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The Sustainable Nutrient Management Decision Support System (SNM-DSS) has been developed to provide farmers with advise on minimising Nitrogen and Phosphorous loss from land spread organic and mineral fertilisers. Within the SNM-DSS, the Hybrid Soil Moisture Deficit (SMD) model (Schulte et al., 2005) is used to assess when slurry spreading is safe and will not pose a risk of pollution to surface water. The Hybrid SMD model assumes a flat field situation where the only input of water is from rainfall, and drainage is not defined thus could be any combination of leaching or runoff. The model has been shown to be practical because it is good at predicting local water status and requires very little data to use. However, in reality fields receive water from upslope areas as well as from rainfall. Research is establishing whether Topographic Wetness Index (TWI) can be accurately calculated and then combined with the Hybrid SMD model to better predict the presence of gravity moveable water at any time. Time Domain Reflectometery (TDR) sensors have been installed on four hill slope transects to record soil volumetric water content over a 6-month period. The relationship between TDR data and predicted SMD has been examined and then linked to the modelled TWI at each location. Successfully linking TWI and the Hybrid SMD model will improve the SNM-DSS resulting in more accurate predictions of periods when there is a transport vector to link pollution sources and environmental targets and to better define safe slurry spreading periods.