Water balance is the area that deals with how the mass of water in a given volume of soil is conserved when all sources and sinks are considered. Water balance over land surfaces involves sources like precipitation, irrigation, run-on, or rise of the water table. The sinks for water are the loss of water due to evaporation and transpiration of vegetation, the loss due to run-off, and the loss due to percolation below the zone of interest. The water stored in the soil matrix is modified as these sources and sinks come into play. In the area of water balance, we have developed measurement techniques to measure soil water in the root zone of managed and non-managed ecosystems. We are also measuring those weather variables that affect evapotranspiration (e.g., solar radiation, wind speed, humidity, and temperature) in a network of more than 60 sites in Nebraska. At nearly half of these sites, we also have soil water measurements under grass. Models that predict soil water and evapotranspiration have also been developed for these ecosystems.

Water balance is re-emerging as an area important to a better understanding of man's influences on the climate. This is because the energy available at the earth's surface must generally go into evaporation or sensible heat. Does irrigation or land use affect the climate? Irrigation decreases the sensible heat and increases the evaporative component. Will water balance play an important role in the development of better prediction models? How did the water balance of the 1930s drought compare to today's water balance? Many exciting questions wait to be answered.