Help Needed: Deriving Monthly Seasonal Water Yield Raster from InVEST Outputs (Baseflow + Quickflow)

Science, Research & Data
1

Dear NatCap Community,

I am working on a research project using the Seasonal Water Yield (SWY) model in the InVEST suite, focusing on the Chambal River Basin in India. The goal is to generate monthly raster maps of total water yield to study temporal patterns in water availability.

Current Progress:

After running the model successfully, I have obtained the following outputs:

  • Monthly Quickflow Rasters (quickflow_Q0.tif, quickflow_Q1.tif, …, quickflow_Q11.tif)
  • Annual Baseflow Raster (baseflow.tif)
  • Watershed summary shapefile (swy_result.shp)

My Research Goal:

I would like to compute monthly Seasonal Water Yield (SWY) rasters using the formula:

ini

CopyEdit

SWY_monthly = Quickflow_monthly + Baseflow_monthly

However, I understand that the baseflow is only provided as a single annual raster. To proceed, I need to estimate monthly baseflow values.

My Questions:

  1. Is it methodologically valid to compute:

java

CopyEdit

Monthly SWY = Monthly Quickflow + Estimated Monthly Baseflow?
  1. What is the recommended approach to distribute the annual baseflow raster into monthly components?
  • Can this be done proportionally based on monthly precipitation or another proxy?
  • Would this still be consistent with the assumptions of the InVEST SWY model?
  1. Has anyone implemented a workflow, methodology, or script to derive monthly baseflow rasters for this purpose?

Additional Context:

  • I am using CHIRPS monthly precipitation and ERA5-Land PET as inputs.
  • My analysis focuses on the temporal dynamics of water yield, so having monthly SWY rasters is essential.

Any guidance, recommended practices, or shared experience would be highly appreciated, as this is a critical component of my research framework.

Thank you very much for your time and help.

Warm regards,
Swarnajit Roy

2

Hi @abir -

We have been discussing these same issues in your other thread here.

In that thread, I listed two options for deriving monthly baseflow:

  1. Divide annual Baseflow equally 12 ways for each month
    All you need to do is calculate (annual baseflow / 12) and use that raster for all monthly calculations.This is most simple, and might be more appropriate in places where the rainfall is not highly seasonal.

  2. Divide annual Baseflow proportionally, based on each month’s rainfall depth
    For this, you need to add up all of the monthly rainfall rasters to get a total annual value, then calculate the proportion of each month’s rainfall with (monthly rainfall proportion = (monthly rainfall / annual rainfall)) then use the result to calculate (monthly rainfall proportion x annual baseflow). This takes an extra couple of calculations, but might be better for places with highly seasonal rainfall, like monsoons.

While the model was not made for calculating monthly baseflow, we do use it that way occasionally, being very clear in our reporting that the baseflow values are very approximate, and describing exactly how we calculated them.

~ Stacie

1 Like