Donald K. McCool

ERODIBILITY AND CRITICAL SHEAR OF A PREVIOUSLY FROZEN SOIL

Many areas of the northern United States and southern Canada, and particularly the four million ha of nonirrigated cropland of the Northwestern Wheat and Range Region in the United States, suffer severe erosion under thawing soil conditions. Ability to predict soil erosion in these areas is hampered by lack of knowledge of soil properties and hydraulic resistance of thawing soils. This study was conducted to determine erodibility parameters of soil frozen and thawed under controlled moisture tension. A tilting flume was designed to allow near-natural freezing and thawing of a soil mass and apply shear stress from flowing water. Tests were conducted under soil moisture tensions of 50, 150, and 450 mm. A linear relationship was found between detachment and shear stress. Critical shear stress values showed little change with time. Rill erodibility was related to soil moisture tension and changed rapidly during the course of a 90-min test flow. At a tension of 450 mm, the erodibility was similar to that found in field tests under summer conditions. This study indicates the transient nature of rill erodibility during soil freezing and thawing that will influence the accuracy of continuous simulation erosion models for winter conditions.

Low Intensity Rainfall with a Rotating Disk Simulator

ABSTRACT THE Palouse Rainfall Simulator was designed to simulate storms of the Pacific Northwest. Such storms are characterized by their low intensities, long durations, and small drop sizes. A drop size distribution similar to that of storms with rainfall intensities of 2.5 to 10 mm/h was achieved by nozzle selection. Rainfall intensities of 6 to 46 mm/h were obtained by use of slotted rotating disks. The rotating disks had no significant effect on the drop size distribution. Application uniformity was slightly lower at the lowest intensity tested..

Critical Conditions for Rill Initiation

Quantifying critical conditions of rill formation can be useful for a better understanding of soil erosion processes. Current studies lack a consensus and related rationale on how to describe these conditions. This study was based on the concepts that (1) the shear stress available for erosion at any given point is a function of the runoff rate, the slope steepness, and hydraulic characteristics of the surface; (2) rill incision begins when overland flow shear stress exceeds soil critical shear stress; and (3) the distance from the top of the slope to the point where rills form can be measured and analyzed as length to rill initiation and decreases with increase in slope and rainfall intensity. These concepts were tested with a representative silty-clay soil from the Loess Plateau in northwestern China on a large sloping indoor plot (8 × 3 m), with five different slopes using simulated rainfall at three rainfall intensities. Values of several hydraulic parameters at rill initiation were determined from the experimental data. The results showed relationships among slope steepness, rainfall intensity, and location of rill initiation. It was found that slope was relatively more important than rainfall intensity in determining the location of rill initiation. Soil critical shear stress determined in this study ranged from 1.33 to 2.63 Pa, with an average of 1.94 Pa. Soil critical shear stress was inversely related to slope and was not influenced by rainfall intensity. The results of this study were comparable with those of previous investigators.

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.

Mt. St. Helens volcanic ash: effect of incorporated and unincorporated ash of two particle sizes on runoff and erosion

Abstract The effect of ash from the 1980 Mt. St. Helens eruption on runoff and rill erosion of cultivated land was studied in 1981 and 1982. Ash from the Pullman (fine particles) and Yakima, WA (coarse particles), areas was applied to a silt loam soil, and water was applied. Treatments were: bare soil (fine-silty, mixed, Mesic Pachic Ultic Haploxerolls); ash incorporated with soil; and unincorporated ash on the soil surface. Sediment concentration in runoff first increased with time to a maximum, then decreased to an approximate steady state. Sediment concentrations were higher for unincorporated ash as compared to ash incorporated with the underlying soil, and were even lower for bare soil. Sediment concentration was higher with Yakima ash as compared to Pullman ash or bare soil because of the less cohesive nature of Yakima ash. Rills became wide and shallow with Yakima ash, as compared to deeper and narrower rills for Pullman ash, because of the relatively more cohesive nature and finer particle size of Pullman ash. The finer Pullman ash caused formation of a surface seal, thus restricting the infiltration rate and producing a higher runoff rate than from rills with Yakima ash or bare soil.

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.

Applying Online WEPP to Assess Forest Watershed Hydrology

Abstract. A new version of the online Water Erosion Prediction Project (WEPP) GIS interface has been developed to assist in evaluating sediment sources associated with forests and forest management within the Great Lakes basin. WEPP watershed structure and topographical inputs for each watershed element are generated from the USGS 30 m National Elevation Dataset (NED), soil inputs are automatically retrieved from the USDA-NRCS SSURGO database, and land use and management inputs are selected from the WEPP database based on the USGS National Land Cover Database 2001 (NLCD2001). Additionally, ground cover and soil properties of the WEPP management and soil input files can be customized to represent site-specific conditions. Daily climate inputs are generated from long-term climate parameters using CLIGEN, a stochastic climate generator embedded in the online interface. Alternatively, a registered user can upload and use observed daily climate data for online WEPP simulation. Long-term observational data, including runoff and water chemistry, from two mature forest watersheds of the Fernow Experimental Forest in West Virginia were used to assess the online WEPP GIS interface. Online WEPP simulations were carried out using both observed and CLIGEN-generated climate inputs, and model performance was examined by comparing simulated and observed runoff and simulated and estimated (from measured water chemistry data) sediment yield. The online WEPP reasonably simulated average annual runoff and the annual maximum runoff series for both watersheds, but overpredicted sediment yield for the annual average and annual maximums. The online WEPP simulation results accurately reflected the differences between the two watersheds in their hydrological characteristics. The online WEPP GIS interface is a user-friendly, web-based computer package that can be used by scientists, researchers, and practitioners as a cost-effective simulation tool for watershed management.

DEM Resolution Effects on Hillslope Length and Steepness Estimates for Erosion Modeling

The use of Geographic Information Systems (GIS) has greatly enhanced the capability of predicting soil loss using hillslope and watershed models. Selecting the appropriate Digital Elevation Model (DEM) resolution is important for estimating slope length and steepness values and subsequently estimating soil erosion and sediment delivery rates. Slope length and steepness estimates from 10-m and 30-m DEMs in ArcGIS were compared to those of observed hillslope rill patterns. The observed data were collected in the springs of 1973 through 1983 from several fall-seeded winter wheat fields in eastern Washington and northern Idaho, an area of steep and variable topography. Estimated erosion rates from the Revised Universal Soil Loss Equation, Version 2 (RUSLE2) model for the observed and DEM topographic data from ArcGIS were also compared. Statistical analyses resulted in the quantified rank of error for estimated slope steepness and slope length data in general ascending order as: DEM resolution (10-m, 30-m); and slope length estimation (Flow Length, Flow Accumulation). RUSLE2 soil loss estimates from 10-m DEM resolution underestimated 15%, and those from 30-m DEM resolution underestimated 22% as compared to RUSLE2 estimates from the observed topographic data. For the conditions of the study area, the impact of slope steepness on soil loss was more significant than that of slope length. The erosion estimates are site-specific, but rank of error should be regionally applicable to hillslope and watershed models using ArcGIS-based extensions to estimate slope steepness or flow path length. Given the lack of accuracy in erosion estimates with the better resolution 10-m DEM, consultants and agencies such as Natural Resources Conservation Service may need to develop a method to adjust GIS steepness estimates on steep, irregular topography in order to make realistic field-scale erosion estimates for planning purposes.

Digitizing Chart Recorder Data: Coordinate System Conversion for Rain Gauges and Similar Recording Instruments

Though chart recorder data are widely available for many environmental parameters over long periods of record, the nonorthogonal coordinate systems of many types of recorder charts complicates data extraction such that it is almost always manual; a tedious chore involving hand-counting tiny red boxes. Perhaps the most common data available on these types of charts are precipitation from weighing-type recording rain gauges. Unfortunately, because most digitizer software assumes the data being digitized have an orthogonal coordinate system, the nonorthogonality inherent in these charts prevents simple digitization of the data. If methodologies exist for electronically extracting rain gauge data, they are not widely known; recent texts still outline manual break-point tabulation in detail with no reference to any software or published methodologies for electronic data extraction. A simple procedure is presented here to transform digitized chart recorder data into meaningful, accurate data using readily available software. A comparison of manually read precipitation data and precipitation data extracted with a digitizer showed average differences of less than half the smallest marked increment on the charts for both precipitation amount and time. Differences were almost exclusively due to human bias in locating break points rather than true error.