Estimating Groundwater Recharge Using the Oklahoma Mesonet.

Groundwater supplies a large portion of water used in Oklahoma, and accurate and current information regarding groundwater recharge rates is essential for long-term, sustainable groundwater management. The Oklahoma Mesonet has provided soil moisture data at 120 monitoring stations for nearly two decades. Using these data in conjunction with site-specific soil hydraulic properties, we have estimated potential groundwater recharge, hereafter called drainage, at a depth of 60 cm for 78 Mesonet sites. Our working hypothesis is that these drainage rates are greater than or equal to the amount of groundwater recharge at each location. Calculated mean drainage rates from 1996-2012 ranged from 4 mm yr^-1 at Hinton to 275 mm yr^-1 at Bristow, with a state-wide median of 61 mm yr^-1. To clarify the relationship between these site-specific drainage rates and actual, independently-estimated groundwater recharge rates, HYDRUS-1D was used to model water flow and root water uptake below 60 cm at four focus sites during a two-year calibration period as well as a 16-year validation period. Modeled water flux at the 60-cm depth was higher than Mesonet-based drainage at two sites and lower than Mesonet-based drainage at the other two sites during the validation period. Modeled flux rates ranged from 9.3 to 166 mm yr^-1. Simulations also showed that a significant portion of water passing the 60-cm depth may be lost to root water uptake before reaching the 3-m depth. Overall, Mesonet-based drainage estimates seem to provide a reasonable upper limit for recharge rates at a regional scale, but may overestimate recharge in some areas.