Stoch Environ Res Risk Assess (2011) 25:139–150
DOI 10.1007/s00477-010-0428-6
ORIGINAL PAPER
Comparison of evapotranspiration variations between the Yellow
River and Pearl River basin, China
Qiang Zhang • Chong-Yu Xu •
Yongqin David Chen • Liliang Ren
Published online: 8 July 2010
Ó Springer-Verlag 2010
Abstract Based on daily meteorological data at 43
gauging stations in the Pearl River basin and 65 gauging
stations in the Yellow River basin, we analyze changing
properties of actual evapotranspiration (ETa), reference
evapotranspiration (ETref) and precipitation in these two
river basins. In our study, Pearl River basin is taken as the
‘energy-limited’ system and the Yellow River basin as the
‘water-limited’ system. The results indicate decreasing ETa
in the Pearl River and Yellow River basin. However, different changing properties are detected for ETref when
compared to ETa. The middle and upper Yellow River
basin are characterized by increasing ETref values, whereas
the Pearl River basin is dominated by decreasing ETref
values. This result demonstrates enhancing drying force in
the Yellow River basin. ETa depends mainly on the
Q. Zhang (&)
Department of Water Resources and Environment, Sun Yat-sen
University, Guangzhou 510275, China
e-mail: zhangqnj@gmail.com
C.-Y. Xu
Department of Geosciences, University of Oslo, PO Box 1047,
Blindern, 0316 Oslo, Norway
Y. D. Chen
Department of Geography and Resource Management,
The Chinese University of Hong Kong,
Hong Kong, Hong Kong, China
Y. D. Chen
Centre of Strategic Environmental Assessment for China,
The Chinese University of Hong Kong, Hong Kong,
Hong Kong, China
L. Ren
State Key Laboratory of Hydrology-Water Resources
and Hydraulic Engineering, Hohai University,
Nanjing 210098, China
changes of precipitation amount in the Yellow River basin.
In the Pearl River basin, however, ETa changes are similar
to those of ETref, i.e. both are in decreasing trend and
which may imply weakening hydrological cycle in the
Pearl River basin. Different influencing factors are identified behind the ETa and ETref in the Pearl River and Yellow
River basin: In the Pearl River basin, intensifying urbanization and increasing aerosol may contribute much to the
evapotranspiration changes. Variations of precipitation
amount may largely impact the spatial and temporal patterns of ETa in the Yellow River basin. The current study is
practically and scientifically significant for regional
assessment of water resource in the arid and humid regions
of China under the changing climate.
Keywords Reference evapotranspiration Actual evapotranspiration The humid and arid regions Hydrological cycle China
1 Introduction
The increasing temperature and its influence on human
society and the hydrologic cycle have caused considerable
concern to investigators in recent years (e.g., Xu et al.
2005; Huntington 2006). Significant evidence (including
precipitation, runoff and soil moisture variations; Alan
et al. 2003; WMO 2003; Zhang et al. 2009) shows that the
projected global climate changes have the potential to
accelerate the global hydrologic cycle. Evaporation is the
only hydrological variable that connects water balance and
energy balance, and it provides a sound understanding of
hydrologic cycle (Xu and Singh 2005). Xu et al. (2006)
summarized the key parameters describing evaporation in
the literatures: (1) free water evaporation, ETo, evaluates
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the amount of evaporation from open/free water surface;
(2) actual evapotranspiration, ETa, describes all the processes that liquid water at or near the land surface becomes
atmospheric water vapor under natural conditions; (3)
potential evapotranspiration introduced in the late 1940s
and 1950s by Penman (1948, 1956) and is defined as ‘‘the
amount of water transpired in a given time by a short green
crop, completely shading the ground, of uniform height and
with adequate water status in the soil profile’’. Reference
evapotranspiration, ETref (Allen et al. 1998) is defined as
‘‘the rate of evapotranspiration from a hypothetical reference crop with an assumed crop height of 0.12 m, a fixed
surface resistance of 70 s m-1 and an albedo of 0.23,
closely resembling the evapotranspiration from an extensive surface of green grass of uniform height, actively
growing, well-watered, and completely shading the
ground’’. Evapotranspiration is perhaps the most difficult
of all hydrological variables of the hydrologic cycle due to
complex interactions among the variables of the land-plantatmosphere system (Xu and Singh 2005).
It is generally expected that, as the average global
temperature increases, the air will become drier and
evaporation from terrestrial water bodies will increase.
However, many studies showed decreased pan evaporation and ETref (e.g., Hobbins and Ramı́rez 2004). This is
usually called the pan evaporation paradox. Peterson
et al. (1995) demonstrated that the evaporation of water in
the United States and the former Soviet Union has
decreased. They attributed the decreased evaporation of
water to decreasing diurnal temperature range (DTR).
Roderick and Farquhar (2002) believed that the decrease
in evaporation is the result of the observed large and
widespread decreases in sunlight resulting from increasing
cloud coverage and aerosol concentration. However,
Brutsaert and Parlange (1998) indicated that, in nonhumid environments, measured pan evaporation was not a
good measure of potential evaporation; moreover, in
many situations, decreasing pan evaporation actually
provides a strong indication of increasing terrestrial
evaporation, implying intensifying hydrological cycle in
large regions. Hobbins and Ramı́rez (2004) indicated that
the ‘‘Pan Evaporation Paradox’’ is no more than a manifestation of the complementarity between ETa and
potential evapotranspiration. They also pointed out that
although pan evaporation is a very useful concept, it can
be misleading if used by itself to indicate climatic trends.
The previous studies only focused on evaporation processes in specific regions or specific evaporation variables
such as reference evaporation (e.g. Xu et al. 2006). It
should be noted here that evaporation changes depend
heavily on the availability of water and energy. In this
sense, comparisons of evaporation changes in humid and
arid regions can be expected to improve and enhance
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Stoch Environ Res Risk Assess (2011) 25:139–150
human understanding of hydrological cycle variations
under the influences of climate changes, particularly the
human understanding of ‘‘pan evaporation paradox’’. This
is the major motivation of this current study.
There are some studies available concerning evapotranspiration variations in China. Xu et al. (2006) investigated the spatial patterns of ETref and pan evaporation of
the Yangtze River basin. Chen et al. (2005) explored
differences in estimates of ETref by using Thornthwaite
method, pan data and Penman–Monteith method. Gao
et al. (2007) studied ETa trends by modified water balance
methodology in China. Of course, there are also other
similar studies and no complete list is provided here for
the sake of limited space of this paper. The current study
is different from the foregoing researches in that the
current study focus on the evaporation changes in the
humid and arid regions characterized by different water
and energy conditions. Availability of water and energy is
the key factor influencing the evaporation, ETref or ETa.
Besides, major factors influencing the evaporation changes in the arid and humid regions are discussed. These
points clarify the novelty of this current analysis. In this
case, we analyze changes of ETref and ETa in the arid and
humid regions of China, i.e. the Yellow River and the
Pearl River with the major objectives as: (1) analysis of
spatial patterns of ETref and ETa; (2) investigation of
trends of ETref and ETa in the humid and arid regions
of China; and (3) explore possible causes behind the
changing patterns of evapotranspiration in the humid and
arid regions respectively in China.
2 Study regions
2.1 The Pearl River basin
The Pearl River basin (97°390 E–117°180 E; 3°410 N–29°150 N)
(Fig. 1) is the third largest river in China with drainage area of 7.96 9 105 km2. The Pearl River basin constitutes three major tributaries: West River, North River
and East River. The West River is the largest tributary
accounting for 77.8% of the total drainage area of the
basin. The North River is the second largest tributary with
a drainage area of 46710 km2. The East River accounts
for 6.6% of the total area of the Pearl River. The Pearl
River basin is located in the tropical and sub-tropical
climate zone with the annual mean temperature ranging
between 14 and 22°C and the precipitation mainly concentrating during April–September (Zhang et al. 2009),
accounting for 72–88% of the annual precipitation
(PRWRC 1991). The average annual humidity is 71–80%.
In this study, the Pearl River basin is regarded as the
humid environment.
Stoch Environ Res Risk Assess (2011) 25:139–150
141
Fig. 1 Location of the study river basins. The points in the river basins show the locations of the gauging stations
2.2 The Yellow River basin
The Yellow River (Huang He) (95°530 E–119°50 E; 32°100 N–
41°500 ) (Fig. 1) is the second largest river in China and the
fifth largest river in the world (Zhang et al. 2008). The
mainstem of the Yellow River is 5464 km in length with an
area of 752440 km2, flowing mainly through the arid and
semi-arid region. The altitude of the river basin ranges
from above 4000 m a.s.l. in the west to below 1000–
2000 m a.s.l. in the middle and lower reaches. The average
annual precipitation is about 466 mm and the annual pan
evaporation ranges from 700 mm to 1800 mm. The annual
mean surface air temperature is 1–8°C in the upper Yellow
River basin, 8–14°C in the middle Yellow River basin and
12–14°C in the lower Yellow River basin. The evaporation
exerts significant impacts on availability of water resources
in the Yellow River basin (Zhang et al. 2008).
3 Data and methodology
3.1 Data
Daily meteorological data (such as temperature, precipitation, relative humidity, sunshine hours, wind speed, etc.)
covering 1960–2005 at 65 meteorological stations in the
Yellow River basin and at 42 stations in the Pearl River
basin have been analyzed in the current study. The missing
data account for\0.001% of the total data and are filled by
using the neighboring stations through the simple linear
regressive method (the R2 is [0.8). If the relation is not
good, e.g. the R2 is \0.8, the missing data will be replaced
by long-term average value. The 1-day missing data are
replaced by the average of the neighboring days. The series
with missing data of more than 1 year will be excluded
from the analysis. We believe that the process of the
missing data satisfies the analysis of the current study.
Furthermore, the data consistency is checked by the double-mass method and the result reveals that all the data
series used in the study are consistent.
3.2 Methodology
There exist some methods proposed for computation of the
ETa (Xu and Singh 2005): (1) Penman–Monteith method,
and (2) complementary relationship. The first method
requires data on aerodynamic resistance and surface
resistance with are not readily available, which limits its
use in practice. The second method is usually preferred
because it requires only standard meteorological variables
and does not require local parameter calibration (Xu and
Singh 2005). Thus, the complementary relationship is used
in the current study.
The computation steps of this study involve:
(I)
Computation of long-term annual average ETa by
using traditional long-term water balance methods. Xu
and Singh (2004) summarized the models for computation of ETa with the long-term water balance
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Stoch Environ Res Risk Assess (2011) 25:139–150
where AE denotes the long-term average ETa, P
denotes the long-term average precipitation, and PE
denotes the long-term average ETref. After this step,
one long-term average ETa value is computed for each
station.
(II) Xu and Singh (2005) evaluated and compared the
performance of three evapotranspiration models in
three study regions, i.e. Central Sweden (seasonally
snow-covered boreal region), Eastern China (subtropical humid monsoon region) and Northwestern
Cyprus (semiarid region) and the models are: (1)
advection-aridity (AA) model (Brutsaert and Stricker
1979); (2) the complementary relationship areal
evapotranspiration (CRAE) model (Morton 1978,
1983); and (3) the complementary relationship model
proposed by Granger and Gray (1989) using the
concept of relative evapotranspiration. Xu and Singh
(2005) named this model as GG model in their study.
More detailed information about these three models
can be referred to Xu and Singh (2005). Xu and
Singh (2005), after thorough analysis, indicated that
in all the three study regions, the CRAE model
produced slightly better results when the recalibrated
model parameters are used. In this case, CRAE
model is used in this current study.
(III) The original parameters of CRAE model are
calibrated based on the long-term average ETa by
Eq. 1 for each station. Then the CRAE model with
calibrated parameters is used to compute the daily
ETa for each station. We analyze the daily ETa and
the ETref data by Penman–Monteith method with
aim to investigate changing properties of evapotranspiration in the arid region (the Yellow River
basin) and the humid region (the Pearl River basin)
of China (Fig. 2). To detect trends within the
evapotranspiration series, we use non-parametric
Mann–Kendall trend test since that non-parametric
trend detection methods are less sensitive to outliers
than are parametric statistics (Kendall 1975; Mann
1945). Moreover, the rank-based nonparametric
Mann–Kendall test can test trends in a time series
without requiring normality or linearity (Wang et al.
2008), and is therefore highly recommended for
general use by the World Meteorological Organization (Mitchell et al. 1966). It is widely used in
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200
100
0
30
Temperature (oC)
ð1Þ
300
2
4
6
8
10
12
20
10
−10
3
2
4
6
8
10
12
2
4
6
8
10
12
3
2
1
0
Pearl River
Yellow River
0
Wind speed (m/s)
P
AE ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P 2ffi
1 þ PE
Vapor pressure (0.1 hPa)
methods. The water balance analysis results indicated
that the formula by Pike (1964) can well describe the
long-term annual average ETa. The formula is written
as:
Precipitation (mm)
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2
4
6
8
10
12
2.5
2
1.5
1
Fig. 2 Comparison of the mean monthly climatological variables for
the two study regions
detection of trends in hydrological series (e.g., Gao
et al. 2007; Zhang et al. 2009). The 95% confidence
level is used to detect significance of trends. The
observed trends are spatially interpolated by applying the Inverse Distance Weighted (IDW) interpolation method. IDW implies that each station has a
local influence that decreases with distance (De By
2001). It is not meaningful to analyze the regional
structure of trends of hydrological components in
this study by applying and comparing different
interpolation methods. Applying other interpolation
methods (e.g. kriging) results in similar output maps
(Gemmer et al. 2004).
4 Results
4.1 Relations between precipitation, ETa and ETref
Figure 3 demonstrates spatial variations of ETa (Fig. 3a),
precipitation (Fig. 3b) and ETref (Fig. 3c). In the Pearl
River basin, precipitation amount is much larger than ETref
and ETa (Fig. 3). Figure 2 also illustrates larger precipitation, temperature and vapor pressure in the Pearl River than
in the Yellow River basin, and it is particularly the case for
the vapor pressure, except the wind speed. Figure 3 also
shows that spatial patterns of ETa are similar to those of
ETref. Large difference can be observed between spatial
distributions of precipitation and those of ETa and ETref. In
the Yellow River basin, however, spatial properties of ETa
are similar to those of precipitation, and comparatively, the
spatial patterns of ETref are distinctly different in comparison with those of ETa and precipitation (Fig. 4). For
further insight into these viewpoints, we randomly select
two stations in the Pearl River basin (Anshun and
Stoch Environ Res Risk Assess (2011) 25:139–150
A
143
A
B
B
C
C
Fig. 3 Long-term average of annual actual evaporation (a), precipitation (b) and reference evapotranspiration (c) of the Pearl River
basin
Fengshan) and in the Yellow River basin (Xixian and
Jingtai) with aim to show relations of precipitation (P), ETa
and ETref. Figure 5 indicates that ETa is similar to ETref in
terms of changing characteristics in the Pearl River basin.
The precipitation amount is much higher than ETa and
ETref. In the Yellow River basin, however, changes of ETa
are close to precipitation, showing considerable influences
of precipitation on ETa. These results are in good agreement with ‘water limited’ and ‘energy limited’ system
(Xu and Singh 2004). Besides, Chi-square analysis of
above hydro-meteorological variables also shows significant difference of humidity conditions of the Yellow River
and the Pearl River basin respectively. In this sense, it is
reasonable to accept that the Pearl River basin is an ‘energy
limited’ system and the Yellow River basin a ‘water limited’ system.
4.2 Trends of precipitation, ETa and ETref
Annual trends of precipitation, ETa and ETref for the Pearl
River basin are demonstrated in Fig. 6. The Pearl River
basin is dominated by decreasing ETa, ETref and
Fig. 4 Long-term average of annual actual evaporation (a), precipitation (b) and reference evapotranspiration (c) of the Yellow River
basin
precipitation. The difference is that the decrease of precipitation in the Pearl River basin is not significant. Most
stations show significant decreasing ETa. However, stations
characterized by significant decreasing ETref are located
mainly in the Pearl River basin between 106E and 110E. In
terms of seasonal precipitation trends (Fig. 7), generally
speaking, seasonal precipitation changes of the Pearl River
basin are characterized mainly by decreasing trends, particularly in spring, summer and autumn. Precipitation in
winter is increasing but is not significant. Decreasing ETref
can be observed in most parts of the Pearl River basin
(Fig. 8) in spring (Fig. 8a), summer (Fig. 8b) and autumn
(Fig. 8c). Difference of ETref changes in winter when
compared to spring, summer and autumn is that some
stations in the upper and lower Pearl River basin show
increasing ETref (Fig. 8d). ETa changes are in similar patterns when compared to ETref. Results by Gao et al. (2007)
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Stoch Environ Res Risk Assess (2011) 25:139–150
300
250
250
Anshun station
(26.25 N, 105.96E)
Fengshan station
(24.55 N, 107.03E)
P
AE
ET
ET, AE and P (mm)
ET, AE and P (mm)
Fig. 5 Long-term monthly
mean reference
evapotranspiration (ET), actual
evaporation (AE) and
precipitation (P) of two gauging
stations randomly selected from
the Pearl River basin (upper two
panels) and the Yellow River
basin (lower two panels)
200
150
100
150
100
50
50
0
200
2
4
6
8
10
0
12
Xixian station
(36.7N, 110.95E)
100
50
0
2
4
6
8
Time (months)
A
B
C
Fig. 6 Trends of actual evaporation (a), precipitation (b) and
reference evapotranspiration (c) over the Pearl River basin. Solid
lines show positive trends and dashed lines denote negative trends
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10
12
4
6
8
10
12
8
10
12
Jingtai station
(37.18N, 104.05E)
ET, AE and P (mm)
ET, AE and P (mm)
150
2
150
100
P
AE
ET
50
0
2
4
6
Time (months)
also indicated decreasing ETa in the Pearl River basin in
terms of annual changes. Our results further show that
seasonal ETa is also decreasing (Fig. 9).
Annual trends of precipitation, ETa and ETref are illustrated in Fig. 10. ETa and precipitation are in decreasing
trends and significant decrease can be identified in the
middle and upper Yellow River basin. These observations
are in good agreement with the results by Gao et al. (2007).
ETref shows different changing properties when compared
to those of precipitation and ETa (Fig. 10c). A majority of
stations in the upper Yellow River basin are characterized
by increasing ETref and significant increasing ETref is
observe in upper Yellow River basin. These results are also
in good line with those by Gao et al. (2007). Changes of
seasonal precipitation, ETa and ETref are demonstrated in
Fig. 11. Figure 11 shows different properties of precipitation in diverse seasons. The Yellow River basin is dominated by decreasing precipitation in spring, summer and
autumn (Fig. 11a–c). Precipitation in winter is in different
changing properties in comparison with those in other
seasons, i.e. the west and north parts of the Yellow River
basin are characterized by increasing trends and the
increasing trends at some stations are significant statistically (Fig. 11d). ETa are in similar spatial patterns with
those of precipitation (Fig. 12a), showing good relations
between precipitation and ETa in the Yellow River basin.
Comparison between Figs. 11c and 12c indicates that
regions with stations characterized by significant decreasing precipitation match well the places with stations featured by significant decreasing ETa. As for precipitation
Stoch Environ Res Risk Assess (2011) 25:139–150
145
A
B
C
D
Fig. 7 Trends of seasonal precipitation in spring (a), summer (b), autumn (c) and winter (d) in the Pearl River basin. Solid lines show positive
trends and dashed lines denote negative trends
A
C
B
D
Fig. 8 Trends of reference evapotranspiration in the Pearl River basin. Solid lines show positive trends and dashed lines denote negative trends
changes in winter (Fig. 11d), majority of stations are dominated by increasing precipitation. Precipitation at some
stations in west parts of the Yellow River basin is in significant increasing trends. Stations with increasing ETa distribute sporadically in the Yellow River basin (Fig. 12d).
Even so, stations having increasing ETa are also located
within the area dominated by increasing precipitation. The
above-mentioned analysis told such a story that ETa in the
Yellow River basin largely but not exactly hinges on spatial
and temporal changes of precipitation. It is reasonable since
that ETa variations are the results of more than meteorological factors, such as wind speed, temperature, soil moisture conditions, vapor pressure and so forth. However, the
results mentioned above still well support the viewpoint that
the Yellow River basin is a ‘water limited system’. ETref
variations present distinctly different spatial patterns when
compared to precipitation and ETa changes (Fig. 13). ETref
in spring, autumn and winter is increasing in the regions
between 102E and 110E of the Yellow River basin.
Increasing ETref in summer is observed mainly in the west
corner of the Yellow River basin.
5 Discussions
Xu et al. (2006) and Gong et al. (2006) analyzed influencing factors of ETref, indicating that the most sensitive
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Stoch Environ Res Risk Assess (2011) 25:139–150
A
B
C
D
Fig. 9 Trends of actual evaporation in the Pearl River basin. Solid lines show positive trends and dashed lines denote negative trends
A
B
C
Fig. 10 Trends of actual evaporation (a), precipitation (b) and
reference evapotranspiration (c) over the Yellow River basin. Solid
lines show positive trends and dashed lines denote negative trends
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variable for the ETref is the relative humidity followed by
the net total radiation, air temperature and wind speed. Due
to considerable dependence of ETa on precipitation, we
also analyze precipitation changes. In this case, we analyzed changes of precipitation, air temperature, wind speed
and sunshine hours for the Pearl River (Fig. 14a–d) and the
Yellow River basin (Fig. 14e–h). Seasonal variations of
these meteorological factors show similar characteristics
when compared to annual changes, therefore, we only
analyzed the annual variations of these meteorological
factors in this study. Figure 14a–d show that, in the Pearl
River basin, precipitation and air temperature are increasing statistically. However, wind speed and sunshine hours
are decreasing statistically. Increasing temperature may
have the potential to enhance evaporation. However, some
scholars indicated that a warmer atmosphere does not
necessarily increase evaporation due to the fact that
hemispheric evaporation is much more substantial in winter than in summer under the present climate (Ohmura and
Wild 2002). Furthermore, analysis results of temperature
extremes indicated significant increasing extreme low
temperature but no significant increasing extreme high
temperature (Zhang et al. 2008). Decreasing wind speed
and sunshine hours may be mainly responsible for
decreasing ETa and ETref.
As for wind speed, we analyzed areal average wind speed
at 200, 500, 850 and 1000 hPa (figures not shown here). The
results indicated increasing wind speed at 200 hPa, and
downward tendency from 200 to 1000 hPa. Therefore, we
speculated that urbanization may be one of the causes
behind decreasing wind speed near the ground surface.
Decreasing sunshine hours may be due to increasing cloud
cover. However, our analysis (figures not shown here)
indicated that the areal average cloud cover is decreasing,
Stoch Environ Res Risk Assess (2011) 25:139–150
147
A
B
C
D
Fig. 11 Trends of precipitation in spring (a), summer (b), autumn (c) and winter (d) in the Yellow River basin. Solid lines show positive trends
and dashed lines denote negative trends
A
B
C
D
Fig. 12 Trends of actual evaporation in spring (a), summer (b), autumn (c) and winter (d) in the Yellow River basin. Solid lines show positive
trends and dashed lines denote negative trends
thus the decreasing sunshine hours could be due to
increasing aerosol across the Pearl River basin (Xu et al.
2006). All these factors combined to cause decreasing ETa
and ETref over the Pearl River basin. Similarly, decreasing
wind speed and sunshine hours are also observed in the
Yellow River basin. Except these factors, evapotranspiration changes are also heavily influenced by precipitation
amount in the Yellow River basin, a ‘water-limited’ system.
Decreasing ETa across the Yellow River basin should be
attributed to areal average precipitation of the Yellow River
basin. Increasing ETref in the Yellow River basin implies
enhancing hydrological cycle in the Yellow River basin (Xu
et al. 2006; Gao et al. 2007).
In this study, the Pearl River basin is taken as the wet
environment and the Yellow River basin the dry environment. Figure 2 well corroborates these assumptions: the
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A
B
C
D
Fig. 13 Trends of reference evapotranspiration in spring (a), summer (b), autumn (c) and winter (d) in the Yellow River basin. Solid lines show
positive trends and dashed lines denote negative trends
Fig. 14 Changes of
standardized a temperature, b
precipitation, c wind speed, and
d sunshine hours for the Pearl
River basin; and e temperature,
f precipitation, g wind speed,
and h sunshine hours for the
Yellow River basin
ETa and ETref approach each other in the Pearl River basin;
precipitation and ETa match well in the Yellow River
basin. Therefore, we can say that the ETa reaches the
evaporation demand (or energy available for evaporation)
in the Pearl River basin, i.e. an ‘‘energy limited’’ system
(Xu and Singh 2004). In this Yellow River basin however,
ETa are controlled mainly by the precipitation amount even
the ETref is much higher than precipitation in the Yellow
River basin. This is called as a ‘‘water limited’’ system
(Xu and Singh 2004). As for the evapotranspiration models
we used in this study, we accepted the CRAE model based
on the previous studies (e.g. Xu et al. 2005). Hobbins et al.
(2001) evaluated the performance of the Complementary
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Relationship Areal Evapotranspiration (CRAE) model and
the Advection-Aridity (AA) model against independent
estimates of regional evapotranspiration derived from longterm, large-scale water balances (1962–1988) for 120
minimally impacted basins in the conterminous United
States, demonstrating that the CRAE model accurately
predicts monthly regional evapotranspiration. All these
previous analysis further confirms the rationality of the
application of CRAE model in the current study. It should
be noted here uncertainty of the models are common in
the hydrological study and the discussion of modeling
uncertainty is out of the scope of this study. Even so, the
results of this study still provide scientifically meaningful
Stoch Environ Res Risk Assess (2011) 25:139–150
conclusions for the further development of human understanding of hydrological cycle in arid and humid regions in
China, and also contribute much to the similar studies of
the world.
6 Conclusions
The evapotranspiration is one of the key components of
hydrological cycle. ETa is attached considerable importance in sound understanding of the way the changing
climate influence the hydrological cycle on a regional and
global scale (e.g. Ohmura and Wild 2002). In the current
study, we analyzed changing properties of ETa, ETref and
precipitation in the arid region (the Yellow River basin)
and humid region (the Pearl River basin). We also investigate possible causes for these changing properties by
investigation changing features of wind speed, sunshine
hours, precipitation and air temperature. Some interesting
conclusions are achieved as the follows:
(1)
(2)
(3)
In the arid and humid regions, decreasing ETa is
observed. However, ETref presents different changing
properties in the arid and humid regions of China.
Majority parts of the Pearl River basin are dominated
by decreasing ETref and some parts of the Yellow
River basin are characterized by increasing ETref,
which may imply enhancing hydrological cycle in the
Yellow River basin when compared to that of the
Pearl River basin.
In the Yellow River basin, spatial patterns of ETa
match well those of precipitation changes, implying
that the changes of ETa in the arid regions largely
hinges on precipitation amount. In the humid regions,
changes of the ETa follow the similar patterns when
compared to those of ETref. Decreasing actual and
ETref may imply weakening hydrological cycle in the
Pearl River basin when compared to the Yellow River
basin. This conclusion has the potential to contribute
to better understanding of influences of climate
changes on the hydrological cycle in the arid and
humid regions respectively in China.
Decreasing wind speed in the Pearl River and Yellow
River basin may be partly due to intensifying
urbanization in that wind speed tends to be increasing
from 1000 to 200 hPa. Decreasing sunshine hours
should be attributed to increasing aerosol because the
cloud cover is increasing over the Pearl River basin.
Therefore, human activities contributed much to the
decreasing ETa and ETref in the Pearl River basin. In
the Yellow River basin, evapotranspiration largely
depends on precipitation amount. Decreasing precipitation should be responsible for decreasing ETa in the
149
Yellow River basin. The current study is practically
and scientifically significant for regional assessment
of water resource in the arid and humid regions of
China under the changing climate. It should be noted
here that in no way does it mean that the current
report perfectly settled down the subject of evapotranspiration. However the current study provides an
important clue for further study of hydrological cycle
in the arid and humid regions of China under the
changing climate, and which is the major contribution
of this study to the hydrological science.
Acknowledgements This work is financially supported by the
Program for Outstanding Young Teachers of the Sun Yat-sen
University (Grant No.: 2009-37000-1132381), the ‘985 Project’
(Grant No.: 37000-3171315), the Key National Natural Science
Foundation of China (Grant No.: 50839005), a grant from the
Research Grants Council of the Hong Kong Special Administrative
Region, China (Project No. CUHK405308) and by Program of
Introducing Talents of Discipline to Universities—the 111 Project of
Hohai University. Cordial thanks should be extended to two anonymous reviewers and the editor, Prof. Dr. Jiaping Wu, for their constructive and pertinent comments which greatly helped to improve the
quality of this manuscript.
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