|
1. Definition
| Name |
RAINFALL
SEASONALITY |
| Brief definition |
The temporal
distribution of rainfall (monthly) in Mediterranean landscapes |
| Unit
of measure |
Three possible units of
measure:
- Season rainfall/total
rainfall (%)
- Monthly rainfall/total
rainfall (%)
- Index: Seasonality
Index (SI) (derived by Walsh and Lawler (1981) or
Markham technique, to determine the months with most rainfall
seasonality
|
| Spatial scale |
|
| Temporal scale |
|
2. Position
within the logical framework DPSIR
3. Target and
political pertinence
| Objective |
The
objective of this indicator is to assess the importance of one
of the climatic conditions that affect desertification, the
occurrence of rainfall. |
| Importance
with respect to desertification |
Rainfall seasonality
affects the soil and vegetation characteristics in an area.
The annual distribution of rainfall can explain the vegetation
distribution and composition and the sensitivity of soil to
erosion processes. Vegetation composition and distribution depends
on the amount of water available, in addition to nutrients in
the soil (soil composition) and other climatic conditions, as
temperature and potential evapotranspiration. The measure of
the rainfall seasonality is important to set the soil capacity
to maintain water storage to be used by plants. It is also important
to determine the hydric stress that occurs in the more dry and
warm months and can determine the presence or absence of vegetation.
It is also very important when viewed in the light of runoff
generation. In semi-arid Mediterranean areas erosion processes
are determined by intense rainfall, more likely to happen when
rainfall is concentrated over a short period of time. This is
the most important process of soil erosion, especially with
high-energy rainfall at the end of the summer or beginning of
autumn. In addition, these processes and their effects can be
increased by the absence of vegetation cover, more likely to
happen in areas where the irregular rainfall distribution makes
hydric stress greater in some months, especially during the
summer. |
| International
Conventions and agreements |
The UNCCD emphasizes
that combating desertification must be tackled within the general
framework of actions to promote sustainable development. |
| Secondary
objectives of the indicator |
Contribution
to the definition and mapping of ESAs and evaluation of the
desertification risk of an area. |
4. Methodological
description and basic definitions
| Definitions
and basic concepts |
Rainfall
seasonality can be defined as the irregular distribution of
rainfall during a normal year. This irregular distribution means
that most of the rainfall occurs in specific months (that can
be identified with specific seasons). This phenomenon is quite
usual in Mediterranean areas, where most of the rainfall occurs
in the autumn and in the spring. Normally this seasonality comes
with irregular distribution of the amount of rainfall in each
episode. Most of the rainfall quantity is accounted for in a
few violent storms. |
| Benchmarks
Indication of the values/ranges of value |
Three different ranges
of values can be used:
Season
rainfall/total rainfall (%).
From 0 to 100%. The statistical theoretic "normal"
value for each season is 25%
Monthly rainfall/total
rainfall (%). An equal distribution would assign approximately
8% to each month. The bigger the difference the more the seasonality.
In the case of using an
index there are other ranges of values:
Seasonality Index (SI),
derived by Walsh and Lawler (1981) (explanation below):
| Sii
|
Precipitation regime |
| <0.19 |
Precipitation spread
throughout the year |
| 0.20-0.39 |
Precipitation spread
throughout the year, but with a definite wetter season |
| 0.40-0.59 |
Rather seasonal with
a short drier season |
| 0.60-0.79 |
Seasonal |
| 0.80-0.99 |
Marked seasonal
with a long dry season |
| 1.00-1.19 |
Most precipitation
in <3 months |
| >1.20
|
Extreme
seasonality, with almost all precipitation in 1-2 months |
Using the Markham technique:
When P%i
is>8% a concentration of precipitation is taking place
in month I, where i=months(1…12)
|
| Methods of
measurement |
The measurement of rainfall
is standardized and the data can be easily obtained.
If Season rainfall/total
rainfall (%) or Monthly rainfall/total rainfall (%) are used
then calculations are very simple. In the case of using an
index some transformations are needed:
Seasonality Index (SI),
derived by Walsh and Lawler (1981)
The following transformation
is needed:
Where Ri
is the total annual precipitation for the particular year
under study and Xin is the actual monthly
precipitation for month n. Higher index values indicate a
great overall departure from an equal distribution of precipitation
trough the year, with near zero values indicating that there
is little or no seasonal variation in precipitation. It is
important to know that although we can calculate this index
using long -term average monthly precipitation data directly
this is not a good option since the resulting index possesses
a lower magnitude, since the process of averaging smoothes
year-to-year "noise" in the monthly precipitation
values. Alternatively, it is much more suitable to do it year
by year and then calculate the mean value for the calculated
period.
One problem with this
index is that it does not indicate when or how wetter periods
are distributed through the year. To overcome this difficulty
we can use a "replicability index" to indicate whether
or not the wettest period occurs over a small range of months
or whether it may occur in any month during the year. Higher
values of the replicability index indicate that the wettest
month of the year generally occurs in the same range of months
every year. Lower values show a distribution spread more evenly
over different months.
Markham technique
This can be used to determine
which are the months with more rainfall seasonality, using
the smoothed percentages index. In this index the median monthly
precipitation is expressed as a percentage of the monthly
rainfalls. The monthly percentages, P%, are then subjected
to a weighted smoothing by considering the months before and
after the one selected:
Where i=months(1…12)
When P%i
is>8% a concentration of precipitation is taking place
in month i.
|
| Limits of the
indicator |
The necessary
data is easily obtained and common in any meteorological data
collection. |
| Linkages with
other indicators |
Rainfall,
Rainfall erosivity, Potential
evapotranspiration, Drought
index, Aridity index,
Rainfall-runoff relationship. |
5. Evaluation
of data needs and availability
| Data
required to calculate the indicator |
Daily
or monthly rainfall |
| Data sources |
Necessary data
are usually available and accessible and the cost/benefit ratio
is reasonable. |
| Availability
of data from national and international sources |
Data can be
obtained from various local, regional, national or international
institutions involved in the collection and the analysis of
meteorological data. |
6. Institutions
that have participated in developing the indicator
| Main
institutions responsible |
University
of Murcia, Spain |
| Other contributing
organizations |
Agricultural
University of Athens, Universities of Basilicata, Amsterdam,
Leeds, Lisbon |
7. Additional
information
| Bibliography
|
Summer, G., Homar, V.,
Ramis, C., 2001. Precipitation seasonality in eastern and
southern Coastal Spain. International Journal of Climatology
21: 219-247 (2001)
Walsh PD, Lawler DM.,
1981. Rainfall seasonality: description, spatial patterns
and change through time. Weather 36: 201-208
López Bermúdez,F.;
Garcia Gomez,J. 2004: Variaciones y tendencias de las temperaturas
en Murcia durante los últimos 140 años. En Historia,
Clima y Paisaje. Estudios geográficos en memoria del
Prof. Antonio López Gómez. Universitat de València,
Universidad Autónoma de Madrid, Universitat D´Alacant.
ISBN: 84-370-5864-3., Valencia, pp.353-362Alonso Sarria,F.;
López-Bermúdez,F.,1994: "Rainfall time
and space variability during short storms in South-East Spain".
Geoökodynamik., XV, 3: 261-278.
Alonso-Sarria,F.; López-Bermúdez,F.;
Conesa-Garcia,C.,2002: Synoptic Conditions Producing Extreme
Rainfall Events along the Mediterranean Coast of the Iberian
Peninsula. In Drylands Rivers. Hydrology ad Gemorphology of
Semi-arid Channels. Edited by L.J. Bull and M.J. Kirkby. ISBN
0-471-49123-3. John Wiley & Sons. Chichester. England,
pp. 351-372.
|
| Other references |
González Hidalgo,
J.C., De Luis, M., Raventós, J. and Sánchez,
J.R., 2001 Spatial Distribution of Seasonal Rainfall trends
in a Western Mediterranean Area. International Journal of
Climatology, 21: 843-860
Garcia De Pedraza ,L.,
1971: Los torrenciales aguaceros en la cuenca mediterránea.
Calendario Meteorológico. Servicio Meteorológico
Nacional. Madrid: 160-166
ICONA, 1988: Agresividad
de la lluvia en España. Valores del factor R de la
USLE. Ministerio de Agricultura, Pesca y Alimentación.
.ICONA. Madrid, 39 99
Lopez Bermudez, F., 1990
: "El clima mediterráneo semiárido como
factor de erosión". Estudios Geográficos.,
Tomo LI, núm 199-200 C.S.I.C., Madrid. pp. 489-506
López Bermúdez,F.;
Romero Diaz, M.A.,1993: "Génesis y consecuencias
erosiva de las lluvias de alta intensidad en la Región
mediterránea". Cuadernos de Investigación
Geográfica, Vol.18-19: 7-28. Universidad de La Rioja.
Logroño, España
López Bermúdez,F.;
Garcia Gómez,J.: Desertification in the Arid and semiarid
Mediterranean Region. A Food Security Issue. In Desertification
in the Mediterranean Region. A Security Issue. J.L. Rubio,
W.Kepner, D. M. Pedrazzini, Eds. NATO - CCMS and Science Committee
(In press)López Gomez, A., 1983: Lluvias catastróficas
mediterráneas. Estudios Geográficos, 170-171:
11-29
Romero Diaz, A.; Gonzalez
Barbera, G.; López Bermúdez,F., 1995: "Relationship
between soil erosion, rainfall and vegetation cover in the
semiarid environment of South East of Iberian Peninsula".
In Erosion and Land Degradation in the Mediterranean. Proceeding
of International Geographical Union. European Commission,
DG-XII. The University of Aveiro. Portugal: 59-73
Wischmeier,W.H:, Smith,D.D.,
1959: A rainfall erosion index for a Universal Soil.-Loss
Equation. Proc. Soil. Sci .Soc. Am.,23: 246-249
|
| Contacts Name
and address |
University
of Murcia
Jorge García Gómez jorgegg@um.es
Pr. Francisco López-Bermúdez lopber@um.es |
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