User's guide of Proceessing Modflow 5.0

Processing Modflow 263
6.4 Automatic Calibration and Pumping Test
6.4.1 Basic Model Calibration Skill with PEST/UCODE
Folder: \pm5\examples\calibration\calibration1\
Overview of the Problem
Groundwater models are usually applied to conceptualize and understand a hydrologic system
or to predict the outcome of a future change to the system. In order to provide some assurance
that the model reflects the behavior or appearance of the flow system, it must be calibrated prior
to use as a predictive tool. Calibration is accomplished by finding a set of parameters, boundary
conditions, and excitations or stresses that produce simulated heads (or drawdowns) and fluxes
that match measurement values within an acceptable range of error. Model calibration can be
performed by the hand-operated trial-and-error adjustment of aquifer parameters or by inverse
models such as PEST, UCODE, MODINV (Doherty, 1990) or MODFLOW/P (Hill, 1992). This
example provides an exercise in model calibration with PEST and UCODE. Specific details of
this example are from Andersen (1993).
Fig. 6.35 shows the idealized flow system and locations of observation bores. The flow
system is a small, confined aquifer which is strongly controlled by the river which across it. The
aquifer is approximately 100 ft thick and is composed primarily of silty sand. The river is not in
direct hydraulic connection with the aquifer, but acts as a leaky boundary condition which can
gain or lose water to the aquifer. Stage data for the river and riverbed elevation are listed in
Table 6.2. Other boundary conditions are no-flow, which surround the square and define the
areal extent of the aquifer.
Given constraints of uniform transmissivity and recharge, and additional data below, obtain
a steady state calibration based on the measurements listed in Table 6.3.
Initial hydraulic head = 100.0 ft
grid size = 15× 15
)x = )y = 500 ft
river base flow at western model boundary = 10 cfs
river base flow at eatern model boundary = 11.125 cfs
2
riverbed conductance = 0.01 ft /s
6.4.1 Basic Model Calibration Skill with PEST/UCODE