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Stream routing using STR1


KevinSpelts
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Greetings:

I have a question I hope you can help me with.

I have two hypothetical models whereby I have a mainstem stream traverse the single layer grid from left to right. 4 separate tributary streams join the mainstem at various and unique locations along the mainstem stream. The main stem and all headwater segments of each of the tributaries have incoming flow. I used GMS 6.0 to execute MODFLOW 2000 and the stream routing STR1 package.

If I understand the operation of the STR1 Package correctly, stream routing and stream discharge is sequentially ordered from lower- to higher-numbered segments. Outflow from segment 1 is routed to the next higher numbered segment; this occurs even if these higher numbered segments form portions of separate tributaries. Thus outflow from segment 5 of tributary one will be routed to segment 6 even if segment 6 is the headwater segment of a tributary two; if my understanding is correct, this seems physically unrealistic.

I can create two separate models to simulate stream-aquifer interactions of my hypothetical model.

(1) GMS allows me to create a simulation with a single coverage containing a main stem stream with 4 tributary streams. The segments that represent the mainstem and tributaries are created from left to right. Thus segment 1 occurs on the left side of the grid whereas segment 30 occurs on the right side of the grid.

(2) On the other hand, GMS will allow me to create a simulation with 5 separate coverages; one for the main stem and separate coverages for each tributary. Thus, I created 5 separate coverages in order to keep the outflow from a low numbered segment of a tributary stream from contributing stream flow to a high numbered headwater segment of a separate tributary stream.

Other than the method I used to depict streams and stream routing, the models are identical in parametrization (e.g., identical parameters of starting head, hydraulic conductivity, specific yield, and so on). It should be noted that the number of stream segments and incoming flow distribution is different for these two models.

I compare the simulation output for each of the models and there is a difference in the rates and volume of stream leakage inputs to and stream leakage outputs from the aquifer between these two simulations (the difference is probably statistically significant for these simulations).

If I understand correctly, stream leakage is simply input to the “W” term of the ground water flow equation.

My question: Is there a prohibition in MODFLOW from depicting tributary streams using separate coverages?

Thank you in advance for your time and attention.

Kevin

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