The water balance compares precipitation received with evapotranspiration and losses through runoff and deep drainage. When inputs exceed outputs, there is water surplus (aquifer recharge); when lower, deficit (water stress).
In Spain
The southern and eastern half of the Peninsula shows water deficit for most of the year. The annual balance is positive only along the Cantabrian coast and mountain areas. Water balance models (Thornthwaite, Penman-Monteith) are fundamental tools for irrigation planning, reservoir management, and drought prediction.