Fix critical unit conversion bug in water access calculations

[k-r-a-s-s]

Summary

• Fixes critical unit conversion bug that caused water access distances to be underestimated by 3.6x
• Updates sensitivity analysis MET value for more accurate results
• Adds improved error handling

The Bug

The velocity data from the mobility models is in meters per second (m/s) but the distance calculation formula was treating it as kilometers per hour (km/h). This missing unit conversion caused all water access distances to be 3.6x smaller than they should be.

Evidence

• Walking velocity data shows ~1.3 m/s (4.7 km/h) which is typical walking speed
• The formula calculated 3.7 km instead of 13.3 km for 5.5 hours of walking
• The sensitivity analysis correctly used the conversion factor and showed the discrepancy

The Fix

Added the proper unit conversion factor * 3600 / 1000 to convert m/s velocities to km distances:

# Before (incorrect)
df_zones["max distance walking"] = df_zones["average_velocity_walk"] * time_gathering_water / 2

# After (correct)
df_zones["max distance walking"] = df_zones["average_velocity_walk"] * time_gathering_water * 3600 / 2 / 1000

Changes

1. src/gis_global_module.py: Fixed unit conversion in lines 840-844
2. scripts/sensitivity_analysis_refactored.py: Updated MET value from 3.5 to 4.5 and improved error handling

Impact

This fix corrects a fundamental calculation error that affected all water access distance estimates in the global model.

🤖 Generated with Claude Code