Ecohydrological effects of water conveyance in a disconnected river in an arid inland river basin

Water system management is a worldwide challenge, especially in arid and semi-arid regions. Ecological water conveyance projects aim to raise the groundwater table, thereby saving natural vegetation and curbing ecological deterioration. Since 2000, these projects have been implemented in the arid zone of northwest China, with generally successful outcomes. Taking a portion of the lower reaches of the Tarim River as the study area, this paper analyzes in detail the ecohydrological effects which have occurred since the launching of artificial water conveyance 20 years ago. The results show that the groundwater table in the upper, middle and lower segments of the Tarim River’s lower reaches has been raised on average 4.06, 4.83 and 5.13 m, respectively, while the area of surface water bodies connected to those sections has expanded from 49.00 km2 to 498.54 km2. At the same time, Taitema Lake, which is the terminal lake of the Tarim River, has been revived and now boasts a water area of 455.27 km2. Other findings indicate that the surface ecological response is extremely sensitive and that the area of natural vegetation has expanded to 1423 km2. Furthermore, the vegetation coverage, vegetation index (NDVI), and Net Primary Productivity (NPP) have increased by 132 km2, 0.07 and 7.6 g C m−2, respectively, and the Simpson dominance, McIntosh evenness, and Margalef richness indices have risen by 0.33, 0.35 and 0.49, respectively, in the monitored sample sites. As well, the carbon sink area has expanded from 1.54% to 7.8%. Given the increasing intensity of the occurrence of extreme hydrological events and successive dry years, similar ecological water conveyance projects should be considered elsewhere in China and in other parts of the world. The water conveyance scheme has generally proven successful and should be optimized to enhance the benefits of ecological water conveyance under water resource constraints.

. Changes in groundwater depth of typical monitoring cross-sections pre-and post-conveyance of water in the lower reaches of Tarim River from 2000 to 2020. Yengsu, Karday, Argan and Yikanbujima are four monitoring sections in the lower reaches of Tarim River. "#1"is the No. 1 groundwater level monitoring well on each monitoring section, which is located 50 m away from the river. www.nature.com/scientificreports/ In the early stages of the water conveyance projects (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010), the groundwater table in the upper and middle segments of the lower reaches of the Tarim River rose to a relatively large extent, while the groundwater table in the lower segment of the river only showed an increasing rising trend after 2011. The monitoring results reveal that after nearly 20 years of ecological water conveyance, the groundwater table in three sections of the lower reaches of the Tarim has been affected at a range of more than 1000 m. The three sections are the Yengsu section in the upper segment, the Karday section in the middle segment, and the Yiganbujima section in the lower segment. Furthermore, the groundwater table has risen by 2.69, 1.38 and 1.59 m, respectively, in these three sections 22 . Within 100 m from the river, the water table depth rose from 7.76, 9.31 and 7.82 m prior to ecological water conveyance to 3.70, 4.48, and 2.69 m, and 4.06, 4.83, and 5.13 m, respectively, after it. Within 500 m from the river, the water table rose by 1.6, 3.99, and 5.26 m, respectively. The shallow groundwater in the lower reaches of the Tarim River has also been recharged to a certain extent, and the lateral influence range is still gradually expanding.
Changes in water body area. The changes in water body area in the lower reaches of the Tarim River are closely related to the amount of water delivered via conveyance. During the past 20 years, the surface water body area, seasonal water body area, and permanent water body area all decreased to the lowest point in 2009, with the river water failing to reach Taitema Lake, the river's terminal, in 2006, 2007, and 2009 23 . The surface water body area, seasonal water body area and permanent water body area in the river's lower reaches fluctuated and increased during the ecological water conveyance process. In particular, the seasonal water body area in the upstream section showed a significant expansion. The area increase rate of surface water, seasonal water, and permanent water in the middle section from Yengsu to Argan is 1.75 km 2 a −1 , 1.58 km 2 a −1 , and 0.16 km 2 a −1 , respectively. Similarly, the area of surface water bodies, seasonal water bodies, and permanent water bodies in the lower section (below Argan) increased at the rate of 13.48 km 2 a −1 , 8.24 km 2 a −1 , and 5.23 km 2 a −1 , respectively. It is worth mentioning that the area of surface permanent water body and seasonal water body in Taitema Lake significantly increased, with the area of the lake waters expanding 417.08 km 2 , from 38.19 km 2 in 2000 to 455.27 km 2 in 2019. This represents a nearly 12-fold increase (Fig. 2).
Vegetation sample site monitoring analysis. The vegetation species in the lower reaches of the Tarim River were sparsely distributed, with P. euphratica and Tamarix sp. as the main established species. In the longitudinal direction, surface vegetation coverage and species number decreased as the water table depth increased from the upper and middle segments to the lower segment. In the lateral direction, surface vegetation shows the same trend, with groundwater table depth increasing the greater the distance from the river 13 .
The surface ecological processes in the lower reaches of the Tarim River have responded positively to the water conveyance project, with density, coverage and the number and diversity of species significantly increasing.  www.nature.com/scientificreports/ However, the response of surface ecological processes to the changes in groundwater table uplift has varied from section to section. In the lateral direction, the groundwater table in areas nearer to the river had a more prominent rise and the response of surface vegetation was stronger, whereas the groundwater table rise in areas farther from the river was smaller and so the response of surface vegetation was weaker. In the longitudinal direction, the same trend was observed from the upper to the lower segments in response to changes in the groundwater table.
In this paper, we analyze the changes in detail by taking a closer look at the Yengsu section, which is located at the beginning of the middle section of the lower reaches of the Tarim River. In so doing, we apply sample site investigation and dynamic monitoring of the groundwater table to the study area.  In the downstream segment, the area covered by all types of vegetation showed an upward trend, with the area covered by low vegetation expanding significantly (Fig. 4). In the lateral direction, the NDVI within 2 km of the water conveyance channel showed a more obvious response with greater increases, while NDVI beyond 2 km from the channel revealed smaller increases 25 . These differences reflect the influence range of the ecological water conveyance.

Changes in net primary production (NPP) of natural vegetation. Net primary production (NPP)
is a key parameter of carbon cycling and energy flow in terrestrial ecosystems. NPP not only reflects terrestrial ecosystem productivity, but also characterizes the quality of terrestrial ecosystems and plays an important role in global change and carbon balance 26,27 . The results of our study show that the area of natural vegetation in the lower reaches of the Tarim River with highly significant and significant increases in NPP during the study period accounted for 31.93% (P < 0.01) and 11.49% (P < 0.05), respectively, with an increase rate of 0.40 g C·m −2 ·a −1 . The increases were greater in the upper and middle segments of the lower reaches of the river than in the lower segment. In terms of vegetation type, the magnitude of the multi-year mean NPP was in the order of Tamarix spp. community > P. euphratica community > herbaceous community. The largest increase in NPP was observed in the Tamarix spp. community, rising 350.20% from 2001 to 2019 28 .
Area changes in vegetation carbon sink area. The ecological water conveyance project in the lower reaches of the Tarim expanded the vegetation coverage and enhanced the carbon sequestration capacity of the region through photosynthesis. The lower reaches of the river are dominated by desert and sparse vegetation, and the ecosystem carbon sinks are mainly low carbon sinks. The monitoring results of the study show that the vegetation carbon sink area in the river's lower reaches indicate a gradual expansion under the influence of the ecological water conveyance 29 , increasing from 1.54% of the study area in 2001 to 7.8% in 2020. As well, the Net Ecosystem Productivity (NEP) of the area's vegetation showed an increasing trend at a rate of 0.541 g C·m −2 ·a −1 , with the largest increase -0.406 g C·m −2 ·a −1 -occurring in summer 29 and no significant carbon sink area in winter.
Furthermore, in order to quantitatively investigate the degree of influence of ecological water conveyance on the carbon sink area in the lower reaches of the Tarim, a linear fit of cumulative water conveyance and carbon sink area was performed (Fig. 5). Based on the results, a strong linear correlation was found between cumulative water conveyance and carbon sink area (R 2 = 0.958, p < 0.005). The data points were all within the 95% confidence interval, indicating that, with the increase of ecological water conveyance in the lower reaches of the river, the carbon sink area appeared to respond positively, taking into account a lag effect of about one year.

Conclusion
River cutoff and ecological degradation in the lower reaches of inland river basins in arid regions is a growing problem. In order to salvage and restore damaged ecosystems of disconnected rivers, ecological water conveyance projects were implemented in several watersheds in the arid zone of northwest China, starting in 2000. Since then, significant ecological benefits have been achieved. For example, in the Tarim River's lower reaches, the groundwater table on both sides has risen by 3-5 m, and the range of influence exceeds 1000 m from the river. Additionally, the response range of natural vegetation has expanded from 492 km 2 to 1423 km 2 , and the river's terminal, Taitema Lake, has been revived, forming a water area of 455.27 km 2 . These results clearly indicate that the continuous ecological deterioration along the banks of the lower reaches of the Tarim River has successfully been curbed.
However, because the conveyance projects have mainly taken the form of natural drainage along the river channel, the ecological response has been slow and the impact relatively limited. Only the groundwater www.nature.com/scientificreports/ near both sides of the river has effectively been raised. The three main manifestations of this process in the upper and middle segments of the lower reaches of the Tarim River are as follows: (1) The analysis of changes in groundwater level shows that the recent groundwater table rise has slowed significantly and has tended to reach an equilibrium state at some sections. Therefore, the intensity of the impact of the artificially introduced water on surface vegetation is weakening. (2) The natural water conveyance method along the river channel cannot achieve the purpose of plant seeding and regeneration. Further, this method will likely not help the region obtain ecological sustainability, as it is even more difficult to make a large and rapid recovery of the growth of grass and shrub vegetation on both sides of the riverbank. (3) A large amount of river water has flowed into the terminal lake (Taitema Lake). Although this has led to an expansion of the water area, most of the water bodies are being dissipated by evaporation. Given the above challenges, we suggest taking the following course of action to remedy them. (1) The water conveyance currently proceeds on a "linear" path along the natural river to maintain the rivers ecological flow. However, the subdivision of surface overflow water supply from top to bottom could be engineered to promote the seeding and regeneration of P. euphratica, Tamarix spp. and other species, as well as the recovery of grass and shrub vegetation on both sides of the riverbank. (2) In areas with soil characterized as seed-poor, river overflow could help disperse artificial floating seeds to promote the growth of surface vegetation. This would rapidly increase the surface coverage and expand the scope and effect of the ecological water conveyance area. (3) Taitema Lake needs to be scientifically assessed and its ecological function and suitable water area determined. At the same time, the uncertainty brought by the increase in frequency and intensity of extreme hydrological The Tarim River Basin is located in the south of Xinjiang, China, with the Tianshan Mountains to the north, the Kunlun Mountains to the south, and the Pamirs Plateau to the west. It consists of 9 major headwaters and 114 water systems, covering an area of about 102 × 10 4 km 2 . Due in part to its inland remote positioning, the basin is characterized by the duality of relatively abundant natural resources and an extremely fragile ecological environment. The main stream of the Tarim River is 1,321 km long and is roughly divided into the upper, middle and lower reaches (Fig. 6), the latter which is located between the Taklamakan Desert and the Quruq Desert, with the Qiala hydrological station as the connector node. The terminal of the Tarim River is Taitema Lake. The lower reaches of the Tarim have a continental warm temperate desert arid climate, with dry  www.nature.com/scientificreports/ and windy weather and average annual precipitation ranging from 17.4 to 42.0 mm. The average annual evaporation is as high as 2500-3000 mm, making it one of the driest regions in China 30 . The vegetation type in the Tarim's lower reaches is mainly desert riparian. Accordingly, the plant community has low diversity and is mostly composed of species with high salt tolerance and drought resistance. There is also saline desert vegetation in localized areas with a single structure and poor species. Among the natural plants, the trees mainly include Populus euphratica, and the shrubs and herbs mainly include Tamarix ramosissima, Tamarix hispida, Lycium ruthenicum, Alhagi sparsifolia, Apocynum venetu, Karelinia caspica, Glycyrrhiza inflata, and Phragmites communis.
Since the 1960s, the Tarim River Basin has been under the influence of intensive human economic and social activities centered on the exploitation of soil and water resources. As a result, a 321 km stretch of the lower reaches of the Tarim River was cut off, the river's terminal lakes (Lop Nor and Taitema Lake) dried up in 1970 and 1972, respectively, and the groundwater table dropped significantly 13 . Furthermore, in the Yengsu section of the river, the groundwater table depth is generally below 8 m, and even below 12 m in some sections. There has also been a precipitous decline in the region's natural vegetation, such as P. euphratica and Tamarix spp. forest, accompanied by an increase in wind erosion and sanding and an intensification of land desertification. In short, the overall ecosystem has been severely damaged 30 , mainly by human activities. At the same time, the "Green Corridor" (it refers to the desert riparian forest ecosystem in the lower reaches of the Tarim River Basin, which is a verdant ecological corridor.) of the lower reaches of the Tarim, which is sandwiched between the Taklamakan and Quruq deserts, has shrunk dramatically and is on the verge of disappearing.
The increasing ecological problems and water crisis in the Tarim River Basin have become a hot research topic in relation to ecological and environmental problems in western China. In 2000, the Chinese government implemented an ecological water conveyance project aiming to raise groundwater levels, save the Green Corridor, and curb the manmade ecological deterioration 13 .
Data acquisition. Plant sample monitoring. In order to understand the changes in groundwater table depth and surface vegetation during the water conveyance process, we established nine monitoring sections in 2000 (Fig. 6) along the lower reaches of the Tarim River water conveyance channel (the Qiwenkuoer River). The distance between each section is about 20 km. From the Daxihaizi reservoir downward, the monitoring sections are Akdun (I), Yahopmarhan (II), Yengsu (III), Abudali (IV), Karday (V), Tugmailai (VI), Argan (VII), Yiganbujima (VIII), and Korgan (IX). Among these, the spacing of the latter three cross-sections is 45 km (Fig. 6).
Fifty-five long-term ecological sample plots in total were constructed at the monitoring sections. Each sample plot is 50 m × 50 m and focuses on the number of individual plants as well as plant coverage, diameter at breast height, diameter at base, and the height and crown width of each tree and shrub. We surveyed the plant community in July every two or three years from 2000 to 2021. The plant sample monitoring was done using the methods described in Chen et al 5 . All of these methods were performed according to the relevant guidelines and regulations set forth by the Chinese government.
Groundwater table depth monitoring. Fifty-five groundwater monitoring wells (8 ~ 17 m depth) were installed near the sample sites of the nine monitoring sections to track the dynamic changes of groundwater table depth and water salinity during the water conveyance process. Initially, changes in the groundwater table depth were measured manually by conductivity. However, a HOBO water level logger (Campbell Scientific, Logan, UT, USA) was later installed at each monitoring well and record pressure-head readings (absolute pressure) were made every 18 h. Groundwater table depth from the soil surface is calculated using the corrected pressure-head measurement and the known sensor elevation.
Ecological water conveyance. The ecological water conveyance project of the lower reaches of the Tarim River was launched in May 2000. By 2020, 21 intermittent water conveyance channels had been implemented in the 321 km disconnected river channel of the lower reaches of the river. Except for the years 2006, 2007 and 2009, the river water reached the river's terminal, Taitema Lake, most of the time, delivering 84.45 × 10 8 m 3 of ecological water to the lower reaches of the river ( Table 1). The total amount of groundwater and aeration zone recharged through river infiltration along the course of the river was about 30.6 × 10 8 m 3 and 42.15 × 10 8 m 3 , respectively, and the amount of water entering Taitema Lake was about 11.7 × 10 8 m 3 , accounting for 36.23%, 49.92% and 13.85% of total water delivered, respectively 31 .

Methodology. Species diversity indices.
These indices measure several different factors, such as number, evenness and richness. The Simpson diversity index reflects the species' variation in number and biological diversity, the McIntosh index reflects the species' evenness, and the Margalef index reflects their richness. In the sample site survey, the following methods were used in the calculations 31,32 .
Simpson index : =1− where N is the total number of individuals of all plants, A is the area of the sample plot, and a i is the canopy area of the i-th plant. Additionally, using multi-source remote sensing MOD13Q1 and MCD12Q1 data with a temporal resolution of 16 d and a spatial resolution of 250 m, we calculated the basin-scale vegetation index (NDVI) and vegetation coverage changes since the implementation of the ecological water conveyance in the lower reaches of the Tarim River. To do so, we employed the Google Earth Engine (GEE) computing platform and multivariate statistical analysis to analyze changes in surface water area based on images from Landsat 5, 7 and 8 and ecological water conveyance data. Further, we applied the Ames Stanford Approach (CASA) model to estimate changes in NPP in natural vegetation, and the modified soil microbial respiration model R H (Heterotrophic Respiration) to estimate the NEP (Net Ecosystem Productivity) of vegetation. Based on these techniques, were able to analyze the spatial variations in vegetation carbon source/sink.

Data availability
All of the data used in the present study are available from the corresponding author upon request.