HYDRO-CLIMATOLOGICAL TRENDS IN THE CONTINENTAL UNITED-STATES, 1948-88

Spatial patterns in trends of four monthly variables: average temperat ure, precipitation, streamflow, and average of the daily temperature r ange were examined for the continental United States for the period 19 48-88. The data used are a subset of the Historical Climatology Networ k (1036 stations) and a stream gage network of 1009 stations. Trend si gnificance was determined using the nonparametric seasonal Kendall's t est on a monthly and annual basis, and a robust slope estimator was us ed for determination of trend magnitudes. A bivariate test was used fo r evaluation of relative changes in the variables, specifically, strea mflow relative to precipitation, streamflow relative to temperature, a nd precipitation relative to temperature. Strong trends were found in all of the variables at many more stations than would be expected due to chance. There is a strong spatial and seasonal structure in the tre nd results. For instance, although annual temperature increases were f ound at many stations, mostly in the North and West, there were almost as many downtrends, especially in the South and East. Among the most important trend patterns are (a) increases in March temperature at alm ost half of the stations; (b) increases in precipitation from Septembe r through December at as many as 25 percent of the stations, mostly in the central part of the country; (c) strong increases in streamflow i n the period November-April at a maximum of almost half of the station s, with the largest trend magnitudes in the north-central states; (d) changes in the temperature range (mostly downward) at a large number o f stations beginning in late spring and continuing through winter, aff ecting as many as over half of the stations. The observed trends in st reamflow are not entirely consistent with the changes in the climatic variables and may be due to a combination of climatic and water manage ment effects.