Question on low sensitivity of water balance to rooting depth root profile parameters in ATS

[Joe Lee]

Hello ATS developers,

I am running the latest version of ATS and experimenting with the vegetation root parameters using one of the official ATS example cases.

I would like to ask for some clarification regarding the apparent low sensitivity of the integrated water balance to rooting depth–related parameters.

Setup summary

• ATS version: latest (current main / release)

• Case: official ATS example case (unmodified baseline)

• Daymet: data/daymet_TaylorRiverCO.h5

• Two sets of experiments:

  1. Modify rooting depth max (e.g., from 15 m to 10 m) while keeping all other settings identical.

  2. A custom sensitivity test with shallower effective rooting depths and different root profile parameters (alpha / beta).

Diagnostics examined

• Integrated water balance quantities:

  • Sum P, Sum ET, Sum Q

  • End-of-period storage

• Daily water balance outputs (water_balance.dat)

Observed behavior

• For both experiment sets, the integrated quantities (Sum ET, Sum Q, End Storage) show very little to no difference across cases.

• This remains true even when rooting depth and root profile parameters are changed appreciably.

• The “Root” diagnostic reported by ATS does change across experiments, suggesting that at least part of the root parameterization is being applied.

Questions

1. Is this behavior expected under energy-limited or water-abundant conditions, where ET is primarily controlled by atmospheric demand rather than root water availability?

2. In ATS, under what hydrologic or climatic regimes does rooting depth or root profile typically exert a strong influence on integrated water balance quantities?

3. Have there been any prior sensitivity studies (internal tests, example cases, or publications) focusing on root parameters (rooting depth, alpha/beta) that you would recommend as references?

4. Are there specific diagnostics you would recommend examining (e.g., root water uptake by depth, plant stress factors) to better quantify the impact of root parameters beyond integrated water balance metrics?

I am happy to share:

• Input file, input file diffs

• Baseline and modified configuration files

• Output diagnostics and time series comparisons

Thank you very much for your time and for developing ATS.

Best regards,

Bing

[Coon, Ethan]

Hi Joe,

First, you should realize that the “rooting depth max” parameter really has almost no control on actual rooting distribution unless you make it very shallow.  Those are controlled by alpha and beta, the two shape profiles.  It is really just intended to be a cutoff for the exponentials (which otherwise are non-zero but tiny for large depths).  So ignore that one.

On to your questions:

1. Yes, absolutely.  In energy-limited or water-abundant states, rooting profile parameters only control where the water is taken from (at what depth) and do not affect the total amount of water removed through ET. 
2. Only in water-limited, very dry conditions do we expect down regulation of water (where the total ET is less than the potential ET) to play a role.
3. Not that I am aware of.
4. I think some useful indicators would be T / potential T (which may almost always be 1 if you’re seeing no effect) is the right indicator for integrated quantities.  If you looked at WHERE (in z) the water was being taken from, you would see a stronger signal, but I’m not sure what scientifically it would be useful for.  

I think it’s useful to point out that this is a relatively simple model compared to a full plant hydraulics model, where things like plant properties (stomate properties, xylem taper, wilting points, etc) start to come into play.  These shape parameters are from CLM/ELM, and have been used extensively in non-water-limited cases, but really only for large-scale land surface models.  I don’t believe that CLM/ELM have enough lateral flow to drive truly water limited cases.

Ethan 

From: ats-...@googlegroups.com <ats-...@googlegroups.com> on behalf of Joe Lee <lee19...@gmail.com>
Date: Thursday, February 5, 2026 at 7:47 AM
To: Amanzi-ATS Users <ats-...@googlegroups.com>
Subject: [EXTERNAL] Question on low sensitivity of water balance to rooting depth root profile parameters in ATS

This Message Is From an Untrusted Sender

You have not previously corresponded with this sender. If suspicious, use the Report Phish button in Outlook.

 

--
You received this message because you are subscribed to the Google Groups "Amanzi-ATS Users" group.
To unsubscribe from this group and stop receiving emails from it, send an email to ats-users+...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/ats-users/b79f5f09-53ae-4165-9c91-5e3562ce3fb5n%40googlegroups.com.