Demographic Transition in Natural Watersheds: Evidence from Population Aging in the Yellow River Basin Based on Various Types of Migration

Environmental phenomena in natural watersheds have attracted much attention, while where demographic transition, especially population aging, have not. Therefore, we try to analyze regional evolution of population aging in the Yellow River Basin from the perspective of population migration during 1990–2020, in order to explain the laws and mechanism of demographic transition in natural watersheds. Population aging in the Yellow River Basin began in its downstream cities in 1990 and spread to its middle and upper reaches, showing positive spatial correlation. Aging population in the Yellow River Basin forms obvious geographic agglomeration, namely a nonstandard inverted M-shaped agglomeration pattern. During 2000–2020, regional evolution of population aging in the Yellow River Basin is affected by various types of population migration, whose extent varies greatly, especially for the scale of an aging population. Among them, the scale of an aging population in a slow and deep emigration area ( SDE ) and a slow and shallow emigration area ( SSE ) is significantly affected by migration speed ( Ms ), which is positive. However, the migration rate ( Mr ) has a negative impact on population aging in a slow and deep emigration area ( SDE ), slow and deep immigration area ( SDI ), slow and shallow emigration ( SSE ) and slow and shallow immigration area ( SSI ), whose degree of influence slightly differs. Only the power function graph of aging population ( AP ) in a slow and shallow immigration area ( SSI ) about migration speed ( Ms ) is convex, and that in other types about migration rate ( Mr ) or migration speed ( Ms ) is monotonically decreasing, while the inclination degree of whose graphs varies greatly.