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Current rangeland erosion modeling efforts do not adequately account for vegetation induced variability in interrill erosion processes. Research examples are presented that support the premise that rangeland interrill erosion is spatially and temporally distributed because of the influence of different vegetation growth forms, spatially distributed bio-mass, and a variable climate on surface soil properties. The surface soil parameters of shrub dominated landscapes display greater spatial than temporal variability, but bunch-grass and sodgrass dominated landscapes exhibited greater temporal variability than spatial. Improved model parameter estimating techniques are needed to account for the interrill erosion variability found on rangelands.
The Water Erosion Prediction Project (WEPP) model is intended to replace the Universal Soil Loss Equation for predicting soil erosion. The WEPP is a fundamental process-based model that operates on a daily time step to estimate land, soil and vegetation conditions when a rainfall event occurs, and then uses this information to predict the hydrology and erosion of single events. The WEPP is used in conjunction with an input climate data file, long term estimates are based on the accumulated erosion occurring during the period of record covered by the input climate file. This chapter describes the representation of rangelands for making estimates of the land, soil, and vegetation conditions, and their effect on soil erosion estimates. Additionally, shortcomings and advantages of WEPP for erosion prediction on rangelands is discussed.
A rainfall simulation study was conducted on sagebrush rangeland to quantify the small scale spatial variability in soil, plant, and hydrologic characteristics between four different surface soil-vegetation-microtopographic microsites (coppice, moss-grass, bare, and vesicular crust). The impact that this small scale spatial variability in hydrologic characteristics has on predictions of runoff and erosion from sagebrush rangeland was also investigated. The coppice and moss-grass microsites had significantly lower runoff and interrill erosion rates than the bare and vesicular crust microsites. Two averaging tech-niques (arithmetic mean and area weighted mean) were used to estimate the runoff and erosion response from a larger integrated area using measurements of runoff and erosion from the four surface microsites. The area-weighted average approach provided significantly better integrated estimates of infiltration, runoff, and interrill erosion than the arithmetic mean approach. Both averaging approaches produced poor integrated estimates of interrill erosion. These results have a significant impact on how hydrologic and erosion processes are modeled on rangelands. The commonly used assumption of viewing a hill-slope as a uniform plane that can be modeled using a single set of parameters would appear to be adequate for modeling infiltration and runoff on sagebrush rangeland, but not for modeling interrill erosion. Data presented in this chapter indicate that only a small portion of the soil particles that are detached in the interrill erosion process are actually delivered to the bottom of the hillslope. This suggests that the erosion process is transport limited and not detachment limited as often assumed.
Climate induced temporal variation, spatial patterns of vegetation and microenvironments, plant growth forms, soils and geology, and topographic factors influence hydrologic processes in rangeland environments. In Part I of this study, a gradient analysis of 13 environmental variables identified temporal and spatial gradients in sagebrush coppice and interspace soil surface cover types. Spatial cover types and temporal cyclic variations were distinct for both soil surface cover types. Part II of the study identified different spatial patterns for several sagebrush species. Wyoming big sagebrush (Artemisia tridentata Beetle & Young wyomingensis) and mountain big sagebrush (A. tri-dentata Rydb. veseyana) were both associated with uniform distribution patterns. Low sagebrush (A. arbuscula Nutt. arbuscula) exhibited a random pattern. Spatial patterns of vegetation (random, clumped, and uniform distribution) effect the degree of tortusoity of flow paths and hydraulic roughness on rangelands. Additional refinements to the Chezy friction coefficient that incorporates estimates of roughness coefficients for rills and interrill areas should be considered through additional resistance factors such as plant dispersion coefficients.
Rainfall simulation experiments conducted on large plots at various rangeland sites in southeastern Arizona were used to determine temporal variability in rangeland soil erosion. Measured soil erodibility varied monthly, seasonally, and yearly and appeared to depend on vegetation and soil type. Short term (monthly or seasonally) variability was greater than year to year variability unless treatment effects were interacting. The RUSLE K factor, computed within the RUSLE model from an algorithm based on frost-free period and annual R-values, cycles differently than the rainfall simulator measured erodibility; RUSLE estimates of K were the highest when measured erodibilities were the lowest. Time related changes in erosion rates associated with rangeland treatment need to be evaluated during a multiyear period using multiplot studies.
Surface runoff and erosion from frozen soils are widespread phenomena that have been reported in most regions of the world that have significant soil freezing. In some regions, such as the interior Pacific Northwest, frozen soil is associated with the majority of flooding and erosion events. The processes involved in frozen soil erosion are somewhat different from those in normal, unfrozen soil erosion. When the soil solution freezes, some portion of the total water content remains as liquid water. This is critical because the amount of ice formed largely determines the impact of soil freezing on the soil properties that affect erosion. Soil with high ice content may be essentially impermeable so that when the soil surface thaws it becomes highly erodible. These conditions can generate runoff and erosion with little or no rainfall (e.g., if there is snowmelt). Accurate estimation of runoff and erosion from frozen soils requires knowledge of soil freezing occurrence and depth, the effect of freezing on infiltrability and surface runoff, and the effect of freezing on soil erodibility. Current models can accurately predict frost occurrence and depth but not infiltrability or erodibility. Field observations of frozen soil runoff have shown extreme spatial and temporal variability over a range of scales. Accurate description of the effects of soil freezing on surface runoff and erosion on rangelands will ultimately require that models incorporate landscape scale processes.
The complexity and diversity of rangeland is epitomized by significant small scale spatial variability in soil hydrologic condition. Some variability in hydrologic condition may be due to the spatial and temporal effects of surface crusting. Presently, the Water Erosion Prediction Project (WEPP) model contains hydraulic conductivity algorithms that are based on crusting theories developed under agronomic rather than rangeland conditions. Crust thicknesses are fixed and physical relationships are based upon a structural crust produced by raindrop impact. Realistically, many rangelands retain durable microbiotic or physico-chemical crusts of varying thicknesses surrounded by shrub coppice dunes. A field technique was adapted to estimate in situ effective saturated hydraulic conductivity (Ke) on typically crusted rangeland soils. The data provides some insight into water flow through multilayered, unplowed soils. These actual Ke measurements on rangeland, from intact surface soils within shrub interspaces and beneath shrubs should be used to further enhance parameterization of the infiltration component of the WEPP model.
Surface erosion on pinyon pine (Pinus edulis Engelm.)-juniper (Juniperus spp.) dominated rangelands varied spatially and temporally due to the confounding effects of erratic climate, topographic changes, incongruities of soil and geologic substrate, and various other perturbations. Measurement of spatial variation was affected by plot size. Small plots (1 m2) were influenced by differences in soils, geologic substrate, and plant community structure. Therefore, many plots were needed to stratify vegetation, soil, and geologic differences. Runon and runoff processes between coppice dune and dune interspaces could not be measured. Large watershed size plots (a few hectares) were highly influenced by topographic features such as watershed slope, aspect, and shape. Plots that were 4 by 25 m could be located to minimize topographic changes, yet were large enough to include changes in soil, geologic substrate, and plant community structure.