Bowen Ratio Energy Budget Research Articles

Application of Arduino and CMOS temperature-humidity sensor to evapotranspiration calculation by Bowen ratio energy budget method

Application of Arduino and CMOS temperature-humidity sensor to evapotranspiration calculation by Bowen ratio energy budget method

Evaluation of the Performance of Different Methods for Estimating Evaporation over a Highland Open Freshwater Lake in Mountainous Area

Lake evaporation is an important link connecting the water cycle and the surface energy cycle and remains one of the most uncertain terms in the local catchment’s water balance. Quantifying lake evaporation and its variability is crucial to improve water resource management and understand the response of the lake system towards climate change. In this study, we evaluated the performances of nine evaporation methods at different timescales and calibrated them by using the continuous eddy covariance (EC) observation data during 2015–2018 over Erhai Lake, a highland open freshwater lake situated in the Dali valley, China. The nine evaporation methods could be classified into combination methods (Bowen-ratio energy budget, Penman, Priestley–Taylor, DeBruin–Keijman and Brutsaert–Stricker), solar radiation-based methods (Jensen–Haise and Makkink) and Dalton-based method (mass transfer and Ryan–Harleman) based on their parameterization schemes. The Dalton-based Ryan–Harleman method is most suitable for estimating evaporation at daily to weekly scales, while the combination methods and solar radiation-based method had good estimates at monthly timescale. After calibration, the biases of the Jensen–Haise and Ryan–Harleman method were slightly reduced, while the biases of the Makkink and mass transfer methods were reduced substantially. The calibrated Jensen–Haise method with small annual bias (−2.2~2.8%) and simple input variables was applied to estimate the long-term trend of evaporation during 1981–2018. The annual total evaporation showed an insignificant increasing trend of 0.30 mm year−1, mainly caused by the significant rising air temperature. This study showed the performance of evaporation methods over water bodies had large discrepancies on different time scales, which indicated the importance of the choice of evaporation methods and provided instruction for water resource management of this region under climate change.

Pan coefficients for estimating open-water surface evaporation for a humid tropical monsoon climate region in India

Pan coefficients have been developed for the Sukhna lake region in Chandigarh, India that has a humid tropical monsoon climate, using the optimisation technique for annual, monthly and seasonal time scales. Evaporation estimated using the Bowen ratio energy budget method has been considered as actual evaporation. Results show that the pan coefficient for the study area varies significantly both by month and season. The month-wise variation is in the range of 0.72–1.40 and the seasonal variation is in the range of 0.81–1.16. Pan coefficients obtained using various existing models such as Cuenca, Snyder, Modified Snyder, Pereira, Orang, FAO-56 and Wahed–Snyder have also been compared with the developed coefficients. Comparative analysis indicates that the pan coefficients obtained using the Snyder model overestimate evaporation significantly, while the rest of the models significantly underestimate evaporation. The study concludes that the developed pan coefficients are observed to estimate the open-water surface evaporation with a fair degree of accuracy for the study area while the pan coefficient value of 0.7 being used by most field organisations in India give high errors. However, since pan coefficients vary spatially due to the variation in the relative significance of various meteorological parameters, the pan coefficients developed in the present study need to be further evaluated for their suitability to other similar climatic regions of India.

Comparison of methods to calculate evaporation from reservoirs

ABSTRACTLarge volumes of water evaporate from reservoirs in arid and semi-arid regions. Such water losses are substantial in the regional water balance, and must be considered in the management of water resources. This paper evaluates 12 reservoir evaporation methods that are applied to calculate daily evaporation from Karkheh reservoir (Iran) created by the world’s sixth largest dam. The Bowen ratio energy budget (BREB) formula is chosen as the reference method for the purpose of evaluating 12 alternative evaporation methods. The evaporation methods are ranked based on their accuracy, sensitivity to input variables, and simplicity of application by means of the technique for order of preference by similarity to ideal solution (TOPSIS) multi-criteria decision analysis method. Our results from a multi-year data set for Karkheh reservoir indicate that the best-ranked methods by TOPSIS considering the performance indices are the solar radiation-temperature based methods and the Blaney and Criddle method that is based on temperature and daylight duration variables.

Evaluation of ten methods for estimating evaporation in a small high-elevation lake on the Tibetan Plateau

To quantify lake evaporation and its variations in time, ten methods for estimating evaporation at a temporal resolution of 10 days over a small high-elevation lake in the Nam Co lake basin of the Tibetan Plateau (TP) were evaluated by using eddy covariance (EC) observation-based reference datasets. After examination of the consistency of the parameters used in the different methods, the ranking of the methods under different conditions are shown to be inconsistent. The Bowen ratios derived from meteorological data and EC observations are consistent, and it supports a ranking of energy-budget-based methods (including the Bowen ratio energy budget, Penman, Priestley-Taylor, Brutsaert-Stricker and DeBruin-Keijman methods) as the best when heat storage in the water can be estimated accurately. The elevation-dependent psychometric constant can explain the differences between the Priestley-Taylor and DeBruin-Keijman methods. The Dalton-type methods (Dalton and Ryan-Harleman methods) and radiation-based method (Jensen-Haise) all improve significantly after parameter optimization, with better performance by the former than the latter. The deBruin method yields the largest error due to the poor relationship between evaporation and the drying power of the air. The good performance of the Makkink method, with no significant differences before and after optimization, indicates the importance of solar radiation and air temperature in estimation of lake evaporation. The Makkink method was used for long-term evaporation estimation due to lack of water temperature observations in lakes on the TP. Lastly, long-term evaporation during the open-water period (April 6 to November 15 from 1979 to 2015) were obtained; the mean bias was only 6%. A decreasing-increasing trend in lake evaporation with a turning point in 2004 was noted, and this trend corresponds to the published decreasing-increasing trend in reference evapotranspiration on the Tibetan Plateau and can be explained by variations in related meteorological variables.

Evapotranspiration and water balance in a hot pepper (Capsicum frutescens L.) field during a dry season in the tropics

Evapotranspiration and water balance in a hot pepper (Capsicum frutescens L.) field during the 2nd dry season were analyzed in this study. Actual evapotranspiration (ET) was estimated by Bowen Ratio Energy Budget (BREB) method, potential evaporation (EP) was calculated by Penman method, and irrigation volume of water was measured manually. Meteorological instruments were installed in the experimental field during hot pepper cultivation. Leaf area index increased during the growing stages where the highest LAI of 1.65 in the generative stage. The daily average of ET was 1.94 and EP was 6.71 mm resulting in low Kc. The Kc values were significantly different between stage to stage under T-test analysis (α = 0.05). Moreover, Kc in every stage could be related to soil water content (SWC) in logarithmic function. Totally, ET during hot pepper cultivation was 179.19 mm, while rainfall was 180.0 mm and irrigation water was 27.42 mm. However, there was a water shortages during vegetative and generative stages. This study suggested that consumptive water of hot pepper was complimented by soil and groundwater under the condition of water shortages in the vegetative and generative stages during the 2nd dry season.

Evaluating Micrometeorological Estimates of Groundwater Discharge from Great Basin Desert Playas.

Groundwater availability studies in the arid southwestern United States traditionally have assumed that groundwater discharge by evapotranspiration (ETg ) from desert playas is a significant component of the groundwater budget. However, desert playa ETg rates are poorly constrained by Bowen ratio energy budget (BREB) and eddy-covariance (EC) micrometeorological measurement approaches. Best attempts by previous studies to constrain ETg from desert playas have resulted in ETg rates that are within the measurement error of micrometeorological approaches. This study uses numerical models to further constrain desert playa ETg rates that are within the measurement error of BREB and EC approaches, and to evaluate the effect of hydraulic properties and salinity-based groundwater density contrasts on desert playa ETg rates. Numerical models simulated ETg rates from desert playas in Death Valley, California and Dixie Valley, Nevada. Results indicate that actual ETg rates from desert playas are significantly below the uncertainty thresholds of BREB- and EC-based micrometeorological measurements. Discharge from desert playas likely contributes less than 2% of total groundwater discharge from Dixie and Death Valleys, which suggests discharge from desert playas also is negligible in other basins. Simulation results also show that ETg from desert playas primarily is limited by differences in hydraulic properties between alluvial fan and playa sediments and, to a lesser extent, by salinity-based groundwater density contrasts.

Dead Sea evaporation by eddy covariance measurements vs. aerodynamic, energy budget, Priestley–Taylor, and Penman estimates

Abstract. The Dead Sea is a terminal lake, located in an arid environment. Evaporation is the key component of the Dead Sea water budget and accounts for the main loss of water. So far, lake evaporation has been determined by indirect methods only and not measured directly. Consequently, the governing factors of evaporation are unknown. For the first time, long-term eddy covariance measurements were performed at the western Dead Sea shore for a period of 1 year by implementing a new concept for onshore lake evaporation measurements. To account for lake evaporation during offshore wind conditions, a robust and reliable multiple regression model was developed using the identified governing factors wind velocity and water vapour pressure deficit. An overall regression coefficient of 0.8 is achieved. The measurements show that the diurnal evaporation cycle is governed by three local wind systems: a lake breeze during daytime, strong downslope winds in the evening, and strong northerly along-valley flows during the night. After sunset, the strong winds cause half-hourly evaporation rates which are up to 100 % higher than during daytime. The median daily evaporation is 4.3 mm d−1 in July and 1.1 mm d−1 in December. The annual evaporation of the water surface at the measurement location was 994±88 mm a−1 from March 2014 until March 2015. Furthermore, the performance of indirect evaporation approaches was tested and compared to the measurements. The aerodynamic approach is applicable for sub-daily and multi-day calculations and attains correlation coefficients between 0.85 and 0.99. For the application of the Bowen ratio energy budget method and the Priestley–Taylor method, measurements of the heat storage term are inevitable on timescales up to 1 month. Otherwise strong seasonal biases occur. The Penman equation was adapted to calculate realistic evaporation, by using an empirically gained linear function for the heat storage term, achieving correlation coefficients between 0.92 and 0.97. In summary, this study introduces a new approach to measure lake evaporation with a station located at the shoreline, which is also transferable to other lakes. It provides the first directly measured Dead Sea evaporation rates as well as applicable methods for evaporation calculation. The first one enables us to further close the Dead Sea water budget, and the latter one enables us to facilitate water management in the region.

Dew formation and its variation in Haloxylon ammodendron plantations at the edge of a desert oasis, northwestern China

Dewfall may be a critical source of moisture in desert environments and may determine sustainability of sand-stabilizing planted vegetation. However, little is known about factors responsible for dew formation, the relative importance of dew as a source of water, and its variability in plantations. During June and October of 2013, the dew amounts and duration were estimated by using the Bowen ratio energy budget technique (BREB), and the dew variability on sand dunes planted with Haloxylon ammodendron 5, 20, and 40 years before were measured by microlysimeter. We quantified dew formation characteristics in a sand-stabilizing H. ammodendron plantation at the edge of a desert oasis, northwestern China. The results indicated that the average daily amount of dew in the H. ammodendron plantations during the observation period based on BREB was 0.13mm, and the dew duration lasted from 1 to 9.5h. Dew occurred on 77% of growing season days, the number of days with dew amounts of >0.03mm comprised 95% of the total dewfall days, and the cumulative amount of dew for those days was 16.1mm. Air temperature, relative humidity, the difference between air temperature and dew point, and wind speed had significant effects on dew formation. The thresholds of the dew formation were RH >50% and wind speed <4.27m/s. As a result of larger canopy area and lower Sky View Factors to 20- and 40-year-old H. ammodendron, the accumulated amount of dew was always significantly greater, and its night-time variability was almost 3 times greater for 5-year-old than for 20- and 40-year-old shrubs. In addition, near-ground dew amounts at the inter-space of three ages of H. ammodendron exhibited higher values than that under the canopy, while dew formation lagged and the maximum cumulative amount of dew was observed 2h later under the canopy of shrubs. The Bowen ratio method estimated actual dew reasonably well. It is concluded that dew may be a frequent and stable water resources in H. ammodendron plantations at the edge of a desert oasis, and there is a mutually reinforcing effect between dewfall and the sand-fixing vegetation system.

Evaporation from a shallow, saline lake in the Nebraska Sandhills: Energy balance drivers of seasonal and interannual variability

Despite potential evaporation rates in excess of the local precipitation, dry climates often support saline lakes through groundwater inputs of water and associated solutes. These groundwater-fed lakes are important indicators of environmental change, in part because their shallow water levels and salinity are very sensitive to weather and climatic variability. Some of this sensitivity arises from high rates of open-water evaporation, which is a dominant but poorly quantified process for saline lakes. This study used the Bowen ratio energy budget method to calculate open-water evaporation rates for Alkali Lake, a saline lake in the Nebraska Sandhills region (central United States), where numerous groundwater-fed lakes occupy the landscape. Evaporation rates were measured during the warm season (May – October) over three consecutive years (2007–2009) to gain insights into the climatic and limnological factors driving evaporation, as well as the partitioning of energy balance components at seasonal and interannual time scales. Results show a seasonal peak in evaporation rate in late June of 7.0mmday−1 (on average), with a maximum daily rate of 10.5mmday−1 and a 3-year mean July-September (JAS) rate of 5.1mmday−1, which greatly exceeds the long-term JAS precipitation rate of 1.3mmday−1. Seasonal variability in lake evaporation closely follows that of net radiation and lake surface temperature, with sensible heat flux and heat storage variations being relatively small, except in response to short-term, synoptic events. Interannual changes in the surface energy balance were weak, by comparison, although a 6-fold increase in mean lake level over the three years (0.05–0.30m) led to greater heat storage within the lake, an enhanced JAS lake-air temperature gradient, and greater sensible heat loss. These large variations in water level were also associated with large changes in absolute salinity (from 28 to 118gkg−1), with periods of high salinity characterized by reductions in mass transfer estimates of evaporation rate by up to 20%, depending on atmospheric conditions and absolute salinity. Energy balance estimates of evaporation, on the other hand, were found to be less sensitive to variations in salinity. These results provide regional insights for lakes in the Nebraska Sandhills region and implications for estimation of the energy and water balance of saline lakes in similar arid and semi-arid landscapes.

Estimation of evaporation for Lake Van

An evaporation study was conducted at Van Lake, one of the largest soda lakes on Earth, which is located on a high plateau in the Eastern Anatolia region of Turkey. Evaporation on a daily basis for the Gevas station located at the southeastern shore of the lake was calculated using the Bowen ratio energy budget (BREB), Priestley–Taylor, deBruin–Keijman, Penman, and deBruin methods. The calculated evaporation values were then compared and tested with previously calculated and measured evaporation rates. Compared with available measured evaporation data, the best evaporation estimations (within ±0.5 mm/day) were made by the BREB method. The BREB calculations made in this study were statistically significant at the confidence level of 99.9 % for estimation of evaporation at the Gevas station. Relationships between the calculated BREB evaporations and historical climatic records were also examined to propose a new empirical equation for evaporation estimation for the case of Lake Van. The best regression equation with a coefficient of determination (R 2) of 0.9399 was found between BREB evaporation values and incoming solar radiation (Q s) values. A power regression model proposed to estimate daily evaporation rates from Q s values was statistically significant at the confidence level of 99.9 %. Additionally, a few regression models were also examined to predict the surface water temperature of Lake Van from recorded air temperature. The highest coefficient of determination (0.974) and moderate standard error (3.12 °C) values were obtained from a polynomial air–water temperature model. Estimated lake water temperatures using the polynomial model were within ±10 % of observed lake water temperatures.

Exploring the long-term balance between net precipitation and net groundwater exchange in Florida seepage lakes

The long-term balance between net precipitation and net groundwater exchange that maintains thousands of seepage lakes in Florida’s karst terrain is explored at a representative lake basin and then regionally for the State’s peninsular lake district. The 15-year water budget of Lake Starr includes El Niño Southern Oscillation (ENSO)-related extremes in rainfall, and provides the longest record of Bowen ratio energy-budget (BREB) lake evaporation and lake-groundwater exchanges in the southeastern United States. Negative net precipitation averaging −25 cm/yr at Lake Starr overturns the previously-held conclusion that lakes in this region receive surplus net precipitation. Net groundwater exchange with the lake was positive on average but too small to balance the net precipitation deficit. Groundwater pumping effects and surface-water withdrawals from the lake widened the imbalance. Satellite-based regional estimates of potential evapotranspiration at five large lakes in peninsular Florida compared well with basin-scale evaporation measurements from seven open-water sites that used BREB methods. The regional average lake evaporation estimated for Lake Starr during 1996–2011 was within 5% of its measured average, and regional net precipitation agreed within 10%. Regional net precipitation to lakes was negative throughout central peninsular Florida and the net precipitation deficit increased by about 20 cm from north to south. Results indicate that seepage lakes farther south on the peninsula receive greater net groundwater inflow than northern lakes and imply that northern lakes are in comparatively leakier hydrogeologic settings. Findings reveal the peninsular lake district to be more vulnerable than was previously realized to drier climate, surface-water withdrawals from lakes, and groundwater pumping effects.

Thermal structure and Energy Budget for Alghadir reservoir, Iran

Knowing the rate of evaporation from surface water resources such as channels and reservoirs is essential for precise management of the water balance. Few detailed evaporation studies exist for small lakes or reservoirs in arid regions of the world. In this study, monthly evaporation was determined by Bowen Ratio Energy Budget technique (BREB) for Alghadir reservoir (Saveh, Iran) from 1995–2007 and compared with class-A pan. Daily data were obtained from IMO (Iran Meteorological Organization) weather station, located near the reservoir, for all of these years. Some variations have been observed on the estimated evaporation of the energy budget at different years, ranging from 151 cm for 1997 to 174 cm for 2006. There is no specific evaporation pattern for all the years, but evaporation rates were low in winter (January and February) and highest during the summer (July and August). However, the times and month of highest evaporation rates varied during the study period. Surveying of reservoir thermal structure showed that, inflow and outflow energy fluxes to this water volume have large variations. In addition, incoming solar shortwave radiation, incoming atmospheric longwave radiation, and longwave atmospheric radiation emitted from the water surface are largest energy fluxes for this small and deep reservoir. Results of this study could explain the importance and sensible contribution of the evaporation rates as a fundamental parameter in the hydrological cycle of reservoirs ecosystem in the arid regions of Iran.

Evaporation Studies of Canopy-Covered On-Farm Reservoir

The present study used fifteen existing evaporation models that use flux gradient, energy balance, and mass transfer approaches to estimate the rate of evaporation from a regular-sized open on-farm reservoir (OFR). Statistical analysis was performed to screen the models. The root mean square error (RMSE), percent deviation, coefficient of determination (R2), and scatter plot analysis reveal that three of fifteen models—namely, the Bowen ratio energy budget (BREB), Priestley-Taylor, and de Bruin–Keijman models—predict the evaporation rate from an open on-farm reservoir that is close to the actual evaporation recorded by a class A pan evaporimeter. These selected models were further used to predict the evaporation rate from a canopy-covered on-farm reservoir, taking into consideration the weather parameters in their microenvironment. The depletion of the water level in the OFR, which was recorded daily by a graduated staff gauge, and the change in the water spread area were measured together to compute the actual evaporation rate from the water bodies. The daily average evaporation rate from the canopy-covered OFR was approximately 1.18 mm less than the evaporation rate of a similar open OFR. During winter season (October to February), the 35% reduction in evaporation loss owing to the canopy cover over the OFR in comparison with the evaporation loss in an open OFR indicates the efficacy of the biological measure in mitigating the loss of scarce natural resources (e.g., rainwater) in rainfed areas.

Assessment of the sensitivity and prediction uncertainty of evaporation models applied to Nasser Lake, Egypt

Summary This paper examines the sensitivity and uncertainty of evaporation estimates from Nasser Lake, south of Egypt using data collected at the floating station Raft, 2 km upstream of the Aswan High Dam, and 12 evaporation models. Based on their approach the evaporation models were ranked in five classes, respectively the Bowen ratio energy budget (BREB) approach, three energy–aerodynamic based methods, a mass transfer method, two temperature and five solar radiation–temperature approaches. State dependent parameter models were used to estimate the sensitivity indices of the different evaporation estimation methods at daily and monthly frequencies. A drastic reduction in the cost of the sensitivity analysis was obtained when coupled with efficient sampling methods, producing quasi-Monte Carlo sequences (Latin Hypercube samples). Analysis of the sensitivity indices ( S i ) revealed that the energy and advective parameters ( Q x and Q v ) in the Bowen ratio energy budget method most strongly affect evaporation estimates. The most sensitive parameters for the energy–aerodynamic methods (Penman, Priestley–Taylor and the deBruin–Keijman methods) are the change in heat storage ( Q x ) and solar radiation. For the other methods the most sensitive parameters were solar radiation, air temperature and vapour pressure, but with lower sensitivities. After investigation of the first and second-order sensitivity indices, it was found that the first-order sensitivity indices capture more than 90% of the total variance of the evaporation estimates. Interactions between input parameters were found limited with a maximum contribution to variance of less than 3.3% for daily frequency and less than 8.5% for the monthly frequency. Results indicated that many parameters in the 12 evaporation models could be fixed without appreciably affecting the output of these models. This helped to reduce the computational load of these models. The uncertainty results show that the Dalton group is subject to the highest error. The overall uncertainty ranged from 9% (BREB method) to 15% (mass transfer method) of the mean evaporation rate.

Evaporation estimates from Nasser Lake, Egypt, based on three floating station data and Bowen ratio energy budget

Nasser Lake is located in a hyper-arid region in the south of Egypt. Evaporation is by far the most important factor in explaining the water losses from the lake. To obtain better management scenarios for Nasser Lake, an accurate estimation of the lake evaporation losses thus is essential. This paper presents an update of previous evaporation estimates, making use of local meteorological and hydrological data collected from instrumented platforms (floating weather stations) at three locations on the lake: at Raft, Allaqi, and Abusembel (respectively at 2, 75, and 280 km upstream of the Aswan High Dam). Results from six conventional evaporation quantification methods were compared with the values obtained by the Bowen ratio energy budget method (BREB). The results of the BREB method showed that there is no significant difference between the evaporation rates at Allaqi and Abusembel. At Raft, higher evaporation rates were obtained, which were assumed to be overestimated due to the high uncertainty of the Bowen ratio (BR) parameter. The average BR value at Allaqi and Abusembel was used to eliminate this overestimates evaporation. Variance-based sensitivity and uncertainty analyses on the BREB results were conducted based on quasi-Monte Carlo sequences (Latin Hypercube sampling). The standard deviation of the total uncertainty on the BREB evaporation rate was found to be 0.62 mm day−1. The parameter controlling the change in stored energy, followed by the BR parameter, was found to be the most sensitive parameters. Several of the six conventional methods showed substantial bias when compared with the BREB method. These were modified to eliminate the bias. When compared to the BREB-based values, the Penman method showed most favorably for the daily time scale, while for the monthly scale, the Priestley–Taylor and the deBruin–Keijman methods showed best agreement. Differences in mean evaporation estimates of these methods (against the BREB method) were found to be in the range 0.14 and 0.36 mm day−1. All estimates were based calculations at the daily time scale covering a 10-year period (1995–2004).

Modeling the water budget in a deep caldera lake and its hydrologic assessment: Lake Ikeda, Japan

The hydrologic assessment of a lake water budget can be helpful in achieving proper water management and sustainable water use. A model to analyze a lake water budget was developed and verified for Lake Ikeda, Japan. Lake evaporation was estimated by numerical analyses of lake water temperature and the lake energy budget. Inflow from the lake catchment area and leakage from the lake bottom were estimated based on the tank model and Darcy's law, and the model parameters were optimized by the shuffled complex evolution method. The estimated monthly lake evaporation rate is consistent with the evaporation rate estimated by the energy budget Bowen ratio method based on in situ data from 2004 to 2005. Moreover, the calculated time series of daily lake levels agrees well with those of measured lake levels during 1983 to 1999. Thus, the model is useful for evaluating the lake water budget. Numerical analysis reveals seasonal and annual variation characteristics in the water budget components. Precipitation, inflow from the catchment area, and river water supply are generally high during the rainy season from June to July with substantial annual variation. Lake evaporation is greatest in October and least in April, but the annual variation is relatively small. Agricultural water use is relatively high from April to September. There are no marked seasonal changes in leakage and drinking water use. The lake level is generally highest in September and lowest in March, which is characterized by seasonal changes in water budget components. The model was also applied to 17-year simulations under hypothetical hydrologic conditions to examine the effect of water use and agricultural water management on the lake level. Results indicate that river water supply, provided under the agricultural water management system, effectively compensates for the decrease in lake water resulting from agricultural water use.

Energy balance and its closure over a young larch forest in eastern Siberia

To elucidate the energy balance over a forest, seasonal changes in the water and energy exchanges over a young larch forest (E-YL) in eastern Siberia were investigated and compared with the reported characteristics of a mature larch forest (W-ML) in the same region. Observations of the energy fluxes were conducted during the growing season from 15 April through 14 September 2000. The sum of the turbulent heat fluxes H and λE, ground heat flux G and heat storage change rate Δ S was generally in good agreement with the net radiation flux R NET, except when the friction velocity u * was <0.2 m s −1. Atmospheric instability also reduced the energy balance ratio EBR in the daytime. The EBR was defined as the ratio of the turbulent heat flux H + λE toward the available energy input R NET − G − Δ S for the reference horizontal plane, whereas the energy closure fraction CF was defined as the ratio of the net ecosystem heat exchange H + λE + Δ S atmosphere to the available energy input R NET − G − Δ S biomass for the biomass surface. EBR and CF had different features in their relationship to u * at night. The threshold to reject nighttime data at low u * values was estimated to be 0.2 m s −1 by the EBR, but >0.3 m s −1 by the CF. The averaged features of the CF relationships with u * and atmospheric stability ζ were similar to those reported for Canadian boreal forests. In contrast, hourly CF and hourly EBR were widely scattered. The probability functions of EBR for different wind velocities were consistent with previous results from large eddy simulation. To determine the seasonal course of daily flux values, the Bowen ratio energy budget method was used to estimate the turbulent heat fluxes. The daily latent heat flux maintained a value of approximately 50 W m −2 during the summer, whereas the daily sensible heat flux varied with changes in the available energy. The Bowen ratio had a clear “U” shape over the course of the growing season. The minimum values of approximately 1.0 occurred in the summer from June to August. The general features of seasonal changes at E-YL in 2000 were similar to those at W-ML in 1998, except for the lower Bowen ratio in August at E-YL.

Comparison of 15 evaporation methods applied to a small mountain lake in the northeastern USA

Summary Few detailed evaporation studies exist for small lakes or reservoirs in mountainous settings. A detailed evaporation study was conducted at Mirror Lake, a 0.15 km2 lake in New Hampshire, northeastern USA, as part of a long-term investigation of lake hydrology. Evaporation was determined using 14 alternate evaporation methods during six open-water seasons and compared with values from the Bowen-ratio energy-budget (BREB) method, considered the standard. Values from the Priestley–Taylor, deBruin–Keijman, and Penman methods compared most favorably with BREB-determined values. Differences from BREB values averaged 0.19, 0.27, and 0.20 mm d−1, respectively, and results were within 20% of BREB values during more than 90% of the 37 monthly comparison periods. All three methods require measurement of net radiation, air temperature, change in heat stored in the lake, and vapor pressure, making them relatively data intensive. Several of the methods had substantial bias when compared with BREB values and were subsequently modified to eliminate bias. Methods that rely only on measurement of air temperature, or air temperature and solar radiation, were relatively cost-effective options for measuring evaporation at this small New England lake, outperforming some methods that require measurement of a greater number of variables. It is likely that the atmosphere above Mirror Lake was affected by occasional formation of separation eddies on the lee side of nearby high terrain, although those influences do not appear to be significant to measured evaporation from the lake when averaged over monthly periods.

Determination of a crop coefficient for evapotranspiration in a sparse sorghum field

AbstractThe FAO basal crop coefficient Kcb and the soil evaporation coefficient Ke for a sparse sorghum field in one growing season were estimated using the compartment model (Shuttleworth–Wallace model; S–W model), which separately estimates transpiration from crops and evaporation from the soil. The Kcb had a low value of 0.10 (Kcb, ini) during the initial period, increased rapidly to a maximum of 0.98 (Kcb, mid) at midseason, and decreased rapidly to 0.53 (Kcb, end) by the end of the growing season. The Kcb increased with increasing leaf area index (LAI) rapidly and slowly, respectively, below and above an LAI of 1.0. The Ke showed a positive relationship to increasing soil water content (SWC) at a depth of −15.0 cm, but the soil water content indicated a negative relationship to increasing LAI. Using the dual crop coefficients derived by their relationships to the LAI and soil water content, the actual evapotranspiration (ETa) was estimated and compared to the ETa derived by the Bowen ratio energy budget (BREB) method. The root mean square error (RMSE) of the ETa obtained from the simulated crop coefficient to the ETa obtained by the BREB method was 0.84 mm day−1. Thus, the adjusted relationship of Kcb and Ke allowed a good estimation of the evapotranspiration in an irrigated field with sparse vegetation cover. Copyright © 2006 John Wiley &amp; Sons, Ltd.