Historical evaporation estimates using a form of the Penman-Monteith equation are computed from hourly first order station data at a network of northeastern U.S. sites. Temporal trends in modeled, actual, potential and pan evaporation are compared with each other as well as a limited set of observed pan evaporation data. Trends in modeled evaporation can result from time dependent changes in either temperature, humidity, solar radiation, wind speed or some combination of these variables. The influence of changes in the individual parameters is evaluated by artificially detrending the time series of a single variable, while allowing the others to vary naturally. The resulting trends are compared to those obtained from the unaltered meteorological series.

Our preliminary results indicate that modeled pan evaporation has shown a consistent increase from 1970 through the 1990s at stations ranging from Washington, DC to Syracuse, NY. Pan evaporation observations at Ithaca and Geneva, NY and New Brunswick, NJ do not show such a trend.