CATCHMENT AREAS. The term "catchment area" is used to describe the collecting area from which water would flow to a stream or river, the boundary of the area being deter mined by the ridge separating water flowing in opposite directions. The amount of water collected within the catchment area would depend on the extent of that area, the amount of rain which has fallen on the surface, and the proportion of that rainfall which has been lost by evaporation or absorption.
The term "run-off" has been adopted to describe that portion of the rainfall which ultimately finds its way to the stream, and the ratio between the rainfall and the "run-off" varies widely, according to the climatic conditions. • In England and Scotland the average annual rainfall varies from a little over coin. to 175in., and the annual loss by evapora tion from a land surface varies from about i o to i 8in., being less in the north, increasing towards the south.
The following average figures for the period 1883-190o, illus trate the effect of these various factors on the proportion of the rainfall which flowed over Teddington weir from the Thames catchment area above that point.
The total annual loss by evaporation in Britain is less in regions of high rainfall than in areas of low rainfall, and the seasonal loss varies in a similar manner. The influence of climatic condi tions on the evaporation is so marked, that in tropical countries the proportion of the rain which flows off the ground is small.
It has been held that forests tend to increase the discharge of a river by reducing evaporation, but such evidence as has been obtainable fails to support this view. Shade cast by the trees would reduce evaporation, but rain which would otherwise flow from the ground, is absorbed by the trees. Although forests have but little effect on the annual loss by evaporation, their presence has an influence in delaying the flow of water from the hillsides, and when a catchment area has been cleared of timber, floods tend to become more intense.
Evaporation from a water surface is greater than from a land surface, especially in tropical countries, and is a matter of impor tance when lakes occupy a considerable proportion of the catch ment area.
When the surface is impermeable, there would be no loss by percolation.
When the surface is permeable but is underlain by impermeable rock, water percolates downwards till that rock is reached and then travels underground in the direction of the steepest incli nation of the rock surface, which is generally towards the stream, water lost by percolation reappearing as springs. Such percola tion has little influence on the annual run-off, but may have a marked effect on its variations throughout the year. Thick beds of permeable material, such as sand or gravel, absorb large vol umes of water, acting as natural storage reservoirs. During periods of abundant rainfall the beds would become saturated, and during periods of dry weather water so absorbed would he gradually discharged, thus maintaining a considerable flow in the stream.
This point is of great importance when it is desired to utilize the water of a stream without constructing a storage reservoir to balance its fluctuations, as the quantity of water which could be taken during certain periods would be limited to the dry weather flow.
When the surface is permeable and is underlain with permeable or fissured rock, such as chalk, the entire rainfall on the catch ment area might percolate downwards, reappearing in the form of springs issuing either within or without the limits of the catchment area.
In the former case there would be no surface flow above the point where the springs break out, and in the latter, the whole catchment area would be void of streams. (W. J. E. B.)