HILLSLOPE AND CLIMATIC CONTROLS ON HYDROLOGIC FLUXES

The position and shape of the hillslope water table has long been considered to be a useful diagnostic both for field analysis of the spatial structure of hydrologic behavior and for initializing hydrologic forecasting models. Water table positions near the surface often indicate areas which contribute to surface runoff generation, yield evaporation at the climate-demanded rate, and produce a net discharge of saturated zone groundwater. Deeper water tables typically indicate drier areas where evaporation is suppressed and infiltration enhanced, therefore promoting net groundwater recharge. The long time integral of these fluxes over an area forms the local climatic hydrologic cycle. Continuity (throughout a hillslope) of modeled long-term mean moisture fluxes at the land-atmosphere interface within the unsaturated and saturated zones provides closure to the climate-hillslope system, and it is used to determine the equilibrium water table position and the corresponding spatial structure of mean recharge, discharge, evaporation, and surface runoff production. This method of analysis provides a tool for the systematic study of how characteristics of the atmospheric forcing (e.g., mean precipitation, evaporative demand, and storm intermittency and intensity), soil properties, and geologic features (e.g., topography) interact to yield observed spatial patterns of distinct hydrologic behavior at various scales. Application of the methodology to simplified and idealized hillslope geometries reveals spatial patterns in good qualitative agreement with field observations. Case studies demonstrate that the wide range of behavior observed in nature is reproducible within an observed range of climatic, geologic, and soil parameters. Partial analysis of the full system yields a few dimensional parameter groups, the relative values of which are indicative of transitions between geologic-, soil-, and climate-controlled conditions and the presence or absence of characteristic hydrologic zones such as seepage faces, recharge areas, hinges, midline zones, discharge areas, and partial areas of surface runoff production. Under conditions of limited lateral transmissivity and slope, the midline region (over which the long-term mean net exchange between the saturated and unsaturated zones is zero) can dominate the hillslope and lead to an invariance of area-averaged surface runoff and evaporation with respect to hillslope length scale.