Water demand/withdrawal describes the total amount of water withdrawn from surface water or groundwater source. The measurements of this withdrawn water help evaluate demands from domestic, industrial, and agricultural users.
Water consumption is the portion of the withdrawn water permanently lost from its source. This water is no longer available because it evaporated, transpired or used by plants, or consumed by people or livestock. Irrigation is by far the largest water consumer. Globally irrigated agriculture accounts for 70% of the total water used, and almost 50% is lost either by evaporation or transpiration.
Understanding both water demand/withdrawal and consumption is critical to evaluating water stress properly. Measurements of water withdrawal indicate the level of competition and dependence on water resources. Water consumption estimates help to quantify the impact of water withdrawals on downstream availabilities and are essential to evaluate water shortage and scarcity. For example, most water used by households is not consumed and flows back as return flow and can be reused further downstream. However, water is rarely returned to the watershed after being used by households or industry without changing the water quality, increasing water stress levels.