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) high-mountain elevations. Comparison of monthly vegetation phenology with precipitation and snowmelt dynamics within the same basins over a 10-year period (2000–2009) 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 oasis-vegetation 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.
Highlights
River basins not connected to oceans occupy about 13 % of the total land surface of the earth (Meybeck et al, 2001) and generate about 2.3 % of global runoff (Shiklomanov, 1998)
Changes in vegetation development patterns in the upper mountains of the river basins may occur as a result of localised melting of the snowpack during a time when atmospheric temperatures are favourable for plant growth
Convergent cross-mapping of oases time series data of enhanced vegetation index with evaporation indicates feedback between the two variables (p values < 0.05; Fig. 5a and b), with plant-mediated evaporation providing marginally stronger control over plant growth, i.e. Pearson’s correlation coefficient at the point of convergence for “B causes A” is greater than that for “A causes B”, where A represents changes in enhanced vegetation index and B, changes in oasis evaporation (Fig. 5b)
Summary
River basins not connected to oceans (endorheic basins; Meybeck, 2003) occupy about 13 % of the total land surface of the earth (Meybeck et al, 2001) and generate about 2.3 % of global runoff (Shiklomanov, 1998). Most of these basins are located in water-limited regions of the world, generally in the middle of continents remote from oceanic sources of atmospheric moisture or blocked by mountain ranges (Meybeck et al, 2001; Warner, 2004). Understanding the water cycle in these areas is extremely important for the long-term sustainability (Pilgrim et al, 1988) of desert oases in north-west China
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