USDA LTAR Common Experiment measurement: Depth to water table v1
Authors: A. Snyder Keirith 
Published: August 2024

Groundwater is water held beneath the land surface in rock and soil pore space. The upper surface of this saturated water zone is called the water table. Depth to groundwater, also referred to as the water table, can change in response to management activities and environmental factors. Groundwater is measured in observation wells (i.e., piezometers or non-pumping wells) using manual methods or with pressure transducers. The water table is the level in the saturated zone at which the hydraulic pressure is equal to atmospheric pressure and is represented by the water level in observation wells. Below the water table, the hydraulic pressure increases with increasing depth. Water table aquifers, also known as unconfined aquifers, are influenced by the unsaturated zone, and the water table is free to rise and decline. Wells with pipes screened to allow water to flow laterally within the pipe measure the position of the water table in the surrounding aquifer. Manual methods rely on an observer with a linear measuring device (e.g., a steel measuring tape with wettable chalk or beeping electric tape) dropped into the well and subtracted from a permanent reference point above the land surface. These measurements are performed at certain points in time and are known as synoptic measurements. In networks such as the LTAR network, it is common practice to use automated pressure transducers with an internal or external data logger. Transducers have the advantage of continuous in situ monitoring. Submersible pressure transducers are installed at a fixed depth in a well (typically 2” polyvinyl chloride (PVC) pipe) - refer to manuals on groundwater well design) and measure pressure exerted by the water column and atmosphere against a membrane in units of pressure (Pascals, pounds per square inch, or equivalent). This pressure measurement is converted into the height of the water column above the transducer (or water depth) using a conversion formula (e.g., 1 psi = 2.303 ft of water). Submersible transducers are vented or non-vented. The former type has a tube from the transducer whose end is maintained above the water surface to automatically exclude the atmospheric pressure from the pressure measurement, thus providing a direct measure of the water depth. In contrast, a non-vented transducer reads the combined pressure of the water column over the transducer and the atmospheric pressure; hence, atmospheric pressure must be subtracted from non-vented transducer data. The atmospheric pressure can be obtained from a nearby weather station or by installing a transducer above the water surface in a well. Monitoring well layout and installation procedures are designed to ensure that the water level inside the well is identical to the water table level. In addition, wells will likely need to be periodically de-silted. Another approach to measuring the depth to the water table (in surface water bodies) is to use an ultrasonic range finder (Maxbotix) placed on the underside of a bridge or structure pointing directly down, overlooking the water surface of interest (canal, lake, etc.). This device transmits an ultrasonic wave that travels downward and gets reflected back to the sensor from the water surface; the distance to the water surface is the product of the speed of the wave (speed of sound) and the time taken for this wave to travel and return. This device is considerably more economical than submersible transducers, interfaces easily with open-source data loggers, and is used in monitoring networks globally where funds are limited. However, scattering of the ultrasonic wave from emergent aquatic vegetation or waves in choppy conditions poses limitations to this method. In addition, scattering from the inner surfaces of a narrow well (e.g., 2” PVC tube) poses a limitation to the use of these rangefinders to measure groundwater levels in wells.