This chapter shows how the knowledge from the previous chapters can be combined to understand and manage processes at the land–atmosphere interface. First, attention is paid to the estimation of crop water requirements using the crop factor method and to the direct measurement of evapotranspiration using lysimeters. Then it is shown how in a semiarid region the water productivity of irrigated crops can be studied and improved. Finally, the response of different vegetation types (grass and forest) to heat wave conditions is studied.

Crop Water Requirements

Evapotranspiration determines to a large extent the hydrological cycle and the environmental conditions near the soil surface. There is a direct relation between the ratio of actual to optimal transpiration and the ratio of actual to optimal crop yield. Irrigation water requirements are determined by the amount of evapotranspiration relative to the amount of natural rainfall and readily available soil moisture. Groundwater recharge and soil salinization also depend largely on the amount of evapotranspiration. In the context of agricultural practice the water required to grow a crop does not only include the water loss due to evapotranspiration, but also the water needed to leach salts and to compensate for nonuniform application of the water (Allen, 1998).