We determined the amount of information needed to estimate watershed-scale transpiration in a Japanese cedar (Cryptomeria japonica D. Don) forest from sap flow measurements of individual trees. Measurements of tree biometrics (diameter at breast height (DBH) and tree sapwood area (AS_tree)), and tree-to-tree and radial variations in xylem sap flux density (Fd) were made in two stand plots, an upper slope plot (UP) and a lower slope plot (LP), during a growing season characterized by significant variations in environmental factors. We then investigated how mean stand sap flux density (JS) and a tree stem allometric relationship (DBH-AS_tree) varied between the stands. Appropriate sample sizes for estimating representative JS values were determined. Both a unique and a general function allowed description of the allometric relationship along the slope, but the data for its formulation was required for both the UP and LP. Values of JS in the UP and LP were similar during the study period despite differences in tree density and size between the plots, implying that JS measured in a partial stand in a watershed is a reasonable estimate of JS in other stands in the watershed, and that stand sapwood area calculated from AS_tree is a strong determinant of water use in a forest watershed. To estimate JS in both the UP and LP, it was necessary to sample at least 10 trees in each plot.