The water table depth, surface water body area, and surface ecological processes have all changed significantly during the 20 years the ecological water conveyance projects have been underway in the lower reaches of the Tarim River. Specifically, there has been a notable increase in the water table, surface water body area, vegetation density and coverage, the vegetation index (NDVI), Net Primary Production (NPP) of natural vegetation, and ecosystem function and health. The following sections provide details on these changes.
Changes in groundwater table depth
Groundwater (soil water) is the most important water source for maintaining natural vegetation in the lower reaches of the Tarim River, as the climate is extremely arid and atmospheric precipitation has little ecological significance. The changes in water table depth are directly related to the composition, distribution, and growth of the natural vegetation of the desert riparian forest, which in this case is mainly P. euphratica5. During the past 20 years, the ecological water conveyance in the lower reaches of the Tarim has been intermittent, and the groundwater table elevation has been closely related to the water conveyance. From the analysis of the groundwater table’s rise in the upper, middle, and lower reaches of the Tarim River (Fig. 1), the magnitude of the uplift is clearly related to four crucial factors: the groundwater table depth prior to the water conveyance, the volume of water discharge, the duration of the transfer, and the water head location.
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.
In the early stages of the water conveyance projects (2000–2010), 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 sections22. 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 200923. 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 km2 a−1, 1.58 km2 a−1, and 0.16 km2 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 km2 a−1, 8.24 km2 a−1, and 5.23 km2 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 km2, from 38.19 km2 in 2000 to 455.27 km2 in 2019. This represents a nearly 12-fold increase (Fig. 2).
Spatial distribution of water surface area in lower reaches of Tarim River in 2000 and 2019. The subfigures were generated in R 4.0.2 (https://cran.r-project.org/bin/windows/), and then merged in Microsoft PowerPoint 2013 (https://www.microsoft.com/).
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 river13.
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. 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.
Changes in vegetation density and coverage
The results of our sample site monitoring show notable positive changes in groundwater depth between 2000 and 2021 as a direct result of the ecological water conveyance initiative. At 150 m from the river, the groundwater table depth rose from 8.47 m to 4.34 m, respectively, representing an uplift of 4.13 m (Fig. 3c). Moreover, the vegetation coverage and density increased from 18.77% and 0.016 plants/m2 to 46.51% and 0.049 plants/m2, and the number of species doubled from three to six.
Changes in vegetation coverage, density and number of species (a), species diversity indices (b), and groundwater depth (c) for each site at Yengsu section in the lower reaches of Tarim River.
At 250 m from the river, the groundwater table depth rose from 8.07 m in 2000 to 4.85 m in 2021, representing an uplift of 3.22 m. The vegetation coverage and density increased from 10.89% and 0.020 plants/m2 to 31.24% and 0.160 plants/m2, respectively, and the number of species jumped from five to seven.
At 350 m from the river, the water table rose 2.48 m between 2000 and 2021. The vegetation coverage and density increased from 3.69% and 0.010 plants/m2 to 22.27% and 0.022 plants/m2, respectively, and the number of species increased from two to three. It is worth noting that the expansion in vegetation cover in the first three sample sites was mainly due to the increase in the number and canopy width of herbs and shrubs that occurred as a direct result of the ecological water conveyance process.
At 750 m from the river, the groundwater table depth rose from 5.96 m to 4.98 m between 2005 and 2021, respectively, representing an uplift of 0.64 m, while the vegetation coverage and density increased from 20.07% and 0.011 plants/m2 to 26.43% and 0.019 plants/m2, respectively.
At 1050 m from the river, the sample site had an elevated water table of 1.22 m. The vegetation coverage and density increased from 2.41% and 0.004 plants/m2 in 2005 to 5.89% and 0.0148 plants/m2 in 2021, respectively (Fig. 3a). Among them, the increase in canopy area of Tamarix sp. and P. euphratica in the sample site was the main reason for the expansion in coverage.
Changes in species diversity indices
Plant richness and evenness in the lower reaches of the Tarim River were low, with species diversity indices showing significant changes in response to the ecological water conveyance and the rise in the groundwater table (Fig. 3b). For example, at the Yengsu section, the Simpson dominance index, McIntosh evenness index and Margalef richness index, which reflect changes in species diversity, decreased from 0.58, 0.45 and 0.74 in 2005 to 0.46, 0.03 and 0.03, respectively. These changes occurred in response to the increase in groundwater depth from the first sample site at 150 m to the sixth sample site at 1050 m from the river channel. After 20 years of ecological water conveyance, the Simpson dominance index, McIntosh evenness index and Margalef richness index had increased on average by 0.33, 0.35 and 0.49, respectively, in the first three sample sites (Fig. 3b).
Vegetation index (NDVI) changes
The Normalized Difference Vegetation Index (NDVI) is an important indicator of vegetation growth24. The study results reveal that the NDVI of the lower reaches of the Tarim River increased from 0.14 in 2000 to 0.21 in 2020, representing a rise of about 33.3%. The ecological water conveyance expanded the river region’s natural vegetation 188%, from 492 km2 in 2000 to 1423 km2 in 2020. Specifically, the area of low, medium, and high vegetation cover expanded by 277 km2, 537 km2 and 132 km2, representing increases of 20.8%, 448% and 190%, respectively. Further analysis of changes in vegetation coverage at different river sections indicate that the area of low vegetation coverage in the upper and middle segments showed a decreasing trend, whereas the area of medium and high vegetation coverage in the upper and middle segments showed an increasing trend. This latter trend was especially prominent in the middle segment, where the increase in the area covered by medium and high vegetation was relatively large.
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 increases25. These differences reflect the influence range of the ecological water conveyance.
Variation of vegetation cover in the lower reaches of Tarim River. Spatial distribution of fraction of vegetation cover in (a) 2000, (b) 2010 and (c) 2020. Trends of (d) high fraction of vegetation cover, (e) middle fraction of vegetation cover and (f) low fraction of vegetation cover in different river sections. (g) Vegetation area and (h) change trend at different distances from the river.
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 balance26,27. 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 201928.
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 conveyance29, 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 summer29and 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 (R2 = 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.
Linear fitting chart of cumulative water conveyance and cumulative carbon sink in the lower reaches of Tarim River.
Source: Ecology - nature.com
