2.1. Study Area—The Agri River Basin
The proposed analysis was applied to all of the Lucanian river networks (Basilicata region, Southern Italy), but the present study refers to the case study of the Agri River as a suitable example representing all the problems observed in the Lucanian river systems (
Figure 1).
In detail, the study area consists of the Agri River basin. The Agri River is one of the six major rivers of the Basilicata region, with a drainage basin area of over 1715 km2 and varying morphology from mountainous and hilly, in the medium-high upstream part, to low hilly and flat, in the downstream portion.
The hydrographic network is substantially ramified, presenting a main stream of about 113 km in length, whose mountainous reach has an NNW–SSE trend, crossing the intermontane depression of the Alta Val d’Agri and then assuming a fairly regular W–E trend, reaching the Ionian coast of Basilicata.
The average annual rainfall is quite homogeneous along the basin, following the distributions aligned to the NW–SE ridge, with a reference value of about 900 mm/y (
Figure 2).
The Agri basin is representative of a generic river system, where several pressure elements are present in terms of both anthropogenic activities and infrastructural intervention (Pertusillo dam, hydropower station, oil district, extensive farming, urban areas, etc.)
2.2. Advanced Integrated Hydrological Modeling (AIHM) for Water Resource Management
The catchment data were obtained by carrying out Advanced Integrated Hydrological Modeling (AIHM) for water resource management at the basin scale, developed based on multisource data paired with a distributed hydrological numerical model [
42]. The AIHM proposes the implementation of an advanced computing hydrological routing on a regional spatial decision support system (SDSS) integrating land and resource management as well as human and natural risks. In detail, the project envisages the application of a hydrological computational module of an expandable SDSS based on open-data catalogues, able to manage and display both the basic information, including the relative metadata, and post-processing data with open-source change-detecting codes and hydrological modeling.
The SDSS includes the development of integrative methodologies for systematic and continuous catchment monitoring, interfaced in an open-source WebGIS environment dialoguing with the Regional Spatial Data Infrastructure (RSDI). The general SDSS as well as the single hydrological routine provides the integration of ground and remotely sensed data with open-source information technologies for basic and advanced analyses as well as web publishing of geographic data for a simple and intuitive end-user consultation.
Geographic data are processed through the interoperability WMS standards defined by the OGC by implementing data processing techniques to obtain territorial information, based on PSInSAR and change detection methodologies widely and robustly implemented in the analysis of extended targets such as basin and sub-basin areas and/or coastal zones [
43,
44,
45]. In such a context, the proposed AIHM firstly provides a detailed historical analysis of the entirety of the data and measures available from different sources and refers to distributed rainfall, temperature, water discharge, and evapotranspiration gauges coupled with a multitemporal change detection analysis of the DEM, technical map, land use, soil properties, and urban areas as well. All the analyses refer to a 20 × 20 grid as the optimal spatial resolution derived from multisource data, including satellite, topographic, and Lidar data.
The second step leads to the assessment of the effective runoff, based on detailed geological and pedological studies and implementing the Soil Conservation Service method to provide suitable values of CN all over the catchment. The final values of CN adopted in the simulation correspond to medium-high saturation ground (CNIII) (
Figure 3). Furthermore, the processing routine of the catchment hydrological model was developed using the open-source HEC-HMS implemented in the Q-GIS environment (
Figure 4).
The numerical results were validated based on observed time series of water discharge data measured on the inlet cross-section of the Pertusillo reservoir along the Agri River. The routines implemented refer to the monthly and yearly water balance as well as event-scale analysis robustly supporting the short and medium planning and decision making phases as pre-operational and operational tools for both fields of water resource management at the catchment scale.
2.3. Methodology
The evaluation of the hydrological regime alteration of a watercourse, both in terms of characterization and quantification, is a problem still not fully solved, leaving significant degrees of arbitrariness for decision makers, and depending on direct and indirect assessments of hydrological functional scenarios.
In the literature, several methodologies have been proposed for evaluating the alteration of the hydrological regime [
46], as summarily reported in
Table 1.
All of the procedures mentioned in
Table 1 are generally based on the comparison between an undisturbed condition and an “altered” condition, both characterized by the value assumed by several descriptive parameters for different aspects of the hydrological regime.
In particular, ISPRA (2011) [
40] proposes the implementation of the combined use of the “Indicators of Hydrologic Alteration” (IHA) and the “Hydrological Regime Alteration Index” (IARI) as a useful tool for assessing changes in the hydrological regime of a watercourse induced by anthropogenic pressures such as dams, diversions, hydroelectric plants, or any other type of action affecting the naturalness of the river system [
39,
50]. Obviously, the present study focuses on the ISPRA approach, which is the methodology suggested for Italian basins, and the produced results might be immediately adopted by the regional authorities.
The analysis of the hydrological regime alteration of a watercourse can be carried out at a cross-section of the watercourse on the basis of the IARI methodology, which provides a measure of the deviation of the hydrological regime, assessed on a daily or monthly scale, compared to the natural datum corresponding to the absence of any anthropogenic pressure.
Furthermore, the IHA can be calculated using parametric (mean/standard deviation) or non-parametric (percentile) analysis, starting from the time series of water discharge. The IHA parameters and their influence on the ecosystem are attributable to five main hydrological groups, HG, [
6,
7] and five different environmental flow components, EFC (
Table 2).
The entire range of flow conditions represented by the EFC components (globally 33 parameters) must be maintained in order to ensure the river ecological integrity.
The IARI is determined from the flow data, by comparing the “altered” flow rates with the corresponding natural flow rates, as indicated by the ISPRA [
40] according to the following criteria:
- (1)
Provide a quantitative measure of the deviation of the observed hydrological regime from the natural one that would occur in the absence of anthropogenic pressures;
- (2)
Take into account the general and widespread scarcity and/or absence of data;
- (3)
Be able to use all available hydrological information;
- (4)
Use tools, methods, and results already available from the competent entities that carried out the hydrological and water balance in the water protection plans;
- (5)
Be easy to implement and calculate with the usual calculation tools.
The procedure has the following characteristics:
- (a)
It is defined at successive levels of in-depth analysis;
- (b)
It is defined primarily on the basis of the monthly average flow rates to take into account the effect of seasonality and to use the results of the water balance of the protection plans;
- (c)
It is defined differently for river sections with or without flow measurement instrumentation;
- (d)
It is derived from the IHA method, and the statistics used in the procedure can be easily calculated with the corresponding open-source software IHA.
With regard to the reference values for the IARI, it is useful to point out that, conventionally, the following ranges are adopted:
Thus, the procedure for the assessment of the status of the hydrological regime through the determination of the IARI is divided into three phases (
Figure 5):
- 1.
Phase 0—preliminary analysis: An analysis of the basin-scale pressures shall be carried out in order to identify the detectable conditions in the considered section by selecting one of the following conditions:
No or negligible pressure on the hydrological regime—it can be assumed that it is unchanged;
Significant or non-negligible pressures leading to impacts that cannot be assessed a priori—a necessary assessment must be made on an objective basis.
- 2.
Phase 1—calculation of the index: If in Phase 0, the identified conditions show an impact on the hydrological regime due to pressure, the quantitative assessment of the alteration is carried out through the calculation of the IARI index.
- 3.
Phase 2—direct evaluation or consultation: This step is activated whenever the results obtained in Phase 1 reveal critical elements. In such a case, a detailed analysis essentially based on the qualified information given by experts is provided in order to explain the causes and to confirm the exposed criticalities.
In detail, in each control cross-section, based on the monthly time series relating to the “undisturbed” condition, the 25th and 75th percentiles, XN0.25,i and XN0.75,i, must be computed for each ith IHA parameter (e.g., the monthly flow discharge).
Subsequently, for each ith parameter, the characteristic value, Xi,k, corresponds to the kth reference period in which the altered condition occurs, e.g., in the present case, this corresponds to the value of the monthly flow rate assumed as the monthly ecological flow.
Furthermore, through the comparison between the value X
i,k and the X
N0.25,i and X
N0.75,i, the term p
i,k is calculated according to the procedural scheme reported in the following equation:
where i refers to the ith IHA parameter, k is the reference period, X
i,k is the characteristic value of the ith parameter corresponding to the kth reference period in which the altered condition occurs, X
N0.25,i is the 25th percentile of the ith IHA parameter in the natural condition (unaltered condition), and X
N 0.75,i is the 75th percentile of the ith IHA parameter in the natural condition (unaltered condition).
In other words, if the value of the parameter Xi,k falls within the band delimited by the percentiles 25% and 75%, the term pi,k is assumed to be zero, corresponding to a condition of ordinary fluctuation; otherwise, it is equal to the minimum distance, normalized on the amplitude of the interval, from the limits of the band. IARI is therefore defined as the average of the values assumed by the terms pi,k.
In order to identify the groups of elements which have the greatest influence on the alteration of the regime, and also to plan any intervention measures, the IARI was estimated for each group, and the average was then calculated as reported in the general form of Equation (2) based on the number of parameters belonging to the HG, j = 1, 2, …,5:
In this framework, Eflow is considered as the hydrological alteration induced in the water body. Thus, the hydrological Eflow arises from the assessment of the levels of acceptability of the Indicators of Hydrological Alteration (IHA) and the determination of the Hydrological Regime Alteration Index (IARI) at a generic river cross-section.
In the present study, therefore, the ecological flow was defined as the ith percentile of the distribution of the average monthly flows, which provides a value of the IARI corresponding to a “good” level at least, and Equation (2) becomes
Whenever no time series of water discharge are available, and a rain gauge network is present, classical catchment rainfall–runoff modeling can be used. In the present study, the HEC-Hydrological Model System (HEC-HMS) was employed using a suitable rainfall time series available from the regional civil protection gauge network. The HEC-HMS simulates the complete hydrologic processes of river catchment systems and includes several traditional hydrologic analyses such as infiltration, evapotranspiration, unit hydrographs, and hydrologic routing [
51,
52].
The significance and the validation of the reconstructed data were evaluated using two criteria: the root mean square, E
RMS, and mean absolute percentage error, E
MA, expressed as follows:
where X
o is the observed value, and X
c is the value estimated by the application of HEC-HMS.
Following such an approach, the concept of ecological flow goes far beyond the commonly reductive idea in which the minimum ecological flow rate corresponds to a fixed percentage of the average annual flow rate. Eflow, indeed, represents the monthly water discharge inducing acceptable alterations in the hydrological regime, ensuring the functionality of the water body.
The methodology is spatial scaling and applicable to a generic cross-section of any watercourse.