The impact of enhanced cover crop coverage on water quality at three spatial scales.

Water quality in the Choptank Basin is significantly impacted by agriculture, which covers 61% of the land area. Historically, nitrogen (N) and phosphorus (P) concentrations have been increasing from the upper portion of the watershed since 1970, when USGS monitoring started. We have been monitoring water quality at the outlets of 15 small (8.5 to 51.4 km²), agriculturally dominated watersheds since 2003, and we have been intensively monitoring within four of these watersheds since 2013. In 2015, we started encouraging and financially assisting farmers to implement additional best management practices (examples: cover crops, controlled drainage structures) to the properties they own or farm. Within one of these watersheds, we have had 85% of the active agricultural area (predominantly corn and soybeans) planted in winter cover crop for the past two years, and 75% was under cover crop in 2015. Since 2006, the % cover crop has increased by 2.0% y^-1, primarily due to the Maryland Cover Crop program prior to the start of our enhanced BMP push. Stream total nitrogen (TN) concentrations started to decrease in 2011, and annual average stream TN concentrations decreased by 10.0% from 2011 to 2017. With the intense application of cover crop over the past two years, average annual stream TNdecreased from 324 μM in 2014 to 214 μM in 2017 at an intermediate monitoring site within the watershed. Prior to 2015, this area had limited cover crop coverage. TNconcentration in three ditches on a farm planted in winter cover crop in fall 2015, 2016, and 2017 (but not prior to these years) decreased by an average of 37% from pre-cover crop conditions to post. Preliminary analysis suggests a significant positive impact of wide-spread cover crop application in this watershed. However, one of the other intensively monitored watersheds had 85% of active agriculture under cover crop by 2015-2017 and stream TN concentrations at the watershed outlet continue to increase, indicating that more than just cover crop coverage impacts stream TN concentrations. We have one more year of funded monitoring, and we hope to observe decreasing N concentrations during this time. Significant decreases in stream N draining from small streams in the upper Choptank are necessary to improve downstream water quality in the estuary.