User's guide of Proceessing Modflow 5.0

Processing Modflow 95
MODFLOW < Time-Variant Specified-Head
For transient simulations, the Time-Variant Specified-Head package (Leake and Prudic, 1991)
allows fixed-head cells to take on different head values for each time step. The following
required data are spcified by using the Time-Variant Specified-Head Package dialog box (Fig.
3.26) of the Data Editor.
- Flag [-]. A non-zero value indicates that a cell is specified as a time-variant specified-head
boundary.
- Start Head h [L]. This value is the head in the cell at the start of the stress period.
s
- End Head h [L]. This value is the head that will be assigned to the cell for the last time
e
step in the stress period.
This package does not alter the way fixed-head boundaries are formulated in the finite-difference
equations of MODFLOW. It simply sets the element in the IBOUND array to a negative value
for all cells where a time-variant specified-head boundary is selected (Flag…0). The package
linearly interpolates boundary heads h for each time-variant specified-head boundary cell by
using the equation:
h ' h s % (h e & h s )@ PERTIM
PERLEN
Fig. 3.26 The Time-Variant Specified-Head Package dialog box
(3.25)
where PERTIM is the staring time of a time step in a stress period and PERLEN is the length
of the stress period. The interpolated heads are constant during a time step. If a cell is specified
as a time-variant specified-head boundary for a stress period and ommitted in the specification
for a subsequent period, it remains a fixed-head boundary with a head equal to that at the end of
the pervious period.
3.6.1 MODFLOW