Submitted to: Journal of Hydrology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/1/2012
Publication Date: 5/8/2012
Citation: Kennedy, C.D., Bataille, C., Liu, Z., Ale, S., Vandevelde, J., Roswell, C.R., Bowling, L.C., Bowen, G.J. 2012. Dynamics of nitrate and chloride during storm events in agricultural catchments with different subsurface drainage intensity (Indiana, USA). Journal of Hydrology. DOI: 10.1016/j.hydrol.2012.05.002. Interpretive Summary: Subsurface drainage of many poorly drained agricultural fields in the Midwestern U.S. is believed to “short circuit” nitrate flow paths to streams, degrading downstream water quality and contributing to the occurrence of harmful algal blooms. Although much is known about the mechanisms controlling this regionally pervasive practice of artificial drainage (popularly known as “tile drainage”), an integrative assessment of the catchment-scale effect of drainage density (i.e., the number of tile drains per unit area) on the transport of nitrogen in streams is lacking. In this study, we found that drainage density affected the contribution of subsurface hydrological pools to streamflow during storm events: higher drainage density resulted in increased transport of near-surface water following dry periods and increased transport of soil water following wet periods. With respect to nutrient management, these findings suggest that the impact of drainage density on the transport of nitrate depends, in part, on antecedent hydrological conditions and the timing of storm events.
Technical Abstract: Grids of perforated pipe buried beneath many poorly drained agricultural fields in the Midwestern U.S. are believed to “short circuit” pools of nitrate-laden soil water and shallow groundwater directly into streams that eventually discharge to the Mississippi River. Although much is known about the mechanisms controlling this regionally pervasive practice of artificial (popularly known as “tile drainage”) at the field-plot scale, an integrative assessment of the effect of drainage density (i.e., the number of tile drains per unit area) on the transport of nutrients and solutes in streams at the catchment scale is lacking. In this study, we used hydrometric, chemical, and isotopic data to quantify stream exports and to infer hydrological transport pathways of two solutes (nitrate and chloride) derived from different sources (fertilizers and de-icing road salts) and exhibiting different chemical properties (chemically non-conservative and conservative) during a winter storm for two catchments located in north-central Indiana and lying within the Wabash River Basin, a major source of nitrate to the Mississippi River that discharges to the Gulf of Mexico. The proximally located catchments differ primarily in drainage density (70 vs. 31%, by catchment area), with essentially all other characteristics remaining equal. Our study revealed two significant relationships between increased drainage density and stream nitrate and chloride dynamics during storms: (1) more near-surface storm event water (dilute in both nitrate and chloride) was transported early in the storm (preceding an exceptionally dry period), and (2) higher transport of chloride-laden pre-event soil water relative to shallow groundwater elevated in nitrate occurred in response to the “wetting” of the catchments later in the storm. These patterns are consistent with a proposed conceptual model relating drainage density to the physical interception of infiltrating soil water by tile drains, in which increased drainage density results in (1) greater transport of soil water to streams and (2) delayed rise in the water table. With respect to nutrient management implications, these finding suggests that increased drainage density impacts subsurface pools of chloride and nitrate differently, providing the impetus for future work aimed at better understanding soil/ground water interactions in artificially drained agricultural catchments.