Inverse modeling approach for determining soil hydraulic properties as affected by application of cattle manure

Numerical codes are extensively used in the modeling of water and solute transport in the vadose zone. The application of these codes depends on knowledge of soil hydraulic properties such as soil water retention curve and hydraulic conductivity. Application of cattle manure to the soil can increase soil organic matter (SOM) contents. Increases in SOM associated with changes in the structure and adsorption properties of soil and, thus, their hydraulic properties. In this study the effect of cattle manure on soil hydraulic properties was investigated using inverse method. Applied inverse method was based on Levenberg-Marquart optimization algorithm to estimate hydraulic properties of soil in transient condition using C++ programming language along with forward model (HydroGeoSphere) as a numerical code. Nine iron cylinders of 57 cm in inner diameter and about 40 cm in height were filled with Sandy clay loam soil of 30 cm in height. Cattle manure applied at 0, 30, and 60 Mg/ha at three replications in a completely random design. One year after cattle manure application, saturated hydraulic conductivity, porosity, and water retention curve parameters (van Genuchten function, alpha and beta) were estimated using inverse method. Statistical analysis showed that the automatic calibration is sensitive to alpha more than the other parameters. The results showed that porosity, saturated hydraulic conductivity, residual water content, alpha and beta increased significantly (P<0.05) with application 30 and 60 Mg/ha cattle manure. But there was no significant difference (P<0.05) in beta between application of 30 and 60 Mg/ha cattle manure. The study also indicated that a was 25.0% and 50.0% higher and beta was 9.6% and 12.6% lower than control treatment in 30 and 60 Mg/ha treatments. In addition, application cattle manure showed positive effect on hydraulic parameters of soil.