| PREVIOUS | NEXT | NEW SEARCH |
The Evolution of the Conservation Movement, 1850-1920
Conservation by sanitation; disposal of waste (including a laboratory guide for sanitary engineers) by Ellen H. RichardsA. Prospecting for Additional Water Supplies
This field work, sanitary survey as it is sometimes called, has two objects; (1) the search for quantity; (2) the inference as to lasting quality.
Since the total supply of water on the earth is limited by the rainfall and the available supply by atmospheric and geological conditions, the engineer must avail himself of meteorological data--rainfall records of at least fifty years, cycle of wet and dry years, etc. The quantity of rainfall in a given year may be distributed somewhat evenly in time and downpour, or may come in deluges at long intervals--the total quantity may be two inches or less over desert and alkali plain, or three hundred to six hundred inches in tropical countries. Panama and Costa Rica are giving illustrations of excess of water.
The fertile temperate regions of the globe, the great food belt of the world, receive annually forty sixty inches of rain, equivalent to about six hundred to eight hundred million gallons of available water per square mile.
Page 93 { page image }
Since this area contains much arable land, it is probable that about one-half the rainfall will sink some three feet below the surface; a portion, depending on character of rock formation, position of strata, etc., will penetrate depths hundreds of feet below the surface. About one quarter of the rainfall in the temperate zone may flow over the surface without having soaked the ground for more than six inches in depth or six hours in time. The remaining quarter will, in forested and arable regions, be evaporated from the leaves of trees and crops and from bare soil.
In one of the western states, it has been estimated that one hundred tons of water are needed to raise one ton of crops. The desert air has been shown to take up from water surfaces seven or eight a year where the rainfall was less than ten inches.
The study of water circulation is a science by itself, hardly established as yet, but of the utmost importance, as will be considered later.
The prospector is concerned with the evidence he can collect from observation and in some cases by boring.
The rain water, as it falls on the earth, tends at once to find the lowest level and to flow back to the ocean whence for the most part it came. The speed it makes is lessened by the obstructions it meets--impervious rock, broken strata, deep fissures, fine clay, etc. The rate of surface motion is greatly modified by the slope of the land. The force of gravity pulls water down, while capillarity pulls it up, hence evaporation from arable soil and plants.
The underground circulation of water is a mystery to the average man, and his ignorance is the cause of many of his ills. Although he knows that surface water flows approximately horizontally, it seems to him that the perpendicular rain must continue down in the same direction instead of being deflected almost at once in a lateral direction. Else, why does the place wells and cesspools within a few feet of each other and express such indignant surprise at the proof that their waters mingle?
The art of a Sherlock Holmes is often needed to draw out the
Page 94 { page image }
facts affecting soil and water pollution, the habits of the community, and their attitude toward the need of conservation and toward the legal aspects of the care of watersheds.
Just as in prospecting for ore deposits, so in looking for drainage horizons, boring, cross-cutting, and trenching are often needed, and bore holes, perhaps ditches, and certainly wires and sounding lines come into use.
One visit will not suffice. Unexpected appearance, exceptional meteorological conditions must be taken into account, the cycle of wet and dry years, the position of the time of examination in that cycle. All these things take time; not less than two years, and probably longer, will suffice for a reasonably clear idea of the quality of a given watershed as a source of supply. No genuine mining company is satisfied with a less thorough report and no town should risk the lives of its citizens with less. Tests should be continued over a long enough time to prove either variability or permanence. One test or several in one season is not enough.
Information about SGML version of this document.
| PREVIOUS | NEXT | NEW SEARCH |