Wells may increase the water supply available at the river at a
given time step if well return flows exceed the stream depletion.
StateMod checks for such a condition and re-operates to allow senior
ditches to benefit from the additional water supply.
Wells may require two or more delay patterns to represent the delay
associated with return flows and depletions. The data for both types of
delays are specified in the unit response file (*.dly or *.urm). When
the sum of return flows to the river is less than 100%, the balance is
treated as a loss (e.g evaporation or phreatophytes). Similarly when the
sum of depletions to the river is less than 100%, the balance is assumed
to come from ground water storage.
Wells may cause river flows to go negative when the well’s estimated
depletion to the river exceeds the streamflow. StateMod treats such an
occurrence as an indication that pumping impacts have depleted ground
water storage rather than the stream flow. Under such a case, StateMod
allows the pumping to occur and accounts for the source of water as originating
from ground water storage. This water is presented in the column From/To GW Stor
for each river node in the diversion summary output (*.xdd) and for the
whole basin in the water budget report (*.xwb). Note the quantity of water
supplied by ground water storage in a simulation time period is taken out of
the stream the next time period before any water allocation occurs. The control
file (*.ctl) variable iwell = 2 or 3 allows the repayment of this water to be
limited to a maximum amount to represent a stream ground water system that are
disconnected. Also, since data for this term is generally not observed, baseflow
calculations may be influenced by this lack of data.
Well information is presented in four columns of the diversion summary
report (*.xdd). The column titled From Well describes the total amount of
water pumped and made available to a diversion. The column titled Well Depletion
represents the impact of a previous months pumping on the river. The column titled
To/From GW Stor was described above. The column titled River by Well represents
the impact of the current months pumping on the river. The Well Depletion and
River by Well data are separated because the impact of a previous months pumping
on the river influences the water supply available to all users before any
diversions occur while the impact of the current months pumping impacts water
rights that are junior to the well only. Note by definition, a well structure
that is not tied to a diversion has no data under the column From Well. However,
the columns titled Well Depletion and River by Well include the impact of all
well pumping on the river.