The water balance of a defined area tallies up the full tally of inflows from precipitation, surface water and groundwater entering into the region, weighed against all the outflows like evaporation and transpiration by plants, water withdrawal for agricultural, industrial and municipal uses, and surface runoff leaving the region.
Constructing this basic water accounting sheet allows water managers to evaluate how balanced water supply and demands currently are, and how sustainable the balance is into the future. Excess demand leading to chronic storage depletions signals that limits to consumption may become necessary.
The water balance also provides a means to quantify whether storage reserves are being adequately replenished each year, or whether more precipitation disappearing as fast as it arrives. The volumes in the balance equation are commonly tracked over monthly or yearly cycles. More frequent major imbalances point to substantial risks in the region’s water security that water managers will need to address.