Page 29 - Water Engineering Hydraulics, Distribution and Treatment

1.6 Groundwater
Solution 1 (US Customary System):
1. The following conversion factors and approximations are being employed:
2
1in. rainfall∕km = 17.378 MG
2
Hence, 20 in.∕km annually = 20 × 17.378 = 348 MG or 348∕365 = 0.952 MGD.
2
2. A stream flow of about 1 MGD/km is a good average for the well-watered sections of North America. Not all of it can be
adduced economically by storage.
2
2
For 75% development (0.75 MGD/km , or 750,000 gpd/km ), about half a year’s supply must generally be stored. For
2
a catchment area of 100 km , therefore
2
2
Storage = (0.75 MGD∕km )(100 km ) × (0.5 × 365 days) = 13,688 MG = 13.5BG (billion gallons) approximately. 7
In semiarid regions storage of three times the mean annual stream flow is not uncommon, that is, water is held over from wet
years to supply demands during dry years.
2
3. For an average consumption of 150 gpcd, the drainage area of 100 km and impoundage of 13.5 BG will supply a population
of 100 × 750,000∕150 = 500,000 persons.
4. For water supply by continuous draft, low water flows rather than average annual yields govern.
2
3
In well-watered sections of North America, these approximate 0.1 ft /s or 64,600 gpd/km .
2
A catchment area of 100 km , therefore, can supply without storage
100 × 64,600∕150 = 43,000 people.
This is compared against 500,000 people in the presence of proper storage.
Solution 2 (SI System):
1. The following conversion factors and approximations are being employed:
2
1cm∕km = 67.12 ML (million liters)
2
Hence, 19.6 cm/km annually = 19.6 × 67.12 = 1315.6 ML annually = 3.6MLD.
2
2. A stream flow of about 1.46 MLD/km is a good average for the well-watered sections of North America. Not all of it can
be adduced economically by storage.
2
For 75% development (0.75 × 1.46 MLD∕km ), about half a year’s supply must generally be stored.
2
For a catchment area of 259 km , therefore
2
2
Storage = 0.75(1.46 MLD∕km )(259 km )(0.5 × 365) = 51,758 ML = 51.758 BL (billion liters).
In semiarid regions storage of three times the mean annual stream flow is not uncommon, that is, water is held over from wet
years to supply demands during dry years.
2
3. For an average consumption of 567.8 Lpcd, the drainage area of 259 km and impoundage of 51.758 BL will supply a
population of
2 2
(0.75 × 1.46 MLD∕km )(259 km )(1,000,000 L∕ML)∕(567.8 Lpcd) = 500,000 persons.
4. For water supply by continuous draft, low water flows rather than average annual yields govern. In well-watered sections of
2
North America these approximate 28.32 L/s or 0.094316 MLD/km .
2
A catchment area of 259 km , therefore, can supply without storage
2 2
(259 km )(0.094316 MLD∕km )(1,000,000 L∕ML)∕(567.8 Lpcd) = 43,000 people.
This is compared against 500,000 people in the presence of proper storage.