Evaluation of the upgraded SPUR model (SPUR2.4)

SPUR (Simulation of Production and Utilization of Rangelands) is a grassland ecosystem simulation model. SPUR2.4 integrates all previous versions of SPUR. The changes from SPUR91 make the model more applicable to the southern Great Plains. The forage submodel from SPUR2 predicts forage intake and diet selection by grazing herbivores, and the cow-calf submodel from SPUR2 simulates all individuals in a herd for their full life cycle, based on defined genetic traits. The soil organic matter submodel from CENTURY was added to improve soil carbon and nitrogen cycling. SPUR2.4 output is compared with observed values from the Texas Experimental Ranch, Throckmorton, Texas. The changes made in creating SPUR2.4 have considerably improved the utility and accuracy of the SPUR model for north Texas. Soil moisture predictions are improved (SPUR91 is 137% and SPUR2.4 is 106% of observed, R = 0.79-0.86), but run-off is still not adequate (R = 0.65-0.76) and monthly evaporation is lower (SPUR91 is 94% and SPUR2.4 is 95%) than observed. Soil carbon level predictions by the CENTURY submodel are within range of the data, and output is stable over the simulation period. Predictions of individual plant species productivity compared with observed data are improved with SPUR2.4 (R = 0.79-0.98). Between-season plant growth and long-term persistence are simulated well. The addition of the CENTURY soil organic matter submodel increased nitrogen level predictions resulting in more accurate predictions of animal weight gain. Steer weight gains are typical for north Texas. Simulation of cow-calf mass was good (R > 0.98) as was production per hectare and per cow. The addition of the cow-calf model increases the utility of the model. The paper identifies portions of the model that need further validation and field research to improve model utility and credibility for use in natural resource management. (C) 1999 Elsevier Science B.V. All rights reserved.