Has the hydrologic cycle decelerated during the past half century? Professor
Wilfried Brutsaert. School of Civil & Environmental Engineering. Cornell
University.
Has the hydrologic cycle decelerated during the past half century? Professor Wilfried Brutsaert School of Civil & Environmental Engineering Cornell University The water budget of a natural river basin is usually formulated as P − Q − E = dS / dt where P is the precipitation rate, Q the outflow rate per unit area, E the evaporation rate and S the water stored per unit area in the basin. The variables P and Q can be and have been measured directly and many long-term data sets are available for basins all over the world, with which their evolution over time can be studied in great detail. The construction of reliable long term data sets for E and S for climate change purposes is more challenging; indeed, the direct measurement of these variables is still difficult, so that in practice to gain information on past trends they must invariably be estimated by indirect methods. In the case of E, attempts have been made to estimate past trends of landscape evaporation from available pan evaporation records. Pan evaporation has been decreasing for the past half century or so over many regions of the Earth. However, because pan and landscape evaporation are intrinsically different, the significance of this negative trend, as regards terrestrial evaporation, has been controversial, and its implications for the global water cycle have remained unclear. The controversy stemmed from the alternative views that these evaporative changes resulted, either from global radiative dimming, or from the complementary relationship between pan and terrestrial evaporation. It stands to reason that these factors need not be mutually exclusive but can act concurrently. Nevertheless, if the presently available data records are taken at face value, despite global dimming, the observed decreases in pan evaporation are generally evidence of increased terrestrial evaporation. This is consistent with independent hydrologic budget calculations for several large river basins, and likely further evidence of an accelerating hydrologic cycle in many areas, where pan evaporation has been decreasing.
