Overview: The goal of this project is to provide assessments of groundwater conditions in basins throughout the Southern District. The results of the assessments will be made available in a set of maps showing groundwater elevation contours, changes in groundwater elevation, and changes in groundwater in storage. We will endeavor to update these maps periodically and increase the number of basin assessments available.

Groundwater Elevation Contour Maps: These maps are constructed using groundwater level measurements obtained from various agencies. Groundwater elevation contour lines are interpreted from these data. These measurements may vary in accuracy and precision depending on the method of measurement and whether or not the levels were affected by pumping in the vicinity of the well. In general, we have accepted the measurement data provided by the agencies as representing the elevation of the unconfined or water table aquifer at the time and location of the measurement. These maps represent a generalized water table map of groundwater for the basin study.

Groundwater level measurements are compiled for a basin based on the date of measurement. It is preferable to have many wells distributed uniformly throughout a groundwater basin all measured during a short period (e.g., one day) and at the same time each year (e.g., each spring or each fall). When such data are not available, we compile measurements over longer periods and choose a contour interval that reflects this added ambiguity in the water level. Dashed contour lines are used to indicate less certainty or less supporting data for the interpreted contours. These maps are best used as a general guide to the elevation of the water table at the time and location of the measurements. The interpreted water level elevations are expected to be within about one contour interval for the time indicated on the map, however, the elevations may not be the same later.

Change in Groundwater Elevation Maps: Contours of change in groundwater elevation are interpreted from data obtained directly from hydrographs of water levels in wells and by comparing groundwater elevation contour maps. The period of comparison for water level change is picked from patterns of recorded precipitation. The base period is picked to begin and end during a similar trend in rainfall and the period to have an average rainfall close to the long-term average precipitation for the location. This helps ensure that there should be a similar amount of water in transit toward the water table at each end of the base period. These maps are useful in viewing medium- to long-term trends in water level elevation in a basin.

Change in Groundwater Storage Maps: Change in groundwater storage in a basin is computed by using change in groundwater elevation maps. Areas of equal change in water level are interpreted from these maps and the average value for each area is noted. The area (in acres) is multiplied by the average water level change (in feet) and by the average specific yield (in percent) interpreted for the aquifer underlying that area. The result of this calculation is a value of the volume of water gained (+) or lost (-) in the aquifer underlying the area, as expressed in acre feet (i.e., an acre of land covered by water to a depth of one foot or 325,850 gallons). The change in storage for each portion of aquifer is summed to achieve a total change in storage within the basin during the base period. An average gain or loss in storage can be derived by taking the total change and dividing by the number of years in the base period.

These maps are helpful in discussing the patterns of change in storage for a basin. The precision of the values obtained by this method is dependent on the precision of the water level measurements, the water level elevation maps, and the specific yield values used. The precision of the values is three significant figures or less.