Groundwater recharge in the oasis-desert areas of northern Tarim Basin , Northwest China

Groundwater is an important source for maintaining desert ecological processes in arid areas. With the increasing intensity of climate change and human activities, the rivers in Tarim Basin are severely dried-up. Aiming at the dried-up river, vegetation degradation and oasis maintenance in the middle and lower reaches of dried-up river basin, groundwater recharge and groundwater-surface water interaction have become hotspots, but are not well known. We examined spatial distributions and controlling factors of groundwater stable isotopes and recharge at oasis scale using data from 247 samples surveyed in the four headwaters in the northern Tarim Basin. Stable isotopes of surface water and groundwater were different from each other, and varied among sampling sites. Surface water and groundwater isotopes generally became enriched towards the east throughout the study area, while surface water isotopes showed enrichment towards the upstream direction within each catchment, mainly due to cultivated area expansion. Surface water mainly originated from precipitation, groundwater, and meltwater, while shallow groundwater derived from lateral groundwater flow, river and irrigated water infiltration, and little precipitation. The mainstream water was directly recharged by the headwaters. The results could provide a new insight into groundwater cycling in oases of dried-up river basins, which is helpful for regional groundwater management.

b; Guo et al. a). Groundwater depletion in the regions is further challenging regional water and ecological security, causing severe cutoff of river channels and ecological degradation in the downstream (Chen et al. a, b, c). Therefore, it is essential to improve our understanding of groundwater sources and recharge mechanisms to uniformly regulate water resources in the arid oasis region of the Tarim Basin, focusing on water resources preservation, ecosystem restoration and sustainable economic development (Pang et al. ; Steward & Allen ).
At the watershed scale in the Tarim Basin, groundwater is an important source of agricultural irrigation and maintaining ecological processes in arid desert oasis regions (Pang et al. ; Chen et al. b). Previous studies on the groundwater in the Tarim River, using hydrological models, stable isotopic data, and hydrological monitoring methods, have investigated groundwater table dynamics (Chen et  and resultant hydrological processes are spatially heterogeneous, which could hardly be captured by point-scale investigation (Li et al. b). It is also difficult to use point-based samples to interpret the groundwater-surface water relationship at the watershed scale (Li et al. b Li et al. a), which could provide direct information on the water cycle and circumvent substantial uncertainties in hydrological simulation (Gibson et al. ). Using stable isotope analysis, Li et al. (b) found that streamflow was sourced from similar groundwater reserves across four large catchments in China's Loess Plateau.
We examined the spatial distributions and controlling factors of groundwater stable isotopes and sources of groundwater in oasis areas of four headwaters in the northern Tarim Basin at the catchment scale using data from 247 water samples surveyed during the period July and November 2018. The objectives of our study were to analyze the spatial distributions of groundwater isotopes in the oases, to examine the influence of environmental factors on groundwater isotopic values, and to determine the sources of groundwater. The result would be expected to advance our understanding of the regional groundwater cycle mechanism, and provide a theoretical foundation for regional water resources management in arid desert oasis regions of the dried-up river basin in Tarim Basin.

Study area
The Tarim Basin is located in the northwest arid region of July to September (Pang et al. ). This study focused on the oasis-desert areas of four headwaters in the northern Tarim Basin, that is, the Dina River, Weigan-Kuqa River, Akesu River, Yarkand River, and the mainstream of Tarim River (Figure 1(a)). The four headwaters cover a large area (18.69 × 10 4 km 2 ) with variable physical conditions, such as climate, topography, soil, vegetation, and hydrogeology (Table 1). The elevation in the study area ranged from 868 to 8,354 m above sea level, mean annual air temperature ranged from 1.8 to 7.5 C, and mean annual precipitation was 119-248 mm ( Figure 1 and Table 1; data from the Chinese National Meteorological Centre (2018)). Precipitation in the region has a seasonal distribution, with 70% occurring from June to October (Fang et al. ).
The study region is covered by sediments of different hydrogeologic units, including pore water in friable rocks, pore-fissure water in clastic rocks, fissure-karstic water in carbonate rocks, fissure-karstic water in carbonate-clastic rocks, fissure water in magmatic rocks, fissure water in metamorphic rocks, and glacier-snow cover (Figure 1(e)).
The hydrogeological conditions in the oasis-desert areas are mainly dominated by pore water in friable rocks with weak water-abundance for Dina River, Weigan-Kuqa River, and Akesu River, and are dominated by pore water in friable rocks with very weak water-abundance for Yarkand River ( Figure 1(e)). In general, the water abundance of aquifer systems in the oasis regions is better than that in the desert regions ( Figure 1  Afriable rock, pore water, strong water-abundance; Bfriable rock, pore water, moderate water-abundance; Cfriable rock, pore water, weak water-abundance; Dfriable rock, pore water, very weak water-abundance; Eclastic rock, pore-fissure water, weak water-abundance; Fcarbonate rock, fissure-karstic water, moderate water-abundance; Gcarbonate-clastic rock, fissure-karstic water, moderate water-abundance; Hmagmatic rock, fissure water, moderate water-abundance; Imetamorphic rock, fissure water, weak water-abundance; Jglacier, snow cover.

Field data
Water samples of surface water and groundwater were col-

Spatial distributions of groundwater stable isotopes
For the oasis areas of northern Tarim Basin (Dina River, Weigan-Kuqa River, Akesu River, Yarkand River, and the mainstream of Tarim River), surface water was generally more enriched isotopically than groundwater ( Table 2).
The mean isotopes of surface water were -52.0 and -8.2‰   water isotopes showed enrichment towards the upstream direction ( Figure 2). This may be because the three rivers were dried-up river basins, in which more than 90% of the upstream river water was transported for irrigation, leading to more depleted isotopes in channel water by reducing the evaporation period and increasing groundwater withdrawal (Wu et al. ). However, the Akesu River was not a driedup river basin, the much longer river length may lead to longer evaporation, and thus in turn lead to more enriched isotopes in the downstream river water (Li et al. b).    and Table 2). This may be because Akesu River was not a dried-up river basin, while the other rivers were dried-up in recent decades (Chen & Xu ). The purple and blue dotted lines represent the local meteoric water line (LMWL) for the Xiehela Hydrological Station and Shaliguilank Hydrological Station in the Aksu River basin, respectively (data from Sun (2015)).
groundwater in this region (Sun ). Similarly, the stable isotopes of shallow groundwater also exhibited significant seasonal difference in the sampling period within the four headwaters, with more depleted isotopes in the dry season and more enriched isotopes in the wet season, mainly because shallow groundwater was recharged by evaporated surface water infiltration in the desert oasis region in the wet season (Sun ). However, this study did not address this aspect, and further studies on this topic are needed.
Electrical conductivity (EC) of the surface water and groundwater samples are presented in Figure 6, and vary sig-

Implications for groundwater resources management
The above analyses showed that in the wet season, surface water was mainly recharged from precipitation, meltwater, and groundwater, while shallow groundwater was mainly recharged from river infiltration, irrigated water infiltration, and lateral groundwater flow in the desert oasis areas. In the dry season, stream runoff primarily originated from lateral groundwater flow, while shallow groundwater in the desert oasis region was also primarily recharged from lateral groundwater flow and little local precipitation. Furthermore, shallow groundwater in the oasis areas was primarily supplied by river water and irrigated water infiltration, while precipitation and lateral groundwater flow were the primary sources of deep groundwater (Wang et al. ; Guo et al. b). The mainstream water was directly recharged by water from the four headwaters, mainly including river water, lateral groundwater, and salt water from the drainage system (Sun ). At the same time, surface water and As mentioned above, river infiltration, irrigated water infiltration, and lateral groundwater flow were the main sources of shallow groundwater in the desert oasis areas, which have important implications for regional groundwater resource management. The amount of infiltration from river and irrigated water to shallow groundwater has gradually decreased in the past decades, mainly due to the increased canal water use coefficient and popularized water-saving irrigation technology in the arid oasis region (Han & Feng ). Meanwhile, overexploitation and utilization of deep groundwater for irrigation in the oasis region could lead to the decrease of semi-confined or confined groundwater levels (a slow decline), thus causing a gradual decrease in the recharge of lateral groundwater flow to shallow groundwater (Guo et al. b). This in turn resulted in a downward trend in shallow groundwater levels in this region in recent decades (Chen et al. b). Therefore, close attention should be paid to river runoff inputs, exploi-  the east throughout the study area, while surface water isotopes showed enrichment towards the upstream direction within each catchment, due to climatic conditions and human activities. Moreover, surface water in the four headwaters mainly originated from precipitation, groundwater, and meltwater, while shallow groundwater in the desert oasis areas may be derived from lateral groundwater flow, river and irrigated water infiltration, and little precipitation, implying a frequent interaction between surface water and groundwater in the oasis region. The mainstream water was directly recharged by water from the four headwaters, mainly including river water, lateral groundwater, and salt water from drainage systems. These results provide additional information on groundwater cycling in the desert oasis regions, and thus should be helpful for regional water resources management. Further research is needed to quantify the sources of groundwater and advance a deeper understanding of groundwater recharge mechanisms in the oasis areas of dried-up river basin in the Tarim Basin.