Heavy precipitation and high streamflow in the contiguous United States: Trends in the twentieth century

Changes in several components of the hydrological cycle over the contiguous United States have been documented during the twentieth century: an increa se of precipitation, especially heavy and very heavy precipitation, and a s ignificant retreat in spring snow cover extent over western regions during the last few decades. These changes have affected streamflow, including the probability of high flow. In the eastern half of the United States a signi ficant relationship is found between the frequency of heavy precipitation a nd high streamflow events both annually and during the months of maximum st reamflow. Two factors contributed to finding such a relation: 1) the relati vely small contribution of snowmelt to heavy runoff in the eastern United S tates (compared to the west), and 2) the presence of a sufficiently dense n etwork of streamflow and precipitation gauges available for analysis. An in crease of spring heavy precipitation events over the eastern United States indicates with high probability that during the twentieth century an increa se of high streamflow conditions has also occurred. In the West, a statisti cally significant reduction of snow cover extent has complicated the relati on between heavy precipitation and streamflow. Increases in peak stream flo w have not been observed here, despite increases in heavy precipitation eve nts, and less extensive snow cover is the likely cause.