Use of remotely sensed soil moisture content as boundary conditions in soil‐atmosphere water transport modeling: 1. Field validation of a water flow model

Abstract

A physically based heat and mass flow model is presented and compared with experimental data measured on a bare soil (27.2% clay, 61.7% silt, 11.0% sand) under field conditions. Both liquid and vapor phases were taken into account, and soil temperature and water pressure head were the descriptive variables. The model was directly driven by soil surface temperature and water pressure head (derived from moisture content), which were used as boundary conditions. Coupled equations were solved using a numerical finite element method, from the soil surface to 1‐m depth. The experiment was conducted on a bare soil (0.1 ha) for a 7‐day period. The period was dry for 5 days (calibration phase) and then rainy (validation phase). Soil water balance was determined from gravimetric water content, neutron probe profiles, and tensiometer measurements. The unsaturated hydraulic conductivity/volumetric water content relationship was measured under field conditions, and the apparent thermal conductivity/water content relationship was derived from the thermal profile analysis. Results showed that the proposed model described soil temperature and water content variations versus time quite well: After a calibration phase, differences between the measured and calculated temperatures and volumetric water contents were below 1.5° and 0.03 m3 m−3, respectively. Analysis of the errors involved in both initial and soil surface boundary conditions showed that these errors induced moderate effects on actual evaporation calculations. Although the vapor phase contributed largely to the total water fluxes, differences between coupled heat and water transport or isothermal liquid phase models were very small in regard to the actual evaporation or infiltration estimates. This was explained by the use of soil surface moisture content as boundary conditions which induced increasing soil surface pressure head gradients when the simplified isothermal liquid phase water flow model was used.