Energy and vapour transfer in evaporation processes over saturated bare soil and water surface‐lysimeter studies

Abstract

AbstractPotential evaporation (PE) is a significant input in many hydrological models for the estimation of actual evaporation. Evaporation from water (PEw) is generally considered equivalent to evaporation from saturated bare soils (PEs). The influences of the underlying surface on PE as well as the energy and vapour transfer in potential evaporation processes over different surfaces are rarely discussed. In this research, lysimeter experiments were set up to measure the diurnal cycles of evaporation from two saturated sandy soils and water at a high temporal resolution in the Guanzhong Basin, China. Evaporation from Class A Pan, meteorological variables and temperatures were also measured during the experiment. Observation results show that PEs is ~12% higher than PEw on a yearly scale. There were also some clear differences in diurnal and seasonal PE dynamics between saturated bare soils and water. In summer, PEs is higher than PEw at day but smaller at night, with the peak value of PEw lagging ~4 h behind PEs. These observed PE dynamics and energy transfer processes can be quantitative explained on the basis of a full analysis of the energy balance equation. A comprehensive description of the flux transfer processes showed that these differences in PE are governed by differences in available energy (including albedo and thermal properties) between soils and water. Moreover, the observed differences in PE and vapour transfer processes were reproduced and described by improving the vapour diffusion equation, with considering the influence of different surfaces and boundary layer thicknesses. It is found that dynamics in PE were mainly characterised by surface temperature, which further determined the vapour gradients between the evaporation surface and air flow. The results suggested that differences between PEs and PEw cannot be neglected in hydrological applications. This study can act as both an experimental and theoretical reference for estimating potential evaporation rates.