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Hammond, John C ¹ ; Harpold, Adrian A ² ; Kampf, Stephanie K ³

¹ Colorado State University
² University of Nevada Reno
³ Colorado State University

Snowmelt is the dominant source of streamflow generation and groundwater recharge in many high elevation and high latitude locations, yet we still lack a detailed understanding of how snowmelt is partitioned between the soil, deep drainage, and streamflow under a variety of soil, climate, and snow conditions. Here we use Hydrus 1-D simulations with historical inputs from five SNOTEL snow monitoring sites in each of three regions, Cascades, Sierra, and Southern Rockies, to investigate how inter-annual variability on water input rate and duration affects soil saturation and deep drainage. Each input scenario was run with three different soil profiles of varying hydraulic conductivity, soil texture, and bulk density. These scenarios were then run with all inputs converted to rain and for multiple soil profile depths. We also created artificial input scenarios to test how snowmelt intermittence, conversion of snow to rain input, and soil profile depth affect deep drainage. Results indicate that precipitation is the strongest predictor (R2 = 0.83) of deep drainage below the root zone, with weaker relationships observed between deep drainage and snow persistence, peak snow water equivalent, input rate, and the concentration of input in time. The ratio of deep drainage to precipitation shows a stronger positive relationship to melt rate and input concentration suggesting that a greater fraction of input becomes deep drainage at higher input rates. For a given amount of precipitation, rapid, concentrated input may create greater deep drainage below the root zone than slower, intermittent input. Deep drainage requires saturation below the root zone, so saturated hydraulic conductivity serves as a primary control on deep drainage magnitude. Deep drainage response to climate is mostly independent of soil texture because of its reliance on saturated conditions. Mean water year saturations of deep soil layers can predict deep drainage and may be a useful way to compare sites in soils with soil hydraulic porosities. The volume of surface runoff often is often greater than deep drainage at daily and annual timescales, as snowmelt exceeds infiltration capacity in near-surface soil layers. These results suggest that processes affecting the duration of saturation below the root zone could compromise deep recharge, including changes in snowmelt rate and duration as well as the depth and rate of ET losses from the soil profile.