Quantifying Subsurface Flow and Solute Transport in a Snowmelt‐Recharged Hillslope With Multiyear Water Balance

• Published in: Water resources research 2022-12, Vol.58 (12), p.n/a
• Main Authors: Tokunaga, Tetsu K., Tran, Ahn Phuong, Wan, Jiamin, Dong, Wenming, Newman, Alexander W., Beutler, Curtis A., Brown, Wendy, Henderson, Amanda N., Williams, Kenneth H.
• Format: Article
• Language: English
• Subjects: Drought; Environmental monitoring; ENVIRONMENTAL SCIENCES; evapotranspiration; Exports; Groundwater; Groundwater flow; Groundwater table; hillslope hydrology; Hydraulic conductivity; Moisture content; Nitrogen; Pore water; Predictions; Rainfall; Recharge; Recharging; Rivers; snow drought; Snowmelt; Soil water; Solute transport; Solutes; Subsurface flow; Transmissivity; transmissivity feedback; Transport; Water balance; Water chemistry; Water monitoring; Water quality; Water supply; Water table; Watersheds; Weathering; Weathering zone
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2022WR032902

Quantifying flow and transport from hillslopes is vital for understanding water quantity and quality in rivers, but remains obscure because of limited subsurface measurements. Using measured hydraulic conductivity K profiles and water balance over a single year to calibrate a transmissivity feedback model for a hillslope in the East River watershed (Colorado) proved unsatisfactory for predicting flow over the subsequent years. Well‐constrained field‐scale K were obtained by optimizing subsurface flux predictions over years having large differences in recharge, and by including estimates of interannual transfer of excess snowmelt recharge. Water and solute exports during high snowmelt recharge occur predominantly via shallow groundwater flow through weathered rock and soil because of their enlarged transmissivities under saturated conditions. Conversely, these shallow pathways are less active in snow drought years when the water table remains deeper within the weathering zone. Hillslope soil water monitoring showed that rainfall does not infiltrate deeply during summer and fall months, and revealed water losses consistent with model ET predictions. By combining water table‐dependent fluxes with pore water chemistry in different zones, time‐dependent rates of solute exports become predictable. As an example, calibrated K were combined with dissolved nitrogen concentrations in pore waters to show the snowmelt‐dependence of reactive nitrogen exported from the hillslope, further supporting the recent finding that the weathering zone is the dominant source of reactive nitrogen at this site. Subsurface export predictions can now be obtained for wide ranges of recharge based on measurements of water table elevation and profiles of pore water chemistry. Key Points Large contrasts in annual precipitation are essential for constraining hydraulic conductivity profiles estimated by transmissivity feedback Flow and transport during snowmelt occur predominantly within shallow depths because water table rise amplifies transmissivities A framework is presented for estimating subsurface flow and transport on hillslopes based on sparsely distributed measurements