Estimating evapotranspiration parameters by inverse modelling and non-linear optimization

The use of models like HYDRUS in planning irrigation and fertigation scheduling is increasingly researched. Simulation of the root zone processes using HYDRUS largely depends on characterizing parameters related to unsaturated hydraulic conductivity, root growth and water uptake dynamics. A methodology for estimating evapotranspiration parameters for drip irrigated paddy is developed in the present work. The methodology involves soil moisture logging of root zone followed by modelling the dynamics of soil water movement in the root zone by inversely solving the Richards' equation and evapotranspiration parameters estimation by non-linear programming. All the computer programming was done with MS-EXCEL and it is found that this tool is versatile for this purpose. The results of this work are very useful for using HYDRUS in developing optimal irrigation and fertigation schedule. A threshold level of soil water suction pressure for irrigation scheduling of paddy was also obtained through this work. Temporal changes in the location of the average volumetric water content and average evapotranspiration in the root zone were evaluated. These data are useful for developing guidelines for soil water sensor placement in automatic irrigation triggers.