Joan Q. Wu

Effects of DEM resolution and source on soil erosion modelling: a case study using the WEPP model

Digital elevation models (DEMs) vary in resolution and accuracy by the production method. DEMs with different resolutions and accuracies can generate varied topographic and hydrological features, which can in turn affect predictions by soil erosion models, such as the WEPP (Water Erosion Prediction Project) model. This study investigates the effects of DEMs on deriving topographic and hydrological attributes, and on predicting watershed erosion using WEPP v2006.5. Six DEMs at three resolutions from three sources were prepared for two small forested watersheds located in northern Idaho, USA. These DEMs were used to calculate topographic and hydrological parameters that served as inputs to WEPP. The model results of sediment yields and runoffs were compared with field observations. For both watersheds, DEMs with different resolutions and sources generated varied watershed shapes and structures, which in turn led to different extracted hill slope and channel lengths and gradients, and produced substantially different erosion predictions by WEPP.

SWAT modeling with uncertainty and cluster analyses of tillage impacts on hydrological processes

The impacts of tillage practices, majorly conventional tillage (CT) and no-till (NT), on soil hydraulic properties have been studied in recent decades. In this paper, we incorporated an auto-calibration algorithm into the Soil and Water Assessment Tool (SWAT) model and calibrated the model at eight field sites with soil water content (SWC) observations in the Pataha Creek Watershed, WA, USA. The Green–Ampt method in SWAT was chosen to determine infiltration and surface runoff. Parameter uncertainty was quantified by “relatively optimal” parameter sets filtered by a critical objective function value. Cluster analysis was adopted to obtain equal-sized parameter sets for each site and to compare parameter sets between tillage practices. The centers of these clusters were employed as a sample of parameter values. The clustered parameter sets were then used in scenario analysis to examine the impacts of cropland tillage practices on lateral flow, runoff and evapotranspiration (ET). The model parameters (e.g., soil hydraulic properties) were significantly different between CT and NT. In particular, higher bulk density, larger available water capacity, and higher effective hydraulic conductivity were found for NT than for CT. SWCs at three depths of the NT sites were significantly higher than those of CT sites, which could be attributed to tillage practices. However, higher available water capacity at NT sites indicated that the NT soil had a higher capacity to hold water. Thus the mean net changes in SWC during a year were not significantly different between CT and NT. The statistically different model parameters neither resulted in statistical differences in annual outputs (e.g., runoff and ET) nor substantial differences in monthly outputs. Our study indicates that the tillage impacts on hydrological processes are site-specific and scale-dependent.

Development of a GIS Interface for WEPP Model Application to Great Lakes Forested Watersheds

This presentation will highlight efforts on development of a new online WEPP GIS interface, targeted toward application in forested regions bordering the Great Lakes. The key components and algorithms of the online GIS system will be outlined. The general procedures used to provide input to the WEPP model and to display model output will be demonstrated.

Soil water and temperature in chemical versus reduced-tillage fallow in a Mediterranean climate.

A 2-year rotation of winter wheat (Triticum aestivum L.)-summer fallow is a dominant cropping system in the dryland region of the Pacific Northwest United States. Traditional, tillage-based summer fallow relies on a soil mulch to disrupt capillary continuity to conserve seed-zone water for early establishment of winter wheat. However, tillage to create the soil mulch and to subsequently fertilize and control weeds often results in unacceptable levels of wind erosion due to the burial of crop residues and the exposure of fine soil particles. Chemical (no-till) fallow (CF) and reduced-tillage fallow (RT) are two alternatives for reducing wind erosion. Our objectives were: (i) to assess the effects of CF and RT on seed- and root-zone temperature and water regimes; and (ii) to test the Simultaneous Heat and Water (SHAW) model for simulating management effects on soil temperature and water. Weather data, soil temperature, and water content were monitored in paired CF and RT treatments during April 2003-March 2004. The RT treatment was observed to retain more seed-zone water over summer compared to CF, consistent with relevant literature for Mediterranean environments and of critical importance to farmers. During the wet winter, CF gained more water than RT because of later planting of winter wheat, and thus less water use. Observed soil temperatures were higher in the CF due to its lower dry soil albedo, higher bulk density and thermal diffusivity than in the RT. SHAW-simulated water contents followed the general trend of the field data, though it slightly under-predicted soil water content for CF and over-predicted for RT. SHAW under-predicted soil temperature during the dry summer and over-predicted for the wet (November-December) period yet the overall trend was properly described with differences between simulations and observations decreasing with soil depth. Overall, SHAW proved adequate in simulating seed-zone and whole-profile soil water and temperature, and therefore may serve as a useful modeling tool for tillage and residue management.

Modifying WEPP to improve streamflow simulation in a Pacific Northwest watershed

Abstract. The assessment of water yield from hillslopes into streams is critical in managing water supply and aquatic habitat. Streamflow is typically composed of surface runoff, subsurface lateral flow, and groundwater baseflow; baseflow sustains the stream during the dry season. The Water Erosion Prediction Project (WEPP) model simulates surface runoff, subsurface lateral flow, soil water, and deep percolation. However, to adequately simulate hydrologic conditions with significant quantities of groundwater flow into streams, a baseflow component for WEPP is needed. The objectives of this study were (1) to simulate streamflow in the Priest River Experimental Forest in the U.S. Pacific Northwest using the WEPP model and a baseflow routine, and (2) to compare the performance of the WEPP model with and without including the baseflow using observed streamflow data. The baseflow was determined using a linear reservoir model. The WEPP-simulated and observed streamflows were in reasonable agreement when baseflow was considered, with an overall Nash-Sutcliffe efficiency (NSE) of 0.67 and deviation of runoff volume (D v ) of 7%. In contrast, the WEPP simulations without including baseflow resulted in an overall NSE of 0.57 and D v of 47%. On average, the simulated baseflow accounted for 43% of the streamflow and 12% of precipitation annually. Integration of WEPP with a baseflow routine improved the model’s applicability to watersheds where groundwater contributes to streamflow.

Residue Management Impacts on Field-Scale Snow Distribution and Soil Water Storage

Spatial variation of soil water affects crop performance, fertilizer use efficiencies, and other important economic and environmental factors. Soil water storage could be increased and field variability reduced by residue management practices such as no tillage (NT), as surface residues can retain more snow, enhance water infiltration, and reduce evaporation as compared to conventional tillage (CT). Our objectives were to evaluate the residue effects on snow distribution and the spatial variation of soil water storage for two adjacent fields near Pullman, Washington: one under NT, and the other under CT. The fields were surveyed during the winter and spring of 2007-2008 to assess topographic variations in snow depth, snow water equivalent (SWE), and soil water storage. Standing stubble under NT retained 10 to 20 cm more snow on ridge tops and steeply sloped ground than CT, and the snowpack was distributed more evenly with less spatial variation. SWE followed the same pattern of larger spatial variation in CT than in NT. Soil water (0 to 1.5 m) in the spring was lowest for ridge tops and highest in valleys for NT and CT. Under NT, however, soil water varied less across different field locations than under CT, and overall water storage was 60, 29, and 13 mm more for NT than CT on ridge top, south slope, and valley locations, respectively. Although many factors can contribute to the spatial variation of soil water, standing wheat residue under NT retained more snow on ridge tops and steeply sloped areas, reduced soil water spatial variation, and increased soil water recharge.

IMPROVING FROST-SIMULATION SUBROUTINES OF THE WATER EROSION PREDICTION PROJECT (WEPP) MODEL

Erosion models play an important role in assessing the influence of human activities on the environment. For cold areas, adequate frost simulation is crucial for predicting surface runoff and water erosion. The Water Erosion Prediction Project (WEPP) model is a physically based erosion prediction software program developed by the USDA. One of the major components of WEPP is the simulation of winter processes, which include snow accumulation and melt as well as soil freeze and thaw. WEPP is successfully used in the evaluation of important natural resource issues throughout the U.S. and in a number of other countries. However, previous studies revealed problems in the winter component of the WEPP model, especially the routine for frost simulation. The main purpose of this study was to improve the WEPP model (v2006.5) by changing the soil profile discretization and computation of key thermal and hydraulic parameters in the frost simulation routines so that the model can adequately simulate soil freeze-thaw and winter runoff and erosion. WEPP v2006.5 and the modified version (v2010.1) were applied to experimental plots in Pullman, Washington, and Morris, Minnesota. The simulated snow and frost depths as well as runoff and sediment yield were contrasted and compared with field observations; the results from v2010.1 showed substantial improvement compared to those from v2006.5.

WEPP simulations of dryland cropping systems in small drainages of northeastern Oregon

Computer simulation models are essential tools for evaluating soil erosion potential over large areas of cropland. Small-plot and field-scale evaluations are commonly conducted for federal farm program compliance, but producers are now faced with off-farm water quality concerns. Predicting the potential contribution of upland sediment is of interest to producers and state and federal agencies. The purpose of this study was to evaluate the applicability of the Water Erosion Prediction Project (WEPP) model for quantifying hydrological and erosion processes in the semiarid croplands of the Columbia Plateau. Two headwater drainages managed using conventional inversion tillage (CT) or no-tillage (NT) management techniques were monitored from 2001 through 2007 in the dryland cropping region of northeastern Oregon. The WEPP model was parameterized primarily from field data, including management and weather data. Crop parameters (above-ground biomass and crop yield), water balance components (volumetric soil water, evapotranspiration [ET], and surface runoff), and soil loss were observed and subsequently used to evaluate WEPP simulations. This detailed dataset allowed for a unique opportunity to evaluate not only the WEPP routines for runoff and erosion but also the routine for crop growth, which greatly influences the erodibility and hydraulic conductivity of top soil layers. Graphical and goodness-of-fit analyses indicate that WEPP generated satisfactory estimates for volumetric soil water and crop yields in NT and CT and above-ground biomass production in NT. Gross patterns of ET simulated by WEPP were compatible with those determined using observed precipitation and soil water data. Observed annual runoff and erosion values from both drainages were low (NT: 0.1 mm [0.004 in], 2.5 kg ha−1 [0.001 tn ac−1]; CT: 0.9 mm [0.04 in], 72.0 kg ha−1 [0.03 tn ac−1]). On average only 0.3% and 0.03% of total precipitation left the catchment as runoff during the six-year study period for CT and NT, respectively. No runoff was predicted by WEPP when default input values for a Walla Walla silt loam soil were used in the model. Simulated runoff and erosion agreed well with field observations after the effective surface hydraulic conductivity Keff and rill erodibility Kr were calibrated. With minimal calibration, the WEPP model was able to successfully represent the hydrology, sediment transport, and crop growth for CT and NT cropping systems in northeastern Oregon during years of below normal precipitation, mild weather, and little runoff.

Seasonal Change of WEPP Erodibility Parameters for Two Fallow Plots on a Palouse Silt Loam

Abstract. In cold regions, frozen soil has a significant influence on runoff and water erosion. In the U.S. Inland Pacific Northwest, major erosion events typically occur during winter as frozen soil thaws and exhibits low cohesion. Previous applications of the WEPP (Water Erosion Prediction Project) model to a continuous bare tilled fallow (CBF) runoff plot at the Palouse Conservation Field Station (PCFS) in southeastern Washington State showed that WEPP reproduced the occurrence of the large erosion events, but the amount of sediment yield was either under- or overestimated. The inability of WEPP to reproduce the magnitude of field-observed erosion events at the PCFS suggests the need for an examination of the dynamic changes in soil erosion properties and for improving the representation of such dynamics. The objective of this study was to evaluate the seasonal changes of rill erosion parameters for two CBF runoff plots at the PCFS. Field-observed runoff and erosion events during 1984-1990 were used to estimate WEPP hydraulic and erosion parameters, including soil effective hydraulic conductivity, critical shear stress (I„ c ), and rill erodibility (K r ). The parameters for each event were best-fitted using WEPP single-event simulations to reproduce the observed runoff and sediment yield on both plots. The results suggest that the adjustment factors for I„ c and K r of frozen and thawing soils in the WEPP model could be modified to better describe the changes of these parameters in winter.

Effects of DEM resolution on forest hydrologic and erosion prediction using WEPP

The recent modification of WEPP (Water Erosion Prediction Project) has improved the original model’s applicability to hydrology and erosion modeling in forest watersheds. To generate reliable topographic and hydrologic inputs for the WEPP model, carefully selecting Digital Elevation Models (DEMs) with appropriate resolution and accuracy is essential because topography is a major factor controlling water erosion. LIght Detection and Ranging (LIDAR), a new remote sensing technology, provides an alternative for generating fine and high-quality DEMs. This study applies WEPP (v2006.201) for hydrological and erosion simulation under forest conditions and evaluates the effects of DEM resolution and accuracy on watershed hydrology and water erosion prediction at a watershed scale. Stream flow and total suspended solids (TSS) in two small forest watersheds located in northern Idaho were collected and processed. A total of six DEMs from three sources (NED, SRTM, and LIDAR) at three resolutions (30 m, 10 m, and 4 m) were obtained and used to calculate topographic parameters as inputs to the WEPP model. WEPP-simulated hydrologic and erosion results using the six DEMs were compared with the field-observed data. For both study watersheds, DEMs with different resolutions and sources generated varied topographic and hydrologic attributes, which in turn led to significantly different erosion predictions by WEPP.