Population circulation refers to the circular movement of people, goods and technology within a region - in this context I'll focus on population. To simplify, you start at one place, move away from that place, and then return to that place. A common example is within developing countries where you'd expect to see migration from the rural to the urban (urbanisation), but for many reasons the population may once again return to the rural areas they came from. This tends to be a seasonal change in population, for example, outside of growing season people may choose to leave their villages to find work in the urban informal sector. During harvesting season, people may return back to their villages to help to cash their crops. In essence they operate between the two areas on a semi-permanent basis.