Monthly flow estimation of a catchment is a major task for the design of an irrigation system in a required commanded area. We generally estimate runoff as annual or monthly flow, extremely low flow & flood flow. Annual or monthly flow is required for the design of irrigation water requirements whereas extreme low flow & flood flow are determined for the design of hydraulic structures & river training works. Here, in this article, we are mainly concerned with the estimation of monthly flow in the catchment area of study. When long-term flow data of the study are available, we use the frequency analysis method. In absence of such records, we can apply regional & empirical methods 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 techniques. The 80% reliable flow is obtained as:
Q80=Qmean−SD∗0.8418
Example:
(Q80)January=(Qmean)January−SD∗0.8418
WECS/DHM 1990
This method was developed by WESCS based on the study of DHM's records, the so-called WESCS/DHM method. The equation developed is given below:
Qmean=C∗(BasinArea)A1
∗(BasinAreaBelow5000+1)A2
∗(MeanMonsoonPrecipitation)A3
∗(BasinAreaBelow5000+1)A2
∗(MeanMonsoonPrecipitation)A3
Here, C, A1, A2 & A3 are coefficients whose values are given in Table 1.
Table 1: WECS/DHM predication equation constants
Table 1: WECS/DHM predication equation constants
MIP Method
This method uses non-dimensional hydrographs developed for Nepal dividing the whole country into seven different regions & requires at least a single dry weather flow. April flow is obtained by dividing the dry weather flow by corresponding non-dimensional ordinate & then, each ordinate of the required region is multiplied by the April flow to get the monthly hydrograph. Non-dimensional hydrographs are available for Q80 & mean monthly flow.
Table 2: Non-dimensional hydrograph for mean monthly flow
Table 3: Non-dimensional hydrograph for Q80
Table 2: Non-dimensional hydrograph for mean monthly flow
Table 3: Non-dimensional hydrograph for Q80
MHSP 1997
MHSP suggested the following formula to calculate the mean monthly flow of a catchment.
Qmean=C∗(BasinArea)A1
∗(MeanMonsoonPrecipitation)A2
Table 4: MHSP 1997 predication equation constants
Qmean=C∗(BasinArea)A1
∗(MeanMonsoonPrecipitation)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 the 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:
Rm=Pm−Lm
Where,
Rm = Monthly runoff in cm
Pm = Monthly rainfall in cm
Lm = Monthly losses in cm
Tm = Mean monthly temperature of the catchment
(Subramanya,2013)
This is really useful.😀
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