Modelling soil water dynamics in the unsaturated zone — State of the art

Abstract

This paper reviews the principles underlying water dynamics in the unsaturated zone and gives an overview of simulation modelling of soil water flow in the unsaturated zone. The basic relations describing kinetics of flow and governing equations of flow in the unsaturated zone are presented in a general form considering unsteady multidimensional anisotropic and nonhomogeneous flow in the presence of sinks/sources. The influence of overburden potential, characterizing swelling soils, on water transport through the unsaturated zone is discussed. Outlines of thermally-induced water flow are presented with an extension to the specific case of frozen soils. Complex relationships describing the hydrological system are reduced into the one-dimensional vertical flow cases, for which the mathematical models are defined. The entire model has the form of a set of partial differential equations together with auxiliary conditions, that describe the system's geometry, parameters, boundary conditions and in case of transient flow also the initial conditions. Simulation is defined as operation with such a mathematical model. Numerical approximations to solve governing equations of unsaturated water flow are emphasized. Recently-introduced numerical methods are discussed pointing out their advantages and limitations. Atmospheric boundary conditions of the modelled system are described with a maximizing procedure of the flux through the soil-air interface. Several options to define lower boundary conditions are discussed and a special type of lower boundary condition is given for the case that unsaturated flow is coupled with a regional groundwater model. Simulation models require data concerning input, calibration and verification. The current status of collecting model parameters is discussed as well as measurement of common verification data, i.e. phreatic surface, matric head, water content and actual evapotranspiration. Preferential flow of water through unsaturated soil has considerable consequences for simulating the field water balance. The attempts to simulate such hydrological systems are also evaluated. Some transient flow problems are affected by hysteresis. Successful attempts to build hysteresis into dynamic simulation models are still scarce, though recently a number of solutions have been presented. Finally, several practical examples of simulation of flow problems are presented. These examples are taken from everyday water management practice and document the wide range of applicability of simulation models.