Seasonal Variations In Evapotranspiration Research Articles

Net Radiation Drives Evapotranspiration Dynamics in a Bottomland Hardwood Forest in the Southeastern United States: Insights from Multi-Modeling Approaches

Evapotranspiration (ET) is a major component of the water budget in Bottomland Hardwood Forests (BHFs) and is driven by a complex intertwined suite of meteorological variables. The understanding of these interdependencies leading to seasonal variations in ET is crucial in better informing water resource management in the region. We used structural equation modeling and AIC modeling to analyze drivers of ET using Eddy covariance water flux data collected from a BHF located in the Russel Sage Wildlife Management Area (RSWMA). It consists of mature closed-canopy deciduous hardwood trees with an average canopy height of 27 m. A factor analysis was used to characterize the shared variance among drivers, and a path analysis was used to quantify the independent contributions of individual drivers. In our results, ET and net radiation (Rn) showed similar variability patterns with Vapor Pressure Deficit (VPD) and temperature in the spring, summer, and autumn seasons, while they differed in the winter season. The path analysis showed that Rn has the strongest influence on ET variations via direct and indirect pathways. In deciduous forests like BHFs, our results suggest that ET is more energy dependent during the growing season (spring and summer) and early non-growing season (autumn) and more temperature dependent during the winter season.

Hydrological losses and soil moisture carryover affected the relationship between evapotranspiration and rainfall in a temperate semiarid shrubland

Evapotranspiration (ET) represents a major pathway of water loss from terrestrial ecosystems. However, large uncertainty exists regarding the relationship between ET and precipitation at seasonal and interannual timescales in drylands. Using the eddy-covariance technique, we investigated temporal variations of ET in a semiarid shrubland of northern China during January 2012–December 2020. Daily ET was close to zero during winter and reached up to 2–4 mm day−1 in summer. Monthly ET was positively and linearly correlated with the normalized difference vegetation index and surface conductance (gs), and showed a saturating relationship with monthly rainfall. Analyses based on bin-averaged half-hourly values revealed increases in midday ET and gs with increasing soil water content, and decreases in midday gs and saturated ET with increasing vapor pressure deficit. Annual ET for ecohydrologic years (July 1–June 30) ranged 207–297 mm yr−1 (with a multi-year average of 242 mm yr−1), and accounted for 62%–105% of annual rainfall. Aggregated ET over the study period (2163 mm) accounted for 78% of total rainfall. Rainfall amount was a poor predictor of year-to-year differences in annual ET, wet-season (July 1–November 30) ET, and dry-season (December 1–June 30) ET. Wet-season ET (mm season−1) was generally lower than rainfall while dry-season ET tended to be higher than rainfall. Our results suggest that biotic factors played an essential role in regulating seasonal variations in ET, while air and soil dryness imposed additional constraints on ET at finer timescales. Moreover, hydrological losses (e.g., to subsurface runoff and/or deep drainage) during wet seasons and soil moisture carryover from wet to dry seasons may be nonnegligible in the studied semiarid shrubland, and thus affected seasonal and interannual relationships between ET and rainfall.

Seasonal variation of evapotranspiration, Priestley-Taylor coefficient and crop coefficient in diverse landscapes

Priestley-Taylor equation (PT) and the Penman-Monteith equation (PM) are commonly used methods for regional evapotranspiration monitoring, using the PT coefficient (αa) and PM crop/ vegetation coefficient (Kc). This paper investigates the seasonal changes in αa and Kc at five sites in Australia and China, to understand the relationship between environmental conditions and evapotranspiration when applying different evaporation estimation methods. The research shows that higher actual evapotranspiration does not lead to higher αa and Kc values. αa and Kc perform similarly in cropland and forest environments in both China and Australia. Both αa and Kc continuously increase to a peak during the growing season and then decrease to their lowest values during the winter season. Considering Kc’s similar performance to αa and its greater data processing requirements, Kc has few advantages for estimating regional evapotranspiration. Applying the Priestley-Taylor equation with a regional α indicator will enhance the accuracy and reduce the workload when estimating regional evapotranspiration for similar landcover types based on remote sensing.

Integration of evapotranspiration remote sensing products based on Bayesian model averaging: An example from Three-River-Source National Park

Evapotranspiration (ET) is a fundamental flux in land surface hydrothermal process. Because of the differences in basic concepts, assumptions, application scales, different models have induced varying uncertainties to the estimation and simulation of evapotranspiration. With the Three-River-Source National Park as an example, we used the Bayesian model averaging (BMA) method to integrate the ET estimations from five models of PT-JPL, ARTS-GIMMS3, ARTS-MODIS, MODIS global evapotranspiration product (MOD16), and SSEBop, and tried to improve the estimating accuracy of evapotranspiration. The results showed that the five models could well capture the seasonal variations in evapotranspiration at Haibei Flux Station, with an explanation range of 64%-86% variability in the observed ET, and a root means square deviation (RMSD) ranged from 0.47 mm·(8 d)-1 to 0.76 mm·(8 d)-1. BMA-based ET greatly improved its explanation to 89% and decreased the RMSD to 0.43 mm·(8 d)-1. The Three-River-Source National Park experienced an overall insignificant increasing trend in its inter-annual ET from 2003 to 2015. At the regional scale, the effects of temperature and precipitation on evapotranspiration were not significant, but were significant in the Yangtze River Source Park. Temperature and precipitation had positive impacts on evapotranspiration. The evapotranspiration showed different trends due to the geographi-cal differences between parks. This study provided a method reference for other multi-source data integration analysis. The integrated evapotranspiration data could effectively reduce the uncertainty of the original models and provide a more accurate data basis for the study of regional water heat change, which is of great significance to better understand water cycle under climate changes.

Evapotranspiration and its components over a rainfed spring maize cropland under plastic film on the Loess Plateau, China

Aim of study: To determine seasonal variations in evapotranspiration (ET) and its components; and ascertain the key factors controlling ET and its components in a rainfed spring maize field under plastic film.Area of study: Shouyang County in Shanxi Province on the eastern Loess Plateau, ChinaMaterial and methods: Eddy covariance system combined with micro-lysimeters and meteorological observing instruments were used in the field. The manual method was used to measure the green leaf area index (GLAI) during the growing season.Main results: In 2015 and 2016, the growing season ET accounted for 80% and 79% of annual ET, respectively. Soil evaporation (E) accounted for 36% and 33% of the growing season ET in 2015 and 2016, respectively. The daily crop coefficient increased with increasing GLAI until a threshold of ~3 m2 m−2 in the canopy-increasing stage, and decreased linearly with decreasing GLAI in the canopy-decreasing stage. At equivalent GLAI, daily basal crop coefficient and soil water evaporation coefficient were generally higher in the canopy-increasing and -decreasing stages, respectively. During the growing season, the most important factor controlling daily ET, T, and E was net radiation (Rn), followed by GLAI for daily ET and T, and soil water content at 10-cm depth for daily E; during the non-growing season, daily ET was mainly controlled by Rn.Research highlights: The daily crop coefficient and its components reacted differently to GLAI in the canopy-increasing and -decreasing stages.

Evapotranspiration and gross primary productivity of secondary vegetation in Amazonia inferred by eddy covariance

The conversion of primary forest (PF) to other types of land cover, such as pasture and agriculture, in Amazonia, affects regional carbon and water balances, significantly contributing to increased carbon emissions and reduced evapotranspiration. However, secondary forest (SF) growth, resulting from the abandonment of low-productivity pasture areas, offers a potential alternative to counterbalance the effects of deforestation on carbon release to the atmosphere and evapotranspiration reduction. In this work, we present four years of eddy flux measurements of a SF that is approximately 20 years old, located in Central Amazonia, and we compare these measurements with those of a PF in the same region, analyzing daily and seasonal variations in evapotranspiration, gross primary productivity of carbon and water use efficiency. On average, evapotranspiration is 20% higher in the SF (3.6 mm day−1) than in the PF (3.1 mm day−1), while gross primary productivity is only 5% higher in the SF (8.1 gC m−2 day−1) than in the PF (7.8 gC m−2 day−1). Despite robust evidence of higher evapotranspiration and gross primary productivity in SF, the estimated uncertainty range of WUE is large to reach definite conclusions about the differences on carbon gain per water loss between the sites. Nonetheless, the significantly higher evapotranspiration and gross primary productivity of SF may counterbalance both water and C losses from deforestation and has important implications for regional budgets.

Evaluation of seasonal evapotranspiration of winter wheat in humid region of East China using large-weighted lysimeter and three models

Water consumption demand of winter wheat remains uncertain in humid region of East China. A better prediction of water consumption can improve agricultural water management. Based on the evapotranspiration (ET) data determined by large-weighted lysimeter and the meteorological data during 2011–2018, the actual ET of winter wheat was compared with the ET simulated by Priestley-Taylor (PT), Penman-Monteith (PM) and FAO-56 single crop coefficient (Kc) models, and the relationships between ET of winter wheat and its driving environmental factors were assessed to localize the model’s parameters. The results showed that the seasonal variations of ET in winter wheat could be divided into three stages with the canopy development, i.e., Stage I: from sowing to wintering when hourly ET maintained a low level (0.03–0.35 mm/h); Stage II: from revival to dough when hourly ET increased rapidly to 0.60–1.50 mm/h during daytime; Stage III: the ripening stage with hourly ET decreasing to 0.01–1.00 mm/h. The ET of winter wheat was mainly determined by wind speed at Stage I and by net solar radiation (Rn) at Stage II and III by Besides primary determining factor net radiation at the whole season, vapor pressure deficit and air temperature was the second factor that dominated ET at Stages I-II and Stage III, respectively. The PM model performed excellently for hourly ET modelling at Stage II with R2 of 0.944 during 2017–2018. All models did not perform well at Stage I and III on hourly time scale. PM model was the best among the three models for calculating daily ET at Stage II, while both PM and PT models could be used to simulate daily ET at Stage III. After local parameterization, the calibrated PTx and Kcx models were validated with daily ET simulation with the coefficients of 1.52 and 1.25 at Stage I, 1.81 and 1.80 at Stage II, and 1.12 and 0.78 at Stage III, respectively. The PTx model was suitable to calculate daily ET at Stage I and III with R2 of 0.19 and 0.55, MAE of 0.39 and 0.88 mm/d, GPI of 0.59 and 2.93, respectively, while Kcx model was the best model at Stage II, with R2 of 0.86, MAE of 0.74 mm/d and GPI of 0.22. Moreover, the PTx at Stage I, Kcx at Stage II, and PTx at Stage III appeared to give reasonable results in evaluating seasonal ET of winter wheat, with values of 53.8, 375.8 and 21.0 mm at Stage I, II and III in 2017–2018, respectively. Total ET suggested that the main water management period should be Stage II. The results can improve the simulation of seasonal ET and may be used as a benchmark for water management of winter wheat in East China.

Efecto de la diversidad funcional sobre la tasa de evapotranspiración: un análisis anual de tres bosques templados chilenos

There is a link between the diversity of forest ecosystems and the evapotranspiration (ET) ecosystem process. However, there is little knowledge of this link in temperate forests of southern South America. Given the influence of ET in relevant ecosystem services, such as hydrological or climatic regulation, it is important to understand the effect of the change in species composition on this process. The objective of this research was to determine the relationship between functional diversity and the seasonal variation of ET using three native forest ecosystems that differ in structure, composition of dominant species and proportion of phenological strategies as a study model. The general pattern shows that there is a significant negative relationship between the community weighted average of the specific leaf area attribute; CWM-SLA and the ET process. We found seasonal changes in ET in the three ecosystems, which was greater in spring-summer. There is a positive relationship between the percentage of evergreen species in the canopy and total annual ET of each ecosystem. This would explain the negative relationship between CWM-SLA and ET, since the ecosystem with the highest proportion of evergreen species corresponds to the one with the lowest CWM-SLA. The results of this study contribute to giving relevance to the functional diversity of native forest ecosystems over a crucial ecosystem process for the provision of various ecosystem services. Subsequent studies could assess the effect of the loss of functional diversity on ecosystem processes in order to relieve the need to conserve and/or restore the native forest from this perspective.

Seasonal variation of evapotranspiration and its effect on the surface energy budget closure at a tropical forest over north-east India

This study uses 1 yr of eddy covariance (EC) flux observations to investigate seasonal variations in evapotranspiration (ET) and surface energy budget (SEB) closure at a tropical semi-deciduous forest located in north-east India. The annual cycle is divided into four seasons, namely, pre-monsoon, monsoon, post-monsoon and winter. The highest energy balance closure (76%) is observed during pre-monsoon, whereas the lowest level of closure (62%) is observed during winter. Intermediate closure of 68% and 72% is observed during the monsoon and post-monsoon seasons, respectively. Maximum latent heat flux during winter (\(\hbox {150 W m}^{-2}\)) is half of the maximum latent heat (\(\hbox {300 W m}^{-2}\)) flux during the monsoon. ET is a controlling factor of SEB closure, with the highest rates of closure corresponding to the periods of the highest ET. The Bowen ratio ranges from 0.93 in winter to 0.27 during the monsoon. This is the first time the role of ET in the seasonal variation of SEB closure has been reported for any ecosystem in north-east India using EC measurements.

Energy and water vapor exchange over a young plantation in northern China

Northern China’s plantations have long played a role in providing ecological services to the people of China. Reliable detection and attribution of ecosystem water and energy exchange is a precondition to the development of strategies for the sustainable management of these plantations. Here, the seasonal and interannual variability in surface energy exchange and evapotranspiration (ET) over a young pine plantation was investigated using eddy-covariance measurements collected over a four-year period (2012–2015). Seasonal patterns in net radiation (Rn), latent (LE) and sensible heat fluxes (H) were largely similar over the four years. H was the dominant energy component, with LE exceeding H only during the mid-growing season. A significant share of Rn was allocated to H during the 2014 summer drought. Energy exchange for the young pine plantation was characterized by high Bowen ratios (β = H/LE; 6.28, 6.42, 5.65, and 5.34 for the four years), high H/Rn (0.37, 0.36, 0.36, and 0.30), and low LE/Rn (0.15, 0.17, 0.17, and 0.16), with mean annual values of 5.92, 0.35, and 0.16 for β, H/Rn, and LE/Rn, respectively. Daily maximum and annual ET were 4.8, 4.5, 4.1, and 3.3 mm day−1 and 328, 371, 290, and 326 mm for the four years, with a mean annual value of 329 mm. Seasonal variation in ET was strongly controlled by biological factors (i.e., bulk surface conductance and vegetation greenness and density, as characterized by the normalized difference vegetation index) regulated by soil water availability and water vapor pressure deficit (VPD). Evapotranspiration varied interannually and the evapotranspiration-to-precipitation ratio (ET/P) ranged from 0.79 to 1.62. Soil water replenishment through precipitation during the non-growing period of the previous year and mid-growing season of the current year was responsible for the interannual variation observed in ET. These results clearly indicate the importance of precipitation timing and soil moisture carry-over from previous years in controlling ecosystem energy and water vapor exchange. Irrigation during the spring and dry periods of the year is highly favorable for growth of plantation trees in northern China.

Energy and water balance of a treatment wetland under mediterranean climatic conditions

During 2013–2014 period, the surface renewal (SR) – energy balance (EB) method combined with the water balance approach was adopted to define the energy and water budget of a treatment wetland (TW) in Eastern Sicily (Italy). The main objective was to evaluate the differences between energy and water balances and understand the underlining mechanisms of environmental control. Results shown that the seasonal variation in evapotranspiration (ET) had similar trend as the seasonal variation of leaf area index (LAI) and net radiation (Rn). During the study period, the daily rate of ET by EB ranged from −0.1 mm d−1 to 9.3 mm d−1, with a total ET of about 1837 mm in 17 months. The crop coefficient Kc (e.g. obtained using the FAO-56 mean Kc approach) for the TW ranged from 0.5 to 2.4, with a mean annual value of 1.24. On a 10-day mean basis, ET fluxes from the TW water balance were 15% higher than ET by EB.

Actual and Reference Evapotranspiration in a Cornfield in the Zhangye Oasis, Northwestern China

Evapotranspiration (ET) is an important component of the surface energy balance and water cycle, especially in arid and semiarid regions. The characteristics of the actual evapotranspiration (ETa), which was calculated using the eddy covariance method, and the reference evapotranspiration (ET0), which was estimated using the Food and Agriculture Organisation (FAO) Penman–Monteith method, were analysed. This work focussed on the seasonal variations in evapotranspiration and crop coefficient (Kc) above the heterogeneous canopy of an arid oasis ecosystem in a cornfield of the Zhangye oasis in northwestern China. The results showed that in 2008, the total net radiation (Rn) was 2457.73 MJ∙m−2 and that the rainfall was 117 mm. The average wind velocity, air temperature, and specific humidity, which were observed 2 m above the ground surface, were 1.23 m∙s−1, 7.07 °C, and 3.66 g∙kg−1, respectively. The total ETa and ET0 were 654.69 mm and 1039.92 mm, respectively; thus, the ET0 was higher than the ETa. The difference between the ET0 and ETa was high in summer and autumn, and low in winter and spring. The ETa was greatly influenced by irrigation events, whereas the ET0 was not influenced by irrigation. The ETa and ET0 were both greatly influenced by meteorological elements. The Kc values were less than 0.5 outside of the maize-growing stage and greater than 0.5 during the entire maize-growing stage (from 20 April to 22 September 2008). The Kc values were 0.63, 0.75, 0.78, 0.76, 0.61 and 0.71 at the seedling, shooting, heading, filling, and maturity stages and the entire growth stage, respectively.

Monitoring the variations of evapotranspiration due to land use/cover change in a semiarid shrubland

Abstract. Evapotranspiration (ET) is an important process in the hydrological cycle, and vegetation change is a primary factor that affects ET. In this study, we analyzed the annual and inter-annual characteristics of ET using continuous observation data from eddy covariance (EC) measurement over 4 years (1 July 2011 to 30 June 2015) in a semiarid shrubland of Mu Us Sandy Land, China. The Normalized Difference Vegetation Index (NDVI) was demonstrated as the predominant factor that influences the seasonal variations in ET. Additionally, during the land degradation and vegetation rehabilitation processes, ET and normalized ET both increased due to the integrated effects of the changes in vegetation type, topography, and soil surface characteristics. This study could improve our understanding of the effects of land use/cover change on ET in the fragile ecosystem of semiarid regions and provide a scientific reference for the sustainable management of regional land and water resources.

表层阻力和环境因素对杨树(Populus sp.)人工林蒸散发的控制

PDF HTML阅读 XML下载 导出引用 引用提醒 表层阻力和环境因素对杨树(Populus sp.)人工林蒸散发的控制 DOI: 10.5846/stxb201502250381 作者: 作者单位: 北京林业大学,北京市八达岭林场,北京市园林绿化局,北京林业大学,北京林业大学,北京林业大学,北京市农业环境监测站,北京林业大学 作者简介: 通讯作者: 中图分类号: 基金项目: 国家林业行业科研专项（201204102）；北京市教育委员会共建项目；北京市林业碳汇工作办公室观测运行基金；中美碳联盟USCCC国际合作项目 Control of evapotranspiration by surface resistance and environmental factors in poplar (Populus×euramericana) plantations Author: Affiliation: Beijing forestry university,,,,,,,Beijing forestry university Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:在水资源短缺地区大面积栽植高耗水的人工林相比于低矮农作物会加剧地区的水分短缺，因而其可持续性正受到越来越多的关注。但是，在不同地域复杂的水、能量和气候条件下的人工林蒸散发的控制机制仍不清楚。基于涡度相关（EC）系统和微气象系统对北京市大兴区杨树（Populus euramericana CV.“74/76”）人工林生态系统与大气间水分交换的连续监测，（a）分析了2006-2009年生长季中生态系统蒸散发（ET）、表层阻力（Rs）、气候阻力（Ri）和空气动力学阻力（Ra）在干湿年份间的变化动态；（b）以偏相关分析法探讨了干旱和湿润年份中不同土壤水分条件下生物因素Rs和环境因素（Ri和Ra）对杨树人工林ET的直接控制作用。研究结果表明：在年际尺度上，干旱年份杨树人工林的日平均ET（2.23±1.30）mm/d低于湿润年份约17%，对应地，干旱年份的表层阻力（Rs ：LAI）高于湿润年份（71.2 s/m）约50%，而Ri和Ra未表现出干湿年份间的差异。在季节尺度上，季节性的干旱胁迫显著影响杨树人工林的ET和Rs、Ri的变化，水分供应（降雨量与灌溉量之和）是该尺度上影响杨树人工林ET的主导因素，其解释了ET变化的71%（P<0.01）。偏相关分析结果表明，除了在土壤水分严重胁迫（REW<0.1）情况外，其他土壤水分条件下表层阻力Rs是日尺度上控制ET变化的主导因素，其与ET呈负相关关系，二阶相关系数（SOCC）变化范围为-0.518--0.293（P<0.01），且干旱年份中Rs对ET的控制程度高于湿润年份；环境因素中气候阻力Ri和空气动力学阻力Ra各自对ET的控制作用远小于表层阻力Rs；相对土壤含水量（REW）只在干旱年份中干旱胁迫时段（REW<0.4）直接影响ET（Pearson相关系数为0.217-0.323，P<0.01），其他情况下则是通过影响表层阻力Rs、气候阻力Ri和空气动力学阻力Ra对ET的作用来间接影响ET的。另外，相比于偏相关分析，简单的相关性分析会对各因素对ET的控制作用造成估计偏差。 Abstract:There is increasing concern about the sustainability of large-scale plantations in water-limited regions, with most studies indicating that the higher water use of forest plantations compared to herbaceous crops exacerbates water shortages in such areas. However, the mechanisms that control evapotranspiration in forest plantations under complex water, energy, and climatic conditions and across diverse geographical regions remain unclear. Here, we report continuous water flux data for a poplar plantation (Populus×euramericana ‘74/76’) in Daxing District, Beijing, China, collected with an eddy covariance (EC) and microclimate monitoring system. Our objectives were to a) quantify the dynamics of evapotranspiration (ET), surface resistance (Rs), and climatological resistance (Ri) over dry and wet years; b) examine the direct controlling effects of biological and environmental factors on ET by using partial correlation analysis under different soil moisture conditions over dry and wet years. On the interannual scale, average daily ET in dry years (2.23±1.30) mm/d was 17% lower than that in wet years; surface resistance (Rs:LAI) increased by 50% in dry years, but there were no significant differences in Ri and Ra between dry and wet years. At the seasonal scale, seasonal drought stress had a discernible impact on ET, Rs, and Ri of the poplar plantation, and water supply (precipitation+irrigation) caused 71% of seasonal variation in ET (P<0.01). Partial correlation analysis indicated that Rs was the main factor controlling daily ET, except under severe water stress (REW<0.1), and daily ET was negatively related to Rs (second-order correlation coefficient of -0.518 to -0.293, P<0.01). The effect of Rs on ET was stronger in dry years than in wet years, and the effects of Ri and Ra on daily ET were irregular and weaker than that of Rs. Daily ET of the poplar plantation was directly affected by relative extractable soil water (REW) only under water stress (REW<0.4) in dry years (Pearson coefficient 0.217-0.323, P<0.01), and it was indirectly influenced by REW under other soil water conditions. Compared to partial correlation analysis, correlation analysis would incorrectly evaluate the effects of Rs, Ri, and Ra on ET. 参考文献 相似文献 引证文献

Spatial–temporal dynamics of gross primary productivity, evapotranspiration, and water-use efficiency in the terrestrial ecosystems of the Yangtze River Delta region and their relations to climatic variables

Moderate Resolution Imaging Spectroradiometer (MODIS) products and climate data collected from meteorological stations were used to characterize the spatial–temporal dynamics of gross primary productivity (GPP), evapotranspiration (ET), and water-use efficiency (WUE) in the Yangtze River Delta (YRD) region and the response of these three variables to meteorological factors. The seasonal patterns of GPP and WUE showed a bimodal distribution, with their peak values occurring in May and August, and April and October, respectively. By contrast, the seasonal variation of ET presented a unimodal pattern with its maximum in July or August. The spatial distribution of ET and GPP was similar to higher values occurring in the south. From 2001 to 2012, GPP in the eastern YRD decreased, while GPP in the western part increased. In comparison, over the 12 years, ET in the northern part of YRD decreased, while ET in the southern part increased. The spatial distribution and spatial variation of WUE were both similar to those of GPP. This implies that the changes in WUE are primarily controlled by the variations in GPP. The annual average WUE over vegetation types followed the order of: evergreen broadleaf forest (1.95 g C kg−1 H2O) > deciduous broadleaf forest (1.87 g C kg−1 H2O) > evergreen needle leaf forest (1.70 g C kg−1 H2O) > deciduous needle leaf forest (1.68 g C kg−1 H2O) > grassland (1.66 g C kg−1 H2O) > cropland (1.61 g C kg−1 H2O). Both GPP and ET increased with increasing annual mean temperature (Ta) and annual mean precipitation across all of the plant function types. WUE decreased as vapour pressure deficit (VPD) increased in all of the biomes. Interestingly, the relationship between WUE and VPD was the most significant in broadleaf forest. Whether this phenomenon is universal should be investigated in future studies.

Comparison of ET estimations by the three-temperature model, SEBAL model and eddy covariance observations

The three-temperature (3T) model is a simple model which estimates plant transpiration from only temperature data. In-situ field experimental results have shown that 3T is a reliable evapotranspiration (ET) estimation model. Despite encouraging results from recent efforts extending the 3T model to remote sensing applications, literature shows limited comparisons of the 3T model with other remote sensing driven ET models. This research used ET obtained from eddy covariance to evaluate the 3T model and in turn compared the model-simulated ET with that of the more traditional SEBAL (Surface Energy Balance Algorithm for Land) model. A field experiment was conducted in the cotton fields of Taklamakan desert oasis in Xinjiang, Northwest China. Radiation and surface temperature were obtained from hyperspectral and thermal infrared images for clear days in 2013. The images covered the time period of 0900–1800 h at four different phenological stages of cotton. Meteorological data were automatically recorded in a station located at the center of the cotton field. Results showed that the 3T model accurately captured daily and seasonal variations in ET. As low dry soil surface temperatures induced significant errors in the 3T model, it was unsuitable for estimating ET in the early morning and late afternoon periods. The model-simulated ET was relatively more accurate for squaring, bolling and boll-opening stages than for seedling stage of cotton during when ET was generally low. Wind speed was apparently not a limiting factor of ET in the 3T model. This was attributed to the fact that surface temperature, a vital input of the model, indirectly accounted for the effect of wind speed on ET. Although the 3T model slightly overestimated ET compared with SEBAL and eddy covariance, it was generally reliable for estimating daytime ET during 0900–1600 h.

Incorporating an Urban Irrigation Module into the Noah Land Surface Model Coupled with an Urban Canopy Model

Abstract The current research examines the influence of irrigation on urban hydrological cycles through the development of an irrigation scheme within the Noah land surface model (LSM)–Single Layer Urban Canopy Model (SLUCM) system. The model is run at a 30-m resolution for a 2-yr period over a 49 km2 urban domain in the Los Angeles metropolitan area. A sensitivity analysis indicates significant sensitivity relative to both the amount and timing of irrigation on diurnal and monthly energy budgets, hydrological fluxes, and state variables. Monthly residential water use data and three estimates of outdoor water consumption are used to calibrate the developed irrigation scheme. Model performance is evaluated using a previously developed MODIS–Landsat evapotranspiration (ET) and Landsat land surface temperature (LST) products as well as hourly ET observations through the California Irrigation Management Information System (CIMIS). Results show that the Noah LSM–SLUCM realistically simulates the diurnal and seasonal variations of ET when the irrigation module is incorporated. However, without irrigation, the model produces large biases in ET simulations. The ET errors for the nonirrigation simulations are −56 and −90 mm month−1 for July 2003 and 2004, respectively, while these values decline to −6 and −11 mm month−1 over the same 2 months when the proposed irrigation scheme is adopted. Results show that the irrigation-induced increase in latent heat flux leads to a decrease in LST of about 2°C in urban parks. The developed modeling framework can be utilized for a number of applications, ranging from outdoor water use estimation to climate change impact assessments.

Spatial and seasonal variations in evapotranspiration over Canada's landmass

Abstract. A 30 yr (1979–2008) dataset of actual evapotranspiration (ET) at 1 km resolution was generated over Canada's landmass by integrating remote sensing land surface data and gridded climate data using the EALCO model run at a 30 min time step. This long-term high-resolution dataset was used to characterize the spatiotemporal variations in ET across Canada. The results show that annual ET varied from 600 mm yr−1 over several regions in the south to less than 100 mm yr−1 in the northern Arctic. Nationally, ET in summer (i.e., June to August) comprised 65% of the annual total amount. ET in the cold season remained mostly below 10 mm month−1 over the country. Negative monthly ET was obtained over the Arctic region in winter, indicating EALCO simulated a larger amount of condensation than ET. Overall, the mean ET over the entire Canadian landmass for the 30 yr was 239 mm yr−1, or 44% of its corresponding precipitation. Comparisons of available ET studies in Canada revealed large uncertainties in ET estimates associated with using different approaches. The scarcity of ET measurements for the diverse ecosystems in Canada remains a significant challenge for reducing the uncertainties; this gap needs to be addressed in future studies to improve capabilities in climate/weather modeling and water resource management.

Spatial‐temporal variations of evapotranspiration and runoff/precipitation ratios responding to the changing climate in the Pacific Northwest during 1921‐2006

Water resources in the Pacific Northwest (PNW) are very sensitive to climate change. There are still big knowledge gaps on how evapotranspiration (ET) varies in responding to changing temperature (T) and precipitation (P) over different zones in terms of supply and demand regime for ET. Here, we employ the Variable Infiltration Capacity (VIC) hydrologic model and a high‐resolution meteorological data set to quantify spatial and seasonal variations of ET and the runoff (R)/precipitation (P) ratio over the PNW and attribute effects of T and P. We evaluate modeled ET and R with eddy covariance measurements, upscaled regional ET, and reconstructed natural streamflow. Simulation results indicate that water‐limited (annual potential ET (PET) ≥ P) and energy‐limited zones (annual PET &lt; P) have different responses to changing climate. In general, water‐limited zones tend to be more associated with increasing ET and decreasing R/P than do energy‐limited regions. With controlled simulation experiments, we document that trends in annual and warm‐season ET and R/P are dominantly controlled by P, while in the cool season they are mainly controlled by T. During an entire cycle of the Pacific Decadal Oscillation (1947‐2006), the PNW experienced a substantial increase in ET and a decrease in R and R/P under the trends of warming and drying. In this snowmelt‐dominated region where warming‐induced changes to the snowpack are impacting seasonal freshwater availability, decreases in R/P could further aggravate water scarcity. To reduce uncertainties, high‐resolution meteorological data sets and intensive model calibrations and evaluations against ET are required.

Long-Term Eddy Covariance Monitoring of Evapotranspiration and Its Environmental Factors in a Temperate Mixed Forest in Northeast China

AbstractOn the basis of eddy covariance measurements, the results of three years (2005–2007) of direct evapotranspiration (ET) measurements over a mixed temperate forest in Northeast China are presented. The diurnal and seasonal variations in ET and its main driving factors were analyzed. Annual ET values for the forest were 437, 506, and 632 mm in 2005, 2006, and 2007, respectively. The contribution of ET during the dormant season was not negligible, ranging from 17 to 23% of the annual ET for the study years. On an annual course, the increase in ET is associated with the increasing air temperature (Ta) and plant growth in late April and early May, peaking in July or August with monthly mean rates of 2.9 mm day−1 (July 2005), 3.4 mm day−1 (July 2006), and 3.8 mm day−1 (August 2007). Priestley-Taylor parameter α also varied seasonally, with its minimum and maximum values occurring in the dormant and growth seasons, respectively. During the growth season, the values of α were generally between 0.6 and 0...