Figure.5 Oasis transition network principal components in different periods
Table 1 showed the results of node ratios, out-degree and in-degree from the oasis transition network to quantify the conversions among various oasis types. A ratio less than 1 indicated a transfer-in oasis type in the sub-period; if the ratio declined during 1990 and 2020, the transfer-in area of the oasis type accelerated. A ratio greater than 1 showed a transfer-out oasis type; if the ratio grew during 1990 and 2020, the transfer-out area of the oasis type sped up.
Natural oasis and artificial oasis were expressed as transfer-in types from 1990 to 2020, with ratios of 0.46 and 0.52, respectively. Throughout the study periods, grassland had the highest out-degree and in-degree values and the ratios were steadily round 1, implying the most active land type in the oasis transition network. Shrubland had a low ratio and increasing tendency in out-degree and in-degree throughout the study period. The ratio of shrubland dramatically dropped from 1.10 to 0.19 from 2015 to 2020, indicating the shift from transfer-out to transfer-in. The ratio of forest varied during the study period and it peaked at 0.23 during 2015 to 2020. Generally, water body was a transfer-out type, and the ratio fluctuated during 1990 and 2020. The ratio of cropland was a transfer-in type with the ratio smaller than 1 after 1995. The out-degree and in-degree maintained in a high level and changed slightly from 1990 to 2020. Built-up had the lowest ratio (0.00-0.02), suggesting that the area of built-up was fixed, and it was difficult to be converted into other land types.
Tab.1 Node ratio, out-degree and in-degree in oasis transition network