Energy and vapour transfer in evaporation processes over saturated soil textures and water surface

Evaporation from water (PE_w) is generally considered equivalent to evaporation from saturated bare soils (PE_s) as a starting point to estimate actual evaporation E_a. This simplification considers that the PE value is mainly determined by meteorological variables. The influences of the surface type on PE, as well as the energy and vapour transfer in evaporation processes over saturated soil textures and water surface have so far received little attention. In this research, evaporation over two saturated sandy soils including coarse sand (PE_coarse), fine sand (PE_fine) and water were assessed for lysimeters installed in the Guanzhong Basin, China. Evaporation from Class A Pan (PE_pan), meteorological variables and temperatures in soil and water were also captured at a high temporal resolution (5 min.) for more than 14 consecutive months. Observed PE rates demonstrated evident differences in both absolute values and diurnal dynamics between saturated soils and water. PE_s is ~12% higher than PE_w on a yearly scale. Annual PE_fine exceeded PE_coarse by 7.3%, with the differences more obvious during daytime in spring and summer. The cumulative evaporation rates over water column and Class A Pan showed minor differences. PE_s is higher than PE_w at day but smaller at night, with the peak value of PE_w lagging ~4 hours behind PE_s. Compared with PE_w-curve, the PE_pan-curve resembles more the PE_s-curves over a sub-daily scale.

Our research revealed that these observed PE dynamics and energy transfer processes can be quantitatively explained with detailed calculations of the surface energy balance. It is found that differences in PE are governed by differences in available energy (related to different albedos, different thermal properties and different surface temperature T) 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. PE dynamics were mainly characterized by the surface temperature T, which further determined the vapour gradients between the evaporation surfaces and airflow. Previous research considered surface temperature T to be an independent external forcing that determines ‘wet surface’ evaporation. Our research suggests that T is a significant internal forcing for both energy and vapour transfer during the evaporation process since it influences the redistribution of energy fluxes at the surface (the ground heat flux G and the variation in water heat storage N), the outgoing longwave radiation (R_lu), as well as the vapour gradients above the surface (Δe).