Thomas G. Franti

Water Erosion Prediction Project (WEPP): Development History, Model Capabilities, and Future Enhancements

The Water Erosion Prediction Project (WEPP) was initiated in August 1985 to develop new-generation water erosion prediction technology for use by federal action agencies involved in soil and water conservation and environmental planning and assessment. Developed by the USDA-ARS as a replacement for empirically based erosion prediction technologies, the WEPP model simulates many of the physical processes important in soil erosion, including infiltration, runoff, raindrop and flow detachment, sediment transport, deposition, plant growth, and residue decomposition. The WEPP project included an extensive field experimental program conducted on cropland, rangeland, and disturbed forest sites to obtain data required to parameterize and test the model. A large team effort at numerous research locations, ARS laboratories, and cooperating land-grant universities was needed to develop this state-of-the-art simulation model. WEPP project participants met frequently to coordinate their efforts. The WEPP model can be used for common hillslope applications or on small watersheds. Because it is physically based, the model has been successfully used in the evaluation of important natural resources issues throughout the U.S. and in many other countries. Upgrades to the modeling system since the 1995 DOS-based release include Microsoft Windows operating system graphical interfaces, web-based interfaces, and integration with Geographic Information Systems. Improvements have been made to the watershed channel and impoundment components, the CLIGEN weather generator, the daily water balance and evapotranspiration routines, and the prediction of subsurface lateral flow along low-permeability soil layers. A combined wind and water erosion prediction system with easily accessible databases and a common interface is planned for the future.

FLOW PATHWAYS AND SEDIMENT TRAPPING IN A FIELD-SCALE VEGETATIVE FILTER

Vegetative filters (VF) are a best management practice installed in many areas to control sediment movement to water bodies. It is commonly assumed that runoff proceeds perpendicularly across a VF as sheet flow. However, there is little research information on natural pathways of water movement and performance of field-scale VF. The objectives of this study were: (1) to quantify the performance of a VF where the flow path is not controlled by artificial borders and flow path lengths are field-scale, and (2) to develop methods to detect and quantify overland flow convergence and divergence in a VF. Our hypothesis is that flow converges and diverges in field-scale VF and that flow pathways that define flow convergence and divergence areas can be predicted using high-resolution topography (i.e., maps). Overland flow and sediment mass flow were monitored in two 13 × 15 m subareas of a 13 × 225 m grass buffer located in Polk County in east-central Nebraska. Monitoring included a high-resolution survey to 3 cm resolution, dye tracer studies to identify flow pathways, and measurement of maximum flow depths at 51 points in each subarea. Despite relatively planar topography (a result of grading for surface irrigation), there were converging and diverging areas of overland flow in the buffer subareas. Convergence ratios ranged from -1.55 to 0.34. Predicted flow pathways using the high-resolution topography (i.e., map) closely followed actual flow paths. Overland flow was not uniformly distributed, and flow depths were not uniform across the subareas. Despite converging and diverging flow, the field-scale VF trapped approximately 80% of the incoming sediment.

Modeling sediment trapping in a vegetative filter accounting for converging overland flow

Vegetative filters (VF) are used to remove sediment and other pollutants from overland flow. When modeling the hydrology of VF, it is often assumed that overland flow is planar, but our research indicates that it can be two-dimensional with converging and diverging pathways. Our hypothesis is that flow convergence will negatively influence the sediment trapping capability of VF. The objectives were to develop a two-dimensional modeling approach for estimating sediment trapping in VF and to investigate the impact of converging overland flow on sediment trapping by VF. In this study, the performance of a VF that has field-scale flow path lengths with uncontrolled flow direction was quantified using field experiments and hydrologic modeling. Simulations of water flow processes were performed using the physically based, distributed model MIKE SHE. A modeling approach that predicts sediment trapping and accounts for converging and diverging flow was developed based on the University of Kentucky sediment filtration model. The results revealed that as flow convergence increases, filter performance decreases, and the impacts are greater at higher flow rates and shorter filter lengths. Convergence that occurs in the contributing field (in-field) upstream of the buffer had a slightly greater impact than convergence that occurred in the filter (in-filter). An area-based convergence ratio was defined that relates the actual flow area in a VF to the theoretical flow area without flow convergence. When the convergence ratio was 0.70, in-filter convergence caused the sediment trapping efficiency to be reduced from 80% for the planar flow condition to 64% for the converging flow condition. When an equivalent convergence occurred in-field, the sediment trapping efficiency was reduced to 57%. Thus, not only is convergence important but the location where convergence occurs can also be important.

Macroinvertebrate drift density in relation to abiotic factors in the Missouri River

Changes in flow management to restore ecosystem health have been proposed as part of many restoration projects for regulated rivers. However, uncertainty exists about how the biota will respond to flow management changes. The objectives of this study were to estimate the relative importance of key abiotic predictor variables to aquatic macroinvertebrate drift densities in the Missouri River and to compare these results among reaches of the river. A multi-year, multi-location database of spring macroinvertebrate drift net sampling was used to develop relations between drift density and variables representing discharge, temperature, and turbidity in the Missouri River from Fort Randall Dam, South Dakota to the mouth of the Little Nemaha River, Nebraska. Multimodel inference using generalized linear mixed models and an information theoretic approach were used to estimate the relative importance of the predictor variables and the parameters. The results varied by reach. Discharge-related factors were more important at the upstream end of the study area, and turbidity was more important at the downstream end of the study area. Water temperature or degree days were also important predictors in the upstream reaches. The results below Gavins Point Dam suggest that increased macroinvertebrate drift densities are a response to reduced habitat and food availability. The results identify important variables for drift density that could be used in future experimental studies of flow manipulation for the Missouri or other large, regulated rivers.

Reducing Long-Term Atrazine Runoff from South Central Nebraska

Heavy reliance on chemical weed control in field crops of South Central Nebraska has resulted in the appearance of atrazine at concentrations greater than established drinking water standards. Our objective was to evaluate the best management practices for atrazine runoff for the tillage and herbicide management practices common to the region under study. Field experiments were performed to measure edge-of-field atrazine and water loss from disk-till, ridge-till, and slot plant (no-till) management systems. Results indicated less water runoff from no-till (34% less) and ridge-till (36% less) than from disk-till. Similarly, atrazine loss was also less: 24% less for no-till and 17% less for ridge-till than for disk-till. GLEAMS (Groundwater Loading Effect of Agricultural Management Systems) simulations were calibrated using field-measured inputs and verified against observed data from two independent sites. Fifteen different combinations of herbicide application and tillage practices were simulated using 50 years of rainfall data. Compared to pre-emergent broadcast + post application on corn with disk-till, annual reductions in simulated atrazine mass loss for the alternative practices ranged from 17% to 77%. The percent of annual atrazine lost ranged from 0.57% to 1.2%. During the 50-year simulation, annual losses from 7 to 10 years constituted >50% of the cumulative 50-year loss for broadcast and banded application. Based on recurrence interval evaluation, pre-emergent incorporation and pre-emergent banding were most effective at reducing long-term atrazine losses.

Tillage Effects on Soil Quality Indicators and Nematode Abundance in Loessial Soil under Long-Term No-Till Production

Abstract: Soil quality indicators and nematode abundance were characterized in a loessial soil under long‐term conservation tillage to evaluate the effects of no‐till, double‐disk, chisel, and moldboard plow treatments. Indicators included soil electrical conductivity (EC), soil texture, soil organic matter (SOM), and total particulate organic matter (tPOM). Nematode abundance was positively correlated with EC, silt content, and total POM and negatively correlated with clay content. Clay content was the main source of variation among soil quality indicators and was negatively correlated with nematode abundance and most indicators. The gain in SOM in the no‐till system amounted to 10887 kg over the 24 years or 454 kg ha−1 year−1, about half of this difference (45%) resulting from soil erosion in plowed soils. The balance of gain in SOM with no till (249 kg ha−1 year−1) was due to SOM sequestration with no till. No‐till management reduced soil erosion, increased SOM, and enhanced soil physical characteristics.

Impact of initial soil water content, crop residue cover, and post-herbicide irrigation on herbicide runoff

Herbicide loss in runoff is strongly influenced by rainfall immediately following herbicide application and by environmental conditions, such as crop residue cover and soil water content. A laboratory rainfall simulator was used to quantify the impact of initial soil water content (0.12 and 0.24 kg kg–1), crop residue cover (10% and 30% surface cover), post–herbicide irrigation (4 to 8 mm), and timing of first runoff event (1, 8, and 15 days) on atrazine and metolachlor runoff. Herbicides were applied (1.3 kg ha–1 a.i. atrazine; 1.6 kg ha–1 a.i. metolachlor) to the surface of self–contained soil trays (0.55 U 0.28 m), and simulated rainfall was applied at 55 mm hr–1. Herbicide concentration and mass loss in runoff were evaluated after 13, 25, 38, and 51 mm of rainfall, but treatment effects were independent of rainfall depth. Greater initial soil water content substantially increased herbicide concentration. When initial soil water content was 24% (versus 12%), 2 to 3 times more herbicide mass loss was observed when runoff occurred 1 and 8 days after herbicide application. After 51 mm of simulated rain, 30% crop residue cover resulted in 22% to 29% less water runoff and 35% to 50% less atrazine mass loss than 10% residue cover. Average herbicide concentrations were similar for both residue levels, indicating that differences in herbicide mass loss resulted from different water runoff volumes. The post–herbicide irrigation (“rain–in”) reduced atrazine mass loss by 33% on day 1, largely from reduced concentration, but no mass loss reduction was seen on days 8 and 15, when soil crusting is believed to have increased runoff volume. These results demonstrate the importance of soil water content during the first runoff following herbicide application and quantify how low antecedent moisture, greater crop residue cover, and a light post–herbicide irrigation can reduce herbicide runoff.

An Overland Flow Sampler for Use in Vegetative Filters

Vegetative filters (VF) are used to remove contaminants from agricultural runoff and improve surface water quality. Techniques are needed to quantify the performance of VF in realistic field settings. The goal of this project was to develop and test a relatively simple and low cost method for sampling overland flow in a VF. The 0.3 m wide sampler has the capacity to sample a flow rate of 1.3 L/s and a total runoff volume of about 20 000 L. The sampler was tested in the laboratory, in field experiments, and using a simulation model. Overall the sampler split ratio (SR) is 2180. The SR is essentially constant with flow rate in the range of 0.1 to 1.0 L/s. Computer simulations of overland flow using MIKE SHE indicate that the sampler does not cause significant convergence or divergence of flow when sampling at the downstream edge of the VF. Because of the higher roughness in the VF relative to the row-cropped contributing watershed, longer wing walls are needed to avoid flow convergence when sampling at the leading edge of the buffer. The required length of the wingwall is dependent on land slope, flow rate, and the hydraulic roughness of the filter. The runoff volume and runoff hydrographs that were derived from the sampler agreed well with the measurements taken with flow measurement flumes. Equations were developed to help interpret the data collected with the sampler.

Yard waste compost as a stormwater protection treatment for construction sites.

Runoff water quality improvement from three yard waste compost erosion control treatments were compared with two conventional treatments and an untreated control on plots of 3:1 slope during two growing seasons, using natural events and simulated rainfall. Runoff volume, suspended solids, nutrients, biomass, turf shear strength, and turfgrass color scale were monitored. The most effective compost treatment, a 5-cm thick blown compost blanket, produced 12.7 times less runoff and 9.8 times less sediment load than a straw mat and silt fence treatment. The compost treatments generated eight times more biomass than the straw mat treatments. Root development was significantly better on the compost treatments based on turf shear strength measurements. Tilled-in compost was not as effective as a compost blanket at reducing sediment loss, particularly before the establishment of grass on the plot. The cost of compost treatments was similar to that of straw mat with silt fence treatments.

Green-Ampt Infiltration Parameters in Riparian Buffers

Riparian buffers can improve surface water quality by filtering contaminants from runoff before they enter streams. Infiltration is an important process in riparian buffers. Computer models are often used to assess the performance of riparian buffers. Accurate prediction of infiltration by these models is dependent upon accurate estimates of infiltration parameters. Of particular interest here are Green-Ampt infiltration parameters, satiated hydraulic conductivity (Ko) and wetting front suction (hf). The objectives of this research were to (i) modify the Smith sorptivity procedure so that it can be used to estimate Green-Ampt infiltration parameters, (ii) Determine the relative closeness of Ko estimated by the inverse sorptivity and inverse Green-Ampt procedures and hf estimated by Rawls and Brakensiek (1985) to the laboratory-determined standards and (iii) Compare Ko estimates of the inverse sorptivity and inverse Green-Ampt procedures to those estimated by pedotransfer functions. This project was conducted at six sites in Nebraska, at which soil type and land use varied. The results of this study suggest that the inverse Green-Ampt procedure can be used to provide Ko estimates, even in the presence of macropores. Generally, pedotransfer function predictions did not estimate Ko well. Finally, hf as predicted by pedotransfer function was lower than laboratorydetermined hf. These predicted infiltration parameters were used in the Green-Ampt infiltration equation to illustrate their effect on cumulative infiltration. Cumulative infiltration based on the inverse Green-Ampt procedure parameters resulted in the closest match to cumulative infiltration prediction from laboratory-based infiltration parameters at five of the six sites.