Evaluation of uncalibrated preferential flow models against data for isoproturon movement to drains through a heavy clay soil

The uncalibrated predictive ability of four preferential flow models (CRACK -NP, MACRO/MACRO_DB, PLM, SWAT) has been evaluated against point rates of d rainflow and associated concentrations of isoproturon from a highly structu red and heterogeneous clay soil in the south of England. Data were availabl e for four plots for a number of storm events in each of three successive g rowing seasons. The mechanistic models CRACK-NP and MACRO generally gave re asonable estimates of drainflow over the three seasons, but under-estimated concentrations of isoproturon over a prolonged period in the first season and over-estimated them in the two remaining seasons. CRACK-NP simulated ma ximum concentrations of isoproturon over the first two events of each of th e three seasons of 156, 527 and 24.4 mug litre(-1), respectively, and match ed the observed data (465, 65.1 and 0.65 mug litre(-1)) slightly better tha n MACRO (69.1, 566 and 58.5 mug litre(-1). Automatic selection of parameter s from soils information within MACRO_DB reduced the emphasis on preferenti al flow relative to the stand-alone version of MACRO. This gave a poor simu lation of isoproturon breakthrough and simulated maximum concentrations wer e 0, 50.1 and 35.1 mug litre(-1), respectively. The capacity model PLM gave the best overall simulation of total drainflow for the first two events in each season, but over-estimated concentrations of isoproturon (967, 808 an d 51.3 mug litre(-1). The simple model SWAT represented total drainflow rea sonably well and gave the best simulation of maximum isoproturon concentrat ions (140, 80.2 and 8.2 mug litre(-1)). There was no clear advantage here i n using the mechanistic models rather than the simpler models. None of the models tested was able to simulate consistently the data set, and uncalibra ted modelling cannot be recommended for such artificially drained heavy cla y soils. (C) 2001 Society of Chemical Industry.