Floodplain restoration increases floodplain exchange and is implemented to mitigate watershed-scale water quality impairments such as eutrophication. However, the effect of multiple restoration projects on watershed-scale nutrient loads is not well studied. Here we evaluated the effects of Stage-0 and bankfull floodplain reconnection scenarios on watershed nitrate removal in a generic 4 th -order Virginia Piedmont watershed for small and sub-annual (monthly to 2-year) storms. We built a zero-order nitrate removal model atop an existing watershed HEC-RAS hydraulic model of cumulative and spatially varying restoration. We found watershed nitrate removal increased with percent of channel length restored, and was highly sensitive to floodplain nitrate removal rate ( k ). Yet removal was generally small, ranging up to 1% or 19% of the watershed load for full watershed restoration, for the median and 90 th -percentile k , respectively. Removal varied with channel network location (Strahler stream order) of both restoration and where effects on nitrate loads were evaluated, with greatest effects in the stream order where restoration occurred, and diminishing effects downstream. The effectiveness of restoration for nitrate load reduction at the outlet of the 4 th -order watershed increased with channel size (stream order) where restoration occurred due to greater flow generation and floodplain inundation durations in larger channels. The converse was also true, that headwater channels, popular for restoration, had minimal effect at the watershed scale. Overall, our results highlight the potential value and limitations of floodplain restoration in reducing nitrate export at the watershed scale.
Oehler et al. (Fri,) studied this question.