Mapping clay content for subsurface drainage in the Nile Delta

Drain envelope requirements for large-scale subsurface drainage projects are often estimated based on the surface area with clay percentages less than a given threshold value. Contouring clay percentage data can be done by computer using spatial interpolation methods. Cross-validation was used to evaluate the precision of estimation for clay percentage using five interpolation techniques (closest neighbor, moving average, weighted moving average, kriging, and cokriging) for a study area of 33,500 ha in the Nile Delta. The clay content was measured at 485 sites. Cokriging used the correlation between clay percentage and saturated hydraulic conductivity, which had been measured at 3488 sites. The lowest mean absolute difference (MAD) between observed and interpolated clay percentage values was obtained for cokriging (12.01). MAD values were very close to that of cokriging in the case of moving average (12.13), weighted moving average (12.23), and kriging (12.21), whereas for the closest neighbor method a MAD value of 16.44 was obtained. Surface areas under 30 and 40 percent clay were over-estimated for the closest neighbor method, and under-estimated for the moving average method. Surface areas and contour maps based on weighted moving average, kriging, and cokriging were very similar. Use of moving average resulted in a loss of precision due to the smoothing effect.