Evaluation of a Site Conservation Rating System in Southeastern Arizona

Abstract

The objectives of this study were to identify a Site Conservation Threshold, the point at which accelerated erosion occurs, and to examine the usefulness of the Water Erosion Prediction Project (WEPP) model in determining the Site Conservation Threshold on a clay loam upland rangeland site. Sixteen sample sites were chosen to represent a wide range of species composition, ground cover, biomass and apparent soil stability to determine which of these factors could be used to evaluate the site conservation status of a clay-loam upland ecological site on a southern Arizona semi-arid desert grassland. A Site Stability Rating based on observable vegetation and soil-surface characteristics (i.e. ground surface cover and distribution of plants) was developed. A Site Stability Rating was estimated for 100 quadrats per sample site and averaged for the entire sample site. The Water Erosion Prediction Project model was used as an objective index of soil stability to assess the degree of site protection. The Soil Conservation Service soil loss tolerance value (T) was used with the sediment yield predicted by the WEPP model to establish a threshold value for the Site Stability Rating. The objective measures of standing biomass, basal cover, average distance to the nearest perennial plant, and frequency of quadrats with no rooted perennial plant showed strong relationships to the subjective Site Stability Rating. Site Conservation Thresholds were identified for standing biomass (750 kg/ha), basal cover (8%), average distance to the nearest perennial plant (15 cm), and frequency of quadrats (20 X 20 cm) with no rooted perennial plant (13%).