Bowen Ratio Energy Balance Technique Research Articles

The water balance of a first order catchment in the montane grasslands of South Africa

Measurements of daily actual evaporation, streamflow, rainfall and soil water storage were measured for a two-year period in a 97 ha grassland catchment located in the Natal Drakensberg. Actual evaporation was measured using the Bowen ratio energy balance technique. Annual precipitation and discharge data were also used to estimate evaporation over a number of years. Results show that in normal years precipitation is equally split between evaporation and streamflow. In dry years evaporation is the dominant component of the water balance. The data were used to develop simple expressions to calculate annual streamflow and evaporation. Good agreements between actual and modelled trends of streamflow using the ACRU hydrological model were found ( r=0.80). In general ACRU underestimated streamflow by 15%.

Modelling evapotranspiration from a barley field over the growing season

This study tested a modified version of the two-layer evaporation energy combination scheme of Shuttleworth–Wallace (S–W). The first modification was the application of a two-layer soil module, which included the soil surface resistance for evaporation and soil moisture in the rooting layer controlling the canopy resistance. Another modification was the inclusion of the surface temperature in the calculation of the aerodynamic resistance. The surface temperature controls both the stability and the canopy resistance. The modelled total evapotranspiration was compared with evaporation calculated from the Bowen-ratio energy-balance technique over two growing seasons of a barley field, which provided a wide range of canopy densities and variation of driving factors with rainy, dry, warm and cold periods. The S–W scheme result on a daily and hourly basis was generally in good agreement with the measured evapotranspiration. The largest deviation between the measured and simulated evaporation occurred in a late phase of the growing season with a yellow non-transpiring canopy and, after harvesting, with stubble. The simulated surface temperature was mainly within ±2 K of direct measurements, both in bare soil and dense canopy situations. The study showed that the scheme is applicable for practical use, since the weather input parameters required are limited to those which are available as normal synoptic observations.

Maize canopies under two soil water regimes: IV. Validity of Bowen ratio–energy balance technique for measuring water vapor and carbon dioxide fluxes at 5-min intervals

The feasibility of determining at 5-min intervals the fluxes of water vapor and CO 2 using a Bowen ratio/energy balance/CO 2 gradient (BREB+) apparatus was studied. Details of the BREB+ apparatus are given. The results obtained over maize canopies at the short 5-min signal averaging time are examined in several aspects. One aspect is the fact that on days when net radiation ( R n) changed rapidly because of cloud movement, latent heat flux ( λE) fluctuated nearly in unison at 5-min intervals with the fluctuations in independently measured R n, Bowen ratio, and canopy temperature. The direction of the fluctuations is consistent with the principles of energy balance and gradient transport. Hence, the results support the use of the short signal averaging time. Another aspect is the coincidental fluctuations in the measured differential in water vapor (Δ q) and in carbon dioxide (ΔCO 2) at two heights above the canopy on clear days when the trend in radiation was steady, which appeared to be the result of measurement noise at first glance. It turned out that Δ q and ΔCO 2 fluctuated largely in unison, indicating the likelihood of real accumulation of water vapor and depletion of CO 2 near the top of the canopy at times of low turbulence, and the opposite effects of reduced accumulation and depletion at times of high turbulence. The calculated values of eddy transfer coefficient for water vapor, K w, is consistent with this interpretation. In spite of the fluctuations in Δ q, the time trend in λ E was steady on clear days. In contrast, the time trend in the 5-min canopy CO 2 flux ( A cf) often exhibited substantial short term peaks and dips. Parts of these fluctuations in A cf may be attributed to the measurement noise in ΔCO 2 and the inherent larger errors in calculating A cf. The error contributed by the changes in CO 2 and water vapor stored within the canopy air volume from one measurement interval to the next were evaluated. The limited results show that the errors due to storage were largely minor and were similar in relative size for λ E and for A cf. The 5-min mean data were compared with the 30-min mean data to evaluate whether the shorter averaging time is sufficient to make horizontal divergence of energy components insignificant. The comparison indicates negligible mean horizontal movements in λE and sensible heat H and demonstrates the fulfillment of the assumption of one-dimensional transport underlying the BREB+ technique. On days of variable clouds, it is shown that the 30-min mean values distorted or obliterated the expected responses of the canopy to sudden changes in radiation. Some limitations and other inaccuracies of the BREB+ technique are discussed. Overall, the results are strongly supportive of the validity of the BREB+ technique in general, and the use of the novel averaging time of 5 min for λ E and H over field crops in particular. The necessary averaging time for A cf is less certain and further study is desirable.

Estimating Water Use and Irrigation Requirements of Coffee in Hawaii

Crop evapotranspiration (ETc) was measured as evaporative heat flux from drip-irrigated coffee (Coffea arabica L. cv. Yellow Catuai) fields at different stages of canopy development using the Bowen ratio-energy balance technique. Irrigation requirements were determined by comparing the ETc values obtained against reference values (ET0) derived from a modified Penman equation, and expressed as the ETc/ET0 ratio, or crop coefficient (Kc). In 1991, the average Kc was 0.75 to 0.79 for fields containing 2- to 4-year-old plantings. This ratio was 0.58 for a field containing a 1-year-old planting. Crop coefficient was 30% lower in 1992 due to higher ET0 values and lower stomatal conductance. Measurements made between July and August and again between September and November 1991 suggested that Kc may vary seasonally. Crop transpiration (T), determined with the stem heat balance technique, comprised from 40% to 95% of ETc as the leaf area index increased from 1.4 to 6.7. Behavior of Kc and T during a 25-day soil drying-reirrigation cycle indicated that the crop was able to maintain relatively high levels of gas-exchange activity during periods of severe water deficit.

Energy balance and latent heat flux partitioning in coffee hedgerows at different stages of canopy development

The energy balance of drip-irrigated coffee ( Coffea arabica L. cv. Yellow Catuai) hedgerows was evaluated at different stages of canopy development using the Bowen ratio-energy balance technique. Simultaneous measurements of mass flow of water through the coffee stems using the heat balance method allowed total latent heat flux (λ E) to be partitioned between crop canopy (λ Ec) and soil and interrow vegetation (λ Es) components. The average Bowen ratio decreased from 0.92 at leaf area index ( L) of 1.4 to 0.36 at L = 6.7. Differences in the Bowen ratio between 2 consecutive years appeared to be related to stomatal response to leaf-to-air vapor pressure difference ( V) and variations in net radiation ( R r). Latent heat loss was the most important component of the energy balance at all stages of canopy development, while sensible heat flux ( H) remained relatively constant. Soil heat flux ( G) invariably declined as L increased. λ Ec was the major form of latent heat loss at all stages of canopy development except at L = 1.4, where λ Es accounted for 60% of λ E. However, the magnitude of λ Es declined with canopy development, with λ Ec accounting for nearly 100% of total λ E at L = 6.7. Withholding irrigation dramatically influenced the partitioning of energy, strongly reducing λ Es and increasing H. After irrigation was discontinued λ Ec dropped rapidly, and the contribution of λ Es to λ E became more important. After irrigation was resumed, a rapid recovery of all energy balance components to their previous values was observed. At low L, energy balance and latent heat partitioning of coffee hedgerows grown under wide spacing resembled the behavior of sparse row crops, but the development of a tall, dense canopy caused available energy to be partitioned in a way typically found in closed canopies such as those of forests.

Energy budget measurements using eddy correlation and Bowen ratio techniques at the Kinosheo Lake tower site during the Northern Wetlands Study

Fluxes of heat and water vapor were measured on a 20‐m tower at Kinosheo Lake in the Hudson Bay lowlands using eddy correlation and Bowen ratio energy balance techniques. The study period was June 25 to July 28, 1990. Measurements were made over a peat bog consisting of a mixture of sphagnum moss and lichen hummocks and black pools. About 200 m west of the tower were several shallow ponds. The hummocks had a dry, insulating surface and were underlain by an ice layer near 50 cm depth until mid‐July. At the beginning of the period the black pools were covered with water, and although the free water gradually disappeared over the study period, they remained saturated to the end of July. The depth of peat near the tower was about 3 m. Despite the ice layer under the hummocks, their daytime surface temperatures were high, near 35°C, and after the middle of July, above 40°C. Inspection of temperature, precipitation, and radiation data showed that the midsummer period of 1990 was warmer, drier, and sunnier than usual at Moosonee and so by inference at Lake Kinosheo. When all the data were combined to yield average diurnal energy balance components, the eddy correlation fluxes accounted for 90% of the available energy. Latent heat flux averaged 46% of the total available energy and the sensible heat flux averaged 34%. Daytime Bowen ratios were near 1 for the experimental period, suggesting that the bog behaved more like a dryland than a wetland. Eddy correlation measurements of sensible heat and latent heat flux were less than those measured using the Bowen ratio energy balance technique, the average ratios being 0.81 and 0.86, respectively. These differences were possibly due to the difficulty in measuring energy balance components of net radiation and ground heat flux over the mosaic surface.

Performance of Bowen ratio systems on a slope

The Bowen ratio energy balance technique was used to assess the energy fluxes on inclined surfaces in several recent surface-atmosphere interaction studies. Since air flow over a sloping surface may differ from that over a flat terrain, it is important to examine whether Bowen ratio measurements taken on sloping surfaces are valid. In this study, the suitability of using the Bowen ratio technique on sloping surfaces was tested by examining the assumptions that the technique requires for valid measurements. This was accomplished by studying the variation of Bowen ratio measurements along a selected slope at a site of the First ISLSCP Field Experiment (FIFE). In September 1988, four Bowen ratio systems were set up in a line along the 22° north-facing slope with northerly air flow (air flowed up the slope). In July of 1989, six Bowen ratio systems were similarly installed with southerly air flow (air flowed down the slope). Results indicated that, at distances between 10 and 40 m from the top of the slope, no temperature or vapor pressure gradient parallel to the slope was detected. Uniform Bowen ratio values were obtained on the slope, and thus the sensible or latent heat flux should be similar along the slope. This indicates that the assumptions for valid flux measurements are reasonably met on the slope. The Bowen ratio technique should give good estimates of the energy fluxes on slopes similar to that in this study.

Regulation of transpiration in field-grown sugarcane: Evaluation of the stomatal response to humidity with the Bowen ratio technique

Parallel time courses of net radiation, canopy conductance and evapotranspiration in a sugarcane canopy demonstrate the interaction of environmental and physiological factors in regulating water loss. The Bowen ratio-energy balance technique was used to evaluate the stomatal response to humidity in the absence of potential artifacts associated with porometer and leaf chamber measurements. Canopy conductance ( g c) responded to leaf-air vapor pressure difference ( V), supporting the validity of available physiological data showing clear stomatal responses to V in porometers and leaf chambers. Several factors were found to obscure the stomatal response to V under field conditions. The stomatal closing stimulus, V, and the opening stimulus, photon flux density ( I), were positively correlated. This caused offsetting stomatal responses to V and I, which reduced the net stomatal response when V changed with radiation-driven changes in leaf temperature. Normalization of g c by I revealed the expected hyperbolic decline of g c with increasing V. Low boundary layer conductance attenuated V imposed at the leaf surface relative to V measured with reference to humidity in the bulk atmosphere. This reduced the stimulus for stomatal response to V, below that indicated by agrometeorological measurements. Use of air saturation deficit ( D) rather than V to express evaporative demand overestimates V, minimizing apparent stomatal sensitivity to evaporative demand, when leaves are cooler than the air. These factors may result in small stomatal responses to V that escape detection by indirect measures of stomatal movement, such as those based on the temperature of exposed leaves.

Similarity Moisture Dew Profiles within a Corn Canopy

Abstract The amount of dewfall and dewrise to a corn canopy has been estimated over 7 nights by using the Bowen ratio energy balance technique and the soil diffusivity technique, respectively. In addition, the distribution of free liquid water within the plant community has been measured by using Leick plates. By combining the Leick plate results with the foliage area distribution of the plant canopy, the total amount of dew deposition within the plant canopy can also be estimated. On average, both techniques did not show a systematic difference and agreed to within 10%. Moreover, it appeared that the nondimensionized dew profiles within the stand showed a more or less similar shape for all nights. The potential dew formation was calculated for all nights. On average, the potential dew was 30% higher than the actual dew. This offers the opportunity to make reliable total dew estimates, and also, due to similarity, dew profile estimates for a corn canopy from simple micrometeorological data. Model calculat...

Net radiation, sensible and latent heat flux densities on slopes computed by the energy balance method

Energy balance components obtained over five grass-covered sloping surfaces near Manhattan, KS, using the Bowen ratio energy balance technique with the instruments mounted horizontally were compared with calculated values when the instruments were mounted parallel to the surfaces. Hourly values of the components changed when the instruments were parallel to the surfaces. The changes were larger at low solar angles (spring and fall) and on steeper slopes. An area average of daylight totals, assuming that all aspects were equally represented, changed only 0.1% on June 6 and 2.3% on October 11. The calculations, extended to steeper slopes, indicated small changes in the daylight totals for slopes of less than 10 deg.

Moisture distribution within a maize crop due to dew.

During 16 nights in Sept. and Oct. 1986, the process of dew formation was studied within a crop of maize cv. Vivia. With the Bowen ratio energy balance technique the total dewfall from above the canopy was measured. By estimating the soil characteristics and measuring the soil moisture and temp. gradients of the topsoil, an assessment of the dewrise from the soil could be made. To estimate the distribution of free water within the crop, the liquid water profile was measured with Leick plates at 5 levels. Dewfall proved to contribute most to the total dew deposition. As an av. for the experimental period, nearly 5% could be attributed to dewrise and 25% could be attributed to dewfall. On the 10 occasions, the total amount of dew ranged between 0.01 and 0.41 mm per night. To obtain a simple but rough indication for the amount of dew, the difference between the min. and midday dew point temp. could be applied, or, if the soil was well-watered, the topsoil temp. gradient could be applied. (Abstract retrieved from CAB Abstracts by CABI’s permission)

Application of an energy combination model for evaporation from sparse canopies

This study tests the evaporation energy combination model developed by Shuttleworth and Wallace (1985) with data collected in a subarctic wetland. The modelled evaporation was compared with evaporation calculated from the Bowen ratio energy balance technique over a range of leaf area indices (LAI) from non-vegetated to fully vegetated conditions. The Shuttleworth-Wallace (SW) model was in excellent agreement with the measured evaporation for hourly and day-time totals for all values of LAI. This gives a particular advantage to the SW model compared to the simple Penman-Monteith combination equation. A comparison of measured and modelled total evaporation for all days yielded a root mean square error and mean bias error of 0.98 and −0.13 MJ m −2 day −1, respectively. The model also shows good agreement with the measured evaporation on an hourly basis. Although the results of this study are encouraging, we are cautious because this test is not truly independent. The need for additional investigation and testing of certain model parameters is recognized. In this study, we assume that eddy diffusivity within the canopy decreased exponentially and was controlled by a decay constant which varied with LAI. However, there is little information available to validate this treatment. Net radiation at the soil surface was computed from net radiation over the canopy and an exponential function of LAI, which was held constant over the course of the growing season.

Stomatal response to humidity in a sugarcane field: simultaneous porometric and micrometeorological measurements*

Abstract. Gas exchange data obtained with wellventilated leaf cuvettes provide clear evidence of a stomatal response to leaf‐air vapour pressure difference (V). In contrast, remotely sensed leaf temperatures with specific assumptions regarding canopy boundary layer characteristics, have been interpreted to mean that stomata do not respond to V. We address this apparent discrepancy in a sugarcane field by simultaneous application of a single‐leaf, porometric technique and a whole‐canopy, Bowen ratioenergy balance technique. These methods indicated significant stomatal response to V in well‐irrigated sugarcane. Stomatal responses to V in the field were obscured by strong covariance of major environmental parameters so that opening responses to light and closing responses to V tended to offset each other. Low boundary layer conductance significantly uncoupled V at the leaf surface (Vs) from V determined in the bulk atmosphere (Va). This reduced the range of the stimulus, Vs, thereby reducing the range of the stomatal response, without indicating low stomatal sensitivity to V. Stomatal responses to Va may be smaller than expected from V response curves in cuvettes, since Vs rather than the conventionally measured Va is analogous to V in a well‐stirred cuvette. Recently published conclusions that remotely sensed canopy temperatures are inconsistent with stomatal response to V may be based on erroneous estimates of canopy boundary layer conductance and thus of Vs, use of air saturation deficit rather than V to express evaporative demand, and investigation at higher levels of evaporative demand than those eliciting maximal stomatal response.

Surface Energy and Radiation Balance Systems: General Description and Improvements

Abstract Surface evaluation of sensible and latent heat flux densities and the components of the radiation balance were desired for various vegetative surfaces during the ASCOT84 experiment to compare with modeled results and to relate these values to drainage winds. Five battery operated data systems equipped with sensors to determine the above values were operated for 105 station days during the ASCOT84 experiment. The Bowen ratio energy balance technique was used to partition the available energy into the sensible and latent heat flux densities. A description of the sensors and battery operated equipment used to collect and process the data is presented. In addition, improvements and modifications made since the 1984 experiments are given. Details of calculations of soil heat flow at the surface and an alternate method to calculate sensible and latent heat flux densities are provided.

Evapotranspiration from pasture: a comparison of lysimeter and Bowen ratio measurements with Priestley-Taylor estimates

Abstract The objectives of this study were to compare evapotranspiration (ET) data obtained using 2 independent measurement techniques, and to compare measured ET with ET estimates obtained using a commonly-applied formula. The measurements were made in a mixed-pasture paddock near Palmerston North. The soil — Manawatu fine sandy loam — contained adequate plant-available water during the study. One set of ET measurements was made using a weighing lysimeter with surface area 2 m2 and a soil depth of 1 m. The lysimeter had a suction-operated drainage system at its base. The other set of ET data was obtained using the Bowen ratio-energy balance technique, with duplicate sets of Bowen ratio apparatus, net radiometers, and soil heat flux sensors. Daily ET estimates were made with the PriestleyTaylor formula, using measured net radiation and air temperature values. The daily ET for wellwatered full-cover pasture measured by the lysimeter on rain-free days was in good agreement with the ET measured using the Bow...