a mosphe e
A icle
Flash D ough Response o P ecipi a ion and A mosphe ic
E apo a i e Demand in Spain
I án Nogue a 1, Fe nando Domínguez-Cas o 2,3 and Se gio M. Vicen e-Se ano 1,*


Ci a ion: Nogue a, I.;
Domínguez-Cas o, F.;
Vicen e-Se ano, S.M. Flash D ough
Response o P ecipi a ion and
A mosphe ic E apo a i e Demand in
Spain. A mosphe e 2021,12, 165.
h ps://doi.o g/10.3390/
a mos12020165
Academic Edi o : Sonia
Raquel Gámiz-Fo is
Recei ed: 5 Janua y 2021
Accep ed: 24 Janua y 2021
Published: 27 Janua y 2021
Publishe ’s No e: MDPI s ays neu al
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Copy igh : © 2021 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
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A ibu ion (CC BY) license (h ps://
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1Ins i u o Pi enaico de Ecología, Consejo Supe io de In es igaciones Cien í icas (IPE–CSIC),
E-50059 Za agoza, Spain; [email p o ec ed]
2Depa men o Geog aphy, Uni e si y o Za agoza, 50009 Za agoza, Spain; dominguez@uniza .es
3ARAID Founda ion, 50018 Za agoza, Spain
*Co espondence: [email p o ec ed]
Abs ac :
Flash d ough is he esul o s ong p ecipi a ion de ici s and/o anomalous inc eases
in a mosphe ic e apo a i e demand (AED), which igge s a apid decline in soil mois u e and
s esses ege a ion o e sho pe iods o ime. Howe e , li le is known abou he ole o p ecipi a ion
and AED in he de elopmen o lash d ough s. Fo his pape , we compa ed he s anda dized
p ecipi a ion index (SPI) based on p ecipi a ion, he e apo a i e demand d ough index (EDDI)
based on AED, and he s anda dized e apo a ion p ecipi a ion index (SPEI) based on he di e ences
be ween p ecipi a ion and AED as lash d ough indica o s o mainland Spain and he Balea ic
Islands o 1961–2018. The esul s show la ge di e ences in he spa ial and empo al pa e ns o
lash d ough s be ween indices. In gene al, he e was a high deg ee o consis ency be ween he lash
d ough pa e ns iden i ied by he SPI and SPEI, wi h he excep ion o sou he n Spain in he summe .
The EDDI showed no able spa ial and empo al di e ences om he SPI in win e and summe ,
while i exhibi ed g ea cohe ence wi h he SPEI in summe . We also examined he sensi i i y o he
SPEI o AED in each mon h o he yea o explain i s con ibu ion o he possible de elopmen o
lash d ough s. Ou indings showed ha p ecipi a ion is he main d i e o lash d ough s in Spain,
al hough AED can play a key ole in he de elopmen o hese du ing pe iods o low p ecipi a ion,
especially in he d ies a eas and in summe .
Keywo ds:
lash d ough ; sensi i i y; a mosphe ic e apo a i e demand; p ecipi a ion; s anda dized
d ough indices; SPEI; Spain
1. In oduc ion
D ough is one o he mos complex na u al haza ds a ec ing na u al and human
sys ems [
1
–
3
]. Typically, d ough is conside ed o be a me eo ological phenomenon ha
is slow o de elop, aking many mon hs o e en yea s o each maximum in ensi y [
4
,
5
].
Howe e , ecen s udies ha e p o ed ha d ough may also de elop in he sho - e m as a
esul o la ge p ecipi a ion de ici s and/o anomalous inc eases in a mosphe ic e apo a i e
demand (AED) [
6
]. The e m “ lash d ough ” has become popula in he scien i ic li e a u e
o desc ibe hese d ough episodes cha ac e ized by apid onse and in ensi ica ion. Flash
d ough s usually begin as a me eo ological d ough ha becomes an ag icul u al d ough
wi hin a sho ime, due o a decline in soil mois u e and an inc ease in ege a ion s ess [
7
],
wi h a s ong po en ial impac on ag icul u e and he en i onmen [8].
P ecipi a ion is he mos impo an ac o in he de elopmen o d ough s [
9
]. How-
e e , he occu ence o lash d ough s has been commonly ela ed o d as ic inc eases
in AED and he associa ed land–a mosphe e eedbacks (e.g., inc eased land e apo an-
spi a ion) ha can igge o ein o ce d ough condi ions [
10
]. Nume ous s udies ha e
e idenced ha AED can play a key ole in he de elopmen and in ensi ica ion o ce ain
d ough episodes [
11
–
13
]. The e o e, he AED can be a c ucial a iable in explaining he
apid deple ion o soil mois u e and ege a ion s ess caused by lash d ough s [
12
,
14
–
16
].
A mosphe e 2021,12, 165. h ps://doi.o g/10.3390/a mos12020165 h ps://www.mdpi.com/jou nal/a mosphe e
A mosphe e 2021,12, 165 2 o 22
Howe e , gi en complex land–a mosphe e eedbacks in ol ing d ough [
10
], he ole o
AED in he de elopmen o lash d ough s could be e y complex and a iable in ime and
space [17].
E o s o de elop objec i e me hodologies and ea ly wa ning sys ems o iden i y lash
d ough s ha e inc eased in ecen yea s [
7
,
14
–
16
,
18
–
26
], bu none is widely accep able [
6
].
Mos me hods p oposed o iden i ying lash d ough s mainly ocus on apid changes in
e e ence e apo anspi a ion (ET
o
) o soil mois u e [
7
,
19
,
23
,
24
], wi hou including p e-
cipi a ion as an inpu a iable. O he au ho s, such as Mo and Le enmaie [
16
,
22
] o
Zhang e al. [
27
] sugges ed di e en ia ing wo ypes o lash d ough s: “hea wa e lash
d ough s” linked o high empe a u es and “p ecipi a ion de ici lash d ough s” linked o
anomalously low ain all.
In his wo k, we s udied he ole o p ecipi a ion de ici and a mosphe ic e apo a i e
demand inc ease on lash d ough s in Spain, whe e lash d ough s a e a equen phe-
nomenon wi h app oxima ely ou in e e y 10 d ough e en s (i.e., a e u n pe iod o 1 in
10 yea s) esul ing in a lash d ough [
18
]. Se e al s udies showed he high occu ence o
se e e d ough s episodes in Spain h ough his o ical [
28
–
30
] and ins umen al eco ds [
31
].
Spain has s ong clima ic con as s wi h ma ked spa ial and seasonal di e ences in p e-
cipi a ion [
32
–
35
] and AED [
36
–
39
]. No he n Spain is cha ac e ized by i s humid oceanic
clima e wi h abundan p ecipi a ion o e he yea , while no heas e n and sou heas e n
egions a e mainly cha ac e ized by semia id condi ions wi h annual p ecipi a ion gen-
e ally below 300 mm [
40
]. On he o he hand, he complex opog aphy gi en by he
p esence o nume ous moun ain chains esul s in a s ong con inen al ea u es in cen al
Spain. This clima ic complexi y is also e lec ed in he spa ial and empo al a ia ion
o d ough s [
41
,
42
] and he a ie y o a mosphe ic mechanisms igge ing hem [
43
–
45
],
which enable an e alua ion o he ole o p ecipi a ion de ici s and AED in he de elopmen
o lash d ough s o e a wide ange o clima e condi ions.
Fo his pu pose, we compa ed h ee s anda dized d ough indices based on di e en
clima e a iables o lash d ough analysis. The objec i es o his s udy we e wo old:
(i) o compa e he spa ial and empo al pa e ns o lash d ough s iden i ied by h ee
obus s anda dized d ough indices based on di e en clima ic a iables: he s anda dized
p ecipi a ion index (SPI), e apo a i e demand d ough index (EDDI), and s anda dized
e apo a ion p ecipi a ion index (SPEI); and (ii) o analyze he ole o p ecipi a ion de ici s
and AED excess in lash d ough s in Spain o e he pe iod 1961–2018.
2. Me hods
2.1. Da a
This esea ch used a high spa ial esolu ion (1.21 km
2
) g idded clima e da ase wi h
co e age o mainland Spain and he Balea ic Islands om 1961 o 2018 a weekly e-
quency. The clima e da ase included da a on p ecipi a ion, maximum and minimum ai
empe a u e, ela i e humidi y, sunshine du a ion, and wind speed. The g idded da ase
was c ea ed based on all daily obse a ional in o ma ion om he Na ional Spanish Me eo-
ological Se ice (AEMET) by means o an in e pola ion scheme o uni e sal k iging using
as inpu he me eo ological da a measu ed in he di e en me eo ological s a ions and he
e ain ele a ion. The clima e se ies we e subjec ed o a homogeniza ion p ocess and a
ca e ul quali y con ol (see de ails in Tomas-Bu gue a e al. [
46
]). Addi ional in o ma ion
abou he da ase de elopmen , in e pola ion me hodology, and alida ion a e a ailable in
Vicen e-Se ano e al. [
47
]. The e e ence e apo anspi a ion (ET
o
), as a me ic o he AED,
was calcula ed om he maximum and minimum empe a u e, ela i e humidi y, wind
speed, and sunshine du a ion, using he FAO-56 Penman–Mon ei h equa ion [48].
2.2. Compu ing he D ough Indices: SPI, EDDI, and SPEI
S anda dized d ough indices a e commonly used in d ough analysis and moni o ing
ac oss he wo ld. Among hese, some o he mos widely used o d ough analysis a e he
s anda dized p ecipi a ion index (SPI; see Mckee e al. [
49
]) based on p ecipi a ion and he
A mosphe e 2021,12, 165 3 o 22
s anda dized e apo a ion p ecipi a ion index (SPEI; see Vicen e-Se ano e al. [
50
]) based on
he di e ence be ween p ecipi a ion and AED. In pa icula , he SPI and SPEI also p o ed o
be obus me ics o iden i ying lash d ough s [
51
]. Recen ly, o he s anda dized d ough
indices, such as he e apo a i e demand d ough index (EDDI; see Hobbins e al. [
52
]) based
exclusi ely on AED, was de eloped and ecommended o lash d ough analysis [
19
].
The EDDI quan i y d ough se e i y as AED inc eases unde wa e -limi ed condi ions,
which is e y use ul du ing pe iods o low p ecipi a ion o soil mois u e and impo an
land–a mosphe e coupling [17].
The SPI and SPEI we e calcula ed using pa ame ic app oaches, i ing he da a o
Gamma and Log-logis ic dis ibu ions, espec i ely. Mo eo e , he EDDI was calcula ed
by a pa ame ic app oach based on Log-logis ic dis ibu ion [
53
]. The SPI, EDDI, and
SPEI a e compa able in ime and space and can be calcula ed on di e en ime scales
o adap he esponse o hyd ological, ag icul u al, and en i onmen al sys ems o he
clima e a iabili y [
54
]. Howe e , he sho e scales be e cap u e he apid a ia ions in
p ecipi a ion and/o a mosphe ic e apo a i e demand ha can igge a lash d ough ,
since he anomalies accumula ed o e pas clima e condi ions do no a ec he index alues.
The e o e, we calcula ed he SPI, EDDI, and SPEI a a sho ime scale (1-mon h) and a
weekly equency o each g id poin o he clima e da ase om 1961 o 2018 (see de ails in
Nogue a e al. [18]).
2.3. Iden i ying Flash D ough s Based on S anda dized D ough Indices: SPI, EDDI, and SPEI
Flashd ough e en swe eiden i ied ollowing heme hodologyp oposedbyNogue ae al.[
18
].
The o iginal app oach was based on he SPEI a a sho ime scale (1-mon h) and high-
equency da a (weekly) o iden i y he onse o lash d ough episodes. This me hod
ocuses on he apid de elopmen cha ac e is ic o lash d ough s, which esul s in a
sudden, e y sha p d op in he index alues. Thus, lash d ough is de ined as: (i) a
minimum leng h o ou weeks in he de elopmen phase; (ii) an
∆
SPEI (in 4 weeks) equal
o o less han
−
2 z-uni s; and (iii) a inal SPEI alue equal o o less han
−
1.28 z-uni s
(i.e., a 10-yea e u n pe iod). This me hod was used he e o iden i y lash d ough s based
on he h ee s anda dized d ough indices: SPI, EDDI, and SPEI.
2.4. Compa ison o he SPI, EDDI, and SPEI
We analyzed he spa ial and empo al a iabili y o he SPI, EDDI, and SPEI by
compa ing he spa ial and empo al beha io o he alues o each index in each mon h
o he yea . Pea son’s co ela ion coe icien (95% con idence le el) was used o examine
he ela ionship be ween SPI, EDDI, and SPEI se ies o e ime and space. Gi en ha each
weekly da a is calcula ed based on he p ecipi a ion, AED, o clima e balance (D=P
−
AED)
da a accumula ed o e ou weeks (co esponding o a 1-mon h ime scale), we used he
weekly da a o he las week o each mon h o calcula e he co ela ion be ween he indices
in o de o e lec he a iabili y o e he whole mon h. The ela ionship be ween he
equency o annual and seasonal lash d ough se ies eco ded by he SPI, EDDI, and
SPEI o each g id poin om 1961 o 2018 was also examined by means o Pea son’s
co ela ion coe icien (95% con idence le el). In addi ion, we analyzed he ela ionship
be ween he o al equency o lash d ough s iden i ied by each index and he associa ed
signi icance. The signi icance o Pea son’s coe icien s was es ima ed using a Mon e Ca lo
app oach, in which he o al numbe o lash d ough s eco ded by SPI, EDDI, and SPEI
was co ela ed in 1000 andom samples o 30 poin s om he en i e da ase a annual and
seasonal scales. We also examined he end o annual and seasonal lash d ough se ies
eco ded by he non-pa ame ic Mann–Kendall s a is ic. Au oco ela ion was included
in he end analysis by he modi ied Mann–Kendall end es , which e u ned co ec ed
p- alues a e accoun ing o empo al pseudo eplica ion [
55
,
56
]. To assess he magni ude
o change in he equencies o lash d ough s, we used a linea eg ession analysis be ween
he se ies o ime (independen a iable) and he seasonal and annual se ies o d ough
equencies (dependen a iable). The slope o he eg ession indica ed he amoun o
A mosphe e 2021,12, 165 4 o 22
change in he numbe o e en s pe yea , wi h highe slope alues indica ing g ea e
a ia ion. In o de o iden i y changes be ween he equencies o lash d ough s eco ded
o each index, we also calcula ed he di e ences (e en s/ o each g id poin ) be ween he
lash d ough se ies ob ained h ough he SPI, EDDI, and SPEI annually and seasonally
and also examined hei ends using he non-pa ame ic Mann–Kendall s a is ic.
2.5. E alua ion o Sensi i i y o Flash D ough s o AED
To analyze he ole o p ecipi a ion de ici s and a mosphe ic e apo a i e demand
(AED) posi i e anomalies in he de elopmen o lash d ough e en s in Spain, we cal-
cula ed he sensi i i y o SPEI o he AED a a sho ime scale (1-mon h) and compa ed
i wi h he o al equency o lash d ough e en s eco ded in each mon h o he yea
in he pe iod om 1961 o 2018. The sensi i i y o SPEI o AED di e s be ween clima e
condi ions [
9
,
17
]. The SPEI is based on s anda diza ion o he clima e balance (D) esul ing
om di e ences be ween p ecipi a ion and AED (D=P
−
AED), making i possible o
quan i y he con ibu ion o p ecipi a ion and AED o he a iabili y o SPEI alues, ollow-
ing he me hodology p oposed by Tomas-Bu gue a e al. [
9
]. Thus, using he p ecipi a ion
and AED se ies employed o compu e SPEI, we calcula ed he pa ial de i a i es o he
clima e balance (D) o de e mina e he ela i e con ibu ion o bo h a iables in each mon h
o e he pe iod 1961–2018. The se ies o p ecipi a ion and AED we e de ended p io o
making he analysis o a oid he possible e ec s o ends on he esul s (see mo e de ails
in Tomas-Bu gue a e al. [9]).
3. Resul s
3.1. Spa ial and Tempo al Va iabili y o he SPI, EDDI, and SPEI
In gene al, he indices p esen high in e annual a iabili y in all mon hs (Figu e 1).
The SPI and EDDI show non-signi ican co ela ion om No embe o Janua y. On he
con a y, he e is a signi ican co ela ion be ween he SPI and EDDI om Ma ch o June
and Sep embe –Oc obe (Pea son’s > 0.7). Feb ua y, July, and Augus e u ned lowe
co ela ion alues, al hough he e was a signi ican co ela ion. Co ela ion be ween he
EDDI and SPEI was also low om No embe o Feb ua y, al hough wi h sligh ly highe
co ela ion alues han hose ound be ween he SPI and EDDI. F om Ma ch o Oc obe ,
he e was a high co ela ion be ween EDDI and SPEI, eaching a maximum in May, June,
and July (Pea son’s
≥
0.9). Co ela ion be ween he SPI and SPEI was e y high and
signi ican in all mon hs o he yea , al hough i was sligh ly lowe in July and Augus .
The spa ial pa e n o he co ela ion and associa ed signi icance be ween mon hly
se ies o he SPI, EDDI, and SPEI o e he pe iod 1961–2018 also show some ele an
di e ences (Figu e 2). In gene al, he e is a low, non-signi ican co ela ion be ween he
SPI and EDDI in no hwes e n egions om No embe o Janua y, while i is high and
signi ican in he Medi e anean coas land and sou he n Spain. F om Feb ua y o June,
also in Sep embe and Oc obe , he e is a high, signi ican co ela ion be ween SPI and
EDDI. In con as , co ela ion be ween he SPI and EDDI was low and non-signi ican in
July and Augus in la ge a eas o sou he n Spain. Co ela ion be ween he EDDI and SPEI
was also low and non-signi ican in no hwes e n egions, bu i was high and signi ican
in sou he n and eas e n Spain om No embe o Janua y. F om Feb ua y o Oc obe ,
he e was a signi ican co ela ion be ween he EDDI and SPEI o e mos o he s udy a ea,
eaching a maximum om May o July. As expec ed, he e was e y high and signi ican
co ela ion be ween he SPI and SPEI in e e y mon h o e almos all o he s udy a ea, wi h
only some a eas in sou he n Spain, whe e p ecipi a ion is e y low in summe , e u ning
low alues in July and Augus .
A mosphe e 2021,12, 165 5 o 22
A mosphe e 2021, 17, x FOR PEER REVIEW 5 o 24
Field Code Changed
Field Code Changed
and signi ican co ela ion be ween he SPI and SPEI in e e y mon h o e almos all o
he s udy a ea, wi h only some a eas in sou he n Spain, whe e p ecipi a ion is e y low
in summe , e u ning low alues in July and Augus .
Figu e 1. Tempo al e olu ion o mon hly se ies o he s anda dized p ecipi a ion index (SPI), e apo a i e demand
d ough index (EDDI), and s anda dized e apo a ion p ecipi a ion index (SPEI) based on a e age da a on mainland
Spain and he Balea ic Islands o e he pe iod 1961–2018 a a sho ime scale (1-mon h). The mon hly se ies includes he
weekly da a o he las week o each mon h in each yea .
Figu e 2. Spa ial pa e n o he co ela ion (Pea son’s ) and associa ed signi icance be ween mon hly se ies o he SPI,
EDDI, and SPEI o e he pe iod 1961–2018. The mon hly se ies include he weekly da a o he las week o each mon h
in each yea .
3.2. Spa ial Dis ibu ion and T ends in Flash D ough
Figu e 3 shows he spa ial dis ibu ion and equency o lash d ough s (e en s/ o
each g id poin ) iden i ied by he SPI, EDDI, and SPEI om 1961 o 2018 a annual and
seasonal scales. The e we e no able di e ences in he spa ial dis ibu ion o lash
d ough s iden i ied by he SPI, EDDI, and SPEI (Figu e 3a), as well as in he a e age e-
quency o e en s eco ded o each index (Figu e 3b). The a e age equency o lash
d ough s ob ained by he SPI (≈70 e en s/ o each g id poin ) was conside ably highe
han o he EDDI and SPEI (≈40 e en s/ o each g id poin ) a an annual scale. In gen-
e al, he SPI iden i ied a high occu ence o lash d ough s in mos o he s udy a ea a
Figu e 1.
Tempo al e olu ion o mon hly se ies o he s anda dized p ecipi a ion index (SPI), e apo a i e demand d ough
index (EDDI), and s anda dized e apo a ion p ecipi a ion index (SPEI) based on a e age da a on mainland Spain and he
Balea ic Islands o e he pe iod 1961–2018 a a sho ime scale (1-mon h). The mon hly se ies includes he weekly da a o
he las week o each mon h in each yea .
A mosphe e2021,17,xFORPEERREVIEW5o 24

andsigni ican co ela ionbe ween heSPIandSPEIine e ymon ho e almos allo 
hes udya ea,wi honlysomea easinsou he nSpain,whe ep ecipi a ionis e ylow
insumme , e u ninglow aluesinJulyandAugus .

Figu e1.Tempo ale olu iono mon hlyse ieso  hes anda dizedp ecipi a ionindex(SPI),e apo a i edemand
d ough index(EDDI),ands anda dizede apo a ionp ecipi a ionindex(SPEI)basedona e ageda aonmainland
Spainand heBalea icIslandso e  hepe iod1961–2018a asho  imescale(1‐mon h).Themon hlyse iesincludes he
weeklyda a o  helas weeko eachmon hineachyea .

Figu e 2.
Spa ial pa e n o he co ela ion (Pea son’s ) and associa ed signi icance be ween mon hly se ies o he SPI,
EDDI, and SPEI o e he pe iod 1961–2018. The mon hly se ies include he weekly da a o he las week o each mon h in
each yea .

A mosphe e 2021,12, 165 6 o 22
3.2. Spa ial Dis ibu ion and T ends in Flash D ough
Figu e 3shows he spa ial dis ibu ion and equency o lash d ough s (e en s/ o
each g id poin ) iden i ied by he SPI, EDDI, and SPEI om 1961 o 2018 a annual and
seasonal scales. The e we e no able di e ences in he spa ial dis ibu ion o lash d ough s
iden i ied by he SPI, EDDI, and SPEI (Figu e 3a), as well as in he a e age equency
o e en s eco ded o each index (Figu e 3b). The a e age equency o lash d ough s
ob ained by he SPI (
≈
70 e en s/ o each g id poin ) was conside ably highe han o he
EDDI and SPEI (
≈
40 e en s/ o each g id poin ) a an annual scale. In gene al, he SPI
iden i ied a high occu ence o lash d ough s in mos o he s udy a ea a an annual scale,
which was also no ed in some a eas o cen al and no heas e n Spain, while he Medi e -
anean coas land eco ded he lowes numbe o e en s. The EDDI and SPEI eco ded he
highes numbe o lash d ough e en s in no he n and no hwes e n Spain, al hough he
EDDI also eco ded a g ea many in some a eas o he Medi e anean coas land.
A a seasonal scale, he e we e also spa ial di e ences be ween pa e ns o he a e age
equency o lash d ough s eco ded by he SPI, EDDI, and SPEI. In win e , simila spa ial
pa e ns we e ound o SPI and SPEI, wi h he highes occu ence ound in he no he n
and eas e n egions o he Ibe ian Peninsula and in he Balea ic Islands. Howe e , he
a e age equency o lash d ough s iden i ied by he SPI (
≈
16 e en s/ o each g id poin )
was subs an ially highe han by he SPEI, which eco ded he lowes a e age equency
in his season (
≈
9 e en s/ o each g id poin ). On he o he hand, he EDDI eco ded a
g ea many lash d ough s in la ge a eas o cen al Spain in win e , eaching i s maximum
a e age equency o he season, wi h app oxima ely 13 e en s/ o each g id poin . In
sp ing, he spa ial dis ibu ion and a e age equency o lash d ough s eco ded by he
SPI showed g ea dispa i ies wi h he EDDI and SPEI. The a e age equency o he lash
d ough s eco ded by he SPI (
≈
17 e en s/ o each g id poin ) was conside ably highe
han by he EDDI and SPEI (
≈
10 e en s/ o each g id poin ). The SPI eco ded a high
incidence o lash d ough s in sp ing in mos o cen al and wes e n Spain, and also in some
a eas o he Py enees. In con as , he EDDI eco ded a low numbe o lash d ough s in
hose egions, and mo e e en s we e ound in ce ain a eas o he Medi e anean coas land
and no he n and no hwes e n Spain. Simila ly, he SPEI iden i ied a high occu ence
o lash d ough e en s in no he n and no hwes e n egions bu also in la ge a eas o
sou he n Spain. In summe , he SPI and EDDI eco ded hei lowes a e age equency,
while he SPEI eached i s maximum a e age equency (
≈
12 e en s/ o each g id poin ).
The e is no clea spa ial pa e n o lash d ough iden i ied by he SPI in summe , wi h
a high numbe o lash d ough s occu ing in cen al, no hwes e n, and no heas e n
Spain. The EDDI iden i ied he highes numbe o lash d ough s in no he n and wes e n
egions, while cen al and eas e n Spain had ew e en s. In gene al, he SPEI eco ded
a high equency o lash d ough e en s in summe , wi h wide a eas o he sou h and
no hwes exceeding 15 e en s/ o each g id poin . In au umn, he pa e n o spa ial
dis ibu ion o lash d ough s iden i ied by he SPI and SPEI was simila in no he n Spain,
and bo h eco ded a low numbe o e en s in he Medi e anean coas land. Howe e , he
SPI also iden i ied a high numbe o e en s in la ge a eas o cen al Spain, eaching i s
maximum a e age equency in his season wi h app oxima ely 19 e en s/ o each g id
poin . Unlike he SPI and SPEI, he EDDI eco ded he highes incidence o lash d ough s
in he Medi e anean coas land in au umn, bu i only eco ded a ew in wes e n Spain.
In gene al, he e was non-signi ican spa ial co ela ion be ween he o al numbe o
e en s iden i ied o each index and he sha ed a iance was no high, due o no able spa ial
di e ences annually and seasonally (Figu e 4). A an annual scale, he e was a nega i e
and non-signi ican co ela ion be ween he spa ial dis ibu ion o he o al numbe o lash
d ough s eco ded by he SPI and EDDI. The highes co ela ion was ound be ween he
EDDI and SPEI a annual scale, al hough i was non-signi ican . The SPI and SPEI also
e u ned a posi i e, bu non-signi ican , co ela ion annually.
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Figu e3.Annualandseasonal(a)spa ialdis ibu ion o  he equencyo  lashd ough s(e en s/ o eachg idpoin )
iden i iedby heSPI,EDDI,andSPEIo e  hepe iod1961–2018,(b)boxplo ssumma izing heannualandseasonal
Figu e 3.
Annual and seasonal (
a
) spa ial dis ibu ion o he equency o lash d ough s (e en s/ o each g id poin )
iden i ied by he SPI, EDDI, and SPEI o e he pe iod 1961–2018, (
b
) box plo s summa izing he annual and seasonal
equencies ob ained wi h he di e en indices. In he box plo , he le axis ep esen s he annual da a and he igh axis
ep esen s he seasonal da a.
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Figu e4.Sca e plo sshowing heannualandseasonal ela ionshipbe ween he o alnumbe o 
lashd ough e en s eco dedby heSPI,EDDI,andSPEI.Thecolo s ep esen  hedensi yo 
poin s,wi h eddeno ing hehighes densi y.
Figu e 4.
Sca e plo s showing he annual and seasonal ela ionship be ween he o al numbe o lash d ough e en s
eco ded by he SPI, EDDI, and SPEI. The colo s ep esen he densi y o poin s, wi h ed deno ing he highes densi y.
A mosphe e 2021,12, 165 9 o 22
A a seasonal scale, he e was a nega i e and non-signi ican co ela ion be ween he
spa ial dis ibu ion o he o al numbe o lash d ough s eco ded by he SPI and he EDDI
in all seasons, excep in win e . Co ela ion be ween he EDDI and SPEI in win e was
nega i e and non-signi ican , while i was posi i e and non-signi ican in sp ing, summe ,
and au umn. The highes co ela ion be ween he EDDI and SPEI was ound in summe
(Pea son’s = 0.33). The spa ial dis ibu ion o he o al numbe o lash d ough s eco ded
by he SPI and SPEI showed a posi i e co ela ion in all seasons, eaching he maximum
alues in win e (Pea son’s = 0.58) and au umn (Pea son’s = 0.41), al hough i was only
signi ican in win e .
Figu e 5depic s he spa ial dis ibu ion o he magni ude o change and signi icance o
ends in annual and seasonal equency o lash d ough s eco ded by he SPI, EDDI, and
SPEI o e he pe iod 1961–2018. A an annual scale, he e was a signi ican decline in he
numbe o lash d ough s iden i ied by he SPI ac oss wide a eas o cen al Spain, and only
small a eas o he Medi e anean coas and no he n Spain showed signi ican inc eases
in lash d ough s. The EDDI also eco ded a dec ease in lash d ough s in some a eas
o cen al Spain, bu his decline was gene ally non-signi ican . In con as , he e was a
signi ican inc ease in he equency o lash d ough e en s iden i ied by he EDDI in some
a eas o sou he n and no hwes e n Spain. The SPEI also iden i ied signi ican inc eases in
lash d ough s in sou he n Spain annually, as well as in some a eas o he Medi e anean
coas land. Simila o he SPI and EDDI, he SPEI also eco ded a gene al dec ease in lash
d ough s in cen al and no he n Spain, al hough his was only s a is ically signi ican in a
ew a eas.
A a seasonal scale, mos o he s udy a ea showed non-signi ican ends in he
occu ence o lash d ough s eco ded by he SPI, EDDI, and SPEI o e he pe iod 1961–
2018. The h ee indices iden i ied a gene al decline in lash d ough e en s in cen al
Spain in win e , al hough only he SPI and EDDI ound s a is ically signi ican dec eases
in some o hese a eas. In sp ing, simila ends we e ound by he SPI, EDDI, and SPEI,
wi h nega i e and non-signi ican ends in mos o he s udy a ea. Fu he mo e, he
h ee indices eco ded inc eases in he equency o lash d ough s in he Medi e anean
coas land and Balea ic Islands in his season, bu only he SPEI and he SPI showed posi i e
and s a is ically signi ican ends. In summe , he e we e no able di e ences in lash
d ough ends eco ded by he SPI, EDDI, and SPEI. The SPI iden i ied nega i e, non-
signi ican ends in mos o he s udy a ea, while he EDDI and SPEI showed a gene al
inc ease in lash d ough s. The SPEI iden i ied some signi ican inc eases in lash d ough s
in sou he n egions in summe , while he EDDI eco ded s a is ically signi ican inc eases
in a ew small a eas o wes e n Spain. In au umn, he SPI e u ned a gene al dec ease in
lash d ough s in mos o he s udy a ea, al hough his was only signi ican in no heas e n
Spain. On he con a y, he EDDI and SPEI iden i ied posi i e, non-signi ican ends in
he occu ence o lash d ough in mos o he s udy a ea, wi h he excep ion o no he n
egions, whe e nega i e, non-signi ican ends we e no ed.
The empo al e olu ion o he a e age equency o lash d ough e en s iden i ied by
he SPI, EDDI, and SPEI on mainland Spain and he Balea ic Islands o e he pe iod 1961–
2018 showed a high a iabili y a annual and seasonal scales (Figu e 6). The annual se ies
o lash d ough s eco ded by he SPI, EDDI, and SPEI exhibi ed s a is ically signi ican
co ela ions among hem, al hough he a e age equency o lash d ough s iden i ied by
he SPI was subs an ially highe un il he yea 2000. The e was a signi ican decline in lash
d ough s eco ded by he SPI annually, while he annual se ies ob ained by he SPEI and
EDDI showed non-signi ican ends.
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Figu e9.Mon hly equencyo  lashd ough e en s/ o eachg idpoin ob ained om heSPEI
andi ssensi i i y(%) oAEDonmainlandSpainand heBalea icIslandsineachmon ho e  he
pe iod1961–2018a asho  imescale(1‐mon h).F equencieso  lashd ough e en swi ha e‐
sidualnumbe o casesa eno  ep esen ed.
4.Discussion
In hiss udy,weiden i ied lashd ough sbasedondi e en d ough indicesin‐
cluding heSPI, heEDDI,and heSPEI.Thisenabled he oleo p ecipi a ionandAED
in hede elopmen o  lashd ough s obees ablished,inaddi ion ohow his olede‐
e mines hespa ialand empo alpa e nso  lashd ough sinSpain.Themainad‐
an ageo  heses anda dizedindexesincompa ison oo he me icsis ha  heya e
Figu e 9.
Mon hly equency o lash d ough e en s/ o each g id poin ob ained om he SPEI and i s sensi i i y (%) o
AED on mainland Spain and he Balea ic Islands in each mon h o e he pe iod 1961–2018 a a sho ime scale (1-mon h).
F equencies o lash d ough e en s wi h a esidual numbe o cases a e no ep esen ed.
4. Discussion
In his s udy, we iden i ied lash d ough s based on di e en d ough indices including
he SPI, he EDDI, and he SPEI. This enabled he ole o p ecipi a ion and AED in he
de elopmen o lash d ough s o be es ablished, in addi ion o how his ole de e mines
he spa ial and empo al pa e ns o lash d ough s in Spain. The main ad an age o
hese s anda dized indexes in compa ison o o he me ics is ha hey a e compa able

A mosphe e 2021,12, 165 17 o 22
in ime and space, making i possible o apply he same me hodology and o ob ain
compa able esul s based on indices om di e en a iables. Fu he mo e, hese indices
also enable lash d ough analyses in he long- e m, since hey a e based on widely a ailable
clima e in o ma ion.
Nume ous s udies p o ed he use ulness o widely used, s anda dized d ough in-
dices such as he SPI o SPEI in iden i ying di e en ypes o impac s [
57
–
60
]. P e ious
s udies also p o ed he obus ness o s anda dized d ough indices, such as he SPI, EDDI,
and SPEI in iden i ying and cha ac e izing lash d ough s o e di e en egions o he
wo ld
[18,19,51,61].
Al hough i is no easy o de e mine which me ic is mo e sui able o
he analysis o lash d ough s, we hink ha he implemen a ion o di e en indices o
lash d ough analysis can be use ul o a mo e comp ehensi e unde s anding.
The spa ial and empo al beha io o lash d ough s in Spain is highly complex and
a iable [
18
]. The esul s ob ained in his esea ch sugges ed ha lash d ough s in Spain
can de elop om bo h p ecipi a ion de ici s and anomalous inc eases in AED, al hough
he e is a di e en ia ed seasonal and spa ial beha io . This is e lec ed in he no able
di e ences ound be ween he spa ial and empo al pa e ns o lash d ough s iden i ied
by he SPI, EDDI, and SPEI. Thus, e en when he d ough indices showed high spa ial and
empo al co ela ion in mos mon hs o he yea , he spa ial and empo al equency o
lash d ough e en s was dependen on he index used, since al hough he e is consis ency
among he h ee indices, he e a e la ge seasonal di e ences. Fo example, in win e , in
which AED is low and d ough condi ions a e closely ela ed o p ecipi a ion de ici s,
co ela ion be ween he EDDI and SPI is low. The e o e, i is no ecommended o use he
EDDI o assess d ough se e i y and, pa icula ly, o iden i y lash d ough s du ing win e .
In au umn and sp ing, when he AED is sligh ly highe bu p ecipi a ion is also he main
d i e con olling a ia ions in index alues, co ela ion o he EDDI wi h he SPI and SPEI
was gene ally high, especially wi h he SPEI. This sugges s an inc ease in he in luence o
he AED on d ough se e i y. In summe , he e is highe consis ency be ween he EDDI
and SPEI, wi h co ela ions simila o hose ound be ween he SPI and SPEI. This means
ha changes in AED can be as ele an o e en mo e so han p ecipi a ion de ici s in he
esponse o indices du ing he summe , pa icula ly in he d ies a eas. This indica es ha
in summe , he use o indices based exclusi ely on p ecipi a ion is no sui able o assess
d ough se e i y, and hey a e pa icula ly unsui able o iden i y lash d ough s. The SPI
showed a high spa ial and empo al cohe ence wi h he SPEI in almos e e y mon h o he
yea , e lec ing he ac ha he SPEI esponds mainly o a ia ions in p ecipi a ion, excep
in he d y summe pe iod. These indings p o e he sensi i i y o he SPEI o changes in
AED du ing d y pe iods, and hey a e e y consis en wi h he pa e ns in he esponse o
he SPEI o AED shown by Tomas-Bu gue a e al. [9] a a global scale.
In gene al, he e a e no iceable spa ial di e ences in he occu ence o lash d ough s
iden i ied o each index. The spa ial pa e ns eco ded by he SPI and SPEI we e s ongly
cohe en in win e , pa icula ly in no he n Spain. This sugges s ha du ing we pe iods,
in which he ole o p ecipi a ion de ici s in d ough de elopmen is no in doub [
17
],
p ecipi a ion is he main d i e o lash d ough s. Howe e , he ole o he a mosphe ic
e apo a i e demand (AED) on d ough s is mo e complex and exhibi s la ge spa ial and
seasonal di e ences [
17
,
62
], making i di icul o de e mine i s ole in he de elopmen o
lash d ough s [
63
]. Se e al s udies showed ha lash d ough s can be d i en by anomalous
inc eases in AED associa ed wi h hea wa es and land–a mosphe e eedbacks ha cause
o ein o ce he apid deple ion o soil mois u e [
12
,
14
–
16
]. Thus, he occu ence o lash
d ough is usually associa ed wi h s ong anomalies o AED [
6
,
19
]. He e, we ha e shown
ha AED can play an impo an ole in he wa m season. In summe , when p ecipi a ion is
gene ally e y low in Spain [
40
,
64
], he e we e no able di e ences in he spa ial pa e ns o
lash d ough s ound by he SPI and SPEI. Compa ed o he SPI, he SPEI showed a g ea e
spa ial consis ency wi h he EDDI, indica ing ha du ing he wa m season, he AED is a key
a iable in explaining he occu ence o lash d ough s. The physical p ocesses explaining
he impo ance o his a iable can be di e se. Some s udies showed ha in pe iods o
A mosphe e 2021,12, 165 18 o 22
e y low soil mois u e and s ong land–a mosphe e coupling, AED would be d i en by he
limi ed la en hea luxes om a d y soil, which could a o some sel -in ensi ica ion o
d ough condi ions [
10
,
62
]. Howe e , in Spain, i is mo e p obably due o he dominan
ole o wa m, d y ai ad ec ions o igina ing in he Saha a, which a e e y equen in
summe [65].
In sp ing and au umn, opposi e spa ial pa e ns we e ound o he lash d ough s iden-
i ied by he SPI and EDDI. This could be ela ed o a a iable con ibu ion o p ecipi a ion
and AED in he de elopmen o he lash d ough s, which seems easonable conside ing
he wide spa ial and empo al a iabili y o p ecipi a ion [
32
–
35
] and AED
[36–38]
du ing
hese seasons. Thus, i is possible ha AED plays an impo an ole in he occu ence o
lash d ough s in he Medi e anean coas land and no heas e n Spain whe e he EDDI
eco ded a high incidence in sp ing and, o a lesse ex en , in au umn. Howe e , i would
be seconda y o p ecipi a ion in mos o Spain. This is also e lec ed in he spa ial pa e ns
o he numbe o lash d ough s iden i ied by he SPEI, since he e is spa ial consis ency
wi h he SPI in mos o he s udy a ea in sp ing and especially in au umn, bu he SPEI
also epo ed a high equency o lash d ough s in he Medi e anean coas land in sp ing,
which is a pa e n also ound in he EDDI. Al hough in some cases, he di e en indices
showed simila spa ial pa e ns a an annual and seasonal scale, he sha ed a iance was
no gene ally high. This seems o con i m ha he lash d ough s in Spain can be igge ed
by di e en d i e s, as well as he ole o p ecipi a ion and AED being seasonally and
spa ially a iable.
Only he SPI epo ed a nega i e and signi ican end in he equency o lash
d ough s, whe eas he EDDI and SPEI eco ded a non-signi ican end o hese e en s
om 1961 o 2018. Howe e , he EDDI and SPEI epo ed signi ican inc eases in he
equency o lash d ough s in some a eas o sou he n and sou heas e n Spain. Since
he SPI epo ed non-signi ican ends in summe o e mos o he s udy a ea, hese
inc eases mus necessa ily be ela ed o an inc ease in he con ibu ion o he AED o he
de elopmen o lash d ough s in hese a eas. This hypo hesis seems cohe en wi h a ious
s udies ha eco ded inc eased AED in Spain du ing he summe [
36
,
38
]. Some a eas o he
Medi e anean coas land also showed inc eases in lash d ough s in sp ing, bu conside ing
ha only he SPI and SPEI epo ed a s a is ically signi ican end, hese inc eases could
ela e o a ia ions in p ecipi a ion.
In addi ion o he ma kedly seasonal cha ac e indica ing he ole o AED in he
de elopmen o lash d ough s, we also ound a s ong spa ial componen de e mining
he ole o AED in hese e en s in Spain. The AED shows s ong spa ial di e ences in
Spain, wi h a clea no h–sou h g adien be ween he humid egions o he no h and he
d ie egions o sou he n and cen al Spain [
36
,
39
]. Se e al s udies sugges ed ha he
con ibu ion o AED is much highe in d y egions han in humid ones, since i s ole is only
impo an du ing pe iods o low p ecipi a ion o limi ed soil mois u e [
9
,
17
]. This could
explain he con as s ound be ween he d ie egions o cen al and sou he n Spain and he
humid ones in no he n Spain, which, e en in summe , exhibi ed e y low sensi i i y o
AED. Thus, du ing summe , when he AED in Spain is high and p ecipi a ion is gene ally
e y low [
40
], s ong AED anomalies play a key ole in he de elopmen o lash d ough s,
especially in d y a eas such as he Eb o Dep ession and sou he n Spain. Al hough he
inc eased in AED was p obably he main d i e o changes in lash d ough equency
in summe , we mus s ess ha we ha e demons a ed ha p ecipi a ion is s ill he main
d i e in he empo al a iabili y o d ough s in Spain, and i is also he main con ibu ing
ac o in igge ing lash d ough s. P ecipi a ion de ici s a e he majo clima ic a iable
igge ing d ough condi ions [
66
,
67
], and i seems easonable o expec ha p ecipi a ion
also has an essen ial ole in he de elopmen o lash d ough s. We ha e demons a ed his
pa e n in win e as well as in sp ing and au umn. Howe e , i is possible ha he AED has
a ele an ole in he de elopmen o ce ain lash d ough e en s in he la e sp ing.
The esul s o his esea ch canno be easily ex apola ed. Spain is cha ac e ized by
high spa ial and seasonal a iabili y in p ecipi a ion and AED, so i was expec ed ha
A mosphe e 2021,12, 165 19 o 22
hei ole in he de elopmen o lash d ough s would be s ongly a iable. Howe e ,
he ole o p ecipi a ion and AED anomalies may exhibi signi ican changes in o he
egions o he wo ld, and lash d ough s could de elop unde di e se condi ions. Fo
example, Hobbins e al. [
52
] and Pende g ass e al. [
19
] poin ed ou ha he occu ence
o lash d ough s is ypically ela ed o a change om ene gy-limi ed o wa e -limi ed
condi ions. This can be ue in some cases, bu i canno be conside ed as a si ua ion
cha ac e is ic o mos lash d ough e en s wo ldwide. A a global scale, p e ious s udies
ound simila pa e ns in he con ibu ion o AED du ing pe iods cha ac e ized by low
le els o p ecipi a ion in sub-humid egions [
9
]. Speci ically, ce ain s udies also indica ed
ha p ecipi a ion is he main a iable explaining he occu ence o lash d ough s in he
Uni ed S a es [
63
]. Howe e , conside ing he complexi y o he ole played by AED in
d ough de elopmen , and i s spa ial and seasonal a iabili y [
17
], u he esea ch in o
hese issues is needed.
5. Conclusions
This s udy ocused on he analysis o he ole o p ecipi a ion and a mosphe ic e apo-
a i e demand (AED) in he occu ence o lash d ough s in Spain. Fo his pu pose, we
analyzed he spa ial and empo al pa e ns o lash d ough iden i ied h ough di e en
d ough indices based on p ecipi a ion (SPI), AED (EDDI), and bo h (SPEI). We also exam-
ined he sensi i i y o he SPEI o AED o cla i y he possible con ibu ion o p ecipi a ion
de ici s and anomalous inc eases in AED o he de elopmen o lash d ough s. The main
conclusions om his s udy a e as ollows:
•
S anda dized d ough indices such as SPI, EDDI, and SPEI a e obus me ics o he
iden i ica ion o lash d ough s. Howe e , he use o indices based exclusi ely on
p ecipi a ion o AED may ha e some limi a ions unde ce ain ci cums ances.
•
The spa ial and empo al pa e ns o lash d ough s can be highly a iable, depending
on he me ics used in analysis.
•
Flash d ough s in Spain can be igge ed by bo h p ecipi a ion de ici s and inc eases
in AED, bu hei con ibu ion o he de elopmen o lash d ough s is highly a iable
spa ially and seasonally.
•
P ecipi a ion is he main a iable d i ing lash d ough s in Spain, al hough AED
anomalies can play a c ucial ole in he de elopmen o some lash d ough e en s,
especially in a id a eas du ing he wa m season.
•
The sensi i i y o he SPEI o AED du ing d y pe iods enables he d ough condi ions
igge ed by anomalous dec eases o p ecipi a ion and/o inc eases o he AED o
be cap u ed, making i possible o iden i y and cha ac e ize lash d ough s o e e y
di e en clima ic condi ions seasonally and spa ially.
Supplemen a y Ma e ials:
The ollowing a e a ailable online a h ps://www.mdpi.com/2073-443
3/12/2/165/s1, Figu e S1: Tempo al e olu ion o he annual and seasonal di e ences (e en s/ o
each g id poin ) be ween he lash d ough se ies eco ded by he SPI, EDDI and SPEI on mainland
Spain and he Balea ic Islands o e he pe iod 1961–2018, Figu e S2: Mon hly equency o he
sensi i i y (%) o he SPEI o AED on mainland Spain and he Balea ic Islands o e he pe iod
1961–2018 a a sho ime scale (1-mon h). The mon hly se ies include he weekly da a o he las
week o each mon h in each yea .
Au ho Con ibu ions:
Da a cu a ion, I.N., F.D.-C. and S.M.V.-S; Fo mal analysis, I.N., F.D.-C. and
S.M.V.-S.; Funding acquisi ion, S.M.V.-S.; So wa e, I.N.; Supe ision, I.N., F.D.-C. and S.M.V.-S.;
W i ing—o iginal d a , I.N., F.D.-C. and S.M.V.-S; W i ing— e iew & edi ing, I.N., F.D.-C. and
S.M.V.-S. All au ho s ha e ead and ag eed o he published e sion o he manusc ip .
Funding:
Resea ch p ojec s CGL2017-82216-R, PCI2019-103631, and PID2019-108589RA-I00 inanced
by he Spanish Commission o Science and Technology and FEDER; CROSSDRO p ojec inanced
by he AXIS (Assessmen o C oss(X)—sec o al clima e Impac s and pa hways o Sus ainable
ans o ma ion), JPI-Clima e co- unded call o he Eu opean Commission and INDECIS which is pa
A mosphe e 2021,12, 165 20 o 22
o ERA4CS, an ERA-NET ini ia ed by JPI Clima e, and unded by FORMAS (SE), DLR (DE), BMWFW
(AT), IFD (DK), MINECO (ES), ANR (FR) wi h co- unding by he Eu opean Union (G an 690462).
Ins i u ional Re iew Boa d S a emen : No applicable.
In o med Consen S a emen : No applicable.
Da a A ailabili y S a emen : Da a used in his s udy is a ailable unde eques .
Acknowledgmen s:
The i s au ho would like o hank he p edoc o al ellowship “Sub enciones
des inadas a la con a ación de pe sonal in es igado p edoc o al en o mación (2017)” awa ded by
he Gobie no de A agón (Go e nmen o A agón, Spain).
Con lic s o In e es : The au ho s decla e no con lic o in e es .
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