New study evaluates effects of agricultural conservation practices on nitrogen in streams of the Chesapeake Bay Active
Issue: Adaptive management in support of Chesapeake Bay restoration is complicated by uncertainty about the effects of agricultural management practices on water quality. Despite increasing investment, effects of agricultural conservation practices on regional water quality remain difficult to quantify due to factors such as groundwater travel times, varying modes-of-action, and the general lack of high-quality spatial datasets representing practice implementation. Given the major role that agriculture plays in the nitrogen budget of the Chesapeake Bay watershed, achieving planned load reductions depends on effective implementation of agricultural conservation practices. Although the net effects of such conservation actions suggest substantial expected declines in nitrogen fluxes in the watershed, observed nitrogen trends in bay tributaries have been mixed.
USGS Study
The regional effects of “high-impact” and “other-impact” agricultural conservation practices on nitrogen fluxes in streams of the Chesapeake Bay watershed were estimated in this study. Spatially referenced regression (SPARROW) modeling was used to relate detailed records of agricultural conservation practices acquired from the U.S. Department of Agriculture (USDA) along with known sources and other watershed conditions to mean-annual flow-normalized nitrogen fluxes estimated at selected locations on watershed tributaries. The model was then used to estimate effects of conservation practices on nitrogen fluxes to the bay and in each of more than 80,000 tributary stream reaches within the bay watershed. An additional SPARROW model developed using agricultural conservation practices averaged over individual counties supports inferences about the importance of the spatial resolution of available information on conservation practices to such regional estimates.
Major Findings
Conservation practices classified as “high-impact” were estimated to be effective at reducing nitrogen loads to streams of the Chesapeake Bay watershed in areas where groundwater ages are expected to be less than 14-years old. Watershed-wide, these high-impact practices were estimated to reduce nitrogen loads to streams by 1.45%, with up to 60% reductions in areas with shorter groundwater ages and larger amounts of implementation (Fig 1). Effects of “other-impact” practices and practices in areas with groundwater ages of greater than 14 years were less apparent. A model recalibrated using high-impact agricultural conservation practice data summarized by county suggests effects may also be detectable using implementation data available at such coarser resolution.
Reductions in nitrogen loads delivered to streams due to high-impact conservation practices are expected throughout the watershed, with larger reductions in areas of Pennsylvania and the Shenandoah Valley in Virginia.
Management Applications
The results of this study provide support for the effectiveness of “high-impact” conservation practices at reducing nitrogen fluxes to streams while demonstrating that current tools can detect a statistically significant relationship between conservation practice implementation and decreased nitrogen fluxes when they occur. This knowledge can improve decisions regarding the geographic placement and intensity of implementation to maximize the potential for measurable benefits.
That the discernable impact of agricultural conservation practices was limited to areas with a median groundwater age of less than 14 years does not imply that conservation practices are ineffective in areas with older groundwater ages. Rather, these SPARROW models had more difficulty detecting the effect of these selections of conservation practices in those areas when compared to contemporary (2012) water quality and the effects of these practices would be expected to take longer in areas with older groundwater ages.
While changes in water quality due to the implementation of high-impact conservation practices were detected, our models did not associate a significant reduction of N fluxes with the “other-impact” group of conservation practices that were expected to have a more moderate effect. This analysis does not indicate that conservation practices in the other-impact classification are not effective but that this SPARROW model does not provide evidence of their efficacy. This group represents a wider range of BMP types, increasing the likelihood of greater variability in effectiveness or other factors that might interact with BMP function. The other-impact BMPs are expected to be less effective than high-impact BMPs, potentially requiring more implementation for an effect that is detectable in monitoring data in 2012.
Finally, an analysis of the delivery variation factor (DVF) for “high-impact” agricultural conservation practices suggests that a relatively small amount of high-impact conservation practice implementation can result in substantial decreases in delivery nitrogen to streams with younger groundwater ages. However, there are also diminishing returns ¬for reductions in nitrogen flux with increasing conservation practice implementation in a single stream segment. This supports a strategy of more practices spread out over a larger targeted region rather than more intense implementation in an individual reach.
For More Information
Sekellick AJ, Ator SW, Devereux OH, Keisman JL (2023) Estimated reduction of nitrogen in streams of the Chesapeake Bay in areas with agricultural conservation practices. PLOS Water 2(5): e0000108. https://doi.org/10.1371/journal.pwat.0000108
Issue: Adaptive management in support of Chesapeake Bay restoration is complicated by uncertainty about the effects of agricultural management practices on water quality. Despite increasing investment, effects of agricultural conservation practices on regional water quality remain difficult to quantify due to factors such as groundwater travel times, varying modes-of-action, and the general lack of high-quality spatial datasets representing practice implementation. Given the major role that agriculture plays in the nitrogen budget of the Chesapeake Bay watershed, achieving planned load reductions depends on effective implementation of agricultural conservation practices. Although the net effects of such conservation actions suggest substantial expected declines in nitrogen fluxes in the watershed, observed nitrogen trends in bay tributaries have been mixed.
USGS Study
The regional effects of “high-impact” and “other-impact” agricultural conservation practices on nitrogen fluxes in streams of the Chesapeake Bay watershed were estimated in this study. Spatially referenced regression (SPARROW) modeling was used to relate detailed records of agricultural conservation practices acquired from the U.S. Department of Agriculture (USDA) along with known sources and other watershed conditions to mean-annual flow-normalized nitrogen fluxes estimated at selected locations on watershed tributaries. The model was then used to estimate effects of conservation practices on nitrogen fluxes to the bay and in each of more than 80,000 tributary stream reaches within the bay watershed. An additional SPARROW model developed using agricultural conservation practices averaged over individual counties supports inferences about the importance of the spatial resolution of available information on conservation practices to such regional estimates.
Major Findings
Conservation practices classified as “high-impact” were estimated to be effective at reducing nitrogen loads to streams of the Chesapeake Bay watershed in areas where groundwater ages are expected to be less than 14-years old. Watershed-wide, these high-impact practices were estimated to reduce nitrogen loads to streams by 1.45%, with up to 60% reductions in areas with shorter groundwater ages and larger amounts of implementation (Fig 1). Effects of “other-impact” practices and practices in areas with groundwater ages of greater than 14 years were less apparent. A model recalibrated using high-impact agricultural conservation practice data summarized by county suggests effects may also be detectable using implementation data available at such coarser resolution.
Reductions in nitrogen loads delivered to streams due to high-impact conservation practices are expected throughout the watershed, with larger reductions in areas of Pennsylvania and the Shenandoah Valley in Virginia.
Management Applications
The results of this study provide support for the effectiveness of “high-impact” conservation practices at reducing nitrogen fluxes to streams while demonstrating that current tools can detect a statistically significant relationship between conservation practice implementation and decreased nitrogen fluxes when they occur. This knowledge can improve decisions regarding the geographic placement and intensity of implementation to maximize the potential for measurable benefits.
That the discernable impact of agricultural conservation practices was limited to areas with a median groundwater age of less than 14 years does not imply that conservation practices are ineffective in areas with older groundwater ages. Rather, these SPARROW models had more difficulty detecting the effect of these selections of conservation practices in those areas when compared to contemporary (2012) water quality and the effects of these practices would be expected to take longer in areas with older groundwater ages.
While changes in water quality due to the implementation of high-impact conservation practices were detected, our models did not associate a significant reduction of N fluxes with the “other-impact” group of conservation practices that were expected to have a more moderate effect. This analysis does not indicate that conservation practices in the other-impact classification are not effective but that this SPARROW model does not provide evidence of their efficacy. This group represents a wider range of BMP types, increasing the likelihood of greater variability in effectiveness or other factors that might interact with BMP function. The other-impact BMPs are expected to be less effective than high-impact BMPs, potentially requiring more implementation for an effect that is detectable in monitoring data in 2012.
Finally, an analysis of the delivery variation factor (DVF) for “high-impact” agricultural conservation practices suggests that a relatively small amount of high-impact conservation practice implementation can result in substantial decreases in delivery nitrogen to streams with younger groundwater ages. However, there are also diminishing returns ¬for reductions in nitrogen flux with increasing conservation practice implementation in a single stream segment. This supports a strategy of more practices spread out over a larger targeted region rather than more intense implementation in an individual reach.
For More Information
Sekellick AJ, Ator SW, Devereux OH, Keisman JL (2023) Estimated reduction of nitrogen in streams of the Chesapeake Bay in areas with agricultural conservation practices. PLOS Water 2(5): e0000108. https://doi.org/10.1371/journal.pwat.0000108