Testing the α and β methods of estimating evaporation from bare and vegetated surfaces in class

Abstract

Surface soil wetness determines whether evaporation occurs at the potential rate or is limited by soil water supply. Many land surface models calculate evaporation by parametrizing the relative humidity at the soil surface (α method) or the soil water diffusion resistance (β method). The relationships of α and β to the average moisture content of soil surface layer thicknesses ranging from 0.5 to 10 cm were examined using Bowen ratio/energy balance measurements of evaporation from a bare loam/silt‐loam soil at two adjacent sites, one of which was culti‐packed while the other was disc‐harrowed. It was found that the relationships were sensitive to the surface layer thickness and tillage treatment and became better defined with larger thicknesses. Simulations of evaporation by CLASS (Canadian Land Surface Scheme) using Philip's relationship (currently used in CLASS to determine α with a 10‐cm thick surface soil layer) and our proposed relationships of α and β were compared with measurements made during a 7‐day drying period for the culti‐packed soil. Evaporation was significantly overestimated by Philip's relationship when the soil was dry. The α and β relationships gave much better estimates of evaporation than Philip's relationship; however, evaporation under conditions of high air humidity (e.g., night‐time) was significantly suppressed by the α relationship. In long‐term simulations, there were large differences between simulated and measured evaporation rates due to differences between actual and calculated soil moisture. When CLASS was forced to use measured soil moisture, evaporation rates calculated using the β relationship agreed remarkably well with the measured values, while those calculated using Philip's relationship were significantly overestimated. The effectiveness of the β relationship was also tested with a 2‐cm thick surface layer using data collected from another bare silt loam soil and for a 10‐cm thick surface layer using data from a crop and two forests with different soil types. It gave better estimates of evaporation from these surfaces than Philip's relationship. In the simulation with CLASS, the β relationship appeared less sensitive to possible errors in modelled drainage than Philip's relationship, and was recommended for use in the model as replacement for Philip's relationship.