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44 Chapter 2 Water Sources: Surface Water
Surface area

Elevation of water surface above reservoir bottom or above datum Volume of water stored, V a dH
Surface area a

V a dH
Figure 2.10 Surface Area of a
Reservoir and Volume of Water
Storage volume Stored

For uniform contour intervals h (ft or m) and successive contour areas a 0 , a 1 , . . . , a n ,
2
3
(acre or m ), the volume V of water (acre-ft or m ) stored up to the nth contour is:
V 1>2 h [(a a ) (a a ) . . . (a n–1 a )]
n
1
0
1
2
n-1
+ a + 2 ab (2.8)
V 1>2 h aa 0 n a
1
For general use, surface areas and volumes are commonly plotted against contour ele-
vations as in Fig. 2.10. Note that volumes must be determined from the surface-area curve
by planimetering the area enclosed between the curve and its ordinate.
In reservoir operation, a small amount of water lies below the invert of the reservoir
outlet. Constituting the dregs of the impoundage, this water is of poor quality. The associ-
ated reduction in useful storage is offset, in general, by bank storage released from the soil
as the reservoir is drawn down. Moreover, the water below the outlet sill does form a con-
servation pool for fish and wildlife.
Surface areas and volumes enter not only into the solution of hydrologic problems but
also into the management of water quality, such as the control of algae by copper sulfate
and destratification by pumping or aeration.

2.7 MANAGEMENT OF CATCHMENT AREAS
The comparative advantage of developing surface rather than underground waters is offset,
in large measure, by the unsteadiness of surface runoff, both in quantity and quality, and
the recurrence of flow extremes. Those hydrologic factors that enter strongly into the de-
velopment of surface water supplies must, therefore, be kept clearly in mind in their design
and operation, with special reference to:

1. The principles of selecting, preparing, and controlling catchment areas
2. The choice and treatment of reservoir areas and the management of natural ponds
and lakes as well as impounding reservoirs
3. The siting, dimensioning, construction, and maintenance of necessary engineering
works, including dams and dikes, intake structures, spillways, and diversion works.
Also keep in mind that river systems may have to be developed for multiple pur-
poses, not just for municipal uses.
The gathering grounds for public water supplies vary in size from a few hundred acres
to thousands of square miles, and in character from sparsely inhabited uplands to densely