森林流域における年消失量と年降水量・標高の関係
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概要
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Forests have two important effects for precipitation -streamflow relationship. One of them is leveling of discharge and another water losses consist of interception, evaporation and transpiration. In the watershed with small precipitation, water losses are important problems. Then we investigated about the relationship between annual water losses and precipitation on 57 watersheds locating in 22 experimental areas in the northern hemisphere. The watersheds are 31 in The United states of America, 22 in Japan, 2 in Switzer land and 2 in Formosa. Annual precipitation and runoff records for a period of several to 10 years on each forested watershed were used in this investigation. Generally speaking, annual water losses increases in proportion to annual precipitation, but in detail it may be gathered the tendency that the losses approach to a fixed value or decrease with more than 1, 800mm of precipitation. These records are classified into two groups with a few exceptions. If the watersheds locate in south of latitude 38°N and mean altitude under 1,000 meters or in south of latitude 33°N and mean altitude over 1,000 meters the relationship should be expressed as following equation 1) L=846 log P-1,753 In the same way, if the watersheds locate in north of latitude 39°N and 35°N～38°N and mean altitude over 1, 000 meters the relationship should be expressed as following equation 2) L=464 log P-857 where : L=annual water losses (mm) P=annual precipitation (mm) On the other hand, it has been already known that the precipitation increase in proportion to altitude. So as an example Coweeta's records indicate that annual water losses or annual water loss rate decrease in proportion to the increase of altitude. The formula is expressed as follows: L =2,236-0.6776 P l =156-0.0560 P L =1,827-0.9472 H l =114-0.0690 H P=0.9853 H+1,006 where : L (mm) =annual water losses l (%) =annual water loss rate P (mm) =annual precipitation H (m) =mean altitude of watershed
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