Francesco Viola

Comparative analysis of different techniques for spatial interpolation of rainfall data to create a serially complete monthly time series of precipitation for Sicily, Italy

Abstract The availability of good and reliable rainfall data is fundamental for most hydrological analyses and for the design and management of water resources systems. However, in practice, precipitation records often suffer from missing data values mainly due to malfunctioning of raingauge for specific time periods. This is an important issue in practical hydrology because it affects the continuity of rainfall data and ultimately influences the results of hydrologic studies which use rainfall as input. Many methods to estimate missing rainfall data have been proposed in literature and, among these, most are based on spatial interpolation algorithms. In this paper different spatial interpolation algorithms have been evaluated to produce a reasonably good continuous dataset bridging the gaps in the historical series. The algorithms used are deterministic methods such as inverse distance weighting, simple linear regression, multiple regression, geographically weighted regression and artificial neural networks, and geostatistical models such as ordinary kriging and residual ordinary kriging. In some of these methods, the elevation information, provided by a Digital Elevation Model, has been added to improve estimation of missing data. These algorithms have been applied to the mean annual and monthly rainfall data of Sicily (Italy), measured at 247 raingauges. Optimization of different settings of the various interpolation methods has been carried out using a subset of the available rainfall dataset (modeling set) while the remaining subset (validation set) has been used to compare the results obtained by the different algorithms. Validation results indicate that the univariate methods, neglecting the information of elevation, are characterized by the largest errors, which decrease when the elevation is taken into account. The ordinary kriging of residuals from linear regression between precipitation and elevation, which has provided the best performance at annual and monthly scale, has been used to complete the precipitation monthly time series in Sicily.

Climate change effects on the hydrological regime of small non-perennial river basins.

Recent years have been witnessing an increasing interest on global climate change and, although we are only at the first stage of the projected trends, some signals of climate alteration are already visible. Climate change encompasses modifications in the characteristics of several interrelated climate variables, and unavoidably produces relevant effects on almost all the natural processes related to the hydrological cycle. This study focuses on potential impacts of climate variations on the streamflow regime of small river basins in Mediterranean, seasonally dry, regions. The paper provides a quantitative evaluation of potential modifications in the flow duration curves (FDCs) and in the partitioning between surface and subsurface contributions to streamflow, induced by climate changes projected over the next century in different basins, also exploring the role exerted by different soil–vegetation compositions. To this aim, it is used a recent hydrological model, which is calibrated at five Sicilian (Italy) basins using a past period with available streamflow observations. The model is then forced by daily precipitation and reference evapotranspiration series representative of the current climatic conditions and two future temporal horizons, referring to the time windows 2045–2065 and 2081–2100. Future climatic series are generated by a weather generator, based on a stochastic downscaling of an ensemble of General Circulation Models. The results show how the projected climatic modifications are differently reflected in the hydrological response of the selected basins, implying, in general, a sensible downshift of the FDCs, with a significant reduction in the mean annual streamflow, and substantial alterations in streamflow seasonality and in the relative importance of the surface and subsurface components. The projected climate change impact on the hydrological regime of ephemeral rivers could have important implications for the water resource management and for the sustainability of many riparian Mediterranean ecosystems.

Co-evolution of hydrological components under climate change scenarios in the Mediterranean area

The Mediterranean area is historically characterized by high human pressure on water resources. Today, while climate is projected to be modified in the future, through precipitation decrease and temperature increase, that jointly and non-linearly may affect runoff, concerns about water availability are increasing. For these reasons, quantitative assessment of future modifications in the mean annual water availability are important; likewise, the description of the future interannual variability of some hydrological components such as runoff and evapotranspiration are highly wished for water management and ecosystems dynamics analyses. This study investigates at basin spatial scale future runoff and evapotranspiration, exploring their probability density functions and their interdependence as functions of climatic changes. In order to do that, a parsimonious conceptual lumped model is here used. The model is forced by different future climate scenarios, generated through a weather generator based on a stochastic downscaling of an ensemble of General Circulation Models (GCMs) realizations. The use of the adopted hydrological model, under reliable stochastic future climate scenarios, allows to project future values of evapotranspiration and runoff in a probabilistic framework and, at the same time, the evaluation of their bivariate frequency distributions for changes through the Multivariate Kernel Density Estimation method. As a case study, a benchmark Mediterranean watershed has been proposed (Imera Meridionale, Italy). Results suggest a radical shift and shape modification of the annual runoff and evapotranspiration probability density functions. Possible implications and impacts on water resources management are here addressed and discussed.

Wind speed and temperature trends impacts on reference evapotranspiration in Southern Italy

In this study, the impacts of both temperature and wind speed trends on reference evapotranspiration have been assessed using as a case study the Southern Italy, which present a wide variety of combination of such climatic variables trends in terms of direction and magnitude. The existence of statistically significant trends in wind speed and temperature from observational datasets, measured in ten stations over Southern Italy during the period 1968–2004, has been investigated. Time series have been examined using the Mann–Kendall nonparametric statistical test in order to detect possible evidences of wind speed and temperature trends at different temporal resolution and significance level. Once trends have been examined and quantified, the effects of these trends on seasonal reference evapotranspiration have been evaluated using the FAO-56 Penman–Monteith equation. Results quantified the effects of extrapolated temperature and wind speed trends on reference evapotranspiration. Where these climatic drivers are on the same direction, reference evapotranspiration generally increases during the growing season due to a nonlinear overlapping of effects. Whereas wind speed decreases and temperature increases, there is a sort of counterbalancing effect between the two considered climatic forcing in determining future reference evapotranspiration.

Adaptation of water resources systems to changing society and environment: a statement by the International Association of Hydrological Sciences

ABSTRACT We explore how to address the challenges of adaptation of water resources systems under changing conditions by supporting flexible, resilient and low-regret solutions, coupled with on-going monitoring and evaluation. This will require improved understanding of the linkages between biophysical and social aspects in order to better anticipate the possible future co-evolution of water systems and society. We also present a call to enhance the dialogue and foster the actions of governments, the international scientific community, research funding agencies and additional stakeholders in order to develop effective solutions to support water resources systems adaptation. Finally, we call the scientific community to a renewed and unified effort to deliver an innovative message to stakeholders. Water science is essential to resolve the water crisis, but the effectiveness of solutions depends, inter alia, on the capability of scientists to deliver a new, coherent and technical vision for the future development of water systems. EDITOR D. Koutsoyiannis; ASSOCIATE EDITOR not assigned

Annual runoff assessment in arid and semiarid Mediterranean watersheds under the Budyko's framework

The solution of many practical water problems is strictly connected to the availability of reliable and widespread information about runoff. The estimation of mean annual runoff and its interannual variability for any basin over a wide region, even if ungauged, would be fundamental for both water resources assessment and planning and for water quality analysis. Starting from these premises, the main aim of this work is to show a new approach, based on the Budykos framework, for mapping the mean annual surface runoff and deriving the probability distribution of the annual runoff in arid and semiarid watersheds. As a case study, the entire island of Sicily, Italy, is here proposed. First, time series data of annual rainfall, runoff, and reconstructed series of potential evapotranspiration have been combined within the Budykos curve framework to obtain regional rules for rainfall partitioning between evapotranspiration and runoff. Then this knowledge has been used to infer long‐term annual runoff at the point scale by means of interpolated rainfall and potential evapotranspiration. The long‐term annual runoff raster layer has been obtained at each pixel of the drainage network, averaging the upstream runoff using advanced spatial analysis techniques within a GIS environment. Furthermore, 2 alternative methods are here proposed to derive the distribution of annual runoff, under the assumption of negligible interannual variations of basin water storage. The first method uses Monte Carlo simulations, combining rainfall and potential evapotranspiration randomly extracted from independent distributions. The second method is based on a simplification of the Budykos curve and analytically provides the annual runoff distribution as the derived distribution of annual rainfall and potential evapotranspiration. Results are very encouraging: long‐term annual runoff and its distribution have been derived and compared with historical records at several gauged stations, obtaining satisfactory matching.

An automatic tool for reconstructing monthly time-series of hydro-climatic variables at ungauged basins

Abstract Integrative information models for filling/reconstructing hydro-climatic time-series are required for a variety of practical applications. A GIS-based model for a rapid and reliable assessment of monthly time-series of several key hydro-climatic variables at the basin scale, is here developed as plug-in and applied to the entire region of Sicily (Italy). The plug-in, once the desired basin outlet section and time-window are selected, uses appropriate spatial techniques and algorithms to identify its drainage area and estimate the corresponding mean areal rainfall and temperatures time-series. A recent regional regressive rainfall-runoff model is successively applied for the assessment of the runoff time-series. Finally, a consolidated temperature-based method is applied to estimate monthly potential evapotranspiration time-series, while, actual evapotranspiration and soil moisture storage time-series are derived through a classical water balance model. The tool, supported by a preliminarily developed database, includes automatic procedures for data retrieving and processing and a user friendly interface.

The state of water resources in major Mediterranean islands

This work analyses the state of water resources in six Mediterranean islands: Corsica, Crete, Cyprus, Mallorca, Malta and Sicily. The analysis is focused on the prevailing social, economic and hydroclimatological characteristics of these islands in order to identify the main common features affecting water availability, demand and use. A detailed analysis of the state of water resources in all the considered islands has been carried out. A background of current conditions relating to freshwater availability, quality and use has been established. Different strategies to face water scarcity and enhance water quality in theislands have been assessed in a context of climate change which is a common threat. The identification of similar problems for all the islands highlights the importance of strengthening inter-islands co-operation in the field of sustainable water resources management.

Salinity and periodic inundation controls on the soil‐plant‐atmosphere continuum of gray mangroves

Salinity and periodic inundation are both known to have a major role in shaping the ecohydrology of mangroves through their controls on water uptake, photosynthesis, stomatal conductance, gas-exchanges and nutrient availability. Salinity, in particular, can be considered one of the main abiotic regulating factors for halophytes and salt tolerant species, due to its influence on water use patterns and growth rate. Ecohydrological literature has rarely focused on the effects of salinity on plant transpiration, based on the fact that the terrestrial plants mostly thrive in low saline, unsaturated soils where the role of osmotic potential can be considered negligible. However, the effect of salinity cannot be neglected in the case of tidal species like mangroves, which have to cope with hyperosmotic conditions and waterlogging. We introduce here a first-order ecohydrological model of the soil-plant-atmosphere continuum (SPAC) of Avicennia marina – also known as gray mangrove – a highly salt tolerant pioneer species able to adapt to hyper-arid intertidal zones and characterized by unique morphological and ecophysiological traits. The A. Marina’s SPAC takes explicitly into account the role of water head, osmotic water potential and water salinity in governing plant water fluxes. A. marina’s transpiration a is thus modelled as a function of salinity based on a simple parameterization of salt exclusion mechanisms at the root level and a modified Jarvis’ expression accounting for the effects of salinity on stomatal conductance. Consistently with previous studies investigating the physiology of mangroves in response to different environmental drivers, our results highlight the major influence of salinity on mangrove transpiration when contrasted with other potential stressors such as waterlogging and water-stress. This article is protected by copyright. All rights reserved.

Budyko’s Based Method for Annual Runoff Characterization across Different Climatic Areas: an Application to United States

Runoff data knowledge is of fundamental importance for a wide range of hydrological, ecological, and socioeconomic applications. The reconstruction of annual runoff is a fundamental task for several activities related to water resources management, especially for ungauged basins. At catchment scales, the Budyko’s framework provides an extremely useful and, in some cases, accurate estimation of the long-term partitioning of precipitation into evapotranspiration and runoff as a function of the prevailing climatic conditions. Recently the same long-term partitioning rules have been successfully used to describe water partitioning also at the annual scale and calculate the annual runoff distribution within a simple analytic framework in arid and semi-arid basins. One of the main advantages of the latter method is that only annual precipitation and potential evapotranspiration statistics, and the Fu’s equation parameter ω are required to obtain the annual runoff probability distribution. The aim of this study is to test the limit and potentialities of the aforementioned method under different climatic conditions. To this aim, the model is applied to more than four hundred basins located in the United States. Catchments were grouped into five different samples, following the subdivision of the continental region in five homogeneous climatic zones according to Köppen-Geiger classification. The theoretical probability distribution of annual runoff at each basin has been compared with that derived from historical observations. The results confirm the capability of the tested technique to reproduce the empirical annual runoff distributions with similar and satisfactory performances across different areas, revealing a good option also in cases characterized by climate and hydrological conditions very different from those hypothesized during the original analytical model design, thus extending the geographical and conceptual limits of applicability of the framework.