CRACK-NP: a pesticide leaching model for cracking clay soils

The CRACK-NP model describes the movement of water and solutes (nitrates an d pesticides) in cracked clay soils. CRACK-NP differs from other models in that it assumes water moves around peds which sorb and desorb water, but wi thin which there is no upward or downward flux. This model is thus restrict ed to those soils in which macropore flow is the dominant mode of water mov ement. The major interaction between macropore and soil matrix components i s controlled by the size of the peds, which is a measurable soil parameter. The water movement and solute (pesticide) components of this model are des cribed. Pesticide degrades according to an exponential function, with coeff icients dependent on temperature and moisture content. Partitioning of pest icide between adsorbed and soluble phases in the soil is described in the m odel by the Freundlich equation. The application of the model to the leachi ng of pesticides at the Brimstone Farm experimental site in Oxfordshire, UK , is described. The model is shown to reproduce the hydrological behaviour of the site for an independent test period, and to replicate some of the ch aracteristics of the pattern of nitrate leaching. Applying the model to the estimation of pesticide leaching for the test periods is identified for th e comparison exercise, the model was capable of predicting the timing of th e main pesticide leaching peak. For the shorter period for which detailed d ata were available, the model predicted the pesticide concentrations in the drainwater moderately well. (C) 2000 Elsevier Science B.V. All rights rese rved.