Rural Hygiene

by Henry N. Ogden

Computation for rain-water storage.

With this for a basis, it may be determined how large a storage tank ought to be, assuming a family of five persons using water at the average rate of 25 gallons per head per day or 125 gallons each day. Doubling this amount to take care of emergencies and of the extra water used in hot weather, let us say that 250 gallons a day must be provided, or 7500 gallons a month. If we could be sure of starting at the beginning of any month with the tank full and that exactly thirty days would be the period of no rainfall, then a tank holding 7500 gallons would be the proper size. Unfortunately, with any month, as August, in which the rainfall may be practically zero, the preceding month may also have been so short of rain that the consumption was equal to or even more than the rainfall, and the month of August would start with no rain in the tank.

But if we take a three-month period, those inequalities will be averaged and the supply will be, so far as one can foresee, ample in amount; that is, we shall take the supply required in three months, namely, 22,500 gallons, and subtract from it the amount of water furnished in the three months, which is presumably two thirds of the average rainfall on the area contributing to the tank. The normal rainfall in three months is three times 3-3/4 inches, or 11-1/4 vertical inches, and if this falls on a roof area of, say, 2000 square feet, the total amount of water is 1850 cubic feet or 13,875 gallons, and two thirds of this is 9250. The tank, then, must hold the difference between the 22,500 gallons and 9250, or 13,250 gallons, whereas a month's supply would be 7500 gallons. The actual tank, therefore, is made to hold a little less than two months' supply. Such a tank would be ten feet deep and fourteen feet square, a good deal larger tank, of course, than one ordinarily finds with a rain water-supply; but the estimate of the use of water has been high and a long period of rainfall has been assumed, so that there is little likelihood of a house with this provision being ever without water.

Computation for storage reservoir on a brook.

In determining the quantity of water that may be taken from a small stream the area of the watershed answers the same purpose as the area of the roof which delivers water into a tank, the only difference being that from the roof all the water is always delivered, except a small proportion that evaporates at the beginning of a rain in summer. From the surface of a watershed, on the contrary, a large amount, and in some cases all of a stream, will be absorbed by the ground and by the vegetation and will never be delivered into the stream which drains an area. On large streams it is fair to assume that, on the average, only one half of the rainfall on the area will reach the stream, while with sandy soils this may be as small as 20 per cent. From December to May inclusive, when the ground is frozen, when there is no vegetation to absorb the water, and when evaporation is very light, practically all of the rainfall reaches the streams. From June to August, on the other hand, when the soil becomes rapidly parched, when vegetation is most active, and when evaporation is high, frequently no rainfall reaches the streams and the ground water sinks lower and lower, so that often streams themselves dry up. It is necessary, therefore, in providing for a definite quantity of water to be taken from a reservoir built on a small stream, to make the reservoir large enough to furnish water from June to September without being supplied with rain. This does not call for a very large dam or a very large storage, and three months' supply will usually be ample.

We have already estimated above that the quantity of water needed for three months will be 45,000 gallons, or about 6000 cubic feet. If the reservoir is built in a small gulley or ravine, its width may be twenty-five feet. If the length of the reservoir or pond formed by the dam is 240 feet, then the reservoir will furnish 6000 cubic feet for every foot of depth, and a reservoir of that size holding one foot of water will tide over a dry season.

Evaporation during these same three months will use up about a foot and a half in depth over whatever area the reservoir covers, so that two and a half feet in depth must be provided above the lowest point to which it is desirable to draw off the water. It would be well to allow a depth of at least ten feet in order to avoid shallow, stagnant pools, and if this depth is provided, even more than the two-and-a-half foot depth mentioned might be withdrawn in extremely dry seasons, though perhaps at some reduction in the quality of the water.

Deficiency from well supplies.

A large number of water-supplies in the country, perhaps the largest number, at present comes from wells, either dug or drilled. It often happens that after plumbing fixtures have been installed with a pump to raise the water to the necessary elevated tank, the increased consumption causes the well to run dry for a number of weeks in the summer. The question then arises, Shall the well supply be supplemented or shall an entirely new supply be developed?

There are two methods of supplementing a dug well supply, and it may be of advantage to point them out. If the sand or gravel in which the water is carried is fine, it may be that the water will not at times of low water enter the well as fast as the pump takes it out. Such a well always has water in it in the morning, but a short pumping exhausts the supply. One remedy here is to provide a more easy path for the water, and that can be done by running out pipe drains in different directions. If there are any evidences that the underground water flows in any direction, then the drains should preferably run out at right angles to this direction, to intercept as much water as possible. The drains must be laid in trenches and be surrounded with gravel, and of course the method is inapplicable if the well is more than about fifteen feet deep, because of the depth of trench involved.