Method for Determining the Modification Coefficient (α) of Hydraulic Conductivity Measured in Fields

Design of pipe drainage for multipurpose paddy fields (II)

Abstract

Our previous paper presented three equations, L=2H√k/D…(1) 1/L²=1/a²+1/b²…(2) k=α·k_s…(3) where L is the spacing of pipe drains, H, depth of the permeable layer, k, hydraulic conductivity, D, requirement for a pipe drain, a and b, spacings of pipe and mole drains for a combined drain system, k_s, hydraulic conductivity observed in fields, and α, the modification coefficient.
On the basis of hypothetical considerations, the authors believe the hydraulic conductivity k in the equation (1) to be very different from k_s, observed in fields. The reason for this is that aggregates, cracks and fractures influence soil structure and effect on hydraulic conductivity. Even the hydraulic conductivity of fractured or blocky structured clayey soil is often much higher than that of sandy soil. Unfortunately, it is very difficult to determine directly the actual k for an entire field, such as by the auger hole method.
The present paper presents a method for determing the actual hydraulic conductivity k for an entire field using the hydrograph of a pipe drain discharge observed in a field already installed with pipe drains on the basis of k_s measured in a field and equation (3). The procedure for obtaining pipe discharge and the estimation of α is presented.