Comparisons of flow time series between preimpact and postimpact periods have been widely used to determine hydrological alterations caused by reservoir operation. However, preimpact and postimpact periods might also be characterized by different climatological properties, a problem that has not been well addressed. In this study, we propose a framework to assess the cumulative impact of dams on hydrological regime over time. The impacts of the Three Gorges Dam (TGD) on the flow regime of the Yangtze River were investigated using this framework. We reconstructed the unregulated flow series to compare with the regulated flow series during the same period (2010 to 2015). Eco-surplus and eco-deficit and the Indicators of Hydrologic Alteration (IHA) parameters were used to examine hydrological regime change. Among 32 IHA parameters, Wilcoxon signed-rank test and principal component analysis identified the October median flow, 1-and 3-day maximum flows, 1-day minimum flow, and rise rate as representative indicators of hydrological alterations. Eco-surplus and eco-deficit showed that the reservoir also changed the seasonal regime of the flows by reducing autumn flow and increasing winter flow. Changes in annual extreme flows and October flows lead to negative ecological implications downstream of the TGD. Ecological considerations should be taken into account during operation of the TGD in order to mitigate the negative effects on the fluvial ecosystem in the middle reach of Yangtze River. The framework proposed here could be a robust method to assess the cumulative impacts of reservoir operation over time.

Groundwater in arid/semiarid regions plays crucial roles in providing drinking water supply, supporting irrigated agriculture, and sustaining important native terrestrial ecosystems. Groundwater depth controls water availability to vegetation and is essential for conserving groundwater-dependent terrestrial ecosystems. Environmental groundwater depth can be defined as a mean depth or a range of depths, satisfying the growth of natural vegetation that is not under stress, either due to lack of water or anoxia or soil salinization. Five methodologies have been reported to estimate environmental groundwater depth: the direct ones rely on response functions that relate vegetation condition, e.g., physiological parameters, appearance frequency, community structure, and remotely sensed physical indexes, to changes in groundwater depth; the indirect one estimates environmental groundwater depth based on the threshold of soil moisture content. To fill a knowledge gap of unique recognized methodology, a conceptual framework was proposed, which involves initial estimation (data collection, response assessment, and estimation) and feedback adjustment (implementation and modification). A key component of the framework is to quantify the linkage between ecological conditions and geohydrological features. This review may provide references for groundwater resources management, ecological conservation, and sustainable development in arid/semiarid regions.

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Hydrological changes of the Irtysh River were analyzed concerning the changes of annual runoff and its distribution features within a year measured by coefficient of variation and concentration degree. Abrupt changes were detected by the heuristic segmentation method.Possible causes of the hydrological changes were investigated considering climate changes and human activities (especially the reservoir operation). The Mann-Kendall method was applied to estimate whether the temperature and precipitation was changed. The increased precipitation in winter may increase the runoff of April. The increased temperature and the decreased precipitation in the flood season may decrease the runoff. At the middle reaches, the impact of the reservoirs at the upper reaches is significant and may be the main factor leading to the abrupt decreases in annual runoff and its intra-annual variability and concentration. The increased water surface area of the reservoirs aggravates the evaporation and leads to annual runoff reduction. The reservoirs regulate runoff by storing water in the flood season and releasing water in the dry season. While at the lower reaches, the annual runoff remained steady and its intra-annual variation and concentration were reduced gradually because the impact of the reservoirs is relative small and the climatic impact may be more relevant.