Scientific irrigation scheduling using crop coefficient methods
Estimating the amount of water evaporated from soil and transpired from vegetation is a necessary process in many engineering and science fields, such as water resources planning, hydrologic analysis, and agricultural water management, as evapotranspiration (ET) controls the moisture content of soil, one of the most important freshwater storages in the earth. A reference evapotranspiration (ET0) rate represents the amount of water that the air can take and hold in a time interval, and it is determined only by weather variables including solar radiation and wind speed. On the other hand, an actual amount of water evaporated and/or transpired can be limited by the soil moisture content and transpiration capacity of the crop plants (Allen 2000). A crop coefficient (Kc) method has served as a convenient way to estimate an actual ET for crop plants (ETc) rate based on a reference evapotranspiration rate estimate and the types of plants located in an area of interest (Irmak and Haman 2003; Zotarelli et al. 2010). The method has been a useful and popular way particularly for ET-based irrigation scheduling due to the simplicity (Kisekka et al. 2009a; 2009b). The crop coefficient method has two different approaches, single and dual crop coefficient(s). The single coefficient method represents averaged effects of evapotranspiration from a cropped surface, while two independent coefficients are required for the dual crop coefficient method taking into account evaporation from soil and transpiration from vegetation separately (Allen et al. 1998). The single crop coefficient approach is relatively simple, however, the dual crop coefficient approach can more effectively consider a soil moisture condition and plant drought stress that can limit evaporation and transpiration, respectively, in an actual evapotranspiration rate estimation. In this document, we introduce the concept of the dual crop coefficient method that allows considering evaporation and transpiration separately in irrigation scheduling and then demonstrate a way of scheduling irrigation by calculating time-varying evapotranspiration rates of a crop field using the method. Material presented in this document is intended to improve the knowledge base of water management and irrigation professionals and can be applied to agricultural production.