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چکیده
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The concept of variable source area (VSA) describes that runoff is generated from a small portion of a watershed rather than the entire watershed (Hewlett, 1961). These areas are hydrologically active, dynamic, sensitive and have higher potential of generating runoff because they are the first places that become saturated (Walter et al., 2000). The measurement of surface runoff and soil moisture are of necessities for studying variable source area concept in a watershed. Analog and digital sensors, as appropriate tools, have been used for various aspects of hydrological studies and environmental engineering during the last decades (Vivoni and Camilli, 2003; Hart and Martinez, 2006; Freiberger et al., 2007); however, few studies have applied sensors for variable source area monitoring and modeling in watersheds. Zollweg (1996) successfully designed and tested a non-automated saturation sensor applicable for delineation of surface saturation areas in VSAs studies. Srinivasan et al. (2000 and 2002) automated the sensors designed by Zollweg (1996) and successfully calibrated and validated them for the purpose of identification of runoff generating areas in a 26-ha watershed in east-central Pennsylvania. Chaubey et al. (2006) and Leh et al. (2008) successfully used automated surface and subsurface sensors designed by Srinivasan et al. (2000 and 2002) for identification of critical source areas of a 1250-ha watershed in Arkansas. Sen et al. (2008) have recently used a paired surface and subsurface runoff sensors to delineate variable source areas in a small pasture watershed in Alabama. The above-mentioned sensors do not have the capability of measuring the depth of surface runoff over different time intervals to be able to monitor the variability of flow with time at various points of the watersheds. Therefore, alternative approaches are needed to study the spatial and temporal variability of hydrologic response of variable source areas in the watersheds. Recent advances
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