After losses, rainfall excess contributes to stream flow. Also, the longest time for water to flow to a discharge point from any point in the watershed. Topographic maps with contours define topographic and surface water conditions.
USGS maps are available for large area studies.
Water always flows perpendicular to the contour lines. A stream is indicated by contours that bend sharply upstream. Large depression storage areas should be located and be excluded from runoff contribution. Land uses and soil types within the boundaries of the watershed should be identified. As slope increases or roughness decreases travel time decreases.