Evaluating watershed-scale effectiveness of traditional agricultural BMPs

The following provides a quick snapshot of key findings.  A more in-depth discussion and references to the scientific literature can be found here.

We explored the impacts of expanding the current use of filter strips, cover crops, and no-till BMPs in controlling runoff with the Soil and Water Assessment Tool (SWAT) and found that when implemented singly and in combinations at levels currently considered feasible by farm experts, these BMPs reduced sediment and nutrient yields by only 0-11% relative to current values. We also found that targeting BMPs in high source locations (see above), rather than randomly, decreased nutrient yields more, whereas reduction in sediment yields was greatest when BMPs were located near the river outlet.

A more detailed analysis of increased BMP implementation strategies for the Maumee watershed pointed to the need for more aggressive implementation of multiple BMPs to substantially reduce loads.  For example, a 20% reduction in TP or DRP load requires implementing the BMPs on more than 50% of the agricultural land.

Figure 16

Climate impacts on BMP effectiveness We also used the SWAT model to explore the impacts of climate change on water flow, sediment yields, and nutrient yields. These watersheds showed consistent increases in sediment yield, with increases being larger under more pronounced climate scenarios.  We also found that under a warmer climate, sediment and nutrient yields would be greater from agricultural (e.g., Maumee and Sandusky) vs. forested watersheds (e.g., Grand in Ohio).  Total annual discharge increased 9-17% under a more pronounced climate scenario and 4-9% under a moderate climate scenario. Stream sediment yields increased by 9% and 23% for moderate and pronounced climate scenarios, respectively.

Figure 18

DRP yields decreased (-2% on average) under the moderate climate scenario and increased slightly (3%) in response to more pronounced climate change.  TP yields increased 4% under moderate climate change and 6% under pronounced climate change.

Importantly, while agricultural BMPs might be less effective under future climates, higher BMP implementation rates could still substantially offset anticipated increases in sediment and nutrient yields.

Figure 19