长江上游森林林冠截留量与林分郁闭度呈正相关，当亚高山冷杉林的林分郁闭度为0．7时(5～7月)，平均截留率为24％，当郁闭度在0．3时(5～7月)，平均截留率降为9．5％；从枯落物持水量来看，箭竹冷杉林最大(6．0mm)，藓类冷杉林最小(2．8mm)，主要因为前者有较多的落叶伴生树种和灌木。岷江冷杉原始林的土壤最大持水量、枯落物最大持水量、苔藓层最大持水量比皆伐后形成的其他森林类型要大2．3～17．2倍，从而具有更好的水源涵养功能。植被对径流的影响初步结论是，森林大流域的年径流量常常大于少林或无林流域的径流量；不同采伐强度径流量比较是皆伐迹地＞择伐迹地＞原始森林。与全国其他森林类型的蒸发散研究比较显示，长江上游森林的相对蒸散率较低，为30％～40％，这主要是由于海拔较高，降水量大所致。

： The canopy density and the interception rate of Sichuan Miyaluo subalpine forest are in positive correlation，the interception rate reaches 24％ when the canopy density is 0．7 and decreases to 9．5％ when the canopy density is 0．3(measured during May and July)litter of arrow bamboo fir mixed stand has the maximum water retaining capacity(6．0 mm)and music fir mixed stand has the minimum(2．8mm)，because the former has more associated broad leaf trees and shrubs．The maximum soil water holding capacity，maximum litter water holding capacity and maximum moss layer water holding capacity of the primitive fir forest of Minjiang is much greater than that of the other vegetation types such as secondary growth after cutting．The runoff of the well coverd valley is greater than that of wasteland or open forestland．As to different cutting methods，the runoff increment is：clear cutting＞ selection cutting＞no cutting．Compared with other forest types of China，the relative forest evapotranspiration ratio(Evapotranspiration／preci pitation)of the upper reaches of the Yangtze River is relatively low(30～40％)because of the high altitude and the greater amount of precipitation．