森林是否减少河流水量(年径流),是人类利用森林措施改造自然的一个实际问题,也是水土保持、涵养水源的一个重要研究课题。

In order to investigate the influence of forest cover upon the annual runoff in detail, we map the annual runoff isogram with the observed data of 1222 station-years from (94) hydrometric stations and the annual precipitation isogram with the observed data of 1534 station-years from 118 raingauge points in the middle reaches of the Yellow River.The annual runoff map shows that the 4epth of mean annual runoff in the wooded areas of the Plateau are less than that of the adjacent woodless areas. It is lower than 25 mm. in wooded area, and 30-40 mm. in woodless areas. Runoff coef-ficient expresses the ratio between runoff and precipitation and reflects the character of runoff formation. The calculation of annual runoff coefficient shown that the annual runoff coefficient in the wooded area is about 30% less than that in the woodless area.We also made a comparison of water balance analysis between five pairs of forested and unforested watersheds.For the purpose of analyzing the influence of fewest cover upon the annual runoff quantitatively, we correlate the forest cover ratio and the mean annual runoff (as well as annual underground runoff) for forested watersheds in Loess Plateau.The results obtained show four pairs in the north of Shensi province having mean annual runoff of the forested watershed less than that of the woodless one. Only one pair of comparative watersheds, the rivers of Venyu and Chongyuan which are of unforested watershed. The geographicals contrary, that is, the mean annual runoff of forested watershed is greater than that of unforested watershed. The geographical condition of this pair differs from other four pairs in that it is covered with stony soil instead of loess.The results obtained by the calculation also prove that the forest cover in Loess Plateau has a negative role for the annual runoff, and in some degree, an active effect on the underground water, that is an action of compensation.