ABSTRACT. In the last four decades, a plethora of models has been developed to simulate nonpoint-source (NPS) odeling nonpoint-source (NPS) pollutant fate and transport processes across multiple scales is fundamental to addressing a number of environmental and natural resource issues, including degradation of soil and contamination of surface and ground waters. Landscape NPS models are currently used for a variety of purposes, including registration of pesticides and other agrochemicals, designing soil conservation practices, water table management, prevention of chemical pollution of surface water bodies and groundwater, protection of aquatic biota and development of Total Maximum Daily Loads (TMDLs). Virtually all federal agencies currently have their own models or support models developed elsewhere. According to Singh and Woolhiser (2002), "In the years ahead, the models will become even more common and will play an increasing role in our day-to-day lives." Since NPS pollutant transport is mainly driven by meteorological events, the early to mid-20th century saw the development of mathematical descriptions of individual hydrologic components (e.g., infiltration, runoff, evapotranspiration, and interception). The digital revolution of 1960s witnessed the integration of individual hydrologic components (Singh and Woolhiser, 2002) into functional models that can be applied at various spatial and temporal scales. The development of NPS models in the U.S. began in response to the Clean Water Act (Arnold and Fohrer, 2005). Examples of these models include AGNPS (Young et al., 1987), AnnAGNPS (Bingner and Theurer, 2003), ANSWERS Dillaha, 1996, 2000), APEX (Williams and Izaurralde, 2005), CREAMS (Knisel 1980), DRAINMOD (Skaggs, 2007, EPIC (Williams et al., 1984), GLEAMS (Leonard et al., 1987;Knisel and Davis, 1999), GWLF (Haith and Shoemaker, 1987), HSPF (Bicknell et al., 2001), HY-DRUS (Šimůnek et al., 2005, 2006, KINEROS (Woolhiser et al., 1990), LEACHM (Hutson and Wagenet, 1992), MAC-RO (Jarvis, 1994), NLEAP (Shaffer et al., 1991), RUSLE (Renard et al., 1991), RZWQM (Ahuja and Hebson, 1992) , SWAT (Neitsch et al., 2002), and WEPP (Laflen et al., 1991). These models tend to be scale dependent and are usually suited for profile/horizon/pedon (point scale), field/farm (field scale), or watershed scale; however, there are a few exceptions.The land phase of the hydrologic cycle is complicated and includes a great deal of uncertainty (Singh and Woolhiser, 2002) because of the spatial and temporal heterogeneity of soil properties, land use, precipitation, etc. Currently, a model that simultaneously operates at multiple spatial scales does not exist (the hierarchies of scales at which NPS models are M