I am currently working on a project to study the regional water conservation capacity. Since the potential evaporation of the water area within the study zone is greater than the precipitation, if I set the kc parameter to a value close to 1, the annual water yield of the water area will be negative and output as 0 in the result.
Based on this result for further calculation, does it mean that the water conservation capacity of the water area is worse than the impervious? If I reduce the kc parameter to a value close to 0, does it violate the principles of the model and parameter setting?
It is a real thing that plants take up water, and subtract it from the yield that we might use, and in impervious areas most of the rainfall just runs off. It’s also the case that the annual water yield model is a very simplified rainfall/runoff model, and does not include any of the other things that happen in the vegetated watershed that benefit us, like groundwater recharge, baseflow in the dry season, etc.
So it often happens that this model produces a result that is not necessarily intuitive, or what we’d like it to say - that we get more water yield from impervious areas than, say, forests. A more complex model would also show the groundwater recharge, baseflow, etc that the forests provide, to show the more nuanced processes that are actually happening in the watershed. Our Seasonal Water Yield model takes it a step further by providing runoff at a monthly level, and an index of which areas provide more infiltration and baseflow.
I would recommend setting the Kc values to the correct evapotranspiration coefficients for the different vegetation types, so you are using the model as it is intended, and you are being clear about the reality of plants using water. Then when reporting your results, you need to emphasize the limitations of the model, and note that it is only showing simple rainfall/runoff, it is not informing us where those areas are providing other hydrologic services that might be very important in that place. If you need a more nuanced view of how the water conservation area affects water yield, you’ll need to use a more complex model.