Use of remotely sensed soil moisture content as boundary conditions in soil‐atmosphere water transport modeling: 2. Estimating soil water balance

A previously calibrated soil/atmosphere heat and water transport model was used to analyze (1) errors involved in soil water balance estimates under drying periods when soil surface moisture contents derived from microwave measurements were used as soil surface boundary conditions, and (2∥ consequences of time periods separating two consecutive soil surface boundary conditions under evaporation or infiltration conditions on soil water balance calculations. Soil moisture and dry bulk density measurements were performed on a 0.4-ha bare field (27.2% clay, 61.7% fine and coarse loam) with simultaneous measurements of backscattering coefficients (5.3 GHz, HH polarization, 15° incidence angle). Regression lines between backscattering coefficient and volumetric water content were calculated taking into account different soil depths (0-1, 0-2, 0-3, 0-4, 0-5, 0-6, 0-7, and 0-10 cm). Two methods for estimating soil surface moisture contents were proposed. In the first one, moisture contents for an arbitrary soil depth were directly extracted from a single “backscattering coefficient/water content” calibration line. The second method combined several calibration lines relative to several arbitrary soil depths. Results showed that (1) under drying periods (5 days) the methods led generally to moderately biased or unbiased water balances, (2) both errors due to the backscattering coefficient measurements and calibration line parameters had little effect on water balance estimations (≤10%), and (3) under evaporation conditions (5 days) or combined evaporation and infiltration phases (15 days), numerous soil surface boundary conditions versus time should be available to avoid wrong water balance estimates. Under evaporation conditions, one soil surface moisture content per day appeared satisfactory when it corresponded to the mean daily soil surface moisture content. Under combined evaporation and infiltration conditions, results depended strongly on the precise position of water content boundary conditions versus time, connected with rainfall sequences.