The role of ground heat flux in estimating evapotranspiration by the Penman-Monteith method on mountain meadow

In part I of this research, it was shown that the simplified bucket method in the PSU/NCAR MM4 system had an apparent tendency to overestimate surface evapotranspiration (ET) when the long-term observational data from the Atmospheric Radiation Measurement program are used for veri fication.It was demonstrated that a Penman-Monteith (PM) method could effectively reduce the degree of overestimating surface ET.An examina tion of the impact of satellite data insertion, using a variational Four-Di mensional Data Assimilation (FDDA) technique proposed by Gal-Chen (1983, 1986), on the model's estimation of surface ET is performed in the second part of this research.It shows that when the bucket method is in use the assimilation of the Geostationary Operational Environmental Satellite (GOES) temperature measurements helps the model make better estima tion of surface ET owing to a significant decrease of potential ET resulting fr om a pronounced decrease of skin temperature and the associated mois ture gradient at the ground surface.When the PM method is in use, the assimilation of GOES data tends to decrease the temperature and the asso ciated mixing ratio depression at the lowest model level during the data assimilation period, and thus, the potential ET is decreased during the suc ceeding simulation period.Therefore, the model using the PM method is able to more correctly estimate latent heat flux afte r the data assimilation period.It reveals that Gal-Chen's FDDA algorithm of assimilating GOES data provides the model with the PM method a greater possibility of yield ing the most accurate estimation of surface ET.The GOES data insertion would allow the model using the bucket method to gain a higher probabil ity of making a more accurate estimation of latent heat flux than the model using the PM method without GOES data insertion.Even only satellite data

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

A surface energy balance was conducted to calculate the latent heat flux (λE) using aerodynamic methods and the Penman–Monteith (PM) method. Computations were based on gridded weather and Landsat satellite reflected and thermal data. The surface energy balance facilitated a comparison of impacts of different parameterizations and assumptions, while calculating λE over large areas through the use of remote sensing. The first part of the study compares the full aerodynamic method for estimating latent heat flux against the appropriately parameterized PM method with calculation of bulk surface resistance (rs). The second part of the study compares the appropriately parameterized PM method against the PM method, with various relaxations on parameters. This study emphasizes the use of separate aerodynamic equations (latent heat flux and sensible heat flux) against the combined Penman–Monteith equation to calculate λE when surface temperature (Ts) is much warmer than air temperature (Ta), as will occur under water stressed conditions. The study was conducted in southern Idaho for a 1000-km2 area over a range of land use classes and for two Landsat satellite overpass dates. The results show discrepancies in latent heat flux (λE) values when the PM method is used with simplifications and relaxations, compared to the appropriately parameterized PM method and full aerodynamic method. Errors were particularly significant in areas of sparse vegetation where differences between Ts and Ta were high. The maximum RMSD between the correct PM method and simplified PM methods was about 56 W/m2 in sparsely vegetated sagebrush desert where the same surface resistance was applied.

Models for predicting hourly canopy resistance (r c) and latent heat flux (LET) based on the Penman–Monteith (PM) and bulk transfer methods are presented. The micrometeorological data and LET were observed during paddy rice-growing seasons in 2010 in Japan. One approach to model r c was using an aerodynamic resistance (r a) and climatic resistance (r *), while another one was based on a relationship with solar radiation (SR). Nonlinear relationships between r c and r *, and between r c and SR were found for different growing stages of the rice crop. The constructed r c models were integrated to the PM and bulk transfer methods and compared with measured LET using a Bowen ratio–energy balance method. The root mean square errors (RMSEs) were 155.2 and 170.5 W m−2 for the bulk transfer method with r c estimated using r * and with a function of SR, respectively, while the RMSEs were 87.4 and 85.7 W m−2 for the PM method with r c estimated using r * and SR, respectively. The r c integrated PM equation provided better performance than the bulk transfer equation. The results also revealed that neglecting the effect of r a on r c did not yield a significant difference in predicting LET.

Abstract. Accurate generation of spatial soil water maps is useful for many types of irrigation management. A hybrid remote sensing evapotranspiration (ET) model combining reflectance-based basal crop coefficients (Kcbrf) and a two-source energy balance (TSEB) model was modified and validated for use in real-time irrigation management. We modeled spatial ET for maize and soybean fields in eastern Nebraska for the 2011-2013 growing seasons. We used Landsat 5, 7, and 8 imagery as remote sensing inputs. In the TSEB, we used the Priestly-Taylor (PT) approximation for canopy latent heat flux, as in the original model formulations. We also used the Penman-Monteith (PM) approximation for comparison. We compared energy balance fluxes and computed ET with measurements from three eddy covariance systems within the study area. Net radiation was underestimated by the model when data from a local weather station were used as input, with mean bias error (MBE) of -33.8 to -40.9 W m-2. The measured incident solar radiation appeared to be biased low. The net radiation model performed more satisfactorily when data from the eddy covariance flux towers were input into the model, with MBE of 5.3 to 11.2 W m-2. We removed bias in the daily energy balance ET using a dimensionless multiplier that ranged from 0.89 to 0.99. The bias-corrected TSEB ET, using weather data from a local weather station and with local ground data in thermal infrared imagery corrections, had MBE = 0.09 mm d-1 (RMSE = 1.49 mm d-1) for PM and MBE = 0.04 mm d-1 (RMSE = 1.18 mm d-1) for PT. The hybrid model used statistical interpolation to combine the two ET estimates. We computed weighting factors for statistical interpolation to be 0.37 to 0.50 for the PM method and 0.56 to 0.64 for the PT method. Provisions were added to the model, including a real-time crop coefficient methodology, which allowed seasonal crop coefficients to be computed with relatively few remote sensing images. This methodology performed well when compared to basal crop coefficients computed using a full season of input imagery. Water balance ET compared favorably with the eddy covariance data after incorporating the TSEB ET. For a validation dataset, the magnitude of MBE decreased from -0.86 mm d-1 (RMSE = 1.37 mm d-1) for the Kcbrf alone to -0.45 mm d-1 (RMSE = 0.98 mm d-1) and -0.39 mm d-1 (RMSE = 0.95 mm d-1) with incorporation of the TSEB ET using the PM and PT methods, respectively. However, the magnitudes of MBE and RMSE were increased for a running average of daily computations in the full May-October periods. The hybrid model did not necessarily result in improved model performance. However, the water balance model is adaptable for real-time irrigation scheduling and may be combined with forecasted reference ET, although the low temporal frequency of satellite imagery is expected to be a challenge in real-time irrigation management. Keywords: Center-pivot irrigation, ET estimation methods, Evapotranspiration, Irrigation scheduling, Irrigation water balance, Model validation, Variable-rate irrigation.

In air quality models, daytime sensible and latent heat fluxes are important factors that influence atmospheric stability. These heat fluxes originate from heat that is generated from solar radiation and is then released from the earth’s surface. Different climates and surface conditions may lead to varying heat flux distributions. Because latent heat flux is influenced by both solar radiation and plant evapotranspiration, it is often difficult to estimate. The objective of this study was to apply thermodynamic concepts to determine an equation that could be used to estimate the Bowen ratio in the absence of latent and sensible heat fluxes. This study showed that, using two meteorological parameters (i.e., absolute temperature and relative humidity), the Bowen ratio for the climate in Taiwan could be obtained and then used to estimate sensible and latent heat fluxes in a series of equations. Furthermore, the approach’s applicability was determined by testing the sensitivities of parameters used in the Bowen ratio equation. A comparison of results determined through the Priestly–Taylor and Penman–Monteith methods with meteorological data for Yilan and Chiayi counties, Taiwan, for the 2006 summer and winter is performed. The results of this study showed that, among the simulated latent heat fluxes in the two study areas, the values estimated using the Penman–Monteith method were the largest, followed by those estimated using the Priestly–Taylor method. Values estimated using the Bowen ratio method were the smallest. Predictions generated by the proposed Bowen ratio equation correlated with those generated by the other models; however, the values estimated with the Priestly–Taylor method were closest to the simulated values.

Oil Palm (Elaeis guinensis Jacq) has a unique morphological characteristics, in particular it has a uniform canopy. As the plant become older, its canopy coverage will completely cover the surface and influence characteristics of its microclimate. Sensible heat flux estimation of oil palm plantation could be used to identify the contribution of oil palm in reducing or increasing heat to its surrounding environment. Determination of heat flux from oil palm plantation was conducted using two methods, Aerodynamic and Penman-Monteith. The result shows that the two methods have similar diurnal pattern. The sensible heat flux peaks in the afternoon, both for two and twelve years oil palm plantations. Sensible heat flux of young plantation is affected by atmospheric stability (stable, unstable and neutral), and is higher than that of older plantation, with mean values of 0.52 W/m2 (stable), 43.53 W/m2 (unstable), 0.63 W/m2 (neutral), with standard deviation of 0.50, 28.75 and 0.46 respectively. Sensible heat flux estimated by Penman-Monteith method in both young and older plantation was higher than the value determined by Aerodynamic method with respective value of 0.77 W/m2 (stable), 45.13 W/m2 (unstable) and 0.63 W/m2 (neutral) and 0.34 W/m2 (stable), 35.82 W/m2 (unstable) and 0.71 W/m2 (neutral).

A multiyear, multitechnique study was conducted to estimate latent heat flux within a temperate mixed forest of broad‐leaved and coniferous trees of Changbai Mountains in northeastern China. Three different methods were used, including eddy covariance (EC), Bowen ratio energy balance (BREB), and Penman‐Monteith (PM), during the growing seasons (May to September) of 2003–2005. BREB‐ and PM‐based latent heat fluxes calculated with micrometeorological variables of different reference levels were analyzed and those of the topmost reference level were the most similar to the EC measurements. The latent heat fluxes (LE) estimated with the three methods showed similar diurnal and seasonal courses. The maximum of monthly averaged daily LE appeared in July and August. Although the three methods gave the roughly consistent result, dispersion among them still existed in the experiment. PM method usually gave the highest latent heat flux among the three methods. Sum of the half‐hourly values from BREB and PM methods of the three growing seasons took 81.2% and 131% of that from EC measurement. The discrepancy between BREB estimates, as well as PM estimates, and EC measurements was analyzed with regard to vapor pressure deficit. Several reasons leading to the uncertainty of BREB and PM methods were discussed, including the assumptions in the two methods, source area and the influence of environmental factors.

Green roof is a technology that consists of the use of soil and vegetation installed in the roof of buildings, being a great solution to combat heat islands. Thus, this study aimed to compare micrometeorological changes and their effect on the energy balance of non-vegetated (slab) and vegetated building roofs by means of a simulation model calculated as a function of the reference evapotranspiration (ETo), determined by the Penman-Monteith method. This research was developed between February 1 and September 30, 2016, in the Charles Darwin Building's Parking Garage, Rio Ave Empreendimentos, Recife, PE, Brazil. For this, a weather station was installed on the external building slab. On the slab, sensible, latent, and soil heat fluxes corresponded to 75, 22, and 3%, respectively, of the energy balance. In the simulated green roof, these fluxes reached values of 6, 87, and 7%, respectively. The simulation model allowed determining the energy balance for the green roof, indicating a lower sensible heat flux (69%) and a higher latent heat flux (55%) when compared to those found in the slab.

The numerous methods for calculating the potential or reference evapotranspiration (ETo or ETP) almost always do for a 24-hour period, including values of climatic parameters throughout the nocturnal period (daily averages). These results have a nil effect on transpiration, constituting the main evaporative demand process in cases of localized irrigation. The aim of the current manuscript was to come up with a model rather simplified for the calculation of diurnal daily ETo. It deals with an alternative approach based on the theoretical background of the Penman method without having to consider values of aerodynamic conductance of latent and sensible heat fluxes, as well as data of wind speed and relative humidity of the air. The comparison between the diurnal values of ETo measured in weighing lysimeters with elevated precision and estimated by either the Penman-Monteith method or the Simplified-Penman approach in study also points out a fairly consistent agreement among the potential demand calculation criteria. The Simplified-Penman approach was a feasible alternative to estimate ETo under the local meteorological conditions of two field trials. With the availability of the input data required, such a method could be employed in other climatic regions for scheduling irrigation.

A study was performed at the Panguilemo Evapotranspiration Investigation Plot (ETIP) located 11 km north of Talca, Chile (35°23' South and 71° West; 110 m above the sea level) to evaluate latent heat flux by using the Penman-Monteith method. Data collected on a 20 minute time intervals included latent heat flux (LE), net radiation (Rn), soil heat flux (G), air temperature (Ta), relative humidity (RH), and wind speed (V). The performance of the Penman-Monteith equation (LEPM) was tested with latent heat flux measurements from the eddy correlation system (LEED) over a grass canopy under different soil moisture conditions. Results indicate that there was a good agreement between LEPM and LEED with an overall standard error of estimate (SEE) of 45 W m · -2 and an absolute relative error of 6.2 %.

An evapotranspiration method comparison was carried out by the International Water Management Institute (IWMI, Sri Lanka), at two locations in the Gediz Basin, Turkey, in the period from May to September 1998. In the IWMI study a number of ground-based techniques were compared with results obtained by remote sensing methods. Recently, a search of the satellite active archive yielded over 70 high quality level 1b images from NOAA/AVHRR over the same time period. The processing of these images with the SEBAL algorithm enabled us to build up a detailed time series of sensible and latent heat fluxes for a period of 120 days. In this paper a comparison is made between the sensible and latent heat fluxes determined from the present series of NOAA-14/AVHRR images and the results obtained earlier from various other prediction methods applied during the 1998 IWMI project. Specifically, the NOAA/SEBAL results are assessed against the scintillometer and temperature fluctuation methods. The results show that the NOAA derived evapotranspiration values follow the seasonal irrigation cycle quite well and correspond closely to the Landsat derived values, although they are lower than the results obtained with the traditional crop factor and Penman–Monteith methods.

산림에서의 차단강수증발(EWC)은 증발산과 강수에 중요한 기여를 한다. 따라서, 산림에서의 수문순환을 이해하기 위해서는 정확한 <TEX>$E_{WC}$</TEX>를 산정하는 것이 중요하다. 본 고찰에서는 <TEX>$E_{WC}$</TEX>의 측정방법을 소개하고, 선행 연구에서 보고된 산림형태(예를 들면, 활엽수림, 침엽수림, 혼효림, 열대림)에 따른 <TEX>$E_{WC}$</TEX> 값과 측정시 고려해야 할 사항에 대하여 논의하였다. 전형적인 <TEX>$E_{WC}$</TEX> 측정에는 물 수지, 에너지 수지 및 Penman-Monteith 방법이 있다. 전반적으로, <TEX>$E_{WC}$</TEX>는 강수량의 5~54%를 차지하였으며, 같은 산림형태내에서도 <TEX>$E_{WC}$</TEX>의 강수량에 대한 기여도는 큰 변동을 보였다. 이러한 변동에는 강수강도, 기상조건, 군락 구조 특성이 영향을 미치는 것으로 나타났다. 따라서 특정 산림형태에서의 <TEX>$E_{WC}$</TEX>의 강수량에 대한 기여도를 정량화하는 것은 어려울 것으로 판단된다. 관측시 발생하는 오차는 <TEX>$E_{WC}$</TEX> 정량화의 불확실성을 증대 시킨다. 물수지 방법의 경우, 풍속의 영향을 받는 강수 관측과 군락 구조의 공간적 비균질성의 영향을 받는 수관통과우 등의 관측 오차를 들 수 있다. 에너지 수지 방법의 경우에는 현열 플럭스와 열저류항의 관측이 주요 오차의 원인이 되며, Penman-Monteith 방법은 공기전도도와 현열의 이류 추정에서 발생하는 오차에 주의를 기울여야 한다. 각 측정방법의 오차를 최소화하고 신뢰할 수 있는 <TEX>$E_{WC}$</TEX>를 얻기위해서는 수문학적 방법과 미기상학적 방법, 즉 물 수지와 에너지 수지 방법을 함께 사용하는 것이 바람직하다. Wet canopy evaporation (<TEX>$E_{WC}$</TEX>) has been recognized as a significant component of total evapotranspiration, especially in forests and therefore it is critical to accurately assess <TEX>$E_{WC}$</TEX> to understand forest hydrological cycle. In this review, I focused on the measurement methods and evaluating the magnitudes of <TEX>$E_{WC}$</TEX> at diverse forest types (e.g., deciduous, coniferous, mixed, and rain forests). I also present the general issues to be considered for <TEX>$E_{WC}$</TEX> measurements. The commonly used measurement methods for <TEX>$E_{WC}$</TEX> include the water balance, energy balance, and the Penman-Monteith (PM) methods. The magnitudes of <TEX>$E_{WC}$</TEX> ranged from 5 to 54% of precipitation based on the literature review, showing a large variation even for a similar forest type possibly related to canopy structure, rainfall intensity, and other meteorological conditions. Therefore, it is difficult to draw a general conclusion on the contribution of <TEX>$E_{WC}$</TEX> to evapotranspiration from a particular forest type. Errors can arise from the measurements of precipitation (due to varying wind effect) and throughfall (due to spatial variability caused by canopy structure) for water balance method, the measurements of sensible heat flux and heat storage for energy balance method, and the estimation of aerodynamic conductance and unaccounted sensible heat advection for the PM method. For a reliable estimation of <TEX>$E_{WC}$</TEX>, the combination of ecohydrological and micrometeorological methods is recommended.

Soil water and heat transport plays an important role in various hydrologic, agricultural, and industrial applications. Accordingly, an increasing attention is paid to relevant simulation models. In the present study, soil thermal conditions at a mountain meadow during the vegetation season were simulated. A dual-continuum model of coupled water and heat transport was employed to account for preferential flow effects. Data collected at an experimental site in the &amp;Scaron;umava Mountains, southern Bohemia, during the vegetation season 2009 were employed. Soil hydraulic properties (retention curve and hydraulic conductivity) determined by independent soil tests were used. Unavailable hydraulic parameters were adjusted to obtain satisfactory hydraulic model performance. Soil thermal properties were estimated based on values found in literature without further optimization. Three different approaches were used to approximate the soil thermal conductivity function, &amp;lambda;(&amp;theta;): (i) relationships provided by Chung and Horton (ii) linear estimates as described by Loukili, Woodbury and Snelgrove, (iii) methodology proposed by C&amp;ocirc;t&amp;eacute; and Konrad. The simulated thermal conditions were compared to those observed. The impact of different soil thermal conductivity approximations on the heat transport simulation results was analysed. The differences between the simulation results in terms of the soil temperature were small. Regarding the surface soil heat flux, these differences became substantial. More realistic simulations were obtained using &amp;lambda;(&amp;theta;) estimates based on the soil texture and composition. The differences between these two, related to neglecting vs. considering &amp;lambda;(&amp;theta;) non-linearity, were found negligible.

Experimento conduzido na Embrapa, em Barbalha, CE, nos anos de 2003 e 2005, objetivou a estimativa da evapotranspiração da cultura e do coeficiente de cultivo do algodoeiro BRS-200 Marrom. Sensores de radiação solar global; saldo de radiação; temperatura do ar (bulbos seco e úmido) e velocidade do vento nos níveis de 0,30 e 1,50 m acima da copa da cultura e fluxo de calor no solo foram instalados e os dados coletados por um sistema automático de aquisição de dados. A evapotranspiração da cultura (ETc) e a evapotranspiração de referência (ETo) foram estimadas pelos métodos da razão de Bowen e Penmam-Monteith, respectivamente, enquanto o coeficiente de cultivo (Kc) foi determinado pela razão ETc/ETo. A ETc da cultura variou em função de sua fenologia, obtendo-se valores médios de 3,8 mm d-1 no período da emergência a 10% da cobertura de solo (Fase I); 5,0 mm d-1 no período do crescimento vegetativo (Fase II); 5,9 mm d-1 no período do desenvolvimento reprodutivo (Fase III) e 5,4 mm d-1 no período de maturação (Fase IV). O Kc pode ser definido em função dos dias após a emergência, pela equação Kc = -0,00006 DAE² + 0,009 DAE + 0,632.