Including Uncertainty of Hydraulic Conductivity into Drainage Design

Abstract

Hydraulic conductivity is one of the most important factors controlling water‐table depth in subsurface drained agricultural land. The current design procedure is to calculate lateral drain spacing using some average of hydraulic conductivity measurements within the area to be drained. In this paper, a procedure is presented to take into account uncertainty in hydraulic conductivity and water‐table depth when delineating areas to which a single lateral drain spacing can be assigned. The procedure uses simple block kriging of log‐transform hydraulic conductivity data to estimate the effective hydraulic conductivity of square blocks with sides equal to lateral drain spacing. (Kriging is a statistical method, based on the theory of regionalized variables and developed by D. G. Krige, for local estimation of soil properties.) Kriged estimates and estimation variances are combined to define the cumulative distribution function of the water‐table depth, which is the basis for deciding whether a given area needs to be subdivided to achieve adequate water‐table control. This procedure can be implemented for interactive use into subsurface drainage design software using computer‐aided design (CAD) and geographical information system (GIS) technology. The method is illustrated with the Hooghoudt's equation and applied to a practical design situation in a 80‐ha area in the Nile Delta of Egypt.