In addition to the general configuration of the valleys, which ought to be deep and with precipitous sides, flanked by lofty hills, there are several other points which require attentive examination in projects for collecting water from drainage areas:
1. The area of water-shed.
2. The geological character of the soil as affecting its capacity to absorb rain, and to allow the infiltration of water through it.
3. The character of the surface soil as affording soluble ingredients which may be taken up by the water and serve to contaminate its quality. In this point of view, districts of decomposing peat, districts of arable agricultural land richly manured, and places thickly covered with population, are often highly objectionable.
4. The rain-fall of the district, and especially the minimum fall in any one year.
5. The nature of the surface-soil as affording facilities for procuring puddle and constructing retentive reservoirs.
6. The consideration of compensation to mill-owners and possibly to land-owners where the water is used for irrigation.
The geological structure is extremely important in estimating the capacity of a drainage area. It is not alone the rain which falls on the sloping surface of the hills, and finds its way by gravitation to the lower levels; but the effect of springs is also very great in augmenting the quantity of water. Many drainage areas are also valleys of elevation, in which the strata dip in opposite or anticlinal directions on opposite sides of the valley. In this case it is evident that much of the rain falling on a porous surface will insinuate itself between the partings of the strata, and flow off in a direction contrary to that of the surface drainage.
From Mr. Beardmore’s work we take the following, as the proportion or percentage of rain-fall which flows off the surface: