Subject: Irrigation Demands Only vs Rainfall Runoff Posted: 6/5/2021 Viewed: 3235 times
Hello! I´ve got a question about the runoff generated from precipitation in catchments modeled with these two options.
I've made a little WEAP model to prove the differences, there's one irrigation demands only catchment and a rainfall runoff one, both with a transmission link from a river and runoff/infiltration link back to it. I gave them 100 ha. area with 20mm/month ETref, Kc=1, irrigation fraction=0.5, and 10 mm/month precipitation with 80% effective precipitation.
When I check results turns out that water demand is the same for both as expected (20mm/month - 0.8 * 10mm/month)/0.5)*100ha= 24000 m3. But the water that returns to the river is different, it's 14000 m3 for the rainfall runoff and 12000 m3 for the irrigation only. I know that 12000 m3 comes from the water not used by crops because of irrigation fraction (24000 m3/ 0.5), and the additional 2000 m3 in the rainfall runoff comes from the fraction of not effective precipitation (0.20 * 10mm).
Now my question is what happens to those 2000 m3 in the irrigation demands only catchment, if it goes out of the model or where does it goes?
Thanks for the help.
Ms. Anne Hereford
Subject: Re: Irrigation Demands Only vs Rainfall Runoff Posted: 7/18/2021 Viewed: 2993 times
The Irrigation Only method should not return any runoff. (From the WEAP Help: The Irrigation Demands Only Method is the simplest [and]...does not simulate runoff or infiltration processes, or track changes in soil moisture). The water that would have been considered runoff disappears from the system (or goes out of the model, as you said it).
With this question you called Jack's attention to a bug in WEAP that allowed runoff when a runoff/infiltration link was added to a model with irrigation fraction less than 100%. This has been fixed in the new version of WEAP.
All the best,
Eng. Rodrigo Urquiza
Subject: Re: Irrigation Demands Only vs Rainfall Runoff Posted: 7/19/2021 Viewed: 2986 times
Thanks Anne, it's nice to know that the question helped to find the bug.
So with this new version if I want to keep water not used by crops in the model and return it to the river by the drainage network (with Runoff/Infiltration links), I should use the Rainfall runoff method right?
Ms. Anne Hereford
Subject: Re: Irrigation Demands Only vs Rainfall Runoff Posted: 7/21/2021 Viewed: 2929 times
Yes, both of the Rainfall Runoff methods will allow you to model runoff staying in the system/returning to the river.
Topic "Irrigation Demands Only vs Rainfall Runoff"