A transfer function model for the prediction of nitrate leaching under field conditions

Nitrate leaching by rainfall in a mole-and-tile drained clay soil catchment was monitored over several seasons. Sample realizations of the stochastic leaching process were observed for individual rainfall events of varying intensity and duration. Rainfall was treated as a pulse input to the soil volume within which existed an operationally defined transport volume, Vst. Within Vst solute could experience various physical, chemical and biological reaction on its passage to the drains. The probability density functions (pdfs) of solute travel times were determined from the normalized rates of loss of chloride or nitrate mass to the drains. The composite pdfs for the whole system (soil plus drains) conformed to a lognormal distribution and could be deconvoluted by equating the mean travel time from the surface to mole drain depth (tL) to the difference between the overall mean travel time (t), and tD, the mean travel time in the drains alone. The fractional transport volume θst for each event was calculated from the mean rainfall intensity q0, the average travel time tL and the depth of the mole drains L. The transport volume Vst was calculated from θst and the whole soil volume. An equation giving the quantity of N leached during each event (ΔNcalc) was developed using Vst from the transfer function, the cumulative drainage during an event and the initial soil solution nitrate concentration C0. The relationship between ΔNcalc and measured amounts of N leached (ΔNmeas) for 10 of the 11 events was virtually 1:1. The exceptional event had a long input of very low intensity rainfall. ΔNcalc obtained using a value of θst averaged over all 11 events, and ΔNcalc obtained when the ratio of total rainfall to mean drainage rate was used to predict tL for each event, were closely related to ΔNmeas for 10 of the 11 events. This indicated that the transfer function approach could be used to predict nitrate leaching losses under variable field conditions, provided that the value of C0 at the start of the leaching season was known and estimates of any major mineral N gains or losses by other processes during the season could be made.

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