WATER BUDGETS: THE FOUNDATION FOR DETERMINING SUSTAINABILITY

To determine the sustainability of a water resource, it is necessary to know how much water is present in a reference volume, such as a reservoir or an aquifer, and how that amount changes with gains from external sources and losses to external sinks. Although the concept of a water budget is simple, practical determination of water budgets is often difficult. The problem relates to limitations in our ability to measure the amount of water stored, the changes in storage over time, and the fluxes to and from a reference volume. In any budget exercise, it is essential to clearly define the reference volume. In the case of ground water, the reference volume could be a volume of the subsurface that includes all rock units through which ground water flows or it could be an aquifer. The former is of interest if the goal is to sustain ground-water supported ecosystems, such as some streams, lakes, and wetlands. The latter is of interest if the goal is to sustain water supplies for human use. In either case, the volume of water in storage and the changes in storage need to be measured. Determining the quantity of water stored in a reference volume requires geologic mapping, test drilling, geophysical surveys, and hydraulic testing of the rocks to determine effective porosity. Measuring changes in storage requires installation of observation wells and monitoring of water levels over time. Determining fluxes to the ground-water system requires measuring precipitation, recharge, and movement from adjacent rocks. Determining discharge from the ground-water system requires measuring evapotranspiration from ground water, discharge to surface water, movement to adjacent rocks, and pumping. All of these activities require instruments and sensors that have some degree of errors inherent in them and in their installation procedures. In addition, all sensors, whether they are rain gauges, wells, or hydraulic tests, measure a very small part of a ground-water reference volume, and those point data need to be distributed mathematically in space and time. These interpretation methods also have various degrees of uncertainty. In summary, all water budgets have some degree of uncertainty. The practical value of a water budget depends on how well the user understands and takes these uncertainties into account in managing a water resource.