Bowen Ratio Energy Balance Measurements Research Articles

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.

Field Evaluation of Polymer Capacitive Humidity Sensors for Bowen Ratio Energy Balance Flux Measurements

The possibility of reliable, reasonably accurate and relatively inexpensive estimates of sensible heat and latent energy fluxes was investigated using a commercial combination thin-film polymer capacitive relative humidity and adjacent temperature sensor instrument. Long-term and unattended water vapour pressure profile difference measurements using low-power combination instruments were compared with those from a cooled dewpoint mirror hygrometer, the latter often used with Bowen ratio energy balance (BREB) systems. An error analysis, based on instrument relative humidity and temperature errors, was applied for various capacitive humidity instrument models. The main disadvantage of a combination capacitive humidity instrument is that two measurements, relative humidity and temperature, are required for estimation of water vapour pressure as opposed to one for a dewpoint hygrometer. In a laboratory experiment using an automated procedure, water vapour pressure differences generated using a reference dewpoint generator were measured using a commercial model (Dew-10) dewpoint hygrometer and a combination capacitive humidity instrument. The laboratory measurement comparisons showed that, potentially, an inexpensive model combination capacitive humidity instrument (CS500 or HMP50), or for improved results a slightly more expensive model (HMP35C or HMP45C), could substitute for the more expensive dewpoint hygrometer. In a field study, in a mesic grassland, the water vapour pressure measurement noise for the combination capacitive humidity instruments was greater than that for the dewpoint hygrometer. The average water vapour pressure profile difference measured using a HMP45C was highly correlated with that from a dewpoint hygrometer with a slope less than unity. Water vapour pressure measurements using the capacitive humidity instruments were not as accurate, compared to those obtained using a dewpoint hygrometer, but the resolution magnitudes for the profile difference measurements were less than the minimum of 0.01 kPa required for BREB measurements when averaged over 20 min. Furthermore, the longer-term capacitive humidity measurements are more reliable and not dependent on a sensor bias adjustment as is the case for the dewpoint hygrometer. A field comparison of CS500 and HMP45C profile water vapour pressure differences yielded a slope of close to unity. However, the CS500 exhibited more variable water vapour pressure measurements mainly due to its increased variation in temperature measurements compared to the HMP45C. Comparisons between 20-min BREB sensible heat fluxes obtained using a HMP45C and a dewpoint hygrometer yielded a slope of almost unity. BREB sensible heat fluxes measured using a HMP45C were reasonably well correlated with those obtained using a surface-layer scintillometer and eddy covariance (slope of 0.9629 and 0.9198 respectively). This reasonable agreement showed that a combination capacitive humidity instrument, with similar relative humidity (RH) and temperature error magnitudes of at most 2% RH and 0.3 °C respectively, and similar measurement time response, would be an adequate and less expensive substitute for a dewpoint hygrometer. Furthermore, a combination capacitive humidity instrument requires no servicing compared to a dewpoint hygrometer which requires a bias adjustment and mirror cleaning each week. These findings make unattended BREB measurements of sensible heat flux and evaporation cheaper and more reliable with the system easier to assemble and service and with reduced instrument power.

Sensible heat measurements indicating depth and magnitude of subsurface soil water evaporation

Most measurement approaches for determining evaporation assume that the latent heat flux originates from the soil surface. Here, a new method is described for determining in situ soil water evaporation dynamics from fine‐scale measurements of soil temperature and thermal properties with heat pulse sensors. A sensible heat balance is computed using soil heat flux density at two depths and change in sensible heat storage in between; the sensible heat balance residual is attributed to latent heat from evaporation of soil water. Comparisons between near‐surface soil heat flux density and Bowen ratio energy balance measurements suggest that evaporation originates below the soil surface several days after rainfall. The sensible heat balance accounts for this evaporation dynamic in millimeter‐scale depth increments within the soil. Comparisons of sensible heat balance daily evaporation estimates to Bowen ratio and mass balance estimates indicate strong agreement (r2 = 0.96, root‐mean‐square error = 0.20 mm). Potential applications of this technique include location of the depth and magnitude of subsurface evaporation fluxes and estimation of stage 2–3 daily evaporation without requirements for large fetch. These applications represent new contributions to vadose zone hydrology.

An Evaluation of Evapotranspiration Model Complexity Against Performance in Comparison with Bowen Ratio Energy Balance Measurements

We investigated the relevance of model complexity in 11 evapotranspiration (ET) models, both for two-step and direct predictions for sum-of-hourly, daily, and seasonal time scales and compared their performances against the Bowen ratio energy balance system-measured ET for corn (Zea mays L.) in south central Nebraska. The simplest model we used was the water equivalent of net radiation, the intermediate was the Priestley-Taylor equation, and the complex models were the ASCE-Penman-Monteith (ASCE-PM) based two-step applications (i.e., grass and alfalfa reference surfaces). We also estimated ET with the soil water balance equation from the soil water content measured using a neutron probe soil moisture meter. By constructing the models from the simple towards more complicated, the effects of different components of the energy balance on ET during the growing season were investigated. We reported coefficients of determination (R2) and Nash-Sutcliffe efficiency (R2NS), and root mean square error (RMSE) to assess the models' performance against measured data. The ET from the soil water balance approach agreed well with those measured with the BREBS throughout the growing season. Over a seasonal time scale, the simplistic radiation-based parameterizations gave similar results as those more complex models. In the early season before canopy closure is attained (stage I) and in the late season after crop physiological maturity (stage III), the model predictions significantly deviated from measured data for both daily and seasonal estimates. The two-step ET calculation procedure was found to work very well during the mid-season (stage II) when the crop canopy was fully developed. In stages I and III, all the methods we used revealed negative R2 and R2NS values between the daily predicted and measured ET. We found significant sensible heat losses, indicating low rates of transpiration in both stages I and III. Compared to stage II, the time-dependent crop coefficients were found less capable in scaling the reference ET to represent daily actual crop ET fluctuations in stages I and III. Both the ASCE-PM-based two-step estimates as well as the direct estimates from the Priestley-Taylor and other methods gave more accurate results under full canopy cover. In this period, a sensible heat parameterization implicit in the models improved the daily predictions approximately 10% to 15%, despite model deficiencies. In the mid-season, our results suggested that the daily ASCE-PM reference grass-based approach provided arguably better estimates with respect to both daily and seasonal total fluxes compared to the sum-of-hourly calculations. Compared to the water equivalent of net radiation method, the complex models achieved up to 17% improvement in explaining the variability in daily ET during the mid-season. However, these methods did not improve seasonal ET estimates. Our results suggested that radiation is the dominant driver of evaporative losses over seasonal time scales. Other meteorological variables gained importance at daily and sum-of-hourly calculations.

Crop coefficients and water-use estimates for sugarcane based on long-term Bowen ratio energy balance measurements

Sugar industries in Australia and Swaziland rely on irrigation to a large extent (60 and 100%, respectively) to produce a viable crop. Some large irrigation schemes are vulnerable to degradation of ground water quality and it is important to have reliable estimates of water use in order to better calculate runoff and drainage losses from sugarcane fields. It is also important to improve estimates of crop water use in order to improve irrigation design parameters and scheduling. Matching water supply and demand on a daily or weekly basis is essential for productivity and sustainability in any irrigation scheme. The United Nations Food and Agriculture Organisation has recently produced clear guidelines (‘FAO 56’) on determination of water requirements for a number of crops including sugarcane. Two process-level models, APSIM and CANEGRO are being used to support various decisions regarding the use of irrigation in sugarcane. The aims of this research were to confirm or otherwise refine FAO 56 crop coefficients for sugarcane, to refine simulation of water use in the APSIM-Sugarcane model and to assess the reference evapotranspiration (ET) procedure from the CANEGRO model now used in Swaziland for irrigation scheduling and system design. Bowen ratio energy balance (BREB) systems were installed in the Burdekin district, Australia and in Swaziland to determine daily ET from well-irrigated sugarcane crops. Automatic weather stations were installed within 1 km of the BREB systems in order to determine ET 0 according to FAO 56. Radiation interception and biomass accumulation were measured in the Australian experiment. The results from the two countries provided a sound basis for confirmation of the current FAO 56 crop coefficients for sugarcane during ‘initial’ (0.4) and ‘mid’ (1.25) growth phases. The FAO 56 crop coefficient for the ‘final’ stage (0.7) was not supported. Instead a value of 1.25 is suggested for working out the water balance. However, 0.7 may be desirable for irrigation scheduling in order to impose some stress on the crop and so enhance sucrose content. Transpiration use efficiency for APSIM-Sugarcane was increased from 8.0 to 8.7 g kPa kg −1 on the basis of the Australian results. The reference ET procedure was supported equally by the new BREB measurements in Swaziland and by the original lysimeter data but the new data indicated that systematic improvements to the reference procedure could be made.

Evaluation of the importance of Lagrangian canopy turbulence formulations in a soil–plant–atmosphere model

The suitability of using K-theory to describe turbulent transfer within plant canopies was evaluated with field measurements and simulations of a detailed soil–plant–atmosphere model (Cupid). Simulated results with both K-theory and an analytical Lagrangian theory (L-theory) implemented in Cupid were evaluated against Bowen-ratio energy balance measurements and the temperature profiles in potato canopies. There was no difference between K- and L-theory in terms of simulating E, H and CO 2 fluxes over the canopy. The model slightly underestimated measured E by 3–8%; the comparison of H contained much scatter and the model slightly overestimated CO 2 flux. When the model was tested by simulating temperature and vapor pressure profiles within the canopy, the difference between the K- and L-theory was much smaller than the difference between each theory and the measurements. From simulated temperature profiles, the near-field correction provided by using L-theory seemed to be significant in canopies where the foliage is concentrated in the upper part, but appeared unnecessary for foliage distributed throughout the canopy depth. The major difference between K- and L-theory was in simulations of canopy radiometric temperature; with foliage distributed through out the depth of the canopy, K-theory consistently predicted higher canopy radiometric temperatures than L-theory by 2–8 °C, depending on leaf area index. More systematic study is required to determine if K-theory or L-theory is inadequate for remote sensing of radiometric temperature of canopies.

Carbon dioxide fluxes over bermudagrass, native prairie, and sorghum

The Bowen ratio/energy balance (BREB) method was used to measure 30 min water vapor and carbon dioxide (CO 2) fluxes over three fields dominated by different C 4 grasses (bermudagrass, tallgrass native prairie, and sorghum) at the Blackland Research Center, Temple, TX. Fluxes were related to biotic and abiotic phenomena. Carbon accumulation rates calculated from BREB measurements were compared with those determined from plant biomass measurements. Thirty min CO 2 fluxes were measured continuously from 22 February to 22 November (days 53–326) in 1993 and from 3 March to 22 November (days 62–326) in 1994. Soil CO 2 fluxes were measured periodically in each field and from a bare soil. In all fields, average daily evapotranspiration varied from less than 1 mm day −1 in March and November to about 5 mm day −1 in June and July. The CO 2 fluxes, which were affected by leaf area, radiation, and soil water content, were near zero in each field in the fall and spring and were maximum in the summer (positive toward surface). Total annual CO 2 flux in the bermudagrass, which was sprigged (planted with grass stolons) in April 1993, was −0.4 and 2.8 kg of CO 2 m −2 year −1 (which are equivalent to −0.1 and 0.8 kg of carbon (C) m −2 year −1) in 1993 and 1994, respectively. The larger flux in the second year was due to greater bermudagrass leaf area. Corresponding annual CO 2 fluxes were 0.2 and 0.3 kg m −2 year −1 (0.05 and 0.08 kg C m −2 year −1) for the prairie, and 0.9 and −0.3 kg m −2 year −1 (0.2 and −0.09 kg C m −2 year −1) for the sorghum. Bare soil CO 2 flux was a small fraction of soil CO 2 flux from all fields, suggesting that a considerable amount of soil CO 2 flux was due to root respiration. The dry matter accumulation rate calculated for each year and field from CO 2 flux measurements was within 20% of the rate calculated from gravimetric biomass measurements. The prairie and sorghum fields were in approximate equilibrium for C storage because estimated annual CO 2 fluxes were near zero. By contrast, the bermudagrass field was a large C sink, primarily in the roots, during the second year. This substantiates other evidence that conversion from continuously cultivated cropland to grassland could create a short-term C sink.

Chamber and micrometeorological measurements of CO 2 and H 2O fluxes for three C 4 grasses

Accurate measurements of surface fluxes of carbon dioxide (CO 2) and water (H 2O) are important for several reasons and can be made using several types of instrumentation. For three C 4 grasses—bermudagrass ( Cynodon dactylon (L.) Pers.), a mixed species native tallgrass prairie, and sorghum ( Sorghum bicolor (L.) Moench.)—we measured evapotranspiration (ET) using a canopy chamber (CC) and Bowen ratio/energy balance (BREB) instrumentation and we measured leaf CO 2 uptake using a leaf chamber (LC), and, after accounting for soil CO 2 fluxes, we calculated leaf uptake using a CC and BREB instrumentation. In addition, soil CO 2 fluxes from bare soil were measured using a CC and soil chamber (SC). Measurements were made on 4 and 5 May 1994 at the Blackland Research Center, Temple, TX. Flux of CO 2 into the leaf was considered positive and was expressed per unit ground area. Half-hour CC ET measurements were consistently and substantially greater than BREB measurements for all grasses, perhaps because of increased soil evaporation due to greater turbulence inside the CC. Leaf CO 2 uptake measured using the three methods showed similar diurnal trends for all grasses (responding, primarily, to changes in photosynthetic photon flux density), but consistently tended to be greatest for BREB measurements. The regression equation for LC CO 2 uptake as a function of BREB uptake had a slope not statistically different from 1.0, with large scatter likely because of limited leaf area sampled. CC CO 2 uptake was consistently the least, partly because we may have underestimated soil CO 2 flux in the CC. Half-hour soil CO 2 fluxes from the CC were significantly greater ( P < 0.05) than those from the SC for about two-thirds of the day on bare soil, perhaps because of large chamber ventilation rates. Differences of daytime soil CO 2 fluxes averaged 0.07 mg m −2 s −1 (1.0 mg m −2 s −1 ≈ 22.7 μ mol m −2 s −1). These results show the consistency, repeatability and, we believe, accuracy of leaf CO 2 uptake and soil CO 2 flux measurements made using all methods.

Accuracy of canopy temperature energy balance for determining daily evapotranspiration

The application of a single-layer canopy temperature energy balance (CTEB) model for determining integrated daily ET rates was tested, with possible applications towards determining irrigation requirements (“how much to irrigate”) as a complement to crop water stress index (CWSI) measurements (“when to irrigate”), an irrigation scheduling tool which uses much of the same data. Evapotranspiration (ET) rates estimated using the CTEB model were compared to Bowen ratio energy balance (BREB) measurements made over substantial portions of the growing seasons of corn and potato crops. Canopy temperature, net radiation and soil heat flux data were collected and analyzed at 20-minute intervals, and ET for each interval was summed to obtain daily and multi-day estimations. Only full canopy conditions were examined. Two methods for atmospheric stability correction were applied to the aerodynamic resistance required by the CTEB model; an iterative procedure proposed by Campbell, and a second procedure proposed by Monteith which uses an adjustment coefficient. To reduce instrumentation requirements for combined CTEB/CWSI data collection, estimates of ET were also determined using net radiation and soil heat flux values estimated from solar radiation measurements. Results showed that uncorrected CTEB ET estimates agreed reasonably well with BREB measurements over corn and potato canopies (RMSE = 0.5 to 0.7 mm day− for observed average ET ranging from 4.8 to 5.5 mm day−, with a trend toward seasonal overprediction with corn. Stability corrections usually lowered the daily RMSE 0.1 to 0.2 mm day−, with seasonal ET more in agreement with BREB ET. The Monteith-based adjustment gave slightly better results. CTEB ET model with estimated net radiation and soil heat flux terms produced similar average and total ET, but somewhat larger daily errors (RMSE=0.5 to 0.9 mm day−). Seasonal total ET by the uncorrected CTEB model generally overestimated within 10% (ranging from 1% to 10%) of the observed BREB total ET, an acceptable error for most irrigation practices. Stability corrections generally caused seasonal ET to be underestimated within 1% to 9%.

Spatial Averaging Bowen Ratio System: Description and Lysimeter Comparison

Bowen ratio energy balance (BREB) measurements near a spatially divergent surface may not correctly partition energy fluxes when simply sampling over time. Thus, a Bowen ratio system was designed and constructed to perform spatial sampling and then evaluated by comparison to a precision weighing lysimeter at Bushland, Texas. Results from a comparison of BREB measurements with lysimeter ET measurements on a day following irrigation showed that daily BREB latent heat flux (lE) underestimated daily lysimeter lE by 8%. However, during the daylight period (sunrise to sunset), BREB lE underestimated lysimeter lE by 1.4%; lysimeter lE was overestimated by 0.2% during the period 0630 to 1845 h. Linear regression of BREB lE on lysimeter lE for the duration of the comparison (5 June to 7 June) produced a slope equal to 1.01, coefficient of determination equal to 0.95 and a standard error of estimate equal to 36.5 W m–2. Operation of the system at Fort Collins, Colorado, during an entire growing season showed that the system was functional over the long-term period with minimal maintenance.

Bowen ratio/energy balance technique for estimating crop net CO2 assimilation, and comparison with a canopy chamber

This paper describes a Bowen ratio/energy balance (BREB) system which, in conjunction with an infra-red gas analyzer (IRGA), is referred to as BREB+ and is used to estimate evapotranspiration (ET) and net CO2 flux (NCF) over crop canopies. The system is composed of a net radiometer, soil heat flux plates, two psychrometers based on platinum resistance thermometers (PRT), bridge circuits to measure resistances, an IRGA, air pumps and switching valves, and a data logger. The psychrometers are triple shielded and aspirated, and with aspiration also between the two inner shields. High resistance (1 000 ohm) PRT's are used for dry and wet bulbs to minimize errors due to wiring and connector resistances. A high (55 K ohm) fixed resistance serves as one arm of the resistance bridge to ensure linearity in output signals. To minimize gaps in data, to allow measurements at short (e.g., 5 min) intervals, and to simplify operation, the psychrometers were fixed at their upper and lower position over the crop and not alternated. Instead, the PRT's, connected to the bridge circuit and the data logger, were carefully calibrated together. Field tests using a common air source showed appartent effects of the local environment around each psychrometer on the temperatures measured. ET rates estimated with the BREB system were compared to those measured with large lysimeters. Daily totals agreed within 5%. There was a tendency, however, for the lysimeter measurements to lag behind the BREB measurements. Daily patterns ofNCF estimated with the BREB+ system are consistent with expectations from theories and data in the literature. Side-by-side comparisons with a stirred Mylar canopy chamber showed similarNCF patterns. On the other hand, discrepancies between the results of the two methods were quite marked in the morning or afternoon on certain dates. Part of the discrepancies may be attributed to inaccuracies in the psychrometric temperature measurements. Other possible causes include the highly artificial air turbulence in the canopy chamber and possible associated stomatal response. More work is necessary to identify conclusively the causes. In spite of these uncertainties, the BREB+ technique appears well suited for the automated and simultaneous tracking of photosynthetic performance and water economy of crops in their virtually undisturbed natural environment.