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Potential evapotranspiration at Simcoe, Southern Ontario

Summary

Several approaches to the evaluation of potential evapotranspiration (PE) are discussed from the point of view of the energy balance equation. Hourly and daily PE were evaluated for an irrigated ryegrass site near Simcoe in Southern Ontario during the summer of 1967 using the Bowen ratio and Penman solution of the energy balance equation, Thornthwaite's temperature equation and the water equivalent of the net radiation. The Bowen ratio values were used as the standard against which the other methods were compared. For the evaluations, measurements of temperature, humidity and wind profiles, net radiation, soil heat flux, screen temperature and humidity and wind run were made.

Excellent results were obtained from the Penman equation (using Businger's with function) for individual hourly periods on days when previous rainfall or irrigation ensured true PE conditions. On other drier days, Penman values were consistently too large. The water equivalent of the net radiation was highly correlated with Bowen ratio values on a hourly basis but overestimated consistently. A poor correlation between Thornthwaite values and the net radiation is demonstrated on a daily basis, which indicates its tenuous connection with the energy balance equation.

The relation between cumulative dry matter productivity of the ryegrass and cumulative PE is shown to be linear, but the productivity rate changed from 1 g/525 mm when the crop was irrigated to 1 g/1287 mm after irrigation had ceased.

Zusammenfassung

Es werden verschiedene Methoden zur Berechnung der potentiellen Evapotranspiration (PE) unter besonderer Berücksichtigung der Energiebilanzgleichung besprochen. Es wurden stündliche und tägliche Werte der PE für eine bewässerte, mit Gras bepflanzte Fläche nahe Simcoe in Süd-Ontario für den Sommer 1967 berechnet, wobei das Bowen-Verhältnis, die Penman-Lösung der Energiebilanzgleichung, Thornthwaites Temperaturgleichung und das Wasseräquivalent der Strahlungsbilanz Verwendung fanden. Die Werte des Bowen-Verhältnisses wurden als Standard verwendet, mit dem die anderen Methoden verglichen wurden. Um die Berechnungen durchführen zu können, wurden Messungen der Temperatur-, Feuchtigkeits- und Windprofile, der Strahlungsbilanz, des Bodenwärmestroms, der Hüttentemperatur und-feuchtigkeit und des Windes vorgenommen.

Es wurden sehr gute Ergebnisse aus der Penman-Gleichung (unter Verwendung der Windfunktion vonBusinger) für einzelne Stunden an Tagen, an denen vorausgehender Regen oder Bewässerung tatsächliche PE-Bedingungen schufen, erhalten. An anderen, trockenen Tagen ergaben sich wesentlich zu hohe Penman-Werte. Das Wasseräquivalent der Strahlungsbilanz war mit dem Bowen-Verhältnis für stündliche Werte hoch korreliert, aber die Werte wurden ständig über-schätzt. Eine schlechte Korrelation zwischen den Werten nach Thornthwaite und der Strahlungsbilanz wird an Hand der täglichen Werte gezeigt, was für ihren nur schwachen Zusammenhang mit der Energiebilanzgleichung spricht.

Es wird gezeigt, daß die Beziehung zwischen der kumulativen Trockensubstanzproduktion des Grases und der kumulativen PE linear ist, aber das Produktionsvermögen fiel von 1 g/525 mm bei Bewässerung auf 1 g/1287 mm nach Aufhören der Bewässerung.

Résumé

On discute ici différentes méthodes de calcul de l'évapotranspiration potentielle (EP) en tenant spécialement compte de l'équation du bilan d'énergie. On calcule pour l'été 1967 les valeurs horaires et journalières d'EP pour une surface irriguée recouverte d'herbe au voisinage de Simcoe dans le sud de l'Ontario. Pour ce faire, on a utilisé le rapport de Bowen, la solution proposée par Penman de résolution de l'équation du bilan d'énergie, l'équation de température de Thornthwaite ainsi que l'équivalent en eau du bilan radiatif. On a utilisé comme référence les valeurs du rapport de Bowen et on leur a comparé les autres méthodes. Afin de pouvoir effectuer les calculs, on a mesuré la température, l'humidité de l'air et le vent à différentes hauteurs, le bilan radiatif, le flux de chaleur dans le sol, la température et l'humidité sous abri et le vent standard.

On a obtenu de très bons résultats de l'équation de Penman (en utilisant la fonction de vent selonBusinger) mais cela seulement pour certaines heures de la journée durant lesquelles de la pluie ou des irrigations antérieures favorisaient effectivement l'EP. Durant les jours secs, les valeurs de Penman étaient au contraire beaucoup trop élevées. L'équivalent en eau du bilan radiatif a donné de hauts coefficients de corrélation avec le rapport de Bowen pour des valeurs horaires. Pourtant celles-ci furent toujours trop élevées. Les valeurs journalières provenant de la formule de Thornthwaite ne concordent que très mal avec le bilan de radiation, ce qui prouve qu'elles n'ont qu'une faible relation avec l'équation du bilan d'énergie.

On démontre en outre qui'il existe une relation linéaire entre la production cumulée de matière sèche de l'herbe et l'EP cumulée, mais que le pouvoir de production est tombé de 1 g pour 525 mm lors de l'irrigation à 1 g pour 1287 mm après que l'irrigation eut pris fin.

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Davies, J.A., McCaughey, J.H. Potential evapotranspiration at Simcoe, Southern Ontario. Arch. Met. Geoph. Biokl. B. 16, 391–417 (1968). https://doi.org/10.1007/BF02243181

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Keywords

  • Potential Evapotranspiration
  • Energy Balance Equation
  • Bowen Ratio
  • Soil Heat Flux
  • Screen Temperature