This chapter develops erosion-soil productivity relationships for a study area within the Palouse Basin. Studies indicate that potential yields have been reduced by erosion. For example, the Palouse Cooperative River Basin Study estimated that without soil erosion, improved technology by the mid 1970s would have increased average wheat yields in the Basin by 30 to 40% above the average yield of 3400 kg/ha. Moreover, most of the present yield increase from technological advances comes from parts of the fields with the least erosion damage. Future wheat yields and yield growth in the Palouse and other areas of the Northwest wheatlands that are subject to erosion will depend more and more on how well erosion is controlled. Major changes in tillage and cropping practices will be required to improve or even sustain productivity on land classes subject to heavy erosion or where past erosion has taken a heavy tool of topsoil.

Seasonally frozen soil strongly influences runoff and erosion on large areas of land around the world. In many areas, rain or snowmelt on seasonally frozen soil is the single leading cause of severe runoff and erosion events. As soils freeze, ice blocks the soil pores, greatly diminishing the permeability of the soil. This is aggravated by the tendency of water to migrate to the freezing front, causing elevated ice content and frost heave. Soil freezing and thawing also plays a role in a variety of other environmental processes. Frost heave poses significant problems for structures, roads, and plant roots. Soil freezing and thawing can create stress fractures and alter soil physical properties including pore continuity and aggregate stability; these alterations can influence soil hydraulic properties and erodibility long after the soil is thawed. Water migration associated with soil freezing can strongly influence solute movement. This paper presents an overview of freezing and thawing processes within the soil, its effect on hydrology, and its interaction with environmental conditions. Topics discussed include the effect of soil freezing on moisture movement, frost heave, infiltration, erodibility, aggregate stabilty, and solute movement. Examples are given illustrating the influence of soil texture on soil freezing dynamics and aggregate stability.

In cold regions, frozen soil has a significant influence on runoff and water erosion. Frozen soil can reduce infiltration capacity, and the freeze-thaw processes degrade soil cohesive strength and increase soil erodibility. In the Inland Pacific Northwest of the USA, major erosion events typically occur during winter from low-intensity rain, snowmelt, or both as frozen soil thaws and exhibits low cohesion. The Water Erosion Prediction Project (WEPP) model is a physically-based simulation tool for water erosion, and has been widely used for conservation planning on agricultural, range, and forest lands. WEPP estimates runoff and sediment yield by simulating major hydrological and erosion processes. Previous applications of WEPP to continuous bare fallow (CBF) runoff plots at the Palouse Conservation Field Station (PCFS) in southeastern Washington State showed that WEPP reproduced the occurrence of the major observed erosion events but the amount of sediment yield was either under- or over-estimated. The inability of WEPP to fully reproduce field-observed erosion events at the PCFS suggests a need for an examination of the dynamic changes in soil properties and for improving the representation of such dynamics. The objective of this study was to evaluate the seasonal changes of rill erosion parameters on a CBF runoff plot at the PCFS.

Roots, cereal crowns, and stems growing beneath the soil surface provide important resistance to soil erosion. Understanding the amount and distribution of this material in the soil profile could provide insight into resistance to soil erosion by water and improve the performance of soil erosion models, such as the revised universal soil loss equation (RUSLE) and the water erosion prediction project (WEPP). Erosion models use built-in or external crop growth models to populate crop yield and live aboveground and associated belowground biomass databases. We examined two data sets from the dryland small grain production region in the Pacific Northwest of the United States to determine root:shoot ratios, the vertical distribution of root and attached belowground biomass, and incorporated residue from previously grown crops. Data were collected in 1993, 1994, 1995, and 2000 from short-term no-till and conventional tillage experiments conducted near Pendleton, Oregon, and Pullman,Washington, and in 1999 and 2000 from long-term experiments representative of farming practices near Pendleton, Oregon. The crops sampled in the short-term data set included soft white winter and spring wheat (Triticum aestivum L.;WW and SW, respectively), spring peas (Pisum sativum L.; SP), and winter canola (Brassica napus L.;WC). Crops sampled in the long-term study included WW SW, and SP. Data were collected at harvest in both data sets and during three phenologic stages in each of the crops in the short-term data set. Soil samples were collected to a depth of 60 cm (23.6 in) in the short-term and 30 cm (11.9 in) in the long-term experiments. In both sets of measurements, we found greater than 70% of root mass is in the top 10 cm (3.9 in) of the soil profile with the exception of SP, which had 70% of root mass in the top 15 cm (5.9 in) of the soil profile.WC produced significantly more biomass near the soil surface than WW SW, or SP Root-to-shoot biomass ratios, in mature wheat ranged from 0.13 to 0.17 in the top 30 cm (11.9 in) of the soil profile, substantially lower than values suggested for use in WEPP (0.25). In the long-term experiments, soil of the conventionally tilled continuous winter wheat (CWW) plots contained significantly greater biomass than soil of conventionally tilled winter wheat/fallow (CR) and no-till winter wheat/fallow (NT) treatments. There was no significant difference between CWW and conventionally tilled winter wheat/spring pea (WP); however, CWW returned more residue to the soil than WP because SP produced less residue and these residues were incorporated with a field cultivator rather than a moldboard plow. More accurate representation of root development, particularly in winter crops, could improve RUSLE and WEPP performance in the Pacific Northwest where winter conditions have proven difficult to model.

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 CLIGENgenerated 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, webbased computer package that can be used by scientists, researchers, and practitioners as a cost-effective simulation tool for watershed management.

Interest in improving the performance of water and wind erosion prediction models, such as the Revised Universal Soil Loss Equation (RUSLE), the Water Erosion Prediction Project (WEPP), and the Wind Erosion Prediction System (WEPS), led to this study of the relationship between the mass of crop residue and crop yield produced on nonirrigated cropland of the Inland Pacific Northwest United States, consisting of eastern Washington, northern Idaho, and northcentral Oregon. Retaining postharvest crop residues as soil surface cover is a primary method for controlling wind and water erosion; accordingly, erosion prediction models are highly sensitive to the amount of surface residue retained as soil cover. Traditionally, crop biomass calculations and erosion prediction models used expected or modeled crop yields and a fixed residue/grain index (R/G Index) value to determine residue quantity. Literature search indicated that cereal breeding efforts that emphasize yield have reduced the amount of residue for each unit. of grain produced. In order to fulfill our objective of improving the relationship between grain yield and residue production, we assembled and examined a large set of regional crop yield and residue production data collected in research plot studies, on-farm field studies, and in available literature from eastern Washington and northcentral Oregon. Results of the study indicated that the R/G Index varies with yield. We also found that residue production versus grain yield for major nonirrigated crops of wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) cereals, as well as for annual legumes and brassicas, can be fit with a linear relationship with a positive intercept and that the slope and intercept of the line are crop specific. Parameter values for specific crops are given. Using the historical fixed R/G Index can result in overestimating residue production of high-yielding winter wheat by as much as 35% and underestimating residue production for low spring wheat yields by as much as 66%. The results provide improved residue-to-grain yield relationships for use in water and wind erosion prediction models applied to the conditions of the Inland Pacific Northwest and adjacent areas. They provide a basis for estimating crop residue production in the region, and in conjunction with carbon sequestration models, a basis for determining if and where residues can be harvested for biomass in the region.

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.

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.

Many areas of the northern United States and southern Canada, and particularly the 4 million ha of unirrigated cropland of the Northwestern Wheat and Range Region in the United States, experience severe water erosion under thawing soil conditions. Modeling soil erosion in these areas is hampered by a lack of knowledge of the relation of soil properties and moisture conditions to hydraulic resistance of thawing soils. This study was conducted to determine hydraulic and erodibility parameters of frozen and thawed soil under controlled moisture tension. A tilting flume was designed and constructed to allow near‐natural freezing and thawing of a soil mass and to apply shear stress from flowing water. Flow tests were conducted for 90 min under soil moisture tensions of 50, 150, and 450 mm. A linear relationship was found between detachment and applied shear stress at a given time and moisture tension. Critical shear stress values showed little change with time. Rill erodibility decreased with increased soil moisture tension but changed more rapidly during tests at 50‐ and 150‐mm tension. At 50‐mm tension, the time‐average erodibility, 689 g N−1 min−1, was about the same, and the critical shear value, 1.53 N m−2, about 60% of that found in tests of a similar Palouse silt loam soil tested under 50‐mm tension without freezing. This study adds to the body of knowledge that indicates that the transient nature of rill erodibility during soil freezing and thawing should be considered to improve the accuracy of continuous simulation erosion models for winter conditions.

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.

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.

Soil erosion by water is detrimental to soil fertility, crop yield, and the environment. For cold areas, knowledge of winter hydrologic processes is critical to determining land‐use and management practices for reducing soil loss and protecting land and water resources. Adequate understanding of winter processes is also essential to developing models as effective predictive tools. This study evaluated the effects of two contrasting tillage practices on winter hydrologic and erosion processes, and the suitability of the Water Erosion Prediction Project (WEPP) model with a newly implemented energy‐budget‐based winter routine for quantifying these processes. Research plots subject to two tillage treatments—continuous tilled bare fallow (CTBF) and no‐till (NT) seeding of winter wheat (Triticum aestivum L. cv. Madsen) after spring barley (Hordeum vulgare L.)—were established at the USDA‐ARS Palouse Conservation Field Station, Pullman, WA. The plots were monitored for runoff, erosion, soil temperature, water content, and depths of snow and freeze–thaw during October to May of 2003–2004 through 2006–2007. The NT plot generated negligible runoff and erosion (0.5 mm, 0.2 Mg ha−1) compared with CTBF (323 mm, 547 Mg ha−1). Frost occurred more frequently and was deeper in CTBF, probably due to its lack of residue and shallower snow depth. The modified WEPP model could reasonably reproduce major winter processes, yet it cannot represent all the complicated winter phenomena observed in the field. Continued efforts are needed to further improve the ability of WEPP to properly account for soil freeze–thaw and thus transient soil hydraulic properties and hydrologic and erosion processes.

The U.S. Pacific Northwest, influenced by maritime air masses for most of the year, generally exhibits low rainfall intensities. The drop-size characteristics of such low-intensity storms are rarely studied. An investigation of natural drop sizes and their distribution was conducted at the Palouse Conservation Field Station near Pullman, WA and at Corvallis, OR. Data was collected from a number of storms by Ozalid paper technique. The distribution of drop sizes, intensity, and total calculated energy of these storms are presented and compared with data and relationships from studies in other areas of the U.S. This data has implications for rainfall simulator design and application of erosion models in areas with low-intensity rainfall that here-to-fore have relied on extrapolation of rainfall characteristics from higher precipitation areas for driving hydrologic and erosion models.

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 x 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.

Soil erosion by water is the wearing away of the earth’s surface by the force of water and gravity, and consists of soil particle dislodgement, entrainment, transport, and deposition. This sequence of events occurs over a wide range of temporal and spatial scales, from raindrop splash moving particles millimeters in milliseconds to suspended sediment and bedload in rivers continuously moving an estimated 15.5 billion metric tonnes per year to the oceans of the world. As a matter of course, soil erosion begins in the uplands where soil is dislodged and moved downslope, progressing from splash saltation, to entrainment in microchannels, to collection in rills, concentrated flow channels, gullies, and ultimately stream channels. Deposition and re-entrainment occurs continuously from ridge top to deltas. The progression from dislodgement and entrainment to deposition can be accelerated or retarded through human manipulation of the soil resource, plant cover, or animal use. In this article, we examine how soil erosion in uplands is influenced by rainfall, climate, topography, soil characteristics, and, most importantly, human activities.

Burn/low-till management of winter wheat (Triticum aestivum) is being practiced by some growers in the higher rainfall areas of the Pacific Northwestern Winter Wheat Region of the US. Residue burning eliminates the numerous seedbed tillage operations that are normally required to reduce residues and control weeds and diseases in continuous winter wheat production. The detrimental effects of burn and till systems on soil erosion are well documented. However, there is little or no data on the effects of burning with no-till or low-till annual cropping on either erosion or soil quality. A 3-year field study comparing winter season erosion resulting from burn/low-till (BLT) seeded winter wheat following winter wheat and conventionally managed (CM) winter wheat following various crops was completed in 1997. Results indicate soil loss from the BLT fields was not significantly different from that of the CM fields with various crops preceding winter wheat. For the BLT fields, soil loss was as closely related to soil disturbance (number of tillage operations) as to the amount of surface residue. When residue and crop cover did not differ with the number of tillage operations, an increased number of tillage operations after burning loosened the soil and resulted in greater soil loss. No adverse effects on soil loss or soil quality from using the BLT with one or two-pass seeding of winter wheat following winter wheat were found in this study. The results have implications for harvesting wheat stubble as a source of biomass, or as an alternative technique for initiating conversion from a conventional tillage to a no-till seeding system, without high initial investment in new seeding equipment.

The occurrence of snow and soil-frost influences hydrology and, in turn, the mechanisms of soil erosion processes in cold regions. For these regions, reliably modeling the dynamics of snow accumulation and melt, and soil frost formation and melt, is necessary prior to accurately predicting runoff and erosion. Only then will methods for predicting the rates and amounts of soil erosion by water be established on a firm hydrological footing. This article examines the potential of an energy budget approach to simulate the magnitude and variations of snow and soil frost depths. It is assumed that the net sum of all energy components in the environment is consumed or compensated by water phase change occurring near or under the ground surface, such as snow melting or soil freezing and thawing. Testing indicates that this energy budget approach demonstrates promise to simulate winter hydrology and to be adapted to erosion prediction models.

Much of the total annual runoff and soil loss from non-irrigated cropland in theNorthwestern Wheat and Range Region of the Pacific Northwest U.S.A. occurs as a result ofrain and snowmelt on frozen and thawing soil. Quantifying the effects of these winter processesand their interaction with crop management is important for hydrologic model development andcalibration, and for designing cropping systems to prevent erosion. An experiment with severalrunoff plots under a number of crop management systems was installed at the PalouseConservation Field Station near Pullman, WA in the fall of 1978. Data was collected fromnatural events on the replicated treatments for 13 years, and from a smaller set of replicatedtreatments for another five years. Frost, thaw, and snow depths were measured regularly.Runoff and sediment samples were obtained from collection tanks on a daily or event basis.This 18-year data set was supplemented with a 43-year data set obtained from monitoringwinter erosion on a large number of sample fields in Whitman Co, WA from Water Year (WY)1941 through WY 1982, to examine historic trends in erosion and corresponding climateconditions. During this period, the winter wheat/summer fallow rotation was used on much ofthe area, including the higher precipitation zone more suitable for annual cropping. Erosion waslow during several consecutive years in the 43-year data set. Weather records for these yearsindicate reduced freeze/thaw activity with little rain or snowmelt during thaw. Data analysisindicated that soil loss was not correlated with diurnal freeze-thaw cycles, or with annualsnowfall, or with snow at the time of thaw. Event soil erosion was positively correlated with precipitation during thaw, as was annual erosion with annual precipitation. Crop managementhad a substantial effect on infiltration and runoff, and an even greater effect on erosion.