Actual evaporation from bare soils - A comparison of numerical modelling and field lysimeter data

Predicting evaporation from drying soils under limited water supply conditions, where water transfer to the atmosphere is limited primarily by soil hydraulic conductivity, is challenging. The parameterization of soil hydraulic properties (SHP) plays a crucial role in reliable predictions of evaporation. In particular, there are expected differences between traditional functions that consider water flow only in capillaries and functions that additionally consider non-capillary processes, i.e., water storage and film flow on particle surfaces and in corners and channels of pores. The non-capillary processes in simulating evaporation from soil surfaces become more important when the soil dries.

The purpose of this study was to investigate the applicability of different soil hydraulic function types in modelling the actual evaporation under water-limited conditions. Data were obtained from a large bare-soil field lysimeter (2.5 m height; 1 m² surface area), where the lysimeter mass and outflow were measured in hourly time intervals. Precipitation and actual evaporation were calculated from the mass changes of the lysimeter, using a simplified version of the AWAT filter approach of Peters et al. (2017). Meteorological parameters to calculate the potential evaporation were taken from the nearest weather station. Potential evaporation rates were obtained by (i) using the FAO-56 version of the Penman-Monteith equation and (ii) scaling these values to match the bare soil potential evaporation.

The evaporation was simulated using two different models for soil hydraulic properties: i) van Genuchten Mualem (VGM) (only capillary storage and flow), and ii) Peters-Durner-Iden (PDI) (capillary and non-capillary storage and flow). The results show a systematic difference in evaporation prediction by applying the PDI and VGM models, with higher evaporation rates for the PDI model under dry conditions.