Unveiling evapotranspiration patterns and energy balance in a subalpine forest of the Qinghai-Tibet Plateau:observations and analysis from an eddy covariance system

Evapotranspiration is an important parameter used to characterize the water cycle of ecosystems.To under-stand the properties of the evapotranspiration and energy balance of a subalpine forest in the southeastern Qinghai-Tibet Plateau,an open-path eddy covariance system was set up to monitor the forest from November 2020 to October 2021 in a core area of the Three Parallel Rivers in the Qing-hai-Tibet Plateau.The results show that the evapotranspira-tion peaked daily,the maximum occurring between 11:00 and 15:00.Environmental factors had significant effects on evapotranspiration,among them,net radiation the greatest(R^(2)=0.487),and relative humidity the least(R^(2)=0.001).The energy flux varied considerably in different seasons and sensible heat flux accounted for the main part of turbulent energy.The energy balance ratio in the dormant season was less than that in the growing season,and there is an energy imbalance at the site on an annual time scale.

A Study on the Assessment and Integration of Multi-Source Evapotranspiration Products over the Tibetan Plateau

Evapotranspiration(ET)is a crucial variable in the terrestrial water,carbon,and energy cycles.At present,a large number of multi source ET products exist.Due to sparse observations,however,great challenges exist in the evaluation and integration of ET products in remote and complex areas such as the Tibetan Plateau(TP).In this paper,the applicability of the multiple collocation(MC)method over the TP is evaluated for the first time,and the uncertainty of multisource ET products(based on reanalysis,remote sensing,and land surface models)is further analyzed,which provides a theoretical basis for ET data fusion.The results show that 1)ET uncertainties quantified via the MC method are lower in RS-based ET products(5.95 vs.7.06 mm month^(-1))than in LSM ET products(10.22 vs.17.97 mm month^(-1))and reanalysis ET estimates(7.27 vs.12.26 mm month-1).2)A multisource evapotranspiration(MET)dataset is generated at a monthly temporal scale with a spatial resolution of 0.25°across the TP during 2005-15.MET has better performance than any individual product.3)Based on the fusion product,the total ET amount over the TP and its patterns of spatiotemporal variability are clearly identified.The annual total ET over the entire TP is approximately 380.60 mm.Additionally,an increasing trend of 1.59±0.85 mm yr^(-1)over the TP is shown during 2005-15.This study provides a basis for future studies on water and energy cycles and water resource management over the TP and surrounding regions.

Correlation Analysis between the Evapotranspiration Quantity and Climatic Factors of Artificial Grassland in Three River Sources Areas

Based on observation data from the mini-type automatic weather station,the evapotranspiration quantity of reference crops from artificial grassland in three river sources areas was estimated by the method of FAO Penman-Monteith.The actual evapotranspiration quantity of grassland was calculated according to the synthetic crop coefficients referenced by FAQ-56,and the change of the actual evapotranspiration quantity of artificial grassland in three river sources areas as well as the relationship between the evapotranspiration quantity and climatic factors were studied.The results suggested that the seasonal change of actual evapotranspiration quantity in grassland was expressed in a single peak curve with the peak in the middle August,and daily transpiration quantity in summer was significantly larger than that in winter.The evapotranspiration was significantly correlated with air temperature,solar radiation and relative humidity,but not significantly correlated with wind speed.Effects of climatic factors on the evapotranspiration quantity of artificial grassland were ordered as follow:air temperature(T)>solar radiation(Ra)>relative humidity(RH)>wind speed(u2).

Forest Evapotranspiration in Pearl River Delta based on Remote Sensing Technology

Evapotranspiration ratio of six major forest types in the Pearl River Delta was studied in this paper, and the six forest types included eucalyptus forest, fruit forest, coniferous forest, shrub forest, coniferous forest, and broad-leaved mixed for- est. The mechanism of PT-T model was analyzed based on Priestley-Taylor ex- tended model, and the result showed that land surface temperature or temperature difference between day and night could be used to evaluate the evapotranspiration factor. The evapotranspiration of 6 forest types was estimated with MODIS data and TM data and weather information during 2008-2009. The result suggested that water consumption of different forest differs insignificantly, and the EF of eucalyptus is not high.

Temporal and Spatial Distribution Characteristics of Potential Evapotranspiration and Its Sensitivity to Meteorological Factors in Guizhou Province

[Objective] The study aims to discuss the changes of potential evapotran- spiration and its sensitivity to meteorological factors in Guizhou Province, so as to provide important references for assessment of water resources, research of agri- cultural water conservancy and climate change. [Method] Temporal and spatial dis- tribution characteristics of potential evapotranspiration in Guizhou Province from 1961 to 2010 were analyzed, and the sensitivity of potential evapotranspiration to meteo- rological factors in Guizhou Province was studied through correlation analysis. [Re- sult] On the whole, potential evapotranspiration in Guizhou Province was higher in the southwest and the west compared with the northeast and the east. In various seasons, it was the highest in summer, followed by spring and autumn, while it was the lowest in winter. In recent 40 years, annual potential evapotranspiration showed an obvious decreasing trend in most stations of Guizhou Province. The main meteo- rological factors influencing changes of potential evapotranspiration in Guizhou Province were sunshine duration, daily maximum temperature, and daily average relative humidity. [Conclusion] Daily average temperature was not the main meteoro- logical factor affecting changes of potential evapotranspiration in Guizhou Province, while sunshine duration, daily maximum temperature, and daily average relative hu- midity had important effects on potential evapotranspiration in Guizhou Province.

Analysis of Reference Evapotranspiration Change and Its Impact Factors in Australia

[ Objective] To discuss regional response of reference evapotranspiration （ ETo ） to global climate change and its influencing factors. [Method] Penman-Monteith equation was adopted to calculate ET0 in Australia from 1998 to 2007. Spatiotemporal change characteristics were analyzed by using GIS spatial analysis tools and relationships between ETo and main climate factors were also analyzed. [ Resultsl The results showed that multi-year average ET0 increased from the east and south part to the northwest part and inland, and its distribution was consistent with climate zones. Multi-year average ETo of the whole region was 1 750 mm, obtaining minimum and maximum values in 2000 and 2002 respectively. Regional ETo decreased in the order of summer, spring, autumn and winter; January and December got the highest monthly ET0, while June got the lowest value 79.55 mm. ETo positively correlated with mean temperature and solar radiation, R2 for each were 0.83 and 0.94, while the relationship between ETo and average relative humidity was negative, and precipitation had no significant relationship with ET0. [ Conclusion] This research could provide important reference for crop water requirement study and making irrigation method for Australia.

Study on Artificial Neural Network Model for Crop Evapotranspiration

Based on potted plant experiment, BP-artifieial neural network was used to simulate crop evapotranspiration and 3 kinds of artificial neural network models were constructed as ET1 （meteorological factors）, ET2（ meteorological factors and sowing days） and ET3 （meteorological factors, sowing days and water content）. And the predicted result was compared with actual value ET that was obtained by weighing method. The results showed that the ET3 model had higher calculation precision and an optimum BP-artificial neural network model for calculating crop evapotranspiration.

Study on Evapotranspiration Regularity of 3 Forages in Eastern Qinghai Province

The transpiration rate and transpiration quantity＇of whole plants and foliages of Bromus innerrnis, Medicago sativa and Agropyron cristatum are measured by using improved quick and continuous weighting after being cut off method （QCWC）. The results show that transpiration rate of branches and foliages of Agropyron cristatum is highest in 3 forages, Medicago sativa is second and Bromus innermis is lowest. From July to September, the average transpiration quantity of whole plants and foliages of 3 forages is highest in August, September is second and July is lowest.

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.

Evapotranspiration and Soil Moisture Balance for Vegetative Restoration in a Gully Catchment on the Loess Plateau, China

Evapotranspiration, soil moisture balance and the dynamics in a gully catchment of the Loess Plateau in China were determined with 6 land use treatments including natural grassland, shrubs （Caragana rnicrophylla）, two woodlands （Prunus armeniaca var. ansu and Pinus tabulaeformis）, cultivated fallow, and farmland （Triticum aestiuum L.） in order to obtain a better understanding of soil moisture balance principles and to improve vegetation restoration efficiency for ecological rebuilding on the plateau. Average runoff from cultivated fallow was very high, reaching 10.3% of the seasonal rainfall. Evapotranspiration under T. aestivurn was not significantly different from natural grasslands. Compared with natural grass, evapotranspiration was significantly greater （P 〈 0.05） in 2002 and there was an increase in soil moisture depleted in the 1-3 m soil under P. armeniaca, P. tabulaeformis and C. microphylla. During the two years of the study the average soil moisture （0-100 cm soil profile） of T. aestivurn was generally the highest, with P. armeniaca, P. tabulaeformis and C. rnicrophylla usually the lowest. Thus, according to the soil moisture balance principle for this area the planned reforestation project was not ecologically reasonable. Reducing human disturbance and restoration with grass could be more effective.

Evapotranspiration of an oasis-desert transition zone in the middle stream of Heihe River, Northwest China

As a main component in water balance, evapotranspiration is of great importance for water saving and irrigation-measure making, especially in arid or semiarid regions. Although studies of evapotranspiration have been conducted for a long time, studies concentrated on oasis-desert transition zone are very limited. On the basis of the meteorological data and other parameters（e.g. leaf area index（LAI）） of an oasis-desert transition zone in the middle stream of Heihe River from 2005 to 2011, this paper calculated both reference（ET0） and actual evapotranspiration（ETc） using FAO56 Penman-Monteith and Penman-Monteith models, respectively. In combination with pan evaporation（Ep） measured by E601 pan evaporator, four aspects were analyzed：（1） ET0 was firstly verified by Ep;（2） Characteristics of ET0 and ETc were compared, while the influencing factors were also analyzed;（3） Since meteorological data are often unavailable for estimating ET0 through FAO56 Penman-Monteith model in this region, pan evaporation coefficient（Kp） is very important when using observed Ep to predict ET0. Under this circumstance, an empirical formula of Kp was put forward for this region;（4） Crop coefficient（Kc）, an important index to reflect evapotranspiration, was also analyzed. Results show that mean annual values of ET0 and ETc were 840 and 221 mm, respectively. On the daily bases, ET0 and ETc were 2.3 and 0.6 mm/d, respectively. The annual tendency of ET0 and ETc was very similar, but their amplitude was obviously different. The differences among ET0 and ETc were mainly attributed to the different meteorological variables and leaf area index. The calculated Kc was about 0.25 and showed little variation during the growing season, indicating that available water（e.g. precipitation and irrigation） of about 221 mm/a was required to keep the water balance in this region. The results provide an comprehensive analysis of evapotranspiration for an oasis-desert transition zone in the middle stream of Heihe River, which was seldom reported before.

Evapotranspiration and Its Energy Exchange in Alpine Meadow Ecosystem on the Qinghai-Tibetan Plateau

To understand the water and energy exchange on the Qinghai-Tibetan Plateau, we explored the characteristics of evapotranspiration （ET） and energy fluxes from 2002 to 2005 over a Kobresia meadow ecosystem using the eddy covariance method. The ratio of annual ET to precipitation （P） of meadow ecosystem was about 60%, but varied greatly with the change of season from summer to winter. The annual ET/P in meadow was lower than that in shrub, steppe and wetland ecosystems of this plateau. The incident solar radiation （Rs） received by the meadow was obviously higher than that of lowland in the same latitude; however the ratio of net radiation （Rn） to Rs with average annual value of 0.44 was significantly lower than that in the same latitude. The average annual ET was about 390 mm for 2002-2005, of which more than 80% occurred in growing season from May to September. The energy consumed on the ET was about 44% of net radiation in growing season, which was lower than that of shrub, steppe and wetland on this plateau. This study demonstrates that the Kobresia meadow may prevent the excessive water loss through evapotranspiration from the ecosystem into the atmosphere in comparison to the shrub, steppe and wetland ecosystems of the Qinghai-Tibetan Plateau.

Sensitivity of Penman-Monteith Reference Crop Evapotranspiration in Tao'er River Basin of Northeastern China

A non-dimensional relative sensitivity coefficient was employed to predict the responses of reference crop evapotranspiration (ET0) to perturbation of four climate variables in Tao'er River Basin of the northeastern China. Mean monthly ET0 and yearly ET0 from 1961 to 2005 were estimated with the FAO-56 Penman-Monteith Equation. A 45-year historical dataset of average monthly maximum/minimum air temperature, mean air temperature, wind speed, sunshine hours and relative humidity from 15 meteorological stations was used in the analysis. Results show that: 1) Sensitivity coefficients of wind speed, air temperature and sunshine hours were positive except for those of air tem- perature of Arxan Meteorological Station, while those of relative humidity were all negative. Relative humidity was the most sensitive variable in general for the Tao'er River Basin, followed by sunshine hours, wind speed and air tem- perature. 2) Similar to climate variable, monthly sensitivity coefficients exhibit large annual fluctuations. 3) Sensitivity coefficients for four climate variables all showed significant trends in seasonal/yearly series. Also, sensitivity coefficients of air temperature, sunshine hours and wind speed all showed significant trends in spring. 4) Among all sensitiv- ity coefficients, the average yearly sensitivity coefficient of relative humidity was highest throughout the basin and showed largest spatial variability. Longitudinal distribution of sensitivity coefficients for air temperature, relative hu- midity and sunshine hours was also found, which was similar to the distribution of the three climate variables.

Actual evapotranspiration of subalpine meadows in the Qilian Mountains, Northwest China

As a main component in water balance,evapotranspiration(ET)is of great importance for water saving,especially in arid and semi-arid areas.In this study,the FAO(Food and Agriculture Organization)Penman-Monteith model was used to estimate the magnitude and temporal dynamics of reference evapotranspiration(ET0)in 2014 in subalpine meadows of the Qilian Mountains,Northwest China.Meanwhile,actual ET(ETc)was also investigated by the eddy covariance(EC)system.Results indicated that ETc estimated by the EC System was 583 mm,lower than ET0(923 mm)estimated by the FAO Penman-Monteith model in 2014.Moreover,ET0 began to increase in March and reached the peak value in August and then declined in September,however,ETc began to increase from April and reached the peak value in July,and then declined in August.Total ETc and ET0 values during the growing season(from May to September)were 441 and 666 mm,respectively,which accounted for 75.73%of annual cumulative ETc and 72.34%of annual cumulative ET0,respectively.A crop coefficient(kc)was also estimated for calculating the ETc,and average value of kc during the growing season was 0.81(ranging from 0.45 to 1.16).Air temperature(Ta),wind speed(u),net radiation(Rn)and soil temperature(Ts)at the depth of 5 cm and aboveground biomass were critical factors for affecting kc,furthermore,a daily empirical kc equation including these main driving factors was developed.Our result demonstrated that the ETc value estimated by the data of kc and ET0 was validated and consistent with the growing season data in 2015 and 2016.

The Limiting Effect of Deep Soil Water on Evapotranspiration of a Subtropical Coniferous Plantation Subjected to Seasonal Drought

Seasonal drought is a common occurrence in humid climates.The year 2003 was the driest year during the period 1985-2011 in southeastern China.The objective of this study was to elucidate the impact of the exceptional drought in 2003,compared with eddy flux measurements during 2004-11,on the dynamics of evapotranspiration （ET） and related factors,as well as their underlying mechanisms,in a subtropical coniferous plantation in southeastern China.It was found that daily ET decreased from 5.34 to 1.84 mm during the intensive drought period and recovered to 4.80 mm during the subsquent recovering drought period.Path analysis indicated that ET was mainly determined by canopy conductance and deep soil water content （50 cm） during the intensive drought and recovering drought periods,respectively.The canopy conductance offset the positive effect of air vapor pressure deficit on ET when suffering drought stress,while the canopy conductance enhanced the positive effect of air temperature on ET during the late growing season.Because the fine roots of this plantation are mainly distributed in shallow soil,and the soil water in the upper 40 cm did not satisfy the demand for ET,stomatal closure and defoliation were evident as physiological responses to drought stress.

Estimation of Cucumber Evapotranspiration in Solar Greenhouse in Northeast China

The solar greenhouse without heating system has been widely used for vegetable production in Northeast China since the 1980s. The experiment was conducted in the solar greenhouse to study the relationship between evapotranspiration （ET） of cucumber and climatic factors. Reference evapotranspiration （ET0） and pan evaporation （Ep） were applied to determine cucumber evapotranspiration by regression methods. Results showed that the ET of cucumber increased with the increasing of solar radiation, air temperature and vapor pressure. The solar radiation was the most important factor that influences ET among environmental parameters. The determination coefficient （R2） was 0.865 between ET and Ep, higher than that between ET and ET0 （application of a constant vegetation coefficient, R2 = 0.46）. The pan coefficient （kp, determined from the ratio of ET to Ep） and the crop coefficients （kc, determined from the ratio of ET to ET0） showed considerable variability and no obvious systematic pattern could be described throughout the study period. Ep was linearly related to ET and 20 cm pan can be well used to determine the water requirement of cucumber under subsurface drip irrigation in this type of solar greenhouse.

Decreasing Reference Evapotranspiration in a Warming Climate-A Case of Changjiang (Yangtze) River Catchment During 1970-2000

This study deals with temporal trends in the Penman-Monteith reference evapotranspiration estimated from standard meteorological observations, observed pan evaporation, and four related meteorological variables during 1970-2000 in the Yangtze River catchment. Relative contributions of the four meteorological variables to changes in the reference evapotranspiration are quantified. The results show that both the reference evapotranspiration and the pan evaporation have significant decreasing trends in the upper, the middle as well as in the whole Changjiang （Yangtze） River catchment at the 5% significance level, while the air temperature shows a significant increasing trend. The decreasing trend detected in the reference evapotranspiration can be attributed to the significant decreasing trends in the net radiation and the wind speed.

Comparison of three evapotranspiration models with eddy covariance measurements for a Populus euphratica Oliv.forest in an arid region of northwestern China

The accurate estimation of evapotranspiration (ET) in arid regions is important for improving the water use efficiency of vegetation. Based on successive observations from May to October of 2014, we estimated the ET of a Populus euphratica Oliv. forest during the growing season in an extremely arid region using the PM (Penman-Monteith), SW (Shuttleworth-Wallace) and SSW (an improved canopy transpiration model) models. Estimated ET values were compared with those of the eddy covariance measurements. Results indicated that the actual ET of the P. euphratica forest was always overestimated by the PM model. The accuracy of the SW model was higher than that of the PM model. However, some data were not easily obtained because of the complicated structure of the SW model. The newly proposed SSW model gave the most accurate ET values, and its accuracy was higher at hourly than at daily time scale. In conclusion, the SSW model is more suitable for sparse vegetation system at large scales in extremely arid regions.

Assessment of an Evapotranspiration Deficit Drought Index in Relation to Impacts on Ecosystems

Ecosystems have increasingly been subject to the challenge of heavy drought under global warming. To quantitatively evaluate the impacts of drought on ecosystems, it is necessary to develop a drought index that can sensitively depict the response of vegetation to drought evolution at a biological time scale. For the ability of direct connection between climate and ecosystem by deficit of evapotranspiration, in the present study, a drought index was defined based on standardized evapotranspiration deficit (SEDI), according to the difference between actual and potential evapotranspiration, to meet the need for highlighting drought impacts on ecological processes. Comparisons with traditional indices show that SEDI can reasonably detect droughts and climatic dry and wet transitions, especially at a monthly time scale, and can also regenerate long-term trends. Moreover, SEDI can more sensitively capture the biological changes of ecosystems in response to the dynamics of drought intensity, compared with the indices of precipitation and temperature. SEDI is more practical than the precipitation and temperature indices to highlight signals of biological effects in climate droughts. Hence, it has potential for use in assessments of climate change and its impact on ecosystems.

Effects of Evapotranspiration on Mitigation of Urban Temperature by Vegetation and Urban Agriculture

The temperature difference between an urban space and surrounding non-urban space is called the urban heat island effect （UHI）. Global terrestrial evapotranspiration （ET） can consume 1.4803x1023joules （J） of energy annually, which is about 21.74% of the total available solar energy at the top of atmosphere, whereas annual human energy use is 4.935× 1020 J, about 0.33% of annual ET energy consumption. Vegetation ET has great potential to reduce urban and global temperatures. Our literature review suggests that vegetation and urban agricultural ET can reduce urban temperatures by 0.5 to 4.0℃. Green roofs （including urban agriculture） and water bodies have also been shown to be effective ways of reducing urban temperatures. The cooling effects on the ambient temperature and the roof surface temperature can be 0.24-4.0℃ and 0.8-60.0℃, respectively. The temperature of a water body （including urban aquaculture） can be lower than the temperature of the surrounding built environment by between 2 and 6℃, and a water body with a 16 m2 surface area can cool up to 2 826 m3 of nearby space by 1℃. Based on these findings, it can be concluded that the increase of evapotranspiration in cities, derived from vegetation, urban agriculture, and water body, can effectively mitigate the effect of urban heat islands.