The quantity of water

Surface water originates mostly from rainfall and is a mixture of surface run-off and ground water. It includes large rivers, ponds and lakes, and the small upland streams which may originate from springs and collect the run-off from the watersheds. The quantity of run-off depends upon a large number of factors, the most important of which are the amount and intensity of rainfall, the climate and vegetation and, also, the geological, geographical, and topographical features of the area under consideration. It varies widely, from about 20% in arid and sandy areas where the rainfall is heavy. Of the remaining portion of the rainfall, some of the water percolates into the ground, and the rest is lost by evaporation, transpiration and absorption.

FACTORS INFLUENCING THE QUANTITY OF WATER

A) Total annual precipitation.

B) Seasonal distribution of precipitation. Both the total annual precipitation and the seasonal distribution of precipitation are best determined by past record.

C) Soil porosity and permeability.

D) Annual and monthly evaporation and transpiration.

One of the first steps -in the selection of a suitable water supply source is determining the demand which will be placed on Sit. The essential elements of water demand include the average dally water consumption and the peak rate of demand. The average daily water consumption must be estimated:

1. To determine the ability of the water source to meet continuing demands over critical periods, when surface flows are low, and ground-water tables are at minimum elevations.

2. For purposes of estimating quantities of stored water which would sustain demands during these critical periods.

The peak demand rates must be estimated in order to determine plumbing and pipe sizing, pressure losses, and storage requirements necessary to supply sufficient water during periods of peak water demand.