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Rodriguez99 - WATER RESOURCES RESEARCH VOL 35 NO 12 PAGES...

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On the spatial and temporal links between vegetation, climate, and soil moisture I. Rodriguez-Iturbe, P. D’Odorico, A. Porporato, 1 and L. Ridolfi 1 Department of Civil and Environmental Engineering and Center for Energy and Environmental Studies Princeton University, Princeton, New Jersey Abstract. The impact of climate fluctuations can be observed in the dynamics of vegetation and most particularly in the sensitive environment of savannas. In this paper we present a model for the local competition for soil moisture among neighboring vegetation. The initial condition for the model is a random field where at each point the soil moisture is the mean water content when there are no spatial interactions between sites. The mean soil moisture values account for stochasticity of climate and losses from evapotranspiration and leakage which depend on the existing water content. A spatial dynamics is then implemented based on the explicit minimization of the global water stress over the region. This approach explains the coexistence of herbaceous and woody plants in savannas as well as the changes in canopy density that have been documented in the southwest of the United States as a function of regional climatic fluctuations. 1. Introduction Soil water availability is recognized as the controlling re- source in the organization and functioning of many ecological systems among which savannas are an important and charac- teristic example. Thus, although fire regime, grazing intensity, and especially nutrient availability are among the factors influ- encing ecosystem function in savannas and other biomes, it is commonly accepted that if water is limited, it becomes the key resource affecting vegetational structure and organization. The mechanisms through which water limitation affects ecological systems are of many different kinds, including well-known ones related to carbon assimilation via control of photosynthesis and stomatal closure as well as nitrogen assimilation through the control of the nitrogen mineralization rate [e.g., Scholes and Walker , 1993]. All these mechanisms are intimately related to soil water availability, and their combined effect on a par- ticular plant is termed water stress. This paper expands the analysis and results presented by Rodriguez-Iturbe et al . [1999a] and provides a new framework for studying the impact of climate fluctuations on patterns of vegetation. Before considering interactions among neighboring plants arising from spatially variable soil moisture availability, it is important to reflect on the water balance dynamics at a point and on its driving elements, climate, soil, and vegetation. The quantitative description of this dynamics is not a trivial task. This paper starts with a description of the soil moisture balance at a point, which allows us to define and quantify the possible presence of water stress at a site when there is no consideration of spatial dynamics. Soil moisture at a point is dependent on a
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