User's guide of Proceessing Modflow 5.0

96 Processing Modflow
MODFLOW < Well
An injection or a pumping well is defined by using the Data Editor to assign two values to a
model cell:
3 -1
- Rechare rate of the well (Q) [L T ] and
- Parameter Number [-]
Negative cell values for the “Rechare rate of the well” are used to indicate pumping wells,
while positive cell values indicate injection wells. The parameter number is used to assign Q as
a parameter for an automatic calibration by the inverse models PEST or UCODE, see PEST <
Parameter List... or UCODE < Parameter List....
The injection or pumping rate of a well is constant during a given stress period and is
independent of both the cell area and the head in the cell. It is implicitly assumed by MODFLOW
that a well penetrates the full thickness of the cell. MODFLOW can simulate wells that
penetrates more than one model layer. In this case, the injection or pumping rate for each layer
has to be specified. The total injection or pumping rate for a multilayer well is equal to the sum
of those from the individual layers. For confined layers, the injection or pumping rate for each
layer (Q ) can be approximately calculated by dividing the total rate (Q ) in proportion to the
k total
layer transmissivities (McDonald and Harbaugh, 1988):
Q k ' Q total @ T k
ET
3.6.1 MODFLOW
(3.26)
where T is the transmissivity of layer k and GT is the sum of the transmissivities of all layers
k
penetrated by the multilayer well.
An other possibility to simulate a multi-layer well is to set a very large vertical hydraulic
conductivity (or vertical leakance), e.g. 1 m/s, to all cells of the well. The total pumping rate is
then assigned to the lowerst cell of the well. For display purposes, a very small pumping rate
-10 3
(say, -1×10 m /s) can be assigned to other cells of the well. In this way, the exact extraction rate
from each penetrated layer will be calculated by MODFLOW implicitly and the value can be
obtained by using the Water Budget Calculator. See Chapter 2 for how to calculate subregional
water budgets.