Spatial and temporal variations and controlling factors of potential evapotranspiration in China:1956-2000

Based on the climatic data of 580 stations in China during 1956 and 2000, potential evapotranspiration are calculated using the Penman-Monteith Method recommended by FAO. The spatial and temporal distributions of the potential evapotranspiration over China and the temporal trends of the regional means for 10 major river basins and whole China are analyzed. Through a partial correlation analysis, the major climate factors which affect the temporal change of the potential evapotranspiration are analyzed. Major results are drawn as follows： 1） The seasonal and annual potential evapotranspiration for China as a whole and for most basins show decline tendencies during the past 45 years; for the Songhua River Basin there appears a slightly increasing trend. 2） Consequently, the annual potential evapotranspirations averaged over 1980-2000 are lower than those for the first water resources assessment （1956-1979） in most parts of China. Exceptions are found in some areas of Shandong Peninsula, western and middle basins of the rivers in Southwest China, Ningxia Hui Autonomous Region as well as the source regions of the Yangtze and Yellow rivers, which may have brought about disadvantages to the exploration and utilization of water resources. 3） Generally, sunshine duration, wind speed and relative humidity have greater impact on the potential evapotranspiration than temperature. Decline tendencies of sunshine duration and/or wind speed in the same period appear to be the major causes for the negative trend of the potential evapotranspiration in most areas.

Sensitivity of Potential Evapotranspiration Estimation to the Thornthwaite and Penman–Monteith Methods in the Study of Global Drylands

Drylands are among those regions most sensitive to climate and environmental changes and human-induced perturbations.The most widely accepted definition of the term dryland is a ratio,called the Surface Wetness Index(SWI),of annual precipitation to potential evapotranspiration(PET)being below 0.65.PET is commonly estimated using the Thornthwaite(PET Th)and Penman–Monteith equations(PET PM).The present study compared spatiotemporal characteristics of global drylands based on the SWI with PET Th and PET PM.Results showed vast differences between PET Th and PET PM;however,the SWI derived from the two kinds of PET showed broadly similar characteristics in the interdecadal variability of global and continental drylands,except in North America,with high correlation coefficients ranging from 0.58 to 0.89.It was found that,during 1901–2014,global hyper-arid and semi-arid regions expanded,arid and dry sub-humid regions contracted,and drylands underwent interdecadal fluctuation.This was because precipitation variations made major contributions,whereas PET changes contributed to a much lesser degree.However,distinct differences in the interdecadal variability of semi-arid and dry sub-humid regions were found.This indicated that the influence of PET changes was comparable to that of precipitation variations in the global dry–wet transition zone.Additionally,the contribution of PET changes to the variations in global and continental drylands gradually enhanced with global warming,and the Thornthwaite method was found to be increasingly less applicable under climate change.

Comparison of Ten Potential Evapotranspiration Models and Their Attribution Analyses for Ten Chinese Drainage Basins

Potential evapotranspiration(EPET)is usually calculated by empirical methods from surface meteorological variables,such as temperature,radiation and wind speed.The in-situ measured pan evaporation(ETpan)can also be used as a proxy for EPET.In this study,EPET values computed from ten models are compared with observed ETpan data in ten Chinese river basins for the period 1961−2013.The daily observed meteorological variables at 2267 stations are used as the input to those models,and a ranking scheme is applied to rank the statistical quantities(ratio of standard deviations,correlation coefficient,and ratio of trends)between ETpan and modeled EPET in different river basins.There are large deviations between the modeled EPET and the ETpan in both the magnitude and the annual trend at most stations.In eight of the basins(except for Southeast and Southwest China),ETpan shows decreasing trends with magnitudes ranging between−0.01 mm d−1 yr−1 and−0.03 mm d−1 yr−1,while the decreasing trends in modeled EPET are less than−0.01 mm d−1 yr−1.Inter comparisons among different models in different river basins suggest that PETHam1 is the best model in the Pearl River basin,PETHam2 outperforms other models in the Huaihe River,Yangtze River and Yellow River basins,and PETFAO is the best model for the remaining basins.Sensitivity analyses reveal that wind speed and sunshine duration are two important factors for decreasing EPET in most basins except in Southeast and Southwest China.The increasing EPET trend in Southeast China is mainly attributed to the reduced relative humidity.

Shortwave radiation balance modulates potential evapotranspiration over China

Shortwave radiation is an influential driver of global hydrological cycle,as its variation will alter evapotranspiration(ET).While climate change discussion extensively examined ET response to temperature,wind speed and land use/cover variations,little is known about the contribution of shortwave radiation balance on ET partly because of data availability.In this study,we applied the newly-released Global LAnd Surface Satellite(GLASS)products to detect changes in shortwave radiation and albedo for 2003–2018 over China,and quantified their contributions on potential evapotranspiration(PET)which is the upper limit of ET and generally the basis to estimate it.The results showed that downward shortwave radiation declined,together with a slight decrease in albedo,causing a decrease of net shortwave radiation.While climate change affected PET,the decreased net shortwave radiation reduced PET during the period.We identified a hotspot area in the north China plain that is one of the main agricultural production regions,as the radiation balance induced a contrasting effect on PET for the warm and the cold seasons.Our study indicates that it is important to consider shortwave radiation balance when estimating PET and ET to make reliable evaluation of climate change on hydrological cycle and ecosystem response.

Spatial and temporal change in the potential evapotranspiration sensitivity to meteorological factors in China （1960-2007）

Potential evapotranspiration （E0）, as an estimate of the evaporative demand of the atmosphere, has been widely studied in the fields of irrigation management, crop water demand and predictions in ungauged basins （PUBs）. Analysis of the sensitivity of E0 to meteorological factors is a basic research on the impact of climate change on water resources, and also is important to the optimal allocation of agricultural water resources. This paper dealt with sensitivity of E0 over China, which was divided into ten drainage systems, including Songhua River basin, Liaohe River basin, Haihe River basin, Yellow River basin, Yangtze River basin, Pearl River basin, Huaihe River drainage system, Southeast river drainage system, Northwest river drainage system and Southwest river drainage system. In addition, the calculation method of global radiation in Penman-Monteith formula was improved by optimization, and the sensitivities of Penman-Monteith potential evapotranspiration to the daily maximum temperature （STmax）, daily minimum temperature （STmin）, wind speed （SU2）, global radiation （SRs） and vapor pressure （SVP） were calculated and analyzed based on the long-term meteorological data from 653 meteorological stations in China during the period 1960–2007. Results show that： （1） the correlation coefficient between E0 and pan evaporation increased from 0.61 to 0.75. E0 had the decline trends in eight of ten drainage systems in China, which indicates that ＂pan evaporation paradox＂ commonly exists in China from 1960 to 2007. （2） Spatially, Tmax was the most sensitive factor in Haihe River basin, Yellow River basin, Huaihe River drainage system, Yangtze River basin, Pearl River basin and Southeast river drainage system, and VP was the most sensitive factor in Songhua River Basin, Liaohe River basin, Northwest river drainage system while Rs was the most sensitive factor in Southwest river drainage system. For the nation-wide average, the most sensitive factor was VP, followed by Tmax, Rs, U2 and Tmin. In addition, the changes in sensitivity coefficients had a certain correlation with elevation. （3） Temporally, the maximum values of STmax and SRs occurred in July, while the maximum values of STmin, SVP and SU2 occurred in January. Moreover, trend analysis indicates that STmax had decline trends, while STmin, SU2, SRs and SVP had increasing trends.

Spatial distribution and temporal trends in potential evapotranspiration over Hengduan Mountains region from 1960 to 2009

Based on the meteorological data of 20 stations in the Hengduan Mountains region during 1961–2009, the annual and seasonal variation of potential evapotranspiration was analyzed in combination with the Penman-Monteith model. With the method of Spline interpolation under ArcGIS, the spatial distribution of potential evapotranspiration was presented to research the regional difference, and the correlation analysis was used to discuss the dominant factor affecting the potential evapotranspiration. The results indicated that the an-nual potential evapotranspiration showed a decreasing tendency since the 1960s, especially from the 1980s to 1990s, while it showed an increasing tendency since 2000. Regional potential evapotranspiration showed a rate of –0.17 mm a？1. Potential evapotranspiration in north, middle and south of the Hengduan Mountains exhibited decreasing trends over the studied period, and its regional trend was on the decline from southwest to northeast.

Evaluation and modification of potential evapotranspiration methods in Beijing,China

In this study,seven widely used potential evapotranspiration(ETo)methods were evaluated by comparing with the FAO-56 Penman-Monteith method(PM method)to provide useful information for selecting appropriate ETo equations under data-limited condition in Beijing,China.Statistical methods and parameters,namely linear regression,root mean squared error(RMSE)and mean bias error(MBE),were used to evaluate the seven ETo methods.Results showed that ETo estimated using Kimberly-Penman method have fairly close agreement with the PM method(referring to standard ETo),considering the coefficient of determination(R^(2))of 0.96,RMSE of 0.42 mm/day,and a coefficient of efficiency(E)of 0.96.Locally calibrated Penman and Doorenbos-Pruitt methods also have better agreement with the PM method,correspondingly with R^(2)of 0.99 and 0.95,RMSEs of 0.24 mm/day and 0.21 mm/day,and coefficients of efficiency of 1.02 and 0.99,respectively.The ETo is the most sensitive to vapor pressure deficit(VPD)and net radiation in the Beijing area.Hence,the VPD-based and VPD-radiation combined ETo methods were developed and calibrated.Results showed that the two developed methods performed well in ETo estimation.By fully considering the data-limit situation,the calibrated Turc method,VPD-based method and VPD-radiation-combined method may be attractive alternatives to the more complex Penman−Monteith method in Beijing.

Division of desertification climate types and determination of potential range of desertification in China

Distribution of desertification climate types in China was analyzed using Thornthwaite's method on calculating potential evapotranspiration (PE), according to the definition provided by the United Nations Convention, by employing meteorological records from 1864 stations in China. The annual PE and the humidity indices were calculated for every station, based on which a distribution map of HI was constructed. The potential range of desertification in China was obtained for the first time, about 3.32 million km2 and makes up about 34.6 percent of the country. The distribution map of HI was compared with the vegetation map of China and the precipitation map of China respectively. In eastern and northern China, the distribution of climate types is basically acceptable, but for QinghaiTibetan Plateau, where Thornthwaite's method on calculating PE is not quite suitable, the isopleths are deviated to the northern side and the potential range of desertification is smaller than expected.

Impact of Drought on Agriculture in the Indo-Gangetic Plain,India

In this study, we investigate the spatiotemporal characteristics of drought in India and its impact on agriculture during the summer season （April-September）. In the analysis, we use Standardized Precipitation Evapotranspiration Index （SPEI） datasets between 1982 and 2012 at the six-monthly timescale. Based on the criterion SPEI 〈 -1, we obtain a map of the number of occurrences of drought and find that the humid subtropical Upper Middle Gangetic Plain （UMGP） region is highly drought-prone, with an occurrence frequency of 40%-45%. This UMGP region contributes at least 18%-20% of India＇s annual cereal production. Not only the probability of drought, but the UMGP region has become increasingly drought-prone in recent decades. Moreover, cereal production in the UMGP region has experienced a gradual declining trend from 2000 onwards, which is consistent with the increase in drought-affected areas from 20%-25% to 50%-60%, before and after 2000, respectively. A higher correlation coefficient （-0.69） between the cereal production changes and drought-affected areas confirms that at least 50% of the agricultural （cereal） losses are associated with drought. While analyzing the individual impact of precipitation and surface temperature on SPEI at 6 month timescale [SPEI （6）] we find that, in the UMGP region, surface temperature plays the primary role in the lowering of the SPEI. The linkage is further confirmed by correlation analysis between SPEI （6） and surface temperature, which exhibits strong negative values in the UMGP region. Higher temperatures may have caused more evaporation and drying, which therefore increased the area affected by drought in recent decades.

Water status in winter wheat grown under salt stress

Properties of the soil surface layer, the temporal pattern of the microclimate variables as well as crop condition were combined to analyze the characteristics of the evapotranspiration from winter wheat fields in a saline soil area. In order to accomplish this analysis, evapotranspiration was divided into evaporation from the soil and transpiration from wheat. Moreover, the effect of soil salinity on evapotranspiration was evaluated through the relationship between actual evapotranspiration and potential evapotranspiration (E a /E o ) and the total soil water potential (Φ) was divided into two components: matric potential (Ψ M ) and osmotic potential (Ψ o ). Two sites with different salinity levels were chosen for this study, located in hebei Province, China. Measurements were conducted in April–May 1997 and May 1998. The Bowen ratio method was used to estimate the actual evapotranspiration (E a ), whereas potential evapotranspiration (E o ) was estimated using Penman’s equation. Measurements of soil evaporation (E s ) were obtained with micro-lysimeters, and transpiration was calculated from the difference betweenE a andE s . The results show that transpiration comprised on average almost 80% of total evapotranspiration. Evaporation from the soil differed slightly between years, but this variation was dominated by the leaf area index (LAI), which ranged from 4 to 5 during the study period of 1997 and 1998. Soil electric conductivity (EC), which is directly related to osmotic potential, ranged from 1.9 to 3.5 mS cm?1 in 1997 and was negligible in 1998. Our results indicate that lower osmotic potential decreases the total soil water potential, thus affecting plant transpiration. Hence, it is possible to say that soil salinity actually decreases evapotranspiration from winter wheat fields.

Long Term Spatio-temporal Variations of Seasonal and Decadal Aridity in India

A comprehensive analysis of climate data(1958-2018)is carried out at the national scale in India to assess spatiotemporal variation in aridity.The aridity is analyzed using UNEP(United Nations Environment Programme)Aridity Index(AI),which is the ratio between Precipitation(P)and Potential Evapotranspiration(PET).Freely available Terra-Climate database,P and PET variables,offered an unprecedented opportunity for monitoring variations in AI and aridity index anomalies(AIA)at inter-seasonal and inter-decadal basis.The study also assesses longer term patterns of P and AI anomalies with vegetation anomalies.The results indicate that significant clustered areas with maximum dryness are located at west-central part of India,the state of Maharashtra.Overall,there is a gradual increase in the extent of arid zone during 60-year period and spatially maximum extent of percentage change in aridity area is observed.The change patterns of AI in India are largely driven by the changing patterns of precipitation.The maximum impact of decline in precipitation on AIA was observed during Kharif season frequently,for every 4-5 years during 1972-1992.The pattern repeated in the last few recent years(2013-2018),the decline in precipitation resulted increased aridity.The study also reveals that the availability and usage of irrigation sources have increased from 2014 to 2018.Thus,despite of less precipitation positive vegetation has been resulted in this period.The findings are important to understand the impacts of climate change on land use pattern,and land and water resource management.

A daily drought index based on evapotranspiration and its application in regional drought analyses

With climate warming, frequent drought events have occurred in recent decades, causing huge losses to industrial and agricultural production, and affecting people’s daily lives. The monitoring and forecasting of drought events has drawn increasing attention. However, compared with the various monthly drought indices and their wide application in drought research,daily drought indices, which would be much more suitable for drought monitoring and forecasting, are still scarce. The development of a daily drought index would improve the accuracy of drought monitoring and forecasting, and facilitate the evaluation of existing indices. In this study, we constructed a new daily drought index, the daily evapotranspiration deficit index(DEDI), based on actual and potential evapotranspiration data from the high-resolution ERA5 reanalysis dataset of the European Center for Medium-Range Weather Forecasts. This new index was then applied to analyze the spatial and temporal evolution characteristics of four drought events that occurred in southwest, north, northeast, and eastern northwest China in the spring and summer of 2019. Comparisons with the operationally used Meteorological Drought Composite Index and another commonly used index, the Standardized Precipitation Evapotranspiration Index, indicated that DEDI characterized the spatiotemporal evolution of the four drought events reasonably well and was superior in depicting the onset and cessation of the drought events,as well as multiple drought intensity peaks. Additionally, DEDI considers land surface conditions, such as vegetation coverage,which enables its potential application not only for meteorological purposes but also for agricultural drought warning and monitoring.

Evaluation of the applicability of multiple drought indices in the core zone of “westerlies-dominated climatic regime”

Drought indices are frequently used to measure the intensity,start and end of droughts.However,the performances of these indices depend on regions and the type of droughts.Therefore,it is necessary to evaluate whether these indices are applicable for a given region.This study evaluated the ability of the self-calibrating Palmer Drought Severity Index(sc PDSI),the Standardized Precipitation Evapotranspiration Index(SPEI),and the Standardized Moisture Anomaly Index(SZI) to describe the dry-wet conditions and drought events in the core zone of the “westerlies-dominated climatic regime”.The results showed that the 12-month SZI(SZI12) and SPEI(SPEI12) have good correlations with precipitation and PET respectively,while the sc PDSI can capture the changes in precipitation and soil moisture.In Xinjiang region of China,the SZI12 and sc PDSI showed that this region gradually became wetter during 1961–2014,which was consistent with increased precipitation,decreased PET,and improved vegetation.However,the SPEI12 showed significant drying due to the strong influence of the PET,suggesting that this index exaggerated the drought conditions in this region.Precipitation in Kazakhstan and the southern Central Asian regions has increased slightly over the past 50 years,but the PET has greatly increased,altogether,all three indices suggested a drying trend in Central Asia.The evaluation of the ability of these drought indices to identify typical drought events in Xinjiang region of China suggested that the sc PDSI can better detect the typical drought events compared with the SZI12.In conclusion,the sc PDSI is the most suitable index for characterizing the long-term trend of hydroclimate conditions and drought events in the core zone of the “westerlies-dominated climatic regime”.This study provides a theoretical basis for the rational utilization and improvement of the drought index,which is critical for monitoring,attributing,and predicting of drought events in arid areas.

1960-2016年中国黄土高原气象干旱趋势的归因

This study uses two forms of the Palmer Drought Severity Index(PDSI),namely the PDSI_TH(potential evapotranspiration estimated-by the Thornthwaite equation)and the PDSI_PM(potential evapotranspiration estimated by the FAO Penman-Monteith equation),to characterize the meteorological drought trends during 1960–2016 in the Loess Plateau(LP)and its four subregions.By designing a series of numerical experiments,we mainly investigated various climatic factors'contributions to the drought trends at annual,summer,and autumn time scales.Overall,the drying trend in the PDSI_TH is much larger than that in the PDSI_PM.The former is more sensitive to air temperature than precipitation,while the latter is the most sensitive to precipitation among all meteorological factors.Increasing temperature results in a decreasing trend(drying)in the PDSI_TH,which is further aggravated by decreasing precipitation,jointly leading to a relatively severe drying trend.For the PDSI_PM that considers more comprehensive climatic factors,the drying trend is partly counteracted by the declining wind speed and solar radiation.Therefore,the PDSI_PM ultimately shows a much smaller drying trend in the past decades.

Variations of the Surface Wettability Index over the Tibetan Plateau During 1971-2005

Based on 1971-2005 monthly mean maximum/minimum temperature,wind speed,relative humidity,sunshine duration,and precipitation data at 25 stations over the Tibetan Plateau,a study of the largest potential evapotranspiration（LPE） is performed by using the Penman-Monteith model.The surface wettability index（SWI） is calculated and examined,together with its space distribution,interannual and seasonal variations,as well as associated causes.The results suggest that the annual area rainfall exhibits a pronounced increasing trend at 15.0 mm per decade;the annual LPE shows a different-degree decrease at-4.6—-71.6 mm/10 yr.In the southwestern Ngari prefecture and Nyalam county,the annual SWI displays insignificant decline trends compared to increasing trends in other areas of Tibet（0.02-0.09 per decade）.For Tibet,on average,the SWI experiences a noticeable rise at 0.04/10 yr,particularly in 1981-2005.On a seasonal basis,the SWI shows increasing trends,especially in summer.In the 1970s-1980s,the interannual variation is characterized mainly by lower temperature and lower humidity.From the 1990s,air temperature keeps on rising,leading to an appreciable increase in SWI,displayed as a type of warm and humid climate.The salient increases（decrease） of precipitation and relative humidity（mean temperature daily range） are the principal causes of the greatly enhanced SWI in the region.The pronounced decrease in mean wind and sunshine duration also plays an active role.

近半个世纪来气候变暖背景下黄河流域潜在蒸散量的变化(英文)

According to the meteorological observation data of 72 stations from 1960 to 2010 in the Huanghe （Yellow） River Watershed, China, the long-term variations of potential evapotranspiration, calculated in the modified Penman-Monteith model of Food and Agricul- ture Organization of the United Nations, were presented, as well as the meteorological causes for the decrease of potential evapotranspiration were discussed. Since 1960, temperature has risen significantly and potential evapotranspiration a decreasing trend, which indicated the existence of ＂Evaporation paradox＂ in the Huanghe River Watershed. This phenomenon was not consistent spatially or temporally with the increase of temperature, potential evapotranspiration decreased in spring, summer and winter, mainly over most parts of Shanxi and Henan, and some parts of Gansu, Ningxia, Inner Mongolia, and Shaanxi. During the recent half century, the trends of temperature and potential evapotranspiration were negatively correlated at most of the stations, while precipitation and potential evapotranspiration exhibited a contrary trend. Calculated in multiple regressions, the contribution to potential evapotranspiration change of related meteorological factors was discussed, including mean pressure, maximum and minimum temperature, sunshine hours, relative humidity and average wind speed. The decrease of wind speed in the Huanghe River Watershed may be the dominating factor causing potential evapotranspiration decreasing.

Applicability evaluation of the SWIM at river basins of the black soil region in Northeast China： A case study of the upper and middle Wuyuer River basin

In this paper, we selected the middle and upper reaches of the Wuyuer River basin in the black soil region of Northeast China as the study area. We adopted the soil and water integrated model （SWIM） and evaluated the parameter sensitivity using partial correlation coefficient. We calibrated and validated our simulation results based on the daily runoff data from Yi＇an hydrological station at the outlet of the river basin and the evaporation data re- corded by various weather stations from 1961 to 1997. Following evaluation of the modeling data against the observed data, we present the applicability of SWIM in the river basin of the black soil region, and discuss the resulting errors and their probable causes. Results show that in the periods of calibration and validation, the Nash-Sutcliffe efficiency （NSE） coeffi- cients of the monthly and daily runoffs were not less than 0.71 and 0.55, and the relative er- rors were less than 6.0%. Compared to daily runoffs, the simulation result of monthly runoffs was better. Additionally, the NSE coefficients of the potential monthly evaporation were not less than 0.81. Together, the results suggest that the calibrated SWIM can be utilized in various simulation analyses of runoffs on a monthly scale in the black soil region of Northeast China. On the contrary, the model had some limitations in simulating runoffs from snowmelt and frozen soil. Meanwhile, the stimulation data deviated from the measured data largely when applied to the years with spring and summer floods. The simulated annual runoffs were considerably higher than the measured data in the years with abrupt increases in annual precipitation. However, the model is capable of reproducing the changes in runoffs during flood seasons. In summary, this model can provide fundamental hydrological information for comprehensive management of the Wuyuer River basin water environment, and its applica- tion can be potentially extended to other river basins in the black soil region.