THIRTY-SEVEN YEARS OF EXPERIENCE WITH DETERMINING SUSTAINABLE YIELDS OF HARDROCK WELLS IN THE SIERRA NEVADA

Pumping rates of many hardrock wells in the Sierra Nevada vary considerably with pumping time and other factors. Early attempts at determining the sustainable yields involved four-hour constant discharge tests. Constant rate pump tests of short duration have proven to be of little value in determining sustainable yields of these hardrock wells. In the mid to late 1970’s, the value of longer-term constant head tests began to be recognized. When a constant head pump test is conducted for 20 to 30 days, the pumping rate normally decreases exponentially with pumping time. The pumping rate is plotted versus the logarithm of time since pumping began. A straight line is developed that can be projected for the normal dry period, and this can be used to determine how much water could be pumped from the well during the period.

Water-level recovery is extremely important for hardrock well pump tests. Hardrock wells vary greatly in the duration required for full recovery, following the cessation of pumping. Water levels in some wells fully recover within only a few hours. Water levels in other hardrock wells only recover after winter precipitation. Lastly, water levels in some hardrock wells take years or longer to fully recover after a pump test, and yields of these wells are not sustainable.

Available recharge should be considered. Precipitation, plant evapotranspiration, and runoff are key parts of the water budget in undeveloped areas. In low elevation, low rainfall areas, there is little runoff, and only small amounts of groundwater can usually be developed. In contrast, at higher elevations there is much more precipitation and runoff, and more groundwater can generally be developed. One of the most important factors is the size of the tributary watershed, which depends highly on the depths of water-producing fractures tapped by the wells. In general, the deeper these fractures, the larger the tributary area that the wells may draw from.