--- title: "Impounded reservoir, 9688 million L per month" subject: "Water Supply Engineering" chapter: 2 key: "semester_5/water_supply_engineering:impounded-reservoir-j66" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/impounded-reservoir-j66 site: BCT Engineering --- # Impounded reservoir, 9688 million L per month Water Supply Engineering · Chapter 2: Sources of Water - Asked in: 2066 Jestha (old course) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/impounded-reservoir-j66 - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-2 ## Question The city has an average water demand of 9688 million liters per month. Calculate the capacity of the impounded reservoir. The flow in the river is shown below. | Month | Inflow (m$^3$/s) | |---|---| | January | 2.51 | | February | 2.11 | | March | 1.20 | | April | 0.00 | | May | 0.11 | | June | 1.00 | | July | 1.30 | | August | 3.00 | | September | 4.00 | | October | 5.2 [?] | | November | 4.00 | | December | 1.80 | ## Short answer Annual river inflow (68.93 million m³) is much smaller than annual demand (116.26 million m³). No impounded reservoir can meet this demand; the given data cannot sustain the required draft. ## Answer 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. | Month | Inflow (Mm3) | Demand (Mm3) | Surplus/deficit | Cumulative | |---|---|---|---|---| | Jan | 6.723 | 9.688 | -2.965 | -2.965 | | Feb | 5.105 | 9.688 | -4.583 | -7.549 | | Mar | 3.214 | 9.688 | -6.474 | -14.023 | | Apr | 0.000 | 9.688 | -9.688 | -23.711 | | May | 0.295 | 9.688 | -9.393 | -33.104 | | Jun | 2.592 | 9.688 | -7.096 | -40.200 | | Jul | 3.482 | 9.688 | -6.206 | -46.406 | | Aug | 8.035 | 9.688 | -1.653 | -48.059 | | Sep | 10.368 | 9.688 | +0.680 | -47.379 | | Oct | 13.928 | 9.688 | +4.240 | -43.139 | | Nov | 10.368 | 9.688 | +0.680 | -42.459 | | Dec | 4.821 | 9.688 | -4.867 | -47.326 | 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why can no impounded reservoir meet this demand? A. Annual inflow (68.93 Mm3) is less than annual demand (116.26 Mm3) B. Monthly inflow is always above demand C. The reservoir is too costly D. Evaporation is neglected Answer: A. A reservoir can only store what the river brings; if annual inflow is below demand, storage cannot fix it. (Key point: Feasibility) 2. What is 9688 ML/month in Mm3/month? A. 96.88 B. 0.9688 C. 9.688 D. 968.8 Answer: C. 1 ML = 1000 m3, so 9688 ML = 9.688 Mm3. (Key point: Demand conversion) 3. What does the cumulative balance do over the year? A. It returns to zero by December B. It keeps falling overall, reaching -47.33 Mm3 at year end C. It rises to a large positive value D. It oscillates around zero Answer: B. Surpluses in Sep to Nov are small compared to deficits, so the balance ends at -47.33. (Key point: Cumulative behaviour) 4. What demand could be supplied by this river on average? A. 9.688 Mm3/month B. 116.26 Mm3/month C. 5.744 Mm3/month D. 68.93 Mm3/month Answer: C. Annual inflow 68.93 divided by 12 = 5.744 Mm3/month. 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