Question

ploughing along the contour lines can decelvate the flow of water down the slopes . decelevate means what

Answer 1

Ploughing along the contour lines can decelerate the flow of water down the slopes. This is called contour ploughing.
It is decelerate not decelevate.

It implies that ploughing along contour lines can reduce the speed of water, flowing down as it creates a water break and allows water to settle down in the soil.