The role of available energy in estimating potential evaporation over different soil textures

The potential evaporation rate (PE) depends on the available energy at the land-atmosphere interface and soil properties. The application of the full-form Penman-Monteith equation (PM) is often simplified. For example, the ground heat flux G is often assumed to be zero for calculating daily evaporation as the value of G is relatively small compared to the net radiation R_n. This and other simplifications consider that the PE value is mainly determined by meteorological variables and independent from soil properties. As the influence of soil textures on PE have so far received little attention, we analyzed data from a lysimeter experiment in the Guanzhong Basin, China. The potential evaporation rate was measured over saturated fine sand (PE_fine), coarse sand (PE_coarse) and gravel sand (PE_gravel) at a high temporal resolution. Meteorological variables, ground heat flux and soil temperature at different depths were observed from July 2018 to August 2019. The measured PE showed clear differences between the three saturated bare soils especially during spring and summer. Our previous research showed that these PE differences are controlled by differences in the available energy, related to differences in the total ground heat flux G for the three materials and different albedos. Both a detailed energy balance and the full-form PM equation can explain the PE differences between the different soil textures on the basis of hourly data. On the other hand, if the full-form PM equation is applied on daily data only minor differences in PE between the three textures are calculated. Our research suggests that the available energy should be calculated as precisely as possible, considering the soil porosity, thermal conductivity as well as albedo for the different soil textures, in order to estimate evaporation.