Water Yield output per pixel increases with deforestation. Why?

Hi all,
Could you please help me to answer the following question?
My question is about the water yield model output. In the model, if I change the type of land use, for example from agricultural land to forest land (but keep all other parameters), the water yield output per pixel is decrease (because of increasing Kc, I guess). following the output, should we change the land use type from forest to agriculture? But In a really life, forest should better than agriculture or grassland, is it true?

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You’re exactly right, @gis4sd, forest generally uses more water (and yes, you can check the Kc value to see if this is true in your case), so you will see a decrease in water yield with reforestation. It’s counter-intuitive, because we want to think that more forest is better for everything. In reality, it may also be true that the forest is contributing to improved baseflow in the dry season, because it provides good infiltration and slows down the water so it arrives in streams later, and this is a benefit that may outweigh the total annual reduction in water yield. But the annual water yield model (is that the one you’re using?) will not show this because it is only an annual model, not monthly or seasonal, and is very simple, it does not calculate baseflow. Even our seasonal water yield model (which does include baseflow) doesn’t always show this dynamic, again because of its simplicity - it provides baseflow at an annual time scale and as an index, not a quantity.

~ Stacie

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Hi Swolny,
Thank you for your clarification.
Yes, I checked Kc value table and Kc is the most affect factor in this case.
Yes, I use annual water yield model.
I really appreciate Invest model but it does not fit my research in the case. what is a pity.
If you have any ideal to help me pass my problem, please help (suggest another approach or model…)
many thank.

Ngô Đăng Trí.

Hi @gis4sd -

The InVEST models will definitely not fit every need. We have done quite a few projects where we (or a partner) uses other models because they are the right tool for the job. One of these is the SWAT model, which works on a daily time step, and includes more of the complexity that’s happening in a watershed. The tradeoff is that it requires a lot more data and time to use. There are other models as well, but I’ve not personally used any of them (including SWAT) and I’m not a hydrologist, so I’ll leave it to others with more experience to make recommendations.

~ Stacie

Hi Swolny,
It is very sorry because of lately reply.
Thank you very much. Your advice is really precious to me.
Wish you have a nice weekend.
Any other advice is appreciate.
Thank you.

Ngo Dang TRI