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Drought Network News (1994-2001) Drought -- National Drought Mitigation Center

August 1999

A Drought Watch System for Southeast

Ramon Garrido Abenza National Institute of Meteorology, Territorial Meteorological Center of , P. O. Box 138, Espinardo. 30071 Murcia, Spain

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Garrido Abenza, Ramon, "A Drought Watch System for Southeast Spain" (1999). Drought Network News (1994-2001). 30. https://digitalcommons.unl.edu/droughtnetnews/30

This Article is brought to you for free and open access by the Drought -- National Drought Mitigation Center at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Drought Network News (1994-2001) by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. A Drought Watch System for Southeast Spain

Introduction Regarding Some Precedents

One of the main climatological characteristics of Up to now, the National Institute of Meteorol- the (11,300 km2), located almost ogy in Spain has used a method of quintiles of the entirely in the Segura Basin (in southeast Spain), is cumulative distribution function (c.d.f.) of precipita- the great temporal and spatial irregularity of its pre- tion, with normal precipitation corresponding to the cipitation. Average annual precipitation values range second and third quintiles (c.d.f. between 40% and between 200 and 500 mm, and coefficients of varia- 60%). Using this method, the region is usually (80% tion (CV) are high, with some values about 50%. It of the time) facing a situation that is not normal. The is a semiarid region (including a small arid area), and watch system for Australia by Gibbs (1987) extends agriculture plays a major role in its economy. Be- the “normal” category from the third through sev- cause of this, drought is one characteristic of the enth deciles (c.d.f. between 30% and 70%), but the region’s climate that has far-reaching consequences, criteria that are used to establish the beginning and from unemployment to social conflicts. end of a drought do not apply in the southeast It is important to define drought and identify Iberian Peninsula. appropriate indicators for the region of Murcia as On the other hand, using the Palmer Drought part of a drought watch system. This system will Severity Index (PDSI) would require more data and define the temporal and spatial limits of drought assume hypotheses that are not always suitable. An- conditions. It would help policy makers and govern- other added inconvenience would be that certain ment officials establish policies for the provision of situations, such as extreme or severe drought, would aid to farmers and cattlemen, as in Australia (White occur more frequently in some areas than in others and O’Meagher, 1995). (Willeke et al., 1994). Furthermore, the category of Because of the wide range of drought impacts, extreme drought occurs too frequently for some re- there are many definitions of this phenomenon. How- gions, in excess of 10% for some areas (Wilhite, ever, one characteristic seems common to all of 1993). them: drought is caused by a deficiency in precipita- The use of the Standardized Precipitation Index tion for a fairly long period of time. For simplicity, (SPI) (McKee et al., 1993) would introduce the and keeping in mind that precipitation is, without great advantage of standardization, allowing us to doubt, the most important variable in the process, the determine the rarity of an episode in terms of prob- watch system developed for the region of Murcia ability. However, the fixed limits among the cate- uses only this variable at the moment, establishing a gories do not arise in an intuitive way (there is a comparison with a climatological reference (1961– mixture of whole and fractional numbers) and the 90) that we consider “normal.” normal category seems too extensive (68% of all occasions), so that some descriptive power would be lost.

Drought Network News 3 The Drought Watch System in the Region of tation Index (NVPI). It can be applied on various Murcia temporal scales, like a month, season, or year. In the region of Murcia, given the great variabil- To incorporate the descriptive power of the PDSI ity and seasonal distribution of the rain, it is essential and the advantages of the standardization of the SPI, to use an index that removes seasonal data, like the we have created the Normalized Precipitation Index precipitation collected in the last 12 months. Such an (NPI), which is calculated with the following equa- index has the advantage of stability, and it allows a tion (Garrido, 1998): prediction (not a prediction in the usual sense of the word, but rather a trend) of its future values, since, at 1 + Erf (NPI / %8) F(x) = any moment, these values can be inferred from cli- 2 matological references. A simulation showed that where F(x) is the empirical c.d.f. and Erf ( ) is the the prediction of the 12-month NVPI can extend for error function (this equation must be solved by means a term of 5 months, with an average absolute error of numerical methods). Imposing threshold values inferior to the unit. of ±4 for the NPI for the extreme categories, as The watch system, which was instituted in Janu- happens in the case of the PDSI, we obtain the ary 1998, consists of two parts. The first part (Figure classification in Table 1. The values of the index and 1) is limited to the consideration of the NVPI on the names of the categories generally coincide with scales of 1 and 12 months, offering information those of the Palmer classification. The most prob- about what happened in the last 4 years, with special able category is the normal one, even using a wider emphasis on the previous 2 years. It also offers a classification that groups drought or wet cases, while prediction of the values of the index, up to 5 months the percentages or probabilities of the moderate, ahead, and it establishes the excess or deficit quanti- severe, and extreme categories coincide with those ties of precipitation in relation to the threshold values of the SPI. In the extreme categories, only 2.3% of of the 12-month NVPI. the situations are included, which is a typical per- In the second part, to study the geographical centage of an extreme event (Wilhite, 1995). distribution, the NPI values are also represented Of course, this methodology can be applied to (Figure 2) on temporal scales of 1 and 12 months. other variables, such as the precipitation volume in a The average value of those indexes in the region region, hydrographic basin, or catchment area, lead- should always be similar to the corresponding NVPI ing, in this case, to a Normalized Volume of Precipi- values.

NPI Category c.d.f. (%) Time in category (%)

4 or more 4: Extremely wet 97.7 (100) 2.3 3 to 4 3: Very wet 93.3 (97.7) 4.4 2 to 3 2: Moderately wet 84.1 (93.3) 9.2 30.9 1 to 2 1: Slightly wet 69.1 (84.1) 15.0 -1 to 1 0: Normal or near normal 30.9 (69.1) 38.3 38.3 -2 to -1 -1: Mild drought 15.9 (30.9) 15.0 -3 to -2 -2: Moderate drought 6.7 (15.9) 9.2 -4 to -3 -3: Severe drought 2.3 (6.7) 4.4 30.9 -4 or less -4: Extreme drought 0 (2.3) 2.3

Table 1. NPI classifications for dry and wet periods.

4 Vol. 11, No. 2, May–August 1999 Trend 5 4 3 2 1 0 -1 -2 -3 -4 -5 12 34 -4 -3 -2 -1 0 1 Excess(+) ordeficit (-) quantities in relation to the threshold values of 12-month NVPI 0

4,000 3,000 2,000 1,000 Volume (hm3) -1,000 Volume -2,000 -3,000 -4,000 6 10 0 6 Drought (<-1) Drought (-1,1) Normal Wet (>1) 8 Last 6 months 12-month NVPI: 2,1(moderately wet) Evolution of 12-month NVPI 12-month of Evolution 18 4 Values of Normalized Volume of Precipitation Index (NVPI) Index Precipitation of Volume Normalized of Values 0 8 8 10 Last 12 months 9 30 1 Feb 96 Jul 96 Jan 97 Jul 97 Jan 98 Feb 98 Jun 98 8 1-month NVPI: 0,6 (normal) 0,6 NVPI: 1-month 8 5 4 3 2 1 0

10 -1 -2 -3 -4 -5

Last 48 months Last 36 months Last 24 months NVPI Last 18 months Number of months within selected time periods with a with periods time selected within months of Number category global each in included NVPI 12-month

Figure 1. Drought watch system in the region of Murcia, January 1998.

Drought Network News 5 a Yecla

Cieza 0

1 Average value within the region: 0.8 Caravaca

1 1 Murcia 1 1

2 1

0 3 2 Lorca 1 1 0 0 Cartagena

Scale Aguilas 0 10 20 30 40 50 Kilometers

Yecla b 3 3

4 2 3 3 2

2

Cieza 1 2 1 2 Caravaca Average value within the region: 1.8

3 2

2 Murcia 1

2 1

2 Lorca 1 Cartagena 2 0 2 Scale Aguilas 0 10 20 30 40 50

Kilometers

Figure 2. Normalized Precipitation Index values, January 1998: (a) 1-month; (b) 2-month.

6 Vol. 11, No. 2, May–August 1999 Future Development

The system will be extended to the whole Segura Basin, and it will be developed to expedite the cap- ture of precipitation data (appealing to automatic observation networks, for example); to increase the number of temporal scales, mainly those of 3 and 6 months time; and to extend the term of the forecasts, incorporating information based on the climatic teleconnections. Designing a watch system based on some type of normalized water balance would also be a future objective.

Ramón Garrido Abenza National Institute of Meteorology Territorial Meteorological Center of Murcia P. O. Box 138, Espinardo. 30071 Murcia Spain

References

Garrido, R. 1998. “Un sistema de seguimiento de la sequía en la Region de Murcia.” In I Asamblea Hispano-Portuguesa de Geodesia y Geogisica. Almería, Spain, in press. Gibbs, W. J. 1987. A Drought Watch System. World Meteoro- logical Organization, TD no. 193, WCP–134. McKee, T. B.; N. J. Doesken; and J. Kleist. 1993. “The rela- tionship of drought frequency and duration to times scales.” Proceedings of the Eighth Conference on Applied Clima- tology; pp. 179–84. American Meteorological Society, Boston. White, D. H.; and B. O’Meagher. 1995. “Coping with excep- tional droughts in Australia.” Drought Network News 7 (2):13–17. Wilhite, D. A. (ed.) 1993. Drought Assessment, Management and Planning: Theory and Case Studies. Kluwer Academic Publishing, Dordrecht, The Netherlands. Wilhite, D. A. 1995. Developing a precipitation-based index to assess climatic conditions across Nebraska. Final report to the Natural Resources Commission, Lincoln, Nebraska. Willeke, G.; J. R. M. Hosking; J. R. Wallis; and N. B. Guttman. 1994. The National Drought Atlas. Institute for Water Re- sources Report 94-NDS-4, U.S. Army Corps of Engineers.

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