J. J. Stone

Soil erosion by surface water flow on a stony, semiarid hillslope

Soil erosion on hillslopes occurs by processes of soil splash from raindrop impacts and sediment entrainment by surface water flows. This study investigates the process of soil erosion by surface water flow on a stony soil in a semiarid environment. A field experimental method was developed whereby erosion by concentrated flow could be measured in predefined flow areas without disturbing the soil surface. The method allowed for measurements in this study of flow erosion at a much wider range of slopes (2·6 to 30·1 per cent) and unit discharge rates (0·0007 to 0·007 m2 s−1) than have been previously feasible. Flow velocities were correlated to discharge and hydraulic radius, but not to slope. The lack of correlation between velocity and slope might have been due to the greater rock cover on the steeper slopes which caused the surface to be hydraulically rougher and thus counteract the expected effect of slope on flow velocity. The detachment data illustrated limitations in applying a linear hydraulic shear stress model over the entire range of the data collected. Flow detachment rates were better correlated to a power function of either shear stress (r2 = 0·51) or stream power (r2 = 0·59). Published in 1999 by John Wiley & Sons, Ltd.

A Rangeland Hydrology and Erosion Model

Soil loss rates on rangelands are considered one of the few quantitative indicators for assessing rangeland health and conservation practice effectiveness. An erosion model to predict soil loss specific for rangeland applications is needed because existing erosion models were developed from croplands where the hydrologic and erosion processes are different, largely due to much higher levels of heterogeneity in soil and plant properties at the plot scale and the consolidated nature of the soils. The Rangeland Hydrology and Erosion Model (RHEM) was designed to fill that need. RHEM is an event-based derivation of the WEPP model made by removing relationships developed specifically for croplands and incorporating new equations derived from rangeland data. RHEM represents erosion processes under disturbed and undisturbed rangeland conditions, it adopts a new splash erosion and thin sheet-flow transport equation developed from rangeland data, and it links the model hydrologic and erosion parameters with rangeland plant communities by providing a new system of parameter estimation equations based on 204 plots at 49 rangeland sites distributed across 15 western U.S. states. RHEM estimates runoff, erosion, and sediment delivery rates and volumes at the spatial scale of the hillslope and the temporal scale of a single rainfall event. Experiments were conducted to generate independent data for model evaluation, and the coefficients of determination (r2) for runoff and erosion predictions were 0.87 and 0.50, respectively, which indicates the ability of RHEM to provide reasonable runoff and soil loss prediction capabilities for rangeland management and research needs.

Fire, Plant Invasions, and Erosion Events on Western Rangelands

Abstract Millions of hectares of rangeland in the western United States have been invaded by annual and woody plants that have increased the role of wildland fire. Altered fire regimes pose significant implications for runoff and erosion. In this paper we synthesize what is known about fire impacts on rangeland hydrology and erosion, and how that knowledge advances understanding of hydrologic risks associated with landscape scale plant community transitions and altered fire regimes. The increased role of wildland fire on western rangeland exposes landscapes to amplified runoff and erosion over short- and long-term windows of time and increases the risk of damage to soil and water resources, property, and human lives during extreme events. Amplified runoff and erosion postfire are a function of storm characteristics and fire-induced changes in site conditions (i.e., ground cover, soil water repellency, aggregate stability, and surface roughness) that define site susceptibility. We suggest that overall postfire hydrologic vulnerability be considered in a probabilistic framework that predicts hydrologic response for a range of potential storms and site susceptibilities and that identifies the hydrologic response magnitudes at which damage to values-at-risk are likely to occur. We identify key knowledge gaps that limit advancement of predictive technologies to address the increased role of wildland fire across rangeland landscapes. Our review of literature suggests quantifying interactions of varying rainfall intensity and key measures of site susceptibility, temporal variability in strength/influence of soil water repellency, and spatial scaling of postfire runoff and erosion remain paramount areas for future research to address hydrologic effects associated with the increased role of wildland fire on western rangelands.

Infiltration and Runoff Simulation on a Plane

A program which computes infiltration and the overland flow hydrograph on a single, homogeneous plane is described. Infiltration is computed by the Green and Ampt equation and the hydrograph is computed by a semi-analytical method of characteristics solution of the kinematic wave model for overland flow. Default parameter estimation values are supplied by the program for both the infiltration and hydrograph models. Use of the model as a tool for parameter selection is illustrated with rangeland rainfall simulator data.

Rainfall Intensity-Dependent Infiltration Rates on Rangeland Rainfall Simulator Plots

Most implementations of infiltration equations with rainfall-runoff models use a hydraulic conductivity parameter that is constant for a given rainfall event. However, plot data from rainfall simulator experiments and natural rainfall events have shown that infiltration rates can increase with increasing rainfall rate instead of decreasing with time or infiltrated depth, as predicted by infiltration models. This has been hypothesized to be a function off the spatial variability of the infiltration capacity across the area. In this article, an exponential model relating steady-state infiltration rate with rainfall intensity and the average areal infiltration rate when the area under consideration is contributing to runoff is evaluated using data from variable-intensity rainfall simulator experiments. The experiments were conducted on five rangeland vegetation-soil associations at the Walnut Gulch Experimental Watershed in southeastern Arizona. The results from 19 rainfall simulation runs show that the increase in infiltration rate with increasing rainfall intensity can be significant and that the exponential model represents the relationship between steady-state infiltration and rainfall intensity. The exponential model coupled with a kinematic wave model also represents the hydrographs better than the Green-Ampt Mein-Larsen infiltration model coupled with the same routing model. The time to the start of runoff is influenced more by rainfall intensity than by initial soil moisture conditions, particularly when the initial rainfall intensity was high. The rapid time to steady-state runoff at the beginning of the simulation run of the observed runoff hydrographs suggests that the infiltration rates become constant more quickly than infiltration theory would suggest.

USING MEASURED DATA AND EXPERT OPINION IN A MULTIPLE OBJECTIVE DECISION SUPPORT SYSTEM FOR SEMIARID RANGELANDS

A Decision Support System (DSS) can be used to structure information in a way that leads to improved decision making for natural resources. The decisions will only be as good as the information on which they are based. As the applications of a DSS are outpacing the available databases and simulation models, there is an increasing reliance on expert opinion for information on resource management systems. As a result, the effect of information source on the outcome from the DSS is an important issue. This article compares the outcomes from a prototype DSS (P-DSS) developed by the USDA-ARS Southwest Watershed Research Center in Tucson, Arizona, when measured data and expert opinion are used to quantify eight decision criteria in the evaluation of four management systems (yearlong and rotation grazing, each with mesquite trees (Prosopis velutina Woot.) retained or removed) for semiarid rangelands. The decision criteria are sediment yield, channel erosion, runoff rate and quantity, rangeland condition, aboveground net production, and wildlife habitat for quail and javelina, although the analysis is not restricted to these criteria. When measured data are used to quantify the decision criteria, rotation grazing with mesquite removed is the preferred management system, whereas yearlong grazing is the preferred system when expert opinion is used. The experts also directly ranked the four management systems. The difference between the experts’ ranking and the P-DSS results based on expert inputs is a concern for future use of decision support system technology, particularly when information sources are blended.

RISK ASSESSMENT OF EROSION FROM CONCENTRATED FLOW ON RANGELANDS USING OVERLAND FLOW DISTRIBUTION AND SHEAR STRESS PARTITIONING

Abstract. Erosion rates of overland flow on rangelands tend to be relatively low, but under certain conditions where flow is concentrated, soil loss can be significant. Therefore, a rangeland site can be highly vulnerable to soil erosion where overland flow is likely to concentrate and exert high shear stress on soil grains. This concept is commonly applied in cropland and wildland soil erosion modeling using predictions of flow effective shear stress (shear stress applied on soil grains). However, historical approaches to partition shear stress in erosion models are computationally complex and require extensive parameterization. Furthermore, most models are not capable of predicting the conditions in which concentrated flow occurs on rangelands. In this study, we investigated the rangelands conditions at which overland flow is more likely to become concentrated and developed equations for partitioning the shear stress of concentrated flow on rangelands. A logistic equation was developed to estimate the probability of overland flow to become concentrated. Total shear stress of rangeland overland flow was partitioned into components exerted on soil, vegetation, and rock cover using field experimental data. In addition, we investigated the vegetation cover limit at which the effective shear stress component is substantially reduced, limiting the erosion rate. The results from the partitioning equations show that shear stress exerted on soil grains was relatively small in sheet flow. Shear stress exerted on soil grains in concentrated flow was significantly higher when bare soil exceeded 60% of the total surface area but decreased significantly when the bare soil area was less than 25% or when the plant base cover exceeded 20%. These percentages could be used as relative measures of hydrologic recovery for disturbed rangelands or as triggers that indicate a site is crossing a threshold beyond which soil erosion might accelerate due to the high effective shear stress.

A Comprehensive Sensitivity Analysis Framework for Model Evaluation and Improvement Using a Case Study of the Rangeland Hydrology and Erosion Model

The complexity of numerical models and the large numbers of input factors result in complex interdependencies of sensitivities to input parameter values, and high risk of having problematic or nonsensical model responses in localized regions of the input parameter space. Sensitivity analysis (SA) is a useful tool for ascertaining model responses to input variables. One popular method is local SA, which calculates the localized model response of output to an input parameter. This article describes a comprehensive SA method to explore the parameter behavior globally by calculating localized sensitivity indices over the entire parameter space. This article further describes how to use this framework to identify model deficiencies and improve model function. The method was applied to the Rangeland Hydrology and Erosion Model (RHEM) using soil erosion response as a case study. The results quantified the localized sensitivity, which varied and was interdependently related to the input parameter values. This article also shows that the localized sensitivity indices, combined with techniques such as correlation analysis and scatter plots, can be used effectively to compare the sensitivity of different inputs, locate sensitive regions in the parameter space, decompose the dependency of the model response on the input parameters, and identify nonlinear and incorrect relationships in the model. The method can be used as an element of the iterative modeling process whereby the model response can be surveyed and problems identified and corrected in order to construct a robust model.

Runoff and erosional responses to a drought‐induced shift in a desert grassland community composition

In contrast, measurements on small runoff plots on the hillslopes of the same watershed showed a significant increase in sediment discharge that continued after E. lehmanniana replaced native grasses. Together, these findings suggest alteration in plant community increased sediment yield but that hydrological responses to this event differ at watershed and plot scales, highlighting the geomorphological controls at the watershed scale that determine sediment transport efficiency and storage. Resolving these scalar issues will help identify critical landform features needed to preserve watershed integrity under changing climate conditions.

Application of a rangeland soil erosion model using National Resources Inventory data in southeastern Arizona

Rangelands comprise a large portion of the western United States. They are important for providing ecosystem services such as sources of clean water and air, wildlife habitat, ecosystem biodiversity, recreation, and aesthetics. The National Resources Inventory (NRI) is a primary data source for ongoing assessment of nonfederal land in the United States, including rangelands, and the data collected during an NRI assessment is typical of rangeland monitoring conducted by managers. This study outlines a methodology for using that type of monitoring data to run a rangeland hydrology and erosion model in order to estimate the relative soil erosion rates across ecosystems located in the American Southwest. The model was run on 134 NRI rangeland field locations with data collected between 2003 and 2006 in Major Land Resource Area 41, the Southeastern Arizona Basin and Range, which is a diverse ecological area of 40,765 km2 (15, 739 mi2) in the transition zone between the Sonoran and Chihuahuan deserts. Results of the study showed that the data collected was adequate to run the model and effectively assess the influence of foliar cover, ground cover, plant life forms, soils, and topography on current soil erosion rates. Results suggested that the model could be further improved with additional measured experimental data on infiltration, runoff, and soil erosion within key ecological sites in order to better quantify model parameters to reflect ecosystem changes and risk of crossing interdependent biotic and abiotic thresholds.