Evidence of Seasonal Changes in Evapotranspiration in Eastern U.S. Hydrological Records

Hydrologic records provide some of the most widespread, long-term data available that can be expected to contain signals of climate variation over time. Yet such data have not been used to identify the widespread effects of spring onset in the forested eastern United States, where streamflow is strongly influenced by vegetation. Three independent runoff characteristics are affected by the enhanced evapotranspiration (ET) that occurs with annual leaf emergence: (i) P − R, the difference between precipitation and runoff, returns to dormant season values and increases due to ET; (ii) the streamflow recession time constant (a measure of the time required after rainfall for streams to return to their baseflow levels) shortens; and (iii) the diurnal streamflow amplitude increases. The smallest watersheds in our dataset (area <200 km2) exhibit the diurnal streamflow signal most often and with the greatest amplitude, in accord with accepted relationships between channel length and watershed area. The dynamics of ET effects on runoff characteristics, illustrated using a simple model, suggest that the recession time constant and diurnal amplitude depend on bulk characteristics of a watershed. Using the P − R, streamflow recession, and diurnal streamflow signal methods, a spring onset date is obtained from the historical hydrologic datasets. The P − R and streamflow recession spring onset dates proceed more slowly than the diurnal amplitude spring onset date and those obtained by independent methods.