Hello WEAP community,

I'm working with a WEAP-MODFLOW model and need guidance on which values to use for water balance reporting.

The Problem: MODFLOW output files (.ccf) show much higher river-aquifer exchange flows than WEAP direct results. The difference is 6-10 times. I've verified my calculations are correct (units, spatial aggregation, sign conventions, linkage, topology, temporal alignment, no double-counting).

What I Need to Know:

1. Why are WEAP values lower than MODFLOW?
- Does WEAP filter flows based on water allocation logic?
- Does WEAP aggregate MODFLOW cells differently?

2. Which values should I report?
- For regulatory water balance reports, should I use:
- MODFLOW values (complete physical exchange)?
- WEAP values (filtered/processed)?

3. Which WEAP output file represents the actual groundwater-surface water exchange?

4. Is this normal behavior? Has anyone dealt with this issue?

Thank you for any guidance!
Carlos

Hi Carlos,

Can you tell me which results you are comparing? Are you looking at Supply and Resources - Groundwater - MODFLOW - Leakage Volume to River? Are you comparing that to results shown in Supply and Resources - Groundwater - Inflows and Outflows?

Or, are you working directly with the ccf files? If so, are you accounting for the timestep length with each result? In order to get the total stream-aquifer exchange for a stress period you need to sum the product of the flow rate and the length of time step for all time steps in the stress period.

Chuck