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Keywords:

  • North Atlantic Oscillation;
  • extreme phases;
  • river discharge;
  • reservoir storage;
  • reservoir management;
  • floods

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[1] This paper analyzes the influence of the extreme phases of the winter North Atlantic Oscillation (NAO) on water resources in the Spanish region of the Tagus River basin. By analyzing a winter NAO index based on station sea level pressure, the years between 1957 and 2003 were classified as normal, positive, and negative NAO years. A statistical test was then applied to monthly data series of precipitation, river discharge, reservoir storage, and reservoir release to analyze the variations in these variables. For all four variables, significant differences were found between positive and negative NAO years, the former resulting in reduced water availability (negative anomalies) and the latter resulting in increased water availability (positive anomalies). The influence of extreme NAO winters was found to act with different time lags on different variables: The effect of extreme NAO winters on precipitation was found to be quite immediate (and significant for December to March), but this effect was observed later in the year and lasted longer for river discharge, reservoir storage, and water release. Positive and negative NAO years were also found to have different effects on these variables, in that the effects of positive years were more sustained and those of negative years were more rapid and less prolonged. In spite of the high variability of the availability of water resources, the strategies for management of the reservoir system of the basin were found in most cases to provide a regular supply that meets water demands. However, our results also indicate that these water management practices are not adequate for the expected scenarios of climate change and increasing water demand.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[2] Management of water resources in the Iberian Peninsula is at present a high priority due to increases in water consumption by agriculture, tourism, and industry. Most Iberian rivers have a marked seasonal and interannual variability, with wet periods characterized by high flows and even catastrophic floods alternating with severe and long droughts [Hisdal et al., 2001] as a consequence of the high precipitation variability in this region [Katz and Acero, 1994; Esteban-Parra et al., 1998]. During droughts, the Mediterranean areas of the Iberian Peninsula experience low precipitation and water storage levels, as well as strong pressures on water resources [Olcina, 2004]. The near-continuous water deficit on the Mediterranean coast is partially solved by the Tagus-Segura water transfer, but the donor basin is also prone to water scarcity. Thus the Tagus-Segura water transfer has only been able to supply about 40% of the projected flows since the beginning of its operation in 1979 [Morales et al., 2005]. The determination of the amount of water available for transfer during the dry years causes severe social and political conflicts. Moreover, the transboundary character of the Tagus basin (Spain and Portugal) results in some difficulties in the management of the flows, mainly during the extremes of large flood events and drought periods.

[3] These considerations show that it is important to improve the understanding of all factors involved in the climatic and hydrological variability of this region, and to use this knowledge for more precise and reliable management of its water resources. In this respect, it has been shown that several large-scale atmospheric patterns (e.g., the North Atlantic Oscillation, the El Niño–Southern Oscillation, the Scandinavian pattern, and the Mediterranean Oscillation) determine most of the variability of precipitation over the Iberian Peninsula [Pozo-Vázquez et al., 2004; Muñoz-Diaz and Rodrigo, 2004; Rodríguez-Puebla et al., 1998; Vicente-Serrano, 2005; Martín-Vide and López-Bustins, 2006]. The North Atlantic Oscillation (NAO) is widely recognized as the most important pattern for explaining the climatic and hydrological variability of the Iberian Peninsula [Zorita et al., 1992; Rodó et al., 1997; Rodríguez-Puebla et al., 1998; Martín-Vide and Fernández, 2001; González-Rouco et al., 2000; Trigo et al., 2004; Xoplaki et al., 2004; Trigo and Palutikof, 2001] and, in particular, of the Tagus basin [Trigo et al., 2004].

[4] The NAO is primarily a north-south dipole characterized by simultaneous out-of-phase geopotential height anomalies between temperate and high latitudes over the Atlantic sector [Hurrell et al., 2003]. The largest amplitude anomalies are recorded over Iceland and the Iberian Peninsula. Most studies of the NAO have focused on the Northern Hemisphere winter months, when the atmosphere is most active and its influence on climate is greatest [Hurrell and Van Loon, 1997]. The NAO controls the direction and intensity of the wind fields and the interaction between air masses in the North Atlantic region, and so influences the weather (precipitation, temperature, etc.) and hydrology over a large part of Europe [Hurrell et al., 2003; Drinkwater et al., 2003; Mysterud et al., 2003; Kingston et al., 2006, and references therein].

[5] Generally, positive NAO index winters are associated with a northeastward shift in the Atlantic storm activity and modest activity in the South [Serreze et al., 1997; Trigo et al., 2002; Hurrell and van Loon, 1997]. This shift usually causes severe and persistent droughts in the Iberian Peninsula although with important spatial variation [Rodríguez-Puebla et al., 1998; Vicente-Serrano and López-Moreno, 2006].

[6] The impact of the NAO on the hydrological system of the Atlantic region, especially in its northern section [Kingston et al., 2006, and references therein], has been addressed by several authors. Kiely [1999] reported that there has been a sharp increase in the streamflow in Ireland since 1975, and that this increase is possibly related to the recent positive trend in the NAO. Phillips et al. [2003] described significant NAO-streamflow relationships in the northwest of Scotland. Shorthouse and Arnell [1997] demonstrated a positive winter NAO-streamflow correlation for most of northwestern Europe. There are few studies devoted to southern Europe, although significant relationships between the NAO and the hydrological cycle have been found in Turkey [Kalayci and Kahya, 2006], Romania [Stefan et al., 2004], the Middle East rivers [Cullen et al., 2002], and the Danube basin [Rimbu et al., 2006]. Although the effect of the NAO on precipitation in the Iberian Peninsula is well known, there are few studies that analyze its effects on precipitation and the different hydrological subsystems throughout the hydrological cycle. Trigo et al. [2004] found decreases in the river discharges of the three Atlantic basins of Spain and Portugal during positive NAO winters.

[7] Most of the studies cited above do not take into account the response times of the components of the hydrological cycle to variation of climate conditions. However, this is an important issue because it is known that different usable water sources have different times of response to decreases and increases in precipitation [McKee et al., 1993; Elfatih et al., 1999; Pandey and Ramasastri, 2001; Peters et al., 2005]. For instance, Trigo et al. [2004] found that correlations between the winter NAO and discharges were significantly higher if a lag of 1 month was taken into account. However, the lag times between climatic anomalies and the responses of the hydrological subsystems can be highly variable [Vicente-Serrano and López-Moreno, 2005]. The lag time depends on the hydrological functioning of the basin (such as its physiographic characteristics and groundwater circulation) and on the water management strategies used in a given sector, which are dependent on the relation between water availability and demand [Vicente-Serrano and López-Moreno, 2005].

[8] In this paper we analyze the effects of the extreme positive and negative phases of the NAO on the various hydrological subsystems in the Spanish region of the Tagus River basin. The objectives of our study were (1) to assess the magnitude and duration of the effects of the extreme phases of the winter NAO on precipitation and surface water resources in the Tagus basin; (2) to determine if there are differences in the impact of the NAO on various hydrological subsystems: precipitation, river discharge, reservoir storage, and reservoir release; and (3) to determine to what extent the management strategies of the reservoirs can reduce the impacts of the extreme phases of the NAO on the availability of water resources.

2. Study Area

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[9] This study investigated the Spanish region of the Tagus River basin (Figure 1). The Tagus River is located in the center of the Iberian Peninsula and flows from east to west for 1009 km (73% of its length is in Spain; 27% is in Portugal), draining a total area of 80,100 km2 (69% of the total area is in Spain; 31% is in Portugal). Close to the border between the two countries is the Alcántara Dam (135 m high), which was built in 1969 for hydropower production. With a storage capacity of 3162 hm3, the Alcántara reservoir is the second-largest reservoir in Europe and significantly modifies the downstream river regime. For this reason, the hydrological records for the Portuguese section of the river were not included in the analysis, and we only analyzed the records for the Spanish section, for which the climatic influence is clearer.

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Figure 1. Study area and location of the stations used: (a) reservoirs (solid squares) and precipitation (open squares); (b) gauging stations. Names and characteristics of reservoirs and gauging stations are indicated in Table 1.

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[10] The total capacity of the reservoirs in the Tagus basin is 14,500 hm3, of which 12,500 hm3 belong to Spain. The discharge of the Tagus River in its outlet to the Atlantic Ocean (in Lisbon) is about 17,730 hm3 yr−1, of which 12,230 hm3 yr−1 are generated in Spanish territory [Confederación Hidrográfica del Tajo (CHT), 2000]. This means that the Tagus is among the most important rivers in Spain and is the most affected by the construction of large reservoirs. In the headwaters, the Entrepeñas (802 hm3) and Buendía (1638 hm3) reservoirs are the largest and provide water for the Tagus-Segura transfer [Gómez-Mendoza and Mata, 1999; Morales et al., 2005]. There are a total of 40 reservoirs with more than 15 hm3 of capacity in the Tagus basin, which affect most of the main tributaries [Flores, 2004].

[11] The Tagus River is one of the main water sources in Spain and is used for both urban (i.e., the cities of Madrid and Lisbon) and agricultural purposes [García and Alcolea, 2006]. About 15.5% of the Spanish population lives within its basin. The headwaters of the Tagus River are located in the Albarracín Range, up to 1500 m above sea level (asl), which is a massif belonging to the Iberian Range. The middle reach runs through the Tagus sedimentary basin. This basin is delimited to the north by the Central Range, with granite peaks that reach 2592 m above sea level (Almanzor peak) in the Gredos massif, and to the south by the Toledo Mountains, a Paleozoic range of moderate altitude (1600 m).

[12] The climate varies from Mediterranean with strong continental influences eastward to an increasing influence of Atlantic conditions westward. The average annual precipitation varies significantly along the length of the river. The headwaters receive about 1100 mm yr−1 on average, but a very important section of the middle reach (south of Madrid) receives less than 450 mm yr−1. The headwaters of the tributaries coming from the north (that is, from the central ranges: the Gredos, Peña de Francia, and Estela sierras) receive more than 1500 mm yr−1. Potential evapotranspiration is estimated at more than 800 mm yr−1 in the central and lower parts of the basin [CHT, 2000]. The mean annual temperature is about 11°C, is even lower in the mountain areas (5°–7°C), and is 14°–15°C in the central part of the basin (i.e., Madrid, 14.1°C; Toledo, 15.4°C).

3. Methods

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

3.1. Data

[13] There is a dense network of gauging stations in the Spanish region of the Tagus River basin, including 104 standard stations (http://www.chtajo.es/redes/aforos/estaciones/informes_aforos.html) and 56 stations at the entrances to the main reservoirs (http://www.chtajo.es/redes/aforos/embalses/informes_embalses.html). The temporal coverage and the amount of missing data for each station vary significantly. With the objective of obtaining a robust database covering the longest time period possible that includes a representative sample of good quality data series (with less than 5% missing values), a careful review of the original data was carried out. The filling in of missing data was carried out by means of a linear regression analysis that considered the most correlated series. Finally, a database with observations from 18 gauging stations, seven reservoirs, and five precipitation observatories was constructed, covering the period between 1958 and 2003 (Figure 1). Tables 1 and 2 show the statistics for the rivers monitored by each gauging station and also for the reservoirs we analyzed. Also listed in Tables 1 and 2 are the general lithological characteristics of the basin recorded at each gauging station and the use of each reservoir.

Table 1. Statistical Characteristics of the Gauging Stationsa
 Station NameMain LithologyAnnual Discharge, hm3
MeanCoefficient of Variation
  • a

    The lithological characteristics of the basin in each gauging station are also shown.

1Peralejos de las Truchaslimestones162.70.41
2Trillolimestones582.50.49
3Ventosalimestones57.40.43
4Mejoradalimestones796.80.58
5Bujalarolimestones109.20.62
6Espinilloslimestones410.00.58
7Belenalimestones144.60.78
8Parque Sindicalgranites96.30.89
9Maseosolimestones67.90.60
10Oruscolimestones364.10.76
11Humaneslimestones308.30.64
12Villalbagranites65.60.54
13Eljertegranites458.80.34
14Entrepeñaslimestones589.80.46
15Buendíalimestones553.10.58
16San Juangranites572.10.49
17Valdecañassedimentary424.90.53
18Gabriel y Galángranites932.00.51
Table 2. Statistical Characteristics and the Use of Each Reservoir
 Reservoir NameMain PurposeMaximum Storage Capacity, hm3Monthly Water Storage, hm3
MeanCoefficient of Variation
1Entrepeñasirrigation/hydropower802.56454.580.40
2Buendíairrigation/hydropower1638.7764.490.51
3San Juanirrigation/water supply/hydropower148.3117.140.17
4Puentes Viejasurban supply49.237.50.25
5El Vadourban supply55.630.60.27
6Borbollónirrigation/hydropower8547.70.26
7El Burguilloirrigation/hydropower208123.330.23

[14] Regional data series for river discharge, reservoir storage, and reservoir release for the whole basin were created from the monthly records for the stations and reservoirs. The purpose was to obtain a regional overview of the main effects of the NAO extreme phases on the water resources of the basin.

3.2. Determination of the Extreme Phases of the North Atlantic Oscillation

[15] Of the several procedures available for characterizing the NAO, we selected the NAO index, which is created from the differences between the standardized pressure anomalies of Gibraltar (south of Spain) and Reykjavik (Iceland). The advantage of using these stations is discussed in depth by Jones et al. [1997]: The records of Gibraltar appear to better represent the southern part of the NAO dipole than those of other commonly used stations, such as Lisbon or Ponta Delgada in the Azores. A temporal series of the NAO index for the period 1957–2003 was obtained from the Climate Research Unit of the University of East Anglia (http://www.cru.uea.ac.uk/cru/data/nao.htm). We calculated the average winter NAO index, which includes the months of December, January, February, and March; this period was selected because the NAO has its highest activity in these months [Hurrell et al., 2003], and the impact of the NAO on climate over the North Atlantic region is better characterized for this period than for other periods [Cook et al., 2002].

[16] Following Muñoz-Diaz and Rodrigo [2004], we calculated the average and standard deviation of the winter NAO index throughout the whole study period (0.40 and 1.16, respectively), and set a threshold of one standard deviation to define the extreme NAO phases. Years with a winter NAO higher than 1.56 were characterized as positive phases, and years with a winter NAO below −0.76 were characterized as negative phases. The positive years were 1961, 1967, 1983, 1989, 1990, 1992, 1994, 1995, and 2000, and the negative years were 1963, 1964, 1965, 1969, 1977, 1979, and 1996. The remainder of the years were characterized as “standard” and were not included in our study. Although the NAO is active mainly in winter, we also considered the spring and autumn NAO phases to know if autumn and spring precipitation could influence the water resources. We also obtained positive and negative years for the autumn and spring NAO following the same threshold of one standard deviation regarding the average.

3.3. Analysis

[17] The monthly data series for precipitation, river discharge, reservoir storage, and reservoir release were standardized (standard deviation units from the average) to allow comparison of records with different magnitudes, and the Wilcoxon Mann-Whitney rank test was used [Siegel and Castelan, 1988] to determine whether significant humid or dry conditions dominate during positive or negative winter NAO phases. Although the Wilcoxon Mann-Whitney test is slightly less powerful than parametric tests such as the t test, it was preferred here because of its robustness against nonnormality of the variables [Helsel and Hirsch, 1992]. The normalized monthly values of the hydrological variables during positive and negative NAO years were compared with the monthly values during (1) normal NAO years and (2) years of opposite NAO sign. The significance level for this test was established at α < 0.05.

[18] The consistency of the responses of the monthly series to positive and negative NAO phases was also checked by examination of the standard error of the monthly averages. A low standard error was considered to be an indication of strong coherence between events, i.e., a guarantee that the influence of NAO extreme phases is not the result of a few major events, but rather the reflection of fairly stable and quantitatively similar results for all of the positive/negative years.

4. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

4.1. Influence of the Extreme NAO Phases on Precipitation

[19] Figure 2 shows the average and standard deviation of the monthly normalized precipitation corresponding to extreme positive and negative winter NAO phases. The significant differences between the positive and negative extreme NAO years and the other years (black columns) are clear. Averages for periods of more than 12 months after the NAO phases were not shown to avoid likely misinterpretation due to difficulties in isolating the effects of the current year from the effects of the previous year.

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Figure 2. Precipitation: monthly average anomalies (bars) and standard error (whiskers) during positive and negative winter North Atlantic Oscillation (NAO) years. Black columns indicate significant differences between positive/negative years and the remainder of years. Crosses indicate significant differences between positive and negative years. Circles indicate significant differences between positive/negative years and normal years.

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[20] The effects of extreme phases of the winter NAO are statistically significant for the majority of weather stations from December to March.

[21] Positive phases of the winter NAO cause negative precipitation anomalies, and negative phases produce the opposite pattern, i.e., positive average anomalies of precipitation. The magnitudes of the anomalies are usually lower during negative phases, which suggests that the response of precipitation to the winter NAO is nonlinear. Some spatial differences were also identified, with a trend to more significant anomalies in the eastern observatories (see the results for Madrid, Toledo, and Cuenca). In Segovia (north) and in Cáceres (west) the numbers of significant anomalies were found to be lower.

4.2. Influence of Extreme NAO Phases on River Discharge

[22] Figure 3 shows the average and standard deviation of the monthly normalized river discharge. As was found in the case of precipitation, during positive NAO phases negative anomalies in river discharge arise, whereas positive anomalies were recorded during negative NAO phases. However, there are several differences between the effects of extreme phases on river discharge and those on precipitation. For example, river discharge anomalies in December did not show significant differences as a function of the NAO phase, whereas significant differences were observed in the precipitation data. This suggests there is a lag of at least 1 month in the hydrological response to climate conditions, as already suggested for the Tagus River by Trigo et al. [2004] and for the north of Spain by Vicente-Serrano and López-Moreno [2005]. Second, it was found that the NAO-induced anomalies in river discharge extended beyond the winter months, indicating a more persistent influence of the winter NAO on river discharge than on precipitation. The influences of the positive and negative NAO phases were found to be significantly different. Thus the response of river discharge to negative phases was very quick and intense but relatively short, in that it was significant only from January to April. In contrast, the intensities of the anomalies arising due to positive NAO phases were more moderate, but were significant even during summer and autumn months.

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Figure 3. River discharge: monthly average anomalies (bars) and standard error (whiskers) during positive and negative winter NAO years for the whole Tagus basin (regional compound series). Legend is the same as in Figure 2.

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[23] Figures 4a, 4b, and 4c show the average anomalies in river discharge at each gauging station during positive and negative NAO phases. The records of the majority of the stations follow the general pattern observed for the regional series: negative anomalies in positive years and positive anomalies in negative years. Nevertheless, the timing and magnitude of the significant anomalies vary, even between neighboring stations.

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Figure 4a. River discharge: monthly average anomalies (bars) and standard error (whiskers) during positive and negative winter NAO years in different gauging stations of the Tagus River basin. Legend is the same as in Figure 2.

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Figure 4b. River discharge: monthly average anomalies (bars) and standard error (whiskers) during positive and negative winter NAO years in different gauging stations of the Tagus River basin. Legend is the same as in Figure 2.

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Figure 4c. River discharge: monthly average anomalies (bars) and standard error (whiskers) during positive and negative winter NAO years in different gauging stations of the Tagus River basin. Legend is the same as in Figure 2.

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[24] The lag in the hydrological response found in the regional series (Figure 3) was detected at all gauging stations, but varied from 1 month (e.g., Peralejos) to 3–4 months (e.g., Trillo). Moreover, the lag times tended to be lower for negative NAO phases than for positive phases.

[25] Generally, stations located in the headwaters of the Tagus River recorded a lower number of months with significant anomalies. At Peralejos (the first gauging station on the Tagus River), significant anomalies were only recorded in January, February, and March. In lower parts of the basin, at stations with larger drainage areas (e.g., Trillo, Entrepeñas, and Buendía), the response to extreme NAO phases is slightly more complex, with a larger number of months with significant anomalies during positive NAO phases. During negative NAO phases, on the contrary, significant anomalies were only found between January and April. The variability between events for negative phases, represented by the standard error of the average, is also higher than for positive phases. Moreover, the response to the NAO was clearly nonlinear, as indicated by the higher magnitude of the anomalies due to positive NAO phases.

[26] The records of the stations located on the Tajuña River (Maseoso and Orusco) adhered to the general pattern observed at other stations during positive NAO phases, but the response at Maseoso (in the headwaters) was found to be quicker and more intense during negative NAO phases than at Orusco (on the final section of the river), where positive anomalies between the negative years and the other years were identified between May and December.

[27] With the exception of Bujalaro station, for which the most important anomalies were identified during positive NAO phases, the stations located on the Jarama and Henares rivers (Humanes, Belena, Espinillos, Parque Sindical, and Mejorada) only recorded significant negative anomalies in the winter and spring months during positive NAO phases, and between January and April during negative NAO phases.

[28] For the Guadarrama (Villalba station) and Alberche (San Juan station) rivers, similar behavior was found to that observed in the basin of the Henares-Jarama. Again, the responses to positive and negative NAO phases were found to be different, with the anomalies during negative phases found to be variable but more intense.

[29] Finally, at the station located in the lowest section of the Tagus River (Valdecañas) and at the two located in the basin of the Alagón River (Gabriel y Galán and Eljerte), very small anomalies were found, although general responses to the positive and negative NAO phases were also detected.

[30] Therefore, although the records of the majority of the stations were found to adhere to a general pattern consisting of negative, less intense, less variable, and more persistent anomalies during positive phases than during negative phases, substantial differences were found between different parts of the catchments. The responses to extreme NAO phases of the headwaters of the Tagus River and of the small rivers were quicker, with some exceptions, becoming more and more moderate toward the middle sections. The records of the stations at Trillo, Entrepeñas, and Buendía on the Tagus River provide the best examples of this retarded response to extreme NAO phases. This is clearest during positive NAO phases, in which this effect was found even in the last months of the year. In the lowest section of the river system, the effect of extreme NAO phases was found to be much more attenuated.

4.3. Influence of Extreme NAO Phases on Reservoir Storage and Water Release

[31] The influence of extreme NAO phases on reservoir storage was noticeably different to the influence on precipitation and river discharge (Figure 5). No significant anomalies were found during the winter months for either positive or negative phases. In contrast, significant anomalies were found during the rest of the year (from February to November), with stronger effects during the summer and autumn months. During positive NAO phases, negative anomalies were identified during 10 months compared with both negative phases and normal years. During negative NAO phases, significant positive anomalies were found from April to December. In general, it can be said that there is a longer lag in the effect of extreme NAO phases on reservoir storage than that found for river discharge, but that these effects were more sustained, affecting almost all the rest of the year.

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Figure 5. Total water storage and reservoir release in the Tagus basin reservoir system: average and standard error of the anomalies during positive and negative winter NAO years. Same legend as in Figure 2.

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[32] As for the amount of water released from the reservoirs, the behavior follows the general pattern observed for other variables: negative anomalies during positive NAO phases and positive anomalies during negative phases. As was observed in the case of river discharge, the influence of the NAO on water release was not linear. The magnitudes of the anomalies were lower during positive NAO phases than during negative phases, although the former anomalies lasted longer. However, the magnitude of the anomalies and the number of months with significant differences between positive and negative phases were lower for reservoir release than for river discharge. This was especially true during positive phases, in which less dry conditions were recorded for reservoir release than for river discharge. Figure 6 shows the anomalies in the storages of the different reservoirs arising in the positive and negative NAO phases. There are some differences between the results for the reservoirs. The largest influence of the extreme NAO phases was seen for the two reservoirs located in the upper section of the Tagus River (Entrepeñas and Buendía). These reservoirs have the largest storage capacity of the analyzed reservoirs. The effect of the NAO is lower in the reservoirs that have less storage capacity (see Table 1 for details).

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Figure 6. Water storage in various reservoirs in the Tagus basin: monthly average anomalies (bars) and standard error (whiskers) during positive and negative winter NAO years. Legend is the same as in Figure 2.

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[33] Figure 7 shows the average anomalies in water release for the seven reservoirs. The differences between the results for the reservoirs can be explained by their contrasting characteristics and operation. The impact of extreme NAO phases on the Entrepeñas and Buendía reservoirs, which are the largest in the basin, is the lowest. The influence of extreme NAO phases on the other smaller reservoirs is larger, and similar to that observed in the case of river discharge. The nonsymmetrical character of the influence of the positive and negative NAO phases is also much clearer for these smaller reservoirs.

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Figure 7. Water release from various reservoirs in the Tagus basin: monthly average anomalies (bars) and standard error (whiskers) during positive and negative winter NAO years. Legend is the same as in Figure 2.

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4.4. Influence of Spring and Autumn NAO Phases on Water Resources

[34] Figure 8 shows the average values of reservoir storages, river discharges, and reservoir releases considering the winter, spring, and autumn NAO phases. The role of the winter NAO is clearly identified during the winter months, but also during several months later. The role of the autumn NAO on water resources is very little; and the spring NAO, although it shows some effects during positive phases, the magnitude of the anomalies is not significant regarding the rest of the years in the majority of the months. Although some strength of the NAO effects could be inferred if negative or positive NAO phases coincide the same year in winter and spring, this pattern is not usual. Therefore the role of NAO in water resources can be considered mainly as a winter phenomenon that affects water resources several months after, whereas autumn and spring NAO conditions have a minimum role.

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Figure 8. Average reservoir storages, reservoir releases, and river discharges for the whole Tagus basin during (left) positive and (right) negative winter, spring, and autumn years.

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[35] Moreover, the pattern observed so far is strengthened during periods of several years of persistence of the NAO in one of the extreme phases. We calculated the anomalies during the two most extreme periods (1964–1965, negative phase, and 1994–1995, positive phase) and found that the pattern is very similar to the single-year extreme phases obtained for the 1958–2003 period (results are not shown). Nevertheless, we found a more pronounced effect as a natural consequence of these periods being the two most extreme events of the NAO identified during the twentieth century.

4.5. Role of Reservoirs in Mitigating the Hydrological Anomalies Associated With Extreme NAO Phases

[36] The results discussed above show that reservoirs have important roles in the management of the hydrological extremes associated with extreme NAO phases. Figure 9 shows the average values of river discharge for the reservoirs, and the real magnitudes (hm3) of the total water stored and total water released in the whole reservoir system. During positive NAO years, the inflows and the water released have completely different behaviors than are observed during the negative years. These differences are especially evident during the winter months, which are responsible for the annual peak discharge during negative NAO phases but represent a secondary minimum during positive phases. The abundant inflow during negative years has two effects on water resources management during winter and spring: (1) the possibility of noticeably increasing the volume of water stored in the reservoir system; and (2) the release of high flows downstream of the dams. During the positive phases, the managers equalize the inflows and the outflows, with the latter noticeably lower than during the negative NAO years. From May to December, the inflows to the reservoirs vary between the different NAO phases, with average inflows of 172.2 m3 s−1 during negative winter NAO phases and 113.4 m3 s−1 during positive winter NAO phases. Outflows are also similar for both types of NAO phases, and so the decreases in the water stored in reservoirs have similar magnitudes (427 and 418 hm3 for negative and positive phases respectively), which were achieved by means of adjustments of the reservoir releases; on average the reservoir releases were 227 m3 s−1 during negative winter NAO phases and 164 m3 s−1 during positive winter NAO phases. The differences between the water stored in winter and spring during the different NAO extreme phases explain the persistence of the significant anomalies in water stored, as shown in Figure 5.

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Figure 9. Average river discharge, total reservoir storage, and total reservoir release for the whole Tagus basin during positive and negative NAO phases.

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[37] Figure 10 shows the average values of the river discharge, reservoir storage, and water release for the reservoirs of the basin. As Figure 6 shows, there are large differences in the effects of extreme NAO phases on reservoir management. The global pattern described above is clearly appropriate for the Entrepeñas and Buendía reservoirs. This is due to the importance of these two reservoirs in the whole system, since they make up the largest fraction of the total storage capacity of the system. The results for the other reservoirs exhibit various levels of discrepancy with respect to the global behavior. Although for most reservoirs the inflows are clearly higher during negative NAO phases, these inflows are not always used for increasing the reservoir storage (e.g., in Barballón, El Vado, San Juan, and El Burguillo). This is because these reservoirs are smaller, and because water release is used for hydropower production and for the infilling of downstream reservoirs. Thus no clear trends in water storage are found during variation of the NAO phases. Differences in water release according to NAO phase are less distinct during summer and autumn, although they can be important for the Buendía, Entrepeñas, and El Burgillo reservoirs.

image

Figure 10. Spatial distribution in the average values of river discharges, reservoir storages and reservoir releases for the whole Tagus basin corresponding to positive and negative NAO phases.

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5. Discussion and Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[38] This study focused on the impact of extreme winter NAO phases on three different subsystems of the water cycle: precipitation, river discharge, and reservoir storage. The results highlight the importance of the NAO extreme phases, with variation of their influence depending on (1) the hydrological subsystem, (2) the occurrence of a positive or a negative phase, and (3) the geographical location within the basin.

[39] Precipitation tends to decrease during positive extreme NAO phases, and tends to increase during negative phases. These results are in general agreement with the pattern of precipitation anomalies in the Iberian Peninsula, which are mainly associated with the winter NAO [Martín-Vide and Fernández, 2001].

[40] López-Moreno and Vicente-Serrano [2007] demonstrated that extreme NAO phases noticeably affect the field pressures over Europe, which explains the regional differences in drought conditions. However, the extreme NAO phases only affect precipitation from December to March, i.e., their impact on precipitation does not continue into spring. Nevertheless, these authors also indicated that although precipitation is not affected, drought conditions last longer in southern Europe during positive phases of the NAO, which is made clear by their analysis of river discharge and reservoir storage in the Tagus basin.

[41] Another important result of this study is the nonlinearity of the responses of precipitation and river discharge to extreme NAO phases. In general, higher anomalies are recorded during negative years. This result might be explained by the influence of extreme NAO phases on rainfall characteristics. Muñoz-Díaz and Rodrigo [2003, 2004] documented significant changes in the frequency distribution of precipitation and the extreme events as a function of the positive and negative phases of the NAO. Gallego et al. [2005] demonstrated that the strong influence of the NAO on southern Europe is due mainly to the effect of the winter NAO on intense rainfall events, which occur more frequently during negative extreme phases. The asymmetrical response of precipitation to various atmospheric circulation patterns has been observed in several regions [Katz and Parlange, 1994; Kiely et al., 1998; Smith and Ropelewski, 1997; Vicente-Serrano, 2005]. Recently, Pires and Perdigao [2007] have assessed asymmetry within the statistical response of the winter precipitation to the NAO over the North Atlantic–European region. They found that near the central North Atlantic, around 40°N–20°W, and southeast of Iceland, the correlation is much stronger in the wet-favorable regime: negative phases of the NAO in the first location and positive phases in the second location. On the contrary, around 42°N, 48°W in the west North Atlantic and in the west Mediterranean near 36°N, the correlation is only relevant for the dry-favorable positive phases. These authors have shown that there are some coherent regions where the nonlinear component of the response of winter precipitation to the NAO is more important.

[42] A possible uncertainty of the results obtained arises from the small size of the sample due to the restrictive threshold used to define the extreme phases of the NAO: ±NAO σ. Therefore only seven negative and nine positive cases were identified, and 30 years fell into the neutral class. Nevertheless, if a less restrictive threshold is chosen (e.g., ±0.5 σ, which results in 12 positive and 11 negative phases), the reservoir storages and reservoir releases anomalies during positive and negative phases are very similar to the ones found for ±1 σ (results not shown). Therefore the criterion used to select the NAO phases does not affect noticeably the results, which show a high degree of robustness against changes in the threshold used to select the NAO phases.

[43] Precipitation is the main variable affecting the interannual variability of river flows [e.g., Dooge et al., 1999; Jones, 1999; Beguería et al., 2003]. However, there is usually a time lag in the conversion of rainfall to runoff, which attenuates the effect of climatic anomalies; this result can be explained by the complexity of the processes involved in runoff generation in a large basin. Thus anomalies in river discharge can be found several months after winter, even if the influence of the NAO on precipitation acts on a much more immediate timescale. This is not the only difference between precipitation and river discharge with respect to the influence of the NAO: The nonlinear character of the response of river discharge to the NAO is higher than that found for precipitation, resulting in very intense but short discharge anomalies during negative NAO phases and significantly prolonged but less marked discharge anomalies during positive NAO phases. This behavior can also be explained in terms of the hydrological processes of the basin. Thus, in negative NAO years, the basin receives more rainfall than usual, the soils become saturated more easily, and a rapid transfer of rainfall to runoff occurs due to this saturation, and this is the cause for high positive anomalies in river discharge. Since this is a rapid process, the effects of increased precipitation do not last very long. During positive NAO years the basin receives less water, and the average surface runoff is drastically reduced, but recharging of the water reserves in soils and aquifers is also diminished.

[44] Although the response of river discharge to the NAO that we have described is a general trend for the basin, there are significant differences in the results for the stations in the magnitude and timing of the anomalies. In general, deviations from the global behavior can be explained in terms of the specific characteristics of each subcatchment. For example, Vicente-Serrano and López-Moreno [2005] showed that the time of response of Pyrenean rivers depends mainly on the characteristics of the basin, the climatic regime, and the water management carried out upstream of the observation sites. Our results show that similar factors seem to have an important influence on the hydrological responses of the subcatchments. For example, a more rapid response was found in small, nonregulated catchments, especially in combination with more impervious substrates such as granite (e.g., Belena); in contrast, a more moderate response was found in large and highly regulated catchments, in particular in those with permeable substrates (e.g., Trillo). However, the different combinations of all these factors in the subcatchments hamper their classification.

[45] The extreme NAO phases affect not only river discharge, but also the water management practices applied to the basin's reservoir system. In general, the reservoirs are managed with the objective of synchronizing water availability with water demand, and this is achieved by adjusting the downstream water release as a function of the river inflow to the reservoir and the desired storage volume. The capacity of reservoir management strategies to adapt to the climatic variability is very high, and usually provides an adequate supply of water even during dry years, as has been highlighted by previous studies [Petts, 1984; McCully, 2001; Bonnacci and Roje-Bonacci, 2003]. The flexibility of these reservoir management strategies is particularly important when long-term hydrological trends are present. For example, López-Moreno et al. [2004] analyzed the case of a large reservoir in the Pyrenees (north of the Iberian Peninsula), where the strategies were adjusted to manage the decreases in water availability that resulted from a generalized revegetation process after farmland abandonment during the second half of the twentieth century [Beguería et al., 2003, 2006]. In the case of the Tagus basin, the number and magnitude of the significant differences in monthly reservoir releases are lower than those observed for the river discharge, thus demonstrating the capacity of reservoir management strategies to manage hydroclimatic variability. However, it was also found that very different management strategies need to be applied during positive and negative NAO years in order to comply with water demands. During negative NAO phases, there is a significant winter peak in the inflows to the reservoirs, which allows for a rapid increase in the water storage level, and consequently a high discharge is released downstream of the dams. In contrast, during positive NAO years there is no winter peak in discharge, so the water released from the dams is reduced to a minimum in order to increase the water storage as much as possible. Since the desired water storage volume is not reached during negative NAO winters, the anomalies continue during the rest of the year, and in the most extreme negative events are probably transferred to the following year.

[46] We also found that not only does the average storage volume differ between positive and negative NAO years, but also the availability of water resources in the year following an extreme NAO year is affected. Thus, during extreme positive NAO years, the total reservoir storage decreases throughout the year, from an average value of 1254 hm3 in December to 874 hm3 in November. In contrast, the negative NAO years enable an increase in the water reserves, ranging from 1573 hm3 in December to 1815 hm3 in November. These results clearly show that water availability will be affected in the year following an extreme NAO year, since the baseline at the beginning of the infilling season will be markedly different.

[47] According to these results, how sustainable is the water resources supply in the Tagus River basin? This question is of great strategic interest due to (1) the importance of the supply to the urban Madrid and Lisbon regions, (2) water diversion to the Mediterranean coast, and (3) the international character of the basin, which must also meet the water demands of the Portuguese sector of the basin. The most likely medium-term scenarios for the basin indicate an increase in the water demand due to urban growth and development in the Madrid metropolitan area and in the Mediterranean coast [Gil Olcina, 1993; Vera-Rebollo and Torres-Alfosea, 1999], together with likely alterations in the hydrological cycle induced by climate and land cover change. Our results show that the capacity of reservoir management strategies to adapt to extreme hydroclimatic events is probably reaching its limit, and that large difficulties will probably arise as the new conditions anticipated by global change scenarios (climate and land-use/plant cover change) emerge since more extreme NAO phases are predicted for the future [Osborn, 2004], as well as changes in the stochastic properties of precipitation and the most extreme events [Katz and Brown, 1992].

[48] Since the construction of new reservoirs in the basin is technically difficult and has a large social impact, delicate adjustments to water management strategies and technologies will be needed. Management of the new circumstances will need to increasingly consider aspects that are now marginal, such as the adoption of more water-efficient practices and technologies, the prizing of water for different uses, an improved protocol of minimum discharge and water sharing between Spain and Portugal, etc. The main problems are as follows: How can integrated water resources management (IWRM) ensure the sustainability of water resources, improve water quality, reduce flood hazards and ensure the protection of natural landscapes and at the same time maintain the welfare of the population? These are difficult questions that require a holistic perspective and the participation of hydrologists, economists, sociologists, and politicians from both Spain and Portugal.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[49] The study has been supported by the following projects: STRIVER (Strategy and methodology for improved IWRM: An integrated interdisciplinary assessment in four twinning river basins) financed by the European Commision (VI framework programme), contract 037141, CGL 2004-04919-C02-01, CGL 2005-04508/BOS, CGL 2006-11619/HID financed by the Spanish Commission of Science and Technology and FEDER, PIP176/2005, PM088/2006 and “Programa de grupos de investigación consolidados” (BOA 48 of 20-04-2005) financed by the Aragón Government. The research activities of the first author were supported by a postdoctoral grant from the Spanish Ministry of Education, Culture and Sports.

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  3. 1. Introduction
  4. 2. Study Area
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information
FilenameFormatSizeDescription
wrcr11260-sup-0001-t01.txtplain text document1KTab-delimited Table 1.
wrcr11260-sup-0002-t02.txtplain text document1KTab-delimited Table 2.

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