Evaporation from a Themeda grassland. 1. Controls imposed on the process in a sub-humid environment.
Evaporation from a weighing lysimeter containing grassland containing 50% T. australis was measured for 3 yr in a sub-humid region of New South Wales, Australia. Evaporation from the lysimeter was similar to that from an unconfined community nearby. For short periods, following dew or rain, evaporation was controlled solely by meteorological influences; this accounted for about 20% of annual water loss. Transpiration which was potentially subject to stomatal and other control mechanisms accounted for about 80% of water loss. Soil water supply and phenology both influenced transpiration. During winter and spring, when plants were dormant, plant mechanisms imposed a large resistance to vapour transport, with as little as 30% of monthly net radiation being used for evaporation. This proportion was even less for short periods when soil water supply was deficient. In the summer growing season about 50% of the net radiation was used for evaporation, when soil moisture levels were adequate, but this fraction decreased as soil moisture was depleted and demand was large.