Understanding the variability and stability of hydrological components under vegetation restoration in watersheds on the Chinese Loess Plateau in past 50 years

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Assessing ecological restoration effects on hydrological regimes is important for watershed management, especially under semi-arid conditions.  The long-term trends of streamflow components were analyzed for three less forested watersheds (LFWs), and for two largely forested watersheds (FWs) within the Beiluo River Basin on the Chinese Loess Plateau.  Three LFWs were undergoing major vegetation restoration, in which the vegetation coverage changed from 16–23% in 1970 to 57–80% coverage in 2019. While as contrast, the two FWs has vegetation coverage from 65–68% in 1970 to 88–92% in 2019. Daily flow data for each watershed and year were normalized by rainfall to eliminate effects of non-stationary rainfall.  Mean annual streamflow totals for the ~60-year study period were 25.1–34.1 and 21.6–48.1 mm y_1 for the LFWs and FWs, respectively.  Average contributions of baseflow to total streamflow were 32–44% for the LFWs and 58–61% for the FWs. Mann-Kendall tests showed significant decreasing trends for annual streamflow and stormflow (0.23–0.54% y_1) from the LFWs throughout.  Mean streamflow from the LFWs between 2000 and 2019 decreased by 58% compared to the pre-2000 period, while the average contribution of stormflow decreased from 66% to 35% (2010s). However, winter baseflow increased with time since start of restoration.  Conversely, streamflow (components) for the FWs showed only slight fluctuations in decadal, annual and seasonal trends.  Both total streamflow and stormflow exhibited strong power relationships with vegetation coverage, illustrating a tendency towards stable flow regimes for vegetation coverage >60–70%.  Large-scale vegetation restoration has fundamentally changed amounts and temporal distribution of streamflow (components) on the Loess Plateau mainly by regulating stormflow. After 20 years of restoration, the trend and proportion of streamflow components from the LFWs approximated those of the FWs.  Since soil erosion is driven by a certain of amount and intensity of runoff, the above results imply that soil erosion and sediment transportation would reach a stability with vegetation restoration to a certain coverage in watersheds. But it is necessary to investigate the difference among regions.  These results underpin the planning of sustainable management of natural resources and socio-economic development during long term ecological restoration on the Loess Plateau.Keywords: Trends of long-term hydrological elements; Surface flow and baseflow; Vegetation restoration; Loess Plateau