The objective is to model on a catchment scale the movement and transformation of nitrate and pesticides in order to evaluate the effectiveness of agricultural management practices in controlling water quality
A modelling system has been designed to investigate field and catchment scale flow transport in a small agricultural watershed. Small scale simulations were carried out at discrete points on a transact using a field water balance model. It was shown that 3 different procedures to calculate the soil moisture characteristic resulted in large differences in simulated moisture contents, pressure heads, and fluxes, when a time series of 2 years was simulated: the use of locally measured points of the moisture characteristic curve, subsequently fitted to van Genuchten's parametric model;
the use of all locally measured points pertaining to the same soil horizon, subsequently fitted to van Genuchten's parametric model;
the use of pedotranfer functions to generate the moisture characteristic curve, starting from information on the particle size distribution and the organic matter content.
When compared to field observations, method (1) gave the best results, while method (3) drastically overestimated the moisture content profile. Large-scale simulations were carried out for the catchment as a whole. A comparison of the simulated discharge and the observed discharge indicated that the fluctuations in discharge were not adequately simulated. The inclusion of drains in the simulation, however, improved the simulated discharges. A comprehensive literature review on existing models for simulation of the transport and the fate of pesticides was carried out.
The realization of the project objective requires the development and testing of an integrated model framework. The framework will be composed of a distributed mechanistic hydrological model (the SHE model) and a mechanistic nitrogen and pesticide leaching model (the SWATNIT and LEACHP model). The SHE model is an advanced, physically based, distributed modelling system. The SWATNIT and LEACHP model is a mechanistic model developed for the simulation of the nitrogen and pesticide fluxes and transformations in the root zone of cropped and bare soils. The model will be tested on the catchment area of the river Ijzer (Belgium) of which a geographic information system is being developed containing the different layers of information needed to run the model framework.
In the project, special emphasis will be given to:
a sensitive analysis of the SWATNIT and LEACHP model as a basis for evaluating: the suitability of the model approach for describing the transformation and transfer processes of nitrates and pesticides at the scale of the unit of the soil map; the relative importance of the different modules used in describing the main processes governing the nitrogen and pesticide transformation and transfer; and the precision with which variables and model parameters have to be measured;
the calibration and validation of the SWATNIT and LEACHP model at a number of data sets collected at field scale;
testing of the SHE model on the Ijzer catchment applying the concept of effective parameter assessment, obtained by stochastic theory, for predicting large scale hydrological behaviour of the unsaturated zone;
the integration of the calibrated and validated SWATNIT and LEACHP model in the SHE model;
the development of pedotransfer functions for relating the reaction rate constants, response functions and model parameters to independent soil variables;
and the evaluation of the performance of the combined model framework, using data of the Ijzer catchment.
Funding SchemeCSC - Cost-sharing contracts