Subject: water year method Posted: 7/21/2021 Viewed: 2017 times
how can compute the water year ratio that I need to input in WEAP. I have done the 5 bins and I know which years are normal, wet, dry. but I cant figure out how to find the ratios.
Ms. Anne Hereford
Subject: Re: water year method Posted: 8/2/2021 Viewed: 1983 times
These ratios are up to you. Suggested values are in WEAP by default. There is really no "right" answer--if you use lower numbers for Very Dry and larger values for Very Wet, it just means that you are modeling more extreme years. You can also vary the ratio throughout the year (by month) if that is more representative of your local weather patterns.
Mr. Mulugeta Taye
Subject: Re: water year method Posted: 10/22/2021 Viewed: 1541 times
The water year values can be determined as stated by Anne once you determined your normal years. My question to you and others is, how these normal years can be identified? Or what factors should be taken into account to determine the normal years? Should we consider demand factors together with water supply/availability?
Ms. Anne Hereford
Subject: Re: water year method Posted: 10/26/2021 Viewed: 1498 times
The water year method designations are only meant to represent environmental inputs/outputs. I.e. the weather. A normal year ends up being everything that falls in between what you designate as "Dry" and "Wet".
I suggest the following:
1) Use historical data to get monthly average streamflow (or precipitation if you don't have streamflow data).
2) Decide where you want to cut off Normal, Wet, and Dry. This will depend on what you're interested in learning from the model and on local conditions. Is there a system of dams that helps buffer climatic conditions? Does a high percentage of impermeable surfaces in your urban watershed mean that even a slightly wet year leads to serious flooding problems? For example, you might choose to designate the middle 50% of years as Normal--25% of years are drier and 25% of years are wetter--and decide that just 5% of years should count as "very dry" (and 5% as "very wet").
3) Go back to your monthly average data and look at the cutoffs. Take the average of the driest year classified as normal and the wettest year classified as dry. Divide this value by the average monthly streamflow (or precip). This will give you the decimal value to input for "Dry" in the Water Year Definitions tab.
4) Determine the values for the other water year types similarly.
While demand is input separately, major fluctuations in demand might have an impact on your historical streamflow values. If you know this has been an issue in your watershed, you could use precipitation data instead.