Bowen Ratio Energy Balance Method Research Articles

Can changes in land use in a semi-arid region of Brazil cause seasonal variation in energy partitioning and evapotranspiration?

Changes to forests due to deforestation, or their replacement by agricultural areas, alter evapotranspiration and the partitioning of available energy. This study investigated seasonal variations in the energy balance and evapotranspiration in landscapes under different levels of anthropogenic intervention in the semi-arid region of Brazil. Micrometeorological data was obtained from September 2020 to October 2022 for three areas of the semi-arid region: preserved Caatinga (CAA, native vegetation), Caatinga under regeneration (REGE) and a deforested area (DEFA). Here, we use the Bowen ratio energy balance method. Measurements were taken of global solar radiation, air temperature, relative humidity, vapour pressure deficit, rainfall, net radiation, latent heat flux, sensible heat flux, soil heat flux, evapotranspiration, volumetric soil water content and Normalised Difference Vegetation Index. Sensible heat flux was the dominant flux in both areas with 66% for preserved Caatinga vegetation, 63% for Caatinga under regeneration and 62% deforested area. The latent heat flux was equivalent to 28% of the net radiation for preserved Caatinga vegetation, Caatinga under regeneration and deforested area. The evapotranspiration in turn responded as a function of water availability, being higher during the rainy seasons, with average values of 1.82 mm day−1 for preserved Caatinga vegetation, 2.26 mm day−1 for Caatinga under regeneration and 1.25 mm day−1 for deforested area. The Bowen ratio presented values > 1 in deforested area, preserved Caatinga vegetation and Caatinga under regeneration. Thus, it can be concluded that the change in land use alters the energy balance components, promoting reductions in available energy and latent and sensible heat fluxes during the rainy-dry transition in the deforested area. In addition, the seasonality of energy fluxes depends on water availability in the environment.

2017–2019年宁夏兰山骄子葡萄园日蒸散发观测数据集

The grapevine is one of important economic crops. Accurate measurements of daily grapevine evapotranspiration (ET) measurements are essential for improving the understanding of crop water demand and optimizing water consumption efficiency. Unfortunately, there is a relative scarcity of daily grapevine ET values in China. This dataset presents daily ET values obtained through using the Bowen ratio energy balance method and the surface renewal method in Lanshan Jiaozi Vineyard on the outskirts of Yinchuan City, spanning the period from 2017 to 2019. Abnormal Bowen ratio values have been removed during the estimation of ET and the quality of each daily ET measurement has been thoroughly assessed. The dataset is freely available, with the primary goal of establishing a foundation for the efficient use of agriculture water resources. Additionally, it is aimed to assess the applicability of the surface renewal method in a highly heterogonous vineyard.

Estimating Completely Remote Sensing-Based Evapotranspiration for Salt Cedar (Tamarix ramosissima), in the Southwestern United States, Using Machine Learning Algorithms

Accurate estimation of evapotranspiration (ET) is a prerequisite for water management in arid regions. Field based methods estimate point-wise ET accurately, but the challenge is in estimating ET over a region with high accuracies. Machine learning based approaches were taken to estimate ET over a large spatial scale using the Bowen Ratio Energy Balance (BREB) technique. The BREB method depends on terrestrial energy balance equations to estimate ET. Thus, remote sensing-based parameters representing variables in the energy balance equation, and vegetation index representing plant health conditions were used in the model. The study was conducted in the arid areas of the southwestern United States, where dense patches of Salt cedar consume water from the primary water source. The preliminary model used enhanced vegetation index (EVI), global horizontal irradiance (GHI), surface temperature (TS), and relative humidity (RH) as parameters. The k-nearest neighbor method consistently generated poor accuracies. When all the parameters were used, accuracies of the other models varied within 90–94%. When one predictor parameter was dropped, the best model produced accuracies between 90 to 93%, which dropped to 87–92% when a second variable was dropped. Random forest and support vector machine with radial kernel consistently produced the best predictive accuracies.

Accuracy of methods for simulating daily water surface evaporation evaluated by the eddy covariance measurement at boreal flux sites

Evaporation from the surface of water is a vital component of the hydrological cycle. Accurately simulating evaporation is crucial for modeling hydrological processes, ensuring ecosystem water efficiency, and managing water resources. Previous methods for evaluating evaporation have generally been limited to a single site and on a regional scale. Based on eddy covariance measurements of 14 boreal flux sites, the accuracy of 26 methods for simulating daily water surface evaporation was comprehensively evaluated in this study. Most methods accurately simulated daily evaporation at most sites with a KGE >0.60. The combination methods with simultaneous calibration of the energy and aerodynamic terms and the double-parameter aerodynamic methods based on the linear and exponential functions showed the best performance, with the median KGE of the evaluated sites ranging from 0.72 to 0.76. The Bowen ratio energy balance (BREB) method embedded with Tw, Rs-based two-variable empirical methods, double-parameter aerodynamic methods, and the combination method of the energy term based on net radiation (Rn) and the aerodynamic term based on the exponential function method performed better than the other methods of the same type. The relative error (RE) simulated by most methods was generally within ± 30 %, with the median RE of all sites within ± 10 % for each method. The combination methods tended to overestimate the level of evaporation, whereas the BREB-type and aerodynamic methods tended to underestimate the extent of evaporation. The simulation accuracy of the daily evaporation showed significant variance among the sites. It showed the best model performance at two FLUXNET and two sites taken from the literature, in which 75 % of the methods were able to accurately simulate daily evaporation with a Kling–Gupta efficiency (KGE) >0.80. All methods performed poorly at the three sites from the literature and were mainly due to the lack of measured variables with a strong correlation with the latent heat flux (LE). The FLUXNET sites generally showed better performance. The observed LE of the sites with the best model performance generally showed a unimodal distribution throughout the year. The simulation accuracy of non-unimodal distribution sites mainly depended on the correlation between the LE and related variables. The combination of the energy term based on Rn and the aerodynamic term based on the power function method showed the best model stability. This finding indicated that the calibrated method was robust and showed high stability in simulating daily evaporation, and it was recommended for application in other sites or regions. The optimization of parameters calibrated in this study may improve the accuracy of simulating the daily evaporation for the application of the Penman and Priestley–Taylor equations when the water heat storage measurement is missing.

Energy Balance partitioning and Evapotranspiration in Sorghum based intercropping system in Western Region of Tamil Nadu

Quantifying energy balance components over cropped area helps in better understanding of the water balance and helps in economic use of water. A field experiment was conducted during summer 2021 at Tamil Nadu Agricultural University, Coimbatore to study the change of energy balance partitioning and evapotranspiration rate at each growth stage of sorghum based intercropping system. The energy balance components and evapotranspiration rate of crops was estimated by Bowen Ratio Energy Balance (BREB) method. The results revealed that the energy balance partitioning varied among crop growth stages. The variation in LE/Rn was 0.61(seedling) to 0.86(flowering).This variation was based on amount of net radiation, foliage cover, soil moisture condition, vapour pressure deficit .The total evapotranspiration for the crop period was observed to be 322 mm. The ET requirement was the highest during vegetative stage when the total LE was found to be maximum.

Calibration and validation of three evapotranspiration models in a tea field in the humid region of south‐east China

AbstractEvapotranspiration (ETc) is the most important part of agricultural water consumption. Accurate determination of the ETc of tea trees is important in the water management of tea fields. In this study, three models, the Penman–Monteith (PM), Priestley–Taylor (PT) and crop coefficient (KC) models, were applied and calibrated using meteorological data of 2017 in a tea field. The calibrated models were validated using the Bowen ratio energy balance (BREB) method with the data of 2018 and 2019 in the tea field. The canopy resistance of the PM model was calibrated by the inverse method with the BREB‐measured ETc using the Katerji and Perrier (KP) submodel with R2 equal to 0.80. The annual mean value of the coefficient in the PT model (α = 1.03) was determined based on the BREB results and meteorological data. In addition, the crop coefficients of tea trees based on the measured ETc and reference evapotranspiration (ET0) were comprehensively evaluated, with values of 0.86, 1.19 and 0.96 at the three growth stages, showing a cyclical evolution within a year. The PT model performed best, with a root mean square error (RMSE) equal to 0.82 mm day−1 and a Nash–Sutcliffe efficiency (NSE) equal to 0.90. The PM model underestimated ETc with RMSE equal to 0.79 mm day−1 and NSE equal to 0.84, while the KC method overestimated ETc.

Comparison of Fourteen Reference Evapotranspiration Models With Lysimeter Measurements at a Site in the Humid Alpine Meadow, Northeastern Qinghai-Tibetan Plateau.

Evapotranspiration is a key component in the terrestrial water cycle, and accurate evapotranspiration estimates are critical for water irrigation management. Although many applicable evapotranspiration models have been developed, they are largely focused on low-altitude regions, with less attention given to alpine ecosystems. In this study, we evaluated the performance of fourteen reference evapotranspiration (ET0) models by comparison with large weight lysimeter measurements. Specifically, we used the Bowen ratio energy balance method (BREB), three combination models, seven radiation-based models, and three temperature-based models based on data from June 2017 to December 2018 in a humid alpine meadow in the northeastern Qinghai–Tibetan Plateau. The daily actual evapotranspiration (ETa) data were obtained using large weighing lysimeters located in an alpine Kobresia meadow. We found that the performance of the fourteen ET0 models, ranked on the basis of their root mean square error (RMSE), decreased in the following order: BREB > Priestley-Taylor (PT) > DeBruin-Keijman (DK) > 1963 Penman > FAO-24 Penman > FAO-56 Penman–Monteith > IRMAK1 > Makkink (1957) > Makkink (1967) > Makkink > IRMAK2 > Hargreaves (HAR) > Hargreaves1 (HAR1) > Hargreaves2 (HAR2). For the combination models, the FAO-24 Penman model yielded the highest correlation (0.77), followed by 1963 Penman (0.75) and FAO-56 PM (0.76). For radiation-based models, PT and DK obtained the highest correlation (0.80), followed by Makkink (1967) (0.69), Makkink (1957) (0.69), IRMAK1 (0.66), and IRMAK2 (0.62). For temperature-based models, the HAR model yielded the highest correlation (0.62), HAR1, and HAR2 obtained the same correlation (0.59). Overall, the BREB performed best, with RMSEs of 0.98, followed by combination models (ranging from 1.19 to 1.27 mm day−1 and averaging 1.22 mm day−1), radiation-based models (ranging from 1.02 to 1.42 mm day−1 and averaging 1.27 mm day−1), and temperature-based models (ranging from 1.47 to 1.48 mm day−1 and averaging 1.47 mm day−1). Furthermore, all models tended to underestimate the measured ETa during periods of high evaporative demand (i.e., growing season) and overestimated measured ETa during low evaporative demand (i.e., nongrowing season). Our results provide new insights into the accurate assessment of evapotranspiration in humid alpine meadows in the northeastern Qinghai–Tibetan Plateau.

Comparison of evaporation estimation methods for water surface under floating coverage in arid areas

The main purpose of this study is to evaluate evaporation estimation methods for water surfaces under floating coverage in arid areas. The natural water surface evaporation methods (namely, the mass transfer method (MT), Bowen ratio energy balance (BREB) method, Penman method, the Penman-Monteith method (PM), and the Priestley-Taylor method (PT)) are corrected to establish the covered water surface evaporation methods, which are based on the micrometeorology near the covered water surface, energy balance components, and Bowen ratio. The radiation term of the natural water surface evaporation model is corrected by introducing the Bowen ratio of the entire surface, which includes the uncovered water surface and the floating cover surface. The mass transfer coefficient and aerodynamic impedance in the aerodynamic terms of the natural water surface evaporation model were corrected. Taking the evaporation of the water surface covered by spheres (coverage rate of spheres is 76.4%) as an example, based on the rose change characteristics of the above parameters, the effects of these modified evaporation models are comprehensively evaluated from three different time scales (even months, odd months and the complete test period). The evaluation methods include the root mean square error (RMSE), index of agreement (D), mean bias error (MBE), and coefficient of determination (R2). The results show that under three different time scales, the Penman-Monteith model is the most promising and superior in estimating water surface evaporation under floating coverage (for the complete test period, RMSE=0.11 mm·d−1, D=0.99, MBE= 0.07 mm·d−1, and R2 =0.99); the Priestley-Taylor model is slightly less effective in estimating evaporation than the Penman-Monteith model, and the mass transfer model and the Penman model are less effective in estimating evaporation. Furthermore, the analysis of the Penman-Monteith model shows that the evaporation of the radiation term (69.5%) is higher than that of the aerodynamic term (30.5%) during the evaporation of the water surface under floating coverage, while the evaporation of the radiation term is dominant. Moreover, during the complete test period, the weights of the radiation evaporation and aerodynamic evaporation changed significantly with time. The proportion of radiation evaporation varies from 54.8% to 81.7%, while that of aerodynamic evaporation varies from 18.3% to 45.2%.

Comparison of evapotranspiration upscaling methods from instantaneous to daytime scale for tea and wheat in southeast China

Accurately converting instantaneous evapotranspiration (λETi) at satellite over-passing time into daily evapotranspiration (λETd) is a key issue of applying remotely sensed data to estimate regional evapotranspiration (λET) from remote sensing satellites, which plays an important role for effective water resource management. The scaling methods that take advantage of the relationship between λET and other environmental factors and can be used to convert λETi into λETd. In this study, five scaling methods of converting λETi into λETd, including the evaporative fraction method (Eva-f method), revised evaporative fraction method (R-Eva-f method), crop coefficient method (Kc-ET0 method), revised crop coefficient method (R-Kc-ET0 method) and direct canopy resistance method (Direct-rc method), were evaluated based on the detailed meteorological data measured from 2016 to 2018 in a tea field and 2018 to 2020 in a wheat field in southeast China. The estimated λETd was compared with the measured λETd by the Bowen ratio energy balance (BREB) method. The results indicated that the Eva-f and R-Eva-f methods with the mean root mean square error (RMSE) and coefficient of efficiency (ε) equaled 9.02 W m−2 and 0.92; 12.06 W m−2 and 0.89, respectively, were superior to the Kc-ET0, R-Kc-ET0 and Direct-rc method; the Kc-ET0 method with the mean RMSE and ε equaled 20.62 W m−2 and 0.79 was also a good option for simulating the λETd of tea and wheat; while the R-Kc-ET0 method simulated the λETd well for the wheat with mean RMSE and ε equaled 36.29 W m−2 and 0.71, but significantly overestimated the tea λETd with the mean RMSE and ε values of 39.61 W m−2 and 0.59 for tea; the Direct-rc method overestimated λETd of tea and wheat for the most of intervals with the mean RMSE and ε of 39.58 W m−2 and 0.62, and was not recommended to use in the present study areas.

Evapotranspiration estimation using a modified crop coefficient model in a rotated rice-winter wheat system

Understanding the process of crop evapotranspiration (ETc) and developing models for estimating ETc are crucial to efficiently schedule irrigation and enhance efficient water use. Here, we investigated variations of ETc and local crop coefficient (Kc = ETc / ETo, where ETo is the reference evapotranspiration) in a rotated flooded rice-winter wheat system using ETc data based on the Bowen-ratio energy balance method from 2016 to 2020. We propose a modified Kc model for estimating daily ETc, which includes a density coefficient (a function of fraction of canopy cover) and incorporates the effect of plant temperature constraint, leaf senescence, and water stress on ETc. Results indicated that the total ETc over whole growth stage for flooded rice and winter wheat field was 500.2 ± 62.5 and 298.6 ± 28.3 mm (means ± standard deviation), respectively. The values of local Kc at the initial, middle, and late stages were 0.83 ± 0.14, 1.11 ± 0.06, and 0.99 ± 0.15, respectively, for flooded rice and 0.71 ± 0.08, 0.86 ± 0.06, and 0.76 ± 0.08, respectively, for winter wheat. There was no water stress over the entire season of the flooded rice-winter wheat rotation system except for some days with water draining in paddy rice field. Heat stress in summer adversely affected the ETc of rice. The modified Kc model can well reproduce the values of daily ETc for both flooded rice and winter wheat, and improved the accuracy by 6~9% compared to the FAO 56 Kc model using tabulated values after adjustment. The regression coefficient, coefficient of determination, root mean squared error and modeling efficiency between measured ETc and estimated by the modified Kc model were 0.99, 0.89, 0.55 mm d−1 and 0.89, respectively, for flooded rice, and 1.03, 0.85, 0.55 mm d−1 and 0.82, respectively, for winter wheat. Therefore, the modified Kc model could reasonably predict ETc for flooded rice and winter wheat and can serve as a useful tool to improve water use.

Augmented Bowen ratio analysis, II: Ohio comparisons

An experiment conducted over a field of maize in Ohio during the growing season of 2015 provided measurements sufficient to compare among three methods for quantifying net ecosystem exchange of sensible heat, latent heat and carbon dioxide: eddy covariance (EC), the familiar Bowen ratio energy balance (BREB) method and a statistical procedure making use of surface temperature measurements by downward-looking infrared thermometry (the augmented Bowen ratio analysis – ABRA). EC and BREB yield run-by-run estimates of fluxes. The third method (ABRA) gives average values over prescribed ensembles of data. The Ohio observations support the conventional understanding that EC is the preferred methodology provided the site is large enough. Both BREB and ABRA can make use of measurements closer to the surface than EC, and hence are more suited for studies of small test plots. The ABRA method provides results agreeing with conventional BREB and EC in daytime except for periods near dawn and dusk when heat storage can be a substantial contributor in the surface heat energy budget. The unattributed heat storage in the morning found in analysis of observations made elsewhere is substantiated. It is concluded that the apparent heat storage term in the surface heat budget is most influenced by heating of crop biomass and the energy demands of photosynthesis. The comparisons provide clear evidence of the difficulties associated with application of all gradient-based methodologies at night (affecting both BREB and ABRA) when atmospheric stability often prohibits reliable association of fluxes with gradients. CO2 measurements reveal substantial differences among the three different methodologies (ABRA, BREB and EC), with EC appearing to be more robust and therefore preferred if the test area is large enough.

Comparison of two canopy resistance models to estimate evapotranspiration for tea and wheat in southeast China

Actual evapotranspiration (λET) can be estimated with meteorological parameters using the Penman-Monteith (PM) model where the canopy resistance (rc) plays an important role but is difficult to determine. The Katerji-Perrier (KP) and Todorovic (TD) models are widely used to determine the rc. A comparative study was conducted to evaluate the performances of two models, on hourly and daily scales, in simulating the λET for a tea and a wheat field in southeast China. The estimated λET was compared with the measured λET by the Bowen ratio energy balance (BREB) method. The results indicated that the KP model simulated the λET of tea and wheat well for both hourly and daily scales, while the TD model simulated the λET well for the wheat for both time scales but significantly overestimated the tea λET by 44.9%. Therefore, the KP model was preferred to estimate λET for both crops, the TD model should be avoided in tea fields.

Determination of Evapotranspiration For Wheat by Using Bowen Ratio Energy Balance Method

Araştırmada, Çukurova yöresinde doğal yağış koşullarında yetiştirilen buğday bitkisinin gerçek su tüketimi, Bowen oranı enerji dengesi (BREB) ve su bütçesi yöntemleri kullanılarak belirlenmiştir. Buğdayın mevsimlik su tüketimi BREB yöntemiyle 477 mm, su bütçesi yöntemiyle 494 mm hesaplanmıştır. Sulanan buğday konusunda ise mevsimlik su tüketimi su bütçesi yöntemiyle 708 mm hesaplanmıştır. Su bütçesi ile BREB yöntemi kullanılarak elde edilen bitki su tüketimi değerleri arasında R2= 0.83 olan doğrusal bir ilişki bulunmuştur. Buğday bitkisine ilişkin bitki katsayıları (Kc), FAO-Penman-Monteith (PM) eşitliği kullanılarak hesaplanmıştır. Kc değerleri başlangıç, bitki gelişimi, mevsim ortası ve mevsim sonu dönemleri için, sırasıyla 0.55, 0.98, 1.47 ve 0.77 olarak hesaplanmıştır. Denemede dane verimi değerleri susuz ve sulanan konuda sırasıyla 661 ve 551 kg.da-1; kuru madde miktarı 1611 ve 1645 kg.da-1 olarak elde edilmiştir. Su kullanma randımanları ise, kuru madde esasına göre (WUEb) susuz ve sulanan konuda sırasıyla 3.38 ve 3.45 g.m-2.mm-1; verim esasına göre (WUEy) 1.39 ve 1.15 g.m-2.mm-1 olarak hesaplanmıştır. Hasat indeksi (HI) değerleri ise, susuz ve sulanan konuda, sırasıyla 0.44 ve 0.34 olarak hesaplanmıştır. Yapılan varyans analiz sonuçlarına göre anılan konuların kuru madde miktarı ve dane verimi değerleri arasında istatistiksel anlamda bir fark bulunmamıştır.

Prediction of actual evapotranspiration by artificial neural network models using data from a Bowen ratio energy balance station

Seven artificial neural network (ANN) models were developed to predict daytime actual evapotranspiration (ET) for Nissouri Creek in Oxford County, Canada, from April to July 2018, using the Bowen ratio energy balance method as target output for the first time. In total, 12 variations of each model were deployed using different combinations of model parameters, including the sigmoid and rectified linear unit (ReLU) activation functions, stochastic gradient descent (SGD), and root-mean-square-propagation (RMSprop) learning algorithms, three different network architectures, and 100 and 500 training epochs. This is the first time that ReLU has been used in ANNs that predict ET and it outperformed sigmoid in six of the seven models. This is particularly significant because until now the sigmoid activation function (or variations therein) had been exclusively employed in the ET literature. RMSprop was also used for the first time and typically demonstrated equivalent performance to that of SGD. The optimal model employs the ReLU activation function, consists of a 4-4-1 network architecture, includes the input parameters of net radiation, air temperature, soil heat flux, and wind speed, and is trained by the SGD learning algorithm for 500 training epochs. This model boasts a coefficient of determination (R2) of 0.997, root-mean-square error (RMSE) of 0.39 mm/day, and mean absolute error (MAE) of 0.18 mm/day. Furthermore, all seven models developed adequately model the ET process, with R2 ranging from 0.988 to 0.997, RMSE from 0.39 to 0.78 mm/day, and MAE from 0.18 to 0.58 mm/day.

Seasonal and Diurnal Cycles of Surface Boundary Layer and Energy Balance in the Central Andes of Perú, Mantaro Valley

The present study presents a detailed analysis of the diurnal and monthly cycles the surface boundary layer and of surface energy balance in a sparse natural vegetation canopy on Huancayo observatory (12.04 ∘ S, 75.32 ∘ W, 3313 m ASL), which is located in the central Andes of Perú (Mantaro Valley) during an entire year (May 2018–April 2019). We used a set of meteorological sensors (temperature, relative humidity, wind) installed in a gradient tower 30 m high, a set of radiative sensors to measure all irradiance components, and a set of tensiometers and heat flux plate to measure soil moisture, soil temperatures and soil heat flux. To estimate turbulent energy fluxes (sensible and latent), two flux–gradient methods: the aerodynamic method and the Bowen-ratio energy-balance method were used. The ground heat flux at surface was estimated using a molecular heat transfer equation. The results show minimum mean monthly temperatures and more stable conditions were observed in June and July before sunrise, while maximum mean monthly temperatures in October and November and more unstable conditions in February and March. From May to August inverted water vapor profiles near the surface were observed (more intense in July) at night hours, which indicate a transfer of water vapor as dewfall on the surface. The patterns of wind direction indicate well-defined mountain–valley circulation from south-east to south-west especially in fall–winter months (April–August). The maximum mean monthly sensible heat fluxes were found in June and September while minimum in February and March. Maximum mean monthly latent heat fluxes were found in February and March while minimum in June and July. The surface albedo and the Bowen ratio indicate semi-arid conditions in wet summer months and extreme arid conditions in dry winter months. The comparisons between sensible heat flux ( Q H ) and latent heat flux ( Q E ), estimated by the two methods show a good agreement (R 2 above 0.8). The comparison between available energy and the sum of Q E and Q H fluxes shows a good level of agreement (R 2 = 0.86) with important imbalance contributions after sunrise and around noon, probably by advection processes generated by heterogeneities on the surface around the Huancayo observatory and intensified by the mountain–valley circulation.

Micrometeorological methods to estimate sugarcane evapotranspiration in coastal northeastern region of Brazil

The aim of the present work was to evaluate the performance of Bowen ratio-energy balance method, as well as the energy balance closure by Eddy covariance technique for a sugarcane crop in Brazilian northeastern region. Micrometeorological measurements were carried out between June 7th and November 17th, 2013. Latent and sensible heat fluxes were determined through Eddy covariance technique (EC) and by the Bowen ratio-energy balance method (BREB), considering two approaches. The first, estimated the air temperature and water vapour pressure gradient in two levels above the canopy. The second method measured the air temperature and water vapour pressure at the first level and the surface temperature from radiometric measurements. Latent heat flux was also estimated as energy balance residue from determinations of the sensible heat flux by Eddy covariance. The degree of energy balance closure was dependent on the time of the day considered. Bowen ratio - energy balance estimated from the first approach, showed the best agreement with the eddy covariance measurements to estimate latent heat flux, while in the second case, when the Bowen ratio was estimated using the surface temperature, the linear relationship was the most discrepant. Therefore, the Bowen ratio conventional method is more suitable for estimating latent heat flux in sugarcane.

Evapotranspiration estimation using a modified Priestley-Taylor model in a rice-wheat rotation system

The rice-wheat rotation system is one of the largest agricultural production systems worldwide. Accurate estimation of evapotranspiration (ET) in the rice-wheat rotation system is critical to enhance efficient irrigation management and water use. The variation of ET for a rice-wheat rotation system during 2015–2018 and its controlling meteorological factors was investigated using the Bowen ratio energy balance and path analysis methods. A modified Priestley–Taylor (PT) model considering soil water stress for soil evaporation (E) (fsw), and plant temperature constraint (deviation of air temperature from optimum for the crops used, ft), leaf senescence for transpiration was developed. The results showed that the diurnal variation of ET rate in different months exhibited a single peak curve with the maximum ET rates of 0.90 and 0.42 mm h−1 for rice and winter wheat, respectively. The total ET of the rice-wheat rotation system over the whole growing season was 765–841 mm, of which 63–67% was consumed by the rice field. The average daily ET rate over the whole growing season was 3.27–4.13 and 1.50-1.65 mm d−1 for rice and winter wheat, respectively. The results of ET partitioning showed that E accounted for 23–32% of the seasonal ET for rice and 48–51% for winter wheat. The ET partitioning of rice and winter wheat was closely linked to leaf area index (LAI). The ratio of E/ET reduced exponentially for rice with the increase of LAI, while it reduced linearly for winter wheat. The path analysis showed that the net radiation (Rn) was the dominant meteorological factor affecting short-term ET of the rice-wheat rotation system through the direct effect. The water vapour pressure deficit (VPD), another important factor influencing ET, showed mainly an indirect effect on ET through path of Rn and had a greater impact on ET for rice than that for wheat. The modified PT model could estimate ET for rice and winter wheat reasonably, with linear regression coefficient of 0.93–1.09 and coefficient of determination of 0.92−0.96. The model was sensitive to the fsw or ft.

A low-cost hydrologic observatory for monitoring the water balance of small lakes.

Global warming portends an accelerated water cycle as increased evaporation feeds atmospheric moisture and precipitation. To monitor effects on surface water levels, we describe a low-cost hydrologic observatory suitable for small to medium size lakes. The observatory comprises sensor platforms that were built in-house to compile continuous, sub-daily water budgets. The variables measured directly are lake stage (S), evaporation (E), and precipitation (P). A net inflow term (Qnet) is estimated as a residual in the continuity equation: ∆S = P - E + Qnet. We describe how to build in-lake stilling wells and floating evaporation pans using readily available materials. We assess their performance in laboratory tests and field trials. A 3-month deployment on a small Wisconsin lake (18 ha, 10 m deep) confirms that continuous estimates of ∆S, E, P, and Qnet can be made with good precision and accuracy at hourly time scales. During that deployment, daily estimates of E from the floating evaporation pans were comparable with estimates made using the more data-intensive Bowen ratio energy balance method and a mass transfer model. Since small lakes are numerically dominant and widely distributed across the globe, a network of hydrologic observatories would enable the calibration and validation of climate models and consumptive use policies at local and regional scales. And since the observatories are inexpensive and relatively simple to maintain, citizen scientists could facilitate the expansion of spatial coverage with minimal training.

Effect of fertilization on water use and yield of hops in mature hop plantation

The effect of decreasing nutrient availability on the ecophysiology and production of hop cones was studied in a plantation of cultivar 'Premiant', in Žatec hop growing region, Czech Republic. The plantation used to be fertilized by 60 t of manure ha(-1) year(-1) until November 2011 when it was discontinued as a part of the measures towards organic farming. Measurements of total evapotranspiration from the plantation using the Bowen ratio energy balance method, transpiration of hop bines by the means of sap flow, soil water and nutrient availability were conducted from 2011 to 2014. The yield of hop cones and the content of alpha acids were monitored in this period too. The concentration of total soil nitrogen (N) dropped from 19.4 mg kg(-1) in March 2012 to 8.2 mg kg(-1) in November 2014. Similarly, the content of N in leaves declined from 4.1% of dry mass in 2012 to 2.5% in 2014. The ratio between total evapotranspiration from the plantation to the potential evapotranspiration (PET) was not affected by the soil nutrient availability and did not vary among the years of study, ranging between average to 91 and 96% of PET. In the contrast to total evapotranspiration and PET, the proportion of hop transpiration on the total evapotranspiration steadily declined from 74 to 46% in 2011 and 2014, respectively. Plant regulation of stomatal conductance changed, bringing along lower stomatal aperture, especially upon the conditions of low vapor pressure deficit. In line with decline in transpiration and stomatal conductance, yield of alpha-acids declined from 204 kg ha(-1) in 2011 to 97 kg ha(-1) in 2014. To sum up, after the stop in fertilizing, total water use of the plantation remained similar as before, but the transpiration and production of alpha acids ha(-1) significantly decreased.

The role of decoupling factor on sugarcane crop water use under tropical conditions

AbstractThe expansion of sugarcane crop to regions with lower water supply in Brazil has increased the importance of correct estimation of crop water requirements. Currently, the irrigation management is generally done using the crop coefficient (Kc) based on the FAO 56 bulletin. Kc is used to determine the potential water demand of the crop for a given period of time and is considered constant for each crop stage. However, some recent studies have shown that Kc can be significantly variable under different evapotranspiration (ETo) rates. This paper aimed to analyse sugarcane water consumption at different scales: plant (sap flow measurements by energy balance method); canopy (Bowen ratio energy balance method); and plant–atmosphere coupling (infrared gas analyser) to reduce the uncertainties on the irrigation practices. Measurements were taken at two experimental sites, where a modern Brazilian cultivar CTC 12 was grown under drip irrigation and an old main Brazilian cultivar (RB867515) was grown under sprinkler irrigation by a central pivot. The mean crop evapotranspiration (ETc) values by the Bowen ratio energy balance method were 2.92 and 3.68 mm d−1 for RB867515 and CTC 12, respectively, resulting in a mean Kc of 0.99 at the full vegetative growth stage. Kc values were dependent on ETo and varied between 0.2 and 1.7 for both cultivars. This occurred in a crop coupled to the atmosphere (Ω = 0.37) and was the same found in other coupled crops such as coffee and citrus. In conclusion, the sugarcane Kc for southeast Brazil presented temporal variability due to coupling conditions according to reference evapotranspiration, and this should be considered in irrigation management.