Bowen Ratio Energy Balance Research Articles

Intercomparison of sensible and latent heat flux measurements from combined eddy covariance, energy balance, and Bowen ratio methods above a grassland prairie

We present a comparison of four different methods of measuring sensible (H) and latent (LE) heat fluxes for a year over a mixed grass prairie ecosystem in the Nebraska SandHills [eddy covariance (EC), energy balance/Bowen ratio (EBBR), residual energy (RES), modified Bowen ratio (MBR) methods]. Additionally, we developed a set of quality control criteria for each method and present a simplification to the traditional EBBR setup. Using EC as reference, all methods yielded similar estimates of yearly H (regression slopes (m) ~ 2% from unity; HEC > HEBBR, HRES, and HMBR). For yearly LE, EBBR and RES yielded similar estimates with EC (m ~ 2% from unity; LEEC < LEEBBR and LERES), while a larger bias was found from MBR (m ~ 8% from unity; LEEC > LEMBR). At shorter time scales (~ hourly), moderate scatter was found about linear regression fits for H between EBBR and EC (R2 = 0.81), with smaller scatter between RES and MBR, and EC (R2 = 0.91). For LE, smaller scatter was also measured between EC, and EBBR and RES (R2 = 0.89 and 0.87, respectively), with the larger scatter between EC and MBR (R2 = 0.65). This suggests methods other than EC may be well suited to longer-term applications (≥ yearly), but have larger uncertainty on individual measurements.

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.

Energy Partitioning and Latent Heat Flux Driving Factors of the CAM Plant Pineapple (Ananas comosus (L.) Merril) Grown in the South Subtropical China.

Elucidation of different vegetation energy partitioning and environmental control factors at the agro-ecosystem levels is critical for better understanding and scientific management of farmland. Pineapple (Ananas comosus (L.) Merril) is a tropical plant widely cultivated in the southern subtropical region of China; however, the energy partitioning of crassulacean acid metabolism (CAM) plants like pineapple and their interactions with the environment remain not well understood. In this study, we investigated the energy partitioning patterns of pineapple fields and latent heat flux (LET) response to environmental factors using the Bowen ratio energy balance system and meteorological observation field data. The results showed that the CAM plant pineapple energy partitioning was significantly different from the common C3 and C4 crops during the study period, which was mainly attributed to the complex interactions between CAM plant transpiration and the environment. Specifically, sensible heat flux was the main component of net radiation (Rn), followed by the LET, accounting for 65.0% and 30.8% of the Rn, respectively. Soil heat flux accounts for a very small fraction (4.2%). The mean values of the Bowen ratio were 2.09 and 1.41 for sunny and cloudy days during the daytime and 0.74 and 0.46 at night, respectively. LET is a key factor in responding to crop growth status and agricultural water management, and the path analysis indicates that its variation is mainly influenced directly by Rn with a direct path coefficient of 0.94 on sunny days, followed by vapor pressure deficit, air temperature and relative humidity, which indirectly affect LET through the Rn pathway, whereas soil moisture and wind speed have a low effect on LET. On cloudy days, the effect of Rn on LET was overwhelmingly dominant, with a direct path coefficient of 0.91. The direct path coefficients of the remaining factors on LET were very small and negative. Overall, this study is an important reference for enhancing the impact of pineapple as well as CAM plants on the surface energy balance and regional climate.

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.

Comparing the eddy covariance and gradient methods for measuring water and heat fluxes in paddy fields

Rice is distributed worldwide; thus, quantitative analyses of water and heat fluxes between the land and air in paddy fields are vital for understanding basin-scale water cycles. In this study, eddy covariance and meteorological gradient systems on a 20-meter flux tower in the Jianghan Plain, Yangtze River Basin, China, were used to measure turbulent and meteorological parameters in paddy fields in 2021. Three micrometeorological methods, namely the eddy covariance (EC), Bowen Ratio-Energy Balance (BREB), and aerodynamic (AERO) methods, were used to calculate the latent heat flux (LE) and sensible heat flux (H); BREB and AERO are collectively referred to as gradient methods. The effects of measurement heights, growth stage, and weather conditions on the consistency among them were analyzed. The difference in meteorological measurement heights affected the results of gradient methods. Among the five height combinations, the error in the 2 and 4 m combination was the smallest, and the coefficients of determination of LEEC–LEBREB/AERO and HEC–HBREB/AERO reached 0.96 and 0.84, respectively. The stratification of the near-surface layer was formed due to the heterogeneous underlying surface of the experimental area, and the instrument needed to be installed within the range between roughness layer and new equilibrium layer. The height difference of gradient methods can be amplified within this range to avoid the influence of instrument resolution. The difference among methods was affected by growth stage, and weather conditions (sunny, cloudy, rainy days and different wind speed class). Both factors influenced atmospheric stability, and the error was the maximum in neutral stratification. This study provides a reference for the selection of flux calculation methods and error analyses for paddy fields in humid areas.

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.

Actual Evapotranspiration for Sugarcane Based on Bowen Ratio-Energy Balance and Soil Water Balance Models with Optimized Crop Coefficients

Evapotranspiration is an important parameter to evaluate soil water deficit and water use efficiency, especially in locations with irregularly distributed precipitation. The aim of this study was to assess the daily actual evapotranspiration (ETa) estimated by the Thornthwaite and Mather soil water balance adapted for crops (ThM) and by the dual Kc method with the crop coefficients optimized from inverse modeling and by the adjustment suggested in the FAO-56 bulletin. The models were optimized and evaluated with actual evapotranspiration determined by the Bowen ratio – energy balance method (ETβ) for sugarcane at full canopy closure grown in Alagoas state, northeastern Brazil. The objective function of the inverse problem was defined in terms of ETβ and ETa estimated by the ThM or dual Kc method by optimizing the single crop coefficient (Kc) and the basal crop coefficient Kcb, respectively. Optimized Kc (1.05) and Kcb (1.03) were lower than those adjusted by the Kc FAO56 method (Kc = 1.25 or Kcb = 1.20), with optimized Kc slightly higher than the Kc obtained experimentally (1.01 ± 0.08). ETa estimated by the ThM and dual Kc method with optimized crop coefficients had similar high precision (r2 > 0.79) and accuracy (dm > 0.93 and RMSE < 0.30 mm d−1). However, using the coefficients adjusted from the FAO56 method overestimated ETa in both models.

Predicting soybean evapotranspiration and crop water productivity for a tropical environment using the CSM-CROPGRO-Soybean model

Prediction of crop yield, evapotranspiration, and crop water productivity are essential aspects for water management and the sustainable intensification of agriculture. The goal of this study was to evaluate the Cropping System Model (CSM)-CROPGRO-Soybean model for simulating evapotranspiration and crop water productivity of soybean grown in a tropical environment. Energy balance evapotranspiration was measured daily using a Bowen Ratio Energy Balance (BREB) system for irrigated experiments that were conducted in Piracicaba, SP, Brazil, during the 2016-2017 and 2017-2018 growing seasons. Evapotranspiration was simulated with the CROPGRO-Soybean model using either the Priestley–Taylor or the FAO-56 Penman–Monteith method for potential ET combined with either the Ritchie-Two-Stage or the Suleiman–Ritchie soil water evaporation methods. The model provided good predictions of daily (D-statistic > 0.7) and cumulative evapotranspiration (RMSE ranged from 8 to 64 mm). FAO-56 Penman–Monteith with the Ritchie-Two-Stage method provided a better fit than the Priestley–Taylor with Suleiman–Ritchie when compared with measured data. Simulated crop water productivity agreed well with observed, but with a systematic underprediction (variation between simulated and measured ranged from -2.2 to -16.8 %). Simulation of long-term scenarios was conducted for different tropical environments, i.e., Piracicaba and Teresina, with soil tillage and water management practices. The results demonstrated that the implementation of no-tillage can increase 0.1 kg/m3 (11%) crop water productivity for grain. This study also showed that when irrigation was triggered at more than 60% of available soil water, irrigation did not result in an increase in yield despite an increase in water supply.

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.

Evaluation of Selected Empirical Schemes of Calculating Sensible Heat Flux from Routinely-Measured Meteorological Parameters in a Tropical Location

The accurate estimation of sensible heat flux (Hs) is imperative in the exchange of energy and mass between the earth’s systems. However, the process of acquiring the sensible heat flux data is challenging due to the complexity in its direct measurement hence, not readily available. The alternative is to use estimations which are derived from specific models. This study evaluated the performances of five selected schemes of estimating Hs from routinely-measured meteorological parameters using statistical methods such as Mean Bias Error (MBE), Mean Percentage Difference (MPD), Root Mean Square Error (RMSE), Index of Agreement (IA), Correlation Coefficient (R) and Coefficient of Determination (R2). The empirical schemes selected are: Berkowicz and Prahm (BP), Bowen Ratio Energy Balance (BREB), Holstlag and Van Ulden (HU), Smith (ST) and Surface Temperature (ST) schemes. The results obtained revealed that in the dry season, MBE values of 6.9 Wm-2, 7.2 Wm-2, 7.5 Wm-2, 6.3 Wm-2 and -3.8 Wm-2 were obtained for BP, BREB, HU, SMT and ST respectively. In the wet season, BP, BREB, HU, SMT and ST had MBE values of 37.4 Wm-2, 14.2 Wm-2, 4.2 Wm-2, 12.0 Wm-2 and -1.1 Wm-2 respectively. The MBEs, RMSEs and MPDs obtained for the schemes were higher in the wet season (about 70 %) than in the dry season; implying that the schemes performed better in the dry season than in the wet season. The study rated BREB as the best method of estimating the sensible heat flux at the study location having the lowest MBE, lowest MPD, lowest RMSE, high R, high R2 and high IA in both seasons.

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.

Alteration in surface energy balance fluxes induced from long-term disk-tilled versus no-till management in maize production

Impacts of tillage-induced surface conditions on modification of the soil-crop-atmospheric interface have not been adequately studied. There is poor quantitative understanding at the intersection of tillage and surface energy balance (SEB), especially for the complete growing season. This research measured and evaluated SEB fluxes, i.e., net radiation (Rn), latent heat (LE), sensible heat (H), and ground heat flux (G) for three growing seasons (2011–2013) in a concurrent fashion in two similarly-managed maize (Zea mays L.) fields, which were under disk-till (DT) and no-till (NT) management for 17 years. To this end, each field was equipped with an identical Bowen Ratio Energy Balance System (BREBS) that collected 30-minute SEB data throughout the growing season. G was the most impacted SEB component, followed by H, LE, and Rn on an average basis, although these comparisons were subject to change during different times in the growing season. Seasonal mean interpretations masked within-season heterogeneity in DT vs. NT SEB dynamics and thus, daily differences for all SEB components, albedo (α), surface soil moisture (θv), and Bowen Ratio (β) were quantified. Greater α in NT field (0.16) was maintained throughout the growing season than DT field (0.14). All SEB components demonstrated greater differences between DT and NT in the early and late growing season. Nevertheless, SEB observations show that tillage impacts during significant crop canopy cover should not be ignored. NT surface (0–0.30 m) θv was as much as 22% greater than that of DT, and held critical importance for regional NT crop production during the extreme 2012 drought. θv also exerted control on albedo, with wetter soils having a lower albedo, which consequently translated into altered Rn across DT and NT fields. The research findings shown here underscore the particular importance of accounting for within-season behavior of albedo and G along with other components of SEB for evaluating soil management-related applications.

Exploring the Influence of Seasonal Cropland Abandonment on Evapotranspiration and Water Resources in the Humid Lowland Region, Southern China

AbstractSeasonal abandonment in the previously rice‐based rotation system is increasingly prevalent worldwide particularly in southern China. Assessing the effect of this abandonment on evapotranspiration (ET) and water resources is of major importance for regional water cycle change and agricultural water management. This study analyzed the ET characterization of rice‐winter rape and rice‐winter fallow rotation systems and its relation to weather factors in a humid lowland region, southern China based on a half‐hourly ET measurement with Bowen ratio energy balance system from 2017 to 2020. Then the strategic modeling experiment combining ET and rainfall‐runoff models was used to explore the land abandoning effect on ET and water resources. The results showed that the daily ET values of the rice, rape, and fallow seasons were 3.30 ± 1.89, 1.31 ± 1.31, and 1.55 ± 1.19 mm/d, respectively. The averaged summed ET values of the rice‐winter rape and rice‐fallow rotation systems were 652.3 and 737.8 mm, respectively. Net radiation (Rn) was identified as the most critical weather factor for ET of all seasons. Air temperature and water vapor pressure deficit mainly affected ET indirectly through the path of Rn. Compared with the previously winter rape, the change in daily ET caused by seasonal abandonment was 0.13 ± 0.11 mm/d. This abandonment‐induced ET change increased the average daily storage deficit by 12.0% and groundwater depth by 4.5%, while reduced the daily runoff by 20.3%.

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%.

Leaf and canopy stomatal resistance, aerodynamic resistance, and evapotranspiration of irrigated continuous no‐till and disk‐till maize

AbstractThe impact of no‐till (NT) management as compared with conventional tillage practices (e.g., disk till [DT]) on plant stomatal behavior in relation to model estimation of crop evapotranspiration (ETc) and plant physiological functions has not been sufficiently evaluated. This research investigated the effects of NT and DT management on maize (Zea mays L.) physiological parameters and ETc losses. We also tested the methodology to estimate ETc using a one‐step Penman–Monteith (PM) model with measured variable resistance from scaled‐up leaf stomatal resistance (RL) to canopy resistance (RC) with Bowen Ratio Energy Balance System (BREBS)‐measured ETc. Photosynthetic photon flux density (PPFD) explained 76 and 79% of the variability in RL for NT and DT maize, respectively, which varied with growth stage. Disk‐till maize RL was 29, 5, 25, and 4% higher than NT maize RL during the vegetative, tasseling and silking, reproductive, and near‐physiological‐maturity stages, respectively. The seasonal average scaled‐up RC values for NT and DT maize were 102.4 and 128.6 s m–1, respectively. The PM‐estimated ETc correlated strongly with the BREBS‐measured ETc, with the PM model estimates being 8 and 6% within the BREBS measurements on an hourly and daily basis, respectively. To the best of our knowledge, this research is the first to develop the response curves (PPFD vs. RL) for maize for different growth stages under NT and DT. The analyses presented here can provide invaluable data and information for plant physiology research, crop modeling, determining crop response to different management, and environmental drivers.

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.