Figure. 3 Oasis area and proportion variations in Tuha Basin from 1990 to 2020.

4.2 Oasis transition process analysis

Oasis transition process was reflected in Figure 4 through nodes and edges in the oasis transition network. The width of the edges in the subgraphs referred to the converted areas from one node to another. The complexity of the network was decided by the numbers of nodes and edges. Generally, the interactions among all the land types became more frequent from 1990 to 2020, with more edges connecting the nodes. Grassland owned the most edges, proving the most active node in the oasis transition network. The interaction between cropland and shrubland was dramatic during the first three periods from 1990 to 2005, and kept steady in the last three periods. There was no transfer-out of built-up to other land types from 1990 to 2000, and the edge of the transfer-in type was substantially wider than the transfer-out one throughout the whole study period.
To reduce the redundancy of the network, principal component analysis was conducted by calculating the main components nodes and edges that had the edge weights with above-average values in the oasis transition network (Figure 5). The primary conversions occurred among grassland, cropland, shrubland, and Bare land. The interaction between grassland and bare land dominated in the oasis transition network throughout the whole study period; the transition between bare land and shrubland was predominant after 1995. The weight of cropland gradually decreased until it was below the average weight, at which point it was eliminated as a primary component in the oasis transition network in the period of 2015-2020.