Quantitative characteristics of irrigation water

2.3 Evapotranspiration

Plants, like all living things require water to sustain their lives. In order to fulfill the basic functions, enough water is needed in the soil containing roots. Otherwise, plant growth decline and deaths occur in later stages. Evapotranspiration (ETc) is consisting of two basic elements. These are: transpiration through stoma of plant leaves and evaporation of open surface of the soil around the plant. Many ways including direct and indirect ways are used in determination of ET. However, reference evapotranspiration (ET0) are used as pre­projecting factor reference. A mathematical relationship is used for this purpose as given bellow. (Doorenboos and Pruit, 1977):

ETc=ETo*kc (1)


ETc: Evapotranspiration of the crop, mm ETo: Reference evapotranspiration, mm kc: crop coefficient

ETc is calculated based on plant that is taken as reference in ETo. This plant is usually alfalfa or meadow pasture. There are many improved equations to calculate the reference Evapotranspiration. These include Penman FAO modification, Penman Montheith, Hargraves, Blaney Criddle and Class A evaporation tank etc. (Tekinel and Kanber, 1988).

The second factor affecting the Evapotranspiration is crop coefficient. Crop coefficient is defined as a ratio of ETc to ETo. Crop coefficient varies depending on factors such as plant type, age, growth period, soil moisture etc (Allen et. al., 1978). Grass plant is usually grown in the fields of landscape. Although this plant has many types, its kc value has been reported to vary between 0.7 and 1.05.