Energy balance of the Arctic

Summary The available aerological material now permits a more accurate estimate than before of the various terms in the heat budget. It is difficult to find an area for which all energy budget terms have been evaluated. The research at McGill University has attempted to fill this need for the Polar Ocean. In such discussions the heat fluxes at two levels must be known: in the present investigation, 300 mb and earth's surface were chosen. The heat budget calculations were carried out for several areas of the Polar Ocean. Independent calculations of all terms having been made, it was possible to check the accuracy. Both for the Polar Ocean and the Norwegian-Barents Sea a satisfactory balance was obtained.The surface energy budget shows that the radiative terms are far greater than all other influences, and the long-wave components are the greatest in all areas and months. The sensible heat flux from atmosphere to ground is negligible. In winter, all energy expenditure is radiative from the Polar Ocean, but 20% is non-radiative over the Norwegian-Barents Sea. There, the readily available energy from the ocean compensates for the progressively smaller input by radiation through the winter, and the energy budget remains extraordinarily stable during the winter. Looking at the tropospheric energy budget over the Arctic, there is a sharp increase in importance of non-radiative terms on the income side, and an even more pronounced decrease on the expenditure side.Calculations for the earth-atmosphere energy budget show that the result of no advection into the North Polar regions would be a temperature drop of 35° C over the Norwegian-Barents Sea and about 50° over the Central Polar Ocean. The various energy currents are represented pictorially, setting the total incoming energy at the top of the atmosphere equal to 100 units. All discussions refer to the average conditions over the Arctic Ocean. It would be most valuable to know which changes in the individual terms are possible and can be realised under the existing conditions of the world in which we live. The data available from the present investigation will be used for such a study of climatic change.

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Zusammenfassung Das verfügbare aerologische Beobachtungsmaterial gestattet heute eine genauere Schätzung der verschiedenen Terme des Wärmehaushalts als früher. Doch ist es schwierig, ein Gebiet zu finden, für das sämtliche Glieder des Energiebudgets bestimmt wurden. Die Untersuchungen der McGill-Universität versuchen, diese Lücke für das Polarmeer auszufüllen. Für solche Untersuchungen müssen die Wärmeströme in zwei verschiedenen Niveaus bekannt sein, und für die vorliegende Untersuchung wurden das 300-mb-Niveau und die Erdoberfläche gewählt. Die Berechnungen des Wärmehaushalts wurden für verschiedene Gebiete des Nördlichen Eismeers durchgeführt. Da unabhängige Berechnungen der einzelnen Glieder durchgeführt wurden, ist es möglich, die Genauigkeit abzuschätzen, und es zeigt sich, daß sowohl für das Eismeer wie für die Norwegen-Barents-See eine befriedigende Bilanz resultiert.Aus der Energiebilanz am Boden ergibt sich, daß die Strahlungsglieder bei weitem größer sind als alle übrigen Einflüsse und daß die langwelligen Komponenten in allen Gebieten und Monaten am größten sind. Der fühlbare Wärmestrom von der Atmosphäre zur Erde kann vernachlässigt werden. Im Winter beruht der gesamte Energieverlust vom Eismeer auf Strahlungsvorgängen, über der Norwegen-Barents-See dagegen nur zu 80%. Hier kompensiert die leichte Wärmeabgabe vom Ozean die progressive Abnahme des Strahlungsgenusses durch den Winter, so daß die Energiebilanz während des Winters außerordentlich gleichmäßig bleibt. Hinsichtlich der Energiebilanz der Troposphäre über der Arktis besteht ein starker Energiegewinn durch die strahlungsfreien Glieder und gleichzeitig eine ausgesprochene Abnahme der Wärmeverluste.Berechnungen des Wärmehaushalts zwischen Erde und Atmosphäre zeigen, daß das Fehlen von Advektion zum Nordpolargebiet zu einem Temperaturabfall von 35° C über der Norwegen-Barents-See und von 50° über dem zentralen Eismeer führen müßte. Die verschiedenen Energieströmungen werden bildlich dargestellt, wobei die gesamte am äußeren Rande der Atmosphäre eintretende Energie 100 Einheiten gleichgesetzt wird. Alle Diskussionen beziehen sich auf durchschnittliche Verhältnisse über dem Eismeer. Es wäre von großem Interesse zu untersuchen, welche Veränderungen der einzelnen Glieder unter den auf der Erde herrschenden Bedingungen möglich und realisierbar sind. Die Resultate der vorliegenden Untersuchung werden für eine derartige Studie über Klimaveränderungen benützt werden.

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Résumé Les observations aérologiques disponibles actuellement permettent une estimation plus précise que jusqu'ici des différents paramètres de calcul du bilan thermique. Il est cependant difficile de trouver une surface d'une certaine dimension pour laquelle tous les termes du bilan énergétique ont été évalués. Les recherches entreprises à l'Université McGill tentent de combler cette lacune pour l'Océan Glacial Arctique. Pour ce faire, il faut connaître les flux de chaleur à deux niveaux; dans la présente étude, on a choisi la surface standard de 300 mb et le sol. Les calculs du bilan thermique ont été effectués pour plusieurs parties de l'Océan Glacial Arctique. Vu que chaque terme de l'équation fut calculé indépendemment des autres, il fut possible d'en contrôler la précision. On a ainsi obtenu un bilan satisfaisant tant pour l'Océan Glacial tout entier que pour la partie située entre les Mers de Norvège et de Barentz.Le bilan énergétique à la surface du sol montre que les paramètres de radiation sont beaucoup plus importants que tous les autres et que leurs composantes se rapportant aux longues ondes sont les plus grandes dans toutes les régions étudiées ainsi qu'au cours de tous les mois de l'année. Le flux de chaleur perceptible de l'atmosphère vers le sol est négligeable. En hiver, toute la dépense d'énergie provient du rayonnement sur l'Océan Glacial Arctique, mais, sur les Mers de Norvège et de Barentz, 20% de ces pertes d'énergie ne proviennent pas du rayonnement. Dans ce second cas, l'énergie venant de la mer et immédiatement disponible compense durant tout l'hiver la diminution progressive du rayonnement reçu, si bien que le bilan énergétique y est extraordinairement stable durant toute cette saison. Quant au bilan énergétique de la troposphère au-dessus de l'Arctique, on constate une forte augmentation de l'importance des termes étrangers au rayonnement du côté des gains en énergie et une décroissante tout aussi importante de ceux-ci du côté des pertes.Des calculs concernant l'échange énergétique entre l'atmosphère et la terre montrent que le résultat de l'absence d'advection vers les régions polaires arctiques serait une chute de température de 35° C sur les Mers de Norvège et de Barentz et de près de 50° C sur le centre de l'Océan Glacial Arctique. Les différents courants d'énergie ainsi calculés sont reportés sur des figures en pour-cent de l'énergie totale reçue au sommet de l'atmosphère. Toutes les dicussions se rapportent à des conditions moyennes régnant sur l'Océan Glacial Arctique dans son ensemble. Il serait cependant très intéressant de connaître quelles sont, pour les différents termes du bilan thermique, les variations possibles et pouvant se réaliser dans les conditions existant dans le monde où nous vivons. Les chiffres résultants de la présente recherche seront utilisés dans une étude consacrée aux modification du climat.

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The research reported in this paper was sponsored in part by the Air Force Cambridge Research Laboratories, Office of Aerospace Research, under Contract AF 19(604)-7415.     

Energy balance of the arctic

Summary Evaporation and sensible heat flux have been calculated for each month over the Polar Ocean and the Norwegian-Barents Sea. Sverdrup's evaporation formula was used, and it was first examined how the K-coefficient in that formula depends on the wind speed frequency distribution. Thus the effect of the Arctic wind conditions could be taken into account. Seasonal maps were constructed of mean wind speed. Previously obtained surface temperatures were used, but some additional examinations were carried out, using various assumptions for extreme surface temperatures in summer and winter.Evaporation and sensible heat flux were calculated separately for the following areas: Central Polar Ocean, Kara-Laptev Sea, East Siberian Sea, Beaufort Sea, and belts of 5° latitude of the Norwegian-Barents Sea.The values for the different areas are presented in tables and figures. Evaporation over ice surfaces has a double maximum—in spring and fall—and a main minimum in winter. Over open water surfaces the evaporation shows a summer minimum and a broad maximum in winter. If small parts of the ocean were to remain open longer in the fall, or during the whole winter, the heat loss would increase very rapidly.Sensible heat flux is often calculated from evaporation by theBowen ratio. The small evaporation values over the Polar Ocean give unreliable values for sensible heat flux, and instead the formula byShuleikin was used. This permits the determination of sensible heat flux independent of evaporation. The characteristic sensible heat flux curves are quite similar to the evaporation curves. The open water areas in the Polar Ocean show very high values for sensible heat flux. One percent open water, from October to May would increase the heat flux from the Central Polar Ocean from 3.7 to 5.2 Kcal cm^–2, year^–1. Open areas must remain small as there is not sufficient energy available to maintain such fluxes.Finally, a table gives the monthly values of the total heat loss for the various areas, by evaporation and sensible heat flux.

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Zusammenfassung Monatswerte für Verdunstung und Wärmefluß wurden für das Polarmeer und für Nordmeer-Barentssee berechnet. Zur Verdungstungsberechnung wurde die Formel vonSverdrup benutzt, deren K-Koeffizient in seiner Windabhängigkeit neu berechnet wurde. Auf Grund neu konstruierter jahreszeitlicher Karten der mittleren Windgeschwindigkeit konnten die arktischen Windverhältnisse berücksichtigt werden. Wegen der Unsicherheit früher bestimmter Oberflächentemperaturen wurden zusätzliche Berechnungen für Extremfälle im Sommer und Winter durchgeführt, um mögliche Fehlerquellen abzuschätzen. Verdunstung sowie Wärmefluß wurden gesondert für die folgenden Gebiete berechnet: Zentrales Polarmeer, Kara-Laptev-See, Beaufort-See sowie für Bänder von 5° Breite im Gebiet Nordmeer-Barentssee.Die Resultate für die einzelnen Gebiete werden an Hand von Diagrammen und Tabellen diskutiert. Über Eis zeigt die Verdunstung ein doppeltes Maximum im Frühling und Herbst und das Hauptminimum im Winter, während sich über offenem Wasser ein Sommerminimum und ein breites Wintermaximum ergeben. Es zeigt sich, daß bereits relativ kleine Wasserflächen, die länger im Herbst oder während des ganzen Winters offen bleiben, im Polarmeer zu sehr hohen Wärmeverlusten führen.Der Wärmefluß wird oft auf Grund der Verdunstung mit Hilfe derBowen-Formel berechnet. Wegen der geringen Verdunstung über dem Polarmeer führt diese Formel jedoch zu unrichtigen Werten, und es wird deshalb hier dieShuleikin-Formel benützt, die eine Bestimmung des Wärmeflusses unabhängig von der Verdunstung ermöglicht; die charakteristischen Kurven des Wärmeflusses sind den Verdunstungskurven sehr ähnlich. Offenes Wasser im Polarmeer führt auch hier zu sehr hohen Werten; eine offene Wasserfläche von 1% in der Zeit von Oktober bis Mai würde den Wärmefluß vom zentralen Polarmeer von 3,7 auf 5,2 Kcal/cm² pro Jahr erhöhen. Offene Flächen müssen daher klein bleiben, da der Energievorrat nicht genügend groß für die Aufrechterhaltung eines solchen Energieflusses wäre. Zum Schlusse werden in einer Tabelle Monatswerte der gesamten Wärmeverluste durch Verdunstung und Wärmefluß für die verschiedenen Gebiete gegeben.

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Résumé On a calculé des valeurs mensuelles de l'évaporation et du flux de chaleur pour l'Océan Glacial Arctique et pour la région située entre la Mer du Groenland et la Mer de Barents. Dans le cas de l'évaporation, on s'est servi de la formule deSverdrup dont on a déterminé à nouveau le coefficient K en tenant compte de sa dépendance du vent. Il a été possible de tenir compte du vent dans les régions arctiques grâce à l'établissement récent de cartes saisonnières de la vitesse moyenne du vent. En raison de l'incertitude des déterminations antérieures de la température de surface, on a procédé à des calculs supplémentaires pour des cas extrêmes en été et en hiver afin d'évaluer les sources d'erreurs possibles. On a calculé séparément l'évaporation et le flux de chaleur pour les régions suivantes: Centre de l'Océan Glacial Arctique, Mer de Kara-Mer de Laptev, Mer de Beaufort ainsi que pour de bandes de 5° de largeur dans la région comprise entre la Mer du Groenland et la Mer de Barents.On discute les résultats obtenus pour ces différentes zones en partant de diagrammes et de tableaux. Au-dessus de la glace, l'évaporation présente deux maximums, l'un au printemps, l'autre en automme et un minimum principal en hiver. Sur la mer libre, on constate au contraire un minimum en été et un maximum très large en hiver. Il en résulte que des surfaces libres de glace relativement peu étendues qui se maintiennent en automne, voire durant tout l'hiver peuvent déjà provoquer des pertes de chaleur considérables dans l'Océan Glacial Arctique.On calcule souvent le flux de chaleur en se basant sur l'évaporation selon la formule deBowen. Cependant, en raison des faibles évaporations constatées sur l'Océan Glacial, cette formule conduirait à des valeurs fausses. On a donc utilisé ici la formule deShuleikin qui permet la détermination du flux de chaleur indépendamment de l'évaporation. Les courbes caractéristiques du flux de chaleur sont très semblables à celles de l'évaporation. Les surfaces libres de glace de l'Océan Glacial conduisent ici aussi à des valeurs très élevées. Une surface d'eau de 1% restant libre de glace d'octobre à mai augmenterait de flux de chaleur de l'océan de 3,7 à 5,2 Kcal/cm² par année. Les surfaces d'eau doivent donc rester très petites, car les réserves d'énergie sont insuffisantes pour maintenir un tel flux d'énergie calorifique. On donne enfin dans une table les pertes mensuelles totales de chaleur dues à l'évaporation et au flux de chaleur et cela pour chacune des régions considérées.

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The research reported in this paper was sponsored in part by the Air Force Cambridge Research Laboratories, Office of Aerospace Research, under Contract AF 19(604)7415.     

Energy balance comparison of sorghum and sunflower

Summary An understanding of the energy exchange processes at the surface of the earth is necessary for studies of global climate change. If the climate becomes drier, as is predicted for northern mid-latitudes, it is important to know how major agricultural crops will play a role in the budget of heat and moisture. Thus, the energy balance components of sorghum [Sorghum bicolor (L.) Moench.] and sunflower (Helianthus annuus L.), two drought-resistant crops grown in the areas where summertime drying is forecasted, were compared. Soil water content and evapotranspiration (ET) rates also were determined. Net radiation was measured with net radiometers. Soil heat flux was analyzed with heat flux plates and thermocouples. The Bowen ratio method was used to determine sensible and latent heat fluxes. Sunflower had a higher evapotranspiration rate and depleted more water from the soil than sorghum. Soil heat flux into the soil during the daytime was greater for sorghum than sunflower, which was probably the result of the more erect leaves of sorghum. Nocturnal net radiation loss from the sorghum crop was greater than that from the sunflower crop, perhaps because more heat was stored in the soil under the sorghum crop. But daytime net radiation values were similar for the two crops. The data indicated that models of climate change must differentiate nighttime net radiation of agricultural crops. Sensible heat flux was not always less (or greater) for sorghum compared to sunflower. Sunflower had greater daytime values for latent heat flux, reflecting its greater depletion of water from the soil. Evapotranspiration rates determined by the energy balance method agreed relatively well with those found by the water balance method. For example, on 8 July (43 days after planting), the ET rates found by the energy-balance and water-balance methods were 4.6 vs. 5.5 mm/day for sunflower, respectively; for sorghum, these values were 4.0 vs. 3.5 mm/day, respectively. If the climate does become drier, the lower soil water use and lower latent heat flux of sorghum compared to sunflower suggest that sorghum will be better adapted to the climate change.Contribution from the Kansas Agricultural Experiment Station. F. Rachidi is now with the Département d'Écologie Végétale et Pastoralisme, École Nationale d'Agriculture, Meknès, Morocco, and E. T. Kanemasu is now with the Department of Agronomy, University of Georgia, Griffin, Georgia, U.S.A.With 5 Figures     

Energy and water balance studies of a snow cover during snowmelt period at a high arctic site

Summary  The predicted global warming is supposed to have an enhanced effect on the arctic regions. How this will influence the water, carbon dioxide and methane balances in the European arctic tundra is the objective of the EU-funded project “Understanding Land Surface Physical Processes in the Arctic” (LAPP), to which where SINTEF is one of several contributors. The snow cover is one of the limiting factors for these exchange processes and knowledge of how it behaves and will behave under a different climate is important. Data collected for water and energy balance studies in an area close to Ny-?lesund at 79°N at Svalbard are the basis of this study. Measurements during the ablation periods since 1992 show an average air temperature for the periods of 2.1 °C, an average incoming shorwave radiation of 230 W/m² and an average measured runoff intensity of 14 mm/day with a maximum of 68 mm/day. Three models of different complexity are tested in order to simulate the water and energy balance of a snow cover on the arctic tundra. The three models are: a complex numerical model (CROCUS), a simple energy balance model and a temperature index model. The simulations were carried out for the melt periods in 1992 and 1996 as these two periods represent very different meteorological conditions. The results of these simulations exposed weaknesses in all the models. The energy balance model lacks calculation of cold content in the snowpack. This influences both the outgoing longwave radiation and the timing of the melt. Due to the effect of compensating errors in the simulations, CROCUS performed better than the simple energy balance model but also this model has problems with the simulation of outgoing longwave radiation. The temperature index model does not perform well for snowmelt studies in regions were radiation is the main driving energy source for the melt. Received September 28, 1999 Revised September 18, 2000     

Global and continental water balance in a GCM

Surface hydrology is recognised as an important component of general circulation climate models. The global and regional climates simulated by such models are demonstrably sensitive to the parameterization of terrestrial hydrologic processes. There exists, therefore, a clear requirement to evaluate different parameterization approaches in terms of the representation of the terrestrial phase of the hydrologic cycle. One potential means of meeting this requirement is by using available continental water-balance summaries. In this study three versions of a GCM, the National Center for Atmospheric Research (NCAR) Community Climate Model Version l (CCM1), differing mainly in spatial resolution and the representation of the surface hydrology, are compared against existing water-balance studies. Additional streamflow data are incorporated as a means of further validating both the water-balance approach and the GCM surface hydrologic parameterization in capturing the gross features of continental-scale hydrology.     

A study of water balance in china

The water balance in C'hina is studied in this paper, which follows the papers on the radiation balance and the heat balance in China. Using the data from more than 200 meteorological stations for 1961-1970 and about 200 hydrometric stations bcforc1972, we, through the equation of water balance, have calculated the monthly and annual amounts of precipitation, evaporation, run-off and soil water variation. The water balance and the hydroclimatological characteristics of China arc given.     

Temporal and spatial variability of the global water balance

An analysis of simulated global water-balance components (precipitation [P], actual evapotranspiration [AET], runoff [R], and potential evapotranspiration [PET]) for the past century indicates that P has been the primary driver of variability in R. Additionally, since about 2000, there have been increases in P, AET, R, and PET for most of the globe. The increases in R during 2000 through 2009 have occurred despite unprecedented increases in PET. The increases in R are the result of substantial increases in P during the cool Northern Hemisphere months (i.e. October through March) when PET increases were relatively small; the largest PET increases occurred during the warm Northern Hemisphere months (April through September). Additionally, for the 2000 through 2009 period, the latitudinal distribution of P departures appears to co-vary with the mean P departures from 16 climate model projections of the latitudinal response of P to warming, except in the high latitudes. Finally, changes in water-balance variables appear large from the perspective of departures from the long-term means. However, when put into the context of the magnitudes of the raw water balance variable values, there appears to have been little change in any of the water-balance variables over the past century on a global or hemispheric scale.     

Energy balance and transpiration from tussock grassland in New Zealand

The energy balance was measured for the dry canopy of narrow-leaved snow tussock (Chionochloa rigida), and measurements of transpiration were obtained from a large weighing lysimeter.Typical maximum summer transpiration rates of 0.21–0.43 mmhr^-1 (140–290 W m^-2) were recorded. The latent heat flux accounted for less than 40% of net radiation. The estimated value of the bulk stomatal resistance (r _ST) for 29 days was 158 s m^-1, and the decoupling parameter () was 0.17. Transpiration rates were not driven directly by net radiation, but were closely linked to the size of the regional saturation deficit imposed at the level of the canopy by efficient overhead mixing, and were constrained by a large bulk stomatal resistance. A linear relationship between r _ST and the saturation deficit is proposed as a realistic method for estimating transpiration for water yield studies of tussock catchments.     

A global monthly climatology of soil moisture and water balance

Global monthly climatology of available soil moisture content is derived on a 4° by 5° grid from observed precipitation and air surface temperature by use of a simple water budget model. The governing equations and methods of calculation for deriving these fields, which follow the formulation of Thornthwaite, are first described and the importance of the various assumptions and simplifications of this approach are discussed. The derived global fields are then presented. A comparison of some of the derived fields with other calculations is also made in order to permit an evaluation of the results: For example, our indirect estimate of the river run-off is generally in good agreement with more direct estimates, except for high latitude regions where the freezing of the soil may play an important role.Yale Mintz died on 27 April 1991. This work was carried out jointly over a number of years preceding his death     

A simple method of estimating the soil water balance

A simple method of computing daily evapotranspiration is described. The main inputs to the model are easily measurable parameters such as rainfall and pan evaporation. The model takes into account evaporative demand and soil and crop factors, and can be used for the estimation of soil water loss in both fallow and cropped situations. In developing and testing the model, both published experimental information and data collected at ICRISAT Center were used. Estimated values of evapotranspiration and soil moisture storage were found to compare favourably with the observed values.     

Estimation of water balance components in the Black Sea

Presented are the estimates of precipitation amount and evaporation for the Black Sea basin based on the data of numerical regional reanalysis. The spatial distribution of considered variables is compared with the results obtained before using the method of extrapolation of measurements at the coastal meteorological stations. The computed water balance components of the Black Sea compared with the available literature data are used for obtaining the estimate of the mean water discharge in the Bosphorus.     

Surface energy- and water balance in a high-arcticenvironment in NE Greenland

Summary  Within the framework of the European LAPP-project (Land Arctic Physical Processes) and as part of the Danish Research Council’s Polar Programme, studies on water- and surface energy balance in NE Greenland were conducted in 1996 and 1997. Eddy correlation measurements of water vapour and sensible heat fluxes above the three dominant vegetation types: fen, willow snowbed, and heath were conducted for the entire growing season. This was supplemented by measurements of evaporation from snow covered areas and from a small pond. The evapotranspiration was found to be relatively high with the maximum from the fen (≈86 mm per season). For the two other vegetation types the evapotranspiration was less, for heath 61 mm per season, while willow snowbed had evaporation rates on intermediate level. By use of the Penman-Monteith equation it was possible to estimate the altitude dependence of the evapotranspiration and calculate the annual evaporation for the whole area to 80 mm per year. By applying a bucket model the evaporation was found to be in accordance with changes in soil moisture as monitored with TDR. The observed surface water balance was compared to river discharge, which shows a glacio-nival regime with an early spring flow (June), determined by the snow melt in the main valley and an July–August maximum determined by melt on higher plateau areas. When balancing the individual hydrological components an annual deficit of 180 mm was observed, but it was found that this deficit could be reduced by correcting for aerodynamic and altitude effects on the precipitation. Finally some of the possible consequences of a global warming is discussed in relation to the water and energy balance in the high-arctic ecosystem. Received November 1, 1999 Revised May 15, 2000     

Response of the water balance in Greece to temperature and precipitation trends

This paper deals with the most recent trends in meteorological and hydrological variables, which include air temperature and precipitation (P), potential and actual (ET) evapotranspiration, surface runoff (RO), water recharge into the soil (R) and water loss from the soil (L). Most hydrological variables were calculated via Palmer's algorithm. For this purpose, two rank-based statistical tests (the Mann?CKendall (MK) and a change-point analysis (CPA) approach) and the basic linear regression-based model were applied on the weekly precipitation and temperature from 17 stations all over Greece, during 1961?C2006. Only in winter, all variables except for R, which showed no clear signal, presented downward trends. The declining trends of P and L in spring and summer were counterbalanced by reductions in RO (and R in the case of summer) as opposed to increases in ET. In autumn, the declining tendencies of P and L were offset by RO reductions and R increases. Annually, the trends in water cycle components were analogous to that of spring, summer and autumn. The number of stations with statistically significant (at 95%) trends greatly varied with season and meteorological/hydrological variable.     

The energy balance of water drops on a leaf surface

The amount of heat conducted to an isolated drop of water on a leaf is described by an equation which includes an effective thermal conductivity coefficient. Measurements of the dimensions of water drops on a horizontal wheat leaf give relationships which allow the volume and exposed surface areas to be obtained from drop diameter. These relationships are used in the experimental determination of the drop boundary-layer resistance and the effective thermal conductivity coefficient for drops on a leaf in a chamber. The boundary-layer resistance of the drop appeared to be independent of drop size and the mean value was about 60% of that for one side of the leaf. For drops with diameters less than 1 mm, conduction of heat to the drop reduced the leaf-to-drop temperature difference by over 50% of the value expected without conduction. Conduction of heat to drops will significantly affect the evaporation rate of surface water from cereal canopies after rain.     

不同生育期水分胁迫对玉米光合特性的影响

利用遮雨棚以夏玉米为对象进行水分胁迫大田试验,通过分析玉米叶片光合测量数据,研究不同生育期水分胁迫对玉米光合特性的影响,为定量分析不同水分胁迫程度对玉米生育的可能机理提供数据和初步的理论支持.结果表明:土壤水分下降会使玉米叶片的光合速率(Pn)、气孔导度(Gs)、蒸腾速率(Tr)降低,而胞间CO2浓度(Ci)和水分利用效率(WUE)会增加;Pn随着光照强度的增加而增加,且随着水分胁迫强度增强,Pn增加速率降低;干旱胁迫会改变Pn、Tr日变化规律,并且对拔节期光合作用的抑制小于成熟期;WUE与Pn存在极显著的正相关关系,与Tr、Ci及Gs存在显著的负相关关系.