Forest Service Department of Agriculture Circular, 54.
Paper-making Materials and their Conservation. Bureau of Chemistry, 41.
[CHAPTER IV]
WATER
Water is an absolute necessity to man, as much as the air he breathes or the food he eats. Water comes to us in the form of rain or snow. We usually think of it as unlimited, but we must come to think of it as a resource that can be abused and wasted or made useful and profitable as is the soil itself.
The amount of water is fixed and passes in an endless round from cloud to river or land and back to the clouds again. The average yearly rainfall of the United States is estimated at thirty inches, about forty inches in the eastern half, an average of eighteen inches in the western part, and in many places not more than ten or twelve inches. One inch of rain would amount to nearly one hundred and one tons per acre, or on a roof twenty feet long by twenty feet wide, one inch of rain would be two hundred and fifty gallons. With a rainfall of forty inches, this would amount to 10,000 gallons in a year, or an average, over every bit of land twenty feet square, of twenty-seven gallons for every day in the year. This is about the quantity that falls in the eastern part of the United States.
It varies slightly from year to year, but there is no more—there is no possible way of adding to it, though we may lessen it by allowing it to rush out to sea, giving no service to the land. As the land waters diminish the rainfall also grows less.
This two hundred trillions cubic feet of water which falls on our land every year constitutes our entire water resource, is the source of all our rivers and streams, of the moisture in the air, of our rains and snows, and our water for plant and animal growth.
To understand how much this is, we may say that it is about equal to ten times the amount of water that flows through the Mississippi River system. The water of the Mississippi and its branches is nearly half of all the water in the United States that flows through waterways to the sea. This water that flows through our streams is sometimes called the run-off. The run-off is increasing every year as we cut our forests and cultivate our land. It is used for navigation, irrigation and power, but the increase is not an advantage for these purposes as might be supposed, because it comes in disastrous floods, tearing away dams, ruining power sites, and not only preventing navigation during the flood season, but by filling up the rivers and changing the channels, making navigation difficult and dangerous throughout the year. The run-off is controlled to some extent and may be brought under almost as complete control as may be desired.