CONTRIBUTION OF DEW IN HYDROLOGIC BALANCE OF A SEMI-ARID GRASSLAND

The hydrologic balance of a semi-arid grassland near Deniliquin, New South Wales, was measured with a precise weighing lysimeter from Jan.-Dec. 1974. Records were analyzed for quantity and duration of dew and actual evapotranspiration. Dewfall was negligible during summer (Jan., Feb., Dec.) but for the remainder of the year it amounted to 1.2% of class A pan evaporation, 2.5% of precipitation or 3.9% of actual evapotranspiration. Since 90% of the dew fell between May-Sept., which coincides with the active growing period of grassland, it possibly plays a role in plant growth and species composition. Maximum dewfalls were received during winter months. In June, dew amounted to 6.1% of pan evaporation, 15.6% of actual evapotranspiration or 29.8% of precipitation. Average monthly maximum dew duration of 17.5 h occurred in this month. During winter, dewfalls of up to 0.56 mm day-1 were recorded, which amounted to 57% of actual evapotranspiration or 27% of pan evaporation. The role of dew in the reduction of evaporative loss of soil water is emphasized. Based on a detailed energy partitioning exercise in May, it was demonstrated that about 80% of net radiation was utilized for evapotranspiration, of this dew used over 22% which would have otherwise been used largely in evapotranspiration of soil water.