Bowen Ratio System Research Articles

High temporal and spatial resolution characteristics of evaporation, transpiration, and evapotranspiration from a subalpine wetland by an advanced UAV technology

Wetland evapotranspiration (ET), which involves the land–atmosphere exchange of energy and water, is dynamic and affects the spatiotemporal distribution of water resources. However, due to the variability and complexity of wetlands, accurate estimation of patch-scale ET and its spatial variability remain insufficiently characterized. To overcome this challenge, an advanced unmanned aerial vehicle (UAV) technology was developed by combining the three-temperature (3T) model, which is robust to estimate transpiration and its spatial variability with UAV-based thermal infrared remote sensing, and Penman equation, which is commonly used to estimate open water evaporation. The combined approach was verified using the Bowen ratio system over a subalpine wetland. The results show that the proposed method is simple and applicable for estimating wetland ET and its spatial variability, with a determination coefficient (R2) of 0.93, mean absolute percentage error (MAPE) of 7.90%, root mean squared error (RMSE) of 0.05 mm h−1, and Nash-Sutcliffe efficiency (NSE) of 0.93. It depicts a large spatial variability in wetland ET with respect to surface vegetation characteristics, water regimes, meteorological factors, and larger transpiration rates than open water evaporation. With its limited inputs and no calibration requirements, the proposed method is concluded to be simple and to easily reveal the high temporal and spatial resolution characteristics of patch-scale ET and its components.

Different Responses of Evapotranspiration Rates of Urban Lawn and Tree to Meteorological Factors and Soil Water in Hot Summer in a Subtropical Megacity

Urban evapotranspiration (ET) is an effective way to mitigate ecological challenges resulting from rapid urbanization. However, the characteristics of urban vegetation ET, especially how they respond to meteorological factors and soil water, remains unclear, which is crucial for urban ET regulation. Therefore, this study measured the actual ET rate of an urban lawn (ETlawn) using the Bowen ratio system and an urban tree (Ttree) by a sap flow system in the hot summer of a subtropical megacity, Shenzhen. The results showed that the daily ETlawn was more restricted by energy (Rs) and diffusion conditions (vapor pressure deficit, VPD), while the daily Ttree was more restricted by VPD and relative extractable water (REW) in the urban area. The daily Ttree decreased when the REW was lower than 0.18, while the daily ETlawn started to decrease when it was lower than 0.14. When REW was lower than 0.11, the Ttree stayed at a relatively low level. The impacts of VPD was more evident on the diurnal Ttree than on the diurnal ETlawn. Wind speed had a scarce impact as it was relatively low in urban areas. This study clarifies the different responses of ETlawn and Ttree to meteorological factors and soil water based on actual ET. The results are of great significance for the knowledge of urban forestry and urban hydrology.

Estimation of evapotranspiration and single and dual crop coefficients of acai palm in the Eastern Amazon (Brazil) using the Bowen ratio system

Estimation of evapotranspiration and single and dual crop coefficients of acai palm in the Eastern Amazon (Brazil) using the Bowen ratio system

Monitoring thermal field, humidity field and energy balance over heterogeneous surfaces in the typical valley-city

Land use and land cover (LULC) alteration has changed original energy balance and heat fluxes between land and atmosphere, and thus affects the structure characteristics of temperature and humidity fields over urban heterogeneous surfaces in different spatio-temporal scales. Lanzhou is the most typical river valley city of China, it is chosen as the case study. Typical river valley terrain, rapid urbanization and severe air pollution have caused unique urban climate and urban heat island (UHI) effects in Lanzhou. Firstly, the spatial structure characteristics and dynamic evolution of temperature and humidity fields in autumn are simulated by mobile measurement experiment and GIS spatial analysis method. The results show that temperature and humidity fields have significant dynamic change within a day, and have multiple center and multiple intensity level characteristics. Then, LULC and normalized difference vegetation index (NDVI) are extracted from remote sensing images, the distribution patterns of temperature and humidity fields have close relationships with LULC and NDVI. Moreover, there is a significant positive correlation between impervious surface area and thermal field intensity. A positive correlation between NDVI value and humidity field intensity has been found as well as a negative correlation between NDVI value and thermal field intensity. Finally, heat fluxes and energy balance characteristics between ground and atmosphere are analyzed based on the Bowen-ratio System experiments. This study could provide theoretical support and practical guidance for urban planning, urban eco-environment construction and air pollution prevention of river valley city.

How precipitation and grazing influence the ecological functions of drought-prone grasslands on the northern slopes of the Tianshan Mountains, China?

Drought-prone grasslands provide a critical resource for the millions of people who are dependent on livestock for food security. However, this ecosystem is potentially vulnerable to climate change (e.g., precipitation) and human activity (e.g., grazing). Despite this, the influences of precipitation and grazing on ecological functions of drought-prone grasslands in the Tianshan Mountains remain relatively unexplored. Therefore, we conducted a systematic field investigation and a clipping experiment (simulating different intensities of grazing) in a drought-prone grassland on the northern slopes of the Tianshan Mountains in China to examine the influences of precipitation and grazing on aboveground biomass (AGB), soil volumetric water content (SVWC), and precipitation use efficiency (PUE) during the period of 2014–2017. We obtained the meteorological and SVWC data using an HL20 Bowen ratio system and a PR2 soil profile hydrometer, respectively. We found that AGB was clearly affected by both the amount and seasonal pattern of precipitation, and that PUE may be relatively low in years with either low or excessive precipitation. The PUE values were generally higher in the rapid growing season (April–July) than in the entire growing season (April–October). Overall, moderate grazing can promote plant growth under water stress conditions. The SVWC value was higher in the clipped plots than in the unclipped plots in the rapid growing season (April–July), but it was lower in the clipped plots than in the unclipped plots in the slow growing season (August–October). Our findings can enhance the understanding of the ecological effects of precipitation and grazing in drought-prone grasslands and provide data that will support the effective local grassland management.

Characteristics of humidity and temperature variations and CO2 exchange of mobile dunes at different space-time scales in Horqin sandy land, China

Terrestrial carbon cycle plays a key role in driving climate change and ecosystem carbon balance. Understanding the variations of humidity and temperature and CO2 exchanges are meaningful to reveal the law and mechanism of regional carbon cycles in deserts. We examined the near surface humidity, temperature variations, and CO2 exchanges by eddy covariance and Bowen ratio systems in a typical mobile dune of Horqin sandy land. We analyzed the relationships between water-heat and CO2 exchanges of 0 to 10 m vertical height at daily and seasonal scales were analyzed. The results showed that the vertical variations of near surface temperature ranged from 0.4 ℃ to 2 ℃ and decreased with the increases of height from April to September, but with an opposite pattern in other months. The seasonal variation of air relative humidity was greater than 40%. During the growing season of 2018, the averaged daily net ecosystem carbon exchange (NEE) was -0.02 mg·m-2·s-1. The annual averaged daily NEE was 0.003 mg·m-2·s-1, indicating that the mobile dunes were carbon sources at the whole year scale. The vertical differences of temperature and humidity well fitted the NEE. The inflexion points of the fitting curve were at 10% humidity and 0.5 ℃ temperature, respectively. At the scalem of the year, the NEE fitting result of temperature was better than that of humidity, with the inflexion points at 17 ℃ and 65% humidity, respectively. In the growing season, the near surface vertical temperature difference was negative, which would inhibit CO2 absorption of mobile dunes. The circumstances of high humidity would promote the absorption of atmospheric CO2. Across different time and vertical height, the variations of humidity and temperature were closely related to CO2 exchanges, which affected carbon sink and source of mobile dunes. Carbon budget was more sensitive to temperature than humidity.

An in-situ measurement method of evapotranspiration from typical LID facilities based on the three-temperature model

Low impact development (LID) facilities have ability to increase evapotranspiration (ET), and thus moderate the hydrological cycle and mitigate urban heat island (UHI) effects. However, it is difficult to measure ET of LID facilities by traditional methods (e.g. weighing lysimeter, Bowen ratio system and eddy covariance system) in urban environment. A recently developed method, called three-temperature model (3T model), has been used to measure actual ET of natural underlying surfaces in situ. However, its applicability in ET measurement for LID facilities has not been investigated. In order to investigate the accuracy of this method to estimate ET of LID facilities and its influencing factors, the experimental devices for vegetated LID (green roof with Zoysia tenuifoli (GRZ) and green roof with Callisia repens (GRC)) and non-vegetated LID (permeable concrete pavement (PCP) and permeable brick pavement (PBP)) were installed. The method was validated by comparing the ET measured by 3T model with that measured by weighing platform (hourly data, n = 223–302 for each facility). After validation, the method was used to measure ET variations of typical LID facilities under moist sub-tropical weather condition in Shenzhen, China. The results indicate the coefficient of determination (R2) between ET measured by 3T model and weighing platform is 0.63, 0.60, 0.70 and 0.65 for GRZ, GRC, PCP and PBP, respectively, and the slopes of regression equations are close to 1 and their intercepts are close to 0. Furthermore, the 3T model has higher absolute relative error (ARE) of ET estimation in the early morning and the late afternoon than that in the rest of daytime. This is mainly due to the effect of energy related factors, such as solar radiation, air temperature and the difference between reference surface temperature and air temperature. In addition, ET rates of LID facilities can maintain at a relatively high level in the first few days after rainfall. However, ET of PCP decreases faster than that of other LID facilities during the drying period. These results demonstrate the ability and accuracy of 3T model to measure variations of ET of LID facilities in situ.

Differences in Eddy‐Correlation and Energy‐Balance Surface Turbulent Heat Flux Measurements and Their Impacts on the Large‐Scale Forcing Fields at the ARM SGP Site

AbstractDifferences in the surface turbulent heat fluxes measured by the eddy correlation flux measurement system (ECOR) and the energy balance Bowen ratio system (EBBR) are examined using 12 years of continuous measurements collected at multiple stations in the Atmospheric Radiation Measurement (ARM) program Southern Great Plains (SGP) surface observational network. The flux measurements are found strongly impacted by their upwind surface types, which vary with wind direction. For collocated ECOR and EBBR at the central facility, the fluxes measured by ECOR and EBBR agree much better when their upwind fetches are over the same surface type. Among all stations at SGP, ECOR measures more over winter wheat fields while EBBR measures mostly over grassland. The different seasonality of growth cycles between winter wheat and grass causes systematic differences in measured fluxes between ECOR and EBBR. These differences impact the derived large‐scale forcing as illustrated in a constrained variational analysis, in which the state variables have to be adjusted according to different fluxes to keep the column‐integrated energy and moisture budgets in balance. This impact prevails in summertime on nonprecipitating days. A single‐column model test shows that model‐simulated boundary layer development is impacted by using the large‐scale forcing data of different surface turbulent fluxes. It is recommended to include both ECOR and EBBR measurements to better represent the domain‐mean turbulent fluxes and atmospheric budgets of energy and water vapor at the SGP.

Aboveground Biomass Dynamics in the Low-Mountain Dry Grasslands of the Tianshan Mountains

Identifying the dynamics of aboveground biomass (AB) is vital for effective grassland management, yet knowledge on this phenomenon remains limited in the low-mountain dry grasslands (LMDG) of the Tianshan Mountains. In this study, systematic observations were conducted from 2014 to 2017 to determine AB dynamics in the LMDG of the Tianshan Mountains. To accomplish this, a cutting experiment, an HL20 Bowen ratio system, and TDR300 and WatchDog1400 systems were used. AB dynamics had unique characteristics, because of the influence of the unique environment of this region. AB increased rapidly in spring (April – May), and then increased slowly in summer (June – August) and autumn (September – October). Plants entered dormancy in summer (primarily August). In autumn, plants continued to grow after dormancy, except in years with low precipitation which limited plant growth. Plants generally suffered from water stress in this region. However, in spring, the limiting factor for plant growth was heat, not water. Heat might also be the limiting factor for plant growth in autumn. The results are expected to enhance the understanding of AB dynamics in the LMDG of the Tianshan Mountains, providing data to support local grassland management.

A Universal Ts‐VI Triangle Method for the Continuous Retrieval of Evaporative Fraction From MODIS Products

AbstractThe triangle method based on the spatial relationship between remotely sensed land surface temperature (Ts) and vegetation index (VI) has been widely used for the estimates of evaporative fraction (EF). In the present study, a universal triangle method was proposed by transforming the Ts‐VI feature space from a regional scale to a pixel scale. The retrieval of EF is only related to the boundary conditions at pixel scale, regardless of the Ts‐VI configuration over the spatial domain. The boundary conditions of each pixel are composed of the theoretical dry edge determined by the surface energy balance principle and the wet edge determined by the average air temperature of open water. The universal triangle method was validated using the EF observations collected by the Energy Balance Bowen Ratio systems in the Southern Great Plains of the United States of America (USA). Two parameterization schemes of EF were used to demonstrate their applicability with Terra Moderate Resolution Imaging Spectroradiometer (MODIS) products over the whole year 2004. The results of this study show that the accuracy produced by both of these two parameterization schemes is comparable to that produced by the traditional triangle method, although the universal triangle method seems specifically suited to the parameterization scheme proposed in our previous research. The independence of the universal triangle method from the Ts‐VI feature space makes it possible to conduct a continuous monitoring of evapotranspiration and soil moisture. That is just the ability the traditional triangle method does not possess.

Estimation of transpiration fluxes from rainfed and irrigated sugarcane in South Africa using a canopy resistance and crop coefficient model

The area under sugarcane is rapidly growing worldwide. The consequences of such growth on basin scale water consumption and competing water resources need to be understood. Conventional models for sugarcane evapotranspiration have shown limitations for different environmental conditions. To improve current estimations of sugarcane water consumption, hourly and daily transpiration of rainfed sugarcane in Kwazulu-Natal (South Africa) and daily transpiration for irrigated sugarcane in Mpumalanga (South Africa) were calculated by using the Penman-Monteith equation (TPM) with a variable canopy resistance. Canopy resistance was calculated with the Jarvis-Stewart model from calibrated environmental stress functions. The classic FAO56 crop coefficient approach (TFAO56) was also investigated and crop coefficient values, crop basal coefficient and water stress coefficient were derived. There were differences between derived crop coefficient values and FAO56 weather-adjusted values. Derived crop basal coefficient (Kcb) was 0.9 for rainfed and irrigated sugarcane, which was lower than FAO56 weather-adjusted values of 1.19 for rainfed and 1.15 for irrigated at mid-stage. The reduction of the crop basal coefficient with the water stress coefficient resulted in an underestimation of transpiration for rainfed sugarcane. This indicates that water uptake under stress conditions is a complex process, not easy to model as water can be extracted from considerable depths. Daily estimates obtained from TPM outperformed those obtained from TFAO56 when compared to Bowen ratio and Surface Renewal system field measurements. For rainfed sugarcane with water-stressed conditions the TFAO56 RMSE was 1.55mmday−1 compared to 0.3mmday−1 for TPM. For rainfed sugarcane with water-unstressed conditions the TFAO56 RMSE was 0.5mmday−1 and the TPM RMSE was 0.22mmday−1 for TPM. For irrigated sugarcane the TPM RMSE of 0.47mmday−1 was slightly lower than the TPM RMSE of 0.49mmday−1, and TPM showed better correlation with an R2 of 0.85 compared to an R2 of 0.64 for TFAO56. This suggests that calibrated variables of the Jarvis-Stewart model for sugarcane proved to be suitable for both rainfed and irrigated sugarcane in South Africa. More research is needed to verify the validity of the calibrated stressed functions in other regions with high intensity of sugarcane plantations.

The Bushland Weighing Lysimeters: A Quarter Century of Crop ET Investigations to Advance Sustainable Irrigation

In 1987-1989, the first irrigated crops were grown on the four large, precision weighing lysimeters at the USDA-ARS Conservation and Production Research Laboratory at Bushland, Texas, on the Southern High Plains (SHP). Thus began >25-years of full- and deficit-irrigated crop growth, energy and water balance, evapotranspiration (ET), yield, and water use efficiency (WUE) studies of major SHP crops, including alfalfa, corn and sorghum for both grain and forage, cotton, soybean, sunflower, and winter wheat. Alfalfa studies supported development of the ASCE Standardized Reference ET methodology. The lysimeter effort, led by Terry Howell, Sr., co-designed with Lynne Ebling and Thomas Marek and constructed by Arland Schneider, eventually grew to include a separate lysimeter to study short grass ET, again for the ASCE standard, and a 48-lysimeter facility to study soil type effects on crop water uptake, ET, and WUE using monoliths of four soils typical of SHP irrigated soils. The large lysimeters were managed to be representative of sprinkler-irrigated fields so as to develop crop coefficients used for irrigation scheduling by clients of ET networks developed by Texas A&M AgriLife in collaboration with the USDA-ARS. In addition, the lysimeters were used to test and further develop several technologies important to irrigation science, including soil water sensors, eddy covariance and Bowen ratio systems, scintillometers, thermal remote sensing based ET models, and hydrologic and crop simulation models. With the installation of subsurface drip irrigation systems on two of the lysimeter fields in 2013, the Bushland lysimeters are entering a new phase of advanced irrigation method and management studies.

绿洲荒漠过渡带风况对波文比和蒸散发的影响研究

PDF HTML阅读 XML下载 导出引用 引用提醒 绿洲荒漠过渡带风况对波文比和蒸散发的影响 DOI: 10.5846/stxb201404250822 作者: 作者单位: 北京大学,北京大学环境与能源学院,北京大学环境与能源学院,北京大学环境与能源学院,北京大学 作者简介: 通讯作者: 中图分类号: 基金项目: 林业公益性行业科研专项(201304305); 国家自然科学基金项目(30972421); 国家自然科学基金项目(91025008) Effects of wind conditions on the bowen ratio and evapotranspiration in an oasis-desert ecotone Author: Affiliation: Energy and environment school, Peking University,,,,Peking 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:波文比-能量平衡法是目前蒸散发研究常用的方法之一。在利用波文比-能量平衡法对荒漠地区,尤其对绿洲荒漠过渡带的蒸散发进行研究时,发现有波文比波动大、蒸散发结果准确性不高等问题。如何提高波文比-能量平衡法在绿洲荒漠过渡带的准确性成为荒漠干旱区精确研究水分收支急需解决的问题。在以前的研究基础上,发现绿洲荒漠过渡带的不同风况对波文比有不同影响,这可能是造成波文比-能量平衡法在绿洲荒漠过渡带精度不高的主要原因。为了证明这个假设,于2010-2012年在西北民勤绿洲荒漠过渡带进行了野外连续观测,观测了不同风况条件下波文比、不同高度的温度差、湿度差以及蒸散发的变化规律,分析了它们的特征及其影响因素。结果表明,研究区观测期内波文比值在-17.3-16.2范围内变化,波动较大,呈"U"型变化,生长季中期波文比值低于初期和末期;在无风天气,绿洲荒漠过渡带温度、湿度梯度不受水平气流的影响,波文比波动小,异常值少,波文比方法测算出的蒸散量较为准确,能够代表实际蒸散量,所测得民勤绿洲荒漠过渡带波文比日均值为0.07,日蒸散量为1.6mm/d;在绿洲风和荒漠风天气,绿洲荒漠过渡带空气温度、湿度结构发生明显改变,波文比波动大,不同风况使得大气处于逆温或逆湿的状态,波文比-能量平衡法所测得的蒸散量负值增多,适用性降低。因而在应用波文比-能量平衡法估算绿洲荒漠过渡带的蒸散发时应该选择合适的天气和观测点,避开风况对观测结果的影响。 Abstract:The Bowen ratio-energy balance method is one of the most commonly used methods for estimating evapotranspiration. However, the Bowen ratio can fluctuate greatly and reduce the accuracy of estimates when it is applied to the study of evapotranspiration in desert areas, especially at an oasis-desert ecotone. Improving the precision of the Bowen ratio-energy balance method has become a major focus of studies aiming to accurately estimate water budgets in arid areas. In this study, we found that wind conditions had obvious effects on the Bowen ratio at an oasis-desert ecotone. This may be the main cause of low precision estimates from the Bowen ratio-energy balance method when used at oasis-desert ecotones. In order to test this hypothesis, we recorded meteorological data from May 2010 to June 2012 through the Bowen ratio system at an oasis-desert ecotone in Minqin, China. We analyzed variation in the Bowen ratio, temperature gradient, humidity gradient, and evapotranspiration between two different heights on windy days. The Bowen ratio was lower in the middle of the growing season than at the beginning or end of the season and fluctuated wildly between -17.3 and 16.2. On windless days, horizontal airflow had little effect, causing small fluctuations in the Bowen ratio, and few abnormal values. Thus, the accuracy of evapotranspiration estimates using the Bowen ratio-energy balance method was high. The daily Bowen ratio calculated by the method was 0.07, and the daily evapotranspiration estimate was 1.6 mm/d on windless days. However, the temperature gradient and humidity gradient changed significantly on windy days. Temperature and humidity inversion led to large fluctuations in the Bowen ratio, and more negative evapotranspiration estimates. In these conditions, the accuracy evapotranspiration estimates using the Bowen ratio-energy balance method was reduced. Therefore, when using the Bowen ratio-energy balance method to estimate evapotranspiration at oasis-desert ecotones, it is important to choose the right measurement points and weather to avoid adverse impacts of wind. 参考文献 相似文献 引证文献

Evapotranspiration estimation of Platycladus orientalis in Northern China based on various models

Platycladus orientalis is one of the most popular afforestation species and greening species for water management in arid and semi-arid regions of northern China. We applied various models to estimate and validate artificial P. orientalis forest evapotranspiration features with the goal of accurately estimating the water use of a P. orientalis plantation. The American Society of Civil Engineers Evapotranspiration–Penman–Monteith model (APM) and FAO56–Penman–Monteith model (FPM) are extensively applied for vegetation evapotranspiration estimation because their reliability has been validated by many scholars. The Priestley–Taylor model (PT) and Hargreaves model (HS) require only the daily maximum temperature, daily minimum temperature and solar radiation to estimate evapotranspiration and are thus widely applied to grasslands but not to forests. We used the Energy Balance Bowen Ratio (EBBR) system to validate the accuracy of the four models. The results indicated that: (1) Compared to the EBBR measurement annual value, APM was the most accurate, followed by FPM, and PT; (2) During the year, the accuracies of the four models varied. APM and FPM underestimated evapotranspiration during June, July and August, whereas PT and HS overestimated evapotranspiration during this period. In the rest of the year, the estimation accuracies were reversed; (3) An analysis of the possible reasons indicated that wind speed, air temperature and precipitation were the most important contributors. High temperatures were measured in June, July and August, which led to an overestimation by PT and HS because these two models only calculated the temperature and radiation without vegetation information. Underestimation also occurred when a low temperature was recorded. Though APM and FPM addressed both meteorological and vegetation factors, slight deviations still existed; and (4) The two models were modified based on EBBR-measured data. Relative humidity was introduced into PT, and parameter “A” in the HS estimation model was amended to 1.41. The accuracy of the modified models significantly increased. The study highlighted the application, comparison and improvement of four models in estimating evapotranspiration and offers more approaches to assess forest hydrological functions.

Assessment of two temporal-information-based methods for estimating evaporative fraction over the Southern Great Plains

Evaporative fraction (EF) is an important index for partitioning surface available energy. Temporal information derived from multi-times observations of remote sensing can reduce the uncertainty of EF estimates caused by the retrieval error of land surface parameters. Two temporal-information-based EF methods were assessed using in situ measurements from the Energy Balance Bowen Ratio system and the outputs from the second phase of the North American Land Data Assimilation System (NLDAS-2) over the Southern Great Plains (SGP). One is a newly developed method for estimating daily EF using a semi-empirical parameterization based on the day–night differences of land surface temperature (LST), air temperature, and incoming solar radiation; the other is the triangular feature space method constructed by the day–night difference of LST and fractional vegetation cover. The results showed that the two methods could reasonably estimate EF over the SGP region in spatial distribution. The EF estimated from the newly developed method was closer to the in situ measurements with a bias of −0.028 and a root mean square error (RMSE) of 0.194, and it was also closer to the outputs of the NLDAS-2 with an RMSE of ~0.13 compared with the results from the feature space method. A strong relationship between the results from the two methods was found with a coefficient of determination up to 0.824 and an RMSE of 0.143. The RMSE for the EF estimates caused by different vegetation indexes in the feature space could reach 0.15, and larger bias mainly occurred on surfaces with low evapotranspiration surface.

Characteristics of soil evaporation, plant transpiration and water budget of Nitraria dune in the arid Northwest China

Nitraria, a widely growing shrub plant in the dry desert area of China, can fix moving sand along its canopy and form a large number of sand dunes. The Nitraria dune is one of the most effective ways to fix moving sand and protect oasis in Northwest China. However, after dune formation, Nitraria plants gradually die and then release the fixed sand which causes damage to the oasis again. A decrease in the ratio of transpiration (T) to evapotranspiration (ET) was assumed to be the main reason for Nitraria dune degradation because more water was used for soil evaporation, however, this assumption has remained untested because of the difficulty in measuring the Nitraria dune transpiration rate. To overcome this challenge, an intensive field experiment was carried out in 2008–2012 in the Minqin, a typical desert-oasis region in Northwest China. Four measurement sites (early growth stage, rapid growth stage, peak growth stage, and senescence stage) represent the different succession stages of a Nitraria dune. Meteorological parameters were measured by Bowen ratio system, vegetation features and soil physical properties were measured by conventional methods, soil evaporation and transpiration by thermal remote sensing and three-temperature model (3T model), soil moisture by gravimetric and neutron probe method, and evapotranspiration (ET) by Bowen ratio energy balance and water balance methods. Results showed that in a wet year (2008), annual ET was 121, 108, 114, and 126mm, for the four stages, respectively. The ratio of ET to precipitation (P) was 103, 92, 97, and 107%, respectively. In a dry year (2010), ET was 75, 89, 79, and 79mm, respectively, while the ET/P was 106, 126, 112, and 112%, respectively. ET accounted for 92–107% of the precipitation in the wet years and 106–126% in the dry years. ET was nearly equal to precipitation in the wet years and greater than precipitation in the dry years, indicating almost all water from precipitation evaporated in all sites. Our results also showed that vegetation coverage in the four stages was 0.15, 0.35, 0.74, and 0.23, respectively. Instantaneous value of T was 0.021, 0.014, 0.033, and 0.003mmh−1, respectively for the four stages. Instantaneous value of soil evaporation (E) was 0.054, 0.013, 0.004, and 0.009mmh−1, respectively. The corresponding ET was 0.075, 0.027, 0.037, and 0.012mmh−1 for the four stages, respectively. The ratio of T/ET was 0.28, 0.52, 0.89, and 0.25, respectively for the four stages. The ratio reached to its maximum value of 0.89 and then began to decrease, which clearly proved our hypothesis. It concluded that the Nitraria plant not only consumes all the rainfall in the growing season, but also some of the water stored in the soil, which gradually consumes all the soil water storage and finally causes the Nitraria plant death. It also concluded that increasing T, decreasing E and keeping a high T/ET ratio is crucial for desert plants to survive. These results show that the above hypothesis is true and it will be useful for vegetation rehabilitation in the desert area.

Discussion and reassessment of the method used for accepting or rejecting data observed by a Bowen ratio system

The Bowen ratio energy balance method often produces extremely inaccurate magnitudes of the flux due to resolution limits of the instruments. We analysed the criteria used for rejecting inaccurate data observed using a Bowen ratio system and the resolution limit of the sensors to analytically determine the reliable values of the Bowen ratio (β) and the latent and sensible heat fluxes. The formula used to calculate the error limit of the Bowen ratio (β) was corrected based on the theory of error analysis. An example was proposed for the common case with 0.2 °C resolution limit of temperature measurement and 0.08 kPa resolution limit of water vapour pressure measurement, to show the steps of accepting or rejecting data observed by a Bowen ratio system. The acceptance or rejection of data observed by a Bowen ratio system is a dynamic process, which should be performed based on the excluded interval of the Bowen ratio and the qualitative relationships among the data observed by a Bowen ratio system. The excluded interval of the Bowen ratio can be structured based on the accuracy of the sensors used. Data are excluded first if they do not satisfy the qualitative relationships between the vapour pressure difference, the temperature difference, Bowen ratio, and the available energy, whereas the data in the rejection region range of the Bowen ratio are excluded second. It is necessary to improve the accuracy of the temperature and humidity probes to improve the acceptance rate for data collected using the Bowen ratio system, apart from improving the observed precision of available energy. Copyright © 2013 John Wiley & Sons, Ltd.

Overview of the Bushland Evapotranspiration and Agricultural Remote sensing EXperiment 2008 (BEAREX08): A field experiment evaluating methods for quantifying ET at multiple scales

In 2008, scientists from seven federal and state institutions worked together to investigate temporal and spatial variations of evapotranspiration (ET) and surface energy balance in a semi-arid irrigated and dryland agricultural region of the Southern High Plains in the Texas Panhandle. This Bushland Evapotranspiration and Agricultural Remote sensing EXperiment 2008 (BEAREX08) involved determination of micrometeorological fluxes (surface energy balance) in four weighing lysimeter fields (each 4.7ha) containing irrigated and dryland cotton and in nearby bare soil, wheat stubble and rangeland fields using nine eddy covariance stations, three large aperture scintillometers, and three Bowen ratio systems. In coordination with satellite overpasses, flux and remote sensing aircraft flew transects over the surrounding fields and region encompassing an area contributing fluxes from 10 to 30km upwind of the USDA-ARS lysimeter site. Tethered balloon soundings were conducted over the irrigated fields to investigate the effect of advection on local boundary layer development. Local ET was measured using four large weighing lysimeters, while field scale estimates were made by soil water balance with a network of neutron probe profile water sites and from the stationary flux systems. Aircraft and satellite imagery were obtained at different spatial and temporal resolutions. Plot-scale experiments dealt with row orientation and crop height effects on spatial and temporal patterns of soil surface temperature, soil water content, soil heat flux, evaporation from soil in the interrow, plant transpiration and canopy and soil radiation fluxes. The BEAREX08 field experiment was unique in its assessment of ET fluxes over a broad range in spatial scales; comparing direct and indirect methods at local scales with remote sensing based methods and models using aircraft and satellite imagery at local to regional scales, and comparing mass balance-based ET ground truth with eddy covariance and remote sensing-based methods. Here we present an overview of the experiment and a summary of preliminary findings described in this special issue of AWR. Our understanding of the role of advection in the measurement and modeling of ET is advanced by these papers integrating measurements and model estimates.

Experimental studies on the effects of the “Conversion of Cropland to Grassland Program” on the water budget and evapotranspiration in a semi-arid steppe in Inner Mongolia, China

Since the late 1990s China has promoted the “Conversion of Cropland to Forest and Grassland Program” (CCFGP) as the largest environmental restoration program in the world. Because more water was required for established vegetation, it was assumed that evapotranspiration (ET) and water use in an older restored grassland would be greater than that in a younger restored grassland and that water shortages would then become more severe for further vegetation establishment in a semi-arid steppe due to the extraction of the established vegetation. To verify this hypothesis, a field experiment was carried out in 2008–2009 at seven sites of different ages after restoration. The hypothesis was tested using the Bowen ratio system together with other comparative methods. Results showed that less rainfall would infiltrate the deeper soil layer as the period of time after restoration increased and that the natural steppe site had the shallowest infiltration depth (10 cm or less). However, the youngest restored grassland site had the deepest infiltration depth and the highest soil water content. These results also showed that transpiration and ET rates were higher in the natural steppe site and older restored sites than those in the younger restored sites. Furthermore, it was found that the ratio between soil evaporation and ET was generally lower for all seven sites, which indicated that transpiration was a dominant component in ET. These results indicated that more water was used for vegetation establishment in the older restored grassland than that in the younger restored grassland. Our hypothesis was proved to be true. Meanwhile, it was also found that the dominant species changed from annual herbs (species had lower drought tolerance ability) in the younger restored grassland to perennial grasses in the older restored grassland (species had higher drought tolerance ability). Therefore, it was concluded that the natural steppe was able to utilize more water for vegetation establishment and it used water more efficiently than the restored steppe, while the older restored steppe used water more efficiently than the younger restored steppe. Because changes of vegetation type from annual to perennial not only increased evapotranspiration, but also changed the water budget, more attention should be paid to water shortage when undertaking the CCFGP in semi-arid steppes.

Estimating evapotranspiration fraction by modeling two-dimensional space of NDVI/albedo and day–night land surface temperature difference: A comparative study

Three simple models estimating the evapotranspiration fraction (EF) were tested, validated and compared with available data from NDVI/albedo and day–night land surface temperature difference (DT). They were analyzed using ground based measurements collected by the energy balance Bowen ratio system at the 11 enhanced facilities located at the Southern Great Planes of the USA between April 2001 and May 2005. One model was superior in predicting the EF. It used an extension of the Priestly–Taylor equation and a relation between the normalized difference vegetation index (NDVI) and DT.