Trends in evaporation and Bowen ratio on urbanizing watersheds in eastern United States

Long-term trends in annual evaporation and watershed Bowen ratio were studied on 51 eastern United States watersheds which had experienced an estimated 0-58% urban/residential development between 1920 and 1990. Annual watershed evaporation was estimated as the difference between annual streamflow and average watershed precipitation, corrected for watershed elevation effects and gage catch error. Annual watershed Bowen ratios were estimated using spatially and temporally averaged net radiation values and annual evaporation. Significant (alpha = 0.1) positive trends in annual watershed Bowen ratio and negative trends in evaporation were found for 12 and 10 watersheds, respectively, of 28 watersheds with >2% urban/residential development and only 2 and 3 watersheds, respectively, of 23 watersheds with <2% development. Trend results lead to estimates of the overall change in water and energy balances which were separately related to the percentage change in urban/residential area for all 51 watersheds. These regional relationships, linking watershed hydrology, energy exchange, and land use changes, showed significant decreases in watershed evaporation and significant increases in sensible heating of the atmosphere with increased urban/residential development on watersheds in the eastern United States. At 100% urbanization the regional relationships predicted a decrease in annual evaporation of 22 cm and an increase in the sensible heating of the atmosphere of 13 W/m(2).