Changes in climate extremes have become a hot issue in the research field of climate change recently. Many studies have reported that climate extremes have occurred more frequently and with increasing intensity in recent decades. In this study, thresholds of precipitation and drought extremes were determined by the cumulative distribution function, and their spatiotemporal changes over the past half-century in China were analyzed by relative change rate. The results show that: (1) precipitation extremes increased in all regions except North China, while increasing trends of drought extremes were detected in all regions except Northwest China and the Qing–Tibet Plateau; (2) the maximum change rates in frequency of precipitation extremes were found in Northwest China and the Qing–Tibet Plateau, with values of 16.13% and 8.12%, and the maximum change rates in frequency of drought extremes were in Southwest and Southeast China, whose increases in intensity of drought extremes were also the maximum; (3) variation in precipitation extremes showed a relatively mixed pattern with higher heterogeneity compared to that of drought extremes; and (4) changes in precipitation and drought extremes relate to mid-intensity, lower-intensity, and annual precipitation.

Increases in climate extremes and their impacts have attracted global attention recently. In this study, the change-point years of precipitation extremes (PEs) and drought extremes (DEs) were investigated by Moving t-Test at 500 stations across the six regions in China. The detailed temporal change processes of them were demonstrated by the cumulative deviation method based on the data from nine typical stations. The results showed that: 1) DEs were more significantly and widely increased than PEs, the stations with increasing trends of PEs and DEs accounted for greater than 52.6% and 61.6% of the total, respectively; 2) increasing trends of DEs were mainly distributed in the east of Hu Huanyong Line. In this area, the increasing change-point years of DEs often occurred in the early 1980s in the south of the Yangzi River, while occurred in the 1990s in the north of the Yangzi River; 3) increasing trends of PEs were mainly distributed in Qing-Tibet Platen, Northwest China, and the southeastern area of Hu Huanyong Line. In these areas, the increasing change-point years of PEs often occurred around 1990 in the southeast of Hu Huanyong Line, while often occurred in the early 1980s in Qing-Tibet Platen. The results indicated that the area in the southeast of Hu Huanyong Line was under the threats of both PEs and DEs, this may produce severe impacts on agriculture, environment, water resources management, human society, etc.

Karst watershed refers to the total range of surface and underground recharge areas of rivers (including subterranean rivers and surface rivers) in karst areas. Karst water resources, as the primary source of domestic water supply in southwest China, are vital for the social and economic development of these regions. It is greatly significant to establish a high-precision hydrological model of karst watershed for guiding water resources management in karst areas. Choosing the Daotian river basin in the Wumeng Mountains of Southwest China as the study area, this paper proposed a method for simplifying karst subterranean rivers into surface rivers by modifying the digital elevation model (DEM) based on a field survey and tracer test. This method aims to solve the inconsistency between the topographical drainage divides and actual catchment boundaries in karst areas. The Soil and Water Assessment Tool (SWAT) model was modified by replacing the single-reservoir model in the groundwater module with a three-reservoir model to depict the constraints of multiple media on groundwater discharge in the karst system. The results show that the catchment areas beyond topographic watershed were effectively identified after simplifying subterranean rivers to surface rivers based on the modified DEM data, which ensured the accuracy of the basic model. For the calibration and two validation periods, the Nash–Sutcliffe efficiencies (NSE) of the modified SWAT model were 0.87, 0.83, and 0.85, respectively, and R² were 0.88, 0.84, and 0.86, respectively. The NSE of the modified SWAT model was 0.09 higher than that of the original SWAT model in simulating baseflow, which effectively improved the simulation accuracy of daily runoff. In addition, the modified SWAT model had a lower uncertainty within the same parameter ranges than the original one. Therefore, the modified SWAT model is more applicable to karst watersheds.

In order to achieve the rational development and utilization of underground water resources in the Dongsheng mining area under coal mining conditions, we selected the upstream area of Beiniuchuan River as a typical region. Through field investigations, sampling tests, and the application of hydrochemical and isotope techniques, we traced the groundwater circulation mechanism in the Dongsheng mining area. The results indicate that the majority of the Quaternary alluvial and Salawusu Formation groundwater is of the HCO₃-Ca type, with a TDS content below 300 mg/L. However, in some areas, the hydrochemical type becomes complex due to anthropogenic contamination. The shallow-buried Yan’an Formation groundwater is either of the HCO₃-Ca·Mg type or the HCO₃·SO₄-Ca·Mg type, with TDS content ranging from 200 to 750 mg/L. The Yan’an Formation at depths greater than 40 m exhibits complex water chemistry, with a TDS content higher than 500 mg/L, and it belongs to the Cl-Na type, with TDS around 700 mg/L. The hydrogen and oxygen isotope results indicate that the local groundwater is primarily recharged via atmospheric precipitation. The ³H and ¹⁴C results show that the Quaternary alluvial and shallow-buried Yan’an Formation groundwater has a fast turnover rate, while the deep-buried Yan’an Formation and Yan’chang Formation groundwater have a slower turnover rate. The regional groundwater circulation can be generalized into three flow systems: shallow, intermediate, and deep. Under the influence of coal mining activities, the water circulation conditions in the study area have undergone significant changes. The sealing integrity of the Yan’an Formation has been compromised, and precipitation and shallow groundwater have enhanced the vertical infiltration capacity of the formation, increasing the proportion of groundwater participating in the intermediate flow system. As a result, the river runoff mainly dependent on the discharge from the shallow flow system has drastically decreased.