# Annual water yield result problem

hi there.Thanks in advance to those who have answered my questions.my model is working succeessfully. but no matter how I adjust the zhang, they result still bigger than it in the real .looking forward your answer.thangyou !

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Hi @lif,

Glad you got the model working. Could you provide more information on the results you’re seeing and what you expect to see? How much bigger are the results than real world data?

Here’s a good summary of the zhang constant and how it interacts with the model: Water yield parameter

We’ll need to know quite a bit more about your inputs and results to help understand why you’re seeing the results you are.

Cheers,

Doug

Hi @lif -

Also remember that this model is very simplified, so it is very unlikely to produce results that match real-world observed data. And while we can use Z for calibration, the model is also dependent on all of the other inputs. So when we calibrate the model against observed data, as the User Guide notes, we will usually perform a sensitivity analysis to see which input(s) the results are most sensitive to, then adjust those inputs (see the referenced paper for more information). Often, the inputs with the most uncertainty are the root depth and Kc values, so trying different values for those can be particularly useful.

~ Stacie

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thank you again. you’re so nice and patient ,I’m really grateful for you.
go to the problem. I have found that when I put the different z parameter between 1 to 30.the results in get is gradually dwindle. but even I have turned my z to 30，the results is like 253710^9m3.and the real runoff is about 198210^9m3 .its so far away from it. I don’t know whether it matters. I mean the erro can be taken.
And I wonder know can I choose to change my kc date to fetch this result？because I checked my other dates and think they are all no mistakes.
and here my results and biophysical table,and the figures in the table refers to other studies.

thank you for your answer. and i want to know can I adjust my kc to help myself to get the result？And now my kc is based on other researcher’s paper.thanks again.

@lif, have you looked at the paper (and Appendix) referenced in the User Guide that talks about Z and Kc, and doing a sensitivity analysis with Kc?

Hamel, P., & Guswa, A. (2015). Uncertainty analysis of a spatially-explicit annual water-balance model: case study of the Cape Fear catchment, NC. Hydrology and Earth System Sciences. doi:10.5194/hess-19-839-2015

While it doesn’t go into a lot of detail, they adjust the Kc value for the dominant land cover type in the basin by -30% - +10%, run the model for each Kc value, and see how that affects the results.

~ Stacie

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Hi @lif . Would revise this paper Redirecting " Empirical validation of the InVEST water yield ecosystem service model at a national scale". The precipitation and evapotranspiration input can be uncertain.