The water that must be supplied through the irrigation system to ensure that the crop receives its full crop water requirement (without including losses in the process of applying the water). It varies considerably with climatic conditions, seasons, crops and soil types: IWR=KcxET0-P-?S, where: Kc is a coefficient varying with crop type and growth stage; ET0 is the reference evapotranspiration, depending on climatic factors; P is the precipitation; ?S is the change in soil moisture from previous month. In the specific case of paddy rice irrigation, additional water is needed for flooding to facilitate land preparation and for plant protection. In that case, irrigation water requirements are the sum of rainfall deficit and the water needed to flood paddy fields.