1, the concept of rain collection area: unit length and width of the size of the water surface of the rain collection, it is called rain collection area. Also called water area, such as around the dike yard, the area within the dike line is the rain-collecting area, alongside the hill area of the yard without skimming ditch, in addition to calculating the area of the plain within the dike, but also to add the area of the hill area to the water. Excavated skimming ditch yard, the skimming ditch outside the hill area of the water area is not counted, rain catchment area in square kilometers as a unit.

The catchment area of a river is both its watershed area and the catchment area of a particular reservoir is the sum of the catchment areas of all the rivers that converge to this reservoir. A sub-basin is the watershed of a tributary of a particular river or water system.

2, the rainfall catchment area of the reservoir: that is, the reservoir's water area, all the water can flow into the reservoir in the plane of a range of areas.

3, the rain catchment area of the outline

In the topographic map to determine (sink) catchment area usually has the following characteristics:

(1) the boundary from a designated point on the river, and finally back to the original point and form a closed loop.

(2) The boundary line passes through each of the saddles separating the different catchment areas.

(3) The boundary line is to be perpendicular to the contour line at all points.

(4) The boundary line often coincides with the ridgeline.

(5) The boundary line can only change direction abruptly at the summit.