Subject: Irrigation return flow Posted: 11/5/2010 Viewed: 22260 times
I am changing catchment nodes representing agricultural land in an existing model to demand site nodes. I plan on calculating agricultural return flow fraction from the demand sites based on results for the respective catchments from the existing model. Because demand site return flow = inflow * (1-consumption), I think I only need to calculate consumption rate for each catchment to calculate the value for each return flow links of the new demand sites. (In this case, inflow is simply the 'supply delivered'.)
I plan to calculate consumption as:
(1 - catchment irrigation return flow fraction) * 100.
If catchment return flow is divided among streams (or between stream and groundwater), I could calculate the 'return flow routing' percent by the relative proportion of catchment return flow fraction to each supply source.
Do think that this approach is appropriate, or am I forgetting something?
Subject: Re: Irrigation return flow Posted: 11/8/2010 Viewed: 22259 times
I think this will work. Are you going to compute the consumption for each timestep (month?), or just an average over the time horizon?
Subject: Re: Irrigation return flow Posted: 11/9/2010 Viewed: 22245 times
I needed to include monthly time variation as well. So, after thinking about it more, I found greater ease in computing and specifying monthly return flows as “Surface Water Inflow” for any particular stream reach instead of using a return flow link. I did this with “MonthlyValues(..)/days * 0.504” as an expression to convert monthly Acre-feet amounts to Cubic Feet/Second.
Hopefully, I didn’t overlook anything here either.
Subject: Re: Irrigation return flow Posted: 11/9/2010 Viewed: 22244 times
And I suppose you will set the demand sites to consume 100% of their inflow, so that you won't double count the return flows?
Subject: Re: Irrigation return flow Posted: 11/9/2010 Viewed: 22241 times
Yes, consumption is 100%. Even if consumption was less than 100%, I am assuming that return flows would only be affected if a return flow link was added to the newly added demand site. Please correct me if I am wrong. I wonder what other effects consumption less than 100% might have on the results though (even without a return flow link).
Subject: Re: Irrigation return flow Posted: 11/9/2010 Viewed: 22240 times
If consumption < 100% but there are no return flow links, WEAP will give a warning when calculating, but the results should be the same as if consumption = 100% (except for the Demand Site Inflows & Outflows report for consumption).
Subject: Re: Irrigation return flow Posted: 11/9/2010 Viewed: 22233 times
I noticed that for any given existing catchment (before converting to demand sites), the runoff/infiltration link is upstream of the transmission link. As a result, it seems that return flows are part of the runoff to the stream reach above the transmission link, effectively making return flows again available for diversion. This would make sense if you are assuming that return flows can be reused within a catchment. Is my interpretation correct?
Subject: Re: Irrigation return flow Posted: 11/9/2010 Viewed: 22232 times
You are correct in your assumption. I would caution against creating a loop like this--depending on the fraction returned, it could allow WEAP to reuse the water many times within each timestep, which may not be realistic.