Pan Evaporation Trends Research Articles

Spatiotemporal variations of pan evaporation in China during 1960–2005: changing patterns and causes

ABSTRACTObserved pan evaporation data from 588 stations covering the entire territory of China are analysed using the modified Mann–Kendall trend test method. Fuzzy C‐Means clustering is conducted for regionalization. Sensitivity analysis is performed to identify the principal influencing factors. Results indicate that: (1) the entire country can be categorized into three parts, i.e. A: southern, central and southwestern China; B: northwestern China and C: strip zone extending from northeastern to southwestern China; (2) significant decreasing pan evaporation can be identified in southern, central, southwestern, eastern and northwestern China. Stations with significant increasing pan evaporation seem to be scattered sporadically across China. Besides, the strip zone extending in the NE‐SW direction is dominated by significant and also nonsignificant increasing trends of pan evaporation and (3) sensitivity analysis indicates that relative humidity is the principal influencing factor for pan evaporation, especially in northwestern, northern and northeastern China. Generally, in northwestern, northern and northeastern China, relative humidity has an adverse relation with pan evaporation, implying pan evaporation paradox and also intensifying hydrological cycle in these regions. In southeastern China, particularly the middle and lower Yangtze River basin and the Pearl River basin, relations between pan evaporation and relative humidity are relatively complex, showing that other factors in addition to relative humidity can have impacts on pan evaporation changes, such as cloud coverage, temperature and aerosol concentration. Local features of aerodynamic and radiative drivers of the hydrological cycle and their regional responses to climate changes, and also different features of ground surface may play considerable roles in pan evaporation changes.

China's dimming and brightening: evidence, causes and hydrological implications

Abstract. There is growing evidence that, corresponding to global dimming and brightening, surface solar radiation and sunshine hours over China have undergone decadal fluctuations during the 1960s–2000s. The results of a number of these analyses are, however, very different. In this study, we synthesize reliable results and conclusively address recent advances and insufficiencies in studies on dimming and brightening in China. A temporally and spatially prevalent dimming trend is noted in surface solar radiation, direct solar radiation and sunshine hours since the 1960s. Meanwhile, the changing trend in diffuse solar radiation is less pronounced. Increasing anthropogenic aerosol loading is regarded as the most plausible explanation for China's dimming. The brightening trend since 1990, which mainly occurs in southeastern China and in the spring season, is weak and insignificant. The reverse in the solar radiation trend is associated with climate change by cloud suppression and slowdown in anthropogenic emissions. The future solar radiation trend in China could largely depend on the development of air quality control. Other potential driving factors such as wind speed, water vapor and surface albedo are also non-negligible in specific regions of China. Hydrological implications of dimming and brightening in China lack systematic investigation. However, the fact that solar radiation and pan evaporation trends in China track a similar curve in 1990 further suggests that the pan evaporation paradox could be partly resolved by changes in solar radiation.

Assessment of Temporal and Spatial Variation of Pan Evaporation with Related Climatological Factors in Bangladesh

Pan evaporation is an effective way to analyze the multidimensional impact of climate change on irrigation water requirement since pan evaporation measures the integrated effect of radiation, wind, temperature and humidity on the evaporation from an open-water surface. The characteristic trends of pan evaporation and related climatological factors, as developed in this paper, indicate that most of the regions of Bangladesh have undergone a significant amount of decrease in evaporation through the years. The reduction in sunshine duration as a consequent of climate change can be attributed to be the principal reason for the decrease in evaporation. The spatial distribution of seasonal variation of pan evaporation along with solar radiation and humidity was analyzed, and solar radiation seemed to have the major influence on evaporation. The study also reveals that summer and spring are the seasons of highest evaporation in most of the regions. The characteristic trend and spatial distribution of seasonal pan evaporation correlated with related climatological factors developed in this study could aid in water resources development and planning for irrigation purposes.

Decrease in evaporation over the Indian monsoon region: implication on regional hydrological cycle

Using surface observations from 58 widely distributed stations over India, a highly significant (99.9 %) decreasing trend of pan evaporation (Epan) of 9.24 mm/a/a is calculated for 1971 to 2010. This constitutes a ~10 % reduction of Epan over the last four decades. While Epan is decreasing during the wet summer monsoon season (JJAS), as well as during the dry rest of the year, the rate of decrease during the dry season is much larger than that during the wet season. Apart from increasing solar dimming, surface winds are also persistently decreasing over the Indian sub-continent at the rate of −0.02 m/s/a resulting in ~40 % reduction over the last four decades. Based on PenPan model, it is shown that both the above factors contribute significantly to the decreasing trend in Epan. On a continental scale, annual mean potential evaporation (Ep) is larger than rainfall (P or Ep-P > 0, moisture divergence) indicating that India is water-limited. However, during wet monsoon P > Ep (or Ep-P < 0, moisture convergence) indicating that India is energy-limited during this season. Long term data shows that annually Ep-P follows a significant decreasing trend indicating that water limitation is decreasing with time. This is largely due to stronger decreasing trend of Ep-P during the dry season compared to weaker increasing trend of Ep-P during the wet monsoon season. The scatter plot of Ep-P versus Ep also conveys that the decrease in Ep leads to increase in moisture convergence in wet season and decrease in moisture divergence in dry season.

The energy balance of a US Class A evaporation pan

Concurrent with the trend of rising global average air temperature, there have been worldwide observations of a decline in pan evaporation over the last 30–50 years. This global phenomenon has since received much attention from the scientific community. Better interpretation of the long-term trend of pan evaporation (involving seasonal and inter-annual variations) requires rigorous experimental investigation of the physics of pan evaporation. To do that, we constructed an instrumented US Class A pan that replicated an operational pan at Canberra Airport in Australia. We subsequently monitored pan evaporation and associated meteorological variables at half-hourly intervals over a three-year period (Oct 2007–Jan 2010).Extending our earlier work on the aerodynamics of pan evaporation, we conducted a theoretical and experimental study on the energy balance of the pan under non-steady state conditions. The theory considers heat exchanges at both the pan water surface and the pan wall. We formulate the radiative balance based on the geometry of the system (the pan diameter, the height of the water level, the height of the rim and solar zenith angle) and account for differences in albedo and emissivity between the pan water surface and the pan wall. The theory is used to compute all relevant energy fluxes and thereby close the energy balance. Integration of the half-hourly fluxes to a daily basis showed that we were able to close the energy budget with a RMSE of ∼18Wm−2 or 7.5% of the net short-wave radiation. We find that evaporation from our pan is dominated (∼80%) by the radiative exchanges at the pan water surface; those at the pan wall are smaller but also important. Contrary to expectation, there was little if any sensible heat exchange across the pan wall. Instead we found that the major sensible heat flux occurs across the pan water surface. Importantly, evaporative cooling at the pan water surface can result in a transfer of sensible heat from the (warmer) air to the (cooler) surface of the water body. The sensible heat flux from the overlying air to the pan water surface is minimal at low evaporation rates, but at higher evaporation rates (e.g., >300Wm−2) it contributes the equivalent of up to 15% of the total evaporative flux.

Dynamic factor analysis and artificial neural network for estimating pan evaporation at multiple stations in northern Taiwan

Evaporation is an important reference for managers of water resources. This study proposes a hybrid model (BD) that combines back-propagation neural networks (BPNN) and dynamic factor analysis (DFA) to simultaneously precisely estimate pan evaporation at multiple meteorological stations in northern Taiwan through incorporating a large number of meteorological data sets into the estimation process. The DFA is first used to extract key meteorological factors that are highly related to pan evaporation and to establish the common trend of pan evaporation among meteorological stations. The BPNN is then trained to estimate pan evaporation with the inputs of the key meteorological factors and evaporation estimates given by the DFA. The BD model successfully inherits the advantages from the DFA and BPNN, and effectively enhances its generalization ability and estimation accuracy. The results demonstrate that the proposed BD model has good reliability and applicability in simultaneously estimating pan evaporation for multiple meteorological stations. Citation Chang, F.J., Sun, W., and Chung, C.H., 2013. Dynamic factor analysis and artificial neural network for estimating pan evaporation at multiple stations in northern Taiwan. Hydrological Sciences Journal, 58 (4), 813–825.

Trends in Annual and Seasonal Pan Evaporation in the Lower Yellow River Basin from 1961 to 2010

The annual and seasonal trends in pan evaporation in the lower Yellow River Basin based on quality-controlled data from 10 meteorological stations in 1961–2010 are analyzed. The causes for the changes in annual and seasonal pan evaporation are also discussed. The results suggest that, despite the 1.15°C increasing in annual mean surface air temperature over the past 50 years (0.23°C per decade), the annual pan evaporation has steadily declined by an average rate of −7.65 mm per year. By comparison, this change is greater than those previously reported in China. Significant decreasing trends in annual pan evaporation have been observed at almost all stations. As a whole, seasonal pan evaporation decreased significantly, especially in summer, whereas seasonal temperature increased significantly, except in summer. Thus, the pan evaporation paradox exists in the lower Yellow River Basin. The trend analysis of other meteorological factors indicates significant decrease in sunshine duration and wind speed, but no significant variations in precipitation and relative humidity at annual and seasonal time scales. By examining the relationship between precipitation and pan evaporation, it did not show a concurrent decrease in pan evaporation and increase in precipitation. The partial correlation analysis discovered that the primary cause of decrease in annual and seasonal pan evaporation is the decrease in wind speed. A further examination using a stepwise regression shows that decrease in wind speed and sunshine duration, and increase in mean temperature are likely to be the main meteorological factors affecting the annual and seasonal pan evaporation in the lower Yellow River Basin over the past 50 years. CitationJi, X.-J., J.-J. Wang, W.-L. Gu, et al., 2012: Trends in annual and seasonal pan evaporation in the lower Yellow River Basin from 1961 to 2010. Adv. Clim. Change Res.,3(4), doi: 10.3724/SP.J.1248.2012.00195.

Spatio-temporal variations of reference crop evapotranspiration and pan evaporation in the West Songnen Plain of China

The spatio-temporal variations of reference crop evapotranspiration (ETref) reflect the combined effects of meteorological variables, primarily wind speed, relative humidity, net radiation and air temperature. This study investigated the spatial distribution and temporal trends of ETref (calculated by the FAO-56 Penman-Monteith equation), pan evaporation (Epan) and pan coefficient (Kp) in a 140 × 103 km2 semi-humid to semi-arid area in China. The results show that: (i) although the spatial distributions of ETref and Epan are roughly similar and their spatial correlation is high over the growing season, Kp varied considerably in space due to high humidity in the east of the region and low humidity in the southwest; (ii) the monthly variations of ETref and Epan are similar to that of net radiation and opposite to that of relative humidity, while the monthly variation of Kp is similar to that of relative humidity and opposite to that of wind speed, and the long-term trend is slightly increasing for ETref and Epan, while significantly (10% significance level) increasing for Kp; and (iii) generally, the time series of ETref and Epan from 1951 to 2001 could be divided into three phases due to variations of meteorological variables. Citation Liang, L.-Q., Li, L.-J. & Liu, Q. (2011) Spatio-temporal variations of reference crop evapotranspiration and pan evaporation in the West Songnen Plain of China. Hydrol. Sci. J. 56(7), 1300–1313.

Regional trend analysis of pan evaporation in Nigeria (1970 to 2000)

The trends pan evaporation (PE) in 4 different climatic regions (Sahel, Midland, Guinea Savannah and Coastal/Tropical Rainforest) covering about 21 tropical stations in Nigeria (during the period 1970 to 2000) were investigated. The influence of meteorological variables, which have significant impact on the spatial and temporal trends of pan evaporation for three consecutive decades across the different regions in Nigeria was tested. Mann-kendal statistical test was carried out to investigate the trend. A decade to decade trend analysis was carried out for 3 consecutive decades over all the regions. A general decrease trend was observed in all the regions except Sahel region in the first decade under consideration. The decade (1970 to1979) coincided with the period when there was a global solar dimming. However, the rest decades witness an increase trend in pan evaporation for all the climatic regions across Nigeria. The general trend analysis of PE for Nigeria shows that about 80% of stations involved in the study experience increase trend (development of arid condition) out of which, only 50% were significant. The rest 20% exhibited decrease trend (creation of humid condition) with none significant. Further trend analysis was carried out on the various seasons and this revealed that all the wet season (April to October) exhibits downward trend for all the regions while similar trend regime was established for dry season months (December to March) in Sahel and Guinea Savannah and upward trend for midland and the coastal or tropical rainforest. Evidence of pan evaporation paradox was also established for certain period in Nigeria. Key words: Pan evaporation, trend, potential evaporation, meteorological variables.

Multi-Time-Scale Climatic Variations over Eastern China and Implications for the South–North Water Diversion Project

Abstract Middle and eastern routes of the South–North Water Diversion Project (SNWDP) of China, which are approximately located within the area 28°–42°N and 110°–122°E, are being constructed. This paper investigates the past climatic variations on various time scales using instrumental and proxy data. It is found that annual mean surface air temperature has increased significantly during the past 50–100 years, and winter and spring temperatures in the northern part of the region have undergone the most significant changes. A much more significant increase occurs for annual mean minimum temperature and extreme low temperature than for annual mean maximum temperature and extreme high temperature. No significant trend in annual precipitation is found for the region as a whole for the last 50 and 100 years, although obvious decadal and spatial variation is detectable. A seesaw pattern of annual and summer precipitation variability between the north and the south of the region is evident. Over the last 100 years, the Haihe River basin has witnessed a significant negative trend of annual precipitation, but no similar trend is detected for the Yangtze and Huaihe River basins. Pan evaporation has significantly decreased since the mid-1960s in the region in spite of the fact that the trend appears to have ended in the early 1990s. The negative trend of pan evaporation is very significant in the plain area between the Yangtze and Yellow Rivers. There was a notable series of dry intervals lasting decades in the north of the region. The northern drought of the past 30 years is not the most severe in view of the past 500 years; however, the southern drought during the period from the 1960s to the 1980s may have been unprecedented. The dryness–wetness index (DWI) shows significant oscillations with periodicities of 9.5 and 20 years in the south and 10.5 and 25 years in the north. Longer periodicities in the DWI series include 160–170- and 70–80-yr oscillations in the north, and 100–150-yr oscillations in the south. The observed climate change could have implications for the construction and management of the SNWDP. The official approval and start of the hydro project was catalyzed by the severe multiyear drought of 1997–2003 in the north, and the operation and management of the project in the future will also be influenced by climate change—in particular by precipitation variability. This paper provides a preliminary discussion of the potential implications of observed climate change for the SNWDP.

Potential evaporation trends over land between 1983–2008: driven by radiative fluxes or vapour-pressure deficit?

Abstract. We model the Penman potential evaporation (PE) over all land areas of the globe for the 25-yr period 1983–2008, relying on radiation transfer models (RTMs) for the shortwave and longwave fluxes. Penman's PE is determined by two factors: available energy for evaporation and ground to atmosphere vapour transfer. Input to the PE model and RTMs comprises satellite cloud and aerosol data, as well as data from reanalyses. PE is closely linked to pan evaporation, whose trends have sparked controversy in the community, since the factors responsible for the observed pan evaporation trends are not determined with consensus. Our particular interest is the temporal evolution of PE, and the provided insight to the observed trends of pan evaporation. We examine the decadal trends of PE and various related physical quantities, such as net solar flux, net longwave flux, water vapour saturation deficit and wind speed. Our findings are the following: Global warming has led to a larger water vapour saturation deficit. The periods 1983–1989, 1990–1999, and 2000–2008 were characterised by decreasing, increasing, and slightly decreasing PE, respectively. In these last 25 yr, global dimming/brightening cycles generally increased the available energy for evaporation. PE trends seem to follow more closely the trends of energy availability than the trends of the atmospheric capability for vapour transfer, at most locations on the globe, with trends in the Northern hemisphere significantly larger than in the Southern. These results support the hypothesis that global potential evaporation trends are attributed primarily to secular changes in the radiation fluxes, and secondarily to vapour transfer considerations.

Identification of dominant climate factor for pan evaporation trend in the Tibetan Plateau

Despite the observed increase in global temperature, observed pan evaporation in many regions has been decreasing over the past 50 years, which is known as the “pan evaporation paradox”. The “pan evaporation paradox” also exists in the Tibetan Plateau, where pan evaporation has decreased by 3.06 mm a−2 (millimeter per annum). It is necessary to explain the mechanisms behind the observed decline in pan evaporation because the Tibetan Plateau strongly influences climatic and environmental changes in China, Asia and even in the Northern Hemisphere. In this paper, a derivation based approach has been used to quantitatively assess the contribution rate of climate factors to the observed pan evaporation trend across the Tibetan Plateau. The results showed that, provided the other factors remain constant, the increasing temperature should have led to a 2.73 mm a−2 increase in pan evaporation annually, while change in wind speed, vapor pressure and solar radiation should have led to a decrease in pan evaporation by 2.81 mm a−2, 1.96 mm a−2 and 1.11 mm a−2 respectively from 1970 to 2005. The combined effects of the four climate variables have resulted in a 3.15 mm a−2 decrease in pan evaporation, which is close to the observed pan evaporation trend with a relative error of 2.94%. A decrease in wind speed was the dominant factor for the decreasing pan evaporation, followed by an increasing vapor pressure and decreasing solar radiation, all of which offset the effect of increasing temperature across the Tibetan Plateau.

Pan evaporation analysis in central México: Trends, self-affinity and important frequencies

Long-term monthly pan evaporation time series registered at 40 meteorological stations located within Mexico’s state of Zacatecas were analyzed in order to identify their trends. In addition, we analyzed the power spectrum signals of evaporation anomaly series in order to identify their important frequencies and its possible connection with periodic phenomena. Results suggest that negative trends are prevailing over positive trends. We found negative linear trend for 27 out of 40 pan evaporation time series; 18 of the 27 decreasing trends were significant at p<0.05. On the other hand, 13 out of 40 pan evaporation trends were positive but only 3 at significant level (p<0.05). Moreover, noise in all of these monthly pan evaporation series tends to be a persistent behavior. Additionally, we found that important frequencies could be related with the yearly cycle, quasi–biannual cycle, ‘El Nino Southern Oscillation’ phenomena and sunspot cycle. Key words: Linear trends, fractal dimension, power spectrum density, yearly cycle, quasi–biannual cycle, El Nino Southern Oscillation, sunspot cycle.

Land‐atmosphere coupling explains the link between pan evaporation and actual evapotranspiration trends in a changing climate

Decreasing trends in pan evaporation are widely observed across the world as a response of the climate system to changes in temperature, precipitation, incoming radiation and wind speed. Nevertheless, we only partially understand how trends in actual evapotranspiration are linked to those trends. Here, we use a model to show that regulation of the near‐surface temperature and humidity by land‐atmosphere feedbacks results in a strong connection between pan evaporation, actual evapotranspiration and vapor pressure deficit (VPD) depending on the climate forcings. When climate change occurs, the feedbacks direct the system towards a different combination of the three variables. If we know the trends in pan evaporation, VPD and wind speed, we can therefore infer the change in the forcings and estimate the trend in actual evapotranspiration.

Pan evaporation and potential evapotranspiration trends in South Florida

AbstractDeclining trends in pan and lake evaporation have been reported. It is important to study this trend in every region to evaluate the validity of the trend and water management implications. Data from nine pan evaporation sites in South Florida were evaluated to see if there is a trend and if the quality of the data is sufficient for such analysis. The conclusion is that pan evaporation measurements are prone to too many sources of errors to be used for trend analysis. This condition is demonstrated in South Florida and in other regions by differences in magnitude and direction between spatially related pan stations and unexplainable observations. Also, potential evapotranspiration (ETp) was estimated with the Simple (Abtew equation) and the Penman–Monteith method. Both cases indicated no decline in evapotranspiration for the period of analysis. Based on the decline in humidity and the increasing trend in vapor pressure deficit for the short period of analysis, 1992–2009, it appears that South Florida is experiencing increase in evaporation and evapotranspiration at this time assuming no systematic error in the weather stations' observations. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Important factors governing the incompatible trends of annual pan evaporation: evidence from a small scale region

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.

Trend in pan evaporation and its attribution over the past 50 years in China

Trends in pan evaporation are widely relevant to the hydrological community as indicators of hydrological and climate change. Pan evaporation has been decreasing in the past few decades over many large areas with differing climates globally. This study analyzes pan evaporation data from 671 stations in China over the past 50 years in order to reveal the trends of it and the corresponding trend attribution. Mann-Kendall test shows a significant declining trend in pan evaporation for most stations, with an average decrease of 17.2 mm/10a in China as a whole, the rate of decline was the steepest in the humid region (29.7 mm/10a), and was 17.6 mm/10a and 5.5 mm/10a in the semi-humid/semi-arid region and arid region, respectively. Complete correlation coefficients of pan evaporation with 7 climate factors were computed, and decreases in diurnal temperature range (DTR), SD (sunshine duration) and wind speed were found to be the main attributing factors in the pan evaporation declines. Decrease in DTR and SD may relate to the increase of clouds and aerosol as well as the other pollutants, and decrease in wind speed to weakening of the Asian winter and summer monsoons under global climate warming.

Geographical and climatic effects on precipitation and pan evaporation

Multiple linear regression (MLR) analyses and Mann-Kendall trend analyses were con- ducted in order to identify the importance of geographical factors (elevation, freshwater area, urban- ization, and proximity to coast) and climatic factors (temperature, wind speed, relative humidity, and solar radiation) on the long-term averages and trends of precipitation and pan evaporation. Annual climate data and monthly climate data for January, April, July, and October, from 1973 to 2006, were collected from 54 weather observatories nationwide in South Korea in consideration of seasonal effects. GIS analyses were used to determine geographical characteristics in the 54 study areas, and land cover data were analyzed to determine the degree of urbanization. According to standard MLR analyses, proximity to coast had the most significant effect on annual and monthly precipitation. Accordingly, land areas that were farther inland experienced less precipitation, except in July, when the reverse was true. Except in April and July, proximity to coast had a greater effect on annual and monthly pan evaporation, and land areas that were farther inland experienced less pan evaporation. Average elevation had a greater effect than other geographical factors on annual and monthly pre- cipitation trends. With regard to the annual pan evaporation trend, proximity to coast was the most important factor; however, the monthly pan evaporation trend was affected differently by geograph- ical factors, depending on the season.

Assessing contributions to panevaporation trends in Haihe River Basin, China

In many places around the world, panevaporation has been detected to decrease with the increase in temperature, which is known as the “panevaporation paradox.” An example of the paradox was found in the Haihe River Basin from 1957 to 2001. To explain the mechanism of the paradox, an approach to quantify the contributions of climate factors to the panevaporation trend has been proposed, in which the individual contribution was defined as the product of the partial derivative and slope of the trend for the concerned variables. Four variables, including temperature, wind speed, solar radiation, and vapor pressure, were selected based on the Penman‐Monteith method to assess their individual contribution to the panevaporation trend. The results showed that an increase in temperature resulted in the increase of panevaporation, but this effect had been offset by an increase in vapor pressure and decrease in wind speed and solar radiation. Wind speed was the dominant factor contributing to panevaporation decreases in the Haihe River Basin.

Warming and drying trends on the Tibetan Plateau (1971–2005)

Annual and seasonal trends in maximum and minimum temperatures, precipitation and vapour pressure deficit (VPD) were examined with the goal of understanding trends in temperature and moisture across the Tibetan Plateau, using meteorological data (1971–2005) collected at 63 stations. Trends in pan evaporation (PE; 1971–2001, 68 stations) and runoff (1971–2002) in the headwater of the Yellow River were also analysed. Positive trends in maximum and minimum temperatures were observed across the Tibetan Plateau. The highest increases were observed during winter, with results from the majority of stations statistically significant at the 95% level. A decrease trend in diurnal temperature range (DTR) was also observed. Trends in annual and seasonal precipitation and VPD were positive, while the trend in PE was negative. However, the increase in precipitation was not as pronounced as the increase in temperature. Although PE decreased during the time series, actual evaporation probably increased because of the warming across the Tibetan Plateau, where the annual potential water loss measured as PE is three to four times the annual water supply by precipitation. Warming was expected to increase evapotranspiration, causing more water vapour to escape into the atmosphere, thus counteracting or even exceeding the slight increase in precipitation. The increases in annual and seasonal VPD trends indicated a drying tendency and were further substantiated by the observed decrease in runoff in the headwater catchment of the Yellow River. The results provided insight into recent climatic changes across the Tibetan Plateau.