Bowen Ratio Energy Balance Research Articles

Estimation of measured evapotranspiration using data-driven methods with limited meteorological variables

Determination of surface energy balance depends on the energy exchange between land and atmosphere. Thus, crop, soil and meteorological factors are crucial, particularly in agricultural fields. Evapotranspiration is derived from latent heat component of surface energy balance and is a key factor to clarify the energy transfer mechanism. Development of the methods and technologies for the aim of determining and measuring of evapotranspiration have been one of the main focus points for researchers. However, the direct measurement systems are not common because of economic reasons. This situation causes that different methods are used to estimate evapotranspiration, particularly in locations where no measurements are made. Thus, in this study, non-linear techniques were applied to make accurate estimations of evapotranspiration over the winter wheat canopy located in the field of Atatürk Soil Water and Agricultural Meteorology Research Institute Directorate, Kırklareli, Turkey. This is the first attempt in the literature which consist of the comparison of different machine learning methods in the evapotranspiration values obtained by the Bowen Ratio Energy Balance system. In order to accomplish this aim, support-vector machine, Adaptive neuro fuzzy inference system and Artificial neural network models have been evaluated for different input combinations. The results revealed that even with only global solar radiation data taken as an input, a high prediction accuracy can be achieved. These results are particularly advantageous in cases where the measurement of meteorological variables is limited. With the results of this study, progress can be made in the efficient use and management of water resources based on the input parameters of evapotranspiration especially for regions with limited data.

Comparison of Satellite Driven Surface Energy Balance Models in Estimating Crop Evapotranspiration in Semi-Arid to Arid Inter-Mountain Region

The regional-scale estimation of crop evapotranspiration (ETc) over a heterogeneous surface is an important tool for the decision-makers in managing and allocating water resources. This is especially critical in the arid to semi-arid regions that require supplemental water due to insufficient precipitation, soil moisture, or groundwater. Over the years, various remote sensing-based surface energy balance (SEB) models have been developed to accurately estimate ETc over a regional scale. However, it is important to carry out the SEB model assessment for a particular geographical setting to ensure the suitability of a model. Thus, in this study, four commonly used and contrasting remote sensing models viz. METRIC (mapping evapotranspiration at high resolution with internalized calibration), SEBAL (surface energy balance algorithm for land), S-SEBI (simplified surface energy balance index), and SEBS (surface energy balance system) were compared and used to quantify and map the spatio-temporal variation of ETc in the semi-arid to arid inter-mountain region of Big Horn Basin, Wyoming (Landsat Path/Row: 37/29). Model estimates from 19 cloud-free Landsat 7 and 8 images were compared with the Bowen ratio energy balance system (BREBS) flux stationed in a center pivot irrigated field during 2017 (sugar beet), 2018 (dry bean), and 2019 (barley) growing seasons. The results indicated that all SEB models are effective in capturing the variation of ETc with R2 ranging in between 0.06 to 0.95 and RMSD between 0.07 to 0.15 mm h−1. Pooled data over three vegetative surfaces for three years under irrigated conditions revealed that METRIC (NSE = 0.9) performed better across all land cover types, followed by SEBS (NSE = 0.76), S-SEBI (NSE = 0.73), and SEBAL (NSE = 0.65). In general, all SEB models substantially overestimated ETc and underestimated sensible heat (H) fluxes under dry conditions when only crop residue was available at the surface. A mid-season density plot and absolute difference maps at image scale between the models showed that models involving METRIC, SEBAL, and S-SEBI are close in their estimates of daily crop evapotranspiration (ET24) with pixel-wise RMSD ranged from 0.54 to 0.76 mm d−1 and an average absolute difference across the study area ranged from 0.47 to 0.56 mm d−1. Likewise, all the SEB models underestimated the seasonal ETc, except SEBS.

Closure in Surface Flux Estimation by Energy Balance Model: Comparison of Priestly-Taylor and Penman-Monteith Computations for a Tropical Site in Ibadan

Increasing demand to further understand complexity in surface energy flux partitioning necessitates the adaptation of numerous estimation methods to fit the site of observation. This is useful for reducing the uncertainty in physically measurable parameters especially those in tropical regions with high human interference in the atmospheric boundary layer. In this study, we used computations from two methods - the Priestley-Taylor (PT) and the Penman-Monteith (PM), based on the Energy Balance model to ascertain closure performance in the surface flux estimations. The study was carried out at the Nigerian Meteorological Experiment III site (7.38 o N and 3.93 o E, 224.2m) located in Ibadan, Southwest Nigeria. Thirty days of a year (2006) dataset were examined using the Bowen ratio (BR) energy balance model to validate the PT and PM methods. The systems were examined across daily and diurnal cycles to better understand the differences in energy partitioning. Results showed that both systems generally favored latent heat flux compared to sensible heat flux perhaps due to above-normal rainfall during the period. The PM method performed better than the PT method with a period average for the sensible heat and latent heat fluxes as 32.05 Wm -2 and 67.66 Wm -2 respectively, accounting for 29.22% and 61.39% of the total net radiation. The PT method underestimates the sensible heat flux by as much as 19.70 Wm -2 compared to the PM method, with a period average of 12.36 Wm -2 representing 11.26% of total net radiation. The PM method also gives a period average Bowen ratio estimate of 0.55, consistent with the standard range for grasslands. The study suggests that the performance of the PM method is related to its response to heat and water vapor transfer over humid regions and would contribute to further research on land-surface interactions over the tropics. Finally, we propose that the measurement of available energy, net radiation, and ground heat flux should be separated for different collocated systems in order to reduce the forcing of closure and aid in proper partitioning of the fluxes. Keywords: surface energy flux, energy balance model, Priestly-Taylor, Penman-Monteith, West Africa, latent heat, sensible heat, NIMEX_3 DOI: 10.7176/JEES/11-5-05 Publication date: May 31 st 2021

Estimation and Validation of Actual Evapotranspiration (ETa) of Maize Wheat Cropping System Using SSEBop Model Over IARI Research Farm, New Delhi, India

Evapotranspiration (ET) is an indispensable component of earth surface energy exchange studies and forms basis for various environmental applications. The present study attempts to estimate Landsat-8 satellite-based spatial distribution of crop ET using Operational Simplified Surface Energy Balance (SSEBop) model and validate through ET estimated using multilevel micrometeorological tower-based Bowen Ratio Energy Balance (BREB). The study was conducted over Indian Agricultural Research Institute farm during the summer (kharif maize) and winter (wheat) season of 2017–2018. The result of BREB showed the intra-seasonal variations in actual ET estimation ranging from maximum ET of 3.84 mm day−1 and 2.64 mm day−1 for maize and wheat during mid-stage to minimum ET of 1.88 mm day−1 for maize in maturity and 1.14 mm day−1 in an initial stage of wheat. It was observed that SSEBop slightly overestimated the crop ET; however, it showed the good agreement with BREB estimated ET (R2 = 0.76) with d-index of 0.92 and root-mean-square error of 0.48 mm day−1. It can be concluded that despite slight overestimation of ET by SSEBop, satellite-based SSEBop model can be recommended for crop ET estimation over large area and can be used for various water management studies.

Evapotranspiration simulation from a sparsely vegetated agricultural field in a semi-arid agro-ecosystem using Penman-Monteith models

Increased competition for water resources for agriculture calls for improved water productivities. Improved water productivities must start with accurate estimation of crop evapotranspiration (ET). Accurate estimation of ET has been a challenge for sparsely vegetated crops especially under varying soil water conditions. In this paper, we aim to improve estimation of ET using the Penman-Monteith (PM) method by (1) improved canopy resistance estimation by better representing soil water (2) improved surface resistance estimation by implementing a segmented approach of surface resistance calibration, and (3) investigating the performance of PM models over sparse vegetation by comparing four different solutions; the standard-PM model (PM-STD), the PM-Coupled (PM-CO) single-layer interactive model, the PM Shuttleworth-Wallace (PM-SW) dual-source interactive model and the PM Two-source Patch (PM-TSP) non-interactive model. The model results are evaluated against ET measurements from in-field Bowen ratio-energy balance observations. The results show that, under water-limiting conditions, correctly representing the soil moisture during the different crop development stages enhanced a PM model's ability to accurately estimate ET. The approach raised the Nash-Sutcliffe Efficiency (NSE) of a PM-STD from 0.72 to 0.75. Applying a segmented surface resistance further improved the PM-STD to an NSE of 0.77. When these improvements were implemented on the other PM models, the PM-SW and PM-CO performed superiorly with NSE of 0.83 and RMSE of 0.07 mm hr−1. The PM-TSP followed with an NSE of 0.79 and RMSE of 0.09 mm hr−1 while the PM-STD model trailed with an NSE of 0.77 and RMSE of 0.08 mm hr−1. The performance of all models highlighted the need to separate canopy and soil resistance terms during surface resistance calibration and the value of implementing a segmented approach for resistance calibration. High performance of PM-SW especially demonstrated the need to implement surface energy partitioning and surface aerodynamics interaction for canopy and soil surfaces in PM models

Quantification and Mapping of Satellite Driven Surface Energy Balance Fluxes in Semi-Arid to Arid Inter-Mountain Region

Crop evapotranspiration (ETc) estimates, on a regional scale, hold enormous potential in managing surface and groundwater resources. This is particularly important for the headwater state of Wyoming, which provides water to found major river basins of the US. In this study, METRIC (Mapping evapotranspiration at high resolution with internalized calibration), a satellite-based image processing model, was used to map and quantify daily, monthly, and seasonal ETc and other energy balance fluxes, i.e., net radiation (Rn), sensible heat (H), and soil heat flux (G) dynamics for different land-use classes. Monthly and seasonal ETc estimated were further used to approximate regional water consumption patterns for different land-use types for nine irrigation districts in semi-arid to arid intermountain region of Big Horn Basin (BHB), Wyoming. The validation of METRIC retrievals against Bowen ratio energy balance system (BREBS) fluxes measured over three vegetative surfaces, viz. sugar beet in 2017, dry bean in 2018, and barley in 2019, indicated high accuracy. The pooled correlation observed between estimated (pooled) and measured instantaneous fluxes had R2 values of 0.91 (RMSE = 0.08 mm h−1, NSE = 0.91), 0.81 (RMSE = 49.6 Wm−2, NSE = 0.67), 0.53 (RMSE = 27.1 Wm−2, NSE = 0.53), and 0.86 (RMSE = 59.2 Wm−2, NSE = 0.84) for ETc, Rn, G, and H, respectively. The biggest discrepancy between measured and estimated monthly ETc values was observed during times when BREBS flux tower footprint was devoid of any crops or the crops at footprint were not actively transpiring. Validation results improved when comparisons were made on monthly scales with METRIC underestimating growing season ETc in the range between 3.2% to 6.0%. Seasonal ETc by land-use type showed significant variation over the study area where crop ETc was 52% higher than natural vegetation ETc. Furthermore, it was found that, in the arid to semi-arid intermountain region of Wyoming, the contribution of irrigation to total seasonal ETc varied in the range of 73–81% in nine irrigation districts that fall within the study area. The high relative contribution of irrigation highlights the importance of identifying and quantifying ETc for improved management in irrigation system design and water allocation.

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.

Augmented Bowen Ratio Analysis –I: Site Adequacy, Fetch and Heat Storage (ABRA)

The addition of measurements of downward-looking infrared surface temperature to the standard suite of Bowen Ratio Energy Balance observations permits a direct examination of the adequacy of fetch. Furthermore, when the observed rates of change of surface temperature are examined (using multiple regression) in the light of the total turbulent heat exchange (the sum of sensible and latent heats) and the total heat energy available (due to net radiation, corrected for the ground heat exchange) a quantification of the rate of heat storage otherwise unaccounted for (e.g. by the canopy) is revealed. The statistical methodology minimizes the difficulties often encountered near dawn and dusk when the ratio of sensible to latent heat fluxes approaches -1. Analysis making use of all available data is then possible, without data exclusions being applied. Four months of data obtained at a site in the Zimbabwe highland region in 2014 are used to illustrate this augmented Bowen Ratio analysis methodology. The analysis reveals that the heat storage terms tend to dominate the heat energy budget for periods following dawn and dusk, and that most of the heat storage is associated with the uppermost layers of soil rather than the vegetative canopy.

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.

Evapotranspiration partitioning and crop coefficient of maize in dry semi-humid climate regime

Guanzhong Plain is one of the most critical maize production areas in Northwest China. It is essential to study the maize irrigation requirement and improve water use efficiency in this area. There is a lack of knowledge about the evaporation portioning and irrigation requirements of crops grown in this region. Based on evapotranspiration observed in a maize cropland using the eddy covariance (EC) technique during four growing seasons (2013, 2014, 2015, and 2017), the seasonal variation of evapotranspiration components and the crop coefficients (Kc) for summer maize in a dry semi-arid area were determined. Energy partitioning has an obvious seasonal variation during growing seasons. The pattern of evapotranspiration partitioning has a clear seasonal variation with the development of the canopy. The pattern of the ratio of transpiration (T) to evapotranspiration (ET) is consistent with the canopy development. For four growing seasons, on a seasonal basis, the ratios of T to ET and E to ET were comparable. In addition, the locally developed crop coefficients were 0.57, 1.01, and 0.50 for the initial, mid, and late stages, respectively. The single crop coefficient derived from local datasets can provide a good prediction of ET. The Kc values reported in this paper were consistent with previous studies conducted in other regions using EC systems but were generally lower than the Kc values derived from ET data measured by lysimeters, the Bowen Ratio Energy Balance system, and the soil water balance method. This indicates that the variability of the locally developed crop coefficient caused by measurement methods is higher than the variability caused by climate.

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.

Effect of drip irrigation on wheat evapotranspiration, soil evaporation and transpiration in Northwest China

Drip irrigation (DI) has been widely used for corn, potato, and soybean, among other crops, and its water-saving effect has been confirmed by numerous studies. However, how DI affects soil evaporation (E), crop transpiration (T) and total evapotranspiration (ET) of wheat in arid regions and how much it can reduce E, T and ET under sufficient irrigation remain uncertain.Our study aimed to reveal the effect of DI on wheat ET by using contrast tests. ET and soil E were continuously measured in a wheat field with two treatments during 2017–2018 in Northwest China by Bowen ratio energy balance (BREB) instruments and micro-lysimeters simultaneously. The two treatments were DI and border irrigation (BI), and the irrigation scheduling for DI and BI was based on the local irrigation scheduling in arid Northwest China. Numerous soil E data were missing for rain and irrigation. We used the classical Shuttleworth-Wallace model to estimate E/ET. By combining the model with the measured ET, we obtained the ET, E and T every day.The results indicated that the total wheat ET, E and T under DI were 340, 50 and 290 mm in 2017 and 520, 99 and 421 mm in 2018, respectively. The ET, E and T under BI were 362, 57 and 305 mm in 2017 and 542, 99 and 443 mm in 2018, respectively. Compared to BI, DI reduced ET, E and T by 22, 3.5 and 18.5 mm on average, respectively. The ET under DI was decreased by 4 %–6 % during 2017–2018. The reduction in ET under DI was primarily caused by the reduction in crop T. These changes were due to DI significantly lowering the leaf area index by 24 % and shortening the crop growth duration by one week.Our study revealed the effect of DI on wheat ET and provided new insight for understanding the effects of DI on field hydrological and crop growth processes.

New Approach to Improve the Soil Water Balance Method for Evapotranspiration Estimation

As an important component of the water budget, quantifying actual crop evapotranspiration (ET) will enable better planning, management, and allocation of the water resources. However, accurate ET measurement has always been a challenging task in agricultural water management. In the upper Midwest, where subsurface drainage is a common practice due to the shallow ground water depth and heavy clayey soil, ET measurement using traditional ground-based methods is more difficult. In this study, ET was measured using the eddy covariance (EC), Bowen ratio-energy balance (BREB), and soil water balance (SWB) methods during the 2018 corn growing season, and the results of the three methods were compared. To close the energy balance for the EC system, the residual method was used. For the SWB method, capillary rise was included in the ET estimation and was calculated using the measured soil water potential. The change of soil water content for ET estimation using the SWB method was calculated in four different ways, including daily average, 24:00–2:00 average, 24:00–4:00 average, and 4:00 measurement. Through the growing season, six observation periods (OPs) with no rainfall or minimal rainfall events were selected for comparisons among the three methods. The estimated latent heat flux (LE) by the EC system using the residual method showed a 29% overestimation compared to LE determined by the BREB system for the entire growing season. After excluding data taken in May and October, LE determined by the EC system was only 10% higher, indicating that the main difference between the two systems occurred during the early and late of the growing season. By considering all six OPs, a 6%–22% LE difference between the EC and the BREB systems was observed. Except during the early growing and late harvest seasons, both systems agreed well in LE estimation. The SWB method using the average soil water contents between 24:00 and 2:00 time period to calculate the daily capillary rise produced the best statistical fit when compared to the ET estimated by the BREB, with a root-mean-square error of 1.15. Therefore, measuring ET using the capillary rise from a shallow water table between 24:00 and 2:00 could improve the performance of the SWB methodology for ET measurement.

PATTERNS OF ENERGY EXCHANGE FOR TROPICAL URBAN AND RURAL ECOSYSTEMS LOCATED IN BRAZIL CENTRAL

Seasonality and inter-annual variation of the energy balance of natural surfaces as the effect of conversion from natural to agricultural areas has been the object of much in-depth research in South America. However, none has assessed the effect of conversion from a native to an urbanized area. Current research was performed in the city of Cuiabá, in the Cerrado-Pantanal ecotone, state of Mato Grosso, central Brazil, featuring a tropical sub-humid climate (Aw). The paper investigates the pattern of energy fluxes exchanges in the rural and urban sites located in Brazilian savannah ecosystem. The heat stored inside the urban canopy (ΔQS) and within the rural area was obtained by the Objective Hysteresis Model (OHM) and by soil flux meter, respectively. Sensible (H) and latent (LE) heat fluxes were estimated by the Bowen Ratio Energy Balance (BREB). The rural and urban sites had different patterns in the micrometeorology variables Wind speed decreased and air temperature and vapor pressure deficit increased from rural to urban site. The urbanization also modified the energy fluxes partition of urban canopy by increasing ΔQs (12%) and H (60%) and by decreasing LE (29%), with β = 2.0 in contrast of rural canopy, where the partition was ΔQs (1.5%), H (29%), LE (64%) and β = 0.45. The ΔQS, H and LE were in phase with Rn, differing from what has been reported for sites in North America and Europe.

PATTERNS OF ENERGY EXCHANGE FOR TROPICAL URBAN AND RURAL ECOSYSTEMS LOCATED IN BRAZIL CENTRAL

Seasonality and inter-annual variation of the energy balance of natural surfaces as the effect of conversion from natural to agricultural areas has been the object of much in-depth research in South America. However, none has assessed the effect of conversion from a native to an urbanized area. Current research was performed in the city of Cuiabá, in the Cerrado-Pantanal ecotone, state of Mato Grosso, central Brazil, featuring a tropical sub-humid climate (Aw). The paper investigates the pattern of energy fluxes exchanges in the rural and urban sites located in Brazilian savannah ecosystem. The heat stored inside the urban canopy (ΔQS) and within the rural area was obtained by the Objective Hysteresis Model (OHM) and by soil flux meter, respectively. Sensible (H) and latent (LE) heat fluxes were estimated by the Bowen Ratio Energy Balance (BREB). The rural and urban sites had different patterns in the micrometeorology variables Wind speed decreased and air temperature and vapor pressure deficit increased from rural to urban site. The urbanization also modified the energy fluxes partition of urban canopy by increasing ΔQs (12%) and H (60%) and by decreasing LE (29%), with β = 2.0 in contrast of rural canopy, where the partition was ΔQs (1.5%), H (29%), LE (64%) and β = 0.45. The ΔQS, H and LE were in phase with Rn, differing from what has been reported for sites in North America and Europe.

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.

An automated multi-model evapotranspiration mapping framework using remotely sensed and reanalysis data

Estimating regional evapotranspiration (ET) is challenging in data-limited regions where a lack of in situ observations constrain model calibration and implementation. Here we developed an ensemble mean surface energy balance (EnSEB) modeling framework that is independent of any ground calibration and applied it in India to understand the magnitude and variability of ET in this agriculturally important region. EnSEB uses daily land surface temperature (LST) and vegetation biophysical inputs from Moderate Resolution Imaging Spectroradiometer (MODIS) and climatic information from the National Aeronautics and Space Administration (NASA) Modern-Era Retrospective Analysis for Research and Applications, version 2 (Merra-2) products and runs seven surface energy balance (SEB) algorithms to estimate ensemble mean ET or latent heat (LE) fluxes at a spatial resolution of 1 km × 1 km. Due to limited access to observed flux data, we conducted three different types of model evaluation: i) instantaneous LE validation using observed SEB fluxes from Bowen ratio energy balance (BREB) measurements in four agroecosystems, ii) annual and seasonal ET comparison with six global products at the regional scale, and iii) by closing the basin-scale monthly water budget (WB) for five large river basins (87,900–312,812 km2) in India. Validation with the BREB measurements revealed hourly EnSEB LE estimates to be within 2% of the observed LE (R2 = 0.57 and RMSE = 59 W m−2) and EnSEB was more accurate than any of the individual SEB models. Annual ET from EnSEB was positively correlated with six widely-used global ET products (r = 0.52–0.83, p-value < 0.001), but EnSEB captured the magnitude of ET in intensively irrigated regions much better. Basin-scale monthly WB miscloures were found to be between −1 and 9 mm month−1 from EnSEB ET estimates, which were better than those from the six global ET products. The gap filling method based on the constant ETrF (ET/reference ET) approach introduced some uncertainties in EnSEB, which presents room for future improvements. Overall, our results suggest that the automated and calibration-free multi-model EnSEB framework, which uses only remote sensing and readily available reanalysis data, has the ability to estimate ET with reliable accuracy in Indian agroecosystems. Such a framework could help us better understand and monitor the water cycle in regions where ground data are limited or non-existent.

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.