Seasonal precipitation trends in the Mediterranean Iberian Peninsula in second half of 20th century

INTERNATIONAL JOURNAL OF CLIMATOLOGYInt. J. Climatol. 29: 1312–1323 (2009)Published online 3 December 2008 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/joc.1778

Seasonal precipitation trends in the Mediterranean IberianPeninsula in second half of 20th century

Martın De Luis,a* Jose Carlos Gonzalez-Hidalgo,a Luis Alberto Longaresa and Petr Stepanekb

a Department of Geography, University of Saragossa, 50009, Saragossa, Spainb Czech Hydrometeorological Institute, Brno, Czech Republic

ABSTRACT: This is a study of the changes in annual and seasonal precipitation amounts and variability, during theperiod 1951–2000, using MOPREDAMES. This dataset includes 1113 complete and homogeneous monthly precipitationtime series from the Mediterranean Iberian Peninsula (IP), and corresponds to the five official Spanish hydrological divisionswhich drain into the Mediterranean Sea. The time series of annual and seasonal precipitation were used to test for trends.The absolute value of the anomaly time series was also tested for trends to identify changes in interannual variability ofprecipitation. The significance of these changes was assessed using the non-parametric Spearman rank test. The intensitiesof observed changes, both on mean values and variability, were estimated by using linear regression techniques. Finally, weanalysed the area affected by different trends by using raster maps and spatial statistics, in addition to calculating the globalbalances for the five hydrological divisions. We detected high variability in precipitation regimes and conditions in the studyarea; nevertheless, a decrease in seasonal and annual precipitation has predominated in the east of the IP during the secondhalf of the 20th century. On an annual scale, precipitation has diminished over 90.1% of the study area. Additionally, ahigh percentage of the territory was affected by diminishing precipitation at a seasonal level: 85% (of territory) in summer,82% in spring, 64% during winter and 61% in autumn. Taking the study area as a whole, seasonal precipitation decreasesare ranked as follows: summer (−22.5%), spring (−19.3%), winter (−7.3%), and autumn (−5.2%), with a decrease in thevalue of the global mean annual precipitation −12.4%. We also detected an increase of precipitation variability in winter(+23.5%) and summer (+11.4%), and a decrease in autumn and spring (−14.9 and −16.8%, respectively) with a globalmean value of +7.8%. Copyright 2008 Royal Meteorological Society

KEY WORDS precipitation trends; precipitation variability trends; precipitation datasets; non-parametric test

Received 5 December 2007; Revised 8 May 2008; Accepted 23 September 2008

1. Introduction

Water is a fundamental resource for society and life,and any modifications in precipitation regimes may affectnot only social, but also natural systems (Randall et al.,2007). The global impact of precipitation changes maybe more intense particularly where the availability ofwater is scarce, because of irregularity, high demand ora combination of both circumstances.

The recent IPCC-2007 report (Solomon et al., 2007)confirmed an increase in precipitation for the period1900–2005 north of 30° latitude, because of globalwarming (Trenberth et al., 2007). It also confirmed adecrease in precipitation from 1970 in tropical areas, andan increase in land affected by drought in tropical andsubtropical areas from the 1970s (Trenberth et al., 2007).In particular, precipitation in the northern subtropics(20–40°N), has not displayed a clear or significanttrend throughout the 20th century and precipitation ischaracterized by sub-decadal variability (New et al.,2001). This latitude band includes the Mediterranean

* Correspondence to: Martın De Luis, Department of Geography, Uni-versity of Saragossa. 50009, Saragossa, Spain. E-mail: [email protected]

basin area, a climate ecotone where it has been suggestedthat change would be noticed before any other area(Esteban-Parra et al., 1998). In this area, models predictthat, at the end of the 21st century, there will be a globaldecrease in the mean values of precipitation coupledwith increase in variability (Christensen et al., 2007).However, these results show a high degree of uncertainty,as a result of the well-documented fact that there ishighly variable precipitation in the Mediterranean basin(Lionello et al., 2006).

The main characteristic of precipitation around theMediterranean basin is its extraordinary variability bothon a temporal and spatial scale (Lionello et al., 2006;Norrant and Douguedroit, 2006). Such variability iscaused, among other reasons, by its latitudinal positionbetween two strongly contrasting masses of water (theAtlantic Ocean and the Mediterranean Sea), and also bytopographical factors, as various mountain ranges aredistributed along the coastal areas from east to west.Consequently, the IPCC-2007 suggests that the sub-regional variability in precipitation should be analysed indepth. Given the extreme variability, a detailed spatialdataset is needed, going back as far as possible intime (Lana and Burgueno, 2000; Huntington, 2006;

Copyright 2008 Royal Meteorological Society

SEASONAL PRECIPITATION TRENDS IN THE IBERIAN PENINSULA 1313

Trenberth et al., 2007). In this study, we used the recentlydeveloped MOPREDAMES dataset that includes 1113complete, homogeneous monthly precipitation time seriesof the Mediterranean Iberian Peninsula (IP) for the1951–2000 period (Gonzalez-Hidalgo et al., in press).

In this paper, we study changes in annual and sea-sonal precipitation amounts and variability during theperiod from 1951–2000. From a climatic point ofview, seasonal analysis may mask monthly heterogeneousbehaviour (Gonzalez-Hidalgo et al., in press) but sea-sonal and annual scales are important in other contexts,such as the response of natural systems and managementof hydrological resources. Likewise, results for precipita-tion trends on a detailed spatial scale, for use in managingwater, are also critically important. In this respect, in spiteof difficulties caused by hydrological divisions which, attimes, integrated areas with different climate characteris-tics and trends, we produced a global balance for each ofthe five hydrological divisions belonging to the Mediter-ranean coast of Spain.

2. Methods

2.1. Study area

The Mediterranean facade of the IP extends from thenorth to the south of Spain and comprises five hydro-logical divisions which drain into the Mediterranean

Sea: Eastern Pyrenees (EPY), Ebro (EBR), Jucar (JUC),Segura (SEG) and Eastern Andalucıa (EAN) (Figure 1).The total area is about 182 000 km2, i.e. 36.6% of con-terminous Spain (approximately 1/3 IP). The Ebro hydro-logical division represents a transition area from northernAtlantic to eastern Mediterranean conditions, because ofits inland location and north-west to south-east distri-bution. The other hydrological divisions are distributedfrom north to south along the coast, so they receive theeffects of the Mediterranean Sea directly. Overall, annualprecipitation of the study area decreases from north tosouth, and east to west. However, these patterns are veryrough and they mask many irregularities and variationson a local scale. For this reason, a dense precipitationdatabase is required for climate studies in order to inter-pret precipitation variability and changes in the easternIP correctly.

2.2. Precipitation databaseWe used the monthly precipitation database (MOP-REDAMES) recently presented by Gonzalez-Hidalgo et al.(in press). To create this database, the total amounts ofmonthly precipitation data held at the Agencia Estatal deMeteorologıa (AEMET) for the study area were analysed.From the nearly 3800 original stations available, thoseless than 10 years old were discarded. This left a total of2669 stations checked for quality, including more than0.9 × 106 monthly data.

Figure 1. Study area with the National Meteorological Agency main hydrological divisions for the eastern Iberian Peninsula. Different symbolsindicate time span of reconstructed series. This figure is available in colour online at www.interscience.wiley.com/ijoc

Copyright 2008 Royal Meteorological Society Int. J. Climatol. 29: 1312–1323 (2009)DOI: 10.1002/joc

1314 M. DE LUIS ET AL.

Table I. Monthly precipitation stations (reconstructed) by hydrological division, hydrological division area and stations density.

Total EBR EPY JUC SEG EAN

Stations 1113 425 132 247 144 165Area (km2) 182 140 86 098 16 493 42 904 18 254 18 391Density (stations/km2) 1/164 1/203 1/125 1/174 1/127 1/111

A quality control analysis was applied by using areference series together with a repeated procedure. Toconstruct the reference series, neighbouring stations wereselected according to the following criteria: distance< 50 km, minimum overlapping of >10 years, all pos-itive monthly correlations and average monthly correla-tions higher than 0.5. To avoid introducing uncontrolledbias during the creation of the reference series, averagestandardizations were applied to all neighbouring seriesby using a common overlap period with the candidateone. Finally, the calculation of each reference series wascarried out by means of a weighted average of (1/d)2,where d is the distance to the candidate in kilometres.

After comparing the original with the reference series,suspicious data were discarded (see details in Gonzalez-Hidalgo et al., in press), and homogeneity was checkedwith standard normal homogeneity test (SNHT) (Alexan-dersson, 1986). The reconstruction of the final precipita-tion time series was performed with a new set of referenceseries from the final homogeneous data using a maximumdistance of 10 km; a second set of references at 25 kmwas used to fill in any gaps. Selection of the reconstructedprecipitation time series was done according to mini-mum gaps, missing data and nearby information in theoriginal data. The overall procedure was performed withspecific software developed for climate analysis (AnClimand ProClim software, Stepanek et al., 2007, 2008). Fur-ther details of methodological procedures can be foundin the article by Gonzalez-Hidalgo et al. (in press).

Finally, we reconstructed 1113 complete, homoge-neous monthly precipitation series for the period1951–2000 (Table I). From these, 338 reach back to1931, 154 to 1911, and 12 go back further than1911. Figure 1 shows the spatial distribution of theoriginal and reconstructed stations. This spatial cov-erage for 1951–2000 with an overall density of 1station/150–200 km2 is currently the highest availablefor the Mediterranean basin and IP, and represents anew tool for detailed spatial analyses. For the previousperiods, the density is also the highest available for theMediterranean facade of the IP, but coverage varies fromone area to another. Given this limitation, for this studywe have restricted our analysis to the period 1951–2000.

2.3. Statistical procedures

Seasonal precipitation amounts were calculated for eachstation. The period included in each season followedthe climatological criterion adopted by the meteorologi-cal agencies of the region, and the one usually adoptedby researchers in climate (Rodrıguez-Puebla et al., 1998;

Romero et al., 1998). Hence, winter is composed ofthe months of December, January and February; springof March, April and May; summer of June, July andAugust; and autumn of September, October and Novem-ber. Annual time series were composed by data fromDecember of the previous year to November of the cur-rent one.

The time series of annual and seasonal precipitation(winter, spring, summer and autumn) was used to testfor trends in mean values and also to detect changesin interannual seasonal precipitation. In these cases, ameasure of variability, as recommended by De Luiset al. (2000), was used. The normalized precipitationanomalies (AR) for a given station ‘i’ and for a givenyear ‘y’, were expressed as the ratio of each precipitationanomaly to the mean for the reference period (Ri) asfollows (1):

ARiy = Riy − Ri

Ri

(1)

The absolute value of this anomaly time series wasalso tested for trends to identify changes in interannualprecipitation.

The intensities of observed changes both on mean val-ues and variability were estimated by using linear regres-sion techniques. The significance of these changes wasassessed using the non-parametric Spearman rank test(p < 0.05 level). Time series were previously smoothedby a low pass filter (lag 9) to filter out year-to-year vari-ations and reveal more persistent trends (Wheeler andMartın-Vide, 1992; Salinger et al., 1995; Suppiah andHennessy, 1998; De Luis et al., 2000; Gonzalez-Hidalgoet al., 2001).

The spatial distribution of results (rate of change mea-sured as the slope of linear models) is shown by spa-tial interpolation (inverse distance weighted methods) ofobserved trends at each of the 1113 stations (Vicente andSaz, 2002; Ninyerola et al., 2007). Finally, we studiedsurfaces affected by different trends and general balancesfor hydrological divisions by using raster maps and areastatistics obtained for each variable, using ArcGis 9.1software.

3. Results

3.1. Seasonal precipitation means and trends

3.1.1. Winter

The highest values of winter precipitation are locatedin mountains of the north and NW and in the extreme



Figure 2. Winter precipitation during 19502000 period. (a) Mean precipitation; (b) coefficient of variation; (c) trends in rainfall amounts; and(d) trends in variability. This figure is available in colour online at www.interscience.wiley.com/ijoc

SW (Figure 2(a)). Interannual variability (coefficient ofvariation) of winter precipitation shows a coast-to-inlandpattern (Figure 2(b)). Thus, values higher than 60% areobserved in most coastal areas, while values of 40% orlower are observed inland.

Trends in winter precipitation amounts also reveal aclear coast-to-inland gradient, and increases in winterprecipitation have been found in most coastal areas fromnorth to south as far as the Cabo de Gata, while largeand even significant decreases are observed inland andin southern areas (Figure 2(c)). In contrast to this, winterprecipitation variability has increased in southern parts,

the central Pyrenees and coastal areas around the Gulf ofValencia (Figure 2(d)).

3.1.2. Spring

Precipitation in spring displays similar spatial patternsto those described for winter, but the effects extendfrom west to east along the Pyrenees and decreasein magnitude to the south (Figure 3(a)). Interannualvariability also exhibits a coast-to-inland pattern, but witha shift in latitude with regard to winter. Thus, the highestvariability values are still found in coastland areas, butfurther to the south of the Gulf of Valencia (Figure 3(b)).



Figure 3. Spring precipitation during the period from 1950 to 2000. (a) Mean precipitation; (b) coefficient of variation; (c) trends in rainfallamounts; and (d) trends in variability. This figure is available in colour online at www.interscience.wiley.com/ijoc

Trends in spring precipitations are considerably differ-ent from those described for winter with a very rougheast–west (E–W) pattern. Thus, the decrease in springprecipitation is mainly located along the coastal area andthe most negative values are found in the south, whilethe NW inland areas return positive results (Figure 3(c)).Decreases in interannual variability are predominant overthe study area. In spite of this general pattern, areas withpositive trends are also identified in northeast and southof the study area (Figure 3(d)).

3.1.3. Summer

The global pattern of summer precipitation is latitudinal.The maximum values are found along the Pyrenees

and the minimum values are south of the Gulf ofValencia. A second area of low precipitation is foundin the lower EBR hydrological division (Figure 4(a)).Interannual variability of precipitation is extremely highin summer, and this is especially true in the southernareas, where the coefficient of variation is higher than75% (Figure 4(b)).

During the study period of 1951–2000, decreases insummer precipitation were dominant in the study area(84.5% of the total territory). Decreases were especiallyintense in the central Pyrenees and the centre of theEBR hydrological division. Positive trends have beenidentified in the NW EBR hydrological division, inlandSE and southern areas (Figure 4(c)). More consistently,



Figure 4. Summer precipitation during the period from 1950 to 2000. (a) Mean precipitation; (b) coefficient of variation; (c) trends in rainfallamounts; (d) trends in variability. This figure is available in colour online at www.interscience.wiley.com/ijoc

variability in summer precipitation increased nearly overalmost all the study area except the NE (Figure 4(d)).

3.1.4. Autumn

The spatial distribution pattern of autumn precipitationis different from the seasons described above. Themaximum precipitations are located along the Pyreneesand along the coastland from the NE tip to the Gulfof Valencia (Figure 5(a)). A second area with maximumprecipitation is located in the extreme south, near theStrait of Gibraltar.

Variability of autumn precipitation is also high over thewhole study area, with this fact being especially true forthe coast from the Gulf of Valencia to the south, wherea roughly E–W pattern can be observed (Figure 5(b)).

Maps of variability and volume show us that autumnprecipitation in coastal areas of the Gulf of Valenciadiffers in nature from other seasonal maxima. As bothmaps show, the seasonal maximum and the maximavariability coincide in this area, a fact not repeated inother sectors and seasons.

Results of trend analyses for autumn precipitationreveal no changes or slight increases in southernareas, with a more complex pattern in northern ones(Figure 5(c)). In general, precipitation has increased inthe central Pyrenees and parts of the EBR hydrologicaldivision, while in the Iberian range and coastal areas pre-cipitation has decreased. Further to the south, the spatialpattern is complex, although positive trends are dominant

Trends in variability are clearer and a generalnorth–south (N–S) pattern can be described. Thus,



Figure 5. Autumn precipitation during the period from 1950–2000. (a) Mean precipitation; (b) coefficient of variation; (c) trends in rainfallamounts; (d) trends in variability. This figure is available in colour online at www.interscience.wiley.com/ijoc

while increases in variability are dominant in southernregions, decreases in variability are dominant in the north(Figure 5(d)).

3.1.5. Annual precipitation trends

The mean annual precipitation for the whole study areais 562 mm; however, maxima higher than 2000 mm/yearare observed in the Pyrenees and most SW mountainareas and minima of 200 mm or less are also detected(the lowest minimum was found in the Cabo de Gata,at 154 mm/year) (Figure 6(a)). Similarly, interannualvariability is especially high in southern regions andalong the coast from the Gulf of Valencia (Figure 6(b)).

The results described above, show an overall decreasein annual precipitation (Figure 6(c)). The exception is

the extreme SE, where a very complex spatial patternemerges, and includes patches of both positive and neg-ative trends. In contrast, trends in variability again dis-play a clear N–S pattern. Thus, while variability mostlyincreased in the south, decreases in variability are domi-nant in northern areas from parallel 40°N (Figure 6(d)).

3.1.6. Balance of precipitation in the period1951–2000

Decreases in seasonal and annual precipitation predom-inated in the eastern IP during the period 1951–2000,although there was high variability in precipitationregimes and conditions. Thus, on an annual scale, pre-cipitation diminished over 90.1% of the territory anddecreases have also been observed over most of the area



Table II. Summary of means, variability and trends in precipitation by hydrological division.

EBR EPY JUC SEG EAN TOT

Mean rainfall (mm) Annual 606 720 506 365 553 562Winter 142 140 127 94 224 153Spring 167 187 136 110 143 142Summer 124 165 79 44 22 98Autumn 174 228 164 117 164 169

Coefficient of variation (%) Annual 26 28 34 37 38 30Winter 54 65 60 61 61 58Spring 42 47 53 60 59 49Summer 50 48 63 85 120 63Autumn 49 47 58 64 65 54

Rainfall change (mm/decade) Annual −15.1 −17.3 −13.8 −2.3 −17.4 −13.9Winter −5.9 3.0 0.3 3.3 −1.5 −2.2Spring −1.7 −8.7 −6.3 −6.6 −17.4 −5.5Summer −5.1 −7.4 −5.4 −0.7 −0.3 −4.4Autumn −2.5 −4.0 −2.5 1.7 1.8 −1.8

Variability change (%) Annual −4.0 −3.7 −3.1 3.5 10.1 −1.6Winter 4.5 6.1 4.1 0.7 10.0 4.7Spring −3.0 −4.8 −4.3 −3.5 −1.2 −3.4Summer 2.6 0.6 3.1 1.7 1.1 2.3Autumn −4.6 −2.7 −2.8 1.0 −0.0 −3.0

Change in volume (% during 1951–2000) Annual −12.5 −12.0 −13.7 −3.2 −15.8 −12.4Winter −20.6 10.6 1.2 17.5 −3.4 −7.3Spring −5.0 −23.3 −23.1 −30.2 −60.9 −19.3Summer −20.5 −22.5 −34.1 −7.5 −6.9 −22.5Autumn −7.2 −8.9 −7.6 7.3 5.3 −5.2

Change in variability (% during 1951–2000) Annual −19.8 −18.4 −15.5 17.5 50.6 −7.8Winter 22.5 30.7 20.5 3.6 50.0 23.5Spring −15.2 −24.0 −21.4 −17.4 −6.2 −16.8Summer 12.9 3.1 15.4 8.6 5.6 11.4Autumn −23.1 −13.3 −14.1 5.2 −0.0 −14.9

at a seasonal level (85, 82, 64 and 61% of territory whereprecipitation decreased in summer, spring, winter andautumn respectively).

In the EBR hydrological division, decreases in seasonalprecipitation are observed at all seasons, and particularlyaffect winter precipitation with a value of −20.6%, whilespring, summer and autumn decreased −5.0, −20.5 and−7.2% respectively. Interannual variability increased inwinter and summer but decreased in autumn and spring(details in Table II). This results in a global decreaseof −12.5% in annual precipitation in this hydrologicaldivision during 1951–2000, associated with a decreaseof −19.8% in variability.

In the central and north-east area (EPY and JUChydrological divisions), decreases in precipitation werealso dominant, except in winter. Decreases in variabilitywere observed in autumn and spring, while increasesmainly predominated in winter and summer (Table II).In the EPY hydrological division, the decrease in annualprecipitation and variability can be quantified as −12 and−18%, respectively in the period of 1951–2000. In theJUC hydrological division, the result is similar with adecrease in precipitation of −13.7% and a decrease invariability of −15.5%.

To the south, in the SEG hydrological division,the strong decrease observed in spring precipitation(−30%) was partially compensated by increases in winter(+17.5%) and autumn precipitation (+7.25%). Interan-nual variability increased, except for spring and annualprecipitation can be quantified as a slight decrease of−3.2%, but with a large increase in interannual variabil-ity (+17.5%; Table II).

Highly similar patterns were observed in the south-ernmost area (Eastern Andalucıa, EAN, division) wheredecreases in precipitation during summer, winter andespecially spring (−60.9%) were detected. A strongincrease in variability was also observed in winter(+50%). As a consequence, there was a decrease of−15.8% in annual precipitation in the EAN hydrologi-cal division from 1951–2000, while variability increasedby +50.1% (Table II).

To sum up, taking the study area as a whole,seasonal ordered precipitation decreases affected sum-mer (−22.5%), spring (−19.3%), winter (−7.3%), andautumn (−5.2%), with a decrease in the value of theglobal mean annual precipitation of −12.4%. We alsofound an increase in precipitation variability in winter(+23.5%) and summer (+11.4%), and a decrease in



Figure 6. Annual precipitation during the period from 1950–2000. (a) Mean precipitation; (b) coefficient of variation; (c) trends in rainfallamounts; (d) trends in variability. This figure is available in colour online at www.interscience.wiley.com/ijoc

autumn and spring (−14.9 and −16.8%, respectively)with global mean value of +7.8%.

4. Summary and conclusions

Global studies around the Mediterranean basin indicatethat winter and spring precipitation increased duringthe second half of the 20th century and decreasedduring summer (Bradley et al., 1987; Maheras, 1988;Dıaz et al., 1989). An annual decrease was reported inregional analyses in eastern (Giuffrida and Conte, 1989;Maheras and Kolyva-Machera, 1990; Amanatidis et al.,1993; Kutiel et al., 1996; Feidas et al., 2007), central(Piervitali et al., 1998; Delitala et al., 2000; Brunetti

et al., 2001a,b), and western areas of the Mediterraneanbasin (Esteban-Parra et al., 1998; De Luis et al., 2000).

The decrease in precipitation around the Mediterraneanbasin has been related to global warming and changesin pressure gradients between the islands of the Azoresand Iceland, associated with the North Atlantic oscillation(Palutikof, 1996; Houghton et al., 2001). However, inthe aforementioned papers, the average spatial densityof observations was low and therefore, spatial variabilityand transient areas might not be correctly identified.Furthermore, the most recent global revision coveringthe whole Mediterranean basin highlighted great spatialvariations and did not detect any clear spatial patternof precipitation trends during the period 1951–2000(Norrant and Douguedroit, 2006).



The latitude of the Mediterranean basin in the sub-tropical band produces natural high spatial and tem-poral variability (Lionello et al., 2006), with markedincreases in the east of the IP. Furthermore the IPis under the simultaneous influence of different atmo-spheric patterns of low variability (teleconnections) ashas been observed previously (Martın-Vide and Lopez-Bustins, 2006; De Luis et al., 2007; Gonzalez-Hidalgoet al., 2007a,b). Thus, a long term annual decrease inprecipitation for the Mediterranean coast and inland, andan increase in northern areas, were observed between1880–1992 by Esteban-Parra et al. (1998) in their studyusing 40 high-quality stations. Similarly, decreases inwinter (north/south), and in spring–summer (south) werereported by Rodrigo and Trigo (2007) during the secondhalf of the 20th century by analysing a high-quality timeseries of 22 stations. In contrast, a seasonal analysis of95 stations in the IP, south of France and North Africashows that winter precipitation increased overall between1899–1989, and decreased during the spring in the south-ern half of IP; no clear trend appeared in summer, andnegative trends predominated in autumn, thus strength-ening the annual cycle (Gonzalez-Rouco et al., 2001).A decrease in spring precipitation (albeit not significant)over the central plateau of IP was also reported in sevenhigh-quality stations between 1900–1996 by Galan et al.(1999). Also negative trends in spring were reported inthe NE by Saladie et al. (2004), on the basis of data from121 observatories from the middle of 19th century. How-ever, an analysis of seven secular stations dating back formore than 100 years along the Mediterranean coast didnot detect any clear trend on annual and seasonal scales(Lana and Burgueno, 2000).

Regional and sub-regional studies also report differ-ent results for trends in various areas and periods of theeastern IP. Thus, along the Mediterranean coast, dur-ing 1964–1990, Romero et al. (1998) found a globaldecrease in winter precipitation in 410 observatories, butGonzalez-Hidalgo et al. (2006) reported a significant pos-itive trend between 1951 and 2000 in the same area. Adecrease in autumn precipitation was reported in the Gulfof Valencia (on the east coast) between 1961 and 1990(Gonzalez-Hidalgo et al., 2001) and an increase in sum-mer precipitation (mainly inland) was observed during1961–1990 by Mosmann et al. (2004) using 333 obser-vatories.

Thus, high natural climate variability and resultsobtained from different datasets of varying densities andcovering different periods make it difficult to define clearpatterns in trends for the eastern IP (De Castro et al.,2005). Notwithstanding this, information is crucial forunderstanding, predicting and mitigating impacts on nat-ural systems, society and economic activities (Morataet al., 2006).

Our results, based on a high-density dataset (1113stations) and covering the five hydrological divisions ofthe eastern IP, describe high variability in precipitationregimes and trends, and constitute a new tool for detailed

spatial analysis. Furthermore, they allow us to identifysome general patterns not previously described.

Thus, an increase in winter precipitation in easterncoastal areas, and decrease inland, associated with anincrease in precipitation variability in all areas, can beclearly defined for the study area during the second halfof the 20th century. A general decrease in spring andsummer precipitation, associated with increased variabil-ity in summer, but a decrease in spring, may also bedefined. A more complex pattern was found in autumnbecause of the more local character of precipitation. Asa consequence of this heterogeneity, annual trends arecharacterized globally by a general decrease in precipi-tation (−12%), with increased variability to the south ofthe Gulf of Valencia, but a decrease in the north in theperiod 1951–2000.

Sumner et al. (2001) suggested that socioeconomicproblems require a detailed analysis of precipitation inthese areas of Spain, which is true, as since 1950, thepopulation has grown by 86.2% over the study area asa whole, with growth in the coastal provinces risingto 107.3%, but only to 27.6% inland. Furthermore, theinland population is mainly concentrated in a smallnumber of towns and the rural exodus continued up tothe early 1980s, particularly in mountain areas. In 1950, aquarter of Spain’s population lived in the Mediterraneancoastland provinces, but this figure has now risen toone third. Hence, the demand for water resources hassteadily increased throughout the study area during thesecond half of the 20th century, notwithstanding thevarying precipitation regimes existing in each region.The main causes of demographic change on the coasthave been population growth, development of tourismand intensive agriculture. Meanwhile, in the inland areas(such as the EBR hydrological division), the demand forwater has risen mainly as a result of increased irrigationin the lowland. Finally, the whole area has been subjectedto very intense socio-political debate because of theNational Water Plan, which proposes to transfer waterfrom inland in the north to the south (from the EBR to theJUC, and SEG hydrological divisions). Therefore, it canbe seen once again that changes in mountain or remoteareas can affect distant populations (Barnett et al., 2005;Graham et al., 2007).

To summarize, during the second half of the 20thcentury in the eastern fringe of IP, the availability ofwater resources has been under continuously increasingpressure not only because of the climate (global negativetrend and mainly increasing variability), but also ofsocioeconomic factors.

Given that precipitation and water resources in theMediterranean IP are of critical importance from aneconomic point of view, if decreases in precipitation(as observed) are maintained in the 21st century aspredicted (De Castro et al., 2005; Christensen et al.,2007), there may be serious consequences for humanactivity in the social context of population increase anddemand for water. Also, from an ecological perspective,the availability of water is the most limiting factor in



the development of vegetation, and observed decreasesmay impact on the sustainability of many Mediterraneanecosystems, with a probable increase in disturbances,such as forest fires or erosion, in an area currently underserious threat of desertification.

Acknowledgements

Spanish Government, Contract Grant Sponsor CGL2005-04270 and CGL2007-65315-CO3-01/CLI. DGA, Gob-ierno Regional de Aragon, Grupo de Investigacion Con-solidado “Clima. Agua, Cambio Global y Sistemas Natu-rales” (BOA 69, 11-06-2007). We also thank the NationalMeteorological Institute for providing the original datasetfor the study area.

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Seasonal precipitation trends in the Mediterranean Iberian Peninsula in second half of 20th century

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