review and index of underground-water literature published in the United States up to and including the year 1905.

Run-off. The run-off is that part of the rainfall which drains off the surface of the watershed in visible streams. It is that part of the rainfall which remains after nature's need of moisture has been supplied in the form of evaporation and absorption.

The close relation between these three subdivisions of rainfall has been referred to in the above, and it follows that the run-off is affected, both directly and indirectly, by the same factors that govern the rate of evaporation and absorption.

It is often important to know the relation between rainfall and runoff, as this may in many instances be the only way to ascertain the flow of a stream. Rainfall observations have been made for many years, and it may be possible, by knowing the ratio between run-off and rainfall for a certain drainage area, to apply this value to a watershed in another place. It is, of course, of the greatest importance, in such comparisons, that the areas compared be of similar character. They must also be of approximately the same size, because smaller drainage areas usually have a wider variation between maximum and minimum run-off than large ones.

It is apparent that there can be no constant relation between the rainfall and the run-off for the whole country, although in this respect the ratio for the Eastern States is much more constant than for the Western States. There are also great variations in the yearly, as well as the monthly and daily, run-off, and it is very difficult to make accurate estimates as to what the two latter may be expected to be; the daily run-off is, of course, almost impossible to foretell. The yearly run-off, however, bears a more nearly uniform ratio to the rainfall, so that with a good knowledge of the presence of forests, character of soil, climate, etc., a fairly accurate estimate of the yearly run-off may be made, based on known values under similar conditions.

As for rainfall, run-off is also usually expressed in inches, and the map in Fig. 17 shows approximately the mean annual run-off for the country. By comparing this map with that of rainfall in Fig. 11, a fairly good idea of the relation between rainfall and run-off may be had. Table XXII furthermore gives the run-off for various watersheds in the United States.

TABLE XXII

MEAN ANNUAL RUN-OFF FOR VARIOUS WATERSHEDS IN UNITED STATES !

Prepared by Newell and Murphy from U. S. Geological Survey Records.

4. STREAM-FLOW

Definition of Terms. The volume of water flowing in a river is generally defined as "stream-flow" and is expressed in various terms, depending upon the particular class of work for which it is to be used. The term used in the reports of the United States Geological Survey are second-feet, second-feet per square mile, acre-feet, and depth in inches. Of these, the first two represent the rate of flow only, while the two latter represent the actual quantity of water. They are defined in the Survey Reports as follows:

"Second-foot" is an abbreviation for cubic foot per second and is the unit for the rate of discharge of water flowing in a stream 1 foot wide, 1 foot deep, at a rate of 1 foot a second. It is generally used as a fundamental unit from which others are computed by the use of the factors given in the following table of equivalents.

"Second-feet per square mile" is the average number of cubic feet of water flowing per second from each square mile of area drained, on the assumption that the run-off is distributed uniformly both as regards time and area.

"Depth in inches" is the depth to which the drainage area would be covered if all the water flowing from it in a given period were conserved and uniformly distributed on the surface. It is used for comparing run-off with 1a nfall, which is usually expressed in depth in inches.

An "acre-foot" is equivalent to 43,560 cubic feet, and is the quantity required to cover an acre to the depth of 1 foot. The term is commonly used in connection with storage for irrigation.

The direct cause of stream-flow is the visible run-off from the watershed and that part of the rain-fall which was absorbed by the soil and which slowly finds its way to the stream bed in the form of an underground flow.

Variation in Stream-Flow. There is a very considerable variation in the flow of rivers, not only during the various months of the year, but from year to year as well; and the variation is greater in some regions than in others. In Fig. 18 are shown some typical hydrograph records of New York streams, which clearly illustrate what may be expected in the way of variations in stream-flows. While they are of entirely different characteristics, it can be seen that there are certain common features in that the flows are heaviest during the spring and early summer and lowest in autumn.

This irregularity of flow is a very important factor in any waterpower development and one that necessitates reckoning with the mini