Kwantifisering en voorspelling van grondwaterverdamping by droëlandgewasproduksie

English: Research results worldwide show that approximately 70% of the annual rainfall is lost due to evaporation from the soil. It is therefore of utmost importance to store as much water in the root zone during the fallow period as possible. The aim of this study was to find methods of reducing evaporation losses through different cultivation practices and to develop procedures to predict soil evaporation. The soil evaporation process was studied in detail by using micro Iysimeters with an inside diameter of 60 mm and a length of 315 mm. Soil water flux and diffusivity of the top 100 mm was studied in 20 mm layers, and from 200 - 300 mm in 50 mm layers. The different stages of evaporation as well as the duration of the constant phase were determined in this study. The potential evaporation was determined with different methods and compared with the class A-pan. Several methods to reduce soil evaporation were investigated. This included different soil manipulation practices and levels of shading. The influence of soil texture on. the evaporation rate and on the cumulative evaporation was also investigated. It was established that the constant evaporation stage (phase I) lasted only a few hours, and was therefore omitted from the development of evaporation procedures. Good correlation was found between the measured evaporation of evaporation trays and the class A-pan. The results of this study showed that soil evaporation continues during the first two nights after wetting (rainfall or irrigation). The atmospheric evaporative demand has an influence on the evaporation of soil up to the third day after wetting. An increase in evaporation due to a temperature increase was higher in undisturbed soil than in disturbed soil. The evaporation rate decreased with an increase in the level of disturbance (looseness) of the soil. During the first week after wetting the evaporation losses were mainly from the top 100 mm. After nine days the soil started drying out from deeper than 100 mm and from all the deeper layers simultaneously. The evaporation due to some of the different soil surface treatments differed significantly. The difference was mainly ascribed to the initial soil water content rather than to the soil manipulation as such. The soil water content available for evaporation (evaporativity) had the greatest influence on the evaporation rate and on the cumulative evaporation of the soil. Shading of the soil surface decreased evaporation losses over the short term for example over the first ten days and 20 days after wetting for sandy and sandy clay loam soils respectively. This decrease increased with an increase in the shading percentage. Over long drying periods, shading had little effect on evaporation losses. The cumulative soil surface evaporation equations of Rose (1968), Ritchie (1972), Kijne (1973) and AI-Khafaf et. al. (1989) fitted the data of the different experiments well. Equations were developed with which the regression coefficients and empirical constants could be calculated from measurable parameters. Such were the silt plus clay content, initial soil water content (0), desorptivity (01-00) and the evaporativity water content (01-00)Z1 of the soil layer. Equations were also developed to calculate the water content at which evaporation stop (00) and the depth of the evaporating soil layer (Z1). The Ritchie equation predicted the actual soil evaporation the most accurately. This equation is therefore recommended for inclusion in computer modeling of evaporation. The following two equation are recommended: See full text for formulae.