The Dominant Driving Force of Forest Change in the Yangtze River Basin, China: Climate Variation or Anthropogenic Activities?

Under the combined effect of climate variations and anthropogenic activities, the forest ecosystem in the Yangtze River Basin (YRB) has experienced dramatic changes in recent decades. Quantifying their relative contributions can provide a valuable reference for forest management and ecological sustainability. In this study, we selected net primary productivity (NPP) as an indicator to investigate forest variations. Meanwhile, we established eight scenarios based on the slope coefficients of the potential NPP (PNPP) and actual NPP (ANPP), and human-induced NPP (HNPP) to quantify the contributions of anthropogenic activities and climate variations to forest variations in the YRB from 2000 to 2015. The results revealed that in general, the total forest ANPP increased by 10.42 TgC in the YRB, and forest restoration occurred in 57.25% of the study area during the study period. The forest degradation was mainly observed in the Wujiang River basin, Dongting Lake basin, and Poyang Lake basin. On the whole, the contribution of anthropogenic activities was greater than climate variations on both forest restoration and degradation in the YRB. Their contribution to forest restoration and degradation varied in different tributaries. Among the five forest types, shrubs experienced the most severe degradation during the study period, which should arouse great attention. Ecological restoration programs implemented in YRB have effectively mitigated the adverse effect of climate variations and dominated forest restoration, while rapid urbanization in the mid-lower region has resulted in forest degradation. The forest degradation in Dongting Lake basin and Poyang Lake basin may be ascribed to the absence of the Natural Forest Conservation Program. Therefore, we recommend that the extent of the Natural Forest Conservation Program should expand to cover these two basins. The current research could improve the understanding of the driving mechanism of forest dynamics and promote the effectiveness of ecological restoration programs in the YRB.

Evaluation of a multisite weather generator on precipitation simulation in the Yangtze river basin

This paper presents the evaluation of a multisite statistical weather generator (MulGETS: Multisite weather Generator of École de Technologie Supérieure) based on its simulation effect of precipitation in the Yangtze River Basin. MulGETS effectively generates spatially correlated sequences of precipitation simultaneously, while maintaining their spatial and temporal distribution characteristics. On the spatial scales, the accuracy of the model varies from station to station, and in general, the errors are lower at stations in the middle and lower reaches of the Yangtze River Basin than in the upper reaches. This difference is likely to exist because of the lower amount of rainfall and more complex topography than those of the upper river basins. On the temporal scales, the simulated values are more precise on the annual scale than on the seasonal scale. Large relative errors occur more frequently in winter, ranging from −35% to 25%. MulGETS can consistently produce precipitation by considering the intensity, magnitude, and duration indices with sub-basin varied observations. However, the precipitation maxima were much lower than the observations. This work shows the general reasonability of the model in downscaling precipitation in the Yangtze River Basin.

Applicability of Difference in Oxygen-18 and Deuterium of Water Sources and Isotopic Hydrograph Separation in a Bamboo Catchment during Different Rainfall Types

Typhoon storm and plum rain are two typical rainfall types in the lower regions of the Yangtze River Basin, which frequently cause flood disasters in China. New information in stable water isotopes offers the opportunity to advance understanding of runoff mechanisms and water source dynamics in response to these two typical rainfall types. We intensively monitored two representative rainfall events in a small bamboo forestry watershed in 2016. Results showed that precipitation isotopic variations during the event were generally larger than those of other monitored compartments (including throughfall, surface overland water, groundwater and river water) and also larger for the plum rain than for the typhoon event (δ18O varied in 5.2‰ and 3.7‰, respectively). Importantly, the differences of isotopic temporal variation between rainfall and throughfall showed significant impacts on the two-component hydrograph separation for both rainfall types (e.g., if not considered, the pre-event water fractions were 26.6% and 15.3% higher for the typhoon and plum rain events, respectively). Furthermore, we evaluated the role of soil water on the three-component isotopic hydrograph separation model; results revealed that soil water accounted for 10.9% and 28.3% of the total discharge in typhoon and plum rain events, respectively. This underpins the important role of soil water dynamics during the rainy season in this humid region.

Observed Quantile Features of Summertime Temperature Trends over China: Non-Negligible Role of Temperature Variability

Changes in temperature variability can have more serious social and ecological impacts than changes in the mean state of temperature, especially when they are concurrent with global warming. The present study examines the summertime temperatures’ trends over China from the quantile perspective. Through fully investigating the quantile trends (QTs) of the maximum (Tmax) and minimum temperature (Tmin) using the homogenized observation data and quantile regression analysis, we identify evident region-specific quantile features of summertime temperature trends. In most of northern China, the QTs in Tmax and Tmin for all percentiles generally show strong uniform warmings, which are dominated by a warm shift in mean state temperatures. In contrast, the QTs of Tmax in the Yangtze River Basin show distinguishable inter-quantile features, i.e., an increasing tendency of QTs from cooling trends in the lower percentile to warming trends in the higher percentile. Further investigations show that such robust growing QTs of Tmax across quantiles are dominated by the temperature variance. Our results highlight that more attention should be paid to the region-specific dominance of temperature variability in trends and the related causes.

Historical and future runoff changes in the Yangtze River Basin from CMIP6 models constrained by a weighting strategy

Based on the ERA5-Land datasets from 1981-2020, a decadal oscillation has been found in the variation of summer runoff in the middle and lower reaches of the Yangtze River Basin (MLYRB). The oscillation suggests that the MLYRB will experience increased runoff in the next few decades after 2020 which saw a record high runoff in the MLYRB. The decadal changes in summer runoff over the MLYRB under various climate change scenarios are then analyzed with direct runoff outputs from 28 general circulation models (GCMs) participating in the sixth phase of Coupled Model Intercomparison Project (CMIP6). Given that the equal-weighted multi-model ensemble mean could not well represent the historical runoff changes in MLYRB, in this paper we introduced a model weighting scheme that considers both the model skill and independence. It turns out that this scheme has well constrain the models to represent the observed decadal changes of summer runoff. The weighted mean projections suggest that the summer runoff in the MLYRB during 2015-2100 under all warming scenarios will be higher than the present-day; and 2021-2040 is likely to be a period with significantly increased summer runoff. Results of the present study are of great implication for flood control and effective water resources management over the MLYRB in the future, and the weighting approach used in this paper could be applied to a wide range of projections at both regional and global scales.