Terrestrial water may be considered as falling into three broad classes:

1. Surface water – the free-flowing water of streams and rivers, and the water of ponds and lakes:

2. Ground water – the water held in saturated strata below:

3. Soil water – the water held in association with air in unsaturated superficial layers of the earth.

Each of these three classes of water is closely related in the hydrologic cycle and the ways in which one of them is used may affect, to varying degrees, both the quantity and the quality of the other classes. Hence in multi-purpose planning they are considered as a unity, not as separate unrelated subjects.

Expanding industry, which requires an increasing amount of hydro-electric power, makes large demands upon the available water both in the actual manufacturing processes and in the disposal of industrial wastes. As far as is humanly possible, problems of water allocation, of possible re-use of water, and of determination of quality, need to be foreseen and their solutions carefully planned. As farming and forestry become more intensive, the value of production per unit area of land must be greater and seasonal deficiencies of soil moisture become increasingly significant in the economy. A greater amount of water will be needed both for full-scale and for supplementary irrigation. The provision of a well-distributed and assured supply of water for stock is today a pressing need on many farms.

Floods present special problems largely because flood plains, with their flat terrain, good soil, and abundant water of all three classes, are desirable places on which to live. Unfortunately the sense of security engendered by flood-protection works tends to attract more and more wealth and people to areas that one day will be endangered by the inevitable large flood. Whether, by vegetative control of the catchment, we can do much to reduce this threat is a moot point, but a great deal can be done towards minimising the amount of debris and silt the flood will carry.

Besides being needed for human consumption and for such purposes as farming, forestry, industry, sanitation, electric power, inland navigation, and recreation, water plays a part in our cultural life and in our sense of aesthetic values. No sane community would seek an economic advantage at the cost of lifeless or stinking rivers. In multi-purpose water planning, therefore, each and every one of these potential needs must be respected and wise decisions made whenever they are in conflict.