Exploring spatio-temporal distribution and evolution of dry-wet alternation using a three-dimensional identification method

Abstract

• Focus on dry-wet alternation in larger time scale and space scales. • A 3D identification method of dry-wet alternation is proposed. • Spatial-temporal distribution and evolutionary characteristics are explored. The dry-wet alternation evolves in both time and space and it has the characteristics of both dry and wet events. Previous studies have focused more on dry-wet abrupt alternation, and little is known about dry-wet alternation in larger time scale and space scales. In addition, previous studies have focused either on time or on space domain without considering spatio-temporal continuity of dry-wet alternation. This study proposes an identification method of dry-wet alternation, which can be used to identify and extract successive dry-wet alternations in a long time series. A series of three-dimensional dry-wet structures is developed to visualize dry-wet alternation and simulate the spatial-temporal evolution. The time dimension is added into the two-dimensional (latitude-longitude) space to better describe dry-wet alternation and extract dry-wet alternation more completely. Consecutive slices are obtained at each unit time interval to characterize the occurrence, development, migration and disappearance of dry-wet alternation in the space domain in order to better understand the spatio-temporal evolution of dry-wet alternation. Six indicators (transition period, transition rate, area, volume, intensity and density of the three-dimensional dry-wet structure) are proposed to quantify the attributes of dry-wet alternation such as the transition period and rate, scope of influence and severity, and statistical methods are used to analyze the complex spatio-temporal evolution of dry-wet alternation. It is found that dry-wet alternation occurs most frequently in the 1980s and then in the 1960s and 2010s in the Huai River basin. There exists a transition in the spatial distribution of dry-wet regions in the Huai River basin in the 1960s–2000s, as it is wet in the north and dry in the south in the 1960s–1970s, but dry in the north and wet in the south in the 1990s–2000s. Drought is more likely to have large influence range and high severity than wet. The transition period and rate reach the extremum in June-August, and the severity and influence range exhibit an increasing trend since 1980s. Our study may contribute to better understanding the formation and development of dry-wet alternation under environmental changes.