A storage reservoir is to be built to store a river flow QR and to supply a stea
ID: 1824400 • Letter: A
Question
A storage reservoir is to be built to store a river flow QR and to supply a steady flow QD = 108 m3 year to a town with a population of 100,000 people and an irrigation project with a total land area A = 108 m2. The town has a consumptive per capita water demand of 200 m3 person year. The annual precipitation and evaporation from the irrigation area are estimated to be P = 0.5 m and E = 1.0 m. respectively. In addition, the irrigation project leaches a mass flow of dissolved salts ms = 107 kg year from the soil. Assuming that the salt concentration in the water leaving the reservoir is negligible, will the flow from the reservoir QD = 108 m3 /year be sufficient to provide the town the needed water, to water the crops, and to insure that the concentration of dissolved salts in the downstream river cs will be less than the maximum drinking water standard which is 500 mg L? [yes - the maximum possible need for water is 9 times 10' m3/ yr]Explanation / Answer
water is coming from the reservoir and supplied to the town and used for irrigation purpose.
total flow of the water coming through the river, QD = 108 m3/year
population of the town = 100000
consumption of water in the town per capita= 200 m3/year per capita.
total consumption of the water in the town,Qtown = 200*100000 = 2*107 m3/year
water required for the irrigation:
area of the land = 108 m3
precipitaion = 0.5 m per year
then total flow due to precipation on the land = 0.5*108 = 5*107 m3/year
evaporation = 1.0 m
total loss (evaporation) from the land = 1*108 m3/year
irrigation project leaches the mass flow of the dissolve salts, m's = 107 kg/year from the soil.
maximum concentration of the dissolve salts in the downstream river, Cs = 500 mg/L = 0.5 kg/m3
therefore water flow from the land to the downstream = m's/Cs = 107/0.5 = 2*107 m3/year
therefore, total water flow needed from the reservoir to the irrigation =water flow for the leaching of the mass of the dissolve salts + total losses due to evoparation - water flow from the precipation
Qirrigation = 1*108 +2*107 - 5*107 = 7*107 m3/year
hence, total water flow required from the reservoir = Qtown + Qirrigation =2*107 + 7*107 = 9*107 m3/year < QD = 108 m3/year ...then it will be OK.
hence, yes, it will be the sufficient to provide water flow to the town and irrigataion purpose.
total water required for town and irrigation purpose both = 7*107 m3/year
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