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.