For more than a decade, eddy covariance measurements of evapotranspiration (ET) have been made at Harvard Forest, a deciduous forest in central Massachusetts. The marked increase in ET that occurs at the time of leaf emergence, previously documented to lead to measurable changes in the boundary layer temperature, humidity, and cloud based on diurnal, synoptic, and seasonal time scales. Reforestation in the eastern United States overthe last century led us to believe that long-term boundary layer state variations might also occur, but reliable data are not available. We resolved to link changes in ET over the first three time scales to streamflow and water table data, in advance of extending the analysis to decadal and longer scales.

A well-defined diurnal signal in streamflow is frequently observed. Decreases in streamflow occur during the day due to transpiring vegetation, using some of the groundwater supply that composes the base flow of the stream. At night, groundwater replaces some of the water that was transpired during the day. We establish an empirical relationship between the amplitude of the diurnal streamflow signal and ET. This estimated ET will then be compared to the eddy covariance measurements of ET at Harvard Forest at the synoptic and seasonal time scales.

Preliminary results from analysis of 15-minute United States Geological Survey (USGS) streamflow data from the Biscuit Brook watershed in the Catskill Mountains of New York showed a marked increase in the amplitude of the diurnal streamflow signal in late May, near the time of leaf emergence. When normalized by streamflow, the amplitude of the diurnal streamflow signal reaches a maximum after day 200, near the time of maximum observed ET at Harvard Forest.

Freedman et. al. (2001) showed through boundary layer synoptic composites that the presence of a net radiation-boundary layer cumulus (BLcu) feedback ensured the appearance of BLcu on each day of a postfrontal sequence. The presence of BLcu provided favorable conditions for forest-amosphere exchange as opposed to few cloud cases by enhancing carbon uptake and water use efficiency. We plan on examining changes in the amplitude and phase of the diurnal streamflow signal for postfrontal sequences and compare our results to those of Freedman et al.

At the seasonal time scale, Fitzjarrald et. al. (2001) determined the onset of spring by analyzing the seasonal trend of the tendency Bowen ratio from climatic records. We plan on analyzing the seasonal trends in both in the diurnal amplitude and the transient e-folding time constant of streamflow and comparing our results to those of Fitzjarrald et. al. to infer the date of leaf emergence.

REFERENCES

Fitzjarrald, D. R., O. C. Acevedo, and K. E. Moore, 2001: Climatic consequences of leaf presence in the eastern United States. J. Climate, 14, 598-614.

Freedman, J. M. and D. R. Fitzjarrald, 2001: Postfrontal airmass modification. J. Hydrometeorology, 2, 419-437.