# How to calculate Z parameter

**URL:** <https://community.naturalcapitalalliance.org/t/how-to-calculate-z-parameter/6224>\
**Category:** Science, Research & Data\
**Tags:** annual-water-yield\
**Created:** [October 6, 2025, 6:58pm UTC](https://community.naturalcapitalalliance.org/t/how-to-calculate-z-parameter/6224 "2025-10-06T18:58:39Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![MarieEveC](https://avatars.discourse-cdn.com/v4/letter/m/a88e57/32.png) [@MarieEveC](https://community.naturalcapitalalliance.org/u/MarieEveC)\
**Post date:** [October 6, 2025, 6:58pm UTC](https://community.naturalcapitalalliance.org/t/how-to-calculate-z-parameter/6224/1 "2025-10-06T18:58:39Z")

</div>

Hi! I keep encountering problems while trying to figure out the value of Z parameter for my Annual Water Yield model.

Here’s what I have done yet and the data I’m using:

- I used the ISRIC SoilGrids 2017 AWC data and used the treatment suggested in the user guide. I have both a version that’s a fraction and one that’s on a 1-100 scale as percents.
- For the root restricting layer depth, I used the ISRIC SoilGrids250m 2017-03 - Depth to bedrock [as suggested by user Esoth here](https://SoilGrids250m%202017-03%20-%20Depth%20to%20bedrock%20(R%20horizon)).
- I used local monthly precipitations data and combined the rasters to get the annual value per pixel.
- I used figure 3A from [this article](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2013GL058324) referenced in the user guide to estimate the _ϖ value._

I’m not sure I understand correctly what to do with these data. Do I just get the mean for my AOI for all the parameters and then proceed to do the equation presented in the user guide?

Here are the mean values I get for my AOI  
Annual precipitations: 842 mm  
PAWC: 0.205361  
AWC: 1878.1066  
ϖ: 1.6

Thanks for your help 🙂

---

<div class="post-metadata">

**Author:** ![jdouglass](https://yyz1.discourse-cdn.com/flex031/user_avatar/community.naturalcapitalalliance.org/jdouglass/32/9_2.png) [@jdouglass](https://community.naturalcapitalalliance.org/u/jdouglass)\
**Post date:** [October 7, 2025, 11:58pm UTC](https://community.naturalcapitalalliance.org/t/how-to-calculate-z-parameter/6224/2 "2025-10-07T23:58:28Z")

</div>

Hello @MarieEveC and welcome back to the forums!

I’m not an expert in the Annual Water Yield model so I will gladly defer to others on the forums here who have used it, but as I understand the [Z parameter section of the AWY user’s guide chapter](https://storage.googleapis.com/releases.naturalcapitalproject.org/invest-userguide/latest/en/annual_water_yield.html#z-parameter), yes, I think you could just take these values you have collected and then place them into those equations to calculate the Z parameter. There are also some other methods for estimating the Z parameter described in the paragraph, and it’s possible that the literature might also contain an equation for calculating Z parameter for your study area.

Note that, as mentioned in the UG chapter, it would also be a really good idea to compare the modeled outputs with real-world outputs and then calibrate the parameter if needed to make the outputs align more closely with reality.

Hope this helps,

James

---

<div class="post-metadata">

**Author:** ![swolny](https://yyz1.discourse-cdn.com/flex031/user_avatar/community.naturalcapitalalliance.org/swolny/32/181_2.png) [@swolny](https://community.naturalcapitalalliance.org/u/swolny)\
**Post date:** [October 8, 2025, 12:27am UTC](https://community.naturalcapitalalliance.org/t/how-to-calculate-z-parameter/6224/3 "2025-10-08T00:27:12Z")

</div>

Hi @MarieEveC -

> Do I just get the mean for my AOI for all the parameters and then proceed to do the equation presented in the user guide?

Yes. Once you’ve determined ϖ, [the User Guide](https://storage.googleapis.com/releases.naturalcapitalproject.org/invest-userguide/latest/en/annual_water_yield.html#z-parameter) says:

> P and AWC should be average values of Precipitation and Available Water Capacity, respectively, in the study area.

Perhaps we can find a way to describe this more clearly in the User Guide.

~ Stacie

---

<div class="post-metadata">

**Author:** ![system](https://canada1.discourse-cdn.com/flex031/uploads/naturalcapitalproject/original/2X/8/8d95591aa074da87ea0fdd61ca47c1c3c0588aa7.png) [@system](https://community.naturalcapitalalliance.org/u/system)\
**Post date:** [January 6, 2026, 12:27am UTC](https://community.naturalcapitalalliance.org/t/how-to-calculate-z-parameter/6224/4 "2026-01-06T00:27:48Z")

</div>

Closed on January 6, 2026, 12:27am UTC

This topic was automatically closed 90 days after the last reply. New replies are no longer allowed.