Quantification of root water extraction under salinity and drought

Abstract

Widely different approaches have been proposed for modelling root water uptake and most of them are essentially empirical and contain parameters that depend on specific crop, soil, and environmental conditions. The existing root water uptake functions can be categorized into microscopic and macroscopic approaches. Microscopic models consider the radial flow of soil water toward a representative root of infinite length, uniform thickness, and uniform absorptivity. Because the required input parameters at the soil-root interface are rather difficult to measure, it has not proven practical to test the proposed microscopic models directly. Macroscopic models regard the root system as a whole and assume that under optimal conditions the root water uptake is simply equal to potential transpiration over the root zone. Under non-optimal conditions, i.e. low soil water pressure head and/or high salinity, the potential transpiration decreases based on specified soil water pressure head-dependent and/or osmotic head-dependent reduction functions. Because the parameters needed in macroscopic models are practical to measure, this concept is most widely used in numerical simulation models. The macroscopic concept remains essentially empirical, however, and the input parameter values need to be derived for different plants and climatic conditions. This paper focuses on macroscopic models for separate and combined soil water osmotic and pressure heads varying in time and space.