Water on each side of a divide flows

Water on each side of a divide flows in the opposite direction from the top to the bottom on each side of the divide. A divide is understood to be a ridge, mountainous area, a shallow basin or a manmade shelter of sorts. Water flow is affected by natural topographical features as well as manmade objects. For example, rainwater will flow down each side of a hill towards a stream or a lake. The shape of the top of the hill will determine in which direction the water will flow. Rainwater will also soak into the ground and flow downwards towards the water table on either side of the hill. 

Ecological footprint is determined by measuring carbon dioxide emissions for a given region.