Plant water requirements and evapotranspiration

Water is critical for life and freshwater resources are limited. The largest share of freshwater resources is used by agricultural activities to support food security. Governments and relevant international organizations require updated information on water resources for their optimal management. There are two sides of water resource problems. One is the supply side which addresses the availability of water for human consumption and addresses issues such as shifts in rainfall patterns, glacier/snow melt, quality and quantity of groundwater. The other side addresses the demand of water resources required by the human activities. This chapter discusses the demand side which is estimated through studying the dynamics evapotranspiration (ET). ET is one of the main components of the water cycle and its accurate estimation needed on large spatial scales for water management at different decision levels. Evaporation is the conversion of water into water vapor and hence the removal of water from a surface, such as a water bodies, soil or wet vegetation, into the air. ET includes evaporation from soils and water bodies, whereas transpiration is removal of water from plants. ET rate is affected by the weather variables, soil and vegetation characteristics. This chapter provides insights into estimating crop water requirements to efficiently manage water resources. Firstly, factors affecting plant water requirement such as weather variables, vegetation and soil characteristics are presented. Secondly, various methods to calculate ET are elaborated. In addition, the latest tools and technologies are discussed, along with traditional methods of estimating ET. Thirdly, two case studies are presented with distinct methodology for estimating ET. Lastly, the chapter provides a method to estimate crop water requirements using ET, along with total irrigation water requirement while considering losses such as deep percolation, runoff and efficiency of the irrigation method. Policy makers and planners of irrigation water can be benefitted through such studies, as they enable to perform demand-driven management of water resources. On the contrary, a huge amount of already dwindling water resources is lost, if its management is supply-driven.