Seasonal and interannual variation in evapotranspiration, energy balance and surface conductance in a northern temperate grassland

The eddy covariance technique was used to measure evapotranspiration and sensible heat flux continuously for 2.5 years (three growing seasons) in a northern grassland near Lethbridge, Alta., Canada. The 1999 growing season had precipitation (226.9 mm) near the 30-year average value (1971-2000; 206.4 +/- 77.5; mean +/- S.D.). In contrast, the 1998 growing season precipitation (295.3 mm) was significantly above average and almost four times greater than the 2000 growing season precipitation (75.1 mm), which was significantly below average. The 1998 growing season had higher peak evapotranspiration (4.5 mm per day) than values observed in 1999 and 2000, when the highest evapotranspiration was approximately 3 mm per day. Evapotranspiration during the winter periods was normally <0.5 mm per day. In 1998, maximum latent heat flux was higher than sensible heat flux during June and early July. In contrast, sensible heat flux dominated the energy budget at midday during the entire 1999 and 2000 growing seasons. The soil heat flux component was quite similar during all 3 years, with maximum daily values reaching approximately 80-100 W m(-2). The decoupling coefficient (ranged between 0.2 and 0.3) indicated that evapotranspiration was strongly controlled by surface conductance in this grassland. The highest lambdaE/lambdaE(eq) values (0.7-0.8) occurred in June and July 1998, before the exhaustion of soil moisture reduced values to <0.4 in late August. Lower values were recorded in 1999 and 2000, often below 0.5. A non-linear model of surface conductance that included functions for vapor pressure deficit, photon flux density, and available soil moisture explained a large proportion of the variation in the surface conductance data. To model surface conductance accurately, separate model coefficients need to be used for the different years. Therefore, our data indicate that evapotranspiration and its physiological control vary in response to interannual changes in precipitation. (C) 2002 Elsevier Science B.V. All rights reserved.