STUDY ON UNSATURATED VERTICAL FLOW, ANTECEDENT PRECIPITATION INDEX AND DISCHARGE OF FOREST SLOPE ON THE BASIS OF WATER CONTENT OF SOIL

To clarify the mechanism of water movement through a natural forest slope, its soil water content was measured and discussed on the relation between its change rate and unsaturated vertical flow, antecedent precipitation index (API) and slope discharge. Soil water content was measured during and after rainfall events by electric reflectometers at 3 depths (from 0.1 to 1.0m) of 3 places and its change rate was calculated. The relation between water content of shallow soil and its change rate suggested that the vertical water percolation was caused under conditions of unsaturated hydraulic conductivity proportional to (θ-θr)n1 (θ: volumetric water content, θr: residual volumetric water content, n1: constant) and hydraulic head gradient equal to 1. The relation between averaged water content and values of several types of API showed that API2(t)=Σib2 P(t-i) (P(t-i): precipitation on the i-th day before the t-th day, b2:contant) is good as a slope API. The slope discharge could be expressed as a function proportional to (θ-θr)n2 (n2:constant) with θ averaged spatially and this function could estimate the discharge from a slope during rainfall.