Subject: Reservoir Release Requirement and Maximum Hydraulic Outflow Posted: 2/18/2015 Viewed: 19491 times
I have a few questions about defining reservoir release requirements and WEAP’s Maximum Hydraulic Outflow.
I have a WEAP model with a reservoir on a river and multiple downstream demands with transmission links from the river to demands representing percolation ponds.
I have a prescribed/required reservoir environmental release requirement (not greater than and not less than) based on reservoir storage at the end of the previous month. For example, if storage is >= 5000 acre-feet, then release 5 cubic feet/second, else if storage >= 2000 acre-feet, then release 2 cubic feet/second, else release 1 cubic feet/second.
I’m using a minimum flow requirement just downstream of the reservoir to define this prescribed release. However, because the downstream demands may exceed the WEAP minimum flow requirement, I’m also using a maximum hydraulic outflow on the reservoir in an attempt to limit the total monthly release for the minimum flow requirement, so that it is equal to the prescribed/required release.
First, is there a better way to do this? I’ve considered varying the demands to equal the prescribed/required release, but this can mask unmet demands, which may be important to know.
Second, it appears that the maximum hydraulic outflow may be exceeded when the downstream demands are greater than the minimum flow requirement/required release. The downstream demands have a higher priority than the reservoir’s storage priority, but I thought the maximum hydraulic outflow could not be exceeded (regardless of priorities), unless the reservoir is spilling. To compensate for this, I’ve revised the reservoir storage priority to be equal to the demand priorities. Is my assumption about how the maximum hydraulic outflow works correct – that it is a physical limitation and should not be exceeded unless the reservoir is spilling?
Lastly, does the maximum hydraulic outflow apply only to releases to the river downstream (through a dam’s outlet works) or does the maximum also include any withdrawals from the reservoir for transfer, via canal, to a neighboring reservoir for example? There are times when I would want a prescribed release downstream of a reservoir, but would also want the ability to transfer water to this other reservoir.
I hope this makes sense.
Mr. Jack Sieber
Subject: Re: Reservoir Release Requirement and Maximum Hydraulic Outflow Posted: 2/20/2015 Viewed: 19462 times
Your approach should work -- set both a minimum flow requirement (below the reservoir) and a maximum hydraulic outflow (MHO) on the reservoir to the same value, and that should be the amount released from the reservoir (unless the reservoir didn't have enough water to release, or the reservoir filled up and began to spill). I do not know why it is not working (the MHO is a constraint, regardless of priorities). If you can send me your area I can take a look.
Instead of using a maximum hydraulic outflow you could have created a diversion (orange line) starting below the reservoir that had the downstream demands drawing from it. Setting the maximum diversion for this diversion would have limited the amount that could be diverted for the demands, which would have limited the amount the reservoir released to satisfy their demands.
And to answer your last question: Yes, the maximum hydraulic outflow applies only to releases to the river downstream (through a dam's outlet works), not the direct withdrawals by transmission link from the reservoir.
Mr. Jack Sieber
Subject: Re: Reservoir Release Requirement and Maximum Hydraulic Outflow Posted: 9/9/2015 Viewed: 17937 times
I have an update on possible problems with MHO's. Because an MHO represents a non-linear threshold, WEAP must use integer variables in the LP (linear program) to model it. The LP solver that WEAP uses sometimes has problems solving models with integer variables, which leads to LP infeasibility. If this happens, WEAP will remove the MHO constraint for the timestep and continue solving. This means that there would be no MHO constraint on the maximum outflow from the reservoir in that timestep, and flows might be abnormally large.
If any MHO's were removed, they would be listed on the Messages tab in the Result View.
Topic "Reservoir Release Requirement and Maximum Hydraulic Outflow"