Nitrogen transformation component for SHETRAN catchment nitrate transport modelling

The capability to simulate nitrogen transformations has been added to the SHETRAN physically based, spatially distributed river catchment modelling system so that it can be used in 3D simulations of coupled flow and nitrate transport. In SHETRAN, the subsurface is a variably saturated heterogeneous region, comprising perched, unconfined, confined and unsaturated systems, and at the surface there is vegetation and water flow overland and in stream networks. Nitrate transport is modelled in SHETRAN using advection–dispersion equations with terms added for adsorption and a two-region (dynamic region and dead-space) representation. The nitrogen transformations taking place in and below the root zone are modelled using NITS (Nitrate Integrated Transformation component for SHETRAN), which was designed to be comprehensive, self-consistent and fully compatible with SHETRAN. NITS has pools for both carbon and nitrogen in manure, litter and humus, and further pools for ammonium and nitrate, and involves the simultaneous solution of seven ordinary differential equations plus several auxiliary equations. NITS and its integration in SHETRAN are described here, as are a series of successful verification simulations for the responses of the carbon and nitrogen pools when straw and manure are added to the soil, and a successful field validation for nitrate generation and leaching in a fertilised barley plot. The NITS equations strictly apply at a point, and are used in SHETRAN with distributed parameters (i.e. each finite difference cell in SHETRAN has its own set of transformation variables and parameters). The intention is that in addition to being used in simulations of nitrate pollution and the effectiveness of proposed remedial measures and changes in agricultural practice, SHETRAN will be used in studies of the ‘upscaling’ of the equations and parameters for nitrate transport using the ‘UP’ approach of Ewen [Ewen, J., 1997. Hydrol. Earth System Sci. 1, 125–136]. In Birkinshaw and Ewen [Birkinshaw, S.J., Ewen, J., 2000. Modelling nitrate transport in the Slapton Wood catchment using SHETRAN. J. Hydrol. 230, 18–33.] SHETRAN is used to simulate the generation of nitrate following the application of fertiliser in the Slapton Wood catchment, Devon, UK, and the subsequent leaching, lateral subsurface transport and discharge of nitrate to the ground surface and its transport in the Slapton Wood stream.