Subject: Irrigated Catchment Water Demand vs Potential ET? Posted: 8/13/2020 Viewed: 2646 times
Hi all, I have a question regarding the difference that is made in the results between the Water Demand (under "Demand" branch) of irrigated catchments and the Potential ET (under the "catchment" branch).
I expected both to be the same as the Demand of catchment should be what its vegetated area is requiring as the sum of PET*kc_i, with "i" the different land uses. But the the Water Demand is 5 times bigger than the ET Potential (both in m3).
Maybe Water Demand in catchments is considering other factors that I haven't understood (losses in the catchment maybe?).
I'd like some documentation about the calculation of both terms if possible. Thanks!
Mr. Eduardo Bustos
Subject: Re: Irrigated Catchment Water Demand vs Potential ET? Posted: 2/10/2021 Viewed: 2076 times
Hola amigo mío!
Just now seeing your question.
You are using "Soil moisture method" for the catchment? or MABIA method?
The main difference between two components could be (for MABIA Method) that Water Demand are related to your irrigation schedule.
For example, if you define the "irrigation trigger" as fixed time interval (e.g. irrigation each 15 days with a "fixed depth" of 50 mm), the Water Demand results will show the demand from the catchment to satisface this water volume, not the water demand from crops. Is that irrigation schedule ok for your crops? It's a suitable representation of the "real" irrigation in the field? ... is in fact some interesting "calibration" issue. (I'm on some like this question now...)
If in MABIA your irrigation trigger / amount is related to the soil moisture (or you are using "Soils Moisture Method" in your catchment) you have another element to consider. The irrigation thresholds are related with the soil moisture/water content, then some part of the irrigation demand will cover the losses from the upper bucket, as percolation or interflow, in addition to the loses as evapotranspiration from crops. So, your water demand could be easily greater than the potential ET of your crops.
Documentation? I don't know... these ideas arise from hit mi head against the model by questions like yours's.
I hope this help.
If not, send me a WhatsApp!
(By the way, with David work together some years ago, here in Chile)
Topic "Irrigated Catchment Water Demand vs Potential ET?"