Impacts of precipitation trends on North Carolina's water supply

Many public water supply systems in eastern North Carolina are facing increasing difficulty in meeting demand. Decreasing availability because of climate change, along with increasing agricultural and human use are both cited as major possible causes. Management responses partly depend on the relative importance of the causes. There has been no clear trend in the total annual precipitation of the state of North Carolina during the 20th century. However, during the second quarter of the century there were seasonal changes in amount, with Fall and Winter getting wetter, Summer becoming drier. Spring became slightly drier in the middle of the period, then amounts increased slightly. Although creating no annual change, the trends were not complimentary. The Summer decrease led to more frequent and larger soil water deficits, while the extra Fall and Winter precipitation commonly fell onto soils already at field capacity, thus giving an increase in surface run off. Annual ground water recharge thus decreased from the amounts common early in the century. This presentation explores the causes of the precipitation trends through a frequency analysis of daily precipitation amounts, linking them where possible with atmospheric flow patterns and giving suggestions of likely future trends.