Modelling mean nitrate leaching from spatially variable fields using effective hydraulic parameters

When using simulation models for estimating the mean nitrate leaching on di fferent soil types, the common approach is to interpret the field as a sing le equivalent soil column using effective hydraulic parameters, which are e stimated from point measurements. The use of effective hydraulic parameters was evaluated on a coarse sandy soil and a sandy loam using the one-dimens ional mechanistic model, DAISY. On each location, texture, soil water reten tion and hydraulic conductivity from 57 points were measured within an area of ca. 0.25 ha. The following approaches for estimation of effective hydra ulic conductivity were examined: (1) geometric mean; (2) arithmetic mean; ( 3) estimated arithmetic mean from a lognormal distribution; and (4) mean es timated from a stochastic large-scale model for water flow, similar to the Richards equation in one dimension, but with large-scale effective paramete rs accounting for the local three-dimensional flow. The approach of interpr eting the field as a number of non-interacting columns was examined by calc ulating the mean of the field as the mean of the 57 soil columns. The nitra te concentrations simulated by DAISY were compared with nitrate concentrati ons measured by ceramic suction cups at the 57 points at 25 cm and 80 cm de pths during the winter period 1989/1990. At both locations, the nitrate con centrations simulated by the geometric mean, the stochastic approach and th e mean of the 57 simulations matched the observed nitrate concentrations wh ile the other approaches gave unreliable results on the coarse sand. Hence, to simplify the calculations the geometric mean can be used. (C) 1999 Else vier Science B.V. All rights reserved.