RIVER IMPOUNDMENT AND CHANGES IN FLOW REGIME, RIVER BASIN, NORTHEASTERN .

RAMON J. BATALLA, CARLOS M. GOMEZ, AND G. MATHIAS KONDOLF

Ramon J. Batalla (corresponding author) Departament de Medi Ambient i Ciències del Sòl Universitat de E-25918 Lleida (Spain) fax ++34 973702613 [email protected]

Carlos M. Gómez Departamento de Fundamentos de Economía e Historia Económica Universidad de Alcalá de Henares E-28802 Alcalá de Henares (Spain) [email protected]

G. Mathias Kondolf Department of Landscape Architecture and Environmental Planning University of California Berkeley CA 94720-2000 (California, USA) [email protected]

Abstract The Ebro River and tributaries (north-eastern Spain) are regulated by over 187 dams, with a total capacity equivalent to 57% of total mean annual runoff. We analysed 38 gauging records from 22 rivers that, by virtue of location within the drainage network and period of record, would reflect reservoir-induced hydrological changes. Most showed reduction in flood magnitude, with Q2 and Q10 reduced over 30% on average, more in rivers with higher values of the Impounded Runoff index, IR (calculated as reservoir capacity divided by mean annual runoff). Annual runoff did not show strong trends, but the variability of mean daily flows was reduced in most cases due to storing of winter floods and increased baseflows in summer for irrigation. Monthly flows ranged from virtually no change post-dam to complete inversion in seasonal pattern, the latter due to releases for irrigation in the summer, formerly the season of lowest flows.

Keywords: Fluvial Hydrology, River Regime, Dams, Impounded Runoff

1. INTRODUCTION

Flow regime (i.e., total discharge, flood flows, baseflows, the shape of the seasonal and flood hydrographs, seasonal and interannual variability) controls many physical and ecological aspects of river form and processes, including sediment transport and nutrient exchange (Poff et al. 1997). Native plants and animals are adapted to seasonal and inter-annual cycles of flooding and base flows for reproduction, hatching, migration, and other life history stages. Annual floods recharge the alluvial water table, provide water to floodplain vegetation, and inundate backwaters where fish spawn and rear, contribute food to the channel, deposit nutrients on flood plains, flush out backwater channels, and rejuvenate spawning gravels (Junk et al. 1989, Ward and Stanford 1995). Base flows maintain riparian vegetation and aquatic ecology, and seasonal low flows largely determine the species that can survive in a given river. Seasonal flow variations, and the rate of change in flow during and after floods, also influences patterns of sediment transport and deposition and ecological processes such as establishment of riparian vegetation (Rood and Mahoney, 1990).

Numerous authors have reported the effects of dams on downstream flow regimes (e.g., Petts 1984, Williams and Wolman 1984). With increasing scrutiny of dam operations in connection with renewal of hydroelectric licenses or authorizations for other diversions, methods for quantifying hydrologic changes and evaluating their potential environmental effects are now widely used. The Index of Hydrologic Alteration (Richter et al. 1995) can be used to quantify dam-induced changes to flow regime where long gauge records exist for the river before and after regulation. Much of the work quantifying environmental effects of altered flow regimes has concerned adjustments in channel form due to hydrologic change (e.g., Williams, 1978), or methods for prescribing flow regimes below dams to maintain ecological resources (Loar and Sale, 1984).

It is intuitive that the larger the reservoir capacity in relation to the natural flow in the river, the greater the hydrologic effect of the reservoir is likely to be. We propose a simple ratio of reservoir capacity to (unimpaired) mean annual flow as the Impounded Runoff index (IR). This ratio can be calculated using the total reservoir capacity where that is the only figure available, but where available, live storage capacity should be a better predictor of dam effects. IR can be calculated for individual reservoirs or using the cumulative capacity of all

reservoirs in the system, although the two ratios can yield different results and which capacity value used should be clearly indicated.

In the case of reservoirs built for flood control, we might expect a consistent relationship between degree of impoundment and change in flow variables. However, for reservoirs built for irrigation and hydroelectric generation, we should expect the relation to be noisy, because flood reduction is not a purpose and the volume of floodwater stored will be a function of how much water happens to be in the reservoir when the flood begins. Floods will tend to be reduced more in dry years and early in the season, when reservoir levels are lower. Moreover, we might expect reservoir effects to be more pronounced in drier climates because of greater storage needs and greater likelihood that the reservoir will be drawndown when floods enter. Because channel form and river ecology in semiarid rivers will be adapted to highly variable flow regimes, dam-reduced flow variability is likely to have a relatively larger effect on river ecology in drier climates, both through reductions in high flows (reduced disturbance) and often increased baseflows (making these environments more suitable for exotic species not adapted to seasonal drying cycles). Also, reservoirs trap all bedload (gravel and sand) and a (variable but usually large) part of the suspended load (sand and finer sized sediment) supplied from the upstream catchment. As a result, sediment loads below dams are reduced. If post-dam flows are sufficiently energetic to transport sediment, the reaches below dams may become sediment starved and consequently may downcut and widen (Kondolf, 1997).

Despite a considerable literature on hydrologic and geomorphic effects of dams on downstream channels, we are aware of no systematic attempt to quantitatively relate the whole set of hydrological changes (on flood magnitude, seasonal flow distribution, etc.) to the degree of impoundment not at the temporal and spatial scale of a large river basin. The objectives of this study were to systematically analyse the hydrological alterations caused by dams in the Ebro River basin, NE Spain, and to relate the degree of hydrologic alteration to the level of impoundment and climatic zone. The Ebro provides an excellent and representative case study because it is the largest river on the and one of the largest in the Mediterranean region, it has been extensively dammed (total reservoir capacity now equivalent to 57% of annual runoff), and pre- and post-dam hydrologic data are available for a reasonable sample of rivers. Although we recognize the importance of sediment

transport and river ecology and their alteration by dams, we restricted this analysis to hydrologic effects measurable from the gauging record.

2. REVIEW OF DAM EFFECTS ON HYDROLOGY AND SEDIMENT TRANSPORT

Dams have a range of effects on river channels and riverine ecosystems. In addition to their obvious role as barriers for fish migration and the consequent impact on production of salmon and other anandromous species (Goldman and Horne, 1983), dams indirectly affect river ecology through changes in flow regime, sediment load, and channel form. The effects of a given reservoir on flow regime will depend on its capacity in comparison with river runoff, its purpose (e.g., irrigation diversions, hydroelectric generation, flood control), and its operating rules, precluding simple generalizations about the post-dam discharge distributions (Williams and Wolman, 1984). The resultant hydrological alterations caused by reservoirs may include changes in flood frequency and magnitude, reduction in overall flow, increased or decreased summer base flows, and altered timing of releases, with a consequently wide range in effects on riverine ecology (Petts 1984). Among the most geomorphologically and ecologically significant are the following:

2.1. Reduction of flood peaks

The storage of water in reservoirs delays and reduces floods downstream, with average annual peak discharges reduced to as little as 9% of their pre-impoundment values below 21 dams in rivers of the western United States (Williams and Wolman, 1984). In the Sacramento-San

Joaquin River system of California, the Q2 (the discharge with a 2-year recurrence interval) was reduced below dams from 35 to 95 percent of pre-dam values, while the Q10 was reduced from 2 to 78 percent, depending on reservoir size and operating rules (Kondolf and Matthews, 1993). Even routing through a reservoir with no available storage may reduce peak discharges by over 50% (Moore, 1969).

2.2. Reduction in total runoff

Not all dams reduce total runoff to downstream. In general, flood-control dams do not reduce total runoff, but tend to reduce seasonal variability and ‘flat-line’ hydrographs. Hydropower dams do not necessarily reduce total flow, but may divert flow into another basin where generating turbines are located, or may divert water around the reach of the river between the dam and the power plant (Morris and Fan, 1997). Dams that divert water to offstream uses (irrigation and urban uses), especially out of basin diversions, will reduce the total downstream flow (e.g., Collier et al., 1995).

2.3. Alteration of base flows

Dams and diversions typically have a large effect on low flows, because demand for water is usually greatest during the dry (baseflow) season and because a given (absolute) change in flow is a greater proportional change in flow at baseflow. Even small diversion dams can completely dewater channels at baseflow. In contrast, reservoir releases for hydroelectric power generation, irrigation, and wildlife needs on some major rivers increase base flow over pre-project levels.

2.4. Reduction of seasonal variability and changed timing of high flows

Reservoirs often convert highly variable natural flows into more stable flows, with reduced monthly and seasonal variations. Irrigation storage may produce short-term variability in flows during peak demands, with otherwise constant flows, whereas hydroelectric dams can produce rapidly fluctuating flows, often on a diurnal cycle (Walker, 1985). The altered seasonal pattern can influence oxygen levels, temperature, suspended solids, drift of organisms, and cycling of organic matter and other nutrients, as well as having direct impacts on biota. The temperature of released water, which will depend on the reservoir outlet level and reservoir stage, may compound these water quality effects. Altered temperature regime can eliminate species of aquatic insects (Ward and Stanford, 1979) and fish species with specific temperatures requirements for spawning (Cadwallader, 1986).

2.5. Sediment Trapping by Dams

Reservoirs trap all bedload sediment (the coarser part of the sediment load, that moves in intermittent contact with the bed by rolling, sliding, and saltating), and a part of the suspended sediment load (finer sediment that is held aloft in the water column by turbulence). Dams thus release sediment-starved, or “hungry” water to downstream reaches, which may transport sand and gravel downstream without replacement from upstream, resulting in coarsening of the surface layer termed 'armouring', and may erode the bed, resulting in incision, and undercut banks, thereby causing widening. The channel changes from hungry water can cause dramatic changes on river ecology and damage to bridges and other infrastructure (Kondolf, 1997).

3. STUDY AREA

Rivers in Spain are highly regulated. Over 1200 large dams (more than any other European country) impound the equivalent of 40% of mean annual runoff from the entire country. The largest river basin, the Ebro, has 187 reservoirs impounding 57% of mean annual runoff. The Ebro River drains 85,530 km2 along the southern-facing slopes of the Cantabrian Range and , with elevations over 3,400 m, and the northern-facing slopes of the Iberian Massif, with elevations up to 2,300 m, and debouches into the Mediterranean at Tortosa, about 180 km south of Barcelona (figure 1). Mean annual precipitation varies from over 2000 mm in the Pyrenees to less than 400 mm in the arid interior (www.oph.chebro.es). Of the total basin area, forest and shrubland cover 25% and 50% is in agriculture (www.oph.che.es). The rest includes badlands, "waste lands", and urban areas. The Ebro drainage network includes more than 12,000 km of major streams (www.oph.che.es). Mean annual runoff at Tortosa (near the mouth) is 13,408 hm3 (hm3 = 106 m3), with a coefficient of variation of 35% (table 1). Maximum- peak flow on the Ebro River was around 12,000 m3/s in 1907 in Tortosa (Novoa, 1984), although other sources report a peak discharge up to 23,484 m3/s (Geografia de Catalunya, 1958). More than 5000 m3/s was estimated for the in Lleida in 1907 (J.C. Balasch, University of Lleida, personal communication). Peak of the November 1982 flood was 3,760 m3/s on the Ebro (Tortosa), and 2,485 m3/s on the Segre (Lleida).

Of the 187 reservoirs, the largest is Mequinenza, with a capacity of 1,534 hm3 (table 2). Reservoirs range in in elevation range from 2,189 m (Llauset Reservoir on the Noguera Ribargorçana) to 40 m (Flix Reservoir on the mainstem Ebro), but most are located in the central and the upper reaches of the tributaries with a mean elevation of about 700 m. Only three major streams, the Ega, Jiloca and Valira, are not regulated. None of the major dams was built for flood control, but the sheer volume of impoundments would likely affect flood magnitude. Diverted water is used mainly for hydropower production (60,000 hm3/year running 240 hydropower stations and producing 6,700 Gwh/year), for irrigation (6,310 hm3/year), cooling water for nuclear plants (3,350 hm3/year), and for industry and domestic use by almost 3 million people (313 hm3/year) (CHE, 1988). Ebro dams have the potential to store 2,500 m3 (0.0025 hm3) of water per person, a similar per capita storage to that in California (Graf, 1999).

Virtually all the dams were constructed during the 20th century, with 67% of reservoir capacity built in the period 1950-1975, when 5200 hm3 of water was impounded (figure 2). Most reservoirs in the Ebro basin are small. Of the one hundred forty-two reservoirs larger than 10,000 m3 (0.01 hm3) capacity, two thirds have capacities above 1 hm3, 18% above 50 hm3 and 3% above 400 hm3. The twenty-four reservoirs with capacity between 50 and 500 hm3 have a total storage capacity of 4,200 hm3, over half the total basin storage and equivalent to 30% of the total annual runoff. Thus they play an important role in regulating flows in the basin.

Three reservoirs have more than 500 hm3 of capacity: Ebro and Mequinenza at the upstream and downstream ends of the Ebro River, respectively, and Canelles in the Noguera Ribagorçana River. The largest reservoir system is the Mequinenza-Ribarroja-Flix chain on the Lower Ebro, controlling 96% of the basin land area and storing 1,750 hm3 (IR=0.13). Other important reservoir systems are the Canelles-Santa Anna in the Noguera Ribagorçana (capacity 915 hm3, IR=1.48) and Mediano-Grado in the (capacity 837 hm3, IR=0. 31).

According to the MIMAM (2000) map of potential alteration, based on the ratio between impounded volume and annual runoff, only the Ebro headwaters tributaries still have a natural flow regime. Fluvial regimes of the main tributaries (Segre, Cinca, Gállego and Aragón Rivers) and the central and lower reaches of the Ebro mainstem are slightly to moderately

altered. The upper reach of the Ebro main stem, downstream of the Ebro dam, and the Noguera Ribargorçana River downstream Canyelles and Santa Ana dams, show the most profoundly altered fluvial regimes in the basin.

Reservoirs trap large volumes of sediment in the Ebro basin. Avendaño et al. (1997) quantified an annual deposition of 10.5 hm3 of sediment for seventeen reservoirs along the basin, including Mequinenza-Ribarroja, Talarn, Yesa, and . Some small reservoirs are already full of sediment (e.g., Pignatelli in the Ebro main stem, constructed in 1790 with an original capacity of 1 hm3 and Escuriza in the Martín River, constructed in 1890, with an original capacity of 6 hm3). In others, sedimentation has been recognized as a problem (e.g. Terradets on the Noguera Pallaressa, constructed in 1935, with an actual capacity of 8 hm3 of an original 23 hm3), and sediment has been sluiced during low flows to clean out reservoirs (e.g. Santa Anna Reservoir on the Noguera Ribargorçana ,and the Barasona Reservoir on the Ésera River).

Several studies have analysed river dynamics of the Lower Ebro, producing data on ecology, hydrology and sedimentology of that system. Ibañez et al. (1996) found a reduction of 29% of mean annual runoff due to water use and evaporation from reservoirs. Muñoz and Prat (1989), and Ibañez et al. (1995) studied the impacts of river regulation on river ecology. Guillén et al. (1992), Sanz-Montero et al. (2001), and Vericat and Batalla (2003) assessed the enormous reduction of the river sediment load along the 20th century (from 15·106 t/y to less than 150,000 t/y) and its effects on river morphology. Only recently, the MIMAM (2000) and García and Moreno (2000) have made a qualitative assessment of the effects of river regulation on fluvial regime of the whole Ebro River basin.

No studies have been found to analyse systematically hydrological alterations caused by dams at the comprehensive temporal and spatial scales (i.e. a whole large river basin) presented in this paper.

4. MATERIALS AND METHODS

4.1. Source of Hydrologic Data

Flow records for 171 gauging stations in the Ebro basin were available on the web page of the Confederación Hidrográfica del Ebro (CHE), a government agency established in 1926 to manage the water resources in the Ebro River basin (www.oph.chebro.es) (Díaz-Marta, 1997), and monthly flow data were available from García and Moreno (2000). Some of the records went back to 1913, but most started during the 1940s, a decade before the boom of reservoir construction in Spain. Virtually all of these records contained gaps of individual or multiple years, and most had multiple gaps. The years of missing data from one gauge did not necessarily correspond to the years missing from other gauge records, except for the period 1939-1943, for which no data are available in Spain due to the Civil War.

The CHE database was organized into separate sections for annual peak flows, mean daily flows, mean monthly flows, and mean annual flows. Each of these sections contained data for a different set of gauges. We first identified gauges that would reflect hydrologic effects of reservoirs by virtue of their locations and years of operation. We then screened the data sets and used only data series with at least five values pre-dam and eight values post-dam. In the resulting data set, six gauges appeared consistently in all four sections of the database, but there were many gauges that appeared in only one or two database sections (table 3).

Of the gauging series appearing in a given section, many were not suitable for all the hydrologic analyses we conducted in this study because of short records and data gaps, though usually they were suitable for some analyses. From the full data set of 171 records, we analysed 38 flow records from 22 rivers to determine the effects of reservoirs (table 3).

4.2. Data analysis

We first divided the Ebro basin into four main climatic areas: the humid Atlantic headwaters at the western end of the basin, with average annual precipitation of about 900 mm, the west- central Pyrenees (about 950 mm), eastern Pyrenees (about 800 mm), and the drier southern Mediterranean zone, in the south-eastern extremity of the basin (about 500 mm) (figure 1).

The west-central and eastern Pyrenees are open to Mediterranean influence and runoff reflects both high elevation snowmelt and lower elevation rainfall, with a distinct summer drought typical of Mediterranean regions. We compiled reservoir capacity and annual runoff data, and analysed flood-frequency, flow duration of mean daily flows, monthly and annual runoff before and after dam construction, and documented changes with distance downstream of the dams on three rivers.

Degree of Impoundment As an indicator of the degree to which reservoirs could change flows, we calculated the Impounded Runoff index (IR), the ratio of reservoir capacity to mean annual runoff, expressed as a dimensionless decimal fraction. The available values were for total reservoir, i.e., including dead storage. Although calculating the IR with values of total reservoir storage could lead to overestimating the potential hydrologic effect of the dams, these were the only data available at the scale of the entire basin. We examined relations between IR and the calculated hydrologic variables.

Flood Magnitude We analysed flood frequency before and after dam construction for 23 gauging stations to obtain the recurrence interval and flood probabilities in each series. Our interest was in changes of flood frequency and magnitude as they might influence channel processes. Therefore, we were more concerned with frequent floods (i.e., return interval of 25 years and less) than with larger floods (e.g., the 100-yr flood). We used Gumbel extreme value distribution (type 1) (Shaw, 1983) to fit lines through the data series (figure 3). Where the Gumbel-fit line departed significantly from the plotted points, we drew a smooth line through the data points and drew flood frequency values from this empirical flood frequency relation.

We acknowledge that the estimation of flood probabilities in regulated rivers should, strictly speaking, be done using total probability methods or other such approaches, rather than common parametric probability functions such as Gumbel (Durrans, 1998). Effects of not using total probability methods on flood frequency estimations are expected to be important in rivers where dams are built up for flood control. However, in the case of water supply dams, such as in the Ebro basin, influence of regulation space in the reservoir can be incidental and, thus, have to be considered conservatively (USACE, 1993). Total probability

methods involve routing of flood hydrographs through the regulating reservoirs to compare unregulated flood characteristics with regulated discharges downstream dams. Such analysis would also require flow series upstream dams, which for many cases are not available in the Ebro valley or are too short to be considered. Moreover, the mean daily or hourly flow data on which the hydrographs would be based are generally not available for the Ebro River prior to the 60s. Instead, our approach involves comparing pre-dam with post-dam flood magnitude within the same flow series downstream of the dam and using the same probability function, as it is generally done by Water Authorities in Spain (Junta d’Aigües, 1994).

The gauging records were short, so we inspected the data series and flood frequency plots to determine if the data were adequate to calculate the flow for a given return interval. Our standard was that there be at least two data points beyond the return interval estimated, and that the data points trend along a clear line to permit the discharge to be estimated unambiguously. Of the 23 data series of annual peak flows, we had sufficient data to estimate the Q2 for all 23 series, the Q10 for 20 series, and the Q25 for 13 series (table 4), where Qi designates the mean annual peak flow with a return period (in years) of i. After we estimated flows for each return period, the ratio between post and pre-dam discharges (δ=Qpost/Qpre) corresponding to 2-year, 10-year and 25-year return periods was calculated. We then plotted δ against IR for all available data, and the average δ values for each climate zone against mean precipitation and against IR for each zone.

Annual runoff We identified 23 series of average annual flows from the data sets on regulated rivers that were of sufficient length for analysis, which we augmented with four data series from unregulated rivers for comparison (table 5). All were normally distributed. We compared mean pre-and post-dam annual runoff values using the Critical Ratio method (for independent elements and small populations), by means of RC = |(QARpre – QARpost)| / √[(σ1/n1) + (σ2/n2)], where QARpre and QARpost are the mean annual runoff before and after the dam construction respectively, σ is the variance for each of the annual runoff populations, and n is the number of values in each population. We compared Critical Ratio values with t Student levels for corresponding degrees of freedom and an accepted a probability of error (p<0.05). We also ran a simple regression analysis on the values of annual runoff over time to identify trends.

Daily flows Of the mean daily flow series available, only ten were located on dammed rivers and suitable for this study. As flow duration analyses would be revealing even if records were relatively short, we relaxed our minimum of thirty years of record and included Arga (152) and Noguera Ribargorçana (119) so that we could analyse all ten series available (table 3 and table 6). Even the longer records were rarely balanced in number of years of record pre- and post-dam. From the flow duration curves, we read values of P05, P16, P50, P84, and P95, where Pi is the percentile from the flow duration curve corresponding to ith percentile, i.e., the discharges equalled or exceeded 5, 16, 50, 84, and 95% of the time, respectively. We also calculated the Flow Standard Deviation (FSD) relative to the median daily discharge to overcome possible particular hydrological trends in the study periods, as FSD = |[(P84-P16)+(P95-P5)]/P50|, adapted from the Standard Graphic Deviation developed by Folk and Ward (1957) for fluvial sediments.

Monthly regime Of the mean monthly flow series, 34 were suitable for analysis (table 7). In most cases, we used one gauge series to analyze the effects of a single dam. In three cases, (the Noguera Ribagorçana (115), Gállego (12), and Arga (4)), however, a second dam was constructed at least thirty years after the first, and thus we could use data from a single gauging station to analyse the effects of two dams on the same river.

We calculated the correlation coefficient for each gauge series (Φpre,post) by dividing the covariance of the pre and post-dam data sets by the product of their standard deviations:

Φpre,post = Covpre,post/(σx · σy), where 1 ≥ Φ≥ -1, Covpre, post = 1/n [Σ (prei -µx) (posti -µy)], σ is the standard deviation, n the number of data, and µ the mean value of the distribution (citation), thus serves as a variable indicating the degree of impact of the impoundment on runoff: a) A positive value near 1 indicates that the post-dam flow regime closely matches the pre- dam regime, and thus the reservoir has had little impact on monthly regime. b) A value near 0 indicates that the post-dam monthly regime is independent of the natural pre-dam pattern.

c) A negative value indicates an inversion of the flow regime, with values close to –1 indicating that the post-dam monthly flow pattern is a mirror image of the pre-dam pattern.

We plotted Φpre,post against IR and fit an ordinary least squares regression model, which implies a linear relation between a marginal increase in IR and the marginal change in monthly flow pattern. Alternatively, there may be critical thresholds in IR above which the impoundment effects are proportionately greater or lesser. If such thresholds exist, they can be identified by cutting the regressor IR into two or more segments (splines) associated with different marginal effects of dam size on monthly flow regime, repeating the analysis for all possible critical values of IR, and estimating the likelihood function for all possible thresholds (Greene, 2000).

Downstream persistence of dam-induced changes We examined the spatial influence of dams with distance downstream by analysing peak flows and mean monthly flows on three rivers that had two or three gauging stations at distances from a few hundred meters to tens of kilometers downstream of a dam: a) the Jalón River (gauge series 125, 9 and 87), b) the Najerilla River (gauge series 34, 48 and 38), and c) the Aragón River (gauge series 101 and 5) (table 8). As we used five of these data series only to assess downstream persistence of dam-induced changes, they do not appear in table 3.

5. RESULTS AND DISCUSSION

5.1. Flood magnitude

Most flow records showed a pronounced reduction in flood frequency and magnitude as a consequence of dam operation (table 4, figure 3). For example, the Ebro's main tributary, the Segre, had 70% of its runoff impounded by 1960, and the 10-year flood peak was reduced in Lleida by 20% between the periods 1913-59 and 1960-90. Most of the Segre reservoirs are for hydropower and irrigation, so floods have been reduced much less than expected with flood- control dams. However, with the completion of the Rialb reservoir in 2000, 90% of the Segre annual runoff is now impounded and, thus, greater flood reduction is likely in the future.

The ratio between post-dam and pre-dam flood values (δ=Qpost/Qpre), averaged 0.65 (i.e., a

35% decrease) for the Q2, 0.67 for the Q10, and 0.59 for the Q25, but the reduction varied greatly. Small floods (Q2 and Q10) downstream of the Ebro dam (Ebro River), Canelles and Santa Anna dams (Noguera Ribagorçana), Cueva Foradada Dam (Martín), Santolea Dam (Guadalope), and Moteagudo Dam (Najima) were the most affected, with average δ values of

0.30 for Q2 and 0.40 for Q10 (i.e., decreases of 70 and 60 % respectively) (table 4). The

Guatizalema River showed an extreme reduction (δ=0.13 for Q2 and 0.22 for Q10), but is an unusual case because of multiple diversions from this reach.

Some of the rivers showed little change in flood magnitude, and three rivers showed an increase in flood magnitude post-dam: the Upper Segre for the Q2, and the lower Aragón and

Cinca for the Q10. We can think of no plausible mechanism by which these reservoirs could cause flood flows to increase, so higher post-dam floods are likely attributable to higher reservoir inflow in the years after dam construction, due either to climatic fluctuations or a change in runoff processes in the catchment. In these cases, flood timing cannot explain the post-dam increase, since a single large reservoir, or two in series in the case of the Cinca River, controls nearly all runoff. On the Segre, the Valira (gauging station 22), the main tributary upstream of Oliana Reservoir also shows both increased floods and annual runoff since 1960: Q2 increased 25% and its Q10 increased 71% after 1960. Over the same period, no change in the flood frequency was evident in the mainstem Segre above the Valira confluence, as indicated by the flow record at La Seu d’Urgell (gauging station 23). The increase runoff in the Valira was coincident with rapid urbanization and forest clearing (beginning in the 1960s) in the catchment, which is occupied by , a small, independent country that experienced annual population growth exceeding 5% in the decade of the 1970s (Enciclopèdia Catalana, 1983). We have no comparable data for the Aragón and Cinca rivers, but a series of wet years post-dam may be responsible for their increased floods.

In general, flood frequency and magnitude decreased with increasing IR. As shown in figure 4, trends are evident despite considerable scatter. The best-fit relations (p<0.05) between δ and IR for three return intervals have similar slopes, suggesting a similar degree of alteration to each of the three studied recurrence intervals:

2 δ2 = -0.37IR + 0.84 (r =0.52, n=20) 2 δ10 = -0.39IR + 0.85 (r =0.60, n=16) 2 δ25 = -0.33IR + 0.78 (r =0.50, n=11)

where δi is the post-dam flood /pre-dam flood for return interval i. The change in flood magnitude produced by a given level of impoundment varied regionally (table 4, figure 5). Zone-averaged mean annual precipitation ranges from 950 mm to 500 mm, and the dam- induced reduction in floods being greater in more arid regions (figure 5a). Reduction in floods (averaged by zone) was least for the wettest parts of the Ebro Basin (West-Central Pyrenees and Humid Atlantic zones) and greatest in the dry Southern Mediterranean zone. Moreover, a given percentage of impoundment produces greater flood reduction in the drier part of the basin than in the more humid zones. Put another way, despite similar levels of impoundment, floods in the low-rainfall Southern Mediterranean zone have been reduced more than in the Humid Atlantic zone. When degree of flood reduction is plotted against IR (figure 5b), the Mediterranean and Pyrenean basins line up, while the Atlantic zone plots above the line. The reasons for this are not entirely clear. However, it appears that water may have longer residence times in the reservoirs in humid-Atlantic-climate catchments than in the more Mediterranean influenced parts of the basin, where reservoirs rarely attain maximum storage capacity (M.A. García, Confederación Hidrográfica del Ebro, personal communication). Thus, dams in the Mediterranean zone are more likely to have empty storage with which to reduce a given flood (whatever its magnitude) than their counterparts in more humid parts of the basin. Consequently, downstream environmental effects of dams in Mediterranean-climate rivers are likely to be more pronounced. The west-central and eastern Pyrenees zones have intermediate values, reflecting Mediterranean climatic influences attenuated by the influence of snowmelt from the Pyrenees. This apparent climatic gradient of reservoir effects on floods observed in the Ebro basin is an effect that could be tested for elsewhere, and which would have implications for water resources management and planning of future river regulations, diversions, and impoundments.

5.2. Annual Runoff

Our analysis of hydrological series of annual runoff in 23 impounded rivers and four non- impounded ones suggests that annual runoff significantly decreased after dams in eleven and

increased in two of the 23 impounded rivers, with critical ratios comparing mean annual pre- dam and post-dam runoff statistically significant at p<0.05 (table 5). In ten impounded rivers, annual runoff did not significantly change after dams. Looking at the four unregulated rivers, only the Jiloca, a Mediterranean zone river, showed a significant change in annual runoff, perhaps reflecting the inherent variability of the Mediterranean zone rivers rather than a real long-term change. All dammed rivers in the Mediterranean zone showed significant reduction, while in the other zones only 40% of rivers showed significant reduction.

In eleven impounded rivers mean annual runoff appears to have declined over the 20th century, with low but significant correlation coefficients (0.20>r2<0.60) (table 5). There is no evidence that rainfall decreased during the 20th century in the Ebro basin as a whole (Ibañez et al., 1996), nor in these catchments.

Integrating the entire basin, the Ebro in Tortosa displayed a reduction in annual runoff from 1960-1992 (r2=0.45, n=29) (figure 6). Mean annual runoff in the period 1975-1992 was 30% lower than the period 1960-1974 (p<0.05, table 5, series 27). Reservoir capacity for the whole basin was 2,000 hm3 in 1960 (IR=0.15), and over 6,000 hm3 by 1975 (IR nearly 0.50). The construction of the Mequinenza reservoir (1966) alone added 1534 hm3 in capacity. Reduction in the mean annual runoff had probably already started before the 1960s, although unusually wet years in that decade may have masked effects of previous impoundments. According to Ibañez et al. (1996), 22% of the decrease in mean annual runoff was attributable to reservoir evaporation, 75% to irrigation, similar to the findings of Collier et al. (1995) for the western US.

The Segre River at Lleida (series 24) displayed a remarkably similar reduction in mean annual runoff from 1960 to 1993 (r2=0.51, n=26). Reservoir capacity was already greater than 1,500 hm3 by 1960 (IR = 0.60), with about the same value in 1975. Mean annual runoff from 1975- 93 was significantly lower than 1960-74 (p<0.05, table 5).

The cases of flow reduction are not difficult to explain, but the two cases of post-dam flow increase (a 37% increase on the Najerilla and a 30% increase on the upper Segre) are more surprising. As with the examples of increased floods discussed in the last section, we can think of no plausible mechanism whereby these reservoirs themselves would increase annual

average flow. There are no interbasin transfers in these rivers. However, higher post-dam flows could result if the years after reservoir construction were wetter than those preceding, from climatic variation or from changed runoff conditions. Another explanation for the Upper Segre could be the forest cleaning and urbanization of the catchment of the Valira tributary catchment discussed above. On the Najerilla are no gauges upstream of the Mansilla reservoir, so we have no way to test for increased post-dam runoff there. There was no significant relation between IR and changes in annual runoff, as indicated by the statistics of the best-fitted regression (r2 = 0.002, p<0.05, n=11).

5.3. Daily Flows

Of the ten mean daily flow series, median discharge (P50) was reduced in seven cases, and high flows (P05) in eight (table 6). Low flows were the most affected, with the P95 being reduced by half on average. Williams and Wolman (1984) reported similar results. In some cases (e.g. series 119 and 125) low flows were reduced by an order of magnitude. Most effects are related to diversions for agriculture but there are also large changes exclusively due to power generation, such as the reduction in median discharge on the Noguera de Cardós River from 5.1 m3/s to 0.5 m3/s after construction of the Llavorsí hydroelectric power station.

Pre-dam flow variability (FSD) (i.e. the standard derivations of mean daily flows) ranged from 5.1 to 13.1 among the ten gauge records, and FSD values were almost identical for gauges located along the same rivers, e.g. the Ebro (1, 26), and the Aragón (84, 101, 159). Six of the ten gauging records showed decreased FSD (table 6), reflecting reduced seasonal flow variability typically resulting from reduced fall and winter peaks and summer releases for irrigation, which increase base flows over natural levels (diurnal fluctuations for hydroelectric generation would not be captured by this statistic). Loss of natural flow variability may lead to undesirable ecological changes by creating suitable habitat for invasive non-native species that otherwise could not survive the highly episodic flow regimes. For example, in Mediterranean-climate rivers of California, the proportion of non-native fish species is higher below dams than in unregulated rivers (Baltz and Moyle, 1993).

Four of the ten gauging records displayed increased flow variability post- dam. Two of these increased substantially: the upper Ebro below the Ebro Dam (28%), and the Piedra River

below La Tranquera Dam (30%). Inspection of flow duration curves shows that the increased variability results mainly from reducing low flows, although on the Piedra the increment of less-frequent mean daily flows (equalled or exceeded less than 30% of time) contributed to the increased variability (figure 7).

There was no statistically significant relation between the post-dam changes in mean daily 2 2 2 flow statistics and IR (r = 0.15 for IR vs. Q5, r = 0.20 for IR vs. Q16, r = 0.08 for IR vs. Q50, 2 2 r = 0.08 for IR vs. Q84, and r = 0.002 for IR vs. Q95, p<0.05, n=10, for all analysis), nor between flow variability and IR, as indicated by best fitted regression parameters (r2 = 0.15, p<0.05, n=10).

5.4. Mean Monthly Flows

The effects of reservoirs on mean monthly flows ranged widely (table 7). There was virtually no change on the Noguera Pallaresa in Collegats, with a correlation coefficient Φpre,post of 0.99. The seasonal flow regime was inverted on the Ebro River below the Ebro Dam

(Φpre,post= -0.84) and the Piedra River (Φpre,post =-0.42) (figure 8), the same rivers that displayed large increases in flow variability post-dam. Overall, Φpre,post averaged 0.65 for the 34 data series, suggesting a moderate impact of dams on the monthly regime of the Ebro basin overall (table 7). Post-dam mean monthly flows were nearly independent of the pre-dam values on the Guadalope River below Caspe Dam (Φpre,post = 0.01) and on the Noguera

Ribargorçana River below Santa Anna and Canelles dams (Φpre,post = 0.07).

When the correlation coefficient between pre- and post-dam mean monthly flows (Φpre,post) is plotted against IR, there is considerable scatter, but a trend towards greater independence of post-dam flow (i.e., lower Φpre,post) with higher IR is apparent (figure 9). A significant linear 2 regression can be fit to the data Φpre,post = 0.98 - 0.0091 IR, r = 0.72, n = 34, with Student t- statistics of 18.9 and -11.9, respectively.

The sample is dominated by data from rivers with small values of IR, raising the question of whether the relatively few data points from highly impounded rivers (i.e., high leveraged observations) might be unduly influencing the results. We tested this by running the model

excluding extreme points, and obtained a very similar IR coefficient (0.008) and comparable values of the t-student statistic (25.9 and 8.7). The linear model implies that marginal increases in IR produce an essentially constant negative effect on Φpre,post. However, the underlying relation may not be linear, as the marginal effect of increasing IR may change with increasing reservoir capacity. As an alternative, a spline model produced a kinked best-fit line (figure 9) with two critical thresholds, at 50 and 75% of IR, estimated by maximum likelihood estimators, and expressed by the following model: Φpre,post = 0.95 - 0.0070IR1 - 0.0172R2 - 2 0.0067IR3, r = 0.72, n = 35,with Student t-statistics, 13.58, -2.46, -2.69, -2.62, respectively, where in logical terms, IR1 = Min (IRi, 50), IR2 = Min [Max {0, IR-50}; Max {25, IR-75}], and IR3= Max (0, IR-75).

The spline model suggests that the effect of IR on monthly regime is cumulative, with relatively small effects for dams capturing up to 50% of annual runoff (regression coefficient of 0.007), but with larger effects on flow regime from dams capturing between half and three quarters of the annual runoff (regression coefficient 0.017). Above reservoir capacity of 75% (IR = 0.75), the incremental effect of further impoundment is comparatively low, as indicated by the third regression coefficient (0.0067).

Although the spline model of two critical thresholds is statistically significant, visual inspection of the plot does not support the hypothesis that these thresholds exist, nor are we aware of any physical reason for such thresholds in flood reduction from reservoir storage. We present the model as an alternative to the linear model, one that may be testable in the future with more complete data sets.

5.5. Downstream Persistence

Between 1960 and 1962, the Najerilla, Jalón and Aragón Rivers were impounded by reservoirs with capacities of 68 hm3, 84 hm3 and 471 hm3 respectively. Flow records from gauging stations ranging from immediately downstream of the dams (series 34 and 125) to more than 100 km downstream of the dams (series 87) show that the effects on monthly flows

(described by Φpre,post) diminish with increasing distance downstream from the dam (table 8). Downstream recovery of pre-dam hydrologic regime is noticeable in all three cases, with dam-induced effects nearly gone in the Najerilla (series 48) and the Aragón Rivers (series 5)

after some tens of km and a doubling of the drainage area. Effects of the La Tranquera Dam (Piedra River, main tributary of the Jalón River) on mean monthly flows in the Jalón River are still evident (though attenuated, Φpre,post = 0.67) more than 100 km downstream and despite the increase in drainage area (five times in gauge 9 and seven times in gauge 87 in relation to gauge 125) and the debouching of the non-regulated Jiloca River in the Jalón. Recovery of flood magnitude with distance downstream of dams follows a similar pattern, with no apparent decrease in floods in the Aragón River (gauge 5) approximately 80 km downstream of the Yesa Reservoir. In the same river, directly below the dam, Q2 was only slightly reduced (δ=0.91), and Q10 increased (δ=1.19), implying minor dam effects and/or wetter years after the dam was built. On the Najerilla River Q2 recovers fast in downstream gauges No. 48 and No. 38. The Q10 was already not severely affected by the Mansilla dam. On the Jalón river, Q2 and especially Q10 recovered rapidly downstream of La Tranquera Dam, with δ for Q10 increasing from 0.33 to 0.93 over 33 km, probably explained the big increase in drainage area from 1,478 km2 at gauge No. 125 to 9,694 km2 at gauge No. 87, including the Jiloca River. While the increased drainage area minimized downstream dam effects on flood magnitude, the monthly flow regime still showed large changes.

6. SUMMARY AND CONCLUSIONS

From 1916 to 2000, 15 reservoirs larger than 100 hm3 (108 m3) and 172 smaller reservoirs were constructed on the Ebro River and its tributaries, bringing total impoundment in the basin to 7,700 hm3, equivalent to 57% of the river’s average annual runoff. We analysed flow records from 38 gauging stations in the Ebro basin documented hydrologic effects of the dams and tested the degree to which the changes could be explained by the Impounded Runoff index, IR (reservoir capacity/annual runoff).

1) 22 of the 23 rivers showed reductions in flood magnitude, but with wide variations. The ratio between post and pre-dam flood discharges averaged 0.69 for the Q2 (the 2-year flood),

0.67 for Q10 (the 10-year flood), and 0.58 for the Q25 (the 25-year flood). 2) Higher values of IR were significantly associated with greater reductions in floods. An IR of 0.5 (impoundment of 50% of annual runoff) produced an average reduction of 35% in Q2

and Q10, and 40% in Q25. An IR of 1.0 produced an average reduction of 50% in Q2, 53% in

Q10, and 55% in Q25 (figure 4). 3) Despite similar levels of impoundment, floods in the low-rainfall southern Mediterranean tributaries were more affected by reservoirs than those in the high-rainfall humid Atlantic zone, with a given percentage of regulation producing twice the flood reduction as in the humid Atlantic zone. 4) Out of 23 regulated rivers, annual runoff was significantly reduced in eleven, and increased in two. Reductions were attributable to diversions for irrigation, hydroelectric power, and evaporation from reservoirs. The increases appear to be due to high reservoir inflows post- dam, in one case related with urbanization and forest clearing in the catchment. 5) Basinwide there was no statistically significant relation between changes in annual runoff and IR. 6) Mean daily flows showed post-dam reduction in median discharge in almost all cases, and reductions in low flows (P95) of half on average and, in some cases, an order of magnitude. Effects were mostly related to diversions for agriculture, although there have also been important changes exclusively due to power generation. 7) Most daily series showed a reduction in flow variability, mainly from reduction of flow peaks for water storage and increased summer flows for irrigation, but two series showed a pronounced increase in variability, primarily from decreased base flows, and in one case also from increased high flows evidently resulting from higher runoff from rapid urbanization in the upper catchment. 8) There was no statistically significant relation between changes in median daily flow and IR, nor between flow variability and IR. 9) Monthly flows ranged from virtually no change post-dam to complete inversion in seasonal pattern, the latter due to releases for irrigation in the summer, formerly the season of lowest flows. The correlation coefficient between pre and post-dam monthly flows, Φpre,post, ranged from 0.99 (no change) to –0.84 (complete inversion in seasonal pattern), averaged 0.65.

Φpre,post decreased with increasing IR, displaying a significant linear relation, Φpre,post = 0.98 - 0.0091 IR (r2 = 0.72). 10) Effects on mean monthly flows and flood magnitude diminished with distance downstream from reservoirs and with decreasing IR. The increase in drainage area appears to be the key factor minimizing downstream dam effects on monthly flow regime and, especially, on flood magnitude.

Our results indicate that reservoirs have produced substantial alterations to the flow regime on many rivers of the Ebro basin. The Impounded Runoff index, IR, explained much of the post- dam changes in flood magnitude and mean monthly flow, but not changes in annual runoff or median daily flows. The actual effect of a reservoir upon flow regime will depend not only upon reservoir capacity but also reservoir operation. Flood control dams can be expected to have greater and more consistent effect on peak flows by virtue of the reservoir capacity reserved for flood storage, even near the end of a wet year, when other reservoirs would likely be full. The fact that none of the Ebro River basin dams were constructed for flood control probably contributes to the noisiness of the relations between IR and flood reduction. IR would also not predict changes in total annual runoff, as these would depend upon the existence of inter-basin transfers, diversions to consumptive uses, afforestation, etc.

Despite this variability in the actual effects of reservoirs, we recommend IR as an easily calculated index of the potential of a reservoir to change flow regime. Accordingly, it can serve as an indicator of the likelihood of dam-induced hydrologic changes, which in turn could affect channel morphology, sediment transport, and aquatic ecology (e.g., Petts 1984, Williams and Wolman 1984, Ligon et al. 1995, Collier et al. 1995, Ward and Stanford 1995, Kondolf 1997). Further analyses on a broader and updated Ebro data base, and similar analyses on data from other large river basins, could serve to test whether the patterns observed here can be generalized to other river systems, and to what degree these results can be used to inform management and ecological restoration efforts in regulated river basins.

7. ACKNOWLEDGEMENTS

This study was undertaken while the first two authors were Visiting Research Fellows at the University of California in Berkeley, supported by grants from the Catalunya-California Research Programme (Generalitat de Catalunya), and Banco de Santander, respectively, and during research stays by the third author to the University of Lleida, supported by the Catalunya-California Research Programme. Hydrological data was downloaded from the Confederación Hidrográfica del Ebro web site. Miguel A. García of the CHE Hydrological Planning Office, provided additional updated data and shared useful insights into the stream gauging network and reservoir operation. Carles Balasch of the Water Authorities of

Catalonia supplied data from historical floods in the Segre. Damià Vericat of the University of Lleida prepared figure 1. Toby Minear and Mark Tompkins at the University of California in Berkeley, and Joan M. Verdú at the University of Lleida undertook a critical review of the manuscript. To all of them authors are indebted.

8. REFERENCES CITED

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Guillén, J., J.I. Díaz and A. Palanques, Cuantificación y evolución durante el siglo XX de los aportes de sedimento transportado como carga de fondo por el río Ebro al medio marino, Rev. Soc. Geol. España, 5(1-2), 27-37, 1992 Ibañez, C, A. Rodrigues-Capitulo and N. Prat, The combined effects of river regulation and eutrophication on the dynamics of the salt wedge and the ecology of the Lower Ebro River (North-East Spain). In: D.M. Harper and A.J.D. Ferguson (eds.), The Ecological Basis for River Management, John Wiley and Sons, Chichester, 105-114, 1995. Ibañez, C., N. Prat, and A. Canicio, Changes in the hydrology and sediment transport produced by large dams on the Lower Ebro River and its estuary, Regulated Rivers: Research and Management, 12, 51-62, 1996. Junta d’Aigües, Recomanacions sobre mètodes d’estimació d’avingudes màximes, Generalitat de Catalunya, 164 p., 1994. Junk, W., P.B. Bayley, and R.E. Sparks, The Flood Pulse Concept in River-Floodplain Systems, Proceedings of the Large River Symposium, Canadian Ministry of Fisheries and Oceans, 110-127, 1989 Kondolf, G.M., Hungry water: effects of dams and gravel mining on river channels, Environmental Management, 21(4), 533-551, 1997. Kondolf, G.M., and W.V.G. Matthews, Management of coarse sediment in regulated rivers of California. University of California Water Resources Center, Riverside, Report No.80, 1993. Ligon, F.K., W.E. Dietrich, and W.J. Trush, Downstream ecological effects of dams, a geomorphic perspective, Bioscience, 45(3), 183-192, 1995. Loar, J.M., and M.J. Sale, Analysis of environmental issues related to small scale hydroelectric development, V, Instream flow need for fishery resources, Rep. ORNL/TM- 7861, Environ. Sci. Div., Oak Ridge Natl. Lab., Oak Ridge, Tenn., 1984. 1984. MIMAM, Ministerio de Medio Ambiente, Libro Blanco del Agua en España, Chapter 3, La situación actual y los problemas existents y previsibles, 2000. Moore, C.M., Effects of small structures on peak flows, In: Moore W.L. and C.W. Morgan, Effects of Watershed Changes on Streamflow, University of Texas Press, Austin, 101-117, 1969. Morris, G.L. and J. Fan, Reservoir Sedimentation Handbook, McGraw-Hill, New York, 1997. Muñoz, I. and Prat, N., Effects of river regulation on the Lower Ebro River. Regulated Rivers: Research and Management, 3, 345-354, 1989.

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Table 1. Mean annual water yield and unit runoff in the Ebro valley Water yield Unit runoff (hm3/y) (mm/y) Location Mean Max. Min. Mean Max. Min. Upper main valley 1,776 3,122 816 381 670 175 (Miranda de Ebro) Central main valley 8,059 13,575 2,454 193 325 59 (Zaragoza) Basin outlet 13,408 22,540 4,283 159 265 50 (Tortosa) (source of data: www.oph.chebro.es) Figure 2: : Number of Reservoirs by Storage Capacity

70

Figure 1: Evolution of reservoir construction during the 20th century in the Ebro 60 River. 50

8000 1 40

30

0,80.8 Number of reservoirs 6000 20 ) 3

Reservoir Capacity 10 Impounded Runoff 0,60.6 0

5 0 5 0 5 0 0 5 0 0 0 <1 25 50 7 25 00 00 75 75 0 4000 10 1 150 17 2 225 25 27 3 325 35 3 40 42 45 4 50 5 > Capacity (hm3) 0,40.4 Impounded Runoff Impounded Reservoir Capacity (hm Reservoir Capacity

2000 0,20.2

0 0 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 Figure 4: : Flood reduction (expressed as pre,post, the ratio of the post-dam flood to the pre-dam flood) as a function of IR (total reservoir capacity over annual runoff) for Q2, Q10 and Q25 the floods with return intervals of 2, 10 and 25 years, respectively. Data points indicated by  (Guatizalema River) and o (Aragón and Cinca Rivers (Q10) and Segre River (Q2)) and data from gauge No. Figure 3: Flood frequency analysis for the Ebro River at Tortosa, near the river 27 (Ebro in Tortosa, near the mouth), which integrates the whole catchment, mouth, and the Guadalope River downstream Santolea Dam. were not included in the regression analysis (see text for discussion). 1.2

1

0.8 10000 0.6 pre-dam post-dam

2 2 0.4 Q Q

0.2

1000 0

1.2

1 /s)

3 0.8

100 dam (m - 0.6 pr e max

Q 10 10 post-dam 0.4 Q Q

0.2 Ebro post-dam (upper lines) 10 0 Ebro pre-dam 1.2 Guadalope post-dam (lower lines) Guadalope pre-dam 1

0.8 dam dam - 1 - 0.6 post pre 25

1 10 100 25 0.4 Q Q Recurrence interval (years) 0.2

0 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

IR (reservoir capacity / annual runoff)

Figure 5: Mean flood reduction for the four Ebro River basin climatic zones. Average ratios of post-to-pre-dam flood magnitudes for Q2, Q10, and Q25 (y-axis) plotted against a) average precipitation for each climatic zone and b) average IR for each climatic zone (x-axis). In (b), data points for the humid Atlantic zone do not line up with data from the other zones, and the expected plotting positions for the points (if they followed the trend of the other data) are indicated by arrows (data from table 4.)

1

Central Pyrenees 0,8 Humid Atlantic

0,6 Eastern Pyrenees Southern Mediterranean

0,4

0,2 Q2 Q10 Q25 0 1000 900 800 700 600 500 400

Precipitation (mm)

1

Central Pyrenees

0,8

0,6 Humid Atlantic

Eastern Pyrenees

0,4 Southern

Qpost/Qpre Mediterranean

0,2

Q2 Q10 Q25 0 0 0,2 0,4 0,6 0,8 1

Impounded Runoff (IR) Figure 7: Pre and post-dam flow duration curves of mean daily flows for the Ebro downstream Ebro Dam, Noguera de Cardós and Piedra Rivers.

1000 Ebro River 1913-31

100

1945-90 Figure 6: Changes in annual runoff in the Ebro River at Tortosa, near the river mouth (series 10 27) during the 20th century.

35000 1 30000

25000

20000 3 0,1 hm 15000

10000

5000 100

0 Noguera de Cardós )

1917.18 1920.21 1923.24 1926.27 1929.30 1932.33 1935.36 1938.39 1941.42 1944.45 1947.48 1950.51 1953.54 1956.57 1959.60 1962.63 1965.66 1968.69 1971.72 1974.75 1977.78 1980.81 1983.84 1986.87 1989.90 1992.93

s /

3 10 m

25000 (

1954-66

w

o

l f 20000

y 1 l

) i

3

a

d

15000 n 1966-90

a e

0,1 M 10000 Annual Runoff (hm

5000 100 Piedra River 0 1960 1965 1970 1975 1980 1985 1990 1995

10 .

1953-61

1

1962-90 0,1 110100 % Time equalled or exceeded Figure 9: Effects of reservoir impoundment on mean monthly flows, expressed by the correlation coefficient between pre- and post-dam mean monthly flows ( pre,post) plotted against Impounded Runoff (IR), the ratio of reservoir capacity to annual runoff. Two models are shown: an ordinary linear regression model (straight line) and a spline model with two critical thresholds (dashed, kinked line).

Figure 8: Effects of reservoir impoundment on mean monthly flows, expressed by the correlation coefficient between pre- and post-dam mean monthly flows ( pre,post) plotted against Impounded Runoff (IR), the ratio of reservoir capacity to annual runoff. Two models are 1 shown: an ordinary linear regression model (straight line) and a spline model with two critical thresholds (dashed, kinked line).

) Critical Thresholds of IR hypothetically affecting River

pre, post Regime (Splin model) 0,6 ro River at Gauge No. 26 Piedra River at Gauge No. 125 mid Atlantic Southern Mediterr anean =1.75 Φ= - 0.84 IR=0.70 Φ= - 0.42 1945-9310 1961-90 /s) 3 1914-45 0,2 1953-60 5 Mean Monthly Monthly Mean Q (m 0 -0,2 adalopeND River J at FMAMGauge No. 99 J JA S JalónONDJFMAMJ River at Gauge No. 9 JAS uther n Mediterranean Southern Med iterr anean 1.06 Φ=0 .01 IR=0.21 Φ=0.50 20 1912-60 /s) 3 15 1992-95 1973-91 -0,6 10

1961-90 ( flows montly pre-post-dam coefficient Correlation 5 Mean Monthly Monthly Mean Q (m 0 ND J FMAM J JA S ONDJFMAMJ JAS -1 00,511,52 IR (%)