Vegetation greening weakened the capacity of water supply to China's South-to-North Water Diversion Project

Abstract

Abstract. Recent climate change and vegetation greening have important implications for global terrestrial hydrological cycles and other ecosystem functions, raising concerns about the watershed water supply capacity for large water diversion projects. To address this emerging concern, we built a hybrid model based on the Coupled Carbon and Water (CCW) and Water Supply Stress Index (WaSSI) models and conducted a case study on the upper Han River basin (UHRB) in Central China that serves as the water source area to the middle route of the South-to-North Water Diversion Project (SNWDP). Significant vegetation greening occurred in the UHRB during 2001–2018, largely driven by the widespread afforestation in the region, with the normalized difference vegetation index increasing at a rate of 0.5±0.1 % yr−1 (p<0.05) but with no significant trends in climate during the same period (albeit with large interannual variability). Annual water yield greatly decreased, and vegetation greening alone induced a significant decrease in water yield of 3.2±1.0 mm yr−1 (p<0.05). Vegetation greening could potentially reduce the annual water supply by 7.3 km3 on average, accounting for 77 % of the intended annual water diversion volume of the SNWDP. Although vegetation greening can bring enormous ecosystem goods and services (e.g., carbon sequestration and water quality improvement), it could aggravate the severity of hydrological drought. Our analysis indicated that vegetation greening in the UHRB reduced about a quarter of water yield on average during drought periods. Given the future warming and drying climate is likely to continue to raise evaporative demand and exert stress on water availability, the potential water yield decline induced by vegetation greening revealed by our study needs to be taken into account in the water resources management over the UHRB while reaping other benefits of forest protection and ecological restoration.

Does non-stationarity induced by multiyear drought invalidate the paired-catchment method?

“…on paired-catchment experiments, and is still used today (Van Loon et al, 2019). However, separating the effects of vegetation change and climate variability on runoff remains a great challenge due to the complex interactions between climate variability and vegetation change (Cavalcante et al, 2019;Jones et al, 2006;Zhang et al, 2021). Moreover, persistent hydroclimatic non-stationary changes observed during the past few decades have increased both temperatures and occurrences of extreme weather events (such as multiyear drought).…”

Divergent Hydrological Responses to Forest Expansion in Dry and Wet Basins of China: Implications for Future Afforestation Planning

“…The other group suggests that forests retain more water through their deep root system via increasing soil water storage (Li et al., 2018; Ward et al., 2021; Yao et al., 2016; W. Zhang et al., 2016; Zhou et al., 2010). Observations support both groups to some extent, indicating that a more detailed exploration of the hydrological response to afforestation is required (Ward et al., 2021; Xiao et al., 2020; J. Zhang et al., 2021; Zhao et al., 2021; Zhou et al., 2010).…”

“…Generally, there are two groups of studies with contrasting conclusions on the effects of afforestation on runoff. One group argues that afforestation has a negative effect on a basin’s water yield mainly owing to an enhanced E (Andréassian, 2004; Brown et al., 2005; Feng et al., 2016; Swank & Douglass, 1974; Xiao et al., 2020; M. Zhang et al., 2021; Zhao et al., 2021). The other group suggests that forests retain more water through their deep root system via increasing soil water storage (Li et al., 2018; Ward et al., 2021; Yao et al., 2016; W. Zhang et al., 2016; Zhou et al., 2010).…”

Attribution Assessment and Prediction of Runoff Change in the Han River Basin, China

“…In a previous study, it was found that vegetation cover can weaken the water supply capacity of the South-North Water Transfer to some extent. For example, Zhang et al [66] found that vegetation greening may exacerbate the degree of hydrological aridity. The increase in forest cover can reduce runoff in the HRB by as much as 0.19%, thus affecting river health to some extent [63].…”