Cryosphere evapotranspiration in the Tibetan Plateau:A review

Land surface actual evapotranspiration is an important process that influences the Earth's energy and water cycles and determines the water and heat transfer in the soil-vegetation-atmosphere system.Meanwhile,the cryosphere's hydrological process is receiving extensive attention,and its water problem needs to be understood from multiple perspectives.As the main part of the Chinese cryosphere,the Tibetan Plateau faces significant climate and environmental change.There are active interaction and pronounced feedback between the environment and ETa in the cryosphere.This article mainly focuses on the research progress of ETa in the Tibetan Plateau.It first reviews the ETa process,characteristics,and impact factors of typical underlying surfaces in the Tibetan Plateau(alpine meadows,alpine steppes,alpine wetlands,alpine forests,lakes).Then it compares the temporal and spatial variations of ETa at different scales.In addition,considering the current greening of cryosphere vegetation due to climate change,it discusses the relationship between vegetation greening and transpiration to help clarify how vegetation activities are related to the regional water cycle and surface energy budget.

Field Measurement of Cotton Seedling Evapotranspiration

Information on cotton evapotranspiration (ET) during the seedling growth stage and under field conditions is scarce because ET is a difficult parameter to measure. Our objective was to use weighable lysimeters to measure daily values of cotton seedling ET. We designed and built plastic weighable micro-lysimeters (ML) that were 0.35 m deep with a soil volume of 6300 cm3. The soil core was obtained in-situ by pushing the ML well casing into the soil using a commercial soil sampler. The soil core was weighed with tension and compression type load-cells, where a change in mass of 18 g·d-1 was equivalent to a water evaporation of 1 mm·d-1. We compared load-cell measurements of changes in mass to values measured with a portable field scale by linear regression analysis, and the slope was equal to 1, indicating no statistical difference (P = 0.05) between the two measurements. We measured and compared seedling height, root length and leaf area of cotton plants in the ML with cotton plants in the surrounding area and this comparison showed that the ML used was suitable to measure cotton seedling ET for the first 30 days after seed emergence. The root mean squared error for crop height was 0.09 cm, for leaf area index (LAI) was 0.03 m2·m-2 and 6.5 cm for root length. Also, soil temperature at a 0.1 m depth was statistically (P = 0.05) the same in and outside the ML’s. For two planting dates, we measured daily values of soil water evaporation (E) and cotton seedling ET. The day following an irrigation event, E was ~ 9 mm d-1 and quickly declined the following days. Results showed that ML’s provide an accurate tool to measure water losses from the soil and cotton plants with a LAI of ≤0.2.

Transpiration of the Tamarind Artificial Forest in the Arid-Hot Valleys of Jinshajiang River,Yunnan

The transpiration characteristics of the tamarind artificial forest in the arid-hot valleys of Jinshajiang River, Yunnan were investigated through the measurement of the transpiration rate, stomatal conductance of tamarind leaf and the related envi- ronmental factors. The results showed that the transpiration of the tamarind in the clear sky had the close positive correlations with photosynthetically active radiation and air temperature and negative correlation with atmosphere moisture in the whole growth period. The daily change tendency of the transpiration rate was the similar with stomatal conductance, and the transpiration had the positive correlation with stomatal conductance. The transpiration rate of tamarind was highest in the rainy season of June and July and was relatively low in the drought season of March and April. The transpiration water consumption in rainy season of June and July was obviously higher than that in drought season of March and April. It fully suggested that the tamarind showed very good drought resistance and adaptability to the arid-hot valleys of Yunnan .

Nighttime transpiration of Populus euphratica during different phenophases

Evidence exists of nighttime transpiration and its potential impact on plant/water relations for species in a diversity of ecosystems. However, relevant data related to typical desert riparian forest species remains limited Accordingly, we measured sap flow velocity of Populus euphratica using the heat ratio method between 2012 and2014. Nocturnal stem sap flow was separated into nighttime and stem refilling using the ‘‘forecasted refilling''method. Nighttime transpiration was observed for each phenophase. The highest value was during the full foliation period but lowest during leaf expansion and defoliation periods. The contribution of nighttime transpiration to daytime transpiration was an average of 15% but this was comparatively higher during the defoliation period. Relationships between nighttime transpiration, vapor pressure deficits, and air temperatures were more closely associated than with wind speed in all phenophases. Moreover, we found that nighttime transpiration linearly correlated to vapour pressure deficit during the first and the full foliation periods, but nighttime transpiration showed exponential correlations to air temperatures during the same phenophases. Additionally, environmental drivers of transpiration were significantly different between nighttime and daytime(P \ 0.05). Driving forces behind nighttime transpiration were characterized by many factors, and integrated impacts between these multiple environmental factors were complex. Future studies should focus on these integrated impacts on nighttime transpiration, and the physiological mechanisms of nighttime transpiration should be investigated, given that this could also influence its occurrence and magnitude during different phenophases.

Temporal Variation in Sap-Flux-Scaled Transpiration and Cooling Effect of a Subtropical Schima superba Plantation in the Urban Area of Guangzhou

Agriculture could suffer the water stress induced by climate change. Because climate warming affects global hydrological cycles, it is vital to explore the effect of tree transpiration, as an important component of terrestrial evapotranspiration, on the environment. Thermal dissipation probes were used to measure xylem sap flux density of a Schima superba plantation in the urban area of Guangzhou City, South China. Stand transpiration was calculated by mean sap flux density times total sapwood area. The occurrence of the maximum sap flux density on the daily scale was later in wet season than in dry season. The peak of daily sap flux density was the highest of 59 g m-2 s^-1 in July and August, and the lowest of 28 g m-2 s-1 in December. In the two periods （November 2007-October 2008 and November 2008-October 2009）, the stand transpiration reached 263.2 and 291.6 ram, respectively. During our study period, stand transpiration in wet season （from April to September） could account for about 58.5 and 53.8% of the annual transpiration, respectively. Heat energy absorbed by tree transpiration averaged 1.4×10s and 1.6×10s kJ per month in this Schima superba plantation with the area of 2 885 m2, and temperature was reduced by 4.3 and 4.7℃ s^-1 per 10 m3 air.

Impact of time lags on diurnal estimates of canopy transpiration and canopy conductance from sap-flow measurements of Populus cathayana in the Qinghai–Tibetan Plateau

Recently, canopy transpiration （Ec） has been often estimated by xylem sap-flow measurements. However, there is a significant time lag between sap flow measured at the base of the stem and canopy transpiration due to the capacitive exchange between the transpiration stream and stem water storage. Significant errors will be introduced in canopy conductance （gc） and canopy transpiration estimation if the time lag is neglected. In this study, a cross-correlation analysis was used to quantify the time lag, and the sap flowbased transpiration was measured to pararneterize Jarvistype models of gc and thus to simulate Ec of Populus cathayana using the Penman-Monteith equation. The results indicate that solar radiation （Rs） and vapor pressure deficit （VPD） are not fully coincident with sap flow and have an obvious lag effect; the sap flow lags behind Rs and precedes VPD, and there is a 1-h time shift between Eo and sap flow in the 30-min interval data set. A parameterized Jarvis-type gc model is suitable to predict P. cathayana transpiration and explains more than 80% of the variation observed in go, and the relative error was less than 25%, which shows a preferable simulation effect. The root mean square error （RMSEs） between the predicted and measured Ec were 1.91×10^-3 （with the time lag） and 3.12×10^-3cm h^-1 （without the time lag）. More importantly, Ec simulation precision that incorporates time lag is improved by 6% compared to the results without the time lag, with the mean relative error （MRE） of only 8.32% and the mean absolute error （MAE） of 1.48 × 10^-3 cm h^-1.

Subtropical Modern Greenhouse Cucumber Canopy Transpiration Under Summer Climate Condition

Greenhouse canopy transpiration not only has effects on greenhouse air temperature and humidity, but also is important for determining the set-point of fertigation. In this study, Penman-Monteith equation was used to calculate the greenhouse cucumber canopy transpiration under summer climate condition. The effects of greenhouse environmental factors on canopy transpiration were analyzed based on the measurements of greenhouse microclimate factors and canopy transpiration. The results showed that Penman-Monteith equation was reliable and robust in estimating greenhouse cucumber canopy transpiration under summer climate condition. Greenhouse cucumber canopy transpiration rate increased linearly with the increase of net radiation and water vapor pressure deficit (VPD) above the canopy. But the maximum value of the canopy transpiration rate occurred at the same time as that of VPD whereas about two hours later than that of net radiation. Based on the results, it was concluded that in addition to radiation, air humidity should also be considered when determine the set-point of fertigation.

Comparison of transpiration between different aged black locust(Robinia pseudoacacia)trees on the semi-arid Loess Plateau,China

Black locust （Robinia pseudoacacia） is widely planted throughout the semi-arid Loess Plateau of China. The spatial distribution of this species at different ages is highly heterogeneous due to restoration and management practices. In this study, we aimed to compare the transpiration levels between different aged black locusts at the tree and stand scales, clarifying the physiological status of this species with different ages. Black locust trees with two representative age classes （12 and 28 years） were selected in the Yangjuangou catchment on the semi-arid Loess Plateau. Sap flux density （Fd） and environmental variables （solar radiation, air temperature, relative humidity and soil water content） were simultaneously monitored throughout the growing season of 2014. Tree transpiration （Et） was the product of Fd and sapwood area （As）, and stand transpiration （Ec） was calculated basing on the stand sap flux density （Js） and stand total sapwood area （AsT）. Stomatal conductance （gs） was measured in a controlled environment and hydraulic conductance was estimated using the relationship between transpiration rate and vapor pressure deficit （VPD）. Our results showed that Et and Ec were higher in the 28-year-old stand than in the 12-year-old stand. The gs and hydraulic conductance of 28-year-old trees were also higher than those of 12-year-old trees, and the two parameters were thus the causes of variations in transpiration between different age classes. After rainfall, mean Fd increased by 9% in 28-year-old trees and by 5% in 12-year-old trees. This study thus suggests that stand age should be considered for estimating transpiration at the catchment and region scales in this area. These results provide ecophysiological evidences that the older black locust trees had more active physiological status than the younger ones in this area. These findings also provide basic information for the management of water resources and forests on the semi-arid Loess Plateau.

Partitioning tree water usage into storage and transpiration in a mixed forest

Background:Water migration and use are important processes in trees.However,it is possible to overestimate transpiration by equating the water absorbed through the plant roots to that diffused back to the atmosphere through stomatal transpiration.Therefore,it is necessary to quantify the water transpired and stored in plants.Method:The δ^(2)H/δ^(18)O technique and heat ratio method were used to explore the water usage of coniferous and broad-leaved tree species,including the proportions of water used for transpiration and water storage.Results:Platycladus orientalis and Quercus variabilis had strong plasticity in their water usage from different sources.Platycladus orientalis primarily used groundwater(30.5%)and the 60-100-cm soil layer(21.6%)throughout the experimental period and was sensitive to precipitation,absorbing water from the 0-20-cm layer(26.6%)during the rainy season.Quercus variabilis absorbed water from all sources(15.7%-36.5%)except from the 40-60-cm soil layer during the dry season.In addition,it did not change its water source but increased its groundwater uptake during the rainy season.The annual mean water fluxes of P.orientalis and Q.variabilis were 374.69 and 469.50 mm·year−1,with 93.49% and 93.91% of the water used for transpiration,respectively.However,nocturnal sap flow in P.orientalis and Q.variabilis was mainly used for water storage in the trunk rather than transpiration,which effectively alleviated drought stress and facilitated the transport of nutrients.Conclusions:The water stored in both species comprised 6%-7% of the total water fluxes and,therefore,should be considered in water balance models.

Estimation of transpiration coefficient and aboveground biomass in maize using time-series UAV multispectral imagery

Estimating spatial variation in crop transpiration coefficients(CTc) and aboveground biomass(AGB)rapidly and accurately by remote sensing can facilitate precision irrigation management in semiarid regions. This study developed and assessed a novel machine learning(ML) method for estimating CTc and AGB using time-series unmanned aerial vehicle(UAV)-based multispectral vegetation indices(VIs)of maize under several irrigation treatments at the field scale. Four ML regression methods: multiple linear regression(MLR), support vector regression(SVR), random forest regression(RFR), and adaptive boosting regression(ABR), were used to address the complex relationship between CTcand VIs. AGB was then estimated using exponential, logistic, sigmoid, and linear equations because of their clear mathematical formulations based on the optimal CTcestimation model. The UAV VIs-derived CTcusing the RFR estimation model yielded the highest accuracy(R^(2)= 0.91, RMSE = 0.0526, and n RMSE = 9.07%). The normalized difference red-edge index, transformed chlorophyll absorption in reflectance index, and simple ratio contributed significantly to the RFR-based CTcmodel. The accuracy of AGB estimation using nonlinear methods was higher than that using the linear method. The exponential method yielded the highest accuracy(R^(2)= 0.76, RMSE = 282.8 g m, and n RMSE = 39.24%) in both the 2018 and 2019 growing seasons. The study confirms that AGB estimation models based on cumulative CTcperformed well under several irrigation treatments using high-resolution time-series UAV multispectral VIs and can support irrigation management with high spatial precision at a field scale.

Estimating plant crown transpiration and water use efficiency by vegetative reflectance indices associated with chlorophyll fluorescence

This research developed estimates of plant crown transpiration and water-use-efficiency using reflectance and derivative indices extracted from remotely sensed chlorophyll fluorescence measurements under natural conditions. Diurnal changes of leaf-level gas exchange (carbon assimilation rate (A), stomatal conductance (gs), transpiration rate (E)), chlorophyll fluorescence and canopy-scale remote sensing were measured on top crown of valley oak (Quercus lobata) in the foothills of central California, USA. The results indicated Q. lobata experienced saturating irradiance (PAR), which induced photoinhibition indicated by a decrease in the quantum efficiency of photosystem II (r2 = 0.648 with Fv ′/Fm′ and r2 = 0.73 with FPSII) and open reaction centers (qP;r2 = 0.699). The excess absorbed quantum energy was dissipated as heat through the Xanthophyll cycle and other processes (photorespiration and the water-water cycle) rather than energy emission as steady state chlorophyll fluorescence (Fs). An increase in leaf temperature caused by the activity of Xanthophyll cycle was correlated to a decrease in Fs (r2 = 0.381) and an increase in evaporative cooling through E (r2 = 0.800) and water use efficiency (WUE;r2 = 0.872).

Responses of phreatophyte transpiration to falling water table in hyper-arid and arid regions,Northwest China

Quantitative assessment of the impact of groundwater depletion on phreatophytes in(hyper-)arid regions is key to sustainable groundwater management.However,a parsimonious model for predicting the response of phreatophytes to a decrease of the water table is lacking.A variable saturated flow model,HYDRUS-1D,was used to numerically assess the influences of depth to the water table(DWT)and mean annual precipitation(MAP)on transpiration of groundwater-dependent vegetation in(hyper-)arid regions of northwest China.An exponential relationship is found for the normalized transpiration(a ratio of transpiration at a certain DWT to transpiration at 1 m depth,T_(a)^(*))with increasing DWT,while a positive linear relationship is identified between T_(a)^(*)and annual precipitation.Sensitivity analysis shows that the model is insensitive to parameters,such as saturated soil hydraulic conductivity and water stress parameters,indicated by an insignificant variation(less than 20%in most cases)under±50%changes of these parameters.Based on these two relationships,a universal model has been developed to predict the response of phreatophyte transpiration to groundwater drawdown for(hyper-)arid regions using MAP only.The estimated T_(a)^(*)from the model is reasonable by comparing with published measured values.

Spatio-temporal variation in transpiration responses of maize plants to vapor pressure deficit under an arid climatic condition

The transpiration rate of plant is physically controlled by the magnitude of the vapor pressure deficit（VPD） and stomatal conductance. A limited-transpiration trait has been reported for many crop species in different environments, including Maize（Zea mays L.）. This trait results in restricted transpiration rate under high VPD, and can potentially conserve soil water and thus decrease soil water deficit. However, such a restriction on transpiration rate has never been explored in maize under arid climatic conditions in northwestern China. The objective of this study was to examine the transpiration rate of field-grown maize under well-watered conditions in an arid area at both leaf and whole plant levels, and therefore to investigate how transpiration rate responding to the ambient VPD at different spatial and temporal scales. The transpiration rates of maize at leaf and plant scales were measured independently using a gas exchange system and sapflow instrument, respectively. Results showed significant variations in transpiration responses of maize to VPD among different spatio-temporal scales. A two-phase transpiration response was observed at leaf level with a threshold of 3.5 k Pa while at the whole plant level, the daytime transpiration rate was positively associated with VPD across all measurement data, as was nighttime transpiration response to VPD at both leaf and whole plant level, which showed no definable threshold vapor pressure deficit, above which transpiration rate was restricted. With regard to temporal scale, transpiration was most responsive to VPD at a daily scale, moderately responsive at a half-hourly scale, and least responsive at an instantaneous scale. A similar breakpoint（about 3.0 k Pa） in response of the instantaneous leaf stomatal conductance and hourly canopy bulk conductance to VPD were also observed. At a daily scale, the maximum canopy bulk conductance occurred at a VPD about 1.7 k Pa. Generally, the responsiveness of stomatal conductance to VPD at the canopy scale was lower than that at leaf scale. These results indicate a temporal and spatial heterogeneity in how maize transpiration responses to VPD under arid climatic conditions. This could allow a better assessment of the possible benefits of using the maximum transpiration trait to improve maize drought tolerance in arid environment, and allow a better prediction of plant transpiration which underpin empirical models for stomatal conductance at different spatio-temporal scales in the arid climatic conditions.

Study on Transpiration and Stomatal Conductance Characteristics of C3 and C4 Plant

The transpiration experiment was done under greenhouse conditions with a C3 plant sweet pepper （Capsicum annuum Linn.） and two C4 plants, sorghum （Sorghum bicolor L.Moench） and maize （Zea mays Linn.）. Three species were irrigated with three different water treatment levels of 100%, 66% and 33% which gave a comparison of tolerance and adaptation to irrigation and two different levels of water stress. The measurements of transpiration rate and stomatal conductance were done between 8.00 a.m. and 16.00 p.m. with measurements about each 1.5 h with an infrared gas analyzer. The results showed that Z. mays probably due to a higher leaf area had very low values and was significantly different （LSD pairwise comparison） from C. annuum and S. bicolor. The hypotheses that C4 plants and C3 plants have different transpiration rates and stomatal conductance could not be shown with the results. However, the hypotheses that for the same species, the highest values in transpiration rate and stomatal conductance were with the 100% irrigation treatment and the lowest values were with the 33% irrigation treatment could be accepted due to the results of this trial.

Comparison of the S-, N- or P-Deprivations’ Impacts on Stomatal Conductance, Transpiration and Photosynthetic Rate of Young Maize Leaves

Seven-day-old maize (Zea mays) plants were grown hydroponically for ten days in deprived nutrient solutions against the corresponding control grown under full nutrition;the effects of S-, N- or P-deprivation on laminas’ mean stomatal conductance (gs), transpiration rate (E) and photosynthetic rate (A) were monitored, along with the impact on the laminas’ total dry mass (DM), water amount (W), length and surface area (Sa). Furthermore, a time series analysis of each parameter’s response ratios (Rr), i.e. the treatment’s value divided by the corresponding control’s one, was performed. Under S-deprivation, the Rr of laminas’ mean gs, E, and A presented oscillations within a ±15% fluctuation zone, notably the “control” zone, whilst those of laminas’ total DM, water amount, surface area, and length included oscillation during the first days and deviation later on, presenting deviation during d10. Under the N-deprivation conditions all Rr time courses except the A one, included early deviations from the control zone without recovering. The deviation from the control zone appeared at d4. Under P-deprivation, all Rr time courses represented oscillations within the control zone. P-deprivation’s patterns resembled those of S-deprivation. Compared to the one of the S-deprivation, the P-one’s oscillations took place within a broader zone. Linear relationships among the various Rr patterns were found between gs-E, gs-A, E-A, DM-W and DM-Sa. In conclusion, the impact of P-deprivation appeared in an early stage and included an alleviation action, the one of N-deprivation appeared early with no alleviation action, whilst that of S-deprivation appeared later, being rather weaker when compared to the impact of the P-deprivation’s impact.

Growth and Transpiration of <i>Jatropha curcas</i>L. Seedlings under Natural Atmospheric Vapour Pressure Deficit and Progressive Soil Drying in Semi-Arid Climate

During the last decade, Jatropha curcas L. (J. curcas) has gained much attention worldwide as biofuel crop. Although its cultivation is promoted in the Sahel, there is a surprising lack of data on its water use regulation and growth in this region. Here, we investigated, in semi-controlled conditions, leaf transpiration and growth of six accessions of J. curcas at seedling stage under natural changing in vapour pressure deficit (VPD) and progressive soil drying in Senegal. The experimental layout was a randomized complete bloc design and after 3 months of growth arranged to a split-plot at the implementation of water stress to facilitate irrigation. Under well water condition, there was no significant difference between accessions for leave transpiration that was positively correlated to VPD with high values recorded between 13 h and 14 h pm. Accessions of J. curcas used in this study showed closed thresholds at which transpiration declined except accession from Ndawene that threshold was lower (0.30). There is no significant difference between accessions for growth during the experimentation period. In 3 months, we recorded 23.57 g for the aboveground dry biomass and seedlings had about 14 leaves and 24.3 cm height. Positive linear correlation was recorded between aboveground biomass and root dry weight (p J. curcas might need complement irrigation for a better growth of seedlings especially during the dry season.

Relationship between stomatal behavior and characteristics of photosynthesis and transpiration of Adenophora lobophylla and A. potaninii at different altitudes

The photosynthesis and transpiration characteristics of Adenophora lobophylla and A. potaninii as well as stomatal behavior such as stomatal size, stomatal density, stomatal open and stomatal conductivity were measured at different altitudes. The relationship between the photosynthesis and transpiration characteristics and the stomatal behavior was analysed by correlation coefficient and path coefficient analysis with altitude changes.The results showed that the inffuences of stomatal behavior were not evident on the photosynthesis and transpiration characteristics of A. Lobophylla, but evident on that of A. potaninii.

Transpiration and growth responses by Eucalyptus species to progressive soil drying

The regulation of plant transpiration is a key factor affecting transpiration efficiency, growth and adaptation of Eucalyptus species to limited water availability in tropical and subtropical environments. However, few studies have related this trait to the performance of Eucalyptus seedlings and none have investigated the influence of vapor pressure deficit (VPD) on transpiration rates and growth. In this study, the transpiration and growth responses of seedlings of Eucalyptus urophylla (S.T. Blake) and Eucalyptus cloeziana (F. Muell.) to progressive soil water deficits were evaluated under semi-controlled conditions using the fraction of transpirable soil water (FTSW) method. In addition, the influence of VPD on seedling transpiration, development and growth was also investigated. The FTSW threshold ranged from 0.40 to 0.99 for the transpiration rate and from 0.32 to 0.97 for the development and growth variables. Little or no changes in the FTSW threshold were detected in response to changes in atmospheric VPD. Both Eucalyptus species presented a conservation strategy under drought stress. In addition, water-conserving mechanisms during the seedling phase were related to rapid stomatal closure, reduced leaf area, and number of leaves.

Dominance-caused differences in transpiration of trees in a Karst broadleaved mixed forest

Estimating stand transpiration of natural forests using traditional methods through up-scaling of sap fl ux density from sample trees based on stand sapwood area only is diffi cult because of the complexity of species,ages,and hierarchical structure of natural forests.To improve stand transpiration estimation,we developed an up-scaling method by considering the tree dominance eff ect based on the assumption that individual tree transpiration is aff ected by crown dominance and species,in addition to factors previously considered such as meteorological conditions,sapwood area,and soil moisture.In this study,the meteorological factors,soil moisture,and sap fl ux density of 15 sample trees of diff erent species and dominance in a natural evergreen and deciduous broadleaved mixed forest were simultaneously monitored from March 2012 to February 2014 in the Karst mountain region in southwestern China.After establishing a single tree transpiration model which considers the eff ects of dominance and species,an up-scaling method was explored to estimate stand transpiration.The results show that the transpiration intensity increased exponentially with increasing tree dominance.The contribution to annual stand transpiration from a few dominant trees(5.4%of trees, 28.2% of basal area) was up to 65.0%. The correspondingcontribution was 16.2% from sub-dominant trees(7.6% of trees, 16.2% of basal area) and 22.8% from middleandlower-layer trees (87.0% of trees, 55.6% of basal area).The variation of individual tree transpiration was mainly(97.9%) explained by tree dominance, but very weakly bytree species. The estimated annual stand transpiration was300.2 mm when using the newly developed method whichconsiders tree dominance, 52.5 mm (14.9%) lower than theestimation (352.7 mm) of traditional method which considersonly the sapwood area eff ect, and 8.5 mm (2.7%) lowerthan the estimation (308.6 mm) which considers the eff ectsof both species composition and sapwood area. The maintree characteristics aff ecting stand transpiration are tree size(sapwood area) and dominance. Consideration of tree dominancewill signifi cantly improve stand transpiration estimationand provide a more solid basis for guiding integratedforest-water management at stand scale.