All water which is open to the atmosphere and subject to surface runoff.  
Notes:
Surface water includes artificial reservoirs, which are man-made reservoirs used for storage, regulation and control of water resources; lakes which are in general large bodies of standing water occupying a depression in the earth's surface; rivers and streams which are bodies of water flowing continuously or periodically in channels. Surface run-off is the portion of total precipitation or snow melt over an area, that finds its way into streams as surface flow