Ecohydrological Implications of Contrasting Slope and Aspect in Complex Terrain

Abstract:

Understanding how complex terrain affects ecohydrological processes is increasingly important as we attempt to understand how water and carbon fluxes are integrated across relatively large domains. Spatial variations of incoming solar radiation are well understood and quantified, but the understanding their impacts on ecohydrologic processes is primarily qualitative. We provide detailed, extensive data quantifying the effects of contrasting slope/aspect on the soil physical environment and document the implications of those differences on ecohydrological processes. The study site, Johnston Draw, is located in the Reynolds Creek Experimental Watershed and CZO in southwest Idaho, USA (43° latitude). Johnston Draw flows over granitic bedrock nearly due east, resulting in steep (25 to 40°) side slopes oriented north-south. At the study elevation (1600 m) approximately 50% of the annual precipitation is snow. We measured meteorological variables, snow depth, soil water (S_W) and temperature (S_T) at three paired locations for two years. Each soil pair consisted of depth profiles from 5 cm to bedrock measured hourly which were supplemented with periodic extensive measurements. Hourly photographs were taken at two of the pairs for one year. Streamflow is monitored at the Johnston Draw outlet and precipitation was measured at stations at the topographic bottom and top of the watershed. Geophysical data were collected in a transect across both slopes. The S_T was warmer all year on the south-facing slope, with a mean annual difference of 5°C. This S_T difference is effectively equivalent to a 1000 m elevation difference in Reynolds Creek. Despite clear differences in evaporative demand and the timing of spring “green up”, the timing of summer S_W decline is similar on both slopes. Deeper soil on north-facing slopes resulted in more plant available water and a longer growing season, which is reflected in the vegetation. Geophysical data indicate much deeper weathering on the north-facing slope. As a result of these varying conditions, there are effectively two different: hydrologic regimes, soil types, plant communities and weathering regimes within the single small watershed. These effects are predictably related to solar radiation, and so can potentially be incorporated in larger-scale models of water and carbon flux.