Download Important factors governing the incompatible trends of annual pan

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
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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
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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
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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.
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