INFLUENCE OF DIURNAL-VARIATIONS IN STREAM TEMPERATURE ON STREAMFLOW LOSS AND GROUNDWATER RECHARGE

We demonstrate that for losing reaches with significant diurnal variat ions in stream temperature, the effect of stream temperature on stream bed seepage is a major factor contributing to reduced afternoon stream flows. An explanation is based on the effect of stream temperature on the hydraulic conductivity of the streambed, which can be expected to double in the 0 degrees to 25 degrees C temperature range. Results are presented for field experiments in which stream discharge and tempera ture were continuously measured for several days over losing reaches a t St. Kevin Gulch Colorado, and Tijeras Arroyo, New Mexico. At St. Kev in Gulch in July 1991, the diurnal stream temperature in the 160-m stu dy reach ranged from about 4 degrees to 18 degrees C, discharges range d from 10 to 18 L/s, and streamflow loss in the study reach ranged fro m 2.7 to 3.7 L/s. On the basis of measured stream temperature variatio ns, the predicted change in conductivity was about 38%; the measured c hange in stream loss was about 26%, suggesting that streambed temperat ure varied less than the stream temperature. At Tijeras Arroyo in May 1992, diurnal stream temperature in the 655-m study reach ranged from about 10 degrees to 25 degrees C and discharge ranged from 25 to 55 L/ s. Streamflow loss was converted to infiltration rates by factoring in the changing stream reach surface area and streamflow losses due to e vaporation rates as measured in a hemispherical evaporation chamber. I nfiltration rates ranged from about 0.7 to 2.0 m/d, depending on time and location. Based on measured stream temperature variations, the pre dicted change in conductivity was 29%; the measured change in infiltra tion was also about 27%. This suggests that high infiltration rates ca use rapid convection of heat to the streambed. Evapotranspiration loss es were estimated for the reach and adjacent flood plain within the ar royo. On the basis of these estimates, only about 5% of flow loss was consumed via stream evaporation and stream-side evapotranspiration, in dicating that 95% of the loss within the study reach represented groun dwater recharge.