Keywords

1 Introduction

The Mediterranean region has been historically densely inhabited. Societies development principally took place near water bodies, such as rivers, where water resources have been exploited long since for human activities, such as irrigation, navigation, drinkable water supply and energy production.

Environmental consequences of water exploitation in rivers are mainly linked to the alteration of natural hydrologic regimes (Batalla et al. 2004) and include numerous changes in biological communities and ecological processes (Sabater and Tockner 2009). On the other hand, water uses have strong importance for local economy, including energy production as an alternative to fossil fuels.

In this context, the need to define management decisions able to protect freshwater ecosystems while maintaining important water uses represents a crucial issue for both science and management.

This work aimed to contribute to this issue, presenting and comparing water management schemes in two large Mediterranean river basins: the Ebro (Spain) and the Po (Italy) basins. Environmental issues linked to management were presented in terms of alterations of the hydrologic regimes and the biological communities, and discussed taking into account different local features, as well as the regulatory framework (in particular the EU Water Framework Directive—WFD—2000/60/EC).

2 Materials and Methods

The present study was conducted in two river basins: the Po basin (Italy) and the Ebro basin (Spain) (Fig. 1). Within these basins, three reaches regulated by dams were selected, all characterized by a gravel-bed single-thread low-gradient channel.

Fig. 1
figure 1

Position of the Ebro and the Po mainstems within the Mediterranean region (down) and detail of the study area of the Ebro River (left) and the Ticino and Adda rivers (right), indicating study sites and relative upstream dams

Full size image

The Po basin is located in northern Italy and drains an area of approximately 71,000 km2 (Fig. 1). The catchment includes intensive agricultural areas and large urban districts; human activities in this area support 40% of the gross national product. As a consequence, the water resource in the Po River catchment has been exploited for a long time, mainly for land irrigation and hydropower production. The Ticino River and the Adda River are the two main Po tributaries for flow rate. In these rivers water diversion for agriculture dates back to at least the 13th century and hydropower utilization began at the end of the 19th century. Within these rivers, two study sites were selected: Somma, located in the Ticino River, downstream of Panperduto Dam, and Merlino, located in the Adda River, downstream of Vacchelli Dam. Both dams withdraw water for multiple off-stream use, however in the former hydropower is the dominant use whereas irrigation prevails in the latter.

In turn, the Ebro basin is located in the northeastern part of the Iberian Peninsula and drains an area of approximately 85,000 km2. The current layout of reservoirs and water-diversion structures dates back to 1950–60s for most of them, and is mainly addressed to hydropower and irrigation. For the present work, one study site in the Ebro River was selected. The site, Ascó, is located downstream from the Mequinenza—Riba-roja—Flix dams’ chain (Fig. 1), which stores water mainly for hydropower production.

For each study site, the mean daily regulated streamflows for the period 2000–2015 were provided by the Adda Consortium, Ticino Consortium and Confederación Hidrográfica del Ebro. Corresponding reference flows (i.e. the theoretical flows in absence of water diversions) were calculated for the same period. Moreover, available information about chemical quality and aquatic biological communities (e.g. Salmaso et al. 2014, 2017) were used to discuss the main ecological effects of the current water management schemes.

3 Results and Discussion

Reference flows in the Ticino and Adda rivers are characterised by two periods of low flows (in winter and summer, Fig. 2). Discharge downstream of the dams shows the same pattern, but values are reduced to approximately 40% of the reference ones on average (Fig. 2).

Fig. 2
figure 2

Mean monthly flows at Somma in the Ticino River downstream of Panperduto Dam (Q, grey bars) and related mean monthly reference flows (QR, white bars). The hydrologic impairment (i.e. Q/QR) is indicated with a dotted line

Full size image

During low-flow periods, hydrologic impairment (calculated as Q/QR, Fig. 2) is more relevant. In the Ticino River monthly mean flows lower than 30% of reference flows were present in January, February, July, August and September. In the Adda River this condition is less frequent, as an effect of both minor withdrawals for hydropower and larger groundwater contribution to the river discharge.

As a mitigating measure, monthly-modulated minimum flows (MFs) ranging between 4 and 10% of the mean annual natural flow (Table 1) are released below the dams since 2009. Before 2009 no MF was prescribed and streamflow could temporarily equal zero downstream of the diversion works. At Somma, during low flow periods, the discharge downstream of the dam often equalled MF; at Merlino, instead, it was often larger than the MF.

Table 1 Main hydrologic characteristics of the study sites in the Ticino, Adda and Ebro rivers
Full size table

The ecological status sensu WFD for the period 2010–2015 was on average good at both study sites. Interestingly, despite lower hydrologic alteration in the Adda River than in the Ticino River, in the former the good ecological status (GES) was occasionally not achieved. Indeed, groundwater contribution limits the instream flow reduction, but, at the same time, it conveys NO3−-enriched water from agricultural lands to the river (Delconte et al. 2014), with multiple ecological consequences (Salmaso et al. 2014).

Discharge in Ascó is characterised by a period of high flow at the end of the winter and low flows mainly during summer. On average, it represents approximately 60% of the mean annual natural flow. Besides mean annual discharge reduction, water exploitation produces a strong reduction of natural peak flows. Thus, discharge at Ascó is less variable than at Somma and Merlino (the latter two having approximately double CVQ than the former, Table 1).

As a consequence, the main environmental concern linked to the dam operation in the Lower Ebro is related to the loss of natural processes of sediment deposition and erosion, with consequent overgrowth of aquatic plants and reduced ecological status (Tena et al. 2016). As a mitigating measure, specifically designed flushing flows were experimented (Batalla and Vericat 2009). Moreover, a MF of 100 m3s−1 (corresponding to 10% of the mean annual natural flow, Table 1) is set as a limit for overabstraction.

4 Conclusion

Main environmental issues linked to water withdrawal in the Po basin differ between a site exploited for hydropower and a site exploited for irrigation. The former shows long-lasting periods of low flows. The latter shows a minor in-stream flow reduction, but NO3−-enriched water due to agronomical practices. In the Lower Ebro basin, the river physical conditions have changed notably in the last decades mainly due to the reduction in floods magnitude and frequency.

The management of rivers affected by water diversions needs to include measures able to restore some of their basic environmental functions.