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The Urbanization of America's Watersheds: Climatic Implications
Dow, C.L. and DeWalle, D.R. 2000. Trends in evaporation and Bowen ratio on urbanizing watersheds in eastern United States. Water Resources Research 36: 1835-1843.

What was done
The authors studied trends in annual evaporation and Bowen ratio measurements on 51 eastern United States watersheds that had experienced various degrees of urbanization (increases in population density) between 1920 and 1990.

What was learned
As residential development progressively occurred on what originally were rural watersheds, watershed evaporation decreased and sensible heating of the atmosphere increased. From relationships derived from the suite of watersheds investigated, it was calculated that complete transformation from 100% rural to 100% urban characteristics resulted in a 31% decrease in watershed evaporation and a 13 W/m2 increase in sensible heating of the atmosphere.

What it means
A rough rule of thumb derived from climate modeling exercises suggests that a doubling of the air's CO2 concentration will result in a 4 W/m2 increase in radiative forcing of earth's surface-troposphere system, which is predicted to produce an approximate 4C increase in mean global near-surface air temperature, for an order-of-magnitude climate sensitivity of 1C per W/m2 change in radiative forcing. Thus, to a first approximation, the 13 W/m2 increase in sensible heating of the near-surface atmosphere produced by the total urbanization of a pristine rural watershed in the eastern United States could well be expected to produce a maximum increase of 13C in near-surface air temperature over the central portion of the watershed, which is consistent with the just-slightly-smaller urban heat island effects observed in large and densely populated - but not totally urbanized - cities, where some transpiring vegetation is typically always present. Hence, a 10% rural-to-urban transformation would likely produce a warming on the order of 1.3C, and a mere 2% transformation could increase the near-surface air temperature by as much as a quarter of a degree Centigrade.

This powerful anthropogenic, but non-greenhouse, effect of urbanization on the energy balance of the watershed and the temperature of the boundary-layer air above it begins to express itself with the very first hint of urbanization and, hence, may be most difficult to remove from instrumental air temperature records that are used in attempts to identify any greenhouse warming that may be present. Indeed, the signal may already be present in many temperature records that have been considered "rural enough" to be devoid of all human influence, when such is not really the case. Hence, in view of the small degree of warming evident in the global surface air temperature record over the past century - only a half-degree Centigrade or so - and the fact that much of that warming occurred early in the century, probably as the final stage of the planet's natural recovery from the global chill of the Little Ice Age, it is possible, if not probable, that there has been no true greenhouse-induced warming over this time period, and especially over the past 70 years, as we have indicated in our Editorials of 15 June, 1 July, 15 July and 2 August 2000.

Reviewed 11 October 2000