WEPP Simulation of Observed Winter Runoff and Erosion in the U.S. Pacific Northwest

The Palouse area of the Northwestern Wheat and Range Region suffers high erosion throughout the winter season. The excessive soil loss is a result of a combination of winter precipitation, intermittent freezing and thawing of soils, steep land slopes, and improper management practices. Soil strength is typically decreased by the cyclic freeze and thaw, particularly during the period of thawing. When precipitation occurs during these freeze–thaw cycles, soil is easily detached and moved downslope. This study was aimed at improving the knowledge of winter hydrology and erosion in the Pacific Northwest (PNW) through combined field experimentation and mathematical modeling. Surface runoff and sediment were collected for three paired field plots under conventional tillage and no-till, respectively. Additionally, transient soil moisture and temperature at various depths were continuously monitored for two selected plots. These data were used to assess the suitability and performance of the USDA’s Water Erosion Prediction Project (WEPP), a physically based erosion model, under the PNW winter conditions. Field observations revealed that minimal erosion was generated on the no-till plots, whereas erosion from the conventionally tilled plots largely exceeded the tolerable rates recommended by the Natural Resources Conservation Service. The WEPP model could reasonably reproduce certain winter processes (e.g., snow and thaw depths and runoff) after code modification and parameter adjustment. Yet it is not able to represent all the complicated processes of winter erosion as observed in the field. Continued field and laboratory investigation of dynamic winter runoff and erosion mechanisms are necessary so that these processes can be properly represented by physically based erosion models. SOIL EROSION RATES in the Palouse and Nez Perce Prairies of the Northwestern Wheat and Range Region (Austin, 1981) vary among the seasons, with as high as 85% of soil loss occurring during winter (McCool et al., 1976; Zuzel et al., 1982; McCool, 1990). This excessive soil loss is a result of a combination of winter precipitation, intermittent freezing and thawing of soils, steep land slopes, and management practices that often leave the soil pulverized and unprotected during the rainy season (Papendick et al., 1983; McCool et al., 1987). Summer fallow followed by winter wheat (Triticum aestivum L.), traditionally a major cropping system in the Palouse Region, has long been a significant contributor to water erosion (Papendick et al., 1995). When fields are summer fallowed for a year under conventional tillage, the fallowed land is clean tilled to control weeds and store seed-zone moisture for the next year’s crop. Without soil surface cover, the surface layer is highly prone to water erosion when it becomes saturated during the winter precipitation season. In the higher precipitation zone of the Palouse, water is generally adequate for annual cropping, and summer fallow has become less frequently used. However, a small percentage of producers still use the practice on an occasional basis (McCool et al., 2001). The unique winter climatic conditions of the inland PNW typified by frequent freeze and thaw cycles further aggravates the already elevated vulnerability to erosion caused by conventional farming practices. Most water erosion in the Palouse region is related to rain on frozen or thawing soils and is often exacerbated by the warm, moist Pacific air masses that cause precipitation combined with rapid thaw (Yoo and Molnau, 1982; Zuzel et al., 1982). During freezing, water moves from deeper soil layers and concentrates in the frozen zone. Frost heave and expansion of soil pores frequently result. When a warming trend occurs and the soil begins to thaw, the expanded surface layers are at a high water content and can be easily detached. When a rainfall event occurs under these conditions, there is essentially no place for the water to go but down the slope, increasing the likelihood of soil erosion. The rate and depth of soil freezing is largely determined by tillage type, surface residue, water content, and water infiltration rate (McCool et al., 2000). Vomocil et al. (1984) discovered that surface residues help reduce soil freezing, but the extent of the effects of different types of residue on frost depth has not been studied thoroughly. Residue also reduces relative heat loss at night, air movement near the soil surface, and freezing depth (McCool et al., 2000). Field management also has a profound affect on the spatial variation of frost depth. Veseth et al. (1986) observed that rough tillage leads to a nonuniform frost depth, as compared with standing stubble or a smooth tilled soil surface. A study conducted near Pendleton, OR revealed a deeper frost depth (|15 mm) under a conventionally tilled field and a much shallower frost depth (,5 mm) under a no-till field with heavy residue (Greenwalt et al., 1983). Additionally, McCool et al. (2000) found that crop management had a major effect on both runoff and soil loss. The effect was greater on soil loss than on runoff for all observed conditions, and the presence of a frost layer reduced the effect of crop management on runoff more than on soil loss. R.C. Greer, J.Q. Wu, and P. Singh, Dep. of Biological Systems Engineering, Washington State Univ., Pullman, WA 99164; D.K. McCool, USDA-ARS-PWA, Pullman, WA 99164. *Corresponding author ([email protected]). Published in Vadose Zone Journal 5:261–272 (2006). SpecialSection:FromFieldtoLandscape-ScaleVadoseZoneProcesses doi:10.2136/vzj2005.0055 a Soil Science Society of America 677 S. Segoe Rd., Madison,WI 53711 USA Abbreviations: CT, conventionally tilled [plot]; ET, evapotranspiration; NT, no-till [plot]; OFE, overland flow element; PCFS, Palouse Conservation Field Station; PNW, Pacific Northwest; WEPP, Water Erosion Prediction Project. R e p ro d u c e d fr o m V a d o s e Z o n e J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 261 Published online March 8, 2006