ModABa MODEL: ANNUAL FLOW DURATION CURVES ASSESSMENT IN EPHEMERAL BASINS

A representation of the streamflow regime for a river basin is required for a variety of hydrological analyses and engineering applications, from the water resource allocation and utilization to the environmental flow management. The flow duration curve (FDC)represents a comprehensive signature of temporal runoff variability often used to synthesize catchment rainfall-runoff responses. Several models aimed to the theoretical reconstruction of the FDC have been recently developed under different approaches, and a relevant scientific knowledge specific to this topic has been already acquired. In this work, a new model for the probabilistic characterization of the daily streamflows in perennial and ephemeral catchments is introduced. The ModABa model (MODel for Annual flow duration curves assessment in intermittent BAsins) can be thought as a wide mosaic whose tesserae are frameworks, models or conceptual schemes separately developed in different recent studies. Such tesserae are harmoniously placed and interconnected, concurring together towards a unique final aim that is the reproduction of the FDC of daily streamflows in a river basin. Two separated periods within the year are firstly identified: a non-zero period, typically characterized by significant streamflows, and a dry period, that, in the cases of ephemeral basins, is the period typically characterized by absence of streamflow. The proportion of time the river is dry, providing an estimation of the probability of zero flow occurring, is empirically estimated. Then, an analysis concerning the non-zero period is performed, considering the streamflow disaggregated into a slow subsuperficial component and a fast superficial component. A recent analytical model is adopted to derive the non zero FDC relative to the subsuperficial component; this last is considered to be generated by the soil water excess over the field capacity in the permeable portion of the basin. The non zero FDC relative to the fast streamflow component is directly derived from the precipitation duration curve through a simple filter model. The fast component of streamflow is considered to be formed by two contributions that are the entire amount of rainfall falling onto the impervious portion of the basin and the excess of rainfall over a fixed threshold, defining heavy rain events, falling onto the permeable portion. The two obtained FDCs are then overlapped, providing a unique non-zero FDC relative to the total streamflow. Finally, once the probability that the river is dry and the non zero FDC are known, the annual FDC of the daily total streamflow is derived applying the theory of total probability. The model is calibrated on a small catchment with ephemeral streamflows using a long period of daily precipitation, temperature and streamflow measurements, and it is successively validated in the same basin using two different time periods. The high model performances obtained in both the validation periods, demonstrate how the model, once calibrated,is able to accurately reproduce the empirical FDC starting from easily derivable parameters arising from a basic ecohydrological knowledge of the basin and commonly available climatic data such as daily precipitation and temperatures. In this sense, the model reveals itself as a valid tool for streamflow predictions in ungauged basins

Evaluating the performances of an ecohydrological model in semi-arid river basins

The EHSM (EcoHydrological Streamflow Model) is a conceptual lumped model aimed to daily streamflow simulation. The model, processing daily rainfall and reference evapotranspiration at the basin scale, reproduces surface and subsurface runoff, soil moisture dynamics and actual evapotranspiration fluxes. The key elements of this numerical model are the soil bucket, where rainfall, evapotranspiration and leakage drive soil moisture dynamics, and two linear reservoirs working in parallel with different characteristic response times. The surface reservoir, able to simulate the fast response of the basin, is fed by rain falling on impervious area and by runoff generated with excess of saturation mechanism while the deep reservoir, which simulates the slow response, is fed by instantaneous leakage pulses coming from the soil bucket. The model has seven parameters, which summarize soil, vegetation and hydrological catchment properties. Parameters can be assessed using simple basic ecohydrological knowledge or Monte Carlo simulations as well. The model has been here calibrated for three semi-arid river basins located in Sicily, Italy with area ranging from 10 up to 1780 Km2 with the aim of investigating how the spatial scale may influence model performances. At the same time, the link between knowledge driven parameters and the calibrated ones is explored, investigating the suitability of a lumped framework for the model as the basin size increases

Parametric uncertainty or hydrological changes?

The model calibration is the way of hydrologists for searching also a physical interpretation of complex interactions acting within a basin. Actually, it can be frequently noticed how model calibration performed on a given time-window may converge to a point in the parameter space that could be distant from another obtainable calibration of the model in the same basin but considering a different time window. Is that again parametric uncertainty or does the trajectory in the parametric space relate about to a slow hydrological basin change? This paper depicts a possible path for detecting changes’ signatures in a streamflow time series. In particular, the paper seeks to draw a way to discern the random variability over different time-windows of the calibrated model parameters set from that induced by the variation in time of some boundary conditions and external forcings. To this purpose, we will refer to a conceptual lumped model for simulating daily streamflow, the EHSM (EcoHydrological Streamflow Model), and to a hypothetical case study. The selected hydrological model requires a total of seven parameters, some of which can be easily related to land use, while others rely on climate variables. The calibration of the EHSM parameters with regard to different time-windows and the analysis of potential impacts of the anthropic variation in land use and/or climatic variability on the calibrated parameters set, will support our investigation

Olive yield and future climate forcings

The rainfall reduction and the temperature increase forecasted for Mediterranean regions would likely increase the vegetation water stress and decrease productivity in rainfed agriculture. Olive trees, which have traditionally been grown under rainfed conditions, are one of the most characteristic tree crops from the Mediterranean not only for economical importance but also for minimizing erosion and desertification and for improving the carbon balance of these areas. In order to simulate how climatic change could alter soil moisture dynamics, biomass growth and fruit productivity, a water driven crop model is used in this study. The model quantitatively links olive yield to climate and soil moisture dynamics using an ecohydrological model, which simulates soil moisture, evapotranspiration and assimilation dynamics of olive orchards. The model is able to explicitly reproduce two different hydrological and climatic phases in Mediterranean areas: the well-watered conditions and the actual conditions, where the limitations induced by soil moisture availability are taken into account. Annual olive yield is obtained by integrating the carbon assimilation during the growing season, including the effects of vegetation water stress on biomass allocation. The numerical model, previously calibrated on an olive orchard located in Sicily (Italy) with a satisfactory reproduction of historical olive yield data, has been forced with future climate scenarios generated using a stochastic weather generator which allows for the downscaling of an ensemble of climate model outputs. The stochastic downscaling is carried out using simulations of some General Circulation Models adopted in the IPCC 4AR for future scenarios. In particular, 2010, 2050, 2090 and 2130 scenarios have been analyzed

Green roof effects on the rainwater response in the Mediterranean area: first results of a Sicilian case study

Over the last decades, we have been witnessing an increasing frequency of urban floods often attributed to the interaction between intensification of rainfall extremes due to climate change and increasing urbanization. Consequently, many studies have been trying to propose different new alternatives to mitigate ground effects of ever more frequent and severe extreme rainfall events in a context of growing urbanization, such as rain gardens, green roofs, permeable parking lots, etc., which are commonly referred to as green infrastructures. With this regard, one of the most promising mitigation solutions is represented by multilayer green roofs. These systems, coupling classical green roofs with a rainwater harvesting system, results in a high capacity in retaining rainwater, thus improving the potential effects acted by classical green roofs on pluvial floods mitigation. These systems are particularly suited for applications in semi-arid climate, where a fraction of the rainwater can be detained during the more severe rainfall events, significantly reducing the pressure on drainage systems, and released in a later moment or reused, for instance, to sustain the vegetation during driest periods. This study describes a multilayer green roof installed at the Department of Engineering of the University of Palermo (Sicily, Italy) and its preliminary results on its capacity to reduce the pressure of rainfall events on drainage systems in a Mediterranean context. The green roof has an extension of almost 35 m2 and is made of three different areas with different soil thickness (a mixture of volcanic material) and different Mediterranean vegetation. The green roof is equipped with multiple sensors to monitor the water level in the storage layer, soil water content, air and water temperature, and rainfall. Besides, a weighted rain gauge, a disdrometer, and a meteorological station for the collection of meteorological data are available as well. An equal size classical roof area bordering the green roof installation is also monitored. Four different thermometers are used to measure the temperatures in different points of the roofs and a system of two rain barrels and two pressure sensors allows to collect and compare the rainwater coming from the green and the original roofs. Such an installation, differently from many others, has the advantage to allow a complete characterization of the potential benefits of a multilayer green roof through a comparison of the rainwater released by the two roof configurations at a rainfall event scale. The study provides the preliminary results arising from the analysis of the two roof configurations' response to a series of rainfall events characterized by different duration and intensity

Soil moisture limiting olive orchard evapotranspiration

Two years of field data concerning soil moisture dynamics and water vapor fluxes over a rainfed olive orchard in Sicily are presented here in order to understand how climate, seasonality, water availability and farming practices drive evapotranspiration in such a peculiar Mediterranean vegetation. Soil moisture has been measured in two different points characterized by a uniformly sandy soil and at multiple depths up to 1.2 m. The observed dynamics are driven by rainfall inputs, which are frequent during the winter season and rare during the growing season, and by vegetation uptakes, which deplete the water stored in the soil. The top layers soil moisture status is much more time dependent and variable than deeper layers, which instead show a smoother signal. Water vapor fluxes have been measured with the eddy covariance method using a sonic anemometer and a gas analyzer set above the canopy. The measured fluxes show a seasonal behavior justified by the vegetation growing activities, which start approximately in April. High evaporative demand is satisfied when soil moisture is not a limiting factor: that happens at the beginning of the growing season and in fall when olive trees are still active and the late summer rainfalls have replenished the soil. Moving through the growing season, when soil water is depleted day by day, the evaporative demand is no more satisfied because of a soil moisture limit. Plants have difficulty in extracting water from the soil, and then reduce their activity by closing the stomata with a consequent reduction of the evapotranspirative fluxes. Fluxes have been found to be also dependent on tillage operations, which remove grass from the soil thus eliminating a cause of water depletion. In this work, soil moisture data and the ratio between actual and potential evapotranspiration has been related in order to depict a single stepwise relation linking water availability in the soil and vegetation evapotranspirative activity

EHSM: a new conceptual model for daily streamflow simulation under ecohydrological framework

A parsimonious conceptual lumped model is presented here with the aim of simulating daily streamflow in semi-arid areas. The model is able to reproduce surface and sub-surface runoff, soil moisture dynamics and evapotranspirative fluxes, averaged over a basin starting from daily time series of rainfall and temperature and from the initial value of soil. The rainfall is partioned in two components: the first, which interests a totally impermeable area, is routed directly on a superficial linear reservoir, while the second passes through permeable soil. If the rainfall input exceeds the soil storage capacity, which is a function of soil moisture at that specific time, this saturation excess is routed on the superficial reservoir as well. When soil moisture is higher than the field capacity, the model simulates the leakage component, which is described as an instantaneous pulse from the soil bucket to a second deep linear reservoir. The two reservoirs work in parallel with different time of response: the superficial reservoir has a time lag of about 1-2 days, while the deep reservoir is characterized by a lag time of weeks. Soil moisture dynamics, which are crucial in determining how much water could be keep or released as streamflow or leakage, are simulated with a simple bucket model feed by rainfall and depleted by evapotranspiration. The latter component is calculated as a stepwise function of soil moisture. When there is no limitation given by water availability in the soil, basin vegetation evapo-transpires at maximum level, which is a function of daily temperature and crop characteristics. When soil moisture decreases under a critical value (similar to a stomata closure point), evapotranspiration linearly decreases to zero. The model has been calibrated using Montecarlo simulations on 23 Sicilian basins with very different hydrological behavior. This calibration method has allowed to adapt the conceptual model framework to the basin characteristics and at the same time to obtain the set of parameters with the higher efficiency in reproducing historical streamflow. Performances have been compared with the ones obtained with the IHACRES model, which is one of the most used models for daily streamflow simulation in semi-arid catchments. EHSM is able to obtain, on the analyzed basins, performances similar or better than IHACRES using a lower number of parameters. At the same time, the proposed model gives reliable estimate of soil moisture traces and evapotranspiration fluxes, variables very useful in support flood alert models or irrigation models

A regional GIS-based model for reconstructing natural monthly streamflow series at ungauged sites

Several hydrologic applications require reliable estimates of monthly runoff in river basins to face the widespread lack of data, both in time and in space. The main aim of this work is to propose a regional model for the estimation of monthly natural runoff series at ungauged sites, analyzing its applicability, reliability and limitations. A GIS (Geographic Information System) based model is here developed and applied to the entire region of Sicily (Italy). The core of this tool is a regional model for the estimation of monthly natural runoff series, based on a simple modelling structure, consisting of a regression based rainfall-runoff model with only four parameters. The monthly runoff is obtained as a function of precipitation and mean temperature at the same month and runoff at the previous month. For a given basin, the four model parameters are assessed by specific regional equations as a function of some easily measurable geomorphic and climate basins’ descriptors. The model is calibrated by a “two-step” procedure applied to a number of gauged basins over the region. The first step is aimed at the identification of a set of parameters optimizing model performances at the level of single basin. Such “optimal” parameters sets, derived for each calibration basin, are successively used inside a regional regression analysis, performed at the second step, by which the regional equations for model parameters assessment are defined and calibrated. All the gauged watersheds across the Sicily have been analyzed, selecting 53 basins for model calibration and using other 6 basins exclusively for validation purposes. Model performances, quantitatively evaluated considering different statistical indexes, demonstrate a relevant model ability in capturing the observed hydrological response at both the monthly level and higher time scales (seasonal and annual). One of the key features related to the proposed methodology is its easy transferability to other arid and semiarid Mediterranean areas; thus, the application here shown may be considered as a benchmark for similar studies. The calibrated model is implemented by a GIS software (i.e. Quantum GIS 2.10), automatizing data retrieving and processing procedures and creating a prompt and reliable tool for filling/reconstructing precipitation, temperature or streamflow time series at any gauged or ungauged Sicilian basin. The proposed GIS plug-in can, in fact, be applied at any point of the hydrographical network of the region, assessing the precipitation, temperature and natural streamflow series (at the monthly or higher time scales) for a desired time-window

Modelling flows duration curves in Mediterranean river basins through an ecohydrological approach

The flow duration curve, representing the relationship between magnitude and frequency of streamflows in a basin, provides an important synthesis of the relevant hydrological processes occurring at the basin scale. It is typically obtained from field observations and, since most of the geographical areas of the world still lack suitable streamflow observations, its reconstruction in ungauged river basins is certainly an open and relevant issue in the hydrological literature. Different theoretical approaches have been developed in recent years, and in particular, a novel ecohydrological framework has provided considerable results. The aim of this study is to test with field data, a recent analytical model for the probabilistic characterization of base flows in river basins using few climatic, ecohydrologic and geomorphologic parameters. The base flow is the slow, subsurface contribution to runoff that in many circumstances, such as in the case of relatively flat, vegetated catchments, represents the major runoff component in terms of discharged volumes. The model, coupling soil moisture balances with a simplified scheme of the hydrological response of the basin, provides the probability distribution function of the daily streamflows and the corresponding flow duration curves. The temporal dynamic of the soil water content is seen as the result of deterministic, state dependent loss processes (e.g., evapotranspiration, leakage) and stochastic increments driven by intermittent rainfall forcings. The episodical exceedence of a certain critical level, comprised between the field capacity and complete soil saturation, for the catchment-averaged soil moisture is seen as the triggering mechanism for water release from soil toward the catchment outlet. According to this approach the derived probability density function of slow component of runoff is well described by a Gamma distribution. In this work the original approach, that was structured in a spatially lumped framework by assuming average properties, is considered and opportunely modified to adapt it to the peculiarities of some Mediterranean regions, where catchments are often relatively small and characterized by significant periods with absence of water discharge, especially during the summer. The model is tested using long daily streamflows series recorded in a small catchment located in southern Italy. A sensitivity analysis of the model to the most relevant parameters is also carried out. After performing a procedure to determinate appropriate model parameters, the flow duration curve predicted by the model is compared to the empirical one. Important implications arising from this comparison are here presented and discussed

Detecting hydrological changes through conceptual model

Natural changes and human modifications in hydrological systems coevolve and interact in a coupled and interlinked way. If, on one hand, climatic changes are stochastic, non-steady, and affect the hydrological systems, on the other hand, human-induced changes due to over-exploitation of soils and water resources modifies the natural landscape, water fluxes and its partitioning. Indeed, the traditional assumption of static systems in hydrological analysis, which has been adopted for long time, fails whenever transient climatic conditions and/or land use changes occur. Time series analysis is a way to explore environmental changes together with societal changes; unfortunately, the not distinguishability between causes restrict the scope of this method. In order to overcome this limitation, it is possible to couple time series analysis with an opportune hydrological model, such as a conceptual hydrological model, which offers a schematization of complex dynamics acting within a basin. Assuming that model parameters represent morphological basin characteristics and that calibration is a way to detect hydrological signature at a specific moment, it is possible to argue that calibrating the model over different time windows could be a method for detecting potential hydrological changes. In order to test the capabilities of a conceptual model in detecting hydrological changes, this work presents different “in silico” experiments. A synthetic-basin is forced with an ensemble of possible future scenarios generated with a stochastic weather generator able to simulate steady and non-steady climatic conditions. The experiments refer to Mediterranean climate, which is characterized by marked seasonality, and consider the outcomes of the IPCC 5th report for describing climate evolution in the next century. In particular, in order to generate future climate change scenarios, a stochastic downscaling in space and time is carried out using realizations of an ensemble of General Circulation Models (GCMs) for the future scenarios 2046-2065 and 2081-2100. Land use changes (i.e. changes in the fraction of impervious area due to increasing urbanization) are explicitly simulated, while the reference hydrological responses are assessed by the spatially distributed, process-based hydrological model tRIBS, the TIN-based Real-time Integrated Basin Simulator. Several scenarios have been created, describing hypothetical centuries with steady conditions, climate change conditions, land use change conditions and finally complex conditions involving both transient climatic modifications and gradual land use changes. A conceptual lumped model, the EHSM (EcoHydrological Streamflow Model) is calibrated for the above mentioned scenarios with regard to different time-windows. The calibrated parameters show high sensitivity to anthropic variations in land use and/or climatic variability. Land use changes are clearly visible from parameters evolution especially when steady climatic conditions are considered. When the increase in urbanization is coupled with rainfall reduction the ability to detect human interventions through the analysis of conceptual model parameters is weakened