Digital archive of the Ozark Plateaus groundwater-flow model version 1.0

Abstract

Groundwater in the Ozark Plateaus aquifer system is an important source for municipal, industrial, agricultural, and domestic water supply needs across much of southern Missouri and northern Arkansas, and smaller areas of southeastern Kansas and northeastern Oklahoma. Recent short-term drought conditions have emphasized the need to better understand the delicate balance between abundance, sustainability and scarcity. A groundwater flow model has been developed as the primary tool for the assessment of groundwater availability in the Ozark aquifer system. The model was developed to benefit concurrent and future investigations involving groundwater-withdrawal scenarios, optimization, particle transport, and monitoring network analysis. The model is also critical to the ongoing work to quantify groundwater in the Ozark aquifer system. The groundwater model simulates 116 years (1900 2015) of hydrologic conditions and the response of the groundwater system to changes in stress. Stress applied to the groundwater systems includes changes in recharge and increased groundwater withdrawals for water supply. Semi-seasonal stress periods were simulated from the later part of 1991 to 2015 and represent higher demand and lower recharge in the spring and summer months, and lower demand and higher recharge in the fall and winter months. Groundwater pumping increases throughout the simulation period, with a maximum rate of about 600 million gallons per day. History matching for the Ozark aquifer system model was accomplished by a combination of manual changes to parameter values and automated calibration methods. Observation data used in the development and evaluation of the model included 19,045 hydraulic-head observations from 6,683 wells within the Ozark model area that were weighted for use in the parameter estimation software. Observation data also included stream leakage estimates summed to calculate a net gain or net loss value for each stream. The majority, but not all, of the recharge component is discharged through streams simulated in the model. The total simulated discharge to streams fluctuates seasonally between 7,500 and 17,500 Mgal/d with a mean outflow of 11,500 Mgal/d. Much of the remaining balance between modeled recharge inflows and stream outflows is made up by water moving into or out of storage in the aquifer system resulting in changes in modeled groundwater levels. The goal of the Ozark model was to develop a model capable of suitable accuracy at regional scales. The intent was not to reproduce individual local-scale details, which are typically not possible given the uniform cell size of 1 square mile. Although the Ozark model may not represent each local-scale detail, the model is relevant, and can be applied for a better understanding of the regional flow system and to evaluate responses to changes in climate and groundwater withdrawals. This USGS data release contains all of the input and output files for the model and calibration simulation described in the associated model documentation report (http://dx.doi.org/xx.xxxx/sirxxxxxxxx). This data release also includes (1) MODFLOW-NWT (v. 1.1.2) source code, (2) PEST++ source code, and (3) processing python scripts and associated instruction files for parameter estimation and model calibration using PEST++.