干旱区生态环境知识资源中心

Glacial meltwater streams in the McMurdo Dry Valleys, Antarctica exhibit daily cycles in temperature with maxima frequently reaching 10-15 degrees C, often 10 degrees C above air temperatures. Hydrologic and biogeochemical processes occurring in these streams and their hyporheic zones strongly influence the flux of water, solutes, and sediment to the ice-covered lakes on the valley bottoms. The purpose of this study was to identify the dominant processes controlling water temperature in these polar desert streams and to investigate in particular the role of hyporheic exchange. In order to do this, we analyzed stream temperature patterns on basin-wide, longitudinal, and reach scales. In the basin-wide study, we examined stream temperature monitoring data for seven streams in the Lake Fryxell Basin. For the longitudinal study, we measured temperatures at seven sites along a 5-km length of Von Guerard Stream.

Maximum temperatures in the Fryxell Basin streams ranged from 8 to 15 degrees C. Daily temperature changes in the streams averaged 6-9 degrees C. Stream temperature patterns showed strong diel cycles peaking at roughly the same time throughout the lake basin as well as along the longitudinal gradient of Von Guerard Stream. Further, temperature patterns closely matched the associated net shortwave radiation patterns. Von Guerard Stream experienced its greatest amount of warming, 3-6 degrees C, in a playa region and cooled below snowfields. Temperatures in several streams around Lake Fryxell converged on similar values for a given day as did temperatures in downstream reaches of Von Guerard Stream not influenced by snowfields suggesting that at a certain point instream warming and cooling processes balance one another. The reach-scale investigation involved conducting two dual-injection conservative tracer experiments at mid-day in a 143-m reach of Von Guerard Stream instrumented with temperature and specific conductance probes. In one experiment, snow was added to the stream to suppress the temperature maximum. Chloride data from monitoring wells installed in the streambed showed that streamwater was infiltrating into the streambed and was reaching the frozen boundary. Temperatures in the hyporheic zone were always cooler than temperatures in the stream. OTIS-P modeling of tracer experiment data indicated that significant hyporheic exchange occurred in both experiments. Reach and cross-sectional heat budgets were established with data obtained from the tracer experiments and from a nearby meteorological station. The budgets showed that net radiation accounted for 99% of the warming taking place in the experimental reach. This result, together with the streams’ shallow depths and hence rapid response to meteorological conditions, explains the close linkage between stream temperature and net shortwave radiation patterns. Cross-sectional heat budgets also indicated that evaporation, convection, conduction, and hyporheic exchange contributed to 30%, 25-31%, 19-37%, and 6-21%, respectively, of the non-radiative heat losses in the experimental reach. Thus these processes all worked in conjunction to limit stream temperatures in the Dry Valleys’ highly exposed environment. This contrasts with other streams in which convection and conduction may play a warming role. The cooling impact of hyporheic exchange was greater in a losing reach than in a gaining one, and increased hyporheic exchange may have lessened the contribution of conduction to the thermal budgets by decreasing streambed temperature gradients. The cooling contributions of evaporation and convection increased with stream temperature and may thus play a role in constraining stream temperature maxima. Finally, our data indicate that streams act as vectors of heat in the Lake Fryxell basin, not only warming the hyporheic zone and eroding the frozen boundary underlying the streams but also accumulating and carrying heat downstream, sending a thermal wave into the lake once a day. (C) 2005 Elsevier Ltd. All rights reserved.