RESPONSES OF ARID AND SEMIARID WATERSHEDS TO INCREASING CARBON-DIOXIDE AND CLIMATE-CHANGE AS SHOWN BY SIMULATION STUDIES

The atmospheric concentration of carbon dioxide is expected to double in the next century causing increased temperatures and decreasing prec ipitation in some regions of the U.S. The increase in CO2 will also di rectly affect stomatal conductance of plants. At the first-order water shed scale, changes in evaporative demand, transpiration, and runoff w ill also occur. Previous modeling studies of the effect of increased C O2 on the water budgets of watersheds have been single-factor exercise s where a single parameter representing stomatal conductance was reduc ed and the results noted. After showing validation results of the hydr ology module, we used a comprehensive ecosystem model to examine the c onsequences of changes in precipitation, temperature, and CO2-induced plant-function characteristics on small-basin runoff. As a result of t he complex interactions and of the compensatory mechanisms simulated b y the model, we conclude that for arid and semiarid watersheds of the western United States, there will be little change or an actual decrea se in surface runoff because of increased CO2 and climate change. This is due to the decrease in precipitation imposed on the model simulati ons. Implementing stomatal closure in the model did not increase runof f from the watersheds when temperatures were increased and precipitati on decreased.