The Ejina Oasis (99.7–101.7∘ E and 40.45–42.5∘ N) is located in the lower reaches of the HRB, the second largest inland river basin of China, in the Inner Mongolia Autonomous Region (Fig. 1). This area is part of the Gobi Desert with a mean elevation of around 1000 m. The region has a typical continental arid climate, with an annual average temperature of 8.8 ∘C over the last five decades. Annual precipitation in this region is scarce, averaging 35 mm over the past 50 years, but varying widely from 7 to 101 mm, whereas annual potential evaporation reaches 2300 mm. With such a dry climate, surface runoff is rare in this region and therefore discharge down the Heihe River is the major water resource for local economic development. The headwaters of the rivers flowing into the Basin are in the Qilian Mountains and flow to the middle reach plains between Yingluoxia (YLX) and Zhengyixia (ZYX) Gauge Stations. The downstream oasis is fed by the Heihe River and its tributaries, the most important of which are the Donghe River and Xihe River (Fig. 1).

This study focused on discharge at YLX and ZYX Gauge Stations to gain insights into hydrological variations as well as the impacts of water regulation in middle reach catchments on the streamflow flowing into the downstream. The YLX station is located at the junction between upper and middle reaches of the HRB and represents the major water resource for sustaining consumption both in middle reach and downstream catchments. ZYX is located at the junction of middle and lower HRB, and represents the proportion of water allocated for downstream developments. The absolute value of streamflow at ZYX (Qzyx), the streamflow consumed in the middle reaches (ΔQ, the difference in streamflow between YLX and ZYX), and the percentage of water allocated for the downstream basin Rzyx (Qzyx / Qylx) were used to understand the interactions between the middle reach regulation and the consequential changes in downstream hydrological conditions. Monthly discharge records between 1987 and 2014 at the YLX and ZYX stations were collected from WestDC (http://westdc.westgis.ac.cn/), maintained by the Cold and Arid Regions Environmental and Engineering Research Institute of Chinese Academy of Sciences (Li et al., 2011). Discharge records for the 1960s were also included as baseline values, reflecting discharge levels with little human intervention in the HRB.

As well as this, precipitation and temperature were selected as two major climatic variables to understand vegetation dynamics in this arid region. Daily precipitation records at Ejina Meteorological Station from 1987 were collected from WestDC to calculate the annual precipitation. Daily mean temperature, which accompanies this dataset, was also processed to obtain annual mean temperature to assist further analysis. Since the landscape in lower HRB was dominated by desert, annual evapotranspiration (ET) in Ejina was usually less than 50 mm (Lian et al., 2015). Only regions near the river showed relatively higher ET levels but they were also supported by the river flows (Luo et al., 2012). Therefore, we excluded ET in this analysis. Furthermore, although groundwater observations covering the entire study period were not available, short term measurements (2010–2012) at a transection located in the Ejina Oasis (Fig. 1) were collected from WestDC to discuss the impact of groundwater on the downstream vegetation dynamics.

The Ejina Basin is an ecosystem consisting of a Gobi Desert zone, vegetation, water-covered regions and cultivated lands. The Gobi Desert zones occupy about 90 % of the region and, owing to the extremely low precipitation, these areas are vulnerable to any climatic changes, especially the regions further away from the river channels which are less affected by the river discharge. Therefore, this study focused on the native vegetation, water-covered regions and cultivated land. Native vegetation is important to maintain series of ecosystem functions and services, including soil/water/nutrient regulation, biological control, wildlife habitats and the provision of food and water for local people. Local wetlands are vital to support the survival of surrounding vegetation and habitats in this arid region. Cultivated land in this study was represented by vegetated areas resulting from human intervention. Although not a natural ecosystem, it can be used to consider the impact of human activities on the vegetation dynamics.

Table 1 shows the metrics used to characterize the above three components. The spatial extent of vegetation, water-covered regions and cultivated lands were derived through image classification to identify distributions under various levels of water availability. The frequency index for vegetation and water-covered regions was introduced to illustrate their spatial distribution during three identified periods with different flow regimes and to follow their changes between the periods. The three periods were defined according to a preliminary inspection of temporal variations in streamflow entering the downstream catchment, namely: steadily decreasing (1987–1999), variably increasing (2000–2007) and relatively stable (2008–2015). For vegetation-covered regions only, fractional vegetation cover (FVC) was calculated using a linear mixture model.

To show the potential influence of different water availability in near-river to distant regions, a series of buffer zones along river channels (100, 400 and 1000 m away from river channel) were introduced to detect the interactions of vegetation dynamics and river flow. These three buffers were determined from previous work illustrating that vegetation grew well within 400 m of the water channels, while the growth almost ceased beyond 1000 m from the channels (Guo et al., 2008). Areas of vegetation-covered regions and the corresponding mean FVC values for the selected buffer zones were extracted for further analysis.

Landsat Thematic Mapper (TM) or Operational Land Imager (OLI) imageries (hereafter simply referred as “Landsat images”) were adopted to derive the selected metrics. Processing of the datasets were detailed in the following sections.

Linear stepwise regression models were applied to link the changes in spatial extent of vegetation, surface water and derived FVC values with regional climatic variables along with detected trends in hydrological variables. The regression procedure can select the best-fit combination of independent variables for dependent variable prediction with forward-adding and backward-deleting variables using a critical F value to check the eligibility of the step forward added variables (Chen et al., 2013). A general form of the model can be described as: yi=β1xi1+…+βnxin+εi=XiTβ+εi, where T denotes transpose; y is the dependent variable; subscript i denotes for vegetation area, lake surface area and cultivated land area in this study; x represents the independent variables including current year's discharge at ZYX (Qc), previous year's annual discharge (Qp), total discharge of current year and previous years' (Qcp), regional average temperature and precipitation; β is a regression coefficient, where a positive βi indicates positive correlation and vice versa.

Two-tailed Pearson correlation was also introduced to test the relationship between total vegetation dynamics (in spatial extent and FVC) and above-mentioned variables within the selected buffer zones.

The streamflow at YLX and ZYX (Fig. 3) showed a synchronous increasing or decreasing trend for most years during the study period, which indicated that water flow in the Heihe River followed the regulation scheme. Overall, annual river streamflow at both YLX and ZYX has increased for the past three decades. However, the annual streamflow at ZYX decreased slightly before 2000, followed by a rapid increase from 2000 to 2007, and has been relatively stable since 2008. Specifically, annual streamflow at YLX has increased by 40 % from about 15.74 × 108 m3 in 1987 to 22.03 × 108 m3 in 2014. The average annual streamflow at YLX for the study period (1987–2014) was 17.26 × 108 m3, which also increased by 17 % when compare with the level in the 1960s (14.71 × 108 m3). Meanwhile, annual streamflow at ZYX also increased by 34 % from 1987 to 2014, whereas the average annual streamflow has decreased by 10 % since the 1960s: from 10.66 × 108 m3 in the 1960s to 9.63 × 108 m3 in the study period. Water consumption in the middle reaches contributed most to this diverse variation.

The water diverted from the middle reaches (ΔQ) increased from about 6.72 × 108 m3 in the 1980s to 8.11 × 108 m3 in the 1990s. A substantial decrease to about 6.67 × 108 m3 was observed in the early 2000s and this level remained fairly constant, until a slight uptrend started around 2007, reaching about 7.98 × 108 m3 in the 2010s. Accordingly, the ratio of streamflow allocated to Ejina Basin (Qzyx / Qylx) also underwent three stage changes: an initial decrease with variations (1987–1999), a substantial increase (2000–2007) and then relative stability (2008–2015).

Vegetation distribution within Ejina Oasis between 1987 and 2015 was highly variable, ranging from 528 to 1025 km2. From 1987 to 1999, there was a decreasing trend, as shown by the 5-year moving average, where annual vegetation reduced by ∼ 30 %, from about 882.7 km2 in early 1990s to about 619.3 km2. For the period from 2000 to 2007, the region showed relative stable vegetation distribution, averaging about 701.5 km2. Since 2007, Ejina has experienced a steady upward trend in vegetation distribution, with an annual increase of 26.4 km2 (Fig. 4).

Within the vegetation-covered regions, the overall mean FVC was 28 % between 1987 and 2015, ranging from 22 to 35 %. The changes in FVC over time shows a two-stage development, rather than three stages as previously identified. A significant downward trend occurs until 2003, with an annual decrease of 0.3 %. After 2003, a non-statistically significant upward trend was observed, with a 0.2 % increase per year (R2 = 0.379).

Landsat images showed that East Juyan lake had dried up for most of the  years between 1987 and 2001 (except 1988, 1989, 1993 and 1998, which had relatively high discharge events, Fig. 4). Streamflow at ZYX for these  years were 10.56 × 108 m3, 15.74 × 108 m3, 10.41 × 108 m3 and 11.20 × 108 m3, respectively, while for the other years before 2000, the average discharge was only 7.55 × 108 m3. Since 2002, increased discharge has caused steady flows into East Juyan Lake and the lake surface increased from about 15.14 km2 in 2002 to 45.73 km2 in 2009. Thereafter, the average lake surface area was about 39 km2, with little variation. The situation was similar for the overall total surface water, which also included the inundated river channels.

Cultivated lands in the Ejina Oasis experienced constant expansion (Fig. 4), even during the period of increased water stress, as demonstrated by decreasing river discharge at ZYX (1987–1999). A more dramatic rise was observed between 2000 and 2006, by which time the cultivated land has almost doubled in area to about 84.3 km2. Since 2007, the rate of increase rate has been very low, with the average total area of cultivated lands at 87.5 km2, with little inter-annual variation.

The linear models developed to demonstrate the relationship between the change of area of vegetation, lake surface and cultivated land in the downstream Ejina Oasis and hydrological and climatic variables are listed in Table 2. None of the hydrological variables showed significant influence on annual vegetation distribution, but regional temperature exerted significant negative effects, and it alone could explain about 52.6 % of the variations in the area of vegetation. No significant relationship was obtained between mean FVC and hydrological or climatic variables. The linear models for Juyan Lake indicates that area of the lake surface was closely dependent on the combined previous and current year's discharge to the downstream basin. Compare with the significant positive effect of previous year's discharge (R2 = 0.365), the combined river discharge of the current plus previous year showed a more robust relationship (R2 = 0.841), which might indicate the importance of accumulated river flows. Agricultural activity relies on water, and this is demonstrated in this region by a significant positive relationship between current river discharge and detected cultivated lands (R2 = 0.530).

As indicated with the per pixel frequency index, most of the vegetation in this area was concentrated in the entry side of the oasis and within the core oasis area located along the lower Donghe River (Fig. 5). Although fed with increasing river discharge in period 2 (2000–2007), there was still obvious signs of decreased frequency of vegetation distribution for most of the areas when compared to period 1 (1987–2009). While for period 3 (2008–2015), increased frequency was observed, especially along Xihe River and in the south part of the core oasis area. More specifically, from period 1 to 2, about 74 % of the oasis vegetation regions experienced some kind of decrease (minor 47 %, moderate 24 % and significant 3 %). Most moderate decreases occurred in the downstream reaches of the Donghe River, the upper reaches of the Nalin River and the entrance of the Donghe River (Fig. 5a, d). Only 21 % of the regions showed minor increases, which were mainly distributed along the river channels. From period 2 to 3, over 75 % of the regions showed increased frequency (Fig. 5b, e). About 30 % of the regions show moderate to significant increase, which were mainly distributed along Xihe River and its branches, around the Juyan Lake and outer circle of the core area of the Ejina Oasis. Around 20 % of the regions experienced minor decreases during this period, which were mostly distributed along the Donghe River and within the core area of the oasis (Fig. 5e). For the whole study period (period 1 to 3), about half (48 %) of the oasis regions experienced decreased frequency of vegetation distribution, which were mainly distributed along Nalin River and within the core oasis area (Fig. 5f). Meanwhile, regions with increased frequency (52 %) were mainly distributed along the Xihe River, around the Juyan Lake and in the southern part of the core area of the Ejina Oasis (Fig. 5f).

Only one or two inundation events along Xihe River were detected in period 1 (Fig. 6a). After the water reform in 2000, the frequency did not show an obvious increase but broader regions were inundated for two to three times in this area (Fig. 6b, c). Meanwhile, the Donghe River and Juyan Lake experienced more frequent inundation, especially during period 3. The difference in frequency index between periods 1 and 3 also revealed that, during the whole study period, most of the increased water inundation was concentrated along the Donghe River channel and east Juyan Lake (Fig. 6d). The frequency maps also indicated that the Donghe River was not connected with the terminal lake for most years, which could be attributed to two possible reasons. The first is that the width of the streams in lower river courses was below the detection limit of Landsat, while the second is that water did not flow constantly into the terminal lake, but depended on several drainage events that were not captured by the Landsat images.

Detailed correlation analysis within the buffer zones also demonstrated different levels of response along the Xihe and Donghe Rivers. Specifically, while temperature showed a similar negative relationship with vegetation distribution in all three buffer zones, the correlation coefficient increased as the distance to the river channel increased. Although none of the hydrological variables (current and previous years' total discharge at ZYX) showed any impact on total vegetation distribution in the Ejina Oasis, the previous 3 to 5 years' total discharge showed significant positive effects over the vegetation expansion in the selected buffer areas, especially along Donghe River. Vegetation in near-river regions showed more significant correlation with the previous year's discharge (Table 3).

As for FVC within the buffer zones, temperature showed no impacts, but the previous 3–5 years' water discharge at ZYX showed significant positive correlation with the FVC values. According to the correlation coefficient detailed in Table 3, vegetation distributed along Donghe River and its tributaries seemed to be more responsive to flow variations. Furthermore, the mean FVC in different buffer zones showed similar decreasing trends before 2003 but with different rates: in near-river regions (within 100 m) the decreasing rate was -0.09 % yr-1 while this rate rose as distance from the river channel increased, and the decreasing rates in all buffer zones along Xihe River were about 0.1 % higher than those along the Donghe River (Table 4). After 2003, an upward trend (rate > 0.2 % per year) was observed within the buffer zones and vegetation along Donghe River showed about a 0.2 % higher increasing rate than that along the Xihe River (Table 4). Near-river-channel vegetation with higher increasing rates was also observed along both the Xihe River and the Donghe River.