Time course of dual angle temperatures: implication for hydraulic properties evaluation

Evaporation rate after rainfall depends on both available energy and soil moisture. During the drying of the land surface, one can define three stages: 1) when the available energy is a limiting factor for both soil evaporation and transpiration, the land surface evaporates at a daily constant potential rate ("energy limited" or "atmosphere controlled"); 2) when surface soil moisture, which drains towards the deepest layers and is removed by soil evaporation, drops below a critical level depending on both surface soil moisture and the hydraulic parameters, soil evaporation begins to decrease and is only related to the soil conditions ("soil controlled"); during that stage the root zone soil moisture continues to sustain a potential transpiration; 3) during the last stage, the root zone soil moisture decreases and the vegetation is stressed: both soil and vegetation evaporate at a soil controlled rate, which decreases to reach an equilibrium if no rainfall occurs beforehand. It has been shown in numerous studies that the time of switching from stage 1 to 2 (which depends on potential evaporation rate and soil hydraulic conditions) corresponds to a sharp difference in surface conditions and is therefore observable from space. Some authors have used temporal series of nadir surface temperature or albedo to evaluate this time, and, through it, the hydraulic parameters. Here we use dual angles temperatures to assess this critical time with a better accuracy than the surface temperature alone, and give analytical expressions to relate it to the soil hydraulic conditions.