Evaporation from bare soil following rainfall
Toth, Brenda Margaret
The semi-arid environment experiences a potential for evaporation that exceeds the supply of precipitation. In addition a significant portion of precipitation falls in small daily amounts that might not be sufficient to counter evaporative demands of the following day. The objective of this project was to establish the relationship between daily rainfall amounts and that portion of the rainfall lost to evaporation in the subsequent 24-hour period. Evaporation is the link between the hydrological cycle and the surface energy budget, therefore the evaporation evaluation techniques were based on both the soil water balance and the energy balance. The energy balance methods included the Bowen ratio, and the Penman-Monteith and G-D equations. The soil water balance was evaluated with precipitation measurements as well as replicates of two different types of microlysimeters that allowed evaluation of the drainage component. The dataset consisted of daily soil moisture changes from microlysimeters, soil moisture measurements, and daily rainfall; additional measurements included net radiation, ground heat flux, air temperatures, relative humidities, and windspeed. This dataset spanned 36 days in August and early September 1994. The microlysimeters that permitted drainage were deemed to be more representative of field conditions, as they allowed the drainage during rainfall to be quantified, but the drainage component after rainfall was difficult to isolate. The replicated microlysimeters produced a spatially averaged estimate of evaporation. Microlysimetric and energy balance methods gave evaporation estimates of 43.1 to 64.4 mm during a time when the precipitation received was 59.9 mm. The two methods deemed to most accurately reflect actual evaporative losses were the G-D and cotton-capped (corrected for drainage) microlysimetric methods which produced evaporative loss estimates of 43.1 and 47.1 mm, respectively. Estimates of subsequent 24-hour evaporation by the two methods indicate 20.1 and 22.2 mm, respectively, of the 59.9 mm of precipitation was lost on the day following rainfall. The dataset was limited and no rigorous regression techniques could be used to establish a quantifiable relationship between daily rainfall amounts and subsequent 24-hour evaporative loss, but the portion of daily rainfall that could be viewed as effective precipitation decreased with decreasing daily depths of rainfall.