Catchment models determine the source and extent of water quality problems in a catchment. Catchment models may be used to identify 'hotspots' in a region and once calibrated these models may be used to test various land management, land use, and climate-change scenarios. SWAT (Soil & Water assessment tool) is a relatively complex model. The development of SWAT is a continuation of USDA Agricultural Research Service (ARS) modelling experience spanning more than 30 years. SWAT has been used with some degree of success by the LERNZ group at The University of Waikato and it has been applied to several New Zealand catchments. The model is capable of producing daily discharge and nutrient and sediment loads to streams at a sub-catchment level. ArcSWAT is an ArcGIS-ArcView extension and graphical user input interface for SWAT.