Effect Of Seasonal Variations In Flow On Woodchip Bioreactor Design In Tile-Drained Watersheds.

The effectiveness of conservation practices are governed in part by the spatial and temporal patterns of water flow as runoff and subsurface (tile) drainage. The variability in patterns of nitrate loss were examined using data from two watersheds located in central Iowa. Woodchip bioreactors reduce nitrate denitrification and existing data suggests that the fraction of nitrate removed is governed by hydraulic residence time.  Nitrate loss at the watershed scale is correlated with flow and we used base flow as an estimate of tile drainage.  Load duration curves showed that days with the top 10% of daily baseflows delivered between 14 and 40% of the total N load.  Sizing bioreactors to treat 80% of daily base flows with an residence time of 0.5 days could result in watershed-scale reductions in nitrate of 20 to 50% based on actual past data.  The cumulative size of bioreactors could reach 0.27% of the total driange area.