Bai, J, Shi, H, Yu, Q, Xie, Z, Li, L, Luo, G, Jin, N & Li, J 2019, 'Satellite-observed vegetation stability in response to changes in climate and total water storage in Central Asia.', The Science of the total environment, vol. 659, pp. 862-871.View/Download from: UTS OPUS or Publisher's site
Ecosystems in arid and semi-arid regions are vulnerable to climatic and anthropogenic disturbances. However, our understanding of vegetation stability (including resistance and resilience, which are the abilities of ecosystems to resist perturbations and return to pre-disturbance structure or function, respectively) in response to environmental changes in dryland ecosystems remains insufficient, particularly in the absence of large-scale observations of water availability. Here we introduced GRACE monthly total water storage anomaly (TWSA) data into an autoregressive model with remote sensed EVI, air temperature and precipitation to investigate the short-term vegetation stability and its influencing factors in Central Asia (CA) during 2003-2015. The results showed that the grid-level vegetation resilience in CA increased logarithmically as mean annual precipitation (R2 = 0.33, P < 0.05) but decreased linearly with increasing mean annual temperature (R2 = 0.41, P < 0.05). Vegetation resilience was not correlated with TWSA, due to the decoupling of TWSA with precipitation both spatially and temporally in the majority of CA. At the biome level, vegetation resilience also increased as a logarithmical function of aridity index (R2 = 0.80, P < 0.05). Vegetation resistance to TWSA showed minor difference across biomes, while vegetation resistance to precipitation functioned as a parabolic curve along the aridity gradient (R2 = 0.59, P < 0.05). Our results suggest that accounting for the effects of total water column instead of precipitation only is critical in understanding vegetation-water relationships in drylands. The steep decrease in vegetation resilience in areas with high temperature and low water availability implies a high risk of collapse for these water-limited ecosystems if there are severe droughts. Furthermore, reduction in total water storage, induced by, e.g., large-scale extraction of surface runoff or shallow-layer groundwater for irrigation, can resul...
Xie, Z, Huete, A, Cleverly, J, Phinn, S, McDonald-Madden, E, Cao, Y & Qin, F 2019, 'Multi-climate mode interactions drive hydrological and vegetation responses to hydroclimatic extremes in Australia', Remote Sensing of Environment, vol. 231.View/Download from: UTS OPUS or Publisher's site
© 2019 Australia has experienced a large frequency of hydroclimatic events since the early 21st century, with multiple large-scale droughts and flooding rains exerting dramatic impacts on water resources and ecosystems. Despite these pronounced consequences, the coupling of ecosystem functioning to extreme climate variability remains elusive due to the lack of complete understanding of hydrological connections. In this study, we investigated the spatiotemporal trends of Australia's hydrological and vegetation responses to three climate modes: El Niño-Southern Oscillation, the Indian Ocean dipole and the Southern Annular Mode, utilizing climate indices, satellite-derived total water storage anomaly (TWSA) from GRACE, precipitation from TRMM and vegetation greenness from MODIS. Using partial cross-correlation and vegetation sensitivity analyses to interpret the interactions among climate modes, water resources and vegetation across Australia, three hydroclimatic extreme events from 2002 to 2017 were analyzed: (i) a prolonged drought (2002–09, colloquially known as the ‘big dry’); (ii) a dramatic wet pulse (2010–11, the ‘big wet’); and (iii) another anomalous El Niño event (2015). Our results showed the entire continent partitioned into three geographic zones with diverse drying and wetting trends in total water storage, precipitation and vegetation greenness, reflecting varying and fundamental influences from the individual climate modes. Ecosystem productivity was found to be better related and more sensitive to TWSA than precipitation across different hydroclimate zones and during both extreme dry and wet conditions. We also observed TWSA increased rapidly during wet extremes, and these gains in water resources persisted for an additional four years (i.e., TWSA remained positive until 2015 following the 2011 ‘big wet’). Lastly, findings from another hydroclimatic event (the 2015 El Niño drought) further confirmed the relationships among climate, water and ecosyst...
Zhang, X, Wang, N, Xie, Z, Ma, X & Huete, A 2018, 'Water loss due to increasing planted vegetation over the Badain Jaran Desert, China', Remote Sensing, vol. 10, no. 1.View/Download from: UTS OPUS or Publisher's site
© 2018 by the authors. Water resources play a vital role in ecosystem stability, human survival, and social development in drylands. Human activities, such as afforestation and irrigation, have had a large impact on the water cycle and vegetation in drylands over recent years. The Badain Jaran Desert (BJD) is one of the driest regions in China with increasing human activities, yet the connection between human management and the ecohydrology of this area remains largely unclear. In this study, we firstly investigated the ecohydrological dynamics and their relationship across different spatial scales over the BJD, using multi-source observational data from 2001 to 2014, including: total water storage anomaly (TWSA) from Gravity Recovery and Climate Experiment (GRACE), normalized difference vegetation index (NDVI) from Moderate Resolution Imaging Spectroradiometer (MODIS), lake extent from Landsat, and precipitation from in situ meteorological stations. We further studied the response of the local hydrological conditions to large scale vegetation and climatic dynamics, also conducting a change analysis of water levels over four selected lakes within the BJD region from 2011. To normalize the effect of inter-annual variations of precipitation on vegetation, we also employed a relationship between annual average NDVI and annual precipitation, or modified rain-use efficiency, termed the RUEmo. A focus of this study is to understand the impact of the increasing planted vegetation on local ecohydrological systems over the BJD region. Results showed that vegetation increases were largely found to be confined to the areas intensely influenced by human activities, such as croplands and urban areas. With precipitation patterns remaining stable during the study period, there was a significant increasing trend in vegetation greenness per unit of rainfall, or RUEmo over the BJD, while at the same time, total water storage as measured by satellites has been continually decreasin...
Xie, Z, Huete, A, Ma, X, Restrepo-Coupe, N, Devadas, R, Clarke, K & Lewis, M 2016, 'Landsat and GRACE observations of arid wetland dynamics in a dryland river system under multi-decadal hydroclimatic extremes', JOURNAL OF HYDROLOGY, vol. 543, pp. 818-831.View/Download from: UTS OPUS or Publisher's site
Xie, Z, Huete, A, Restrepo-Coupe, N, Ma, X, Devadas, R & Caprarelli, G 2016, 'Spatial partitioning and temporal evolution of Australia's total water storage under extreme hydroclimatic impacts', Remote Sensing of Environment, vol. 183, pp. 43-52.View/Download from: Publisher's site
Australia experienced one of the worst droughts in history during the early 21st-century (termed the ‘big dry’), exerting negative impacts on food production and water supply, with increased forest die-back and bushfires across large areas. Following the ‘big dry’, one of the largest La Niña events in the past century, in conjunction with an extreme positive excursion of the Southern Annular Mode (SAM), resulted in dramatic increased precipitation from 2010 to 2011 (termed the ‘big wet’), causing widespread flooding and a recorded sea level drop. Despite these extreme hydroclimatic impacts, the spatial partitioning and temporal evolution of total water storage across Australia remains unknown. In this study we investigated the spatial-temporal impacts of the recent ‘big dry’ and ‘big wet’ events on Australia's water storage dynamics using the total water storage anomaly (TWSA) data derived from the Gravity Recovery and Climate Experiment (GRACE) satellites.
Results showed widespread, continental-scale decreases in TWS during the ‘big dry’, resulting in a net loss of 3.89 ± 0.47 cm (299 km3) total water, while the ‘big wet’ induced a sharp increase in TWS, equivalent to 11.68 ± 0.52 cm (898 km3) of water, or three times the total water loss during the ‘big dry’. We found highly variable continental patterns in water resources, involving differences in the direction, magnitude, and duration of TWS responses to drought and wet periods. These responses clustered into three distinct geographic zones that correlated well with the influences from multiple large-scale climate modes. Specifically, a persistent increasing trend in TWS was recorded over northern and northeastern Australia, where the climate is strongly influenced by El Niño-Southern Oscillation (ENSO). By contrast, western Australia, a region predominantly controlled by the Indian Ocean Dipole (IOD), exhibited a continuous decline in TWS during the ‘big dry’ and only a subtle increase during the ‘big wet’,...
Ma, X, Huete, A, Cleverly, J, Eamus, D, Chevallier, F, Joiner, J, Poulter, B, Zhang, Y, Guanter, L, Meyer, W, Xie, Z & Ponce-Campos, G 2016, 'Drought rapidly diminishes the large net CO2 uptake in 2011 over semi-arid Australia', Scientific Reports, vol. 6.View/Download from: UTS OPUS or Publisher's site
Each year, terrestrial ecosystems absorb more than a quarter of the anthropogenic carbon emissions, termed as land carbon sink. An exceptionally large land carbon sink anomaly was recorded in 2011, of which more than half was attributed to Australia. However, the persistence and spatially attribution of this carbon sink remain largely unknown. Here we conducted an observation-based study to characterize the Australian land carbon sink through the novel coupling of satellite retrievals of atmospheric CO2 and photosynthesis and in-situ flux tower measures. We show the 2010–11 carbon sink was primarily ascribed to savannas and grasslands. When all biomes were normalized by rainfall, shrublands however, were most efficient in absorbing carbon. We found the 2010–11 net CO2 uptake was highly transient with rapid dissipation through drought. The size of the 2010–11 carbon sink over Australia (0.97 Pg) was reduced to 0.48 Pg in 2011–12, and was nearly eliminated in 2012–13 (0.08 Pg). We further report evidence of an earlier 2000–01 large net CO2 uptake, demonstrating a repetitive nature of this land carbon sink. Given a significant increasing trend in extreme wet year precipitation over Australia, we suggest that carbon sink episodes will exert greater future impacts on global carbon cycle.
Huete, A, Xie, Z, Restrepo-Coupe, N, Devadas, R, Davies, K & Waston, C 2015, 'Terrestrial Total Water Storage Dynamics Of Australia's Recent Dry And Wet Events', Proceedings of the 2015 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), 2015 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), IEEE, Milan, pp. 992-995.View/Download from: Publisher's site
Australia recently experienced a long-term continental drought (“big dry”, 2001-2009) followed by an anomalous wet two-year period (“big wet”, 2010-2011). Despite the significance of the two extreme events, continental-wide information regarding the effects of the high and low precipitation conditions on the hydrological components, stress and recovery is not available. In this paper, we use terrestrial total water storage changes (ATWS) from the Gravity Recovery and Climate Experiment (GRACE) and precipitation data from the Tropical Rainfall Measuring Mission (TRMM) spanning from 2002 to 2013, where ATWS represents the main source of water available for human consumption, agriculture and natural ecosystems. We rely on a combination of temporal trend analysis and spatial statistics methods in order to evaluate the terrestrial total water storage (TWS) dynamics and the relationship between TWS and rainfall during the “big dry” and “big wet” events. Here we report the occurrence of hydrological cycle intensification during the study period in Australia which exhibited strong spatial variations: the wet areas (the northern and northeast regions) got wetter while the dry areas (the west and interior of the continent) became drier. By contrast, in southeastern Australia TWS changes over time showed sudden extreme responses to both events. Our results constitute a step beyond quantifying droughts/anomalous wet years that rely solely on precipitation data. This work demonstrates the ability of TWS observations as a significant indicator of hydrological system performance during hydroclimatic events and also an important tool for understanding continental-wide and regional spatial and temporal patterns of water availability.