Relating seasonal dynamics of enhanced vegetation index to the recycling of water in two endorheic river basins in north-west China

Abstract

Abstract. This study associates the dynamics of enhanced vegetation index in lowland desert oases to the recycling of water in two endorheic (hydrologically closed) river basins in Gansu Province, north-west China, along a gradient of elevation zones and land cover types. Each river basin was subdivided into four elevation zones representative of (i) oasis plains and foothills, and (ii) low-, (iii) mid-, and (iv) highmountain elevations. Comparison of monthly vegetation phenology with precipitation and snowmelt dynamics within the same basins over a 10-year period (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009) suggested that the onset of the precipitation season (cumulative % precipitation > 7-8 %) in the mountains, typically in late April to early May, was triggered by the greening of vegetation and increased production of water vapour at the base of the mountains. Seasonal evolution of in-mountain precipitation correlated fairly well with the temporal variation in oasisvegetation coverage and phenology characterised by monthly enhanced vegetation index, yielding coefficients of determination of 0.65 and 0.85 for the two basins. Convergent cross-mapping of related time series indicated bi-directional causality (feedback) between the two variables. Comparisons between same-zone monthly precipitation amounts and enhanced vegetation index provided weaker correlations. Start of the growing season in the oases was shown to coincide with favourable spring warming and discharge of meltwater from low-to mid-elevations of the Qilian Mountains (zones 1 and 2) in mid-to-late March. In terms of plant requirement for water, mid-seasonal development of oasis vegetation was seen to be controlled to a greater extent by the production of rain in the mountains. Comparison of water volumes associated with in-basin production of rainfall and snowmelt with that associated with evaporation seemed to suggest that about 90 % of the available liquid water (i.e. mostly in the form of direct rainfall and snowmelt in the mountains) was recycled locally.

Relating historical vegetation cover to aridity index patterns in the greater desert region of northern China: Implications to planned and existing restoration projects

“…and convergence is absent). This assessment is consistent with the view that in-mountain production of precipitation through orographic-lifting of evaporated water from the oases and surface and shallow subsurface return flow from the mountains is central to sustaining healthy growing vegetation in the oases (Bourque and Matin, 2012;Matin and Bourque, 2015). Added water resources to the oases in the form of glacial meltwater should help sustain the oases for some time into the future, providing that the vegetation cover in the oases (i.e., transpiration pump) is not radically altered by urbanization and other forms of land conversion (Bourque and Hassan, 2009).…”

“…6). Elevated air temperatures may cause the melting of high-elevation glaciers (a major source of water in the inland arid areas of NW China;Wang et al, 2008;Sorg et al, 2012；Hua andMatin and Bourque, 2015) to accelerate (Zhang et al, 2016), potentially increasing the amount of water available for oasis agriculture and vegetation establishment. and convergence is absent).…”

“…Matin and Bourque (2015) show through their work that vegetation cover dynamics and surface evaporation in the lowland oases of two large endorheic (hydrologically-closed) watersheds of central Gansu (expressed in terms of satellite-based estimates of enhanced vegetation index) was causally related to the production of precipitation in the Qilian Mountains (south of the oases) and precipitation in the mountains was causally related to the seasonal phenology and coverage of oases vegetation, with the former relation providing the stronger feedback control. Convergent cross mapping of within-oasis production of precipitation through localized convective processes is shown to have no causal relation with NDVI in the oases (i.e., p > 0.05…”

Remote land use impacts on river flows through atmospheric teleconnections

“…Observation-based methods relate presence of vegetation or irrigation to precipitation or river flows using statistical methods, often in combination with moisture tracking to determine the geographical origin of rainfall (DeAngelis et al, 2010;Kustu et al, 2010Kustu et al, , 2011Spracklen et al, 2012). Limitations of this type of method include variations in data quality, challenges in isolating effects of land use from climate variability, and difficulties establishing causation from correlation (Matin and Bourque, 2015). Key elements missing in all approaches including our own are socio-economic dynamics and landscape resilience, which are complex issues currently explored in experimental model settings (Nitzbon et al, 2017;Reyer et al, 2015).…”

Detecting the Causal Effect of Soil Moisture on Precipitation Using Convergent Cross Mapping

“…However, it is difficult to establish causality based on observation data. In the literature, correlation analysis is commonly used to study causality 20 , 21 , but it is neither necessary nor sufficient for a causal link 22 . Granger causality, which can detect causality between variables based on predictability instead of correlation, is widely applied in econometrics 23 .…”