Effects of soil moisture sensor spacing and zone of influence on recharge calculations

Quantification of groundwater recharge is essential for water resources management. The water balance method is one of several methods for measuring event-based groundwater recharge. Soil moisture measurement data are essential for quantifying recharge using the water balance method. The spacing and zone of influence of soil moisture sensors used in a study are important factors for the accuracy of recharge calculations. A large lysimeter was used to measure the water balance components to quantify groundwater recharge in South Florida as a function of soil moisture and water table levels. Recharge was calculated using sensor spacings of 10 and 20 cm and zone of influence of 10 and 20 cm for seven major rainfall events. Recharge calculated with a sensor spacing of 20 cm had a relative root mean square error (RRMSE) of 33.1% as compared with the 10-cm spacing. Recharge calculated using a zone of influence of 20 cm had an RRMSE of 38.8% as compared with the 10-cm zone of influence. This error is attributed to soil moisture tensor discretization, which results in averaging the soil moisture in a soil layer containing the saturated and unsaturated zones interface, which leads to an overestimation of soil moisture in the unsaturated zone and an underestimation of soil moisture in the saturated zone. Vertical spacing of sensors larger than the 'sensors' zone of influence also gives erroneous results as portions of the soil profile are not measured. For groundwater recharge calculations, selecting a sensor with suitable zone of influence and appropriate sensor spacing can have a significant effect on the accuracy of the calculations.