Study shows reforestation of flood-prone agricultural lands reduces surface water runoff and associated soil erosion

Studies shows reforestation of flood-prone agricultural lands reduces surface water runoff and associated soil erosionA new modeling study conducted by the USDA Forest Service Center for Bottomland Hardwoods Research highlights the role reforestation plays in reducing flooding and the volume of farmland-derived sediments in waterways in the Lower Mississippi River Alluvial Valley. The study has been accepted for publication in the journal Ecological Engineering. Project leads Ying Ouyang, Ted Leininger, and Matt Moran found that as the area of flood-prone agricultural land converted to forest increased, the total volume of water and the mass of sediment flowing from those areas into watersheds decreased. Sediments from agricultural lands are often associated with fertilizers and pesticides, which contribute to hypoxia (the “dead zone” caused by low oxygen levels) in the Gulf of Mexico. Reforestation would therefore tend to help reduce the volume of the nutrients flowing into rivers. The project was commissioned by the U.S. Endowment for Forestry and Communities (the Endowment) and co-funded by the Forest Service, State & Private Forestry.

“This study provides further evidence of the key role forests play in flood control and in reducing sediment flow from agricultural lands into our watersheds,” noted Carlton Owen, President & CEO of the Endowment. “The findings are particularly timely given the ecological enhancement efforts in the Gulf of Mexico’s watersheds associated with Deepwater Horizon oil spill restoration. Growing forests on frequently-flooded agricultural lands, even just the portion of farmlands nearest streams, significantly reduces the potential for fertilizers –a key contributor to hypoxia–to reach streams and rivers, and ultimately the Gulf of Mexico. Accelerating reforestation in the Gulf’s watersheds, where feasible and desired, could reduce hypoxia and the many economic and environmental challenges it creates.”

Because forest restoration doesn’t require as much fertilizer as does row-crop agriculture, nutrient volumes are lower on the same landscapes. Slower runoff of water from forest lands also allows sediments and associated nutrients such as nitrogen and phosphorus that contribute to hypoxia, to “settle out” before reaching the Gulf. An added economic and human safety benefit would result from the new forests reducing the velocity and severity of river flooding in the Lower Mississippi region, such as occurred during the recent Great Flood of 2011. The new forest areas would also provide regional economic benefits for landowners and local economies, along with improved wildlife habitat, recreation opportunities, and many other values.

The modeling study was conducted to investigate the impacts of reforestation (conversion of certain frequently-flooded agricultural land into forests near the streams being modeled) on water outflow attenuation and sediment load reduction. Two Lower Mississippi River Alluvial Valley watersheds were chosen for the study, the Lower Yazoo River Watershed in the southern part of the Yazoo River Basin, and the Peters Creek Watershed from the Yocona Sub-basin. Each offered the availability of field observed data that are necessary for model calibration and validation. The U.S. EPA’s BASIN-HSPF model was used for predicting the water outflow and sediment load.

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