Edward C. Myers, Consultant
Well Level Measurement Case Studies
The continuous measurement of well water level, under normal use, can tell us much about the capacity of the well, and the possibility of running dry. The following case studies compare two different residential wells.
Case 1: Plenty of water, even in drought conditions:
This was a level measurement for a residential well west of Sebastopol city limits. The sonic water level instrument was left overnight to log the level at 1 minute intervals. It measures the distance from the well head to the water surface. The well driller provided the depth of the pump in the well, so we could calculate how much water was above the pump. The more water over the pump, the less likely it is the well will run dry.
The graph below shows the full 24 hours of logged data, imported into Excel for plotting. The "static" water level (the groundwater table elevation) was 130 feet above the pump, and would drop about 10 feet and recover quickly when the pump ran. We concluded the well had plenty of capacity to supply the home and irrigation needs.
Case 2: Low water level, but OK when tested for extended flow:
This home was on the east side of Santa Rosa, outside the municipal water supply area. The graph below shows there was limited water over the pump, and when the pump ran, the water level appeared to come close to the pump. At first blush, it might seem a sustained water demand could temporarily run the well dry. However, the owner demonstrated they could run water from a hose continuously for several minutes, and the well was able to keep up with no problem. During this test, the water level remained just above the pump; groundwater refilled the well as rapidly as it was withdrawn by the pump. So, even though the water level was quite close to the pump, it did not appear to be in danger of running dry.
Case 3: Low Yield Well
This well serves a rural home northeast of Santa Rosa. The well is about fifty years old. It fills a water storage tank, which insures the house has a reliable supply of water for everyday use. The well pump can detect when the water level is close to it, and will shut off automatically to prevent it from running dry, and possible burnout. Once it shuts off due to a low water level, it will not re-start for 40 or so minutes, to prevent frequent starts and stops.
The graph below shows the water level was steady at 26 feet above the pump early in the morning; the pump had not run for most of the previous day and night. The pump then started and rapidly drew the water down, until the level reached the pump, which triggered a shut off. Then, the well refilled very slowly. From the observed rate of refill, we were able to estimate that the well's sustained yield is less than half a gallon per minute (gpm), or about 550 gallons of water a day. This is enough to supply the house itself, but will limit the amount of irrigation possible. Typically, a low yield well is one with a sustained yield less than 1 gpm.
