MIP method for monthly flow in Nepal

The MIP (Medium Irrigation Project) method was developed by the Department of Hydrology and Meteorology (DHM) in 1990 to estimate mean monthly flows of ungauged rivers of Nepal, mainly for small and medium irrigation and hydropower sites. It is a regional regression method based on the catchment area and the monsoon rainfall index of the catchment.

Procedure

1. Delineate the catchment above the site on topographic maps and measure its area $A$ (km$^2$). The method applies to catchments below about 3000 m elevation without major snow or glacier input.
2. Find the Monsoon Wetness Index (MWI) of the catchment from the MWI isoline map prepared from monsoon rainfall data of Nepal.
3. Select the regional equations or curves of the relevant region (Nepal is divided into hydrological regions) which relate the monthly specific discharge to MWI for each month.
4. Compute the discharge of each month (January to December) from the curve and multiply by the catchment area:

$$Q_m = q_m(\text{MWI})\times A$$

5. Plot the 12 values as a mean monthly flow series (flow hydrograph) and, if a short flow record exists nearby, adjust the estimates to it.

The method is simple and quick, but it gives only long-term mean monthly flows, not flood or low-flow extremes, and its accuracy is limited where rainfall data are sparse.