4.4. Estimation of Reservoir Inflow Using Observed Data
Generally, there is no provision for monitoring upstream discharge in smaller or medium irrigation projects. However, the storage level of the reservoir is regularly monitored daily at 08:00 h, but twice a day during a heavy rainfall. Under such data availability conditions (i.e., only daily storage level and capacity), the continuity equation expressed by Eq. (4.17) is used to estimate the reservoir inflow hydrograph.
(4.25)A discrete form of Eq. (4.25) can be expressed through Eq. (4.26) as follows
(4.26)i.e.,
(4.27)
(4.28)
(4.29)Solving Eq. (4.26) for inflow hydrograph at a daily time step (i.e., Δt = 1 day = 86,400 s) with the elaboration of the various inflow and outflow terms will give the following expression for the estimation of inflow hydrograph:
(4.30)where 
is the average inflow (m3/s) to be estimated; Sj+1 and Sj are the reservoir storage at j + 1 and j-th time steps (m3); Rj+1 is the rainfall volume over the reservoir (m3), which is estimated as point rainfall (m) times submergence area of the reservoir (m2); Ej+1 is the evaporation loss (m3), which is estimated as rainfall volume; 
is the inflow from the feeder canal, if any (m3/s); 
is the mean outflow from the weir (m3/s); and Δt is the time step in seconds. The computation to estimate the catchment inflow rate is presented in Table 4.9.
is the average inflow (m3/s) to be estimated; Sj+1 and Sj are the reservoir storage at j + 1 and j-th time steps (m3); Rj+1 is the rainfall volume over the reservoir (m3), which is estimated as point rainfall (m) times submergence area of the reservoir (m2); Ej+1 is the evaporation loss (m3), which is estimated as rainfall volume; is the inflow from the feeder canal, if any (m3/s); is the mean outflow from the weir (m3/s); and Δt is the time step in seconds. The computation to estimate the catchment inflow rate is presented in Table 4.9.4.4.1. Determination of Catchment or Reservoir Yield
Catchment yield is nothing but total runoff computed at the outlet of the catchment. It can be presented in several ways: (1) annual yield; (2) average yield; and (3) dependable year yield.
Annual yield: It is the runoff volume observed in a particular year in m3 or MCM.
Average yield: It is defined as the mean annual runoff from the catchment observed at the outlet. It represents the surface water availability from the catchment.
Dependable year yield: The D% dependable year is defined as the year for which a corresponding magnitude xD at most 100D% of the years exceeds the value of xD. The steps involved in arriving at the dependable year yield are as follows:
1. Let the sample annual runoff data be x1, x2,…, xN with years y1, y2,…, yN.
2. The sample data x1, x2,…, xN will be arranged in descending order and the year is also written, corresponding to xi, i = 1, 2,…, N.
3. Assign the ranks from 1 to N for xi.
4. The dependable year D will correspond to the year at (N + 1)D/100; and corresponding flow will be referred as the D% dependable year flow of the catchment.
Example 4.9:
Use the data given in Table 4.10 and estimate the annual flow of the river corresponding to the 50%, 75%, and 90% dependable years.
Solution:
The data are arranged in descending order as presented in Table 4.11.
The dependable year and corresponding annual flow of the stream is calculated in Table 4.12.
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