INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 29: 1430–1438 (2009) Published online 3 December 2008 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/joc.1805 Dry spell trend analysis of Isfahan Province, Iran M. Nasria and R. Modarresb*,† a Faculty member, Islamic Azad University, Ardestan Branch, Ardestan, Iran b Faculty of Natural Resources, Isfahan University of Technology, Isfahan, Iran ABSTRACT: In the present study the spatiotemporal trend analysis of the annual maximum dry spell length (AMDSL) and the annual number of dry spell periods (ANDSPs) of Isfahan Province in the center of Iran has been conducted based on daily rainfall records taken from 17 rain gauge stations. The autocorrelation functions (ACFs) of dry spell time series showed the randomness of dry spells. The use of Mann-Kendall test for trend revealed two stations with significant negative trend of the AMDSL located in east and west of the province. The trend analysis of the ANDSP showed three stations with significant upward trend in east and one station with significant negative trend in the west of the province. The sequential Mann-Kendall test showed an evidence of an individual abrupt change in the magnitudes of dry spells from 1980s, especially in eastern arid and semi-arid regions of the province. However, the binomial and uniform distribution of Mann-Kendall statistics reveals that individual trend in a region is not strong enough to conclude the large-scale regional climatic effects on dry spell characteristics in the entire Isfahan Province. Copyright 2008 Royal Meteorological Society KEY WORDS dry spell; trend analysis; Mann-Kendall; sequential Mann-Kendall; climate change Received 2 January 2007; Revised 5 October 2008; Accepted 11 October 2008 1. Introduction used daily precipitation and daily temperature based on 494 stations in China in order to identify the trend in There is a growing evidence of hydrological trend and climate extremes. They found increasing trend in the long-term variability in the 20th century. Since the report number of rainy days and decreasing trend in the num- of IPCC (1995) raised the question ‘Has the climate ber of wet days. Schmidli and Frei (2005) investigated become more variable or extreme?’ trends of climate long-term variations and trends of extreme variables, i.e. extremes have received increasing attention from many heavy precipitation events, spells of intense precipitation researchers in a variety of hydrological studies. Most pre- and dry spells, of 100 rain gauges in Switzerland dur- vious studies concerning long-term climatological trends ing 20th century. They found increasing trends for heavy have focused on surface air temperature and precipitation. precipitation and dry spells. Annual and monthly rainfall series are common vari- Extreme temperature series have also received inc- ables used in trend and climate change analysis. IPCC reased attention during the last decades of the 20th (2007) reported that, during recent decades, precipita- century (Balling et al., 1990; Esterling et al., 2000). tion has tended to increase in mid-latitudes, decrease in Temperature records across the world indicate that there the Northern Hemisphere subtropical zones, and increase has been an increase in the mean global temperature generally throughout the Southern Hemisphere. at about 0.6 °C since the start of the 20th century Lettenmaier et al. (1994), Turkes (1996), Zhang et al. (Nicholls et al., 1996; Unkasevic et al., 2005) and that (2000), Gonzalez Hidalgo et al. (2003), Gong et al. this increase is associated more strongly with warming (2004), del Rio et al. (2005), and Partal and Kahya (2006) in daily minimum temperature rather than with a change are some examples of the researchers who have investi- in maximum temperatures (Esterling and Horton, 1997). gated recent rainfall trends in different geophysical fields. Temperature extremes are key aspects of any climate The change in daily extreme, which is very important change because ecosystem and social response are more aspect of precipitation in semi-arid region, has received sensitive to them. Mearns et al. (1984) and Hansen et al. much interest in recent years. Gong et al. (2004) consid- (1988) concluded that relative small change in the mean ered daily extremes, the number of rainy days, precip- temperature could produce substantial changes in the itation intensity, maximum daily rainfall, persistence of frequency of the extreme temperature. daily rainfall, and dry spell duration. Qian and Lin (2005) There is also strong evidence that rainfall changes associated to global warming are already taking place on * Correspondence to: R. Modarres, Faculty of Natural Resources, global and regional scale. The trend was globally positive Isfahan University of Technology, Isfahan, Iran. throughout the 20th century, although large areas were E-mail: m [email protected] characterized by negative trend (IPCC, 2007). † Current affiliation/correspondence: INRS-ETE, University of Ouebec,´ 490 de la Couronne, Ouebec,´ Canada, G1K 9A9. In addition to the natural features of the climate Email: [email protected]. systems, the global climate change is also said to be Copyright 2008 Royal Meteorological Society DRY SPELL TREND ANALYSIS 1431 related to anthropogenic influences (Jain and Lall, 2000; Sharma et al., 2000). The effect of human beings on the long-term trends in hydrological time series has, therefore, received a great attention worldwide. The example of decreasing precipitation because of increasing deforestation was presented by Meher-Homji (1991). Similarly, Kothyari and Singh (1996) showed increasing temperature and decreasing precipitation trends since the mid-1960s as a result of deforestation. Sharma et al. (2000) also showed some evidence of increasing temperature and decreasing precipitation and discharge particularly during low flow season, as a result of anthropogenic effects. Recently, Serra et al. (2006) studied dry spell trend in Figure 1. Digital Elevation Model (DEM) of Isfahan, location of Catalonia (NE spain). They found decreasing trend for Isfahan Province in Iran and measuring stations in Isfahan Province the number of annual dry spells but increasing trend for (the name and the number of stations are listed in Table I). the annual maximum dry spells. In China, Gong et al. (2004) found significant positive trend in the length of rainfall in each year and the second is the number of dry spells in different regions of China. Schmidli and periods without rainfall in each single year. The annual Frei (2005) found a very small number of stations with maximum dry spells increase from 206 in the western statistically significant dry spell trend in Switzerland. region to 322 days in the eastern region of the province. The primary objective of this study is to investigate The number of annual dry spells varies from 21 to 49 dry recent trends in dry spells of Isfahan Province, in the cen- spell periods eastward toward arid and semi-arid regions ter of Iran. The introduction is followed by a description of Iran. These two variables may cause major reduction in of the dry spell dataset used in this study for trend anal- soil moisture and ground water which are important water ysis. During the past decades, numerous parametric and resources for agricultural activities and development in nonparametric techniques for the detection of long-term Isfahan Province, particularly for hydrology cycle of arid trends in time series were developed (Lettenmaier, 1976; and semi-arid region of the province. Hirsch et al., 1991). They will be briefly described in The Exploratory data analysis is based on the cal- Section 3. Section 4 is devoted to the results and discus- culation of descriptive statistics of data. Table I shows sions of trend analysis and a brief conclusion is presented maximum, average, and standard deviation (SD) of the in Section 5. selected variables. The average AMDSL of Isfahan Province is 157 days (44% of the year) and the average ANDSP is 18 periods. 2. Data set Maximum dry spell lengths are observed at Jandagh and Kuhpayeh Stations in eastern semi-arid region of the Isfahan Province is located in the center of Iran province, whereas the minimum dry spell lengths belong (Figure 1). The eastern regions of the province are to Aznaveleh and Tad Stations in western humid region located in the western margins of arid and semi-arid of the province. regions of Iran. The western regions of the province lay in the eastern hill slopes of Zagrous Mountains. The mean annual rainfall of western region is 800 mm, while it is 3. Methods used about 75 mm in eastern arid region. The average coef- ficient of variation (CV) of rainfall in the province is 3.1. Test for randomness 34%. Winter and fall rainfall consists of 48.4 and 27.6% One of the problems in detecting trends in hydrologic of total annual rainfall, whereas it is 23 and 1% for spring time series is the effect of serial correlation. If there is and summer season, respectively. The average maximum a positive serial correlation in time series, the nonpara- temperature in the province varies from 16.2 to 28.2 °C metric test will suggest a significant trend in time series and the average minimum temperature varies from 6.3 to (Burn and Hag Elnur, 2002; Yue et al., 2002; Partal and 1.1 °C. July and August are the warmest and January and Kahya, 2006). To avoid this problem, we check the auto- February are the coldest months of the province. correlation structure of dry spell time series. If the time There are 17 rain gauges in Isfahan Province with series is random, the autocorrelation coefficients are not sufficient length of daily rainfall record, at least 30 years, statistically different from zero. In other words, the auto- which are used in this study to investigate dry spell correlation coefficients do not cut the confidence interval trend in Isfahan Province.