Experimental investigation and conceptual modelling of water-limited transpiration as hydraulic boundary condition in geotechnical engineering problems

Abstract

Water uptake through plant transpiration is a critical hydraulic boundary condition in modelling geostructures under climatic loading. Transpiration occurs in two regimes: energy-limited, controlled by atmospheric evaporative demand; and water-limited, governed by soil hydraulic conductivity around roots. While energy-limited (potential) transpiration is generally modelled using a physically based approach, the water-limited regime is usually modelled macroscopically by way of an empirical transpiration reduction function. This paper first presents an experimental investigation into the influence of soil hydraulic behaviour and potential transpiration rate on water-limited transpiration. Two soil textures were tested – silty sand and clayey-silty sand – planted with Medicago sativa. The use of high-capacity tensiometers enabled the investigation of an unprecedented suction range in clayey soils. Results show that the water-limited branch of the reduction function is non-linear, and its decay is strongly influenced by unsaturated hydraulic conductivity. Furthermore, the suction marking the transition from energy- to water-limited regimes varied with potential transpiration rate and soil hydraulic behaviour. A novel physically based, closed-form reduction function is then formulated to advance water uptake modelling in the water-limited regime. This function is shown to compactly capture the combined effects of soil hydraulic conductivity, root system architecture expressed through root length density and potential transpiration rate on transpiration in the water-limited regime, consistent with observations from both this experimental campaign and the literature data.