15.1 Soil–plant–atmosphere continuum

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Figure 15.1 A simplified global hydrological cycle. Water condenses in clouds and falls as rain on oceans and land. Surface runoff, plus uptake by roots and percolation down to the water table depletes soil water over time. Soil water and groundwater also supply streams, rivers and eventually the sea through lateral How. Water returns to the atmosphere as transpiration from vegetation and as evaporation from oceans. The soil-plant-atmosphere continuum represents a major return path for water from soil to atmosphere. Variations in water content of soil, plant and atmosphere influence the movement of water within this continuum (Based on Swanage 1996)

In a simplified hydrological cycle (Figure 15.1) a water molecule, present as vapour within a cloud, can, over a day, condense into rain, fall to the ground, percolate a short distance into the soil, be taken up by plant roots and then be lost as transpiration as water vapour again. This return path for water from soil to atmosphere, or soil–plant–atmosphere continuum, was formalised by R.O. Slatyer (1967) and emphasises the connectedness of soil, plant and atmospheric water. Phenomena involving water movement within this continuum include root and whole-plant hydraulic conductance, xylem embolism and hydraulic lift by roots.