An evaporation test based on Thermal Infra Red remote-sensing to select appropriate soil hydraulic properties

SummaryPedotransfer functions are the most widely used method to estimate common soil hydraulic properties at regional scale. Since they rely on an empirical link between textural and structural soil properties observed in the laboratory on undisturbed soil samples, one must check whether the pedotransfer functions built elsewhere also apply to the location of interest. Alternative methods to laboratory analysis, such as infiltration tests, exist but are difficult to carry out at large scales. Here we propose a method for selecting the appropriate hydraulic properties based on the physical link between the soil water diffusion properties and the plant water stress, which has been named the “evaporation test”. It consists in (i) detecting water stress from remote-sensing data in the Thermal Infra Red spectrum and a simulated unstressed surface temperature, then (ii) estimating the date of the last irrigation/rainfall event, the water content at the end of this irrigation/rainfall event, the unstressed evapotranspiration rate and the average root depth and (iii) reducing the range of possible values of the hydraulic parameters to those that compute a time-to-stress that is consistent with the observed one, i.e. the difference between the observed water stress date and the date of the end of the last irrigation/rainfall event. The performance of this method is then checked for two sites within the frame of the SudMed and SALSA experiments by comparing the resulting properties to those obtained by other methods, namely the Beerkan infiltration test and the most commonly used pedotransfer functions. While not providing a unique set of hydraulic properties, the “evaporation test” is a good mean to refine the range of appropriate hydraulic parameter values at the scale of the Thermal Infra Red data.