کامران چپی

Non-point sources of pollution are responsible for more than 50% of total water quality impairment (Novotny and Chesters, 1981). As the most important non-point source of pollution, runoff from agricultural lands, construction sites and urban storm water is a major mode of transporting suspended sediments and nutrients to streams and other water bodies, creating major water quality problems (Delaune et al., 2004). To better control the effects of non-point sources of pollution on water resources, one has to identify the areas in the watershed generating surface runoff and to investigate the mechanism by which runoff is generated. Two different mechanisms of runoff generation (infiltration-excess and saturation-excess) were evaluated in this study during three seasons of the years including summer, fall, and spring. Infiltration-excess mechanism (Horton, 1933) states that surface runoff is generated once the intensity of rainfall exceeds the infiltration capacity. When this condition is established, a uniform thin layer of surface runoff is generated over the entire watershed (Amerman and McGuinness, 1967). The validity of traditional Hortonian overland flow is questionable in humid areas, well-vegetated areas and areas with shallow surface soil such as in the East and Northeastern parts of the United States (Walter et al., 2003). In humid, well-vegetated areas, infiltration capacity is high enough to absorb all storm water such that no overland flow is observed (Hibbert and Troendle, 1988). In these areas, some portions of the land become saturated during the storm due to either the rising of groundwater (Sklash and Farvolden, 1979), or seepage (Freeze, 1969), or return flow (Hewlett and Hibbert, 1967) or exfiltrating subsurface stormflow (Hewlett, 1974), or concentration of subsurface moisture owing to concave topographic contours (Anderson, 1982), or direct precipitation (Wickel et al., 2008). The locations of saturated areas in a watershed are determined to be th