Evapotranspiration Estimation Using Soil Water Balance, Weather and Crop Data

The rise in water demand for agriculture, industry, domestic, and environmental needs requires sagacious use of this limited resource. Since agriculture (mainly irrigation) is the major user of water, improving agricultural water management is essential. Efficient agricultural water management requires reliable estimation of crop water requirement (evapotranspiration). Evapotranspiration (ET) is the transfer of water from the soil surface (evaporation) and plants (transpiration) to the atmosphere. ET is a critical component of water balance at plot, field, farm, catchment, basin or global level. From an agricultural point of view, ET determines the amount of water to be applied through artificial means (irrigation). Reliable estimation of ET is important in that it determines the size of canals, pumps, and dams. The use of the terms ‘reference evapotranspiration’, ‘potential evapotranspiration’, ‘crop evapotranspiration’, ‘actual evapotranspiration’ in this chapter is based on FAO-56 (FAO Irrigation and Drainage publication No 56) (Allen et al., 1998). There are different methods of determining evapotranspiration: direct measurement, indirect methods from weather data and soil water balance. These methods can be generally classified as empirical methods (eg. Thornthwaite, 1948; Blaney and Criddle, 1950) and physical based methods (eg. Penman, 1948; Montheith, 1981 and FAO Penman Montheith (Allen et al., (1998)). They vary in terms of data requirement and accuracy. At present, the FAO Penman Montheith approach is considered as a standard method for ET estimation in agriculture (Allen et al., 1998). A case study from a semiarid region of Australia will be used to demonstrate ET estimation for a canola (Brassica napus L.) crop using soil water balance and crop coefficient approaches. Daily rainfall data, soil moisture measurement data using neutron probe, and AquaCrop (Steduto et al., 2009) -estimated deep percolation below the crop root zone will be used to determine actual evapotranspiration of the crop using soil water balance. Reference evapotranspiration ETo will be determined using FAO ETo calculator (Raes, 2009). Crop canopy cover measured using a handheld GreenSeekerTM and expressed as normalized difference vegetation index (NDVI) will be used to interpret evolution of evapotranspiration during the growing season (life cycle) of the canola crop.