RUNOFF RESPONSE IN A SEMI-ARID HEADWATER DRIVEN BY CATCHMENT-SCALE WATER MOVEMENT

While the rate, duration and type of precipitation are key controls of runoff generation, the critical zone is an important mediator of surface water hydrology. We explore interannual variability in runoff production in the Gordon Gulch catchment, a semi-arid headwater catchment at ~2500 m in the Colorado Front Range with a spatially variable, ephemeral snowpack. The forested (upper montane) catchment is outside the limits of Pleistocene glaciation and is underlain by biotite gneiss bedrock. Recent annual peak discharges have occurred during i) spring snowmelt, ii) summer convective storms, and iii) an exceptional synoptic-scale storm. Soil moisture, water table elevation, and runoff data during different rain and snowmelt conditions show that the spatial and temporal scales of water inputs exert important control on runoff production.

The east-west oriented Gordon Gulch receives 500-600 mm of annual precipitation, 28-65% as snow; annual runoff ratios range from 0.08-0.23. Snowpack is intermittent on south-facing slopes, and longer-lived but thin (< 0.5 m) on north-facing slopes. Although annual peak discharge is typically associated with the end of snowmelt on the north-facing slope, our observations reveal that spring precipitation must augment snowmelt to produce a significant discharge peak. Synchronous inputs on both slope aspects increases soil moisture across the catchment, raises the water table into more permeable regolith, and stimulates rapid lateral transmission of water to produce high streamflow. Summer and fall storms may also result in significant water table rise and large runoff responses when storms are of sufficient duration. This finding is in accord with modeling of water delivery to the weathering front in Gordon Gulch (Langston et al., 2015, ESPL). The roles of spatially and temporally variable evaporation and transpiration demands, antecedent moisture, and conductivity structure of the critical zone on water flow and runoff generation remain to be determined. Processes that affect both north and south-facing slopes simultaneously have first order control on catchment scale hydrologic response.