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

The CRACK-NP model describes the movement of water and solutes (nitrates and 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 within which there is no upward or downward flux. This model is thus restricted to those soils in which macropore flow is the dominant mode of water movement. The major interaction between macropore and soil matrix components is controlled by the size of the peds, which is a measurable soil parameter. The water movement and solute (pesticide) components of this model are described. Pesticide degrades according to an exponential function, with coefficients dependent on temperature and moisture content. Partitioning of pesticide between adsorbed and soluble phases in the soil is described in the model by the Freundlich equation. The application of the model to the leaching 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 characteristics of the pattern of nitrate leaching. Applying the model to the estimation of pesticide leaching for the test periods is identified for the comparison exercise, the model was capable of predicting the timing of the main pesticide leaching peak. For the shorter period for which detailed data were available, the model predicted the pesticide concentrations in the drainwater moderately well.