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

Abstract

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, V st. Within V st 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 ( t ̄ L ) to the difference between the overall mean travel time ( t ̄ ), and t ̄ D , the mean travel time in the drains alone. The fractional transport volume θ st for each event was calculated from the mean rainfall intensity q 0, the average travel time t ̄ L and the depth of the mole drains L. The transport volume V st was calculated from θ st and the whole soil volume. An equation giving the quantity of N leached during each event ( ΔN calc) was developed using V st from the transfer function, the cumulative drainage during an event and the initial soil solution nitrate concentration C 0. The relationship between ΔN calc and measured amounts of N leached ( ΔN meas) for 10 of the 11 events was virtually 1:1. The exceptional event had a long input of very low intensity rainfall. ΔN calc obtained using a value of θ st averaged over all 11 events, and ΔN calc obtained when the ratio of total rainfall to mean drainage rate was used to predict t ̄ L for each event, were closely related to ΔN meas 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 C 0 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.