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# Specific yield

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Hi all,

I cant find an explanation on what is the unit that is used in specific yield filed in GMS. Is it the percentage (0-1) or soemthing else. I find getting strange result as the water gets drained faster when Sy is 0.04 in comparisson to where Sy = 0.15.

Does anyone has any idea regarding that?

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Specific yield is explained on the GMS wiki as follows (http://www.xmswiki.com/wiki/GMS:BCF_Package#Storage_coefficients): specific yield also known as the drainable porosity, is a ratio, less than or equal to the effective porosity, indicating the volumetric fraction of the bulk aquifer volume that a given aquifer will yield when all the water is allowed to drain out of it under the forces of gravity.

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Hey Bill,

thanks. I get that, BUT the problem I have trouble understanding is that by my logic, the higher the number (i.e.: efective porosity)  the FASTER you can drain the aquifer, right? But...when one inputs the number 0.05 (5%)  in comparisson to 0.15 (15%) for Sy (open aquifer) in transient model you get that drainage is a lot faster with the 5%....drawdown is a lot faster in 5%  that in 15%....and that i don't understand if it's correct.

Thanks, m

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With a smaller number the water would drain faster because there is less water to drain. An input of 0.05 means that it only takes 5% of the total cell volume in water to fill the cell, so there is less water that needs to be removed than at 0.15 or 15% total cell volume in water.

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