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Impounded reservoir, 4.4 million m3 draft

Short answer

The mass curve method with a constant draft of 4.4 million m³/month gives a maximum cumulative deficit of 11.8 million m³ at the end of March. Minimum storage capacity = 11.8 million m³.

The yield of water from a catchment area is given below. Determine analytically the minimum storage capacity of the impounded reservoir to maintain a constant draft of 4.4 million m3\mathrm{m^3} of water per month. Neglect all losses and wastage.

MonthJanFebMarAprMayJunJulAugSepOctNovDec
Inflow (million m3\mathrm{m^3})1.52.02.55.06.08.297.55.03.53.12.0

The minimum storage is the maximum cumulative deficit of inflow against the constant draft (analytical mass curve method). Losses are neglected. Draft D=4.4D = 4.4 million m3^3/month; total inflow =55.3= 55.3 million m3^3 and total draft =12×4.4=52.8= 12\times4.4 = 52.8 million m3^3, so the yearly yield is sufficient.

The deficit period starts after the reservoir has been filled by the wet-season surplus. September is the last month with a surplus, so the reservoir is full at the end of September.

MonthInflow (Mm3)Demand (Mm3)Surplus/deficitCumulative deficit
Oct3.504.40-0.90-0.90
Nov3.104.40-1.30-2.20
Dec2.004.40-2.40-4.60
Jan1.504.40-2.90-7.50
Feb2.004.40-2.40-9.90
Mar2.504.40-1.90-11.80

The inflow is less than the draft from October to March, so the cumulative deficit increases throughout this period; the reservoir is then refilled from April onwards.

The largest cumulative deficit is 11.8 million m3^3.

Answer: Minimum storage capacity = 11.8 million m3^3.

✓ Answer checked by Claude Opus 5.5 (high effort)

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