2066 Bhadra (old course)

Impounded reservoir, 400 m3/day demand

Short answer

Converting flows to monthly volumes (30‑day months) and applying the mass curve method yields a maximum cumulative deficit of 21,404 m³ from September to February. Reservoir capacity ≈ 21,400 m³.

The water demand of a community is 400 $\mathrm{m^3}$ /day. The flow in the river in various months of the year is given below. If the river is to be used as a source for the water supply system, calculate the capacity of the impounded reservoir.

Month	January	February	March	April	May	June
Inflow (lps)	2.40	4.00	5.60	4.80	5.60	6.40

Month	July	August	September	October	November	December
Inflow (lps)	7.20	6.40	1.12 [?]	0.00	5.60	6.40

Convert the river flow (lps) to monthly volumes using a 30-day month and compare them with the demand. Demand $= 400\ \mathrm{m^3/day}\times30 = 12{,}000\ \mathrm{m^3}$ per month. Volume $= Q(\text{l/s})\times10^{-3}\times86400\times30$ m $^3$ . The September flow (1.12 lps) is used as printed.

Total inflow $= 143{,}908$ m $^3$ and total demand $= 144{,}000$ m $^3$ . The yearly inflow is almost equal to the yearly demand (only 92 m $^3$ short), so the river can just about meet the demand with storage.

Month	Inflow (m3)	Demand (m3)	Surplus/deficit	Cumulative deficit
Sep	2903	12000	-9097	-9097
Oct	0	12000	-12000	-21097
Nov	14515	12000	+2515	-18582
Dec	16589	12000	+4589	-13993
Jan	6221	12000	-5779	-19772
Feb	10368	12000	-1632	-21404

The critical period is September to February. The inflow is below the demand in September, October, January and February, and the surpluses of November and December are not enough to refill the reservoir in between. The reservoir is full at the start of September (after the surplus months of March to August), and the largest cumulative deficit in this period is the capacity needed.

Answer: Capacity of the impounded reservoir $\approx 21{,}404\ \mathrm{m^3}$ (about 21,400 m $^3$ ).

✓ Answer checked by Claude Opus 5.5 (high effort)

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