EFFECTS OF LAND-COVER, WATER REDISTRIBUTION, AND TEMPERATURE ON ECOSYSTEM PROCESSES IN THE SOUTH PLATTE BASIN

Over one-third of the land area in the South Platte Basin of Colorado, Nebraska, and Wyoming, has been converted to croplands. Irrigated cro pland now comprises 8% of the basin, while dry croplands make up 31%. We used the RHESSys model to compare the changes in plant productivity and vegetation-related hydrological processes that occurred as a resu lt of either land cover alteration or directional temperature changes (-2 degrees C, +4 degrees C). Land cover change exerted more control o ver annual plant productivity and water fluxes for converted grassland s, while the effect of temperature changes on productivity and water f luxes was stronger in the mountain vegetation. Throughout the basin, l and cover change increased the annual loss of water to the atmosphere by 114 mm via evaporation and transpiration, an increase of 37%. Both irrigated and nonirrigated grains became active earlier in the year th an shortgrass steppe, leading to a seasonal shift in water losses to t he atmosphere. Basin-wide photosynthesis increased by 80% due to grain production. In contrast, a 4 degrees C warming scenario caused annual transpiration to increase by only 3% and annual evaporation to increa se by 28%, for a total increase of 71 mm. Warming decreased basin-wide photosynthesis by 16%. There is a large elevational range from east t o west in the South Platte Basin, which encompasses the western edge o f the Great Plains and the eastern front of the Rocky Mountains. This elevational gain is accompanied by great changes in topographic comple xity, vegetation type, and climate. Shortgrass steppe and crops found at elevations between 850 and 1800 m give way to coniferous forests an d tundra between 1800 and 4000 m. Climate is increasingly dominated by winter snow precipitation with increasing elevation, and the timing o f snowmelt influences tundra and forest ecosystem productivity, soil m oisture, and downstream discharge. Mean annual precipitation of <500 m m on the plains below 1800 m is far less than potential evapotranspira tion of 1000-1500 mm and is insufficient for optimum plant productivit y. The changes in water flux and photosynthesis from conversion of ste ppe to cropland are the result of redistribution of snowmelt water fro m the mountains and groundwater pumping through irrigation projects.