Mass balance from the vertical water distribution in the unsaturated soil by TDR measurements and analytical solutions of the linearized Richards equation.

Long series of soil moisture data have been collected during field experiments using the Time Domain Reflectometry (TDR) technique. The time evolution of the soil moisture profile highlights features of the different soil physical characteristics. Among the soils considered, a sandy soil shows the presence of an inflection point in the soil moisture profile whereas this has not been observed in a silty loam of a sub-alpine valley. The water budget in the superficial soil layer is computed by means of a mass balance based procedure, using the soil water content measures. The procedure allows estimating the evaporation during drying periods and the infiltration after precipitation events. The method was applied to soil moisture data collected during field experiments realized both in a physical model at the Lido beach of Venice and in natural sites in mountain and suburban environment. Analytical solutions of the linearized Richards equation are derived to model different situations of interest. Solutions representing the experimental data both during infiltration and evaporation periods were obtained for arbitrary initial water content conditions and boundary conditions on flux rate. This permits to obtain the boundary conditions from standard meteorological data: precipitation data (incoming flux) and Bowen ratio data (outgoing flux, obtained by the energy balance using air temperature and relative humidity at two heights). The advantage is that meteorological data are very common, while soil volumetric water content measurements are usually not available exactly at the soil-atmosphere interface.