Subject: Water Demand and Irrigation Posted: 4/11/2017 Viewed: 2023 times
I use the soil moisture method for WEAP model. For irrigation systems I calibrate the model by lower and upper thresholds under the irrigation tab.The data for losses is about 20%. In the agriculture catchment the data for water demand is much bigger of irrigation in the map results (about 2 times). In your opinion, what is the cause? and what is the relation between water demand and irrigation mathematically?
and if there is no constraint for water supply, should the transmission link flow is the amount of water demand?
Subject: Re: Water Demand and Irrigation Posted: 4/11/2017 Viewed: 1926 times
Let me start out by explaining how the upper and lower thresholds work for determining irrigation in WEAP (the relationship between water demand and irrigation). Think of the upper threshold as your goal for soil moisture, but your system has ability to tolerate less soil moisture (but only until the lower threshold).
As your system dries, WEAP won't irrigate until your soil moisture reaches the lower threshold. Then WEAP will demand enough irrigation water to bring the system soil moisture back up to the upper threshold.
This is an imperfect calculation because there are other processes happening at the same time - for example, if there is precipitation in the same time step, the soil moisture in the next time step (after irrigation) will probably have overshot the upper threshold by at least little bit.
So many things matter here. You can imagine that if you have a large gap between the upper and lower thresholds, WEAP will irrigate only occasionally, but in very large amounts. Conversely, if you have a small gap between your upper and lower threshold, the system will frequently demand small amounts of irrigation. It also matters where the gap is: A gap between 10%-20% will demand less water than a gap between 80%-90%.
To answer to your question, you say you have calibrated your upper and lower thresholds. To me that means you already have data for irrigation, and your model matches that data. Is that right?
And you're saying that the map results shows double the amount of irrigation? Are you looking at the same months for your comparison? The map will default to showing you the first time step of your model. Are you looking at water demand results for both?
To answer the last question, if there is no constraint in water supply *for the time step in question* and no constraint on transmission link capacity, the flow should be the amount of water demand. Notice that the Supply Requirement (another result - the water that must be withdrawn from the source to supply demand) will be more to account for the losses in your system.
Subject: Re: Water Demand and Irrigation Posted: 4/13/2017 Viewed: 1919 times
Thank you very much for your answer and patience. I don't notice exactly about the issue. If the model is calibrated correctly for irrigation systems according to upper and lower thresholds and compare the results in the result map annually, should water demand is equal irrigation or not?
Water demand is usually equal to or smaller or larger irrigation?
Is the irrigation same water demand?
Subject: Re: Water Demand and Irrigation Posted: 4/13/2017 Viewed: 1915 times
So think about the structure of the catchment in WEAP. When you first build a catchment, the General Menu asks you "includes irrigated areas?" If you don't click that box, then the catchment has no demand priority, because it has no demand - it's an area modeling how rain turns into streamflow, and it's colored green, like the other supplies in WEAP.
However, as soon as you say that you'll model irrigation, the catchment also becomes a demand, because it has demand for irrigation water. Therefore, in a *catchment*, the irrigation demand is the water demand for that node.
Catchment irrigation is just one type of demand in your WEAP system. You may have many others. So just remember that all catchment demand is irrigation, but there can be other types of demand elsewhere.
Subject: Re: Water Demand and Irrigation Posted: 4/13/2017 Viewed: 1908 times
Thank you very much for your comprehensive answers.