What does total yield 57.41 versus total draft 57.36 show?

The yearly yield is just enough for the draft

Where does the cumulative deficit start?

End of August, when the last surplus ends and the reservoir is full

Asked 2/30
2067 Asar (old course)

Impounded reservoir, 4.78 million m3 draft

Short answer

With a constant draft of 4.78 million m³/month, the mass curve method shows a maximum cumulative deficit of 20.73 million m³ at the end of April. Reservoir storage capacity = 20.73 million m³.

The yield of water from a catchment area during each successive month is given below. Determine analytically the storage capacity of the impounded reservoir to maintain a constant draft of 4.78 million $\mathrm{m^3}$ of water per month.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Inflow (million $\mathrm{m^3}$ )	1.40	2.10	2.80	2.80	8.40	11.90	11.90	7.70	2.52	2.25	1.96	1.68

Analytical (mass curve) method: the storage required equals the maximum cumulative deficit of the yield compared with the constant draft of $4.78$ million m $^3$ /month.

Total yield $= 57.41$ million m $^3$ and total draft $= 12\times4.78 = 57.36$ million m $^3$ , so the draft is within the yearly yield. The reservoir is assumed full at the start of the dry period (end of Aug), when the last surplus ends.

Month	Inflow (Mm3)	Demand (Mm3)	Surplus/deficit	Cumulative deficit
Sep	2.52	4.78	-2.26	-2.26
Oct	2.25	4.78	-2.53	-4.79
Nov	1.96	4.78	-2.82	-7.61
Dec	1.68	4.78	-3.10	-10.71
Jan	1.40	4.78	-3.38	-14.09
Feb	2.10	4.78	-2.68	-16.77
Mar	2.80	4.78	-1.98	-18.75
Apr	2.80	4.78	-1.98	-20.73

The largest cumulative deficit occurs at the end of Apr and equals 20.73 million m $^3$ .

Answer: Storage capacity of the impounded reservoir = 20.73 million m $^3$ .

✓ Answer checked by Claude Opus 5.5 (high effort)

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