Land use changes are identified as major threats for Hawaii's coral reef ecosystem and cause great impact on the sustainability of Hawaii's natural resources and environmental quality. An integrated approach is vital for successful management of watershed ecosystems; one of its main components is numerical modeling, which is becoming increasingly popular among natural resource managers. The main goal of this work is to evaluate the performance of N-SPECT as a management tool to evaluate the effect of land use on pollutant movement through coastal watersheds, using field measured stream flow and sediment data. N-SPECT is a GIS-based screening level tool that uses land cover, soils, topography, and precipitation data to estimate surface runoff, nonpoint-source pollution, and erosion. N-SPECT is intent to facilitate the comparison of management scenarios by permitting rapid evaluation of potential impacts of future and historical changes in land cover and land use. The study was conducted in Hanalei watershed, a 61 km2 area that extends from the Mount Waialeale (1570 m) to Hanalei Bay, located in the northern part of Kauai Island, Hawaii. The watershed is represented by steep mountain slopes and deep fluvial valleys, through which flows the upper 20 km of Hanalei River with its tributaries. The Hanalei watershed supports an array of land uses such as residential, recreational, agriculture, biodiversity, and preservation of a native culture. The calibration and validation used 2003-2004 stream flow and sediment data. The performance of the model compared to measured data is discussed.