An Examination of the Sensitivity of Runoff in the Northeastern US to 20th Century Development

Holistic understanding of the interaction between climate, people, vegetation and water is crucial for society in the northeast region of the U.S. Changes in land use, land cover or water management systems result in complex interactions of various processes which in turn affect the runoff response. Since this region of the U.S. has seen a reduction in forest cover as the first Europeans settlement established and then over the last century witnessed a forest regrowth. Then, understanding the basic hydrologic processes that drive the water balance in response to these changes is vital. This paper examines how runoff is relatively sensitive to land use, land cover and climate change impacts in the northeastern U.S. from West Virginia, Virginia in the south to Maine in the north and states in between. We analyze historical observations of precipitation, streamflow, land cover and temperature drawing upon the climatological ideas of Budyko who found an empirical function that at a broad scale approximates water balance partitioning based on an index of dryness. We used 51 HCDN watersheds with drainage area varies between 77 and 16,900 km2. In each of the study watersheds we examined trends in precipitation, potential evapotranspiration, streamflow and runoff ratio (Q/AP). Mann Kendall trend analysis suggested that only 5 out of the 51 watersheds studied have a significant decreasing dryness index trend during the twentieth century (i.e., watersheds are becoming more humid) while the rest of watersheds did not show a significant trend. With regard to the fraction of precipitation going to evaporation, 10 out of the 51 the watersheds studied have a significant increasing trend, and those watersheds have a significant decreasing runoff ratio trend which corresponds with the land cover changes seen (forest cover increase). These trend analysis results suggest that given the land cover changes seen in the region, the changes in the runoff ratio are related to changes in fraction of precipitated water going to evaporation and not to dryness index. Also 90% of the watersheds studied have not had a significant changes in dryness index. Moreover, there were decreases in runoff ratio that were in general large when percentages of land area that is forest within watersheds exceeded 30% increase.