Runoff estimation for ungauged basins in Nepal

Most rivers in Nepal are ungauged, so the Department of Hydrology and Meteorology (DHM), WECS and project agencies have developed regional methods to estimate monthly flow from rainfall and catchment data.

1. MIP (Medium Irrigation Project) method

Developed in 1990 under the Medium Irrigation Project for catchments without flow records.

1. Delineate the catchment above the site and measure its area ($A$, km$^2$) from topographic maps.
2. Find the monsoon wetness index (MWI) of the catchment from the Nepal-wide MWI map (isolines based on monsoon rainfall).
3. Use the regression curves or equations that relate monthly specific discharge to MWI for each month of the year, and read the flow of each month.
4. Multiply by the area to get the monthly discharge, and plot the 12 values as a flow series (hydrograph) for the site.

It is simple, but it gives only mean monthly flows and is suited to small and medium catchments below about 3000 m elevation.

2. WECS/DHM method (1990)

A regional approach using relationships between mean monthly flow and catchment characteristics (area, mean elevation, rainfall) for the various physiographic regions of Nepal. It is used for medium-sized basins.

3. Hydest (DHM, 2004)

A computer-based regional model. From the catchment area, mean elevation and rainfall index, it gives mean monthly flows, flood estimates and low-flow estimates for ungauged rivers.

4. Catchment-area ratio (transposition) method

If a gauged station exists on a nearby similar river:

$$Q_{ungauged} = Q_{gauged}\left(\frac{A_{ungauged}}{A_{gauged}}\right)^{n}$$

with $n$ close to 1 (0.8 to 1.0), modified by the rainfall ratio of the two basins.

5. Water-balance and rainfall-runoff coefficient method

Monthly runoff is found from rainfall and losses:

$$Q = P - E - \Delta S$$

Use runoff coefficients for the catchment type, soil and land use, and add the snow and glacier melt contribution in the high Himalaya.

The accuracy of these methods depends on the quality of the rainfall network. Estimates should be checked with a few spot discharge measurements at the site.