Soil hydraulic conductivity is the most important parameter affecting the economic and technical feasibility of subsurface drainage projects. The main aim of this research is to select an effective method for smoothing hydraulic conductivity data using several interpolation techniques. The data set consisted of 605 auger hole hydraulic conductivity measurements on a regular 1000-meter square grid from a 47000 ha areas in south east of Iran. The ordinary kriging, weighted moving average and Thin Plate Smoothing Splines (TPSS) have been used in this research, the precision of each method being evaluated by cross-validation technique. Due to availability of two measurement methods (in saturated and unsaturated soils), the related data to each method is studied differently. Also, the adjusted, unsaturated and saturated hydraulic conductivity have been evaluated separately. The results show that the spatial correlation of hydraulic conductivity in this area is low. For example, on regular 1000 m grid, the semi-variogram of saturated hydraulic conductivity was characterized by an impact range of influence equal to 15 km and a nugget effect corresponding to 83% of the sill. Therefore, the kriging method for evaluation of hydraulic conductivity is not recommended. The TPSS method with a power of two had the highest precision (MAE equal to 1.304 m/day) for estimation of soil saturated hydraulic conductivity in this area. The most suitable method for estimation of unsaturated hydraulic conductivity was the TPSS with a power of two or three with MAE equal to 0.166 and 0.241 m/day, respectively. For evaluating the adjusted hydraulic conductivity, the TPSS with a power of two (MAE equal to 1.094 m/day) reveals the best results. The precision of estimated hydraulic conductivity in saturated and unsaturated areas and also adjusted values have been compared with the average amounts. The results show that the evaluation of adjusted hydraulic conductivity method leads to the highest precision.
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