Overall, the water resource available for vegetation restoration (AWR) from 2003 to 2018 was 12±114 (median ± SD) mm in the drylands (Fig.\ref{419773}a), showing a decreasing spatial pattern from south to north. Among them, water deficit areas accounted for 43.26% of the total drylands and had been recorded mainly in the western drylands, especially in the hyperarid areas (median value was -38.04mm). Droughts occurred mainly in central Xinjiang, northern Gansu, and northwestern Tibet. Water-surplus areas accounted for 56.74%, concentrated in the east and south (median value was 69.62mm), especially at the junction of Qinghai, Sichuan, and Tibet.  
In terms of trends, 64.96% of the drylands showed a declining trend in additional water availability, which mainly occurred in the West ( Fig.\ref{419773}b ). The whole region was also generally decreasing in AWR at a rate of 1.18 mm/year (Fig.\ref{419773}b and Fig.\ref{419773}c). What's more, as for changes in proportion, water deficit areas have been expanding from 2003 to 2018, with a significant increase after 2010 (Fig.\ref{419773}c). This suggests that water resource constraints in the region should be taken into account in various decisions.

3.2 Additional GPP potential that could be restored of different vegetation