Nitrate loss in soil drainage waters in relation to by-passing flow and discharge on an arable site

In a well-structured soil at or close to pedal saturation, rainfall intensity in excess of pedal K_sat is predicted to result in by-passing flow through well connected structural voids. The depth of solute penetration (tracer dye and nitrate) under by-passing flow has been predicted by D_f=(P-C_p)/θ_f, where D_f is the depth of penetration of the leading edge of solute in by-passing flow, P is gross precipitation, C_p is the input volume transmitted through the soil peds and θ_f is the operational volumetric moisture content in by-passing. The timing of surface applied nitrate arrival at a channel bank is demonstrated to be related to by-passing flow rather than uniform displacement. The relationship between nitrate load in soil drainage waters (Y, mg s^-1) and water discharge (X, dm³ s^?1) was Y～6X after nitrate fertilizer application but subsequently declined to Y 0.5X It is deduced that the decline resulted from inefficient leaching under by-passing conditions once surface applied nitrate had become relocated within the soil peds.