Increases in water yield following timber harvest roughly conform to the proportion of the area cleared. In high precipitation areas of the Oregon Cascades, clear‐cut logging can increase annual water yield 18 inches. Approximately 80% of the increase occurs during the October to March season.

Logging in the Douglas-fir forest has only minor effect on major peak streamflows which occur when soils are thoroughly wet. Exceptions are the early fall storms following the dry summers characteristic of the west coast climate. At this time, peak streamflow from unlogged areas may be less than in the harvested area because the soil in the unlogged area is drier and has greater moisture storage capacity than in the harvested area. These early fall storms rarely result in major peak streamflows.

Streamflow from small watersheds on the western slopes of the Oregon Cascade Range is strongly influenced by a maritime climate (wet winters and dry summers). Although annual precipitation is high (94 inches in the study area), overland flow is almost unknown. Peak flows result largely from subsurface flow and under conditions in which both retention and detention reservoirs are almost filled during extended periods of low-intensity rainfall. Under these conditions, vegetation appears to exert a minimum influence on high streamflow. Lowest streamflow occurs from late August to mid-November and may follow a 60-to 100-day period with little or no rain. The dense vegetation of this part of the Douglas-fir region appears to exert its major influence at such times. Removal of vegetation from only 30% of a 250-acre watershed has caused a 12-28% increase in minimum streamflow. On a 237-acre watershed on which 80% of the trees were cut, the increase in low flow was 85%.

No abstract

In recent years increasing awareness of the importance of water in our national economy has focused attention on the significance of forests in this important phase of conservation of our natural resources. Much has been written in the past 30 years concerning various aspects of the water relations of woody plants, yet available data concerning total water loss from trees and forests are inadequate. This inadequacy is apparent when it is realized that many of our present-day estimates are based on experiments conducted by voN H6HNEL prior to 1900 in which potted seedlings were used as test plants (6, 7). Such estimates have little value for appraising seasonal use of water by forests. The present study was undertaken to supply needed information concerning a measurement procedure which could be used with facility under field conditions to give reliable estimates of transpiration by forests of the Tennessee Valley.Methods of measuring transpiration have been summarized from the literature (5, 10) and need not be described here. All of these methods are subject to criticism either on theoretical grounds or on the basis of operational difficulties. Most of them are impractical under field conditions, especially when applied to tree species. This is illustrated in the studies of MINCKLER (11) in which the water loss of 12 trees of five species was determined by an adaptation of the air-stream method. This procedure, one of the most acceptable under laboratory conditions, is quite cumbersome in application to field conditions. The cobalt chloride procedure, first proposed by Stahl, has particular appeal for field studies since it requires no elaborate equipment and determinations can be made simply and quickly (4). Numerous studies of plant transpiration have been made with this procedure (4,8,13). MEYER (9) and BLAYDES (1, 2, 3) have brought out a number of practical possibilities and limitations of the method and have emphasized that it does not measure absolute rates of transpiration that occur normally, but that the rates determined depend on certain abnormal conditions which may be imposed on the transpiring leaf surface by the cobalt chloride test paper. They considered that by careful standardization of the test paper the relative rates of