Browsing by Author "Porporato, A"

Now showing 1 - 13 of 13

Open Access
Analysis of soil carbon transit times and age distributions using network theories
(Journal of Geophysical Research: Biogeosciences, 2009-01-01) Manzoni, S; Katul, GG; Porporato, A
The long-term soil carbon dynamics may be approximated by networks of linear compartments, permitting theoretical analysis of transit time (i.e., the total time spent by a molecule in the system) and age (the time elapsed since the molecule entered the system) distributions. We compute and compare these distributions for different network. configurations, ranging from the simple individual compartment, to series and parallel linear compartments, feedback systems, and models assuming a continuous distribution of decay constants. We also derive the transit time and age distributions of some complex, widely used soil carbon models (the compartmental models CENTURY and Rothamsted, and the continuous-quality Q-Model), and discuss them in the context of long-term carbon sequestration in soils. We show how complex models including feedback loops and slow compartments have distributions with heavier tails than simpler models. Power law tails emerge when using continuous-quality models, indicating long retention times for an important fraction of soil carbon. The responsiveness of the soil system to changes in decay constants due to altered climatic conditions or plant species composition is found to be stronger when all compartments respond equally to the environmental change, and when the slower compartments are more sensitive than the faster ones or lose more carbon through microbial respiration. Copyright 2009 by the American Geophysical Union.
Open Access
Boom and bust carbon-nitrogen dynamics during reforestation
(Ecological Modelling, 2017-09-24) Parolari, AJ; Mobley, ML; Bacon, AR; Katul, GG; Richter, DDB; Porporato, A
© 2017 Elsevier B.V. Legacies of historical land use strongly shape contemporary ecosystem dynamics. In old-field secondary forests, tree growth embodies a legacy of soil changes affected by previous cultivation. Three patterns of biomass accumulation during reforestation have been hypothesized previously, including monotonic to steady state, non-monotonic with a single peak then decay to steady state, and multiple oscillations around the steady state. In this paper, the conditions leading to the emergence of these patterns is analyzed. Using observations and models, we demonstrate that divergent reforestation patterns can be explained by contrasting time-scales in ecosystem carbon-nitrogen cycles that are influenced by land use legacies. Model analyses characterize non-monotonic plant-soil trajectories as either single peaks or multiple oscillations during an initial transient phase controlled by soil carbon-nitrogen conditions at the time of planting. Oscillations in plant and soil pools appear in modeled systems with rapid tree growth and low initial soil nitrogen, which stimulate nitrogen competition between trees and decomposers and lead the forest into a state of acute nitrogen deficiency. High initial soil nitrogen dampens oscillations, but enhances the magnitude of the tree biomass peak. These model results are supported by data derived from the long-running Calhoun Long-Term Soil-Ecosystem Experiment from 1957 to 2007. Observed carbon and nitrogen pools reveal distinct tree growth and decay phases, coincident with soil nitrogen depletion and partial re-accumulation. Further, contemporary tree biomass loss decreases with the legacy soil C:N ratio. These results support the idea that non-monotonic reforestation trajectories may result from initial transients in the plant-soil system affected by initial conditions derived from soil changes associated with land-use history.
Open Access
Causality across rainfall time scales revealed by continuous wavelet transforms
(JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 2010-07-31) Molini, A; Katul, GG; Porporato, A
Open Access
Comparative study of ecohydrological streamflow probability distributions
(Water Resources Research, 2010-09-17) Ceola, S; Botter, G; Bertuzzo, E; Porporato, A; Rodriguez-Iturbe, I; Rinaldo, A
We run a comparative study of ecohydrological models of streamflow probability distributions (pdfs), p(Q), derived by Botter et al. (2007a, 2009), against field data gathered in different hydrological contexts. Streamflows measured in several catchments across various climatic regions of northeastern Italy and the United States are employed. The relevance of the work stems from the implied analytical predictive ability of hydrologic variability, whose role on stream and riparian ecological processes and large-scale management schemes is fundamental. The tools employed are analytical models of p(Q) (and of the related flow duration curve, D(Q)) derived by coupling suitable storage-discharge relations with a stochastic description of streamflow production through soil moisture dynamics, and are expressed as a function of few macroscopic rainfall, soil, vegetation and geomorphological parameters. In this work we compare the performances of a recent version of the model (which includes the effects of nonlinear subsurface storage-discharge relations) to those provided by the linear version through the application of the models to 13 test catchments belonging to various climatic and geomorphic contexts. A general agreement between predicted and observed daily streamflows pdfs is shown, though differences emerge between the linear and the nonlinear approaches. In particular, by including the effects of a nonlinear storage-discharge relation the model accuracy is shown to increase with respect to the linear scheme in most examined cases. We show that this is not simply attributable to the added parameter but corresponds to a proper likelihood increase. The nonlinear model is shown to exhibit three basic forms for p(Q) (monotonically decreasing with an atom of probability in Q = 0, bell-shaped with the mode close to zero, bell-shaped with the mode close to the mean), corresponding to different hydrologic regimes which are clearly detectable in field data. Inferences on the nonlinear character of the relation between subsurface storage and discharge from observed p(Q) are finally drawn. Copyright 2010 by the American Geophysical Union.
Open Access
Local kinetic interpretation of entropy production through reversed diffusion.
(Phys Rev E Stat Nonlin Soft Matter Phys, 2011-10) Porporato, A; Kramer, PR; Cassiani, M; Daly, E; Mattingly, J
The time reversal of stochastic diffusion processes is revisited with emphasis on the physical meaning of the time-reversed drift and the noise prescription in the case of multiplicative noise. The local kinematics and mechanics of free diffusion are linked to the hydrodynamic description. These properties also provide an interpretation of the Pope-Ching formula for the steady-state probability density function along with a geometric interpretation of the fluctuation-dissipation relation. Finally, the statistics of the local entropy production rate of diffusion are discussed in the light of local diffusion properties, and a stochastic differential equation for entropy production is obtained using the Girsanov theorem for reversed diffusion. The results are illustrated for the Ornstein-Uhlenbeck process.
Open Access
Natural streamflow regime alterations: Damming of the Piave river basin (Italy)
(Water Resources Research, 2010-06-01) Botter, G; Basso, S; Porporato, A; Rodriguez-Iturbe, I; Rinaldo, A
A novel approach is proposed to estimate the natural streamflow regime of a river and to assess the extent of the alterations induced by dam operation related to anthropogenic (e.g., agricultural, hydropower) water uses in engineered river basins. The method consists in the comparison between the seasonal probability density function (pdf) of observed streamflows and the purportedly natural streamflow pdf obtained by a recently proposed and validated probabilistic model. The model employs a minimum of landscape and climate parameters and unequivocally separates the effects of anthropogenic regulations from those produced by hydroclimatic fluctuations. The approach is applied to evaluate the extent of the alterations of intra-annual streamflow variability in a highly engineered alpine catchment of north-eastern Italy, the Piave river. Streamflows observed downstream of the regulation devices in the Piave catchment are found to exhibit smaller means/modes, larger coefficients of variation, and more pronounced peaks than the flows that would be observed in the absence of anthropogenic regulation, suggesting that the anthropogenic disturbance leads to remarkable reductions of river flows, with an increase of the streamflow variability and of the frequency of preferential states far from the mean. Some structural limitations of management approaches based on minimum streamflow requirements (widely used to guide water policies) as opposed to criteria based on whole distributions are also discussed. Copyright © 2010 by the American Geophysical Union.
Open Access
Role of microtopography in rainfall-runoff partitioning: An analysis using idealized geometry
(Water Resources Research, 2010) Thompson, SE; Katul, GG; Porporato, A
Open Access
Scale-wise evolution of rainfall probability density functions fingerprints the rainfall generation mechanism
(Geophysical Research Letters, 2010-01-01) Molini, A; Katul, GG; Porporato, A
© 2010 by the American Geophysical Union.The cross-scale probabilistic structure of rainfall intensity records collected over time scales ranging from hours to decades at sites dominated by both convective and frontal systems is investigated. Across these sites, intermittency build-up from slow to fast time-scales is analyzed in terms of heavy tailed and asymmetric signatures in the scale-wise evolution of rainfall probability density functions (pdfs). The analysis demonstrates that rainfall records dominated by convective storms develop heavier-Tailed power law pdfs toward finer scales when compared with their frontal systems counterpart. Also, a concomitant marked asymmetry build-up emerges at such finer time scales. A scale-dependent probabilistic description of such fat tails and asymmetry appearance is proposed based on a modified q-Gaussian model, able to describe the cross-scale rainfall pdfs in terms of the nonextensivity parameter q, a lacunarity (intermittency) correction and a tail asymmetry coefficient, linked to the rainfall generation mechanism.
Open Access
The effect of accelerated soil erosion on hillslope morphology
(Earth Surface Processes and Landforms, 2019-12-01) Bonetti, S; Richter, DD; Porporato, A
© 2019 John Wiley & Sons, Ltd. Intensive agricultural land use can have detrimental effects on landscape properties, greatly accelerating soil erosion, with consequent fertility loss and reduced agricultural potential. To quantify the effects of such erosional processes on hillslope morphology and gain insight into the underlying dynamics, we use a twofold approach. First, a statistical analysis of topographical features is conducted, with a focus on slope and gradient distributions. The accelerated soil erosion is shown to be fingerprinted in the distribution tails, which provide a clear statistical signature of this human-induced land modification. Theoretical solutions are then derived for the hillslope morphology and the associated creep and runoff erosion fluxes, allowing us to distinguish between the main erosional mechanisms operating in disturbed and undisturbed areas. We focus our application on the landscape at the Calhoun Critical Zone Observatory in the US Southern Piedmont, where severe soil erosion followed intensive cotton cultivation, resulting in highly eroded and gullied hillslopes. The observed differences in hillslope morphologies in disturbed and undisturbed areas are shown to be related to the disruption of the natural balance between soil creep and runoff erosion. The relaxation time required for the disturbed hillslopes to reach a quasi-equilibrium condition is also investigated. © 2019 John Wiley & Sons, Ltd.
Open Access
The Formation of Clay-Enriched Horizons by Lessivage
(Geophysical Research Letters, 2018-08-16) Calabrese, S; Richter, DD; Porporato, A
©2018. American Geophysical Union. All Rights Reserved. Soils with argillic horizons comprise more than 25% of the Earth's surface. Although their origin is still debated, lessivage is often invoked to explain them, but the long timescales involved hinder its direct experimental verification. We present a parsimonious model of clay transport, formulated for long timescales over which lessivage is modeled stochastically, complemented by detailed field observations. This probabilistic description allows us to predict the clay profile, the depth of the Bt horizon from the surface, and the mean clay residence time. The results are tested with field measurements at different locations in the Calhoun Critical Zone Observatory. Dimensional analysis unveils two dimensionless parameters governing lessivage dynamics, leading to a classification based on erosion rates and lessivage characteristics. We identify static and eluviated regimes, in which erosion or eluviation prevails, and an illuviated regime, in which the balance between lessivage and erosion brings about the formation of a Bt horizon.
Open Access
The rainfall-no rainfall transition in a coupled land-convective atmosphere system
(Geophysical Research Letters, 2010-07-01) Konings, AG; Katul, GG; Porporato, A
A one-dimensional representation of the atmospheric boundary layer (ABL) depth is coupled to a soil moisture bucket model to dynamically explore the relative roles of surface and free atmospheric conditions on convective precipitation occurrence and resulting soil moisture states. This occurrence is taken to depend on the crossing of the ABL height and the lifting condensation level in the presence of pure convective instability. If rainfall occurs (unrealistically) whenever these conditions are met, and free atmospheric conditions are constant, the resulting system state evolves towards a limit cycle with precipitation every day or every few days, or to a completely dry state. The free atmospheric humidity profile has a larger effect on determining the stationary soil moisture state than the temperature profile. The effect of dry air entrainment on surface energy partitioning decreases soil moisture sensitivity to free atmospheric conditions. © 2010 by the American Geophysical Union.
Open Access
The role of tectonic uplift, climate, and vegetation in the long-term terrestrial phosphorous cycle
(Biogeosciences, 2010-07-02) Buendía, C; Kleidon, A; Porporato, A
Phosphorus (P) is a crucial element for life and therefore for maintaining ecosystem productivity. Its local availability to the terrestrial biosphere results from the interaction between climate, tectonic uplift, atmospheric transport, and biotic cycling. Here we present a mathematical model that describes the terrestrial P-cycle in a simple but comprehensive way. The resulting dynamical system can be solved analytically for steady-state conditions, allowing us to test the sensitivity of the P-availability to the key parameters and processes. Given constant inputs, we find that humid ecosystems exhibit lower P availability due to higher runoff and losses, and that tectonic uplift is a fundamental constraint. In particular, we find that in humid ecosystems the biotic cycling seem essential to maintain long-term P-availability. The time-dependent P dynamics for the Franz Josef and Hawaii chronosequences show how tectonic uplift is an important constraint on ecosystem productivity, while hydroclimatic conditions control the P-losses and speed towards steady-state. The model also helps describe how, with limited uplift and atmospheric input, as in the case of the Amazon Basin, ecosystems must rely on mechanisms that enhance P-availability and retention. Our novel model has a limited number of parameters and can be easily integrated into global climate models to provide a representation of the response of the terrestrial biosphere to global change. © 2010 Author(s).
Open Access
Traditional and microirrigation with stochastic soil moisture
(WATER RESOURCES RESEARCH, 2010-03-09) Vico, G; Porporato, A

Browsing by Author "Porporato, A"

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