Trend Analysis of Streamflow to Support Bay and Estuary Restoration in Gulf States

Author(s): Rodgers, K.

The discharge of freshwater from rivers and streams to estuaries is important for biological and economic endpoints. The estuaries of the Gulf of Mexico represent one of the most diverse and important ecosystems in the United States. These systems are also heavily influenced by anthropogenic effects within upstream watersheds. Understanding systematic changes in streamflow can provide decision-support for water resources managers to help ensure that estuaries of the Gulf of Mexico receive the critical supply of freshwater needed. In 2016, the U.S. Geological Survey began an effort to characterize freshwater discharge in U.S. tributaries to the Gulf in support of the initiatives prioritized by the Gulf Coast Ecosystem Restoration Council. As part of this effort, daily mean streamflow data were aggregated to monthly, seasonal and annual means at 1,389 gaging stations for streams that drain to the Gulf of Mexico. These values were used to test for monotonic trends in streamflow using the non-parametric Mann-Kendall Trend test. Streamflow trends were synthesized by watersheds representing four-digit hydrologic-unit codes (HUC4). Initial analysis of monthly mean discharge at 28 gaging stations (the most downstream station in each of the 28 HUCs draining to the Gulf) indicate increasing trends at 14 percent (4 sites) of the sites and decreasing trends at 32 percent (9 sites) of the sites. Fifty-five percent of the sites indicated no trend in streamflow. A calculation of area based on land use in the 28 HUC4s does not indicate a dominant land use classification associated with increasing or decreasing streamflow trends. Future work will also evaluate trends in low and high flows, relate streamflow trends to changes in land use or other causal influences, and examine the relationship between streamflow trends and biological or economic endpoint within the Gulf of Mexico.

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