WATER UPTAKE BY PLANTS UNDER VARYING SOIL MOISTURE CONDITIONS: AN EXPERIMENTAL AND NUMERICAL STUDY FOR UNRAVELING COMPENSATED ROOT WATER UPTAKE AND HYRAULIC REDISTRIBUTION

Abstract

Plants growth and adaptation are modulated by root mediated water and nutrient extraction from the vadose zone. The root water uptake (RWU)is moderated by their root system, soil property, and atmospheric factors in response to diurnal and seasonal variations in rhizosphere moisture content. When soil moisture is heterogeneously distributed, plants adopt different water uptake strategies to sustain transpiration demand and to the mitigate stress induced wilting. The capability of plant roots to cope with this moisture heterogeneity is mediated by compensated root water uptake (CRWU) and hydraulic redistribution (HR). In CRWU, enhanced water extraction occurs from a high soil moisture zone to compensate for the limited supply from a dry zone. Whereas HR is a root mediated reallocation of water in which soil moisture is extracted from the wet region and realised into the dry zone. Despite their interconnected role in the terrestrial water cycle, current RWU models often consider them separately. The current study unveils the magnitude, coexistence and mutual impact of CRWU and HR in Zea mays under heterogeneous soil moisture conditions using both experimental and modeling approaches. A series of practical experiments were conducted first using hydraulically isolated split-root lysimeters under controlled experiments. The CRWU was found a major contributor towards meeting the transpiration demand while the HR was insignificant. A novel RWU term was then developed to simulate CRWU and HR together and compared with existing RWU models by incorporating them into the HYDRUS-1D platform. In general, integrating CRWU and HR into RWU models increased the actual transpiration prediction. Particularly, CRWU contributes significantly towards the daily transpiration demand than HR under limiting soil moisture conditions in shallow rooted plants like Zea mays. However, with an external water source and in deep rooted plants, both CRWU and HR contribute considerably towards the transpiration demand. The upgraded model was found reliable in predicting evapotranspiration, soil moisture dynamics, and the interaction between soil water and atmosphere which would be useful in devising better irrigation and fertigation strategies to minimize impact of seasonal drought. The RWU modeling framework developed in this study useful in the selection of commonly used RWU models with respect to the parameter availability, accuracy and root zone properties. The incorporation of these RWU models into the HYDRUS 1D platform provides a single common platform for predicting accurate water uptake by plants and soil moisture regimes for varying field applications.