COMPREHENSIVE EVALUATION OF THE IMPROVED SPUR MODEL (SPUR-91)

SPUR (Simulation of Production and Utilization of Rangelands) is a phy sically-based model designed to simulate the complexity of rangeland e cosystems (Wight and Skiles, 1987). Results of extensive validation te sting in Texas indicated a need to modify the model to improve submode l integration and procedural guidelines. A description of the modified model (SPUR-91) is provided. A sensitivity analysis was conducted, ex amining the influence of changes in model output corresponding to pert urbations made to individual input parameters. The model was then exte nsively validated using a Texas data set composed of simultaneous meas urements of hydrology, plant, livestock, and meteorological parameters . Results of validation testing of SPUR-91 confirmed that the model: ( 1) predicted initial soil water content within 3% of observed; (2) pre dicted evapotranspiration, even under very low cover conditions, withi n 1% of observed; (3) provided a good correlation of temporal fluctuat ion (R = 0.72) of observed herbaceous production, the observed 4-year mean live standing crop for the major individual species was predicted within 1-13%; (4) could produce a determinant-type growth curve to ap proximate the long-term response of shrubs and trees; and (5) responde d to management (grazing, vegetation manipulation) in a manner consist ent with observations. A discussion of why modifications to SPUR impro ved the performance of SPUR-91 is presented.