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Experimental Investigations Into Processes Controlling Stream and Hyporheic Temperatures, Fryxell Basin, Antarctica

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Experimental Investigations Into Processes Controlling Stream and Hyporheic Temperatures, Fryxell Basin, Antarctica Advances in Water Resources 29 (2006) 130–153 www.elsevier.com/locate/advwatres Experimental investigations into processes controlling stream and hyporheic temperatures, Fryxell Basin, Antarctica Karen Cozzetto a, Diane McKnight a,*, Thomas Nylen b, Andrew Fountain b a Institute for Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, USA b Department of Geology, Portland State University, Portland, OR 97207, USA Received 11 April 2005; accepted 13 April 2005 Available online 5 July 2005 Abstract Glacial meltwater streams in the McMurdo Dry Valleys, Antarctica exhibit daily cycles in temperature with maxima frequently reaching 10–15 °C, often 10 °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 °C. Daily temperature changes in the streams averaged 6–9 °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 °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 streamsÕ 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 ValleysÕ 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 temper- ature and may thus play a role in constraining stream temperature maxima. Finally, our data indicate that streams act as vectors of * Corresponding author. Fax: +1 303 492 6388. E-mail address: [email protected] (D. McKnight). 0309-1708/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.advwatres.2005.04.012 K. Cozzetto et al. / Advances in Water Resources 29 (2006) 130–153 131 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. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Stream temperature; Hyporheic zone; Heat budgets; Antarctica; Tracer experiments; Polar desert 1. Introduction The glacial meltwater streams in the McMurdo Dry Valleys of Antarctica provide a unique hydrologic set- Temperature regime is an important characteristic of ting for studying hyporheic exchange processes because stream ecosystems, influencing the survival of fish and many complicating factors, such as terrestrial vegeta- other aquatic organisms, as well as the solubility of oxy- tion, groundwater inflows, and runoff from precipita- gen, and rates of nutrient cycling [2,8]. For these rea- tion, are absent in the cold desert environment [45]. sons, energy gains and losses that affect stream Dry valley streams flow through well-established stream temperature have been investigated for many years channels [10] for 6–12 weeks during the summer. In [6,47,48,53]. Energy balances or heat budgets in which these streams, hyporheic zone width is generally two the various fluxes are quantified can be used to assess to five times larger than that of the main channel and the importance of individual heat budget terms, and the depth is constrained by the underlying permafrost, such budgets have been successfully used to predict expanding as the active layer thaws during the summer stream temperatures [6,53]. Energy fluxes that take place [11]. During the continuous sunlight of summer, these at the stream surface include radiation, convection, and hyporheic zones are ‘‘hotspots’’ of biogeochemical pro- evaporation or condensation phase changes. Those that cesses, providing nutrients for the extensive cyanobacte- take place at the streambed include conduction and fric- rial mats in the streams and changing the concentrations tion [55,56]. Incoming streamflow, precipitation, tribu- of major ions as the water flows downstream tary inflows, effluent discharges, groundwater inflow [22,28,36,39,40]. Although the period of streamflow is and outflow, evaporative losses, and outgoing stream- limited, hyporheic zone processes are important at the flow are also energy sources and sinks [55]. Recent heat watershed scale because the streams connect the glaciers budget investigations have examined the influence of to closed-basin, permanently ice-covered lakes in the channel morphology, riparian vegetation, substratum valley floors [4,45]. type and other stream characteristics on the heat bud- The purpose of this study was to identify the domi- gets of streams [31,55]. The components of stream heat nant processes controlling stream temperatures and hyp- budgets vary on both daily and seasonal scales [55–57]. orheic zone thermal gradients in a polar desert glacial In a recent study of temperate streams in a forested meltwater stream and to investigate in particular the watershed, Story et al. [50] used an energy balance ap- role of hyporheic exchange. In this cold environment, proach to quantify the cooling effect of hyporheic ex- stream temperature is a critical characteristic of stream change on stream temperature, an energy flux not habitat, controlling rates of microbial and biogeochem- previously included in heat budgets. The hyporheic zone ical processes as well as the thawing of the hyporheic consists of the area below and adjacent to the stream zone [3,10]. Continuous monitoring at stream gauges that exchanges water with the stream. This zone can at stream outlets to the lakes has shown that stream have a strong impact on the biological, chemical, and temperatures range from zero to 15 °C during the sum- physical processes taking place in streams [7,20,49]. mer and are often as much as 10 °C higher than air tem- Poole and Berman [44] suggested that hyporheic ex- peratures. As is the case for other low discharge streams change may play an important role as a stream temper- in arid, alpine, and clear-cut environments, tempera- ature buffer, and Johnson [31] proposed temperature tures can change by as much as 10 °C in a single day buffering mechanisms. In one scenario, increased [12,13]. The magnitude of daily temperature maxima hydraulic retention coupled with a large volume of sub- may influence the rates of instream processes and the surface storage was hypothesized to allow warmer day- overall response of the McMurdo Dry Valleys to climate time water to mix with cooler nighttime water and thus variability. moderate downstream temperatures [31]. Hyporheic ex- change would also increase contact between stream- water and substrates and could enhance conduction 2. Study site from warmer to cooler surfaces [31]. Johnson [31] pointed out that the magnitude of hyporheic influence The McMurdo Dry Valleys region is the largest ice- is related to the proportion of streamwater that flows free polar desert oasis on the coast of Antarctica and through the hyporheic zone. is located in the Transantarctic Mountains between 132 K. Cozzetto et al. / Advances in Water Resources 29 (2006) 130–153 latitudes, 77–78°S and longitude 160–164°E. This region Table 1 is dominated by barren soils, exposed bedrock, well- Characteristics of streams in the Lake Fryxell Basin established stream channels, permanently ice-covered Length (km) Aspect Time of peak flow lakes, and glaciers. The region receives the equivalent Canada Stream 1.5 South 14:30–16:00 of less than 10 cm of water equivalent (WEQ) each year Huey Creek 2.1 South * on average in the form of snow [5].
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