Edoardo Daly

Soil Water Balance and Ecosystem Response to Climate Change

Some essential features of the terrestrial hydrologic cycle and ecosystem response are singled out by confronting empirical observations of the soil water balance of different ecosystems with the results of a stochastic model of soil moisture dynamics. The simplified framework analytically describes how hydroclimatic variability (especially the frequency and amount of rainfall events) concurs with soil and plant characteristics in producing the soil moisture dynamics that in turn impact vegetation conditions. The results of the model extend and help interpret the classical curve of Budyko, which relates evapotranspiration losses to a dryness index, describing the partitioning of precipitation into evapotranspiration, runoff, and deep infiltration. They also provide a general classification of soil water balance of the world ecosystems based on two governing dimensionless groups summarizing the climate, soil, and vegetation conditions. The subsequent analysis of the links among soil moisture dynamics, plant water stress, and carbon assimilation offers an interpretation of recent manipulative field experiments on ecosystem response to shifts in the rainfall regime, showing that plant carbon assimilation crucially depends not only on the total rainfall during the growing season but also on the intermittency and magnitude of the rainfall events.

Stochastic modeling of soil salinity

A minimalist stochastic model of primary soil salinity is proposed, in which the rate of soil salinization is determined by the balance between dry and wet salt deposition and the intermittent leaching events caused by rainfall events. The long term probability density functions of salt mass and concentration are found by reducing the coupled soil moisture and salt mass balance equation to a single stochastic differential equation driven by multiplicative Poisson noise. The novel analytical solutions provide insight on the interplay of the main soil, plant and climate parameters responsible for long‐term soil salinization. In particular, they show the existence of two distinct regimes, one where the mean salt mass remains nearly constant (or decreases) with increasing rainfall frequency, and another where mean salt content increases markedly with increasing rainfall frequency. As a result, relatively small reductions of rainfall in drier climates may entail dramatic shifts in longterm soil salinization trends, with significant consequences e.g. for climate change impacts on rain‐fed agriculture

Coupled Dynamics of Photosynthesis, Transpiration, and Soil Water Balance. Part I: Upscaling from Hourly to Daily Level

The governing equations of soil moisture dynamics, photosynthesis, and transpiration are reviewed and coupled to study the dependence of plant carbon assimilation on soil moisture. The model follows the scheme of the soil‐plant‐atmosphere continuum (SPAC) and uses a simplified model of the atmospheric boundary layer to arrive at an upscaled, parsimonious representation at the daily time scale. The analysis of soil moisture, transpiration, and carbon assimilation dynamics provides an assessment of the role of soil, plant, and boundary layer characteristics on the diurnal courses of photosynthesis and transpiration rates, while the subsequent upscaling at the daily level provides a functional dependence of stomatal conductance on soil moisture that is in good agreement with field experiments. The upscaled dependence of transpiration and carbon assimilation on soil moisture is used in Part II of this paper to explore the impact of soil moisture dynamics on plant conditions when rainfall variability is explicitly considered.

Probabilistic characterization of base flows in river basins: Roles of soil, vegetation, and geomorphology

[1] In this paper we extend previous mathematical results on the probabilistic modeling of base flows driven by spatial and temporal fluctuations of soil moisture and affected by intermittent rainfall forcings and by heterogeneous transport, soil, and vegetation properties. Rainfall is modeled as a zero-dimensional marked Poisson process with exponentially distributed intensity, and various descriptors of spatial heterogeneity are used. The master equation for the probability distribution (pdf) of the base flow (here epitomized by the mean daily flow rate in suitably sized catchments) and its moment-generating function are derived by coupling soil moisture balances with a traveltime formulation of transport. Exact solutions for the flow moments are derived in the following cases: (1) two tributary areas in parallel, (2) the rigorous extension to N subbasins, and (3) a simplified geomorphic arrangement of subbasins. Base flow statistics obtained by naive spatial averages of heterogeneous properties exhibit the same mean of the exact solution but may significantly overestimate higher-order moments. Relatively wet climate conditions seem to enhance the effects of the heterogeneity of soil, vegetation, transport, and geomorphic properties, particularly for low-stage flow regimes. The probabilistic structure of the base flow is explicitly linked to relevant climatic and geomorphologic features in addition to the spatial distribution of soil and vegetation properties, with possible ecohydrological implications on long-term water and nutrient mass balances in river basins.

Coupled Dynamics of Photosynthesis, Transpiration, and Soil Water Balance. Part II: Stochastic Analysis and Ecohydrological Significance

The coupled dynamics of soil moisture, transpiration, and assimilation are studied at the daily time scale by temporally upscaling the hourly time scale results obtained in a companion paper. The effects of soil and vegetation characteristics on soil moisture dynamics at the daily time scale and the parameters characterizing the dependence of transpiration and assimilation on soil water content are analyzed and discussed. The daily leaf carbon assimilation is then coupled to a stochastic soil moisture model to obtain a probabilistic description of the carbon assimilation during a growing season. The rainfall regime, in terms of both frequency and amount of precipitation, controls the mean assimilation during a growing season that reaches a maximum for an intermediate range of daily rainfall probabilities, indicating the existence of a rainfall regime that is most effective for plant productivity. The analysis of the duration and frequency of periods of no assimilation provides a measure of plant water stress as a function of the soil, vegetation, and climate characteristics. The results are in good agreement with the dynamic water stress defined in Porporato et al. on the basis of the crossing properties of the stochastic soil moisture dynamics.

Intensification of future severe heat waves in India and their effect on heat stress and mortality

Heat waves are expected to intensify around the globe in the future, with potential increase in heat stress and heat-induced mortality in the absence of adaptation measures. India has a high current exposure to heat waves, and with limited adaptive capacity, impacts of increased heat waves might be quite severe. This paper presents the first projections of future heat waves in India based on multiple climate models and scenarios for CMIP5 data. We find that heat waves are projected to be more intense, have longer durations and occur at a higher frequency and earlier in the year. Southern India, currently not influenced by heat waves, is expected to be severely affected by the end of the twenty-first century. Projections indicate that a sizable part of India will experience heat stress conditions in the future. In northern India, the average number of days with extreme heat stress condition during pre-monsoon hot season will reach 30. The intensification of heat waves might lead to severe heat stress and increased mortality.

Probabilistic dynamics of soil nitrate: Coupling of ecohydrological and biogeochemical processes

Reference ECHO-ARTICLE-2008-008doi:10.1029/2007WR006108View record in Web of Science Record created on 2009-06-22, modified on 2017-07-16

The hysteresis response of soil CO2 concentration and soil respiration to soil temperature

Diurnal hysteresis between soil temperature (Ts) and both CO2 concentration ([CO2]) and soil respiration rate (Rs) were reported across different field experiments. However, the causes of these hysteresis patterns remain a subject of debate, with biotic and abiotic factors both invoked as explanations. To address these issues, a CO2 gas transport model is developed by combining a layer-wise mass conservation equation for subsurface gas phase CO2, Fickian diffusion for gas transfer, and a CO2 source term that depends on soil temperature, moisture, and photosynthetic rate. Using this model, a hierarchy of numerical experiments were employed to disentangle the causes of the hysteretic [CO2]-Ts and CO2 flux Ts (i.e., F-Ts) relations. Model results show that gas transport alone can introduce both [CO2]-Ts and F-Ts hystereses and also confirm prior findings that heat flow in soils lead to [CO2] and F being out of phase with Ts, thereby providing another reason for the occurrence of both hystereses. The area (Ahys) of the [CO2]-Ts hysteresis near the surface increases, while the Ahys of the Rs-Ts hysteresis decreases as soils become wetter. Moreover, a time-lagged carbon input from photosynthesis deformed the [CO2]-Ts and Rs-Ts patterns, causing a change in the loop direction from counterclockwise to clockwise with decreasing time lag. An asymmetric 8-shaped pattern emerged as the transition state between the two loop directions. Tracing the pattern and direction of the hysteretic [CO2]-Ts and Rs-Ts relations can provide new ways to fingerprint the effects of photosynthesis stimulation on soil microbial activity and detect time lags between rhizospheric respiration and photosynthesis.

A stochastic model of streamflow for urbanized basins

Given the critical role of the streamflow regime for instream, riparian, and floodplain ecosystem sustainability, modeling the long-term effect of urbanization on streamflow is important to predict possible changes in stream ecosystems. Since flow duration curves are largely used to characterize the streamflow regime and define indices for stream ecosystem health, we present two stochastic models, with different levels of complexity, that link the key physical features of urbanized basins with rainfall variability to determine the resulting flow duration curves. The two models are tested against 11 basins with various degrees of urban development, characterized by the percentage of impervious areas in the basin. Results show that the more complex model needs to be used to reproduce accurately the entire flow duration curve. The analysis performed suggests that the transformation of green (i.e., water used in evapotranspiration) to blue (i.e., streamflow) water in urbanized basins is an important long-term source of ecohydrological alteration. The modeling scheme also provides useful links between rainfall variability, urbanization levels, and some streamflow indices of high and low flows.

A review of ion and metal pollutants in urban green water infrastructures

In urban environments, the breakdown of chemicals and pollutants, especially ions and metal compounds, can be favoured by green water infrastructures (GWIs). The overall aim of this review is to set the basis to model GWIs using deterministic approaches in contrast to empirical ones. If a better picture of chemicals and pollutant input and an improved understanding of hydrological and biogeochemical processes affecting these pollutants were known, GWIs could be designed to efficiently retain these pollutants for site-specific meteorological patterns and pollutant load. To this end, we surveyed the existing literature to retrieve a comprehensive dataset of anions and cations, and alkaline and transition metal pollutants incoming to urban environments. Based on this survey, we assessed the pollution load and ecological risk indexes for metals. The existing literature was then surveyed to review the metal retention efficiency of GWIs, and possible biogeochemical processes related to inorganic metal compounds were proposed that could be integrated in biogeochemical models of GWIs.