University of California Water Resources Center

This study uses five years of field data from the Land Retirement Demonstration Project located in western Fresno County of California to develop a comprehensive theoretical and numerical modeling framework to evaluate the specific site conditions required for a sustainable land retirement ecosystem outcome based on natural drainage. Using field data, principles of mass balance in a control volume, the HYDRUS-1D Software Package for simulating one-dimensional movement of water, heat, and multiple solutes in variably-saturated media, and PEST, a modelindependent parameter optimizer, the processes of soil water and solute movement in root zone and the deep vadose zone were investigated. The optimization of unsaturated soil hydraulic parameters and downward flux (natural drainage) from the control volume against observed vadose zone salinity levels and shallow groundwater levels yield difficult to obtain natural drainage rate as a function of water table height within the control volume. The results show that unsaturated soil hydraulic properties and the downward flux from the soil profile are the critical parameters. A ‘natural drainage approach’ to sustainable land management for drainage impaired land is proposed. With this approach it is feasible to design a sustainable land use regimen for drainage impaired lands in general and retired lands in particular.

Further analysis of data on the evolution of vadose zone salinity and perched water levels also show that effective unsaturated soil hydraulic property and the "natural drainage rate" change with average soil water salinity. The results show that at the same pressure head, soil water content is less with higher soil water salinity as compared to lower soil water salinity. It is thus concluded that the use of soil water salinity invariant soil water hydraulic parameters in numerical modeling can seriously compromise prediction, especially for a variable soil water salinity environment.