THE ROLE OF DROUGHT AND RAINFALL INTENSITY ON THE GENERATION OF RUNOFF AND PHOSPHOROUS EXPORT IN AN EPHEMERAL AGRICULTURAL WATERSHED

Reducing excess nutrients in runoff from non-point sources has become a national priority due to the growing dead zone in the Gulf of Mexico and the prevalence of harmful algal blooms. One way to reduce nutrient loading would be to identify sources that act as hotspots to focus remedial efforts. Ephemeral agricultural watersheds in central Kentucky may be hotspots for nutrient loading due to the ubiquity of thin clay soil and lack of upland riparian zones. In this research we quantified how drought length and rainfall intensity contributed to the volume of runoff and peak phosphorous levels for an ephemeral agricultural watershed near Waco, Kentucky. We began by taking flow measurements using a V-notch weir and daily rainfall data from the Kentucky Mesonet to compute a water budget. We compared storm hydrographs with days of drought for storms with similar rainfall intensity and with rainfall intensity for storms with similar drought length. Each storm was then compared to peak phosphorus levels. We found that runoff responses lasted 10-48 hours, but contributed large amounts of phosphorous. The amount of phosphorous increased with the amount of runoff and drought length. However runoff was inversely proportional to drought length, and droughts lasting up to 20 days reduced runoff to below 1% of the water budget. These results allow for the prediction of when and how much phosphorus could be mobilized. This watershed represents a common condition throughout central Kentucky and shows that strategies focusing on these sources could reduce phosphorus from major rivers.