EAE03-A-01294  

GROISMAN, P. Y.; Knight, R. W.; Karl, T. R.; Easterling, D. R.; Sun, B.; 
Lawrimore, J. H.; Bogdanova, E. G.; Ilijn, B. M.     

Changes in the hydrological cycle over the United States during the past century 

    
Over the contiguous United States, precipitation, temperature, streamflow 
(in particular, base streamflow), heavy and very heavy precipitation and 
high streamflow in the East have increased during the 20th century.  In the 
past 50 years, in addition to these changes, increases in evaporation, 
near-surface humidity, total, low, and convective cloudiness; earlier snow 
cover retreat, streamflow peak, and spring onset in the West; and a decrease 
in near-surface wind speed have been documented.  Along the Atlantic coast, 
there was no noticeable century-long change in hurricane-related precipitation, 
while total precipitation and the frequency of "very heavy" precipitation 
unrelated to tropical storms has significantly increased.  During the last 
four years (1999-2002) precipitation was below average over most of the country 
while temperature was well above the average values.   

The up to date consequences of this recent development for the hydrological 
cycle will be presented. Alaska is one of the focal regions of global and 
high latitudinal warming. Here, during the past fifty years a significant 
warming has been accompanied by increases in the duration of the growing 
and frost-free seasons, degree-days above 15 degrees C, the frequency of winter 
thaws, earlier snow cover retreat, and by decreases of heating degree-days.  
Total increase in precipitation over the state has not been spatially 
homogeneous.  Changes have been companied by a redistribution between frozen 
and liquid forms. This redistribution complicates the accuracy of trends 
of precipitation due to changing biases of the cold season precipitation
measurements but accounting for these biases has considered here.  Along the 
northern slope of Alaska, where cold season precipitation has not increased, 
a strong permafrost warming is evident.