Soil moisture evaporative losses in response to wet-dry cycles in a semiarid climate

In terrestrial environments evaporative losses constitute up to 40% of evapotranspiration. While ~ 65% of these evaporative losses are from soil, this component is usually only coarsely estimated for water budget calculations. The most common method of estimating potential maximum evaporative (reference) loss is based on solar radiation and weather parameters but does not consider soil water availability. While such estimates of evaporative loss are usually credible in wet climates, they are likely to be inaccurate under warmer and drier climates characterised by strongly episodic or seasonal rainfall. In this study, we used a weighing lysimeter (Ready-To-Go RTG, Umwelt-Geräte-Technik, Germany) to quantify soil evaporative loss in situ and to assess how evaporative loss varied between wet and dry cycles in the semiarid and subtropical Hamersley Basin of the Pilbara region of northwest Australia. Mean daily evaporative losses from the soil for dry periods varied between 0.2 and 0.7 mm d⁻¹. In contrast, mean daily evaporative losses during the wet period varied between 0.3 and 13.0 mm d⁻¹, averaging 3.7 mm d⁻¹. The commonly used mathematical equations for estimation of potential maximum evaporative (reference) loss, such as the Ventura modified Hargreaves-Samani equation, overestimated soil evaporative losses by about two-fold. The results of this study highlight the importance of accounting for the transition between energy limited Stage 1 evaporation versus water-limited Stage 2 and Stage 3 evaporation, as these latter Stages can dominate for extended periods in semiarid climates.