Hi,

I'm experiencing physically impossible behavior in my WEAP model that appears to be a MILP solver numerical problem.
Setup:
Large Chilean watershed with streamflow gauge "Estacion1" in upper basin
14 demand nodes distributed throughout watershed
Tested with both lpsolve and XA solvers
The Problem:
WITH demand node E3 active (8.15 Mm³/year, near gauge):
NSE = 0.78, KGE = 0.89, excellent calibration
WHEN I ADD nodes E1 and E2 (located several kilometers DOWNSTREAM of gauge):
E1: 121.7 Mm³/year
E2: 39.9 Mm³/year
NSE drops to 0.26
Streamflow at upstream gauge decreases dramatically
This is physically impossible - downstream extractions cannot affect upstream measurements
Key Evidence:
E3 (8.15 Mm³/yr) works perfectly
E1 and E2 are kilometers downstream, spatially separated from gauge
Reducing E1 and E2 to 1 Mm³/yr each recovers NSE to 0.78
Other downstream nodes E4-E14 with smaller demands cause no issues
Problem is magnitude-related, not topology-related
Question:
Can very large demand values cause MILP solver numerical instability that propagates calculation errors throughout the river network, affecting physically independent upstream nodes? This appears to be a poorly scaled model issue where the solver produces results violating basic mass balance principles.
Has anyone encountered this with over-allocated water rights systems? Any solver settings or configuration suggestions?
Thank you,