Useful Drainage Estimates Obtained from a Large‐Scale Soil Moisture Monitoring Network by Applying the Unit‐Gradient Assumption

Abstract

Core Ideas Useful daily drainage estimates obtained from soil moisture measurements at 60 cm. Annual drainage estimates indicate potential recharge rates to groundwater aquifers. Uncertainty in the drainage estimates was relatively high at sites with high precipitation. Groundwater supplies ~20% of global freshwater withdrawals, and accurate information regarding groundwater recharge rates is needed for sustainable groundwater management. Recharge rates are often limited by the rates of drainage from the soil profile, which are influenced by soil moisture conditions. Soil moisture monitoring has expanded dramatically in recent decades with the advent of large‐scale networks like the Oklahoma Mesonet, which has monitored soil moisture statewide since 1996. Using those data with site‐specific soil hydraulic properties and a unit‐gradient assumption, we estimated daily drainage rates at 60 cm for 78 sites for up to 17 yr. Our working hypothesis was that these drainage rates are indicative of potential groundwater recharge rates. Mean annual drainage rates ranged from 6 to 266 mm yr−1, with a statewide median of 67 mm yr−1. These rates agreed well with prior recharge estimates for major Oklahoma aquifers. To provide a further independent check on our results, drainage was modeled using HYDRUS‐1D for four focus sites across 17 yr. Soil‐moisture‐based drainage rates and HYDRUS‐1D drainage rates agreed to within 10 mm yr−1 at the drier two sites but had discrepancies of > 150 mm yr−1 at two sites with > 1000 mm yr−1 precipitation. Simulations also showed that for a semiarid site the unit‐gradient assumption was likely violated at the 60‐cm depth, highlighting the need for deeper soil moisture monitoring. Despite these limitations, this simple method for estimating drainage through long‐term soil moisture monitoring shows unique potential to provide valuable information for hydrology and groundwater management.