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Monthly Flow Estimation | Regional & Empirical Methods in Nepal

Monthly flow estimation of a catchment is a major task for the design of irrigation system in a required commanded area. We generally estimate runoff as annual or monthly flow, extreme low flow & flood flow. Annual or monthly flow are required for the design of irrigation water requirement where as extreme low flow & flood flow are determined for the design of hydraulic structures & river training works. Here, in this article we are mainly concerned to the estimation of monthly flow in catchment area of study. When long term flow data of study are available, we use frequency analysis method. In absence of such records, we can apply regional & empirical method to calculate monthly flows.

Frequency Analysis

The design criteria for irrigation schemes in Nepal is 80% reliability of full supply. The value can be obtained using the mean and standard deviation technique. The 80% reliable is obtained as:
$$Q_{80}=Q_{mean}-SD * 0.8418$$


This method was developed by WESCS based on the study of DHM's records, so called WESCS/DHM method. The equation developed is given below:
$Q_{mean}= C*(Basin Area)^{A1}$
                       $*(Basin Area Below 5000+1)^{A2}$
                       $*(Mean Monsoon Precipitation)^{A3}$
Here, C, A1, A2 & A3 are coefficients whose values are given in Table 1.
Table 1 : WECS/DHM predication equation constans

MIP Method

This method uses non-dimensional hydrographs developed for Nepal dividing the whole country in seven different regions & requires at least single dry weather flow. April flow is obtained by dividing the dry weather flow by corresponding non-dimensional ordinate & then, each ordinate of required region is multiplied by the April flow to get the monthly hydrograph. Non dimensional hydrographs are available for $Q_{80}$ & mean monthly flow.
Table 2 : Non dimensional hydrograph for mean monthly flow
Table 3 : Non dimensional hydrograph for $Q_{80}$

MHSP 1997

MHSP suggested the following formula to calculate mean monthly flow of a catchment.
$Q_{mean} = C*(Basin Area)^{A1}$
                        $*(Mean Monsoon Precipitation)^{A2}$
Table 4 : MHSP 1997 predication equation constants

HSC Method

HSC stands for Hydrologically similar catchments which method is applicable only when two catchments have same yields (discharge/area). If a catchment has long term flow data & another hydrologically similar catchment has data at least for 3 years, then long term flow for the second catchment can be calculated from the data of HSC.

Khosla's Method

Khosla (1960) analysed the rainfall, runoff and temperature data for various catchments in India & USA to arrive at an empirical relationship between runoff and rainfall. His relationship for monthly runoff is:
$$R_{m} = P_{m} - L_{m}$$
$R_{m}$ = Monthly runoff in cm
$P_{m}$ = Monthly rainfall in cm
$L_{m}$ = Monthly losses in cm
$T_{m}$ = Mean monthly temperature of the catchment


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