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Rangeland landscapes are complex mosaics of geology, topography, soils, plants, and climate. Ecological understanding and land management is facilitated by forming homogeneous recognizable groups such as the ecological site. Ecological site concepts are based on stands of native plants with similar climate, soils, and hydrology requirements that occur in a relatively repeated fashion across a landscape. The USDA-NRCS organizes ecological sites within a Major Land Resource Area (MLRA)/Land Resource Unit (LRU). Ecological sites provide a basis for identifying and mapping community dynamics and ecosystem responses to management and disturbances [state-and-transition models (S & T), establishing ecologically based reference information for rangeland health, targeting conservation practices, and communicating regarding ecosystem responses to management]. Since water is the most limiting factor on rangelands; understanding and documenting hydrologic and erosion responses to ecological site changes, as described by the S&T model, are integral to the dynamics of the ecological site. The S&T model can be used to outline trends and changes in hydrology, erosion potentials, and the overall water budget for the respective states.