Mathematical theory for soil erosion and deposition processes on a plane land element was applied to data on runoff and sediment concentration as a function of time for nine runoff events from a small (1.3 ha) arid watershed in Arizona. Sediment concentration (and therefore sediment flux) vs. time measured at the exit to the watershed could be matched by the theory using particular values of two physically defined parameters. These two parameters are essentially the detachability of soil to rainfall, and the efficiency of sediment entrainment by overland flow. The first parameter had greatest effects on sediment concentration early in any runoff event, and the second parameter dominated later concentrations, thus allowing almost independent assessment of both parameters. The variation in soil detachability with mean depth of overland flow derived from the field data bore similarities to controlled laboratory observations.