A three dimensional finite-difference model part of the Mississippi River Valley alluvial aquifer in the Cache area, Arkansas, was constructed to simulate groundwater flow. The simulation period is from 2000 to 2010 with 23 stress periods modeled under transient flow conditions. The model area is 6,869 km2
(which is 8.3% of the total aquifer area in Arkansas) and includes parts of ten counties. The finite-difference grid consists of 294 rows, 149 columns, and one layer with cells that are 0.5 km2
. The model was constructed using Groundwater Vistas software and was calibrated using parameter estimation code and pilot points with regularization and singular value decomposition. Simulated heads were compared to 2,322 measured hydraulic heads from 150 observation wells. The root mean square error between simulated and measured hydraulic head for all observations ranged from 0.94m in 2002 to 1.45m in 2008 and averaged 1.18m for the entire simulation period. Hydraulic conductivity values from pilot point calibration ranged from 42 to172 m/day. Specific yield values ranged between 0.192 and 0.337. Recharge rates ranged between 0.0001and 0.0006 m/day. Three rivers located in the study area (Cache, L'Anguille, and Black) were simulated and the final values of river bed vertical hydraulic conductivity ranged between 0.069 and 0.1 m/day.
The groundwater flow budget for the model indicated that the average inflows to the aquifer were 36%, 35%, 23%, 6% for recharge, storage, rivers, and constant head respectively. The average outflows from the aquifer were 51%, 46%, and 3% for storage, wells, and constant head respectively. Five pumping scenarios were developed and compared to the simulated head from 2010. Scenarios 1, 2 and 3 represent maximum, minimum, and average pumping rates from irrigation wells. In scenarios 4 and 5,average pumping from wells for dry and wet years were used. For most of the scenarios, groundwater levels declined and generated model dry cells, with the most dry cells occurring in scenario 1 (52 km2converted to dry cells). An estimate of the volume of water in the aquifer was calculated from the thickness of the saturated zone for each scenario. Percent of volume of water available in each scenario relative to 2010 water levels were [-23.33, 2.52, -8.26, -8.03, and -7.82(%)] for scenario 1, 2, 3, 4, and 5 respectively.