Scottish Regional and District Estimates of Non-Point Source Nitrate Nitrogen (No3-N) Loadings Using AVHRR Satellite Data

Abstract

Part of the nitrate load in UK surface waters is derived from non-point sources, principally agricultural land. Estimates of agricultural nitrate loss can be derived using an empirical model. This model is developed using catchments where nitrate losses, agricultural and non-agricultural areas are all known, the latter being derived from high resolution LANDSAT TM imagery. Such a model has been developed for Grampian Region in NE Scotland. in extending its use to deriving national estimates of agricultural nitrate loss, problems were encountered in obtaining appropriate cloud-free LANDSAT TM imagery. Low resolution AVHRR was investigated as an alternative source of data on agricultural and non-agricultural areas. By comparing available land cover information with an AVHRR land cover classification using visible and infrared radiance data together with a banded altitude image, a model was developed that accounted for the agricultural/non-agricuttural land mixture within each AVHRR pixel. Using this model, it was then possible to provide preliminary national estimates for nitrate loss on a district by district basis. the preliminary national nitrate loss estimates correlate well with published River Board information. AVHRR National estimates indicate that long term NO₃-N losses through Scottish surface waters are of the order of 54 000 tonnes per annum, equivalent to a fertilizer NO₃-N loss of approx. £17 million). A geographic breakdown of nitrate loss in Scotland indicates that the North Sea receives the bulk of this.