Soil Infiltration and Evaporation Determination Using Heat-Pulse Measurements and Energy Balance Modeling

The soil surface forms a critical interface between the vadose zone and the plant canopy or atmosphere where water fluxes, whether liquid or vapor, are exchanged. The surface properties, (e.g., water content, texture) determine to a large degree the rates of both infiltration and evaporation. Recent developments using inverse modeling of heat pulse measurements have shown promise for soil water flux determination (e.g., snowmelt, rainfall). Heat-pulse methods for measuring soil thermal properties are based on applying a heat pulse via a line source and measuring the temperature response about 6 mm from the source. A penta-needle heat pulse probe (PHPP) employing a central heater needle surrounded by an orthogonal arrangement of four thermistor needles was constructed. Water flux values as small as 1 cm d-1 and as large as 2500 cm d-1 have been resolved using Tri-needle heat-pulse probes, which equal infiltration rates consistent with 50 - 75 percent of NRCS and UNSODA database soils. Applying energy balance theory in conjunction with heat pulse measurements, it is possible to estimate the latent heat component of the energy flux (i.e., evaporation) as the residual from the soil heat flux and storage estimates. Soil heat flux values between the heater and temperature sensors are determined from the estimate of the thermal conductivity, a fitted parameter from the PHPP and the measured temperature gradient. The heat storage is then calculated from the derived soil heat capacity and differences in spatial and temporal measurements of temperature. Detailed measurements in the near-surface at depth increments of 6 mm were shown to be effective in estimating surface evaporation from different soil textures. An array of PHPP sensors located in the near surface soil could provide measures of both infiltration and soil evaporation from periodic heat pulse measurements. Establishment of spatially distributed point-scale measurements (i.e., weather stations) could provide a valuable new method for estimation of soil evaporation needed for local and regional-scale forecasting.