EFFECTS OF UPSTREAM DAMS VERSUS GROUNDWATER PUMPING ON STREAM TEMPERATURE UNDER VARYING CLIMATE CONDITIONS

The relative impact of the presence/absence of a large upstream dam versus in-reach groundwater pumping on stream temperatures were analyzed for humid, semarid, and arid regions with long dry seasons, which represent typical regions where large dams are present, such as the western US or eastern Australia. Stream temperatures were simulated using the CE-QUAL-W2 water quality model over a 110-kilometer model grid, with the presence/absence of a dam at the top of the reach and pumping in the lower 60-kilometers of the reach. Measured hourly meteorological data from three representative locations were used as model input to simulate the impact of varying climate condtions on streamflow and stream temperature.. For each climate condition four hypothetical scenarios were modeled: (1) natural, (2) large upstream boundary dam, (3) dam with in-reach pumping, and (4) dam removed with continuation of pumping, resulting in 12 cases. The effect of dam removal with either the presence or absence of pumping resulted in significant changes in stream temperature throughout the year for all three climate conditions. From March to August 2001, the presence of a dam caused monthly-mean stream temperatures to decrease on average by approximately 3.0, 2.5, and 2.0 ^oC for the humid, semiarid, and arid conditions, respectively; however, stream temperatures generally increased from September to February. Cessation of pumping resulted in smaller changes in stream temperature compared to the dam, due to a smaller pumping-induced change in streamflow relative to the dam. Pumping induced stream temperatures to warm in summer and cool in winter by generally less than 0.5 ^oC. Overall presence/absence of the dam led to greater changes in stream temperature than the presence/absence of pumping, with magnitudes sensitive to climate conditions.