Potential evaporation (PE) from saturated bare surfaces is the basis for estimating actual evaporation (E_s) in agricultural and related disciplines. Most models estimate PE using meteorological data. Thus, the dependence of soil temperature (T) on PE is often simplified in applications. To address this gap, we conducted an in situ lysimeter experiment in the Guanzhong Basin, China, continuously measuring PE, T, and soil heat flux (G) at high temporal resolution over three fully saturated sandy soils. Results show that annual PE over fine sand was 7.1% and 11.0% higher than that of coarse sand and gravel. The observed PE differences across textures can be quantitatively explained using the surface energy balance equation and a radiatively coupled Penman-Monteith equation, accounting for the dependence of T on net radiation (R_n) and G. In contrast, PE estimates diverged from observations when R_n and G were assumed to be independent of T. We further evaluated the influence of T and other influencing variables on PE. The random forest model identified that near-surface heat storage variations (∆S) contribute most significantly to PE estimation (relative importance = 0.37), followed by surface temperature (0.24) and sensible heat flux (0.23). These findings highlight the critical role of near-surface temperature in PE estimation.

Calculating actual bare soil evaporation (ETa) on the basis of potential bare soil evaporation (PE) is a widely followed approach in many disciplines including hydrogeology, hydrology and agricultural sciences. This approach considers that PE is independent from soil properties, and only ETa is affected by soil properties. In this work, in a unique experiment, seasonal and diurnal cycles of PE over saturated bare soil were assessed for lysimeters installed in the Guanzhong Basin, China. The assessment was made for different soil textures including gravel (PEgravel), coarse sand (PEcoarse) and fine sand (PEfine) and also open water (PEwater). Meteorological variables, ground heat flux and soil temperature were captured at a high temporal resolution (5 min.) for more than 15 consecutive months. The daily evaporation rates over saturated bare soil (PEsoil) showed clear differences between gravel, coarse sand and fine sand, with higher PE for fine sand, smaller PE for coarse sand and smallest PE for gravel, during spring and summer. In addition, PEwater was smaller than PE for the saturated bare soil lysimeters. In autumn and winter, the measured PE rates over different surfaces showed only minor differences. The measurement data also revealed that during spring and summer night-time PE was considerable with ~1.0 mm ET per night. These results can be quantitatively explained with detailed calculations of the energy balance, considering the different porosities for gravel, coarse sand and fine sand, as well as the thermal conductivities of the phases which constitute the porous media. 

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 Rn. 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 (PEfine), coarse sand (PEcoarse) and gravel sand (PEgravel) 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. 

Evaporation from water (PEw) is generally considered equivalent to evaporation from saturated bare soils (PEs) as a starting point to estimate actual evaporation Ea. 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 (PEcoarse), fine sand (PEfine) and water were assessed for lysimeters installed in the Guanzhong Basin, China. Evaporation from Class A Pan (PEpan), 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. PEs is ~12% higher than PEw on a yearly scale. Annual PEfine exceeded PEcoarse 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. PEs is higher than PEw at day but smaller at night, with the peak value of PEw lagging ~4 hours behind PEs. Compared with PEw-curve, the PEpan-curve resembles more the PEs-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 (Rlu), as well as the vapour gradients above the surface (Δe).