Abstract:Understanding the variables regulating tile-flow response to precipitation in the US Midwest is critical for water quality management. This study (1) investigates the relationship between precipitation characteristics, antecedent water table depth and tile-flow response at a high temporal resolution during storms; and (2) determines the relative importance of macropore flow versus matrix flow in tile flow in a tile-drained soya bean field in Indiana. In spring, although variations in antecedent water table depth imparted some variation in tile-flow response to precipitation, bulk precipitation was the best predictor of mean tile flow, maximum tile flow, time to peak, and run-off ratio. The contribution of macropore flow to total flow significantly increased with precipitation amount, and macropore flow represented between 11 and 50% of total drain flow, with peak contributions between 15 and 74% of flow. For large storms (>6 cm bulk precipitation), cations data indicated a dilution of groundwater with new water as discharge peaked. Although no clear dilution or concentration patterns for Mg 2C or K C were observed for smaller tile flow generating events (<3 cm bulk precipitation), macropore flow still contributed between 11 and 17% of the total flow for these moderate size storms. Inter-drain comparison stressed the need to use triplicate or duplicate tile drain experiments when investigating tile drainage impact on water and N losses at the plot scale. These results significantly increase our understanding of the hydrological functioning of tile-drained fields in spring, when most N losses to streams occur in the US Midwest.

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Storm losses of N via tile-drainage in the US Midwest are a major concern for water quality in the Mississippi River Basin (MRB). This study investigates the impact of precipitation characteristics on NO 3 -, NH 4 ? and DON concentrations and fluxes for spring storms in tile-drains in a Midwestern agricultural watershed. Bulk precipitation amount had little impact on solute median concentrations in tile-drains during storms, but clearly impacted Mg 2? , K ? and NO 3 -concentration patterns. For large storms ([6 cm of bulk precipitation), large amounts of macropore flow (43-50% of total tile-drain flow) diluted Mg 2? and NO 3 -rich groundwater as discharge peaked. This pattern was not observed for NH 4? and DON or for smaller tile-flow generating events (\3 cm) during which macropore flow contributions were limited (11-17% of total tile-drain flow). Precipitation amount was positively (P \ 0.01) correlated to NO 3 -and NH 4 ? export rates, but not to DON export rates. Limited variations in antecedent water table depth in spring had little influence on N dynamics for the storms studied. Although significant differences in flow characteristics were observed between tile-drains, solute concentration dynamics and macropore flow contributions to total tile-drain flow were similar for adjacent tile-drains. Generally, NO 3 -represented [80% of N flux during storms, while DON and NH 4 ? represented only 2-14% and 1-7% of N flux, respectively. This study stresses the non-linear behavior of N losses to tile drains during spring storms in artificially drained landscapes of the US Midwest, at a critical time of the year for N management in the MRB.

This study investigates phosphorus (P) concentrations and fluxes in tile drains, overland flow, and streamflow at a high temporal resolution during 7 spring storms in anagricultural watershed in Indiana, USA. Research goals include a better understanding of 1) how bulk precipitation and antecedent moisture conditions affect P concentrations and fluxes at the watershed scale; 2) how P concentrations and fluxes measured in tile drains translate to the whole watershed scale; 3) whether P losses to the stream are significantly affected by overland flow. Results indicate that bulk precipitation and antecedent moisture conditions are not good predictors of SRP or TP losses (either concentration or flux) to the stream. However, along with previously published storm data in this watershed, results indicate a threshold-based behavior whereby SRP and TP fluxes significantly increase with precipitation when bulk precipitation exceeds 4 cm. Although total SRP and TP fluxes are very much driven by flow, SRP and TP fluxes are somewhat limited by the amount of P available for leaching for most storms. On average, SRP fluxes in tile drains are 13% greater than in the stream, and stream SRP fluxes account for 45% of TP fluxes at the watershed scale. Our results indicate that when P is the primary concern, best management practices aimed at reducing P losses via tile drains are likely to have the most effect on P exports at the watershed scale