Evaluation of soil water-carrying capacity for vegetation: the concept and the model

The conception of an area's carrying capacity has proven to be a useful concept at various scales ranging from populations to the entire biosphere, and it appears to be a suitable definition for any level. For soil water-carrying capacity for vegetation (SWCCV), the concept evokes density-dependence research in arid and semi-arid regions. The term is increasingly being used among ecological disciplines but remains vague and elusive. We propose the following definition: the SWCCV is 'the maximum biomass of a given type of vegetation, under specific climatic conditions, soil texture, and management regime, that a given arid or semi-arid area can sustain without diminishing the capacity of soil water to support future generations. Models for assessing the SWCCV of an area of interest, though preliminary, can be classified into three types according to their construction: empirical, conceptual, and process models. Neither the empirical model nor the conceptual model can account for the dynamic interactions between soil water and vegetation for heterogeneous environmental matrices in arid and semi-arid regions. The physically based process model would enable us to effectively analyse and simulate the components of the SWCCV. While water cycle and photosynthesis are not properly integrated in such quantitative models, the effects of water stress on transpiration, carbon-assimilation rates, and its feedback into the water cycle are not generally represented. From our point of view, hydrological and biogeochemical cycles need to be modelled in detail and simultaneously, while considering feedbacks between vegetation growth and soil hydrological processes