بررسي تغييرات هدايت هيدروليكي خاك با استفاده از روش معكوس

Evaluating Temporal Variations of Soil Hydraulic Conductivity Using Inverse Optimization Technique.

One of the common causes that make unexpectedly low irrigation system efficiency, is temporal variations of soil hydraulic properties. In this study, the effect of saline and sodic irrigation water was investigated on infiltration rate in a clay loam soil. Ten weekly irrigations were carried out using water with electrical conductivity and SAR of 12dS/m and 32, respectively. The cumulative infiltration rate was measured on the third, sixth and tenth irrigation events using blocked end furrows. Results showed that infiltration rate was significantly decreased during the three irrigations as indicated above. Decreasing in infiltration rate was due to structural disaggregating caused by sodium. The temporal variations of soil hydraulic properties were estimated using inverse option of HYDRUS-1D model. Due to limitation in estimating several parameters simultaneously, we performed a sensitivity analysis and found that saturated hydraulic conductivity (Ks), van Genuchten n, and saturated water content (9S) were the most sensitive soil hydraulic parameters. Therefore, two scenarios were used to estimate the temporal variations of soil properties. First, only Ks and in a second run both Ks and 9S were estimated simultaneously. The results showed that optimized Ks values decreased with time and it was somewhat difficult to optimize Ks and (theta)s simultaneously. Agreement between measured and calibrated cumulative infiltration was satisfactory. As a conclusion, with present enough measured data, inverse technique can be a suitable method for estimating soil hydraulic properties. We also used Philipʹs equation to fit the measured cumulative infiltration data. The agreement between measured and predicted infiltration data by Philipʹs equation was relatively satisfactory during the third and sixth irrigations and somewhat weak during the tenth irrigation round.