Diffuse pollution is often responsible for observed concentrations of agricultural compounds being in excess of the upper limits prescribed by the EU Water Framework Directive (WFD) and reductions in these concentrations will probably require widespread changes in farm practice. One of the aims of the UK RELU Catchment Hydrology, Resources, Economics and Management (ChREAM) study was to assess likely impacts of WFD implementation on agricultural land use and consequent implications for water quality and farm incomes. This has involved combining data from various sources into a hydrological-economic model, incorporating an existing diffuse pollution model updated to reflect present-day land use profiles. Combining agricultural land use data with hydrological spatial units can involve a number of problems arising from the integration of a variety of data formats at a range of spatial and temporal resolutions and the aggregation of source data over different spatial extents. This paper assesses uncertainty arising from areal interpolation of agricultural census data to hydrological units. The work is illustrated through a case study of the River Derwent catchment in north-east England. The study identifies the range of spatial resolutions at which robust estimations of agricultural land use can be made and indicates that the choice of interpolation method becomes important at smaller catchment scales. In this instance, choice of method starts to affect model outcome at the scale of the River Hertford sub-catchment (approximately 8000 ha) and is influenced by catchment shape and orientation with respect to source data. Implications for diffuse pollution modelling in support of policy implementation are examined.