On the Estimation of Potential Evaporation Under Wet and Dry Conditions

Abstract

Potential evaporation (EP) is an important concept that has been extensively used in hydrology, climate, agriculture and many other relevant fields. However, EP estimates using conventional approaches generally do not conform with the underlying idea of EP, since meteorological forcing variables observed under real conditions are not necessarily equivalent to those over a hypothetical surface with an unlimited water supply. Here, we estimate EP using a recently developed ocean surface evaporation model (i.e., the maximum evaporation model) that explicitly acknowledges the inter‐dependence between evaporation, surface temperature (Ts) and radiation such that is able to recover radiation and Ts to a hypothetical wet surface. We first test the maximum evaporation model over land by validating its evaporation estimates with evaporation observations under unstressed conditions at 86 flux sites and found an overall good performance. We then apply the maximum evaporation model to the entire terrestrial surfaces under both wet and dry conditions to estimate EP. The mean annual (1979–2019) global land EP from the maximum evaporation model (EP_max) is 1,272 mm yr−1, which is 11.2% higher than that estimated using the widely adopted Priestley‐Taylor model (EP_PT). The difference between EP_max and EP_PT is negligible in humid regions or under wet conditions but becomes increasingly larger as the surface moisture availability decreases. This difference is primarily caused by increased net radiation (Rn) when restoring the dry surfaces to hypothetical wet surfaces, despite a lower Ts obtained under hypothetical wet conditions in the maximum evaporation model.

Evaporation From Six Water Bodies of Various Sizes in East Asia: An Analysis on Size Dependency

“…The energy‐based equations are size independent, and the reductions in their performance with the averaged values of the parameters ( α PT = 1.27, η = 0.25) (Table S2 in Supporting Information ) are not substantial compared with those with optimized parameters (Table 4). This condition confirms the wide acceptance of the PT equation with α PT = 1.26 for lakes of various sizes and the YR equation with a specific η of 0.24 or 0.27 (Tu et al., 2022; Tu & Yang, 2022; Yang & Roderick, 2019). Although the DK equation performs better than the YR equation, two parameters are needed and not well recognized.…”

Potential Evaporation and the Complementary Relationship

“…For the estimation of E po , we adopt the maximum evaporation approach that explicitly acknowledges the interdependence between evaporation, surface temperature and radiation over wet surfaces. This enables both radiation and surface temperature to be recovered to a hypothetical wet condition (Tu et al., 2022; Yang & Roderick, 2019) such that the calculated E po (i.e., E po_max ) closely follows the E po definition (Tu & Yang, 2022). With E po_max restored to a hypothetical wet (i.e., potential) condition, we develop a new CR model based on the generalized polynomial function of Brutsaert (2015) and the modified boundary condition under extremely dry conditions of Szilagyi et al.…”

“…Nevertheless, R n is always a function of surface temperature according to Stefan‐Boltzmann's law (Monteith, 1981), so R n and T s are physically interdependent. Tu and Yang (2022) demonstrated that merely using T ws (e.g., R. D. Crago & Qualls, 2021; Qualls & Crago, 2020; Szilagyi, 2014, 2021; Szilagyi & Schepers, 2014; Szilagyi et al., 2017, 2022) and ignoring the R n dependence of T s would result in an even lower E po estimate (compared to the conventional Priestley‐Taylor model). To avoid the difficulties in formulating E po , the generalized sigmoid CR formulation (S. Han & Tian, 2018; S. Han et al., 2012) abandons the concept of E pa and E po and uses Penman's potential evaporation ( E Pen ) and its radiative component (equivalent to E rad ) to describe the relationships with E .…”

Response of temperate forest ecosystem services to rainfall: A case study in the forest nature reserves of northern China

“…In the RDA results, NDVI (contribution = 4.5%; p < 0.01) was the important vegetation factor affecting NPP and WY, and was correlated with precipitation significantly and positively. Under the gradient of less than 600-700 mm, vegetation growth and carbon sequestration in temperate forest are limited by water (Mao et al, 2014;Higgins et al, 2023); when rainfall is greater than 700 mm, community succession and vegetation cover Potential evapotranspiration (PET) is comprehensively affected by meteorological factors, higher PET means less water resources available (Donohue et al, 2010;Tu and Yang, 2022). In the RDA results, PET (contribution = 3.2%; p < 0.01) had a significant negative effect on ESs in areas with less than 600 precipitation, indicating that in areas with less precipitation, precipitation has difficulty in meeting the water requirements of vegetation growth, which limit the provision of temperate forest ESs (Rohatyn et al, 2018), and will also exacerbates trade-off between ESs.…”