SEARCH FOR CLIMATE CHANGE EVIDENCIES IN
THE GUADIANA RIVER PORTUGUESE BASIN.
STUDIES ON PRECIPITATION
J. F. Santos Oliveira1, João Morais2, M. Cardoso3, M. Silva3, P. Pereira3, P. Lopes3
1Coordinator,
Full Professor; 2Teaching Assistant, 3Graduate Students
Grupo de Disciplinas de Ecologia da Hidrosfera, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa (GDEH/FCT/UNL)
Campus da FCT, Edifício Departamental
Quinta da Torre
2825-114 Monte de Caparica
Portugal
Phone/Fax: 351 21 2948543; E-mail: [email protected]
INTRODUCTION: The Guadiana basin is located between 37º and 40º lat.-N and 2º and 8º long.-W. It covers an area of 66960 km2 (55260 km2 in Spain and 11700 km2 in the Southeast of Portugal,
Figure 1). The average altitude in the portuguese side is 237 m. The climate is Mediterranean, with continental influence. The summer is very hot and dry and the winter is fresh. The annual precipitation has
values between 500 and 800 mm, concentrated in a short time period (50 to 80 days). Between October and May usually fall 400 to 700 mm (Cary, F., 1985), (Daveau, S. et all, 1985), (DRAA, 1998). This
conditions determine that the flow regime of Guadiana and affluents is very variable and precipitation dependent. Considering the fact that the ground-water reserves in the area are very low, the need of
water for irrigation, energy production and public supply has been solved by the construction of dams in Spain and Portugal (SEARN, 1986). In the last years it has been observed that many of those
reservoirs seem to be over-exploited, show symptoms of eutrophication, rarely reach the maximum storing level and the surface flow of tributaries is not sufficient to refill them. So, empirically, one can say
that climate changes seem to be occurring in the area, once it seems to be more frequent in the present the occurrence of long periods of draught and extreme episodes of precipitation. The extreme climate
aspects are the most interesting to define the attractive or repulsive climate characteristics of a site or region. The precipitation maximum and minimum mean values, the duration, intensity, frequency and
probability associated to some episodes are very important to define if a given site is adequate to a given human-made structure. A characteristic aspect of a climate change is the alteration of quantity and/or
time distribution pattern of precipitation on a site or area. Such alterations will determine changing in the values of the statistic parameters of precipitation chronological series. Other aspect to consider is the
possible transformation of a very improbable extreme episode in a relatively frequent one. This work is aimed to demonstrate that a climate change is happening in the Guadiana portuguese basin, through
the study of the evolution of the chronological series of month precipitation, recorded in 24 sites representative of the area since 1931 to 1996.
METHODS: The detection of a climate change through the analysis of precipitation chronological series demands that those series are long enough to be representative of the general characteristics of
the climate. In this work, 24 stations representative of all basin area, were selected (Figure 2, Table 1), from all the available in the SNIRH web-site (http://www.inag.pt/snirh), under the following
constraints: a) More than 50 years of month precipitation records; b) Last record as recent as possible (less than 10 years ago); c) Number of years missing in the record not greater than 3. The study focused
on the evolution of month and total annual precipitation. In some cases other aggregation periods were used. It was adopted the hydrological year (from October of year X to September of year X+1). When
necessary, the year was divided in quarters or half-year periods. To each site, time series of each month, aggregation period, and total annual precipitation were generated. Each time series was divided in
two sub-series, before and after the year 1963, and the parameters maximum, mean, minimum, moving average, pluviometric quotient,
skewness, Schmidt & Ferguson index, return period for a given precipitation and the magnitude to the N-year event of each sub-series were
calculated and compared through the paired samples t-Student test. The obtained information was aggregated for the basin area.
Moving average: The plotting of the moving average, calculated for 30 and 10-year periods, permitted to analyse the evolution of month
and total annual precipitation. This procedure is able to confer statistical support to the common sense affirmation: “nowadays is raining less
than in the past”, if the moving average calculated for the past 10 years is significantly minor than the one calculated for the past 30 years.
Pluvimetric Quotient (von Hann, 1932 in Mayer, R., 1935): This parameter defines the contribution quote of each month or aggregation
period to the total annual precipitation, for a given site.
Figure 1- Localisation of the portuguese
basin of Guadiana river. (Adapted from
Atlas do Ambiente).
Pr ecipitation period
PQ 
Pr ecipitation year
Days period
Days year
Rainy periods will have PQ > 1.5 and dry periods will have PQ < 0.5.
Table 2
Schmidt & Ferguson index scale
Skewness: Characterises the degree of asymmetry of a distribution around its mean. Positive skewness indicates a distribution with an
asymmetric tail extending toward more positive values. Negative skewness indicates a distribution with an asymmetric tail extending toward
more negative values.
SF < 0.143, Climate class = A, very rainy;
0.143 <= SF < 0.333, Climate class = B;
0.333 <= SF < 0.600, Climate class = C;
0.600 <= SF < 1.000, Climate class = D;
1.000 <= SF < 1.667, Climate class = E;
1.667 <= SF < 3.000, Climate class = F;
3.000 <= SF < 7.000, Climate class = G;
SF > 7.000, Climate class = H, very arid;
n
n
 xi  x 
a


n  1n  2 
s


i 1
Figure 2- The black spots indicate the
localisation of the 24 selected sites. (Adapted
from http://www.inag.pt/snirh).
Table 1
General information about the selected sites
3
For positive values, the greater is the skewness, the greater is the number of observations in the left-hand side of the mean and the
probability of occurrence of extreme precipitation episodes increases.
Schmidt & Ferguson index: It’s an aridity index. For a given site, the number of rainy months (Prec. > 100 mm), R, and dry months (Prec.
< 60 mm), D, is counted and is calculated de quotient D/R for each year of the time series. Then, is calculated the value of arctg(D/R). Then
these values are averaged for the time series and is calculated SF= tg(mean). The index scale defines an ascending order of aridity (Table 2).
Return period for a given precipitation value and magnitude to the N-year event: Those values were calculated for each site, each
month, total annual precipitation and all basin area through the Gumbel distribution (Wilson, E. M., 1990).
Code
Name
First record
Last record
Records
Years
21M/02
Alandroal
September 31
September 95
758
64
29M/01
Alcoutim
March 38
September 95
655
58
RESULTS: Table 3 and Table 4 and Figure 3 synthesise the obtained results for comparisons between sub-series. In what concerns the Return Period and Magnitude to the N-
27K/01
Algodor
September 31
October 96
782
65
28I/01
Almodôvar
September 31
October 96
779
65
24N/01
Amareleja
September 31
October 96
782
65
year event calculi, the results are as follows: The return period for an annual precipitation of 1000 mm ranges from 2.3 years in Barranco do Velho to 98.8 years in Trindade, the
average value to the basin is 41,2 years. The return period for a basin averaged 100 mm of month precipitation ranges from 2.8 years in December to several million years in July
and August. The annual precipitation to the 100-year event ranges from 1001.5 mm in Trindade to 2034.7 mm in Barranco do Velho. The average value to the basin is 1116.3 mm.
The basin averaged month precipitation to the 100-year event ranges from 21.3 mm in July to 282.4 mm in December.
19N/01
Arronches
September 31
September 96
781
65
21K/01
Azaruja
September 31
September 96
781
65
30J/01
Barranco do Velho
April 35
September 95
726
60
768
65
27I/01
Castro Verde
September 31
October 96
779
65
24J/03
Cuba
September 31
October 96
782
65
21N/01
Juromenha
September 31
September 96
781
65
29K/01
Martim Longo
September 41
March 96
652
54
28L/01
Mértola
January 39
October 96
621
58
25L/01
Pedrogão do Alentejo
June 41
October 96
665
55
24K/01
Portel
January 39
October 96
694
58
22L/01
Redondo
September 31
September 96
781
65
23L/01
Reguengos de Monsaraz
September 31
October 96
782
65
23K/01
S. Mansos
April 42
October 96
655
55
26L/01
Serpa
September 31
October 96
782
65
30I/02
Sobreira
May 42
September 95
640
53
25O/01
Sto. Aleixo da Restauração
September 31
September 96
772
65
26J/01
Trindade
September 31
October 96
782
65
21M/01
Vila Viçosa
September 31
September 95
769
Table 3
Results obtained from comparisons performed to data
aggregated to basin area. Paired samples t-Student test. (**) –
Significant to α=0.05; (*) – Significant to α=0.10; (---) – Nonsignificant; F – First sub-series (before 1963); L – Last subseries (after 1963); (n. a.) – not applicable. It is evident that
December, March, May and September are becoming dryer.
The average precipitation recorded in March (48.6 mm) in the
last sub-series, is 50.9% of the one recorded in the first subseries (95.4 mm). The average total annual precipitation
decreased 7.2% in the last sub-series.
64
Precipitation (mm)
October 96
1000
800
600
400
200
0
1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
Years
30 per. Mov. Avg. (total Prec.)
10 per. Mov. Avg. (total Prec.)
Trindade (March)
Alandroal
S&F(S)
S&F(F)
S&F(L)
Class(F)
Class(L)
Comp.(Class)
Comp.(Value)
3,940
3,957
3,789
G
G
=
F>L
5,072
4,727
5,348
G
G
=
F<L
Algodor
5,209
4,225
6,756
G
G
=
F<L
Almodovar
4,409
4,487
4,333
G
G
=
F>L
Amareleja
5,158
4,651
5,783
G
G
=
F<L
Arronches
3,406
2,906
4,090
F
G
F<L
F<L
Azaruja
4,029
3,439
4,844
G
G
=
F<L
Barr. do Velho
1,731
1,550
1,931
E
F
F<L
F<L
Barrancos
4,283
4,097
4,484
G
G
=
F<L
Castro Verde
4,942
4,369
5,680
G
G
=
F<L
Cuba
4,217
3,888
4,530
G
G
=
F<L
Alcoutim
Juromenha
4,267
3,331
5,835
G
G
=
F<L
Martim Longo
5,018
3,974
6,143
G
G
=
F<L
Mértola
7,820
8,132
7,658
H
H
=
F>L
Pedrogão
5,089
4,542
5,545
G
G
=
F<L
Portel
3,298
2,594
4,142
F
G
F<L
F<L
Redondo
3,843
3,093
5,024
G
G
=
F<L
Reguengos
4,645
3,723
6,129
G
G
=
F<L
São Mansos
4,742
4,241
5,158
G
G
=
F<L
Serpa
3,891
3,749
4,047
G
G
=
F<L
300
Precipitation (mm)
September 31
1200
250
200
150
100
50
0
1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
Years
Março
Min
Mean
Pl. Q.
Skew.
Sobreira
1,851
1,729
1,941
F
F
=
F<L
October
November
--F<L**
F>L**
---
F<L**
---
F<L*
---
--F<L**
Santo Aleixo
6,572
8,297
5,432
H
G
F>L
F>L
400
350
300
250
200
150
100
50
0
1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
Years
December
---
F>L**
F>L**
F>L*
---
January
F<L*
---
---
F>L**
F<L**
February
F>L**
F<L**
F<L**
---
F>L**
March
April
F>L**
---
F>L**
---
F>L**
---
F>L**
---
-----
May
F>L**
F>L*
F>L**
F>L**
F>L**
June
F<L**
---
F<L**
F<L**
---
July
---
---
F<L**
F<L**
F>L**
August
September
F<L**
F>L**
-----
F<L**
F>L**
F<L**
F>L**
--F>L**
YEAR
---
---
F>L**
n. a.
---
2,608
2,324
2,894
F
F
=
F<L
Vila Viçosa
2,672
2,314
3,175
F
G
F<L
F<L
BASIN
4,280
3,931
4,779
G
G
=
F<L*
CONCLUSIONS: This work demonstrated the occurrence of climate change in the
portuguese basin of Guadiana river. This climate change is characterised by a
decreasing of month precipitation in December, March, May and September. The
average precipitation recorded in March decreased almost 50% in the last 30 years.
The average total annual precipitation decreased 7.2% in the last 30 years. The arid
character of climate in this area is increasing. The desertification process happening
in this area will probably intensify in the next years.
10 per. Mov. Avg. (Março)
30 per. Mov. Avg. (Março)
Amareleja (March)
250
200
150
100
50
0
1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
Years
Março
Trindade
30 per. Mov. Avg. (Março)
450
Março
Max
10 per. Mov. Avg. (Março)
Barranco do Velho (March)
Precipitation (mm)
Barrancos
Average annual precipitation in the basin
Precipitation (mm)
25P/01
Table 4
Results of the application of the Schmidt & Ferguson index to the studied sites and
integration of data to the basin. S&F(S)- Index value to the series; S&F(F)- Index value
to the first sub-series; S&F(L)- Index value to the last sub-series; Class(F)- Index
classification to the first sub-series; Class(L)- Index classification to the last sub-series;
Comp.(Class)- Class comparison between sub-series; Comp.(Value)- Values comparison
between sub-series; (*)- Significant to =0.05, paired samples t-Student test. The index
values express clear indications of the increasing aridity of the basin.
10 per. Mov. Avg. (Março)
30 per. Mov. Avg. (Março)
Figure 3- Moving average plots to 3 of
the studied sites (month precipitation of
March) and Basin averaged annual
precipitation. In the March plots is clear
the precipitation decrease in the last 30
years. In this time interval is clear, too,
the positioning of the 10 year moving
average, always under the 30 year
moving average.
REFERENCES:
DRAA (1998) - Zonas de interesse ambiental do Alentejo, Doc 1, Guião. Direcção Geral do Ambiente do Alentejo, Centro de Ecologia Aplicada. Évora.
E. M. WILSON (1990) - Engineering Hydrology (Fourth Edition). MacMillan Education Ltd. London.
FRANCISCO CALDEIRA CARY (1985) - Enquadramento e perfis do investimento agrícola no continente português. Banco de Fomento Nacional, Estudos nº 23, 1º Vol. Lisboa.
IPAMB, (sem data) - Recursos hídricos do sul de Portugal, Vol. II. Compilação de informação disponível no IPAMB. Lisboa.
MARIA MANUELA PORTELA e ANTÓNIO CARVALHO QUINTELA (1999) - Indícios de mudança climática em séries de precipitação em Portugal continental. Recursos Hídricos, Vol. 19, pp 41-74. Lisboa.
PEDRO CABRITA e MÁRIO TAVARES (1991) - A bacia do Guadiana. Uma área sensível, in Recuperação de áreas degradadas na bacia do Guadiana, Revista Florestal Vol. V – nº 2. Lisboa.
RUY MAYER (1935) - Contribuição para o estudo dos factores climáticos nas suas relações com a hidráulica agrícola. Anais do Instituto Superior de Agronomia, Vol. VII - Fasc. 1º, pp 64-103. Universidade Técnica de Lisboa.
SEARN (1986) - Monografias hidrológicas dos principais cursos de água de Portugal continental. Secretaria de Estado do Ambiente e Recursos Naturais, Direcção Geral dos Recursos e Aproveitamentos Hidráulicos, Divisão de Hidrometria. Lisboa.