Ignacio Rodriguez-Iturbe

Plants in water-controlled ecosystems: active role in hydrologic processes and response to water stress: II. Probabilistic soil moisture dynamics

A stochastic model for soil moisture dynamics at a point is studied in detail. Rainfall is described as a marked Poisson process, producing a state-dependent infiltration into the soil. Losses due to leakage and evapotranspiration also depend on the existing level of soil moisture through a simplifying but realistic representation of plant physiological characteristics and soil properties. The analytic solution of the steady-state probability distributions is investigated to assess the role of climate, soil, and vegetation in soil moisture dynamics and water balance.

A coupled channel network growth and hillslope evolution model: 1. Theory

This paper presents a model of the long-term evolution of catchments, the growth of their drainage networks, and the changes in elevations within both the channels and the hillslopes. Elevation changes are determined from continuity equations for flow and sediment transport, with sediment transport being related to discharge and slope. The central feature of the model is that it explicitly differentiates between the sediment transport behavior of the channels and the hillslopes on the basis of observed physics, and the channel network extension results solely from physically based flow interactions on the hillslopes. The difference in behavior of channels and hillslopes is one of the most important properties of a catchment. The flow and sediment transport continuity equations in the channel and the hillslope are coupled and account for the long-term interactions of the elevations in the hillslope and in the channels. Sediment transport can be due to fluvial processes, creep, and rockslides. Tectonic uplift may increase overall catchment elevations. The dynamics of channel head advance, and thus network growth, are modeled using a physically based mechanism for channel initiation and growth where a channel head advances when a channel initiation function, nonlinearly dependent on discharge and slope, exceeds a threshold. This threshold controls the drainage density of the basin. A computer implementation of the model is introduced, some simple simulations presented, and the numerics of the solution technique described.

Plants in water-controlled ecosystems : active role in hydrologic processes and response to water stress. III. Vegetation water stress

The reduction of soil moisture content during droughts lowers the plant water potential and decreases transpiration; this in turn causes a reduction of cell turgor and relative water content which brings about a sequence of damages of increasing seriousness. A review of the literature on plant physiology and water stress shows that vegetation water stress can be assumed to start at the soil moisture level corresponding to incipient stomatal closure and reach a maximum intensity at the wilting point. The mean crossing properties of these soil moisture levels crucial for water stress are derived analytically for the stochastic model of soil moisture dynamics described in Part II (F. Laio, A. Porporato, L. Ridolfi, I. Rodriguez-Iturbe. Adv. Water Res. 24 (7) (2001) 707-723). These properties are then used to propose a measure of vegetation water stress which combines the mean intensity, duration, and frequency of periods of soil water deficit. The characteristics of vegetation water stress are then studied under different climatic conditions, showing how the interplay between plant, soil, and environment can lead to optimal conditions for vegetation.

Some Models for Rainfall Based on Stochastic Point Processes

Stochastic models are discussed for the variation of rainfall intensity at a fixed point in space. First, models are analysed in which storm events arise in a Poisson process, each such event being associated with a period of rainfall of random duration and constant but random intensity. Total rainfall intensity is formed by adding the contributions from all storm events. Then similar but more complex models are studied in which storms arise in a Poisson process, each storm giving rise to a cluster of rain cells and each cell being associated with a random period of rain. The main properties of these models are determined analytically. Analysis of some hourly rainfall data from Denver, Colorado shows the clustered models to be much the more satisfactory.

Probabilistic modelling of water balance at a point: the role of climate, soil and vegetation

The soil moisture dynamics under seasonally fixed conditions are studied at a point. The water balance is described through the representation of rainfall as a marked Poisson process which in turn produces an infiltration into the soil dependent on the existing level of soil moisture. The losses from the soil are due to evapotranspiration and leakage which are also considered dependent on the existing soil moisture. The steady–state probability distributions for soil moisture are then analytically obtained. The analysis of the distribution allows for the assessment of the role of climate, soil and vegetation on soil moisture dynamics. Further hydrologic insight is obtained by studying the various components of an average water balance. The realistic representation of the processes acting at a site and the analytical tractability of the model make it well suited for further analyses which consider the spatial aspect of soil moisture dynamics.

Ecohydrology of water-controlled ecosystems : soil moisture and plant dynamics

Ecosystem dynamics in arid and semiarid climates are strongly dependent on the soil water availability which, in turn, is the result of a number of complex and mutually interacting hydrologic processes. This motivates the development of a process-based framework for the analysis of the soil water content in the root zone at the daily time scale. This paper reviews the results that the authors have obtained using a probabilistic–mechanistic model of soil water balance for the characterization of the seasonal regimes of soil moisture with different combinations of climate, soil, and vegetation. Average seasonal soil water content and level-crossing statistics have been used to study conditions of water stress in vegetation. The same framework has been applied to the analysis of the impact of interannual climate fluctuations on the seasonal regime of soil moisture and water stress. 2002 Elsevier Science Ltd. All rights reserved.

Trading-off fish biodiversity, food security, and hydropower in the Mekong River Basin

The Mekong River Basin, site of the biggest inland fishery in the world, is undergoing massive hydropower development. Planned dams will block critical fish migration routes between the rivers downstream floodplains and upstream tributaries. Here we estimate fish biomass and biodiversity losses in numerous damming scenarios using a simple ecological model of fish migration. Our framework allows detailing trade-offs between dam locations, power production, and impacts on fish resources. We find that the completion of 78 dams on tributaries, which have not previously been subject to strategic analysis, would have catastrophic impacts on fish productivity and biodiversity. Our results argue for reassessment of several dams planned, and call for a new regional agreement on tributary development of the Mekong River Basin.

Plants in water-controlled ecosystems: active role in hydrologic processes and response to water stress: I. Scope and general outline

This series of four papers studies the complex dynamics of water-controlled ecosystems from the hydro-ecological point of view [e.g., I. Rodriguez-Iturbe, Water Resour. Res. 36 (1) (2000) 3-9]. After this general outline, the role of climate, soil, and vegetation is modeled in Part II [F. Laio, A. Porporato, L. Ridolfi, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 707-723] to investigate the probabilistic structure of soil moisture dynamics and the water balance. Particular attention is given to the impact of timing and amount of rainfall, plant physiology, and soil properties. From the statistical characterization of the crossing properties of arbitrary levels of soil moisture, Part III develops an expression for vegetation water stress [A. Porporato, F. Laio, L. Ridolfi, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 725-744]. This measure of stress is then employed to quantify the response of plants to soil moisture deficit as well as to infer plant suitability to given environmental conditions and understand some of the reasons for possible coexistence of different species. Detailed applications of these concepts are developed in Part IV [F. Laio, A. Porporato, C.P. Fernandez-Illescas, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 745-762], where we investigate the dynamics of three different water-controlled ecosystems.

On the spatial and temporal links between vegetation, climate, and soil moisture

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.

Positive feedbacks promote power-law clustering of Kalahari vegetation.

The concept of local-scale interactions driving large-scale pattern formation has been supported by numerical simulations, which have demonstrated that simple rules of interaction are capable of reproducing patterns observed in nature. These models of self-organization suggest that characteristic patterns should exist across a broad range of environmental conditions provided that local interactions do indeed dominate the development of community structure. Readily available observations that could be used to support these theoretical expectations, however, have lacked sufficient spatial extent or the necessary diversity of environmental conditions to confirm the model predictions. We use high-resolution satellite imagery to document the prevalence of self-organized vegetation patterns across a regional rainfall gradient in southern Africa, where percent tree cover ranges from 65% to 4%. Through the application of a cellular automata model, we find that the observed power-law distributions of tree canopy cluster sizes can arise from the interacting effects of global-scale resource constraints (that is, water availability) and local-scale facilitation. Positive local feedbacks result in power-law distributions without entailing threshold behaviour commonly associated with criticality. Our observations provide a framework for integrating a diverse suite of previous studies that have addressed either mean wet season rainfall or landscape-scale soil moisture variability as controls on the structural dynamics of arid and semi-arid ecosystems.