Concepts and terms used in previous multidisciplinary studies of tile-drained aquitard-dominated catchments (TDADC) are inconsistent and confusing. We provide a well-defined, comprehensive conceptual model of the subsurface hydrology of TDADC by selecting seven mutually compatible and consistent concepts. These concepts are: (1) groundwater as the main source of baseflow in headwater streams,(2) dominance of 'pre-event' water in stormflow, (3) importance of both macropores and matrix, (4) changes in flowpaths with rate of stream discharge, (5) dominance of shallow, lateral subsurface flow, (6) interactive nature of subsurface water, (7) transpiration of groundwater. This conceptual model was successfully 'field-tested' by examining data collected in a TDADC in a rural area of southern Ontario, Canada.The data consist mainly of chemical and isotopes tracers in water samples (headwater streams, groundwater, precipitation, tile water, soil-surface water), supplemented by water levels and meteorological data.

Nutrient concentrations in riparian groundwater are variable from site to site, making it challenging to use these data to quantify nutrient fluxes contributed by groundwater discharge to headwater streams. Instead, we define event‐excluded baseflow (BFEE), and use stream discharge data and stream samples collected during BFEE to estimate groundwater discharge and loading of nutrients to the streams. Based on this method, in two study catchments (Thames River basin, Ontario, Canada) groundwater discharge during BFEE contributed about 30% of the total streamflow and 20%–30% of the total loading of soluble reactive phosphorus (SRP) and nitrate. Previous estimates of baseflow using hydrographic separation techniques indicate that groundwater discharge likely contributes even larger fractions, ~50%–60% of flow in these catchments. We infer that groundwater likely contributes ~40% of the annual SRP load and ~ 50%–60% of the nitrate‐N load. These results indicate that groundwater discharge plays an important role in the nutrient loading to headwater streams. This reinforces the findings of earlier studies that have inferred a dominant influence of legacy reservoirs of nutrients in the subsurface of similar agricultural catchments on the loads of these nutrients in headwater streams.

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We investigated the influence of land cover on nutrient concentrations (295 samples) in headwater streams over a 2-year period in 10 agriculture-dominated subcatchments (163-8373 ha) in southern Ontario Canada. In this region, monitoring and research on nutrient dynamics in headwater wetlands is sparse. Our results indicated a significant positive correlation (Pearson coefficient ρ = 0.320) between soluble reactive phosphorus (SRP) in the headwater streams and the percentage of wetlands in these agriculture-dominated catchments. This result suggests that headwater wetlands, and other wet riparian zones, are key sources of SRP in the headwater streams.Nitrate concentrations were positively correlated with % agricultural land cover (ρ = 0.316), consistent with previous studies, while SRP concentrations were negatively correlated with % agricultural land cover (ρ = À0.325). There was a significant positive correlation between SRP concentrations and discharge in some of the streams. Seasonal SRP trends appear to be closely related to temperature-dependent seasonal changes in redox conditions, including levels of dissolved O 2 . Surficial geology had some influence on nitrate concentrations in the streams, which tended to be higher in catchments dominated by glacial till (till terrain), compared to catchments with extensive areas of outwash sand, in addition to till terrain.