Patterns of hydrological drought indicators in major US River basins

Climate non-stationarity, changes in land use and water management practices affect regional hydrological extremes. This research considers extreme low streamflow ("low flow" henceforth) and flow deficit characteristics, which are important indicators of water scarcity within the broader context of droughts. The results indicate that the number of stations with positive significant trends in low flows is generally higher than the number of stations with negative significant trends; however, the fraction of total number of stations with negative trends has been increasing in the last four decades while the fraction of total number of stations with positive trends have been decreasing, making a gradual reduction in the wetting tendency in low flows in the conterminous U.S. Regional differences in low flow trends are notable, which echo the local climatic changes and soil moisture trends documented in the recent National Climate Assessment (NCA), as well as the changes in cropping and irrigation practices, and creation of forest plantations. A reversal of trend is seen for some northern regions since 1980s. Patterns in return periods and corresponding return values of low flows are also examined, which suggests changing risk conditions that are important for water-resources decision-making. Persistent low flow conditions in a river can lead to chronic water scarcity-a main driver of societal and cross-boundary conflicts. Thus, this research identifies "hotspot" locations where suitable adaptive management measures are most needed.