Largely unplanned exploitation of water resources for agricultural, industrial and metropolitan uses has resulted in severe ecological deterioration Northwestern arid region of China (Shen and Chen, 2010). The Tarim river, the longest inland river of China, is facing serious ecological problems in the lower reaches where the green belt supported by the river has retreated up to 321 km, and the river channel has become severely ephemeral for around 30 years (Chen et al., 2011). This phenomenon is mainly a result of the massive water use in the upper reaches of each tributary where the artificial oases have been enlarged several times over the past decades. Presently, restoration of the degraded arid river ecosystems is a major challenge faced by local and central governments as well as hydrologists and ecologists in China. Since 2000, there have been 11 major water diversions from Bosten Lake to the downstream reaches of the Tarim River. About 1·8 billion cubic metres of water was delivered to the dried channels to restore the deteriorated riparian ecosystem over this period. Simultaneously, there has been continuous monitoring of the vegetation response to the water diversion, including the ecophysiological responses at micro-scale and the landscape responses at macro-scale in past several years.
This special issue comprises 15 inter-disciplinary papers selected from the presentations at the third International Symposium on Ecohydrology and Sustainable Development in Arid Regions, which was held in Urumqi, China, during 26–29 June 2010. The selected 15 papers in this issue investigate hydrological and ecological processes, and dynamics at the different scales in the arid region, mainly focused on glacier hydrology in the mountain region, plant eco-physiology in the riparian zone, irrigation effects in farmland, isotopic studies of the surface and groundwater hydrology, and vegetation dynamics and responses to hydroclimatic factors at a regional scale.
According to the water cycle process in the arid region, the papers in the special issue were sorted by mountain, riparian vegetation and oasis. The first paper (Sun et al., 2013) presents information on recent glacial contraction and the impact of climate change on glacial hydrological processes using a water balance model and trend analysis. Results show that Urumqi River Glacier No. 1 has experienced a remarkable shrinkage with a tremendous cumulative declining of glacier mass balance from 1962 to 2009, and annual glacier melt runoff and river flow at a gauging site both exhibiting increasing trends over this period. Increases in both air temperature and precipitation from 1959 and 2008 have had a combined effect on glacier mass loss and runoff change.
Papers on plant eco-physiology in the riparian zone provide insights across three scales, including regional and landscape scale, plant community scale and individual plant scale. At regional and landscape scale, Cao et al. (2013) investigate the impact of climatic factors on trends in vegetation cover on the basis of the SPOT/VEGETATION NDVI dataset of the typical semi-arid land, Inner Mongolia, over the period 1998–2008, through an integrated statistical method combing asymmetric Gaussian filtering, seasonal Kendall test, R/S analysis, correlation analysis and regression analysis. Their results showed that temperature and precipitation have a direct influence on vegetation change, acting as the main climatic driving forces for the regional vegetation evolution. In comparison, Wang et al. (2013) investigate the vegetation dynamics and their response to hydroclimatic change in the Tarim River Basin from 1982 to 2006 by using the Mann–Kendall trend test, partial correlation analysis and grey relation analysis. An important finding was that vegetation vigour and cover area had been improved because of more favourable hydroclimatic conditions under the direct and indirect effects of temperature rise over the study period, except in some vulnerable ecological regions. The responses of vegetation to hydroclimatic factors showed strong spatial heterogeneity, with temperature identified as the main stress factor for mountain vegetation and runoff for the oasis vegetation. This has implications for identifying strategies to cope with future climate change in the Tarim River Basin.
At the plant community scale, Chen et al. (2013b) and Li et al. (2013) identify and discuss the physiological and ecological mechanisms of adaptation by desert riparian forest vegetation in response to drought stress. This was carried out by field monitoring and analysing the ecological and physiological parameters, including the cover, density and species diversity, as well as the photosynthesis, fluorescent parameters, leaf water potential, leaf proline (PRO), soluble sugar, malondialdehye, superoxide dismutase (SOD), peroxidase and abscisic acid of the main constructive species at different depths to groundwater in the lower reaches of the Tarim River. They found that the increasing depth to groundwater was the main driving force for vegetation degradation; the critical groundwater depths for onset of degradation appeared to be around 4–6 m for herbs and greater than 6 m for trees The optimal groundwater depth for preserving the appropriate composition and structure of plant communities was in the range of 2–6 m. For the main constructive species Populus euphratica and Tamarix spp., the critical depths were 4 m and 6 m, respectively. Adaptation strategies by P. euphratica and Tamarix spp. to moderate and severe drought included rapid accumulation of PRO, soluble sugar, SOD and abscisic acid in their leaves so as to reduce leaf water potential and stomatal conductance, enhancing osmoregulation and alleviating the esterification of cell membranes, and actively decreasing the yield, electron transport rate and photosynthetic rate to improve the efficiency of water utilization. Additionally, there were complementary actions in resisting drought stress between the SOD activity and the peroxidase activity, and between the contents of PRO and soluble sugar in the leaf cells in P. euphratica and Tamarix spp. Chen et al. (2013a) studied the influence of embankments on riparian forest communities by using data of groundwater depth, groundwater chemistry and vegetation species diversity collected in 2001–2007 at the middle reaches of Tarim River. They found that the groundwater depths increased gradually, total dissolved salts and major ions in groundwater increased sharply, and vegetation coverage and species diversity declined. During the start of the construction, the hydrological connection was more than 1000 m away from the embankments, but by 2007, the region beyond 800 m from the embankments became a water and salinity discharging area. Therefore, they propose that abandoning the embankments should be considered to protect the riparian forest ecosystems.
At an individual plant scale, Hao et al. (2013) found from field observation that P. euphratica Oliv. possesses clear hydraulic lift properties and that the effect of hydraulic lift was clear at 60- to 120-cm soil depths within a distance of 4 m from the trunk. Hydraulic lift of P. euphratica Oliv. could increase soil moisture, replacing approximately 10–20% of daily water utilization in the upper soil layers (0–120 cm), facilitating the co-existence of some herbs with trees in the Populus community. Zhou et al. (2013) investigated the xylem hydraulic conductivity and the response to drought stress. Plants were observed adapting to the mild drought stress through limiting twig xylem hydraulic conductivity to regulate transpiration, but they adapted the severe drought stress through enhancing xylem hydraulic conductivity of dominant branches with strong competitiveness of sap flow and sacrificing non-dominant branches to improve the chances of plant survival. From the view of fluid flow density, Ma et al. (2013) used heat pulse technology to show that sap flux density of P. euphratica was well correlated with variations in depth to groundwater and concluded that the depth to groundwater should not exceed 4·5 m from the soil surface in order to restore P. euphratica and its ecosystem in the middle and lower reaches of the Tarim River. Additionally, from the view point of mycorrhizal association, Yang et al. (2013) investigated the arbuscular mycorrhizal fungi (AMF) of natural P. euphratica rhizosphere in the lower reaches of Tarim River by using traditional morphological methods and molecular techniques. Under drought stress, the rhizosphere soil of natural P. euphratica contained a small amount of similar AMF spores identified as Glomus mosseae. This opens the possibility to accelerate the reestablishment and conservation of the endangered P euphratica in arid desert regions through a mycorrhizal symbiont established by G. mosseae associated with P. euphratica roots.
Oases are unique intra-zonal landscapes in the arid areas and are the most important parts of arid zones for social and economic activities. Although oases occupy only 4–5% of the total land area of the arid and semi-arid regions in China, over 90% of the population and over 95% of the social wealth are concentrated within oases. All of the oases in the arid areas in northwestern China are fed by irrigation, so secondary soil salinization and irrigation water quality are the key factors for oasis productivity. There are two papers focusing on this issue. Using a combination of interpolation by the geostatistical technique of Kriging and a geographical information system approach, Zhou and Li (2013) studied the effects of hydrological processes in the oasis ecosystem on soil salinization in a typical oasis located in the lower Tarim River. This study demonstrated that the relationships between soil salt content in the 0- to 50-cm layer and surface water quality, groundwater depth and groundwater quality were significantly positive. The critical groundwater depth for condensing salt content in groundwater and changing soil salinity distributions was 6 m. The effects of drip irrigation underneath a plastic film with different salinities of irrigation water on soil salt content and cotton growth were examined by Hu et al. (2013) using test-pit experiments. They found that the salt was accumulated at the main cotton root zone when irrigating cotton with saline water of salinity greater than 2·24 g l−1 and the leaf area index, root area index, root length density and root weight density all decrease with an increase in salinity of irrigation water and soil salt content in the root zone.
We also selected three papers on the characteristics of water resources and ecosystem service at regional scale. From the spatial characteristics of water resource, Zhang et al. (2013) analysed the isotope composition for surface water and groundwater in Tarim River Basin on the basis of the isotope balance model and meteorological data. This study provided the distribution equations between the distance and isotope composition for Tarim River and Aksu River and clarified the evaporation loss (0·25, approximately 0·68 × 108 m3 input water in October) of Bosten Lake by the isotopic method. The results were helpful for understanding hydrological processes and water management in the arid and less gauged region. Further, the minimum ecological influx in the lower Tarim River was estimated by Ye et al.(2013) on the basis of river geomorphology data during the fifth and sixth ecological water conveyances via the wetted perimeter method using the curvature technique. Results showed that the minimum ecological influx of the three control sections of the river (Yinsu, Alagan and Yiganbjima) were 2·85, 3·76 and 1·76 m3/s, which accounted for 9·7%, 14% and 6·9% of the multi-year average annual discharges, respectively. Moreover, except for the dry season, annual ecological water demand of the river was 0·79 × 108 m3, excluding evaporation and leakage. Finally, Huang and Ma (2013) estimated the ecosystem service values through a landscape ecology method combined with the aid of GIS and remote-sensing technologies. They found that the area of forest, grassland and water ecosystem exhibited large decreases, but the area of cropland ecosystem increased in intensity. However, the willingness to pay was not proportional to the value of the ecosystem services in the Tarim River Basin.
Because of the hyper arid climate, ecosystems in the study region are vulnerable to pressures originating from both climatic and human perturbations. Ecosystem deterioration already influences the development of the socio-economy in this region. The ecology, economy and society are tightly linked with water in the arid region. The papers collected in this special issue present an overview of arid land ecohydrology in the hyper arid region in northwestern China. The findings should aid ecological conservation and restoration in the extremely water limited region. Future efforts still need to enhance the studies from a trans-disciplinary and integrated perspective, i.e. ecohydrology, which is the key to sustainable development of economy and society for the arid regions of the world.