Optimal sampling density of hydraulic conductivity for subsurface drainage in the Nile delta

The density of hydraulic conductivity measurements is of primary importance for the design of large-scale subsurface drainage projects. This study was conducted to investigate the effect of sampling density on hydraulic conductivity estimation, and to determine the optimal sampling density using results of 3488 hydraulic conductivity tests from a 33 500 ha subsurface drainage project in the Nile Delta of Egypt. Kriging was used to obtain hydraulic conductivity point and block estimates based on the original 300 m sampling grid and on reduced sampling grids with spacings of 600, 900, and 1200 m. Kriged estimates from the complete grid were compared to those from reduced grids. The correlation coefficient between kriged estimates of block hydraulic conductivity based of the 300 m versus the 600 m measurement grid was larger than 0.90, which indicates that the optimal sampling grid spacing is in the order of 600 m. Slopes of the estimation standard deviation versus sampling density showed that the optimal sampling grid spacing was between 400 and 600 m, and could be determined from sampling grids with a spacing of 900 m or less. It appears that, for the Nile Delta, the optimal sampling density of hydraulic conductivity can be determined from a preliminary survey with a grid spacing of 900 m.