Response of water use efficiency of Central Asia ecosystem to drought based on remote sensing data

Abstract: Ecosystem water-use efficiency plays an important role in carbon and water cycles. Drought is a natural phenomenon that occurs when water availability is significantly below normal levels over a long period and the supply cannot meet the existing demand. The intensity and frequency of global drought events will continually increase, and heavily affects the terrestrial carbon cycle. However, to our knowledge, the potential response of ecosystems to this drying trend still remains unclear, especially in arid ecosystem. The studies on the impact of drought on ecosystem water-use efficiency have been the research hotspot and difficulties at regional and global scales. In this study, we analyzed the response of water use efficiency of Central Asia to drought. The data were obtained from the well-established products from the moderate resolution imaging spectroradiometer (MODIS). The data of gross primary productivity (GPP) and evapotranspiration (ET) were from the MOD17A3. The resolution of data was 1 km. The duration was from the year of 2000 to 2010. The data had been validated. Temperature vegetation dryness index (TVDI) was calculated based on the land surface temperature (LST) of normalized difference vegetation index (NDVI). The WUE was the ratio of GPP and ET. Land coverage was classified by CMG products (MCD12C1) with resolution of 1 km. The desert and water body where the ET was almost 0 was ruled out. The relationship between ecosystem WUE and the TVDI at the arid-land ecosystems scale were analyzed. The results showed that about 60.74% of the Central Asia had a drying trend from the year of 2000 to 2014 but the trend was not significant (P>0.05). About 20.01% of the area had significant drying trend from the year of 2000 to 2014, which was mainly distributed in the northwest of Kazakhstan, Tianshan Mountains in Xinjiang and oasis in the southern Xinjiang. The response of WUE to drought was greatly different among various regions and biomes. Severe arid ecosystems had the highest WUE, followed by drought and slight drought ecosystems. The lowest WUE was observed for humid ecosystems; the correlation between annual WUE and TVDI in drought stress period and post drought stress period showed different patterns over Central Asia, both of which had large spatial heterogeneity. The WUE for arid ecosystems was negatively correlated with drought; during the drought stress, the WUE and TVDI showed the negative correlation in over 63.57% of lands. In post drought stress period, the WUE was positively correlated with TVDI in approximately 66.7% of Central Asia lands. During the drought stress, the correlation between WUE and previous year TVDI was higher than that between WUE and the current year TVDI, showing obvious lag effects of drought on ecosystem WUE. However, there was not apparent lag effect after drought stress. In addition, the arid ecosystem WUE showed a sensitive response to abrupt changes in the dry-wet environment. In post drought period, the WUE of the closed shrubland, cropland, forest, grassland and open shrubland approximately increased by 30.03%, 49.57%, 18.39%, 54.71% and 49.28% respectively compared to that of the drought stress period. A rapid increase in the ecosystem WUE occurred when the environment shifts from dryness to wetness. This indicates that the arid ecosystems in the region have relatively stable resilience against the disturbance of drought.