WECS/DHM Methodology — Hydrological Estimation for Ungauged Locations (1990)
- Page status
- Working page
- Updated
- 2026-05-12
Summary
"Methodology for Estimating Hydrological Characteristics of Ungauged Locations in Nepal" (WECS/DHM, 1990, Seq. No. 331, Ministry of Water Resources). The regulatory default methodology for feasibility studies on Nepal's vast number of projects on ungauged tributaries.
Key points
- Empirical regionalization equations derived from DHM gauging data collected up to 1985.
- Simplistic regression: For example, instantaneous flood flows for a 2-year return period use a regression equation based solely on catchment area below 3,000 meters elevation.
- HYMOS dependence: Data processing and gap-filling have historically relied on HYMOS (version 4), a proprietary hydrological software system.
- Stationarity assumption: The models assume hydrological stationarity — that statistical properties of river flows remain constant over time. They do not capture cryospheric melt acceleration, shifting monsoon patterns, or increasing baseflow volatility.
- Partial update: DHM issued "Hydrological Estimation in Nepal" (June 2016), but the 1990 methodology remains the regulatory default for most feasibility studies.
Limitations
- Data age: Empirical regionalization equations are derived from DHM gauging data collected up to 1985; hydrological conditions may have changed significantly.
- Stationarity assumption: Models assume hydrological stationarity and do not capture cryospheric melt acceleration, shifting monsoon patterns, or increasing baseflow volatility.
- Simplistic regression: Instantaneous flood flows use regression equations based solely on catchment area below 3,000 m elevation, which may not capture complex Himalayan terrain.
- Proprietary dependence: Data processing and gap-filling historically rely on HYMOS (version 4), a proprietary software system that limits transparency and reproducibility.
- Partial update: DHM issued a 2016 update, but the 1990 methodology remains the regulatory default; newer hydrological insights are not systematically integrated.