Impounded reservoir, 9688 million L per month

A reservoir can only store water that the river actually brings in. The solution uses the mass curve (analytical) method, with volumes in million m$^3$ (1 ML = 1000 m$^3$, $V = Q\times86400\times$ days).

Demand $D = 9688\ \text{ML/month} = 9.688\ \text{Mm}^3$/month. The October flow in the given data is doubtful (shown as 5.2 m$^3$/s); it has been used as given.

The total inflow in the year is $68.93$ Mm$^3$, while the total demand is $12\times9.688 = 116.26$ Mm$^3$. The yearly inflow is much less than the demand, so the cumulative balance keeps falling and reaches -47.33 Mm$^3$ at the end of the year.

Answer: With the data as printed, no impounded reservoir can meet this demand, since the annual inflow (68.93 Mm$^3$) is smaller than the annual demand (116.26 Mm$^3$). The largest average draft the river can sustain is 5.744 Mm$^3$/month; for a demand at or below this value, the same table method gives the storage as the largest cumulative deficit. The demand figure (or the table) appears to be misprinted.