I’m interested in calculating the total water savings (stored in freshwater features or groundwater recharge) from planting 1000HA forest. Then I would multiply water savings x shadow price (or tariff) to approximate the economic benefits from this.
Would the Invest Water Yield tools allow me to do this?
Well, the thing about planting forest is that it often evapotranspires more water than whatever other land cover type was there before, which leads to a decrease in modeled water supply. Particularly with our models, which are very simple.
It may very well be that there is a good tradeoff between forest using more water, but also contributing to more infiltration, dry season baseflow and aquifer recharge, which may contribute positively to water provision during the dry season, for people using wells, reducing flooding etc.
The Annual Water Yield model does not account for any of this, it does not differentiate at all between the amount of water that runs off quickly (quickflow) versus more slowly (baseflow) versus infiltration (recharge). It essentially calculates precipitation minus evapotranspiration per pixel, and assumes that whatever is left becomes available in some way (quickflow, baseflow, recharge) within a year.
Seasonal Water Yield does provide some differentiation. It maps the areas that provide more or less baseflow annually, and includes a “local recharge” output to indicate where infiltration may be contributing to groundwater recharge (although the model does not include anything specifically about aquifer properties, so it’s an indicator more than a quantifier). If you’re interested in the value of the forest for reducing downstream flooding, a change in the “quickflow” result can be used for this. The quickflow calculation does not involve evapotranspiration (just the properties of the land cover and soil for runoff versus infiltration during or soon after rainfall), so its result is perhaps more intuitive.
One last thing is that if you’re putting a monetary value on the result, you’ll want to calibrate the model with real-world data so you feel more confident that the absolute values are aligned with actual water yield. The User Guide includes a chapter on calibration that can help get started.
It would be great to hear from anyone who has used one of the Water Yield models for economic valuation - how did you approach it?
Thanks very much for this detailed answer. It’s very helpful.
Point noted on forests as a method for increasing water supply. It sounds like the Seasonal Water Yield and “local recharge” output indicator might be the best – very rough – approximation for what I need.
And yes – anyone else who’s addressed this problem please do let me know! I’m really trying to avoid doing a detailed hydrologic model (which somebody else would have to do, since I’m not a hydrologist) since the outputs of my study are really pre-feasibility level general estimates.
Any similar experiences tackling this problem is much appreciated. And, maybe opening the question even wider for anyone who wants to add something: any advice on how to measure/quantify NBS benefits for drought reduction, also very much appreciated.