On the validity of Boulton and Newman's analytic model in the solution of specific yield of coarse-grained unconfined aquifer A NEW METHOD OF DETERMINING THE SPECIFIC YIELD OF LAYERED SOILS IN SHALLOW LAYER UNDER SURFACE 在粗粒无压含水层给水度的求解中解析方法适用性的探讨确定浅部层状土给水度的一种新方法