Increasing Evapotranspiration Trends Over the Mississippi River Basin

Abstract

Recent research suggests that the evapotranspiration rate (ET) over much of North America has increased over the past 50 years; however there is not a consensus on why the rate is increasing. Most of these studies have focused on annual trends starting around 1950. This study considers (1) whether or not published ET rates would be affected by the addition of the 1940s to the dataset, (2) changes in monthly stream flow and precipitation patterns that could affect the rate of ET and whether there are months driving these annual trends, and (3) whether there are certain areas in the watershed driving this trend. This study considers the hydrological cycle for the Mississippi River Basin from 1940-1999 to show that ET rates have increased. However, when considering trends starting in the 1940s, the trends are not nearly as strong as the trends of the 50 years period 1950-1999 with an increase of 0.29 mm/yr rather than 0.88 mm/yr. Monthly analyses showed that summer months, when potential ET is the highest, have a persistent increasing trend in precipitation, which probably contributes most significantly to overall increasing ET. The most dramatic and longest term shifts appear in July, where, for the period of 1940-1999, there is an increase of 0.30 mm/yr, July accounts for nearly 100% ET increases. However, this drops to 15% for the period of 1950-1999. These results reinforce the studies showing an accelerated hydrologic cycle over North America and, surprisingly, that much of the ET increase can be attributed to increases in July. Also it can be seen that the areas of the Great Plains and Rocky Mountains are the areas driving this increase, especially the Arkansas, Red, and White River valleys