| Soil erosion and sediment yield are global problems. Although largely associated with and attributed to cropland agriculture, erosion and sedimentation also degrade and semiarid ecosystems. Currently, simulation models are available to predict hydrologic and erosion/sediment yield processes on semiarid watersheds. While the output of distributed models is a function of the internal watershed processes represented by the model, the performance of these models is generally evaluated at the watershed outlet. An index has been developed which provides an easily computable method of identifying zones of erosion and deposition within alluvial channels. Although the index is not intended as a replacement for much more complex simulation models, it does provide a surrogate for measured data when evaluating the performance of simulation models at internal watershed locations. In sharp contrast to modern process based simulation models, the index depends only on simply computable watershed characteristics and does not depend at all on the particulars of a runoff event or set of runoff events. Runoff and sediment yield were modeled on three semiarid watersheds identified as 63.223 (43.7 ha), 63.216 (84.2 ha), and 63.218 (58.3 ha) on the United States Department of Agriculture-Agricultural Research Service Walnut Gulch Experimental Watershed near Tombstone, Arizona. Following a comparison of available models, the TLOSS model was calibrated, validated, and then used to simulate event runoff for the 1962–1996 period on Watershed 63.223 and from 1966–1996 on Watersheds 63.216 and 63.218. The APOINT model was calibrated, validated, and used to simulate sediment yield for the same time periods. Modeled event sediment yield was used in conjunction with field data and observations to develop and test a simple erosion/deposition index relating the easily obtainable quantities of slope, area, and median particle diameter. Despite the computational simplicity of the index, the linear relationship between computed index values and modeled erosion values is strong. Therefore, a method is now available to identify zones of erosion and deposition within alluvial channels and thereby provide additional information on erosion and deposition processes at internal points on semiarid watersheds. |
