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Climatic Change (2011) 106:303–314 DOI 10.1007/s10584-010-9900-z Important factors governing the incompatible trends of annual pan evaporation: evidence from a small scale region Yuhe Ji · Guangsheng Zhou Received: 17 March 2009 / Accepted: 15 June 2010 / Published online: 14 August 2010 © Springer Science+Business Media B.V. 2010 Abstract Many studies reported the coexisting trends of decreasing and increasing pan evaporation in some large scale regions. This study proved that the coexisting trends also occurred in small scale region as well as in large scale region. To discover the important factors governing the incompatible trends of annual pan evaporation, annual climatic data of ten meteorological stations at the Liaohe Delta over recent 45 years were analyzed by the partial correlation analysis, and the results showed the strongest statistically correlation between annual relative humidity and annual pan evaporation. Researches on two extreme cases suggested there was obvious contrary trend between annual relative humidity and annual pan evaporation for one case, in despite of slight contrary trend for another case. Generally, annual relative humidity most likely was an important factor relating to the trend of annual pan evaporation. At the same time, an expanded urbanization and irrigation were seen around these meteorological stations. Urbanization and irrigation exerted opposite effects on pan evaporation, they therefore were speculated to be the ultimately inducements causing unbalanced relative humidity, and led to incompatible pan evaporation. 1 Introduction Annual pan evaporation is an important climatic variable, and it is often used to estimate potential evaporation (Kirono et al. 2009) and reference evapotranspiration Y. Ji · G. Zhou (B) State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China e-mail: [email protected] Y. Ji Graduate University of Chinese Academy of Sciences, Beijing 100049, China e-mail: [email protected] 304 Climatic Change (2011) 106:303–314 (Chen et al. 2005), as well as to forecast agricultural production (Wang et al. 2009). Changes of pan evaporation will alter hydrological cycle in a region, so the changes of pan evaporation are of great significance for anticipating water balance and planning irrigation (Alvarez et al. 2007; Lowe et al. 2009). With increasing concern on the global warming, pan evaporation has been investigated, since it was hypothesized to be related to increasing temperature (Fu et al. 2009). Studies in various regions reported both decreasing and increasing trends of pan evaporation over the past half century (Linacre 2004). For northern hemisphere, the pan evaporation of four fifths regions showed statistical significant downward trend in the United States of America (USA) and the former Soviet Union from 1950s to early 1990s (Peterson et al. 1995). Similarly, Lawrimore and Peterson (2000) reported that pan evaporation in USA had been decreasing in most regions, but not in humid south-east. In China, most meteorological stations showed a downtrend of pan evaporation in the period 1961–2000 (Zuo et al. 2005). In India, pan evaporation decreased at all meteorological stations in the monsoon and post-monsoon seasons in the period 1940–1990, but the slight increasing trend occurred in east coastal India in pre-monsoon and winter seasons (Chattopadhyay and Hulme 1997). In some smaller countries, a slight increase of pan evaporation was discovered in Israel (Cohen et al. 2002), while a decrease was discovered in the Kingdom of Thailand (Taichi et al. 2005). For southern hemisphere, similar phenomena occurred. Fourteen out of thirty sites showed statistical significant declines, but three sites showed statistical significant increases in Australia in the period 1970–2002 (Roderick and Farquhar 2004). Six out of 19 sites showed statistically significant declines in New Zealand from 1970s (Roderick and Farquhar 2005). Most stations presented an increasing trend of pan evaporation, except for one decreasing, in northeast Brazil in the period 1964–1993 (Vicente and Rodrigues 2004). All these reports showed that the coexisting trends of decreasing and increasing pan evaporation occurred all over the world. It is still unclear why pan evaporation decreased in some regions, while increased in other regions. One of the expected consequences of global warming is that the air near the surface should be drier, and the dried air should result in an increase in the rate of pan evaporation (Fu et al. 2009). Unexpectedly there are the coexisting trends of decreasing and increasing pan evaporation. Many scientists have attempted to explain this puzzling phenomenon. Roderick and Farquhar (2002) discovered that the decrease in evaporation was consistent with decreases in sunlight. Liu et al. (2004) suggested that the reduced pan evaporation in China likely resulted from the decrease in solar irradiance. However, Xu et al. (2007) argued that the decreasing pan evaporation resulted from complex changes of air temperature, relative humidity, solar radiation and wind speed. Although all the researches have cast some light on the causes resulting in the incompatible trends of annual pan evaporation, more evidences are needed to understand the puzzling phenomenon. To understand the incompatible trends of pan evaporation, we examined the trends of annual pan evaporation of ten meteorological stations in the Liaohe Delta. We also attempted to identify the important factors related to annual pan evaporation, and to discover the ultimately inducements causing the incompatible trends. Climatic Change (2011) 106:303–314 305 2 Study area and methods 2.1 Study area The study area locates on the Liaohe Delta (121◦ 10 ∼ 122◦ 30 E, 40◦ 30 ∼ 41◦ 30 N) in Northeast China, which covers an area of approximately 22,000 km2 , bordering on the Bohai Sea (Fig. 1). The Liaohe Delta mainly consists of homogeneous alluvial plains, and most of it has an elevation of less than 50 m. The climate is temperate continental monsoon climate with a mean annual temperature of 8.3◦ C, and the temperature shows a strong increase with global warming. The precipitation occurs mainly in the summer with a mean annual precipitation of 611.6 mm. Throughout the Liaohe Delta, annual pan evaporation ranges from 1,300 to 2,300 mm; the annual wind speed ranges from 2 to 6 m/s; the annual sunshine duration ranges from 2,100 to 3,100 h; the annual relative humidity varies between 50% and 70%. 2.2 Data processing Original data of ten meteorological stations include monthly pan evaporation, precipitation, sunshine hours, temperature, relative humidity and wind speed in the period of 1961–2005. It should be announced that the pan evaporation in China was measured synchronously by two sets of equipments (evaporation pan with 20 cm diameters and E601 evaporation pan) since the 1980s (Chen et al. 2005). The evaporation pan of 20 cm diameters is a round tank in shape, with a high 10 cm and an inner diameter 20 cm. E601 evaporation pan is also a round tank in shape, with a high of 69 cm and an inner diameter of 61.8 cm. The data of pan evaporation in this paper came from the evaporation pan of 20 cm diameters, which has been used since 1961. China Fig. 1 Spatial locations of the study area (Liaohe Delta) and the meteorological stations 306 Climatic Change (2011) 106:303–314 Pan evaporation and precipitation are measured simultaneously at 08:00 local time of Beijing (time zone 8◦ E) every morning. The amount of pan evaporation per time unit is calculated: E = P + (h1 − h2 ), where E denotes daily pan evaporation (mm/day), P is daily precipitation (measured at the same site and the same time, and with the same unit as pan evaporation), and h1 and h2 are water surface heights in the evaporation pans for the previous and present measurements, respectively. Daily wind speed is measured by automatic anemoscope above ground 10 ∼ 12 m. Daily relative humidity, which is defined as the ratio of the actual to the saturation vapor pressures above the ground of 1.5 m, is measured by automatic hygrometer. Daily sunshine hour, which is defined as the durative time of solar radiant intensity going up to or exceeding 120 w·m−2 , is recorded by automatic sunshine recorder. The daily values of temperature, wind speed and relative humidity are the average of many recorded values in one day. The monthly values of them are the average of daily values, and annual values of them are the average from twelve months in one year. While monthly values of pan evaporation, precipitation and sunshine hours were calculated by adding up daily values in one month, and annual values of them were calculated by adding up the values from twelve months in one year. 2.3 Data analysis To explore the variation trends of annual pan evaporation, a time-series analysis was performed over the period 1961–2005 for each one of ten meteorological stations. To identify the important factors relating to the trend of annual pan evaporation, partial correlation analysis between annual pan evaporation and other climatic factors was applied. Each meteorological station had a value of annual pan evaporation for a given year. Therefore, there were 45 values of annual pan evaporation for one station at the 45-year period from 1961 to 2005, added up to 450 values for ten stations. The 450 values along with annual other climatic factors were directly involved into the calculation of the partial correlation analysis. For two extreme opposite cases with maximal increasing and decreasing pan evaporations respectively, the partial correlation analysis was also applied, and the corresponding trend between annual relative humidity and annual pan evaporation was examined year by year after data standardization was done using the SPSS software. To further explore ultimate causes resulting in the trend of pan evaporation, some geographical characteristics of each meteorological station were examined, such as elevation, distance to sea, prevailing wind, annual wind speed. We referred to the Statistic Yearbooks of Liaoning for the irrigated area, and also interpreted the remote sensing images of 1988 and 2006 for the urban area. 3 Results 3.1 Incompatible trends of annual pan evaporation When pan evaporation trends of ten meteorological stations in the period 1961– 2005 were examined one by one, the coexisting trends of decreasing and increasing Climatic Change (2011) 106:303–314 307 Fig. 2 Locations and trends of annual pan evaporations from ten meteorological stations. The arrows show the trends of pan evaporation, and red arrows show the two extreme cases annual pan evaporation were unveiled (Figs. 2, 3). Slightly decreasing trends were seen at eight out of ten meteorological stations, and one (Dawa meteorological station) of them had the maximal negative coefficient (R = −0.497) at 0.0005 level of statistical significance. At the same time, increasing trends were seen at two out of ten meteorological stations, one (Dashiqiao meteorological station) of them had Fig. 3 Trends of annual pan evaporation of each meteorological station in the Liaohe Delta in the period 1961–2005 308 Climatic Change (2011) 106:303–314 the maximal positive coefficient (R = 0.729) at 0.0001 level of statistical significance (Fig. 3). The results suggested that the trends of statistical significant decreasing and increasing annual pan evaporation coexisted in the Liaohe Delta. 3.2 The correlation between pan evaporation and other climatic factors Partial correlation coefficients revealed: there was the strongest statistical correlation between annual relative humidity and annual pan evaporation, with the maximal partial correlation coefficient of −0.561 (P < 0.01), followed by the statistical correlation between annual precipitation and annual pan evaporation with the partial correlation coefficient of −0.238 (P < 0.01). However, no statistical correlations were discovered between annual pan evaporation and other climatic factors (including annual temperature, annual wind speed and annual sunshine hours) because of lower correlation coefficients (Table 1). For two extreme opposite cases (one was Dashiqiao station with maximal increasing pan evaporation; another was Dawa station with maximal decreasing pan evaporation), a statistical significant correlation between annual relative humidity and pan evaporation was discovered for Dashiqiao station with a partial correlation coefficient of −0.528 (P < 0.01). However, no statistical significant correlation was discovered for Dawa station because of small partial correlation coefficient of −0.088 (P = 0.583). When the corresponding trends were examined between annual pan evaporation and relative humidity in the two extreme cases, well contrary trends were seen between them for Dashiqiao station, since pan evaporation went down as relative humidity went up, and went up as relative humidity went down for a given year over the 45-year period (Fig. 4). However, the contrary trends between them were not obvious for Dawa station (Fig. 5). Generally, annual relative humidity most likely was an important factor relating to the trend of annual pan evaporation at annual time scale, because there was the strongest statistical correlation between annual relative humidity and annual pan evaporation whether for integrative ten stations or for the extreme case with increasing trend, and there were well corresponding contrary trends between annual Table 1 Partial correlations between annual pan evaporation and other annual climatic factors in the period 1961–2005 Control variables Variables Relative humidity and precipitation and sunshine hour and wind speed Relative humidity and sunshine hour and temperature and wind speed Relative humidity and precipitation and sunshine hour and temperature Relative humidity and precipitation and temperature and wind speed Precipitation and sunshine hour and temperature and wind speed Temperature Sig. (2-tailed) df .018 .703 438 Precipitation −.238* .000 438 Wind speed .097 .044 438 Sunshine hour .057 .240 438 −.561* .000 438 Relative humidity * denotes that correlation is significant at 0.01 level (2-tailed) Correlation coefficient Climatic Change (2011) 106:303–314 309 pan evaporation ralative humidity The normalized values 3 0 19 61 19 63 19 65 19 67 19 69 19 71 19 73 19 75 19 77 19 79 19 81 19 83 19 85 19 87 19 89 19 91 19 93 19 95 19 97 19 99 20 01 20 03 20 05 -3 Years Fig. 4 The corresponding trends between annual pan evaporation and relative humidity at Dashiqiao station. (The data of pan evaporation and relative humidity had been normalized) relative humidity and annual pan evaporation for the extreme case with increasing trend, though there were a slight correlation and a blurry corresponding contrary trend for the extreme case with decreasing trend. 3.3 The ultimately inducement for pan evaporation trends All of ten meteorological stations are located on a homogeneous low plain, with an altitude no more than 71 m. They had the same NW–SE prevailing wind directions, and the same decreasing trends of annual wind speed (Fig. 6). The longest distance of meteorological station to sea was 105 km, and the shortest distance was 10 km (Table 2). No correlation was discovered between pan evaporation trend and the pan evaporation relative humidity The normalized values 3 0 19 61 19 63 19 65 19 67 19 69 19 71 19 73 19 75 19 77 19 79 19 81 19 83 19 85 19 87 19 89 19 91 19 93 19 95 19 97 19 99 20 01 20 03 20 05 -3 Years Fig. 5 The corresponding trends between annual pan evaporation and relative humidity at Dawa station. (The data of pan evaporation and relative humidity had been normalized) 310 Climatic Change (2011) 106:303–314 Fig. 6 Trends of annual wind speed from each meteorological station in the Liaohe Delta in the period 1961–2005 distance to sea. Generally, the incompatible pan evaporation trend couldn’t be explained ultimately by altitude, distance to sea, wind direction and wind speed. When the urbanization and irrigation were examined, rapid expansions of irrigation and urbanization were seen. Irrigation in the region became increasingly popular since the1960s (Governmental chorography office of Panjin city 1998).The irrigated area was 6.69 × 104 ha among cultivated land of 1.95 × 105 ha in the year of 1992, while it reached 1.026 × 105 ha among the cultivated land of 2.091 × 105 ha in the year of 2005 (Liaoning Bureau of Statistics 1993–2006). Urbanization has been developing along with the expansion of population since the 1960s, in despite of no Table 2 The geographical characteristics of ten meteorological stations in the Liaohe Delta Station Elevation (m) Distance to sea (km) Prevailing wind Annual wind speed (m/s) Trend of pan evaporation (mm/year) Urbanization area (km2 ) 1988 2006 Beining Dashiqiao Dawa Haicheng Liaozhong Panshan Taian Yixian Yingkou Jinzhou 69.2 12.1 4.8 26.5 20.6 4.6 8.5 87 4.3 70.2 84 23 26 54 105 42 84 70 10 27 NW–SE NW–SE NW–SE NW–SE NW–SE NW–SE NW–SE NW–SE NW–SE NW–SE 2.8 ∼ 4.5 2.8 ∼ 4.2 3.0 ∼ 5.2 2.4 ∼ 4.2 2.5 ∼ 4.6 3.0 ∼ 5.8 2.6 ∼ 4.5 2.0 ∼ 3.6 3.0 ∼ 4.8 2.5 ∼ 4.6 −0.1157 12.6954 −5.268 −3.7015 −4.103 −3.2997 −2.1108 −3.0738 −2.6252 2.4034 3.34 12.33 5.45 14.76 6.01 15.09 9.05 8.34 31.49 28.33 5.72 37.18 7.25 39.89 13.01 23.37 18.73 12.17 65.23 67.24 Climatic Change (2011) 106:303–314 311 recorded data. The results from the interpreted remote sensing images showed that all the cities nearby the meteorological stations had experienced a rapid expansion in the period 1988–2006 (Table 2). The rapid extending urbanization had almost surrounded these meteorological stations (Fig. 1), though they all were located suburb when they were built many years before. Theoretically, increasing irrigation put more water vapor into atmosphere, and enhanced relative humidity in the atmosphere, so a smaller vapor pressure gradient between atmosphere and the surface of pan water came into being, which prevented vapor into atmosphere, pan evaporation therefore was reduced. To the contrary, increasing urbanization suppressed water vapor into atmosphere by the hardened land, and reduced relative humidity in the atmosphere, so a larger vapor pressure gradient between atmosphere and the surface of pan water came into being, pan evaporation therefore was promoted. Thus, irrigation and urbanization exerted opposite effect on pan evaporation. The spatial imbalance of irrigation and urbanization most likely resulted in the imbalance of relative humidity, and led to the incompatible pan evaporation ultimately. 4 Discussion 4.1 Increasing temperature under global warming Our results showed that the decreasing and increasing trends of annual pan evaporation coexisted in the Liaohe Delta. The incompatible pan evaporation trends also occurred synchronously around the world (Peterson et al. 1995; Lawrimore and Peterson 2000; Roderick and Farquhar 2002; Hobbins et al. 2004; Zhang et al. 2007; Jovanovic et al. 2008). Therefore, it could conclude that the incompatible trends of annual pan evaporation occurred in small scale region as well as in large scale region. All the places in Liaohe Delta had the increasing annual temperature (Zhao et al. 2009). Overall warming trends theoretically should have resulted in same pan evaporation trends if annual temperature was a major factor governing pan evaporation trend. However, the opposite pan evaporation trend in this region denied the important effect of annual temperature. Furthermore, partial correlation analysis also proved no statistical correlation between annual temperature and annual pan evaporation. Thus, the increasing temperature under global warming couldn’t be an important factor governing the coexisting trends. 4.2 Wind speed Wind speed is an important factor influencing on pan evaporation at daily time scale, but there are disputes at annual time scale. Liu et al. (2004) reported that pan evaporation in China decreased in most sites in the period 1955–2000. At the same time, Xu et al. (2006) reported that annual mean wind speed in China also decreased in the period 1969–2000. It seems there was some correlation between annual pan evaporation and wind speed, but lacking of evidence. Rayner (2007) suggested that wind was the dominant factor affecting pan evaporation trend in Australia. To the contrary, our results indicated no statistically significant correlation between annual wind speed and pan evaporation. In addition, we discovered that all the stations had 312 Climatic Change (2011) 106:303–314 the decreasing wind speed in the period 1961–2005 (Fig. 6). Overall decreasing wind speed couldn’t lead to opposite pan evaporation trend. We therefore argued that wind speed was not an important factor governing pan evaporation. 4.3 Solar radiation Solar radiation was argued to be correlated to pan evaporation by some researchers. Qian et al. (2006) reported that both solar radiation and pan evaporation decreased in most parts of China in the period 1954–2001, but short of reliable evidence that the decreasing pan evaporation resulted from the decreasing solar radiation. Similarly, Roderick and Farquhar (2002) reported that the decreasing evaporation was consistent with the decreasing sunlight, and speculated that the decreasing pan evaporation might be related to the decreasing sunlight. However, our results drew an opposite conclusion, because there was no statistical significant correlation between annual sunshine hour and pan evaporation. 4.4 Precipitation Precipitation seemed to be an important factor relating to pan evaporation. Brutsaert and Parlange (1998) suggested that a complementary relationship exists between actual evaporation and pan evaporation, since increasing actual evaporation could result in decreasing pan evaporation. It means the decreasing pan evaporation could be seen as a signal of increasing actual evaporation (Liu et al. 2009). Thus, precipitation not only had influence on pan evaporation directly, but also had influence on pan evaporation indirectly through actual evaporation. Our results suggested there was a significant statistical correlation between annual precipitation and annual pan evaporation (Table 1). It just proved that annual precipitation was one of important factors rather than the most important factor for annual pan evaporation. 4.5 Relative humidity Many previous studies discovered the significant correlation between relative humidity and pan evaporation. Chattopadhyay and Hulme (1997) analyzed the climatic data from twenty-seven meteorological stations in India in the period 1940–1990, and discovered that relative humidity was most strongly related to pan evaporation. Zuo et al. (2005) analyzed the climatic data from 61 meteorological stations over China in the period 1961–2000, and discovered that relative humidity had the best correlation with pan evaporation. Our results affirmed the best correlation between relative humidity and pan evaporation, because of the strongest statistical correlation as well as corresponding contrary trend between annual relative humidity and annual pan evaporation. Although there was the best correlation between annual relative humidity and pan evaporation, it couldn’t to say that relative humidity was an ultimate cause resulting in the pan evaporation trend. However, the opposite pan evaporation trend occurred in small scale region. This made our considering that the pan evaporation trend probably rooted in human activities at small spatial scale rather than climatic change at large spatial scale. Further researches discovered that the expanded irrigation and Climatic Change (2011) 106:303–314 313 urbanization possibly firstly resulted in a disordered relative humidity, and then led to the incompatible pan evaporation ultimately. 5 Conclusions Our results affirmed the coexisting trends of decreasing and increasing pan evaporations in the Liaohe Delta in the period 1961–2005. The strongest correlation between annual relative humidity and pan evaporation was discovered. For the extreme case with increasing pan evaporation, a well corresponding contrary trend between annual relative humidity and pan evaporation was also discovered. Generally, annual relative humidity most likely was an important factor relating to annual pan evaporation trend. In addition, rapid expansions of irrigation and urbanization were seen, and they exerted opposite effect on pan evaporation. The spatial imbalance of irrigation and urbanization most likely resulted in the imbalance of relative humidity, and then led to the incompatible pan evaporation ultimately. Acknowledgements This research was supported by National Natural Science Funds for Distinguished Young Scholar (40625015), State Key Development Program of Basic Research (2010CB951303) and Open Program Project of the Institute of Atmospheric Environment in Shenyang. The meteorological data were provided by the Institute of Atmospheric Environment, China Meteorological Administration, Shenyang. Ms. Sarah Evans corrected some syntactic errors. We thank her sincerely. References Alvarez VM, Gonzalez MM, Baille A, Molina JM (2007) A novel approach for estimating the pan coefficient of irrigation water reservoirs Application to South Eastern Spain. Agric Water Manag 92:29–40 Brutsaert W, Parlange MB (1998) Hydrologic cycle explains the evaporation paradox. 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