| Seasonal and interannual variability in water energy fluxes was studied comparatively in tallgrass prairie and winter wheat ecosystems in north-central Oklahoma during 1996--2000. Fluxes were examined in terms of relevant controlling variables (e.g., solar radiation, soil moisture, foliage area, canopy development stage). Annual evapotranspiration ranged 640--810 mm for the prairie, and 710--750 mm for wheat. Evapotranspiration (ET) was the major consumer of the energy in both ecosystems. Interannual variability in ET was primarily linked to the effects of soil moisture stress and variations in green leaf area. The ET model of Priestley and Taylor was modified to incorporate these effects, and tests against field measurements indicated a substantial improvement in ET prediction for a wide range of conditions. The water use efficiency (WUE) for the prairie ranged between 0.2 and 2.2 g kg-1 , and was controlled by green foliage area and soil moisture. The wheat WUE ranged between 0.8 and 6.1 g kg-1, and was controlled mainly by green foliage area. Radiation use efficiency (RUE) of the prairie ranged between 0 and 4 g MJ-1, and was influenced by the soil moisture: it was lowest during 1998, the year with severe moisture. In wheat, RUE ranged from 1.5 to 3.8 g MJ-1 and did not seem to have a distinct seasonal pattern. The periods of vegetation activity were 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. The conventional approach failed to detect several periods of vegetation activity. Use of the modified approach, on the other hand, allowed us to determine such periods more accurately and in greater detail, as compared to the conventional approach. Within each period determined via the modified approach, energy fluxes varied in a manner consistent with the vegetation activity. |
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