Abstract
The Ogallala aquifer is an important water resource for
the Rosebud Sioux Tribe in Gregory and Tripp Counties in
south-central South Dakota and is used for irrigation, public
supply, domestic, and stock water supplies. To better understand
groundwater flow in the Ogallala aquifer, conceptual and
numerical models of groundwater flow were developed for the
aquifer. A conceptual model of the Ogallala aquifer was used
to analyze groundwater flow and develop a numerical model
to simulate groundwater flow in the aquifer. The MODFLOW–NWT model was used to simulate transient groundwater conditions
for water years 1985–2009. The model was calibrated
using statistical parameter estimation techniques. Potential
future scenarios were simulated using the input parameters
from the calibrated model for simulations of potential future
drought and future increased pumping.
Transient simulations were completed with the numerical
model. A 200-year transient initialization period was used to
establish starting conditions for the subsequent 25-year simulation
of water years 1985–2009. The 25-year simulation was
discretized into three seasonal stress periods per year and used
to simulate transient conditions.
A single-layer model was used to simulate flow and mass
balance in the Ogallala aquifer with a grid of 133 rows and
282 columns and a uniform spacing of 500 meters (1,640 feet).
Regional inflow and outflow were simulated along the western
and southern boundaries using specified-head cells. All other
boundaries were simulated using no-flow cells. Recharge to
the aquifer occurs through precipitation on the outcrop area.
Model calibration was accomplished using the Parameter
Estimation (PEST) program that adjusted individual model
input parameters and assessed the difference between estimated
and model-simulated values of hydraulic head and base
flow. This program was designed to estimate parameter values
that are statistically the most likely set of values to result
in the smallest differences between simulated and observed
values, within a given set of constraints. The potentiometric
surface of the aquifer calculated during the 200-year initialization
period established initial conditions for the transient
simulation. Water levels for 38 observation wells were used
to calibrate the 25-year simulation. Simulated hydraulic heads
for the transient simulation were within plus or minus 20 feet
of observed values for 95 percent of observation wells, and the
mean absolute difference was 5.1 feet. Calibrated hydraulic
conductivity ranged from 0.9 to 227 feet per day.
The annual recharge rates for the transient simulation
(water years 1985–2009) ranged from 0.60 to 6.96 inches,
with a mean of 3.68 inches for the Ogallala aquifer. This
represents a mean recharge rate of 280.5 ft3/s for the model
area. Discharge from the aquifer occurs through evapotranspiration,
discharge to streams through river leakage and
flow from springs and seeps, and well withdrawals. Water is
withdrawn from wells for irrigation, public supply, domestic,
and stock uses. Simulated mean discharge rates for water
years 1985–2009 were about 185 cubic feet per second (ft3/s)
for evapotranspiration, 66.7 ft3/s for discharge to streams, and
5.48 ft3/s for well withdrawals. Simulated annual evapotranspiration
rates ranged from about 128 to 254 ft3/s, and outflow
to streams ranged from 52.2 to 79.9 ft3/s.
A sensitivity analysis was used to examine the response
of the calibrated model to changes in model parameters for
horizontal hydraulic conductivity, recharge, evapotranspiration,
and spring and riverbed conductance. The model was
most sensitive to recharge and maximum potential evapotranspiration
and least sensitive to riverbed and spring
conductances.
Two potential future scenarios were simulated: a potential
drought scenario and a potential increased pumping
scenario. To simulate a potential drought scenario, a synthetic
drought record was created, the mean of which was equal to
60 percent of the mean estimated recharge rate for the 25-year
simulation period. Compared with the results of the calibrated
model (non-drought simulation), the simulation representing
a potential drought scenario resulted in water-level decreases
of as much as 30 feet for the Ogallala aquifer. To simulate the
effects of potential future increases in pumping, well withdrawal
rates were increased by 50 percent from those estimated
for the 25-year simulation period. Compared with the
results of the calibrated model, the simulation representing an increased pumping scenario resulted in water-level decreases of as much as 26 feet for the Ogallala aquifer.
Groundwater budgets for the potential future scenario simulations were compared with the transient simulation representing water years 1985–2009. The simulation representing a potential drought scenario resulted in lower aquifer recharge from precipitation and decreased discharge from streams, springs, seeps, and evapotranspiration. The simulation representing a potential increased pumping scenario was similar to results from the transient simulation, with a slight increase in well withdrawals and a slight decrease in discharge from river leakage and evapotranspiration.
This numerical model is suitable as a tool that could be used to better understand the flow system of the Ogallala aquifer, to approximate hydraulic heads in the aquifer, and to estimate discharge to rivers, springs, and seeps in the study area. The model also is useful to help assess the response of the aquifer to additional stresses, including potential drought conditions and increased well withdrawals.