WaterResourcesManagement (2006) 20, 37-60

8/11/2019 WaterResourcesManagement (2006) 20, 37-60

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Diferences in spatial patterns o drought ondiferent time scales: an analysis o the IberianPeninsula

SERGIO M. VICENTE-SERRANO

Centre d'tudes Spatiales de la Biosphre (CESBIO)(author correspondence, 18 Avenue Edouard Belin, pi !8"1, #1$"1 %oulouse cede& ,

rance E*+ail ser-iovicente*serrano.cesiocnes/r)

Abstract.The differences in spatial patterns of drought oer a range of ti!e scales "ereanal#sed $# the Standardi%ed &recipitation Inde' (S&I). In a cli!atic area "ith a "iderange of precipitation characteristics (the I$erian &eninsula)* &earson III distri$ution is

fle'i$le enough to calculate the drought inde' on different ti!e scales. The &earson IIIdistri$ution "as adapted to precipitation fre+uencies at ti!e scales of ,* * * ,/* /0 and !onths. Spatial patterns of drought "ere anal#sed $# &rincipal Co!ponent Anal#sis.The nu!$er of co!ponents found increased as the ti!e scale did* "hich indicates greatspatial co!ple'it# in drought anal#sis and uncertaint# in drought classification* !ainl#at scales of /0 or !onths* since the relationships $et"een S&I series of o$seratories

$eco!es !ore distant as the ti!e scale increases. 1e concluded that there "ere noho!ogeneous regions "ith si!ilar drought patterns that could $e used for effectiedrought !anage!ent or earl# "arning.

Key Words. 2rought* Standardi%ed &recipitation Inde'* &earson III distri$ution* Ti!e

scales* &rincipal Co!ponent Anal#sis* I$erian &eninsula.

1. Introduction

2rought causes huge losses in agriculture (Austin et al* ,3345 6uiring and

&apa7r#ia7ou* /88) and da!ages natural ecos#ste!s (9ogan* ,33: and ,33;) and

forestr# (Or"ing and A$ra!s* ,33;5 A$ra!s et al, ,334). It leads to degradation of

soils and desertification (Nicholson et al* ,3345 &ic7up* ,334)* social alar! (Morales et

al.* /888) and fa!ine and i!poerish!ent (Garc


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McCall* /8885 ?lo#d->ughes and Saunders* /88/). Seeral studies anal#sed the spatial

patterns of drought ris7 in order to assist agricultural or eniron!ental !anage!ent

(Vicente-Serrano and =eguero"eer* these efforts hae focused !ainl# on the deelop!ent of drought indices* to

identif# and +uantif# droughts !agnitude* duration* intensit# and spatial e'tent* and to

i!proe techni+ues for drought earl# "arning and !anage!ent (1ilhite and So$oda*

/888). Nu!erous drought indices* using dierse aria$les for drought +uantification*

"ere deeloped during the /8thcentur# (>ei!* /88/5 du &issani et al,,334). Although

te!peratures or eapotranspiration are generall# included in drought inde' calculation*

precipitation is the !ost i!portant para!eter (Oladipo* ,34:5 Gutt!an* ,3345

9e#antash and 2racup* /88/).

The !ost ro$ust and effectie drought inde' is the Standardi%ed &recipitation Inde'

(S&I)* deeloped $# Mc9ee et al. (,33). This inde' enBo#s seeral adantages oer the

others. Calculation of the S&I is easier than on !ore co!ple' indices such as the &al!er

2rought Seerit# Inde' (&2SI5 &al!er* ,3:)* $ecause the S&I re+uires onl#

precipitation data* "hereas the &2SI uses seeral para!eters (Soul* ,33/). Moreoer*

the &2SI has so!e shortco!ings in spatial and te!poral co!para$ilit# (Alle#* ,3405

9arl* ,34;5 Gutt!an* ,334). The S&I is co!para$le in $oth ti!e and space* and is not

affected $# geographical or topographical differences (?ana et al.* /88,).

The S&I allo"s the deter!ination of duration* !agnitude and intensit# of droughts

(>a#es et al* ,333). Its !ain adantage is that it can $e calculated for seeral ti!e

scales (Mc9ee et al.* ,33:5 9o!uscu* ,333) and identifies arious drought t#pesD

h#drological* agricultural or eniron!ental.

The S&I has $een e'tensiel# used for drought anal#sis in countriescontinents such as

the F.S.A. (>a#es et al.* ,333)* Europe (?lo#d->ughes and Saunders* /88/$)* South


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Africa (Rouault and Richard* /88)* >ungar# (2o!on7os* /88)* Ital# (=onaccorso et

al,/88)* East Africa (Ntale and Gan* /88)* Greece (Tsa7iris and Vangelis* /880) and

9orea (Min et al.* /88).

Seeral studies focused on the S&Is calculation procedures* "hich identif# the !ost

appropriate fre+uenc# distri$utions (Gutt!an* ,333)* the effect of ti!e scales on the

para!eters (Ntale and Gan* /88)* and spatial and te!poral co!para$ilit# (9e#antash

and 2racup* /88/). >o"eer* the S&Is spatial sta$ilit# and coherence in relation to ti!e

scales hae not $een anal#sed.

In earl# drought "arning and !onitoring* accurate spatial classifications identif# areas

"ith ho!ogeneous drought patterns in order to opti!ise resources and deelop effectie

!itigation strategies. Although !an# studies hae addressed the spatial classification of

drought patterns (9arl and 9oscieln#* ,34/5 Eder et al.* ,34;5 ?ana et al.* /88,)* it is

essential to chec7 "hether these classifications are sta$le oer different ti!e scales or

not.

This article anal#ses "ith the S&I the sta$ilit# of spatial patterns of drought as a

function of ti!e scales. The setting for the stud# "as the I$erian &eninsula (Spain and

&ortugal)* an area "ith co!ple' precipitation patterns (Rodr* /880) and the rest of the! fro! the Instituto

0acional de 6eteorolo-5a in Spain. 2ata "ere chec7ed $# !eans of a +ualit# control


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process that identified ano!alous records (ANC?IM progra!* tJpKne7* /880). The

ho!ogeneit# of each series "as chec7ed $# !eans of the Standard Nor!al

>o!ogeneit# Test (Ale'andersson* ,345 Ale'andersson and Mo$erg* ,33;)* a

techni+ue "idel# applied in the ho!ogenisation of cli!ate records (9eiser and

Griffiths* ,33;5 Mo$erg and =ergstro!* ,33;). The fe" non-ho!ogeneous series

identified "ere corrected and te!poral gaps "ere co!pleted using linear regressions

"ith respectie reference series. Higure , sho"s the distri$ution of o$seratories in the

I$erian &eninsula.

In the I$erian &eninsula* precipitation is highl# aria$le in space and ti!e (Rodr


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=+=

+=8

l

li

87l

li

7

8i ::9,

*

,/

,-

*,* * if;7, and

+=

=8

78l

li

7

8i :9

,

** * if


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and 0are the ?-coefficients of s7e"ness and 7urtosis* respectiel#* and are calculated

as follo"sD

/

-

-

=

/

00

=

2* 3and 4are the ?-!o!ents of the precipitation series. These "ere o$tained fro!

pro$a$ilit#-"eighted !o!ents (&1Ms)* using the for!ulaeD

8, =

,8/ / =

/,8- .. +=

-/,80 /8-8,/ +=

The &1Ms of orders"ere calculated as underD

==

0

i

i

s

is &(

0 ,),(

,

&iis the data fro! a gien precipitation series andiis the fre+uenc# esti!ator.i"as

calculated follo"ing the approach of >os7ing (,338)D

0

i(

i

-:.8=

"here i is the range of o$serations arranged in rising order* and N is the nu!$er of

data points. Higure sho"s the ?-!o!ent diagra!s for and 0 at the ti!e scales

tested. In general* these statistical alues of the precipitation series of the o$seratories

oscillated around the theoretical cure of the &earson III distri$ution. >o"eer* large

differences "ere o$sered on different ti!e scales. 1hile for ,-!onth series the

statistics fro! the o$seratories "ere near to e'ponential distri$ution* for the longer

scales (i.e. /0 or !onths) the# "ere distri$uted in a cloud of points around nor!al


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distri$ution. This indicates the distinct characteristics of fre+uenc# distri$utions oer

different ti!e scales. >o"eer* irrespectie of the ti!e scales* &earson III distri$ution

adapted "ell to the statistics fro! the o$seratories. In relation to ti!e scale* the

statistics oscillated along the theoretical ?-cure of &earson III distri$ution $et"een the

points that indicated the alues of e'ponential and nor!al distri$utions.

In this paper &earson III distri$ution "as selected for S&I calculation at ti!e scales of ,*

* * ,/* /0 and !onths. The pro$a$ilit# densit# function of a &earson III distri$uted

aria$le is e'pressed asD

=

&

e&

&/,

)(

,)(

"here , and are the shape* scale and origin para!eters* respectiel#* for

precipitation alues& 85 and (P) is the Ga!!a function of . The para!eters of the

&earson III distri$ution* "hen ?-!o!ent ratios hae $een calculated* can $e o$tained

follo"ing >os7ing (,338)D

If ,* then !Q ,- and can $e o$tained using the for!ulaD

);;80:.8:83./;44,./,(

)/:-,.8:3;.8-8;.8(-/

-/

+++

+++

+

+=

If


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and can $e calculated anal#ticall# (Vicente-Serrano and Cuadrat* /88/).

&earson III distri$ution is not defined for&Q 8* "hich is a dra"$ac7 as precipitation

series !a# include !onths in "hich there is no precipitation. 1ith this in !ind* an

adapted statistic4(&)can $e calculated using the follo"ing for!ulaD

)()()( &(=1=&4 +=

"here = is the pro$a$ilit# of %ero precipitation. Ed"ards (/88,) suggested that = can $e

calculated si!pl# as +>n, "here n is the total nu!$er of !onths and +is the nu!$er of

!onths "ith no precipitation.

&earson III distri$ution "as "ell adapted to precipitation on all the ti!e scales. The

&earson III !odels for precipitation series at different ti!e scales in t"o o$seratories

located in different cli!atic regions (North and Southeast) of the I$erian &eninsula are

sho"n in Higure 0.

After calculating 4(&)* the !ean is standardised as 8 and standard deiation as ,. This

standardised aria$le is interchangea$le "ith the S&I* and is co!!ensura$le "ith other

S&I alues oer ti!e and space. An S&I of 8 indicates precipitation corresponding to

:8 of accu!ulated pro$a$ilit# according to &earson III distri$ution.

To transfor!4(&)and o$tain S&I* the approach for!ulated $# A$ra!o"it% and Stegun

(,3:) is usedD

-

-

/

/,

/

/,8

, ?d?d?d

?C?CC?S@I

++

++=

)ln(/ @? = for &8.:

@ is the pro$a$ilit# of e'ceeding a deter!ined precipitation alue*@1*4(u)

If@"*@is replaced $# ,-& and the sign of the resultant S&I is s"itched.

The constants areD C" !11D, C1 "8"!8#, C! ""1"#!8, d1 1$#!D88, d!

"18!, d# """1#"8


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Higure : sho"s the S&I of precipitation series at the Madrid o$serator# fro! ,3,8 to

/888. On the shorter ti!e scales ( or !onths)* the dr# and hu!id periods are short

and high-fre+uenc#. The S&I for these ti!e scales is considered an agricultural drought

inde' (Mc9ee et al.* ,33 and ,33:5 >a#es et al.* ,333) $ecause it indicates the "ater

content of egetation and the soil !oisture conditions (Si!s et al.* /88/5 i and &eters*

/88).

At a ti!e scale of ,/ !onths* droughts "ere less fre+uent* $ut the# lasted longer. The

S&I at ,/ !onths is considered a h#drological drought inde'* $ecause it is used for

!onitoring surface "ater resources* e.g. rier flo"s (S%alai et al, /8885 >a#es et al,

,333). At longer ti!e scales (/0 or !onths)* droughts lasted longer* $ut "ere less

fre+uent* "ith fe" dr# or hu!id periods recorded.

!#* AnalFsis o/ drou-ht spatial patterns over a ran-e o/ ti+e scales usin- @rincipal

Co+ponent AnalFsis

&rincipal Co!ponent Anal#sis (&CA) "as used to deter!ine the spatial patterns of

drought on different ti!e scales (9arl and 9oscieln#* ,34/5 =onaccorso et al* /885

Vicente-Serrano et al.* /880). The application of &CA for cli!atic series can $e

perfor!ed in S or T !odes (Serrano et al.* ,333). S-!ode has $een used to o$tain

general te!poral patterns of cli!atic seriesD the o$seratories are the aria$les5 and the

ti!e o$serations* the cases. The areas represented $# each !ode can also $e identified

$# !apping the factorial loadings (correlation $et"een each original aria$le and the

principal co!ponents e'tracted). S-!ode identifies regions in "hich the te!poral

ariation of cli!atic aria$les has the sa!e pattern.

Hro! the S&I series of the I$erian &eninsula* "e applied si' &CAs* one for each ti!e

scale* to chec7 "hether the spatial patterns of drought changed in relation to ti!e scales


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of drought* and "hether the spatial patterns "ere sta$le. As the S&I is a standardised

cli!atic aria$le* the final co!ponents o$tained can $e rena!ed as the co!!on S&I of

specific areas* identified $# !apping the factorial loadings. This approach has also $een

used $# =onaccorso et al. (/88) in Sicil# and $# ?ana et al. (/88,) in the Northeast of

the I$erian &eninsula.

The nu!$er of co!ponents "as selected according to the criterion of eigenalue ,*

and the co!ponents "ere rotated to redistri$ute the final e'plained ariance. The

Vari!a' rotation "as selected for this purpose (1hite et al.* ,33,5 Serrano et al,,333).

!$* Classi/ication o/ the Ierian @eninsula on the asis o/ drou-ht patterns

2ifferent spatial classifications* $ased on the general drought patterns gien $# &CAs*

"ere o$tained $# grouping the o$seratories through the !a'i!u! loading rule. Each

o$serator# "as assigned to the co!ponent "ith the highest loading alue. This !ethod

has $een used in cli!ate classification $# nu!erous authors* such as 9arl and 9oscieln#

(,34/)* Co!rie and Glenn (,334)* =rring (,344) and Mallants and He#en (,338) in the

FSA* Me'ico* 9en#a and =elgiu!* respectiel#. Si' spatial classifications (one for

each ti!e scale) "ere constructed in this stud#.

!* Evaluation o/ spatial uncertaintF usin- /ra-+entation and diversitF indices

The spatial sta$ilit# of drought patterns for the distinct ti!e scales "as ealuated $#

!eans of frag!entation and diersit# indices. Hor this purpose* categor# !aps "ere

created $# !eans of the Thiessen pol#gons !ethod (Vicente-Serrano et al.* /88).

Seeral indices "ere o$tained fro! these !aps to test the coherence and spatial

ro$ustness of classifications. The indices selected hae $een e'tensiel# used in

landscape ecolog# to anal#se the spatial structure of categor# !aps (i.e. Turner* ,3385


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>argis et al, ,334). The indices "ere calculated "ith the Hragstat . . soft"are

pac7age (McGarigal and Mar7c* ,33:). The follo"ing indices "ere selectedD

The Shannon 2iersit# Inde' (S>NI)D this indicates the spatial concentration of

classes. >igh alues indicate !ore heterogeneit# in drought classification.

=

=+

i

ii @@S4GI,

)ln(

"here@iis the proportion of the surface occupied $# class i.

The !ean area of each class (MA).

The nu!$er of patches (N).

The patch aerageD

C0@A =

"here C is the nu!$er of classes. The inde' indicates the degree of spatial

frag!entation of final classes. >igh alues indicate high spatial co!part!entalisation in

a large nu!$er of patches* "hich indicates !ore uncertaint# in classification.

Edge densit# (E2)D

,8888=A

EEG

This inde' is $ased on contacts $et"een distinct patches* "here E is the total length (!)

of edge on the !ap and A is the total area of the I$erian &eninsula. >igh alues on this

indicator also sho" large spatial co!part!entalisation and* therefore* high

heterogeneit# and spatial uncertaint# in classification.

3. Results

#1 Spatial patterns o/ S@I /or a ran-e o/ ti+e scales


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The nu!$er of &CA co!ponents e'tracted using the S&I series increased as the ti!e

scale $eca!e longer (Ta$le ,).

At a ,-!onth ti!e scale* co!ponents "ere e'tracted* "hich e'plained ; total

ariance of the S&I series. The sa!e nu!$er of co!ponents and a si!ilar percentage of

ariance "ere e'plained using series for a -!onth ti!e scale. Hor and ,/ !onths* the

nu!$er of co!ponents o$tained increased to ; and 3* respectiel#* although the

percentage of total ariance e'plained "as si!ilar to the ariance e'plained at , and

!onths. This o$seration i!plies higher distri$ution of total ariance a!ong the

co!ponents selected. At /0 and !onths* the nu!$er of co!ponents "as significantl#

greater than in the shortest scales* "ith an e'traction of ,8 and ,, co!ponents*

respectiel#. Hor the longer ti!e scales* the e'plained ariance "as !ore distri$uted

than for the shorter ones and the total e'plained ariance "as also greater. The

differences $et"een total ariance grouped $# first co!ponent and the re!aining ones

decreased "ith increased ti!e scale.

The spatial patterns of the factorial loadings o$tained fro! &CAs using the S&I series

for distinct ti!e scales "ere si!ilar at , and !onths (Higures -,,). The patterns "ere

consistent and had a logical spatial structure that "as consistent "ith cli!ate influences

in the I$erian &eninsula (Rod et al, ,33;5 Rodr


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Hor a -!onths ti!e scale* spatial drought patterns "ere si!ilar to the shorter scales*

although a ne" pattern "as identified in the South of the I$erian &eninsula. Moreoer*

the spatial pattern of co!ponent , "as displaced to the 1est. In spite of these s!all

differences* the patterns "ere consistent "ith those o$tained for shorter ti!e scales* and

their spatial organisation "as also si!ilar.

>o"eer* at ,/ !onths* the spatial frag!entation caused $# the higher nu!$er of

co!ponents e'tracted "as greater. At this ti!e scale* the area represented $# each

co!ponent "as s!aller and so!e spaces represented $# a single co!ponent at ti!e

scales of ,* or !onths "ere diided. The drought eolution of these areas at ,/

!onths "as represented $# seeral co!ponents. Hor e'a!ple* at ,* or !onths* the

Northeast of the &eninsula "as defined as a ho!ogeneous area in "hich drought

eolution "as represented $# onl# one co!ponent* "hereas at ,/ !onths the area "as

represented $# t"o co!ponents (; and 4). This o$seration indicates !ore spatial

aria$ilit# in drought eolution at ,/ !onths than for short ti!e scales. In general* for a

,/-!onth ti!e scale the areas represented $# each co!ponent had s!all surfaces* and

ne" co!ponents for s!aller areas "ere identified* "hich indicates !ore co!ple'

spatial patterns of drought eolution.

Hor a /0-!onth ti!e scale* spatial patterns "ere een !ore co!ple' (Higure ,8). The

co!ponent representing inland areas of the I$erian &eninsula* "hich at ,* or !onths

coered a large percentage of total S&I ariance* disappeared at /0 !onths* and drought

eolution in these areas "as represented $# distinct and !ore local co!ponents. The

loading alues decreased significantl# and co!ponents o$tained for this ti!e scale "ere

representatie of !ore concrete areas. Si!ilar spatial $ehaiour of droughts "as also

o$sered at !onths* "here drought patterns "ere er# local (Higure ,,).


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The increase in spatial diersit# "as caused $# the decrease in the correlation of S&I

series $et"een the o$seratories "hen ti!e scale increased. Higure ,/ sho"s the

distri$ution of correlations $et"een pairs of o$seratories (,/;: pairs). The drought

indices in different o$seratories "ere closer "hen the ti!e scale "as short* $ut for

longer periods the drought conditions recorded at these o$seratories* in general*

differed !ore.

#! Ierian @eninsula classi/ication on the asis o/ ti+e scales

The spatial patterns of drought eolution in the I$erian &eninsula differed greatl# in

relation to the ti!e scale (Higure ,). At or !onths* the classifications sho"ed fe"

classes* "hich had a high spatial ho!ogeneit#. These classifications allo"ed drought

!onitoring and earl# "arning of agricultural droughts $# $uilding regional precipitation

series "ithin these ho!ogeneous areas.

The classifications for longer ti!e scales sho"ed higher co!ple'it# $ecause the I$erian

&eninsula "as diided into !ore classes* and !ore spatial frag!entation "as recorded.

This introduces great co!ple'it# into the classifications* and indicates that the te!poral

$ehaiour of droughts is spatiall# !ore co!ple' "hen ti!e scale increases. The

classifications for longer ti!e scales are no use for o$taining infor!ation on the

regionalisation of drought $ehaiour in the I$erian &eninsula. Hor e'a!ple* at ,/

!onths* drought classes are er# frag!ented (Higure ,0).

Ta$le / sho"s the results of the application of diersit# and frag!entation indices to

continuous !aps of S&I classifications oer a range of ti!e scales. The diersit# inde'

(S>2I) indicates an incre!ent of spatial diersit# "hen ti!e scale increases. This

incre!ent indicates greater spatial heterogeneit# in drought classification for longer

ti!e scales. This o$seration can also $e deduced fro! the calculation of the !ean class


15/39

area (MA)* "hich indicates a decrease in its alues "ith increased ti!e scale. Therefore*

at ti!e scales of /0 or !onths* the distinct classes coer less surface.

>o"eer* the higher spatial heterogeneit# o$sered in classifications for longer ti!e

scales "as caused not onl# $# the incre!ent in the nu!$er of classes* $ut also $# the

spatial frag!entation of each drought class. The nu!$er of patches increased fro! at

a ,-!onth ti!e scale (onl# one patch per class) to /8 patches at !onths. Ti!e scales

higher than , !onth had !ore patches than classes* "hich indicates that classes

represent areas that are separated fro! each other. This is er# clear at ti!e scales of ,/*

/0 or !onths* at "hich the patch aerage increased significantl# (,./* ,.; and /*

respectiel#). This finding introduces great spatial heterogeneit# and uncertaint# into

classifications. The edge densit# (E2) confir!s this $ehaiour* as it increased fro! the

shortest ti!e scales (8.,/ at , and !onths) to the longest (8./ at !onths). This

increase is caused $# the increased nu!$er of classes and patches and reaffir!s the

greater spatial uncertaint# in the spatial classifications of drought $ehaiour oer longer

ti!e scales.

4. Discussion and conclusions

This stud# sho"s the spatial patterns of droughts as a function of ti!e scale in the

I$erian &eninsula. 1e used the S&I calculated oer ti!e scales of ,* * * ,/* /0 and

!onths* $ecause of its effectieness in identif#ing h#drological and agricultural drought

(Si!s et al.* /88/5 S%alai et al.* /8885 Ua!oah et al.* /888).

The article sho"s that the spatial patterns and ti!e scales of drought indices !a# differ

greatl#* and that the spatial $ehaiour of the inde'* "hen calculated for long ti!e scales*

is not coherent.


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The spatial patterns o$tained $# !eans of &CA and the diersit# and frag!entation

indices used for testing the spatial coherence of drought classifications sho"ed that the

diision of the I$erian &eninsula as a function of ti!e scales is too co!ple' to $e

applied in drought !onitoring. Moreoer* for the aried ti!e scales* the classifications

are not coherent cli!aticall#* $ecause the# gie spatial patterns that are too co!ple'.

The aria$ilit# of spatial $ehaiour of drought in relation to ti!e scales indicates the

co!ple'it# of the drought pheno!enon. >o"eer* interpretations of so!e indices as

indicatie of true drought conditions !ust $e treated "ith caution.

Gien the great spatial uncertaint# of drought classifications on distinct S&I ti!e scales*

integrated !onitoring s#ste!s that ie" the territor# continuousl# oer a range of ti!e

scales are the $est tools for !onitoring drought conditions (Tsa7iris and Vangelis* /8805

So$oda et al.* /88/)* or een seeral drought indicators used together (Steinne!ann*

/88). Moreoer* "e !ust not discard the possi$ilit# of integrating other para!eters

into drought !onitoring* such as soil !oisture !easure!ents* rier flo"s* "ater leels

in reseroirs* or een egetation indices o$tained fro! satellite data (9ogan* ,33: and

,33;).

In spite of the spatial uncertaint# of drought indices calculated fro! cli!atic data* the#

are essential in accurate drought !onitoring. 2rought !anage!ent plans that do not use

real ti!e infor!ation detect droughts "hen the# reach critical proportions* $ut then it is

too late to ta7e !itigation !easures (1ilhite et al.* ,34).

2ifferences in drought conditions $et"een neigh$ouring o$seratories* "hen ti!e

scales increase* can $e deter!ined $# the "eight that e'tre!e precipitation eents hae

on drought indices. In the Mediterranean area* e'tre!e precipitation eents hae $road

spatial aria$ilit# (Martin-Vide and ?lasat* /888). On shorter ti!e scales (i.e. or

!onths)* intense precipitation has a significant "eight on drought indices onl# for a fe"


17/39

!onths. >o"eer* the nu!$er of !onths in "hich these eents hae a significant

influence on these indices increases in the longer ti!e scales (/0 or !onths). The

spatial location of e'tre!e eents causes significant differences $et"een neigh$ouring

areas in their drought inde' alues oer long periods* if long ti!e scales are used.

Therefore* the S&I co!puted at these ti!e scales !a# not $e an ade+uate indicator of

true drought conditions.

In su!!ar#* the spatial $ehaiour of drought indices in the I$erian &eninsula should

also $e tested in other regions in "hich cli!atic or precipitation aria$ilit# is not so

high* to deter!ine !ore accuratel# their usefulness. Moreoer* although nota$le

adances hae $een achieed in the deelop!ent of drought indices* !ore effectie

tools and s#ste!s to control and !itigate the ris7 of drought need to $e deeloped.

Acknowledgeents

This "or7 has $een supported $# the proBectsD Caracteri%acin espacio-te!poral de las

se+u


18/39

A$ra!s* M.2.* Ruffuer* M.C. and Morgan* T.A.D ,334* XTree-ring responses to droughtacross species and contrasting sites in the ridge and alle# of central&enns#lania*orest Science 44* ::8-::4.

Ale'andersson >.D ,34* XA ho!ogeneit# test applied to precipitation data* Hournal o/Cli+atolo-F "* ,-;:.

Ale'andersson >. and Mo$erg A.D ,33;* X>o!ogeni%ation of S"edish te!eperaturedata. &art ID >o!ogeneit# test for lineal trends* International Hournal o/Cli+atolo-F 1#* /:-0.

Alle#* 1.M.D ,340* XThe &al!er drought seerit# inde'D li!itations and applications*Hournal o/ Applied 6eteorolo-F 23$,,88-,,83.

Austin* R.=.* Cantero-Mart.R. and 1ilhite* 2.A.D ,333* XO$Bectie +uantification of drought seerit# andduration*Hournal o/ Cli+ate 12*/;0;-/;:.

Co!rie* A.C. and Glenn* E.C.D ,334* X&rincipal co!ponents-$ased regionali%ation ofprecipitation regi!es across the south"est Fnited States and northern Me'ico*"ith an application to !onsoon precipitation aria$ilit#* Cli+ate 3esearch 1&*/8,-/,:.

Corder#* I. and McCall* M.D /888* XA !odel for forecasting drought fro!teleconections* ?ater 3esources 3esearch 3"* ;-;4.

Changnon* S.A. and Easterling* 1.E.D ,343* XMeasuring drought i!pactsD the Illinoiscase* ?ater 3esources Bulletin 2'* /;-0/.

2o!on7os* &.D /88* XRecent precipitation trends in >ungar# in the conte't of largerscale cli!atic changes*0atural 4aards 2(* /::-/;,.

2racup* .A.* ?ee* 9.S. and &aulson* E.G.D ,348* XOn the Statistical Characteristics of2rought Eents* ?ater 3esources 3esearch1"* /43-/3.

2u &isani* C.G.* Houch* >.. and Venter* .C.D ,334* XAssessing rangeland drought inSouth Africa*A-ricultural SFste+s '#* ;-48.

Eder* =.9.* 2ais* .M. and Monahan* .H.D ,34;* XSpatial and te!poral anal#sis of the&al!er drought seerit# inde' oer the south-eastern united States* Hournal o/Cli+atolo-F #* ,-:,.

Ed"ards* 2.C.D /88,*6ethodolo-F o/ S@I. httpDccc.at!os.colostate.eduS&I.ht!.

Ed"ards* 2.C. and Mc9ee* T.=.D ,33;* Characteristics o/ !"thcenturF drou-ht in theJnited States at +ultiple ti+e scales. At!ospheric Science &aper No. 0.

Este$an &arra* M..* Rodrigo* H.S. and Castro-2


19/39

HernKnde%-Mills* G.D ,33:* X&rincipal co!ponent anal#sis of precipitation and rainfallregionali%ation in Spain* %heoretical and Applied Cli+atolo-F'&* ,3-,4.

Garcargis* C. 2.* =issonette* .A.* and 2aid* .?.D ,334* XThe $ehaior of landscape!etrics co!!onl# used in the stud# of ha$itat frag!entation* Kandscape

Ecolo-F13* ,;-,4.

>a#es* M.* 1ilhite* 2.A.* So$oda* M. and Van#ar7ho* O.D ,333* XMonitoring the ,33drought using the Standardi%ed &recipitation Inde'* Bulletin o/ the A+erican

6eteorolo-ical SocietF %&* 0/3-04.

>ei!* R.R.D /88/* XA reie" of t"entieth-centur# drought indices used in the FnitedStates*Bulletin o/ the A+erican 6eteorolo-ical SocietF %3* ,,03-,,:.

>os7ing* .R.M.D ,338* X?-Mo!entsD Anal#sis and esti!ation of distri$utions usinglinear co!$inations of order statistics*Hournal o/ 3oFal Statistical SocietF B '2*,8:-,/0.

i* ?. and &eters* A..D /88* XAssessing egetation response to drought in the northern

Great &lains using egetation and drought indices* 3e+ote Sensin- o/Environ+ent %#* 4:-34.

9eiser* 2.T. and Griffiths* .H.D ,33;* X&ro$le!s Associated "ith >o!ogeneit# Testingin Cli!ate Variations Studies. A Case Stud# of Te!perature in the NorthernGreat &lains* FSA*International Hournal o/ Cli+atolo-F 1#* 03;-:,8.

9anti* =.D ,334* XCoping !echanis!s practised $# drought icti!s (,330:) in north=engal* =angladesh*Applied Leo-raphF 1%* ::-;.

9arl* T.R.D ,34;* XThe sensitiit# of the &al!er 2rought Seerit# Inde' and the &al!er%-Inde' to their cali$ration coefficients including potential eapotranspiration*

Hournal o/ Cli+ate and Applied 6eteorolo-F 2'* ;;-4.9arl* T.R. and 9oscieln#* A..D ,34/* X2rought in the Fnited StatesD ,43:-,34,*

Hournal o/ Cli+atolo-F 2* ,-/3.

9e#antash* . and 2racup.* .D /88/* XThe +uantification of droughtD an ealuation ofdrought indices* Bulletin o/ the A+erican 6eteorolo-ical SocietF %3* ,,;-,,48.

9ogan* H.D ,33:* X2roughts of the late ,348s in the Fnited States as deried fro!NOAA &olar-Or$iting Satellite data* Bulletin o/ the A+erican 6eteorolo-icalSocietF #"* ::-4.

9ogan* H.D ,33;* XGlo$al drought "atch fro! space* Bulletin o/ the A+erican6eteorolo-ical SocietF #%* /,-.


20/39

9o!uscu* A.F.D ,333* XFsing the S&I to anal#%e spatial and te!poral patterns ofdrought in Tur7e#*Grou-ht 0et:or7 0e:s11* ;-,.

?ana* Z. and =urgue@o* A.D ,334* X&ro$a$ilities of repeated long dr# episodes $ased onthe poisson distri$ution. An e'a!ple for Catalonia (NE Spain)* %heoretical and

Applied Cli+atolo-F "&* ,,,-,/8.

?ana* Z.* Serra* C. and =urgue@o* A.D /88,* X&atterns of !onthl# rainfall shortage ande'cess in ter!s of the Standardied &recipitation Inde' for Catalonia (NE Spain)*

International Hournal o/ Cli+atolo-F 21* ,3-,3,.

?lo#d->ughes* =. and Saunders* M.A.D /88/* XSeasonal prediction of european springprecipitation fro! El Ni@o-southern oscillation and local sea-surfacete!peratures*International Hournal o/ Cli+atolo-F 22* ,-,0.

?lo#d->ughes* =. and Saunders* M.A.D /88/$* XA drought cli!atolog# for Europe*International Hournal o/ Cli+atolo-F 22* ,:;,- ,:3/.

Mallants 2. and He#en* .D ,338* X&rincipal co!ponent anal#sis of =elgian rainfall*%heoretical and Applied Cli+atolo-F 3(* ,33-/80.

Mart


21/39

Nicholson* S.E.* Tuc7er* C.. and =a* M.=.D ,334* X2esertification* drought and surfaceegetationD an e'a!ple fro! the "est African Sahel*Bulletin o/ the A+erican

6eteorolo-ical SocietF #(* 4,:-4/3.

Ntale* >.9. and Gan* T.D /88* X2rought indices and their application to East Africa*International Hournal o/ Cli+atolo-F 23* ,:-,:;.

Oladipo* E.O.D ,34:* XA co!paratie perfor!ance anal#sis of three !eteorologicaldrought indices*Hournal o/ Cli+atolo-F '* ::-0.

Olcina .D /88,* XTipolog


22/39

Si!s* A.&.* Nigo#i* 2.S. and Ra!an* S.D /88/* XAdopting indices for esti!ating soil!oistureD A North Carolina case stud#* LeophFsical 3esearch Ketters 2(* ,-0.

SNIR>.D /880* Siste+a 0acional de In/or+aao de 3ecursos 45dricoshttpDsnirh.inag.ptinde'.php]idio!aQ

Soul* &.T.D ,33/* XSpatial patterns of drought frecuenc# and duration in the contiguousFSA $ased on !ultiple drought eent definitions* International Hournal o/Cli+atolo-F12* ,,-/0.

Steine!ann* A.D /88* X2rought indicators and triggersD A stochastic approach toealuation* Hournal o/ the A+erican ?ater 3esources Association 3(* ,/,;-,/

tJpKne7* &.D /880*AnCli+ * so/t:are /or ti+e series analFsis (/or ?indo:s)* 2ept. ofGeograph#* Hac. of Natural Sciences* MF* =rno. ,.0; M=.

So$oda* M.* ?eCo!pte* 2.* >a#es* M.* >ei!* R.* Gleason* 9.* Angel* .* Rippe#* =.*Tin7er* R.* &alec7i* M.* Stoo7s$ur#* 2.* Mis7us* 2. and Stephens* S.D /88/* XThedrought !onitor* Bulletin o/ the A+erican 6eteorolo-ical SocietF %3* ,,4,-,,38.

S%alai* S.* S%inell* C.S. and [o$o7i* .D /888* X2rought !onitoring in >ungar#* InEarlF :arnin- sFste+s /or drou-ht preparedness and drou-ht +ana-e+ent1orld Meteorological Organi%ation. ?is$oaD ,4/-,33.

Tsa7iris* G. and Vangelis* >.D /880* XTo"ards a drought "atch s#ste! $ased on spatialS&I* ?ater 3esources 6ana-e+ent 1%* ,-,/.

Turner* M. G.D ,338* XSpatial and te!poral anal#sis of landscape patterns* KandscapeEcolo-F4* /,-8.

Vicente-Serrano* S.M. and Cuadrat* .M.D /88/* X2esarrollo de un !todo anal. and Uarnal* =.D ,33,* XCli!ate regionali%ation and rotation ofprincipal co!ponentes*International Hournal o/ Cli+atolo-F 11* ,-/:.

1ilhite* 2.A.D ,33;* XA !ethodolog# for drought preparedness*0atural 4aards 13*

//3-/:/.


23/39

1ilhite* 2.A. and So$oda* M.2.D /888* X2rought earl# "arning s#ste!s in the conte'tof drought preparedness and !itigation* InEarlF :arnin- s#ste!s/or drou-ht

preparedness and drou-ht +ana-e+ent 1orld Meteorological Organi%ation.?is$oa* ,-/,.

1ilhite* 2.A.* Rosen$erg* N.. and Glant%* M.>.D ,34* XI!proing federal response todrought*Hournal o/ Cli+ate and Applied 6eteorolo-F 2'* /-0/.

Ua!oah* C.H.* 1alters* 2.T.* Shapiro* C.A.* Hrancis* C.A. and >a#es* M..D /888*XStandardi%ed &recipitation Inde' and nitrogen rate effects on crop #ields andris7 distri$ution in !ai%e*A-riculture, EcosFste+s Environ+ent %&* ,,-,/8.

FIGURE AD !A"#E $AP!I%&:

Figure '( &patial distribution o precipitation recording obser)atoriesused or drought analysis in the Iberian Peninsula(Figure *( Fre+uency histograms o precipitation calculated at diferenttime scales, obser)atory o Albacete(Figure -( #.moment ratios diagrams or precipitation series at diferenttime scales(

Figure /( Pearson III models at diferent time scales in t0oobser)atories o the Iberian Peninsula(

Figure 1( E)olution o &PI at diferent time scales( %bser)atory o2adrid(

!able '( Results o P$A analysis at diferent time scales(

Figure 3( #oadings o P$A: &PI at time scale o ' month(Figure 4( #oadings o P$A: &PI at time scale o - months(Figure 5( #oadings o P$A: &PI at time scale o 3 months(

Figure 6( #oadings o P$A: &PI at time scale o '* months(Figure '7( #oadings o P$A: &PI at time scale o */ months(

Figure ''( #oadings o P$A: &PI at time scale o -3 months(Figure '*( Diferences in distribution o R.Pearson statistics bet0eenall pairs o obser)atories at diferent time scales

Figure '-( $lassi8cation o the Iberian Peninsula according to temporal

e)olution o &PI at diferent time scales(


24/39

Figure '/( E9ample o continuos classi8cations according to temporale)olution o droughts at diferent time scales

!able *( &patial heterogeneity statistics or &PI classi8cations atdiferent time scales(

&DI: &hanon;s Di)ersity Inde9, 2A: mean area o each class


25/39

Figure 2.Fre+uency histograms o precipitation calculated at diferent time scales,obser)atory o Albacete(


26/39

Figure 3.#.moment ratios diagrams or precipitation series at diferent time scales(


27/39

Figure 4.Pearson III models at diferent time scales in t0o obser)atories o theIberian Peninsula(


28/39

Figure 5.E)olution o &PI at diferent time scales( %bser)atory o 2adrid(


29/39

Table 1.Results o P$A analysis at diferent time scales(

Total % of variance % accumulated Total % of variance % accumulated

1 month 24 months

12.2 23.9 23.9 9.4 18.4 18.4

7.1 14 37.8 7.8 15.4 33.7

5.5 10.8 48.6 5.1 10.1 43.8

5 9.7 58.3 3.7 7.4 51.1

4 7.9 66.3 3 5.9 57.1

3.5 6.8 73 2.9 5.8 62.8

3 months 2.7 5.3 68.1

12.7 24.9 24.9 2.3 4.4 72.6

5.9 11.7 36.6 2.2 4.4 76.9

5.2 10.2 46.8 1.5 3 79.9

4.8 9.5 56.2 36 months

4.3 8.5 64.7 8.9 17.4 17.4

3.1 6.2 70.9 8.3 16.3 33.7

6 months 5.5 10.8 44.5

13.3 26.2 26.2 3.6 7.1 51.6

5.8 11.4 37.6 3 5.8 57.4

4.7 9.3 46.9 2.9 5.6 63

4 7.9 54.8 2.7 5.3 68.3

3.7 7.2 62 2.6 5.1 73.4

3.2 6.2 68.3 2.2 4.3 77.7

1.9 3.7 72 2 3.8 81.5

12 months 1.3 2.6 84.2

12.6 24.8 24.8

4.6 8.9 33.7

4.1 8.1 41.8

3.8 7.5 49.2

3.7 7.3 56.6

3.4 6.7 63.2

3.1 6.2 69.4

1.9 3.8 73.2

1.4 2.7 75.8


30/39

Figure 6.#oadings o P$A: &PI at time scale o ' month(


31/39

Figure 7.#oadings o P$A: &PI at time scale o - months(


32/39

Figure 8.#oadings o P$A: &PI at time scale o 3 months(


33/39

Figure 9.#oadings o P$A: &PI at time scale o '* months(


34/39

Figure 10.#oadings o P$A: &PI at time scale o */ months(


35/39

Figure 11.#oadings o P$A: &PI at time scale o -3 months(


36/39

Figure 12.Diferences in distribution o R.Pearson statistics bet0een all pairs oobser)atories at diferent time scales


37/39

i-ure 1#Classification of the I$erian &eninsula according to te!poral eolution ofS&I at different ti!e scales.


38/39

Figure 14.E9ample o continuos classi8cations according to temporal e)olution odroughts at diferent time scales


39/39

Table 2.&patial heterogeneity statistics or &PI classi8cations atdiferent time scales(

Time scale SHDI MA N PA ED

1 1.68 969.8 6 1 0.123

3 1.67 969.8 7 1.16 0.1266 1.51 969.8 7 1.16 0.133

12 1.78 727.4 13 1.62 0.163

24 2.03 581.9 17 1.7 0.200

36 2.12 581.9 20 2 0.211

&DI: &hanon;s Di)ersity Inde9, 2A: mean area o each class

Documents

WaterResourcesManagement (2006) 20, 37-60