The amount of water that is depleted by a crop between irrigations that needs to be replenished during the forthcoming irrigation. SWD corresponds to the difference between field capacity (FC) and the soil water content (SWC) at the time of interest, or the amount of water that must be added to the root zone to return it to field capacity. SWD is generally expressed as a volume of water over an irrigated area (e.g. acre--inches/acre or inches).