Year-round measurements of surface energy fluxes, supporting meteorological parameters, soil moisture, and leaf foliage area were made at a tallgrass prairie site in north-central Oklahoma. Here we present the results of the first year (August 1996 to early September 1997) of the study. Non-limiting soil moisture and generally moderate atmospheric conditions throughout the measurement period provided a unique opportunity to observe the role of the prairie canopy in influencing energy fluxes. The periods of vegetation activity are determined by employing two approaches: the conventional approach using green foliage area, and a modified approach using photosynthetically active radiation (PAR) albedo in conjunction with the green foliage area. Performance of the two approaches is examined in explaining the seasonal variation in energy fluxes. The conventional approach failed to detect several periods of vegetation activity (e.g., the end of peak growth in 1996, the entire pre-growth and post-burn periods in 1997). Use of the modified approach, on the other hand, allowed us to determine such periods more accurately and in greater detail. Within each of eight periods determined via the modified approach, energy fluxes varied in a manner consistent with the vegetation activity. On an annual basis, evapotranspiration was the main sink of the energy, consuming about 58% of the net radiation. Evapotranspiration was largest during the peak growth period of 1997 (late May to late August: 388 mm, 37% of the annual ET of 1040 mm). Energy fluxes during other periods (e.g., senescence, post-senescence, pre-growth, early growth) were smaller, but still substantial in magnitude as well as in their relative contributions to the annual energy budget.
