairie and forest to farmland, patches of forest, and urbanized areas. Between 1832 and 1859, the General Land Office surveyed the state of Iowa to aid in the disbursement of land. In 1875, an illustrated atlas of the State of Iowa was published. Using these two data resources for classifying land cover, the hydrologic impact of the land cover change resulting from the first three decades of settlement is presented in terms of the effect on the area-weighted average curve number, a term commonly used to predict runoff from rainstorms. In the four watersheds studied, the area-weighted average curve number increased by a mean of 16.4 from 61.4 to 77.8 with the greatest magnitude of change occurring in the two western Iowa watersheds as opposed to the two more heavily forested eastern Iowa watersheds.

Long‐term effects of management practices on water‐driven soil erosion in an intense agricultural sub‐watershed: monitoring and modelling

AbstractThe overarching objective of this research was to provide an improved understanding of the role of land use and associated management practices on long‐term water‐driven soil erosion in small agricultural watersheds by coupling the established, physically based, distributed parameter Water Erosion Prediction Project (WEPP) model with long‐term hydrologic, land use and soil data. A key step towards achieving this objective was the development of a detailed methodology for model calibration using physical ranges of key governing parameters such as effective hydraulic conductivity, critical hydraulic shear stress and rill/inter‐rill erodibilities. The physical ranges for these governing parameters were obtained based on in situ observations within the South Amana Sub‐Watershed (SASW) (∼26 km2) of the Clear Creek, IA watershed where detailed documentation of the different land uses was available for a period of nearly 100 years. A quasi validation of the calibrated model was conducted through long‐term field estimates of water and sediment discharge at the outlet of SASW and also by comparing the results with data reported in the literature for other Iowa watersheds exhibiting similar biogeochemical properties. Once WEPP was verified, ‘thought experiments’ were conducted to test our hypothesis that land use and associated management practices may be the major control of long‐term erosion in small agricultural watersheds such as SASW. Those experiments were performed using the dominant 2‐year crop rotations in the SASW, namely, fall till corn–no till bean (FTC‐NTB), no till bean–spring till corn (NTB‐STC) and no till corn–fall till bean (NTC‐FTB), which comprised approximately 90% of the total acreage in SASW. Results of this study showed that for all crop rotations, a strong correspondence existed between soil erosion rates and high‐magnitude precipitation events during the period of mid‐April and late July, as expected. The magnitude of this correspondence, however, was strongly affected by the crop rotation characteristics, such as canopy/residue cover provided by the crop, and the type and associated timing of tillage. Tillage type (i.e. primary and secondary tillages) affected the roughness of the soil surface and resulted in increases of the rill/inter‐rill erodibilities up to 35% and 300%, respectively. Particularly, the NTC‐FTB crop rotation, being the most intense land use in terms of tillage operations, caused the highest average annual erosion rate within the SASW, yielding quadrupled erosion rates comparatively to NTB‐STC. The impacts of tillage operation were further exacerbated by the timing of the operations in relation to precipitation events. Timing of operations affected the ‘life‐time’ of residue cover and as a result, the degree of protection that residue cover offers against the water action on the soil surface. In the case of NTC‐FTB crop rotation, dense corn residue stayed on the ground for only 40 days, whereas for the other two rotations, corn residue provided a protective layer for nearly 7 months, lessening thus the degree of soil erosion. The cumulative effects of tillage type and timing in conjunction with canopy/residue cover led to the conclusion that land management practices can significantly amplify or deamplify the impact of precipitation on long‐term soil erosion in small agricultural watersheds. Copyright © 2009 John Wiley & Sons, Ltd.

Getting to better water quality outcomes: the promise and challenge of the citizen effect

Agriculture is a major cause of non-point source water pollution in the Midwest. Excessive nitrate, phosphorous, and sediment levels degrade the Mississippi River and Gulf of Mexico. In this research we ask, to what extent can citizen involvement help solve the problem of non-point source pollution. Does connecting farmers to farmers and to other community members make a difference in moving beyond the status quo? To answer these questions we examine the satisfaction level of Iowa farmers and landowners with their current conservation measures as a proxy for willingness to change. A survey of 360 conservation minded farmers obtained from a random sample of 75 HUC (Hydrologic Unit Code) 12 Iowa watersheds reveals that 27% of the variance among farmers’ perception of adequacy of their conservation practices is explained by a combination of beliefs about the seriousness of water pollution, personal, civic, and expert connections. The more farmers talk with other farmers the more likely they are satisfied with their conservation efforts. However, the more frequently farmers talk to friends and neighbors that don’t farm, the more likely they are to not be satisfied with their conservation efforts. Further, the more social organizations farmers belong to—e.g., more non-farmers they interact with in a group setting—the more likely they are to be dissatisfied with their level of effort being adequate to protect local water bodies. These findings suggest the personal and civic connections among farmers and communities are important in explaining perceptions of how adequate conservation measures are. These perceptions have implications for farmers’ willingness to go beyond current actions and more actively engage in solving local watershed problems and explain why they may not currently be engaged in additional actions.

Upland Erosion Modeling in a Semihumid Environment via the Water Erosion Prediction Project Model

The major water quality impairment in the midwest United States is sediment eroded from agricultural lands. Yet, few understand the spatial and temporal variability of erosion, or soil erosion dynamics, in relation to precipitation, topography, land management, and severe events. The objectives of this paper are to (1) develop a methodology for estimating long-term spatial soil erosion and water runoff losses and (2) explore issues in applying an established physical-based process model, Water Erosion Prediction Project (WEPP), to a large area by establishing a prototype system for the state of Iowa. This study for the first time provides a comparison of the model predictions against long-term measurements of the sediment delivery ratio (SDR) in the South Amana Catchment of the Clear Creek Watershed (CCW), a heavily instrumented watershed that is roughly 10 times the maximum WEPP fold size. To further examine the performance of WEPP in a semihumid environment, such as the CCW, where runoff and raindrop impact to erosion may be significant, the SDR was plotted as a function of the runoff coefficient, defined as the runoff/rainfall ratio. In addition, the WEPP predictions are compared against the statistical relation of SDR vs. runoff coefficient developed by Piest et al. in 1975) for watersheds in Iowa. It is shown that WEPP follows the trend shown by Piest et al. quite closely and performs well for continuous simulations extended up to 300 years.

Simulated long-term nitrogen losses for a midwestern agricultural watershed in the United States

Adequate knowledge on the movement of nutrients under various agricultural practices is essential for developing remedial measures to reduce nonpoint source pollution. Mathematical models, after extensive calibration and validation, are useful to derive such knowledge and to identify site-specific alternative agricultural management practices. A spatial-process model that uses GIS and ADAPT, a field scale daily time-step continuous water table management model, was calibrated and validated for flow and nitrate-N discharges from a 365 ha agricultural watershed in central Iowa, in the Midwestern United States. This watershed was monitored for nitrate-N losses from 1991 to 1997. Spatial patterns in crops, topography, fertilizer applications and climate were used as input to drive the model. The first half of the monitored data was used for calibration and the other half was used in validation of the model. For the calibration period, the observed and predicted flow and nitrate-N discharges were in excellent agreement with r 2 values of 0.88 and 0.74, respectively. During the validation period, the observed and predicted flow and nitrate-N discharges were in good agreement with r 2 values of 0.71 and 0.50, respectively. For all 6 years of data, the observed annual nitrate-N losses of 26 kg ha −1 for the entire simulation were in excellent agreement with predicted nitrate-N losses of 24.2 kg ha −1. The calibrated model was used to investigate the long-term impacts of nitrate-N losses to changes in the rate and timing of fertilizer application. Results indicate that nitrate-N losses were sensitive to rate and timing of fertilizer application. Modeled annual nitrate-N losses showed a 17% reduction in nitrate-N losses by reducing the fertilizer application rate by 20% and switching the application timing from fall to spring. Further reductions in nitrate-N losses require conversion of row cropland to pasture and/or replacement of continuous corn or corn–soybean rotation systems with alternative crops.

Observations of Soils at the Hillslope Scale in the Clear Creek Watershed in Iowa, USA

Observations of Soils at the Hillslope Scale in the Clear Creek Watershed in Iowa, USA

Water and soil resources response to rising levels of atmospheric CO 2 concentration and to changes in precipitation and air temperature

Summary The quantification of the diverse responses of soils and terrestrial fresh water to elevated levels of greenhouse gases and to climate change is crucial for the proper management of natural ecosystems in the future. Despite previous experiments and simulations, there is still a need for the evaluation of the impact of these global changes at the watershed level. The main objective of this paper is to examine the effects of increasing CO2 concentrations and rainfall changes associated with changes in average daily rainfall intensity, and surface air temperature on loads of water, NO3–N and sediments from watersheds exhibiting different environmental conditions. The interactively coupled (CO2–climate–landuse) Soil and Water Assessment Tool (SWAT) was used to predict the effect of variations in precipitation and rainfall intensity (a 10%, 20% and 40% increase or decrease) or surface air temperature (an increase of 0.5 and 3.5 °C in the mean winter temperature) associated with an increase of CO2 concentration from 330 to 950 ppm in two agricultural watersheds in Iowa and Texas. Over a 100-year simulated period: (1) precipitation changes primarily affected flow and sediment discharges while temperature and changes in atmospheric CO2 concentration had a smaller effect; (2) CO2 concentration was the main controlling factor of NO3–N loads; and (3) global changes in the humid watershed had a greater effect on the water and soil resources than in the semi-arid. In particular, increasing precipitations in the humid watershed significantly increased the median and interquartile of outputs of all the variables under consideration. In contrast, changes in surface air temperature had only a slight impact on loads. However higher temperatures in the more humid site tended to reduce the highest loads. These results demonstrate that changes in CO2 concentration and climate, particularly an increase in precipitation, had significant effects on the soil and fresh water resources. The magnitude of these effects differed according to the watershed’s characteristics. Finally, the general applicability of these results and the limits of the approach are discussed.

Water Quality Modeling for the Raccoon River Watershed Using SWAT

The Raccoon River watershed (RRW) in west-central Iowa has been recognized as exporting some of the highest nitrate-nitrogen loadings in the U.S. and is a major source of sediment and other nutrient loadings. An integrated modeling framework has been constructed for the 9,400 km2 RRW that consists of the SWAT (Soil and Water Assessment Tool) model, the interactive SWAT (i_SWAT) software package, the Load Estimator (LOADEST) computer program, and other supporting software and databases. The simulation framework includes detailed land use and management data, such as different crop rotations, and an array of nutrient and tillage management schemes, derived from the USDA National Resources Inventory (NRI) databases and other sources. This article presents the calibration and validation of SWAT for the streamflow, sediment losses, and nutrient loadings in the watershed, and an assessment of land use and management practice shifts in controlling pollution. Streamflow, sediment yield, and nitrate loadings were calibrated for the period 1981-1992 and validated for the period 1993-2003. Limited field data on organic nitrogen, organic phosphorus, and mineral phosphorus allowed model validation for the period 2001-2003. Model predictions generally performed very well on both an annual and monthly basis during the calibration and validation periods, as indicated by R2 and Nash-Sutcliffe efficiency (E) values that exceeded 0.7 in most cases. A set of land use change scenarios depicting conversion of cropland into land set-aside resulted in large reductions of sediment yield at the watershed outlet. A second scenario set found that reductions in nutrient applications of 10% to 20% resulted in similar predicted percentage reductions in nitrate loadings at the watershed outlet and in corresponding corn yield reductions of 3% to 6%.

Effects of Watershed‐Scale Land Use Change on Stream Nitrate Concentrations

The Walnut Creek Watershed Monitoring Project was conducted from 1995 through 2005 to evaluate the response of stream nitrate concentrations to changing land use patterns in paired 5000-ha Iowa watersheds. A large portion of the Walnut Creek watershed is being converted from row crop agriculture to native prairie and savanna by the U.S. Fish and Wildlife Service at the Neal Smith National Wildlife Refuge (NSNWR). Before restoration, land use in both Walnut Creek (treatment) and Squaw Creek (control) watersheds consisted of 70% row crops. Between 1990 and 2005, row crop area decreased 25.4% in Walnut Creek due to prairie restoration but increased 9.2% in Squaw Creek due to Conservation Reserve Program (CRP) grassland conversion back to row crop. Nitrate concentrations ranged between <0.5 to 14 mg L(-1) at the Walnut Creek outlet and 2.1 to 15 mg L(-1) at the downstream Squaw Creek outlet. Nitrate concentrations decreased 1.2 mg L(-1) over 10 yr in the Walnut Creek watershed but increased 1.9 mg L(-1) over 10 yr in Squaw Creek. Changes in nitrate were easier to detect and more pronounced in monitored subbasins, decreasing 1.2 to 3.4 mg L(-1) in three Walnut Creek subbasins, but increasing up to 8.0 and 11.6 mg L(-1) in 10 yr in two Squaw Creek subbasins. Converting row crop lands to grass reduced stream nitrate levels over time in Walnut Creek, but stream nitrate rapidly increased in Squaw Creek when CRP grasslands were converted back to row crop. Study results highlight the close association of stream nitrate to land use change and emphasize that grasslands or other perennial vegetation placed in agricultural settings should be part of a long-term solution to water quality problems.

Small area estimation for spatial correlation in watershed erosion assessment

This article describes the combination of small area estimator and a simultaneously autoregressive (SAR) model applied to the erosion data collected at the Rathbun Lake Watershed in Iowa (USA). The proposed methodology considers and EBLUP estimator with spatially correlated random area effects taking into account the information provided by neighboring areas. The article discusses the gain obtained from modeling the spatial correlation among small area random effects useful in representing the unexplained variation of the small area target quantities. Moreover the estimator of mean squared error of the proposed estimator is presented.

Correction to “Impacts of integrated crop‐livestock systems on nitrogen dynamics and soil erosion in western Iowa watersheds”

Correction to “Impacts of integrated crop‐livestock systems on nitrogen dynamics and soil erosion in western Iowa watersheds”