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58 Chapter 2 Water Sources: Surface Water

150
130
Runoff 0.75 total precipitation 60 50
Yield of catchment area (in thousand gallons) 90 Total precipitation, inches 40 30 20
110

70

50
30 10 Figure 2.18 Yield of
Impervious Catchment Area
Conversion factors: 1 inch
10
25.4 mm; 1 gal 3.785 L;
0 2,000 4,000 6,000 1 square ft 0.0929
Horizontal area of catchment (in square feet) square m

The size of cistern needed will depend on the size of the family and the length of time
between periods of heavy rainfall. The size of the catchment area will depend on the
amount of rainfall and the character of the surface. The safety factor allowed should be for
lower than normal rainfall levels. Designing for two-thirds of the mean annual rainfall will
result usually in a catchment area of adequate capacity (Fig. 2.18).

EXAMPLE 2.5 SIZING OF CISTERN AND REQUIRED CATCHMENT AREA

A farmhouse for a family of four people has minimum drinking and culinary requirements of 120
gpd (454 L/d). The mean annual rainfall is 45 in. (114 cm), and the effective period between rainy
periods is 100 days. Determine:
1. The size of the required rainfall collection cistern
2. The size of the required catchment area
Solution 1 (U.S. Customary System):

1. The size of the required rainfall collection cistern:
The minimum volume of the cistern required will be:
120 gpd 100 days 12,000 gal 12,000>7.48 1,604 ft 3

Say, a 10-ft deep, 13-ft-square cistern.
2. The size of the required catchment area:
Because the mean annual rainfall is 45 in., then the total design rainfall is:
45 2>3 30 in.
The total year’s requirement 365 days 120 gpd 43,800 gal.
Referring to Fig. 2.18, the catchment area required to produce 43,800 gal from a design
2
storm of 30 in. is 3,300 ft .
Say, a 100-ft 33-ft catchment area.