Problem: Satellite passive microwave (SMAP-400m) fails on fewer than 12% of monsoon days over the Harave watershed — precisely when hydrology is most active. Cloud cover blocks the optical downscaling inputs; L-band is attenuated under dense wet canopy.
Approach: Calibrated a dry-season empirical transfer function between Sentinel-1 VV backscatter and SMAP-400m soil moisture, selected from four functional forms using the Bayesian Information Criterion and validated by leave-one-year-out cross-validation.
Key Findings:
- Linear model retained for operational use (yields absolute volumetric estimates)
- Sentinel-2 NDVI did not improve calibration — vegetation structure already embedded in SMAP-400m via VIIRS leaf area index
- Fused record validated against independent rainfall and TDR observations in adjacent Mallaiinupura watershed without recalibration
Output: 129-month continuous monthly soil moisture record, 2016–2026, covering 58 monsoon months where SMAP-400m was unavailable.
Submitted to: International Journal of Remote Sensing (IJRS)