Two-layer model of near-surface soil drying for time-continuous hydrologic simulations

A two-layer soil water balance model is developed to provide an efficient and robust descriptionof land surface dynamics in response to atmospheric evaporative events. Soil, vegetation, and atmosphere arecoupled dynamically under the assumption that soil moisture profiles approximately preserve similarity duringsimultaneous atmospheric drying and gravity drainage. The exfiltration flux at the land surface in response tothe atmospheric evaporative demand is limited by relating the surface resistance to water vapor transfer in thePenman-Monteith equation to the near-surface soil status. In addition, the control of deeper soils on both exfiltrationand drainage is expressed by performing a time compression approximation water balance over the entiredrying profile and by scaling the obtained exfiltration and drainage fluxes to the near-surface soil layers. Thereliability and robustness of the proposed formulation is evaluated with rates of evaporation calculated frommeasurements of the Bowen ratio and soil moisture data obtained from time domain reflectometry measurementsfor a bare soil field in the Zwalmbeek catchment (Belgium).