USE OF A CONTINENTAL-SCALE HYDROLOGIC MODEL (NATIONAL HYDROLOGIC MODEL - NHM) TO INFORM WATER MANAGEMENT IN A SINGLE BASIN

The hydrology of the Kiamichi River, a 4,799 km² watershed located in southeastern Oklahoma and home to several endangered freshwater mussel species, has been altered by two major reservoirs with two additional large projects pending. The upper, northern portion of the watershed is located in the Ouachita Mountains, and is generally underlain by poorly hydraulically conductive Carboniferous shales and sandstones. Sardis Lake impounds a tributary in this part of the watershed ~ 3.38 x 10⁸ m³ of conservation-pool storage. A trans-basin water transfer of ~1.4x10⁸ m³/yr to Oklahoma City municipal area from Sardis Lake storage is planned, releasing water for recovery from the reservoir to the river for recovery at a proposed pipeline 40 miles downstream. This would dramatically alter streamflow through critical mussel habitat. There is a need to understand how this management change will affect critical mussel habitat, and how groundwater-surface water exchange will change, which is a primary control on that habitat. A direct comparison of historic discharge data with planned water takings by the Oklahoma City Water Utilities Trust indicates flows are currently less than the expected water takings at least 75% of the time in the summer months.

The purpose of this study is to investigate and validate the performance of the National Hydrologic Model (NHM) in the Kiamichi watershed and evaluate the viability of using this continental-scale model to inform management decisions in this local-scale watershed. The NHM is a Precipitation-Runoff Modeling System (PRMS) developed by the USGS as a deterministic, physical-process based numerical model with the continental United States as a model domain. Model outputs are compared with USGS daily mean discharge data in the Kiamichi watershed over a 36-year period. The model generally exhibits smaller variance than the observed data, and underestimated cumulative stream discharge, and appears to predict peak flows ~3 days prior to observations. Thus, the model performed well during normal conditions, with performance degrading during extreme events. Overall, the NHM provides reasonable results in this watershed. Groundwater-surface water flux results will be investigated next to inform further study.