Walnut Creek Watershed Research Articles

Rapid-DEM: Rapid Topographic Updates through Satellite Change Detection and UAS Data Fusion

As rapid urbanization occurs in cities worldwide, the importance of maintaining updated digital elevation models (DEM) will continue to increase. However, due to the cost of generating high-resolution DEM over large spatial extents, the temporal resolution of DEMs is coarse in many regions. Low-cost unmanned aerial vehicles (UAS) and DEM data fusion provide a partial solution to improving the temporal resolution of DEM but do not identify which areas of a DEM require updates. We present Rapid-DEM, a framework that identifies and prioritizes locations with a high likelihood of an urban topographic change to target UAS data acquisition and fusion to provide up-to-date DEM. The framework uses PlanetScope 3 m satellite imagery, Google Earth Engine, and OpenStreetMap for land cover classification. GRASS GIS generates a contextualized priority queue from the land cover data and outputs polygons for UAS flight planning. Low-cost UAS fly the identified areas, and WebODM generates a DEM from the UAS survey data. The UAS data is fused with an existing DEM and uploaded to a public data repository. To demonstrate Rapid-DEM a case study in the Walnut Creek Watershed in Wake County, North Carolina is presented. Two land cover classification models were generated using random forests with an overall accuracy of 89% (kappa 0.86) and 91% (kappa 0.88). The priority queue identified 109 priority locations representing 1.5% area of the watershed. Large forest clearings were the highest priority locations, followed by newly constructed buildings. The highest priority site was a 0.5 km2 forest clearing that was mapped with UAS, generating a 15 cm DEM. The UAS DEM was resampled to 3 m resolution and fused with USGS NED 1/9 arc-second DEM data. Surface water flow was simulated over the original and updated DEM to illustrate the impact of the topographic change on flow patterns and highlight the importance of timely DEM updates.

Spatially Disaggregating Satellite Land Surface Temperature With a Nonlinear Model Across Agricultural Areas

AbstractAccurate remotely sensed land surface temperature (LST) is a promising tool for predicting surface evapotranspiration (ET). The spatial resolution of commonly existing daily satellite products (i.e., Moderate Resolution Imaging Spectroradiometer [MODIS] LST) is ~1 km, which remains relatively low for used in estimating ET. This paper developed a model that disaggregates ~1‐km spatial resolution MODIS‐derived LST data to fine spatial resolutions of 250 m. The proposed model was achieved by using a spatial and temporal nonlinear strategy that contains the predictor variables of the Bowen ratio, the photochemical reflectance index, and the normalized difference vegetation index. The proposed disaggregation model was assessed mainly at two agriculture sites, including the Heihe River Basin in China and the Walnut Creek Watershed in the United States, during the growing seasons. The assessment procedure was conducted at both the field scale and the image scale in terms of disaggregated LST and ET. The statistical results demonstrated that the proposed model produced 250‐m LST and ET that matched better with the observed values and achieved more accurate LST and ET relative to other reference ones. Our study shows that surface moisture status and vegetation physiological dynamic are important factors in improving the LST disaggregation over the agriculture region. The results of this study have the potential to improve water resource management and sustainable water use.

Nitrate on a Slow Decline: Watershed Water Quality Response during Two Decades of Tallgrass Prairie Ecosystem Reconstruction in Iowa

The Neal Smith National Wildlife Refuge was established as a tallgrass prairie ecosystem reconstruction in the Walnut Creek watershed (5238 ha), Jasper County, Iowa, with >1200 ha of prairie plantings initiated between 1993 and 2006. This study updates the documented decreases in watershed NO-N losses that accompanied this change in land cover to a 20-yr record. Annual flow-weighted NO-N concentrations declined by 0.15 mg NO-N L yr, which was not significantly different from the rate of 0.07 mg NO-N L yr reported after the first decade of monitoring. There was also evidence ( < 0.1) that prairie reconstruction led to a declining trend in annual watershed water yield, which would have contributed to the trend of decreasing NO-N loads. However, variability in climate, including 2 yr with significant flooding events followed by a major drought during the second decade of monitoring, challenged any notion that a watershed water quality record will stabilize even >10 yr after a substantial change in land cover, in this naturally drained watershed underlain by fine grained glacial deposits that exhibit multidecadal groundwater transport times. Efforts to document progress toward water quality goals will need to consider dominant flow paths and associated travel times, uncertainty in the effectiveness of management changes, and a changeable climate.

Changes in lateral floodplain connectivity accompanying stream channel evolution: Implications for sediment and nutrient budgets

Floodplain storage commonly represents one of the largest sediment fluxes within sediment budgets. In watersheds responding to large scale disturbance, floodplain-channel lateral connectivity may change over time with progression of channel evolution and associated changes in channel geometry. In this study we investigated the effects of channel geometry change on floodplain inundation frequency and flux of suspended sediment (SS) and total phosphorus (TP) to floodplain storage within the 52.2 km2 Walnut Creek watershed (Iowa, USA) through a combination of 25 in-field channel cross section transects, hydraulic modeling (HEC-RAS), and stream gauging station-derived water quality and quantity data. Cross sectional area of the 25 in-field channel cross sections increased by a mean of 17% over the 16 year study period (1998–2014), and field data indicate a general trend of degradation and widening to be present along Walnut Creek's main stem. Estimated stream discharge required to generate lateral overbank flow increased 15%, and floodplain inundation volume decreased by 37% over study duration. Estimated annual fluxes of SS and TP to floodplain storage decreased by 61 and 62% over study duration, respectively. The estimated reductions in flux to floodplain storage have potential to increase watershed export of SS and TP by 9 and 18%, respectively. Increased contributions to SS and TP export may continue as channel evolution progresses and floodplain storage opportunities continue to decline. In addition to loss of storage, higher discharges confined to the channel may have greater stream power, resulting in further enhancement of SS and TP export through accelerated bed and bank erosion. These increased contributions to watershed loads may mask SS and TP reductions achieved through edge of field practices, thus making it critical that stage and progression of channel evolution be taken into consideration when addressing sediment and phosphorus loading at the watershed scale.

An integrated approach to place Green Infrastructure strategies in marginalized communities and evaluate stormwater mitigation

Increasing urbanization augments impervious surface area, which results in increased run off volumes and peak flows. Green Infrastructure (GI) approaches are a decentralized alternative for sustainable urban stormwater and provide an array of ecosystem services and foster community building by enhancing neighborhood aesthetics, increasing property value, and providing shared green spaces. While projects involving sustainability concepts and environmental design are favored in privileged communities, marginalized communities have historically been located in areas that suffer from environmental degradation. Underprivileged communities typically do not receive as many social and environmental services as advantaged communities. This research explores GI-based management strategies that are evaluated at the watershed scale to improve hydrological performance by mitigating storm water run off volumes and peak flows. GI deployment strategies are developed to address environmental justice issues by prioritizing placement in communities that are underprivileged and locations with high outreach potential. A hydrologic/hydraulic stormwater model is developed using the Storm Water Management Model (SWMM 5.1) to simulate the impacts of alternative management strategies. Management scenarios include the implementation of rain water harvesting in private households, the decentralized implementation of bioretention cells in private households, the centralized implementation of bioretention cells in municipally owned vacant land, and combinations of those strategies. Realities of implementing GI on private and public lands are taken into account to simulate various levels of coverage and routing for bioretention cell scenarios. The effects of these strategies are measured by the volumetric reduction of run off and reduction in peak flow; social benefits are not evaluated. This approach is applied in an underprivileged community within the Walnut Creek Watershed in Raleigh, North Carolina.

Inorganic and Organic Phosphorus in Sediments in the Walnut Creek Watershed of Central Iowa, USA

The dynamics of phosphorus (P) reactions in stream water are important because of their potential to trigger eutrophication. This study aimed to explore the nature of P in sediments associated with Walnut Creek, Jasper County, Iowa. The Walnut Creek watershed supports row crop production, grazing, and riparian buffer zones. The alluvial cross section is composed of a sequence of sediments that contribute differentially to the amounts and forms of P entering the stream. Twenty-five sediment samples collected near Walnut Creek (classified as bank, in-stream, and floodplain deposits) were sequentially extracted for P. Across all 25 samples, the inorganic P (Pi) fractions followed the order Fe-bound Pi > Ca-bound Pi > reductant-soluble Pi > Al-bound Pi > soluble and loosely bound Pi. For the organic (Po) fractions, the order was nonlabile Po > fulvic acid-bound Po > humic acid-bound Po > labile Po > moderately labile Po. The ranges of total P (TP), Mehlich-3-extractable P (P-M3), and ammonium oxalate-extractable P (Pox) were 386 to 1134, 5 to 85, and 60 to 823 mg kg−1, respectively. Among the sample groups, the highest concentrations of TP, P-M3, and Pox were measured in in-stream deposits. Total P was significantly correlated with Fe oxides, clay, and soil organic matter, especially in the bank and floodplain deposits. Because of the potential release of P from these sediments, we can speculate that changes in land use within the riparian areas may, at least initially, have little direct effect on soluble or particulate P loads in Walnut Creek.

Distribution and mass of groundwater orthophosphorus in an agricultural watershed

Orthophosphorus (OP) is the form of dissolved inorganic P that is commonly measured in groundwater studies, but the spatial distribution of groundwater OP across a watershed has rarely been assessed. In this study, we characterized spatial patterns of groundwater OP concentrations and loading rates within the 5218ha Walnut Creek watershed (Iowa) over a two-year period. Using a network of 24 shallow (<6m) monitoring wells established across watershed, OP concentrations ranged from <0.01 to 0.58mg/l in all samples (n=147) and averaged 0.084±0.107mg/l. Groundwater OP concentrations were higher in floodplains and OP mass loading rates were approximately three times higher than in uplands. We estimated that approximately 1231kg of OP is present in floodplain groundwater and 2869kg is present in upland groundwater within the shallow groundwater zone (0–5m depth). Assuming no new inputs of OP to shallow groundwater, we estimated it would take approximately eight years to flush out existing OP mass present in the system. Results suggest that conservation practices focused on reducing OP loading rates in floodplain areas may have a disproportionately large water quality benefit compared to upland areas.

Upland and in-stream controls on baseflow nutrient dynamics in tile-drained agroecosystem watersheds

In landscapes with low residence times (e.g., rivers and reservoirs), baseflow nutrient concentration dynamics during sensitive timeframes can contribute to deleterious environmental conditions downstream. This study assessed upland and in-stream controls on baseflow nutrient concentrations in a low-gradient, tile-drained agroecosystem watershed. We conducted time-series analysis using Empirical mode decomposition of seven decade-long nutrient concentration time-series in the agricultural Upper Big Walnut Creek watershed (Ohio, USA). Four tributaries of varying drainage areas and three main-stem sites were monitored, and nutrient grab samples were collected weekly from 2006 to 2016 and analyzed for dissolved reactive phosphorus (DRP), nitrate-nitrogen (NO3-N), total nitrogen (TN), and total phosphorus (TP). Statistically significant seasonal fluctuations were compared with seasonality of baseflow, watershed characteristics (e.g., tile-drain density), and in-stream water quality parameters (pH, DO, temperature). Findings point to statistically significant seasonality of all parameters with peak P concentrations in summer and peak N in late winter-early spring. Results suggest that upland processes exert strong control on DRP concentrations in the winter and spring months, while coupled upland and in-stream conditions control watershed baseflow DRP concentrations during summer and early fall. Conversely, upland flow sources driving streamflow exert strong control on baseflow NO3-N, and in-stream attenuation through transient and permanent pathways impacts the magnitude of removal. Regarding TN and TP, we found that TN was governed by NO3-N, while TP was governed by DRP in summer and fluvial erosion of P-rich benthic sediments during higher baseflow conditions. Findings of the study highlight the importance of coupled in-stream and upland management for mitigating eutrophic conditions during environmentally sensitive timeframes.

Assessing BMP Effectiveness and Guiding BMP Planning Using Process‐Based Modeling

AbstractThere is an increasing need for improved process‐based planning tools to assist watershed managers in the selection and placement of effective best management practices (BMPs). In this article, we present an approach, based on the Water Erosion Prediction Project model and a pesticide transport model, to identify dominant hydrologic flow paths and critical source areas for a variety of pollutant types. We use this approach to compare the relative impacts of BMPs on hydrology, erosion, sediment, and pollutant delivery within different landscapes. Specifically, we focus on using this approach to understand what factors promoted and/or hindered BMP effectiveness at three Conservation Effects Assessment Project watersheds: Paradise Creek Watershed in Idaho, Walnut Creek Watershed in Iowa, and Goodwater Creek Experimental Watershed in Missouri. These watersheds were first broken down into unique land types based on soil and topographic characteristics. We used the model to assess BMP effectiveness in each of these land types. This simple process‐based modeling approach provided valuable insights that are not generally available to planners when selecting and locating BMPs and helped explain fundamental reasons why long‐term improvement in water quality of these three watersheds has yet to be completely realized.

Observations of dew amount using in situ and satellite measurements in an agricultural landscape

Estimating the amount of water on leaf surfaces is an increasing concern for remote sensing and hydrology. Measuring the magnitude and spatial extent of leaf wetness events will provide useful information for water and energy balance modeling and remote sensing. As part of the Soil Moisture Experiments 2005 (SMEX05), the temporal and spatial characterization of leaf wetness over a heterogeneous agricultural domain was investigated. Leaf wetness sensors and physical measurements were collected from 15 June to 3 July 2005 in and around the Walnut Creek Watershed near Ames, Iowa, USA. Comparison of the results of the in situ leaf wetness sensor measurements and the physical sampling revealed a moderate correlation for both corn ( Zea mays L .) and soybeans ( Glycine max Merr.). Regression equations were developed to estimate leaf wetness quantity from these leaf wetness sensors and combined with a vegetation leaf area index map to produce a spatial leaf wetness product hourly during the experiment with an error of approximately 0.05 kg/(m 2 LAI). Using this strategy, future efforts in spatial hydrologic modeling and remote sensing would be able to incorporate quantitative estimates of leaf wetness amount in watershed scale studies using only in situ measurements.

Economic and Environmental Impacts of LSNT and Cover Crops for Nitrate-Nitrogen Reduction in Walnut Creek Watershed, Iowa, Using FEM and Enhanced SWAT Models

Nitrate nitrogen (NO 3 -N) enriched water originates from subsurface drains or tiles that underlay many fields in the Corn Belt and is the primary source of NO 3 -N to surface waters in this region. To better assess the fate and transport of nutrients, such as NO 3 -N, the tile drain and pothole components of SWAT (Soil and Water Assessment Tool) were enhanced and modified in the previous component of this study. In this study, the environmental and economic impacts of various best management practice (BMP) scenarios often adopted by local farmers to reduce sediment and nutrient loadings (in particular NO 3 -N) were evaluated using the modified SWAT (SWAT-M) and FEM (Farm-level Economic Model) models. Measured values of water quality indicators from the Walnut Creek watershed (WCW) located in central Iowa were used to verify the capability of SWAT-M to predict the impact of late-spring nitrate test (LSNT) and rye cover crop management on NO 3 -N reduction at the subbasin level. The results obtained from SWAT-M simulation results, similar to field measurement data, indicated a 25% reduction in NO 3 -N under the LSNT scenario. FEM results indicated a corresponding increased annual cost of $6/ha across all farms in the watershed. Simulation of other scenarios, including winter cover cropping and a combination of LSNT and cover cropping at different adoption rates within WCW, resulted in a progressive reduction in sediment and nutrient losses as adoption rates increased. Use of the rye cover crop added about $25/ha to $35/ha to the annual cost of the average farm, indicating that some cost-share support may be necessary to encourage farmers to use winter cover crops.

Spatial and Temporal Variation of Energy and Carbon Fluxes in Central Iowa

Energy balance and CO2 exchange of agricultural crops has been investigated through limited field studies because of the expense of the monitoring equipment and availability of fields to place equipment. Quantifying the spatial and temporal variation in the energy balance and CO2 dynamics over crop canopies will improve regional‐scale estimates of water and C fluxes. A study was conducted in central Iowa during 2002 as part of the Soil Moisture Experiment to evaluate soil moisture energy exchange across an intensive corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production area near Ames, IA (lat. 41.98380985, long. −93.75497316). Surface energy balance and CO2 flux stations were placed in 12 corn and soybean fields across different soils and landscapes in and around the Walnut Creek watershed. Variability among fields was induced by three factors. Within a day, variations among fields were due to the presence of cumulus cloud formation in the afternoon. Short‐term differences across days among the fields were due to variation in the spatial pattern of rainfall events causing differential drying. Throughout the season, differences among fields were due to soil water availability, which affected crop growth and ground cover. Differences in early season ground cover were correlated with energy balance (r = 0.80). During the growing season, latent heat and CO2 uptake were closely related (r = 0.85). Characterization of the spatial distribution of energy balance and CO2 uptake in an intensive cropping region provides guidance on the confidence that can be placed in interpreting single‐site measurements.

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.

Estimates of the potential temperature profile from lidar measurements of boundary layer evolution

The Soil Moisture‐Atmosphere Coupling Experiment (SMACEX) was conducted in the Walnut Creek Watershed near Ames, Iowa, over the period from 15 June to 11 July 2002. A main focus of SMACEX is the investigation of the interactions between the atmospheric boundary layer, surface moisture, and canopy. A vertically staring elastic lidar was used to provide a high time resolution, continuous record of the mixed layer height at the edge between a soybean and a corn field. The height and thickness of the entrainment zone are used to estimate the vertical potential temperature profile in the boundary layer using surface energy measurements in the Batchvarova‐Gryning mixed layer model. Calculated values of potential temperature compared well to radiosonde measurements taken simultaneously with the lidar measurements. The root‐mean‐square difference between the lidar‐derived values and the balloon‐based values is 1.20°C.

EVALUATION OF SWAT IN SIMULATING NITRATE NITROGEN AND ATRAZINE FATES IN A WATERSHED WITH TILES AND POTHOLES

We evaluated a version of the Soil Water Assessment Tool (SWAT-M) that was modified to more accurately simulatetile drainage and water flow in a landscape dominated by closed surface depressions or potholes at a watershed scale usingten years of measured nitrate-nitrogen (NO3-N) and atrazine data in stream discharge in the Walnut Creek watershed (WCW).The model was calibrated during the period of 1992 to 1995 and validated during the period of 1996 to 2001. Stream sitesin the middle and outlet of the WCW were selected to assess overall performance of the model, while one drainage districtdrain was used for investigating chemical loads in subsurface flows. With the introduction of an independent tile drain lagtime parameter, the performance of SWAT-M for daily flow simulation was improved. In comparison to our previous results,the Nash-Sutcliffe E values for the calibrated daily flow at the mid-watershed and outlet simulated by the enhanced SWATmodel rose from 0.55 to 0.69 and from 0.51 to 0.63, respectively. Of special note, the E value for calibrated flow rose from-0.23 to 0.40 for the drainage district drain, which was dominated by tile and subsurface flow. Both the predicted corn yieldsand N uptake by corn were very similar to the measured data. The predicted yield and N uptake by soybean were relativelylower than the measured values. The monthly NO3-N loads in stream discharges at the center and outlet of the Walnut Creekwatershed were accurately predicted with good Nash-Sutcliffe E values of 0.91/0.80 and 0.85/0.67 in calibration/validation,respectively. Nevertheless, the models simulation of the daily NO3-N loads was not as good as the monthly simulation. Thegood agreement between the simulated and measured monthly NO3-N loads from the drainage district site leads us to concludethat SWAT can reasonably simulate tile flow from pothole-dominated landscapes, although the model needs to be improvedin the simulation of daily subsurface NO3-N fluxes. The enhanced SWAT-M model simulated the NO3-N loads in a watershedwith intensive tile drainage systems much more accurately than the original SWAT2000 version. A second pesticidedegradation half-life in soil was added for SWAT-M, which greatly improved the model performance for predicting atrazinelosses from the watershed. Overall, SWAT-M is capable of simulating atrazine loads in the stream discharge of the WCW andis a much-improved tool over SWAT2000 for predicting both daily and monthly atrazine losses in nearly level, tile-drainedwatersheds.

Relationship of Soil Respiration to Crop and Landscape in the Walnut Creek Watershed

Abstract Soil respiration is an important component of the carbon dynamics of terrestrial ecosystems. Many factors exert controls on soil respiration, including temperature, soil water content, organic matter, soil texture, and plant root activity. This study was conducted to quantify soil respiration in the Walnut Creek watershed in central Iowa, and to investigate the factors controlling this process. Six agricultural fields were identified for this investigation: three of the fields were cropped with soybean [Glycine max (L.) Merr.] and three were cropped with corn (Zea mays L.). Within each field, soil respiration was measured at nine locations, with each location corresponding to one of three general landscape positions (summit, side slope, and depression). Soil respiration was measured using a portable vented chamber connected to an infrared gas analyzer. Soil samples were collected at each location for the measurement of soil water content, pH, texture, microbial biomass, and respiration potential. Field respiration rates did not show a significant landscape effect. However, there was a significant crop effect, with respiration from cornfields averaging 37.5 g CO2 m−2 day−1 versus an average respiration of 13.1 g CO2 m−2 day−1 in soybean fields. In contrast, laboratory measurements of soil respiration potential, which did not include plant roots, showed a significant landscape effect and an insignificant cropping system effect. Similar relationships were observed for soil organic C and microbial biomass. Additional analyses indicate that corn roots may be more important than soybean roots in their contribution to surface CO2 flux, and that root respiration masked landscape effects on total soil respiration. Also, the failure to account for soil respiration may lead to biased estimates of net primary production measured by eddy covariance.

Spatial and temporal variation in evapotranspiration using Raman lidar

The Los Alamos Raman lidar has been used to make high resolution (25 m) estimates of the evapotranspiration rate over adjacent corn and soybean canopies. The lidar makes three-dimensional measurements of the water vapor content of the atmosphere directly above the canopy that are inverted using Monin–Obukhov similarity theory. This may be used to examine the relationship between evapotranspiration and surface moisture/soil type. Lidar estimates of evapotranspiration reveal a high degree of spatial variability over corn and soybean fields that may be associated with small elevation changes in the area. The spatial structure of the variability is characterized using a structure function and correlation function approach. The power law relationship found by other investigators for soil moisture is not clear in the data for evapotranspiration, nor is the data a straight line over the measured lags. The magnitude of the structure function and the slope changes with time of day, with a probable connection to the amount of evapotranspiration and the spatial variability of the water vapor source. The data used was taken during the soil moisture–atmosphere coupling experiment (SMACEX) conducted in the Walnut Creek Watershed near Ames, Iowa in June and July 2002.

Corn Response to Late‐Spring Nitrogen Management in the Walnut Creek Watershed

A 400‐ha subbasin study within the Walnut Creek watershed near Ames, IA, confirmed that using late‐spring soil nitrate nitrogen (NO3–N) test (LSNT) fertilizer recommendations could significantly reduce NO3–N loss in drainage water, but detailed crop response was not reported. Herein we summarize corn (Zea mays L.) response to the LSNT program when implemented across the Clarion–Nicollet–Webster soil association. The LSNT was used to determine the recommended N fertilizer rate that was applied uniformly across each field except on check strips where zero or nonlimiting (&gt;220 kg N ha−1) sidedress N was applied. Leaf chlorophyll (SPAD) readings, end‐of‐season stalk nitrate concentrations, and grain yield and quality (protein, starch, and oil content) showed significant year, field, soil map unit and N rate response. Average grain yield with the LSNT program was significantly lower than the nonlimiting rate in 1997 and 1998 but not in 1999 or 2000. This suggests that although watershed‐scale implementation of the LSNT can reduce nitrate loss through drainage water, it may also increase producer risk, especially when above‐normal rainfall occurs shortly after the sidedress N fertilizer is applied. To encourage adoption of the LSNT program for its water quality benefits, we suggest that federal, state, or private agencies develop affordable risk insurance or some other financial incentives to help producers minimize the potential crop risk associated with this program.

DEVELOPMENT AND APPLICATION OF SWAT TO LANDSCAPES WITH TILES AND POTHOLES

SWAT (Soil and Water Assessment Tool) is a watershed model that has been incorporated into USEPA’s modeling framework called BASINS used for total maximum daily load (TMDL) analysis. It is thus important that SWAT realistically simulate tile flow and pothole landscapes that are common in much of the Corn Belt and Great Lakes states. In this study, SWAT was modified to simulate water table dynamics and linked with a simple tile flow equation. Algorithms were also added to SWAT to include simulation of potholes (closed depressions), surface tile inlets, and aeration stress on plants. Flow interaction between HRUs was introduced in order to simulate pothole water. The modified SWAT model (SWAT-M) was evaluated using eight years of measured flow data from Walnut Creek watershed (WCW), an intensively tile-drained watershed in central Iowa. The model was calibrated during the period of 1992 to 1995 and validated during the period of 1996 to 1999. In addition, comparisons between the enhanced version (SWAT-M) and older version (SWAT2000) of SWAT were conducted. For assessing overall performance of the SWAT models, predictions were compared to data measured at stream sites approximately at the midpoint of the watershed (site 310) and at the outlet (site 330). Nash-Sutcliffe E values for the simulated monthly flows during the calibration/validation periods were 0.88/0.82 and 0.84/0.72 at sites 310 and 330, respectively. The relative mean errors (RME) of the simulated monthly flows during the calibration/validation periods were -2% / -1% and -18% / 10%, respectively, for the same two sites. These statistical values indicate that SWAT-M estimated both pattern and amount of the monthly flows reasonably well for the large flat landscape of WCW containing tile drains and potholes. SWAT-M needs to improve in its daily prediction because of its lower E values (-0.11 to 0.55), compared to the monthly results. In applying the model to a third site (site 210) that was predominantly influenced by tile drainage, it was concluded that the pattern and amount of simulated monthly subsurface flows (E values of 0.61 and 0.70 and RME values of 10% and -9% for the calibration and validation periods, respectively) were relatively close to the measured values. Nevertheless, SWAT-M simulation of daily subsurface flows was less accurate than monthly results. In general, the pattern and amount of monthly flow and subsurface tile drainage predicted by SWAT-M has been greatly improved as compared to SWAT2000.

Upscaling ground observations of vegetation water content, canopy height, and leaf area index during SMEX02 using aircraft and Landsat imagery

Microwave-based remote sensing algorithms for mapping soil moisture are sensitive to water contained in surface vegetation at moderate levels of canopy cover. Correction schemes require spatially distributed estimates of vegetation water content at scales comparable to that of the microwave sensor footprint (10 1 to 10 4 m). This study compares the relative utility of high-resolution (1.5 m) aircraft and coarser-resolution (30 m) Landsat imagery in upscaling an extensive set of ground-based measurements of canopy biophysical properties collected during the Soil Moisture Experiment of 2002 (SMEX02) within the Walnut Creek Watershed. The upscaling was accomplished using expolinear relationships developed between spectral vegetation indices and measurements of leaf area index, canopy height, and vegetation water content. Of the various indices examined, a Normalized Difference Water Index (NDWI), derived from near- and shortwave-infrared reflectances, was found to be least susceptible to saturation at high levels of leaf area index. With the aircraft data set, which did not include a short-wave infrared water absorption band, the Optimized Soil Adjusted Vegetation Index (OSAVI) yielded best correlations with observations and highest saturation levels. At the observation scale (10 m), LAI was retrieved from both NDWI and OSAVI imagery with an accuracy of 0.6, vegetation water content at 0.7 kg m −2, and canopy height to within 0.2 m. Both indices were used to estimate field-scale mean canopy properties and variability for each of the intensive soil-moisture-sampling sites within the watershed study area. Results regarding scale invariance over the SMEX02 study area in transformations from band reflectance and vegetation indices to canopy biophysical properties are also presented.