A rapid shrinkage of Daihai Lake was found in recent decades. The present study analyzed the characteristics of Daihai Lake shrinkage and quantified the contribution of climate and human activities. The results of Mann-Kendall- Sneyers test and moving t-test showed that there was an obvious mutation point of lake level in 2006 and the descending speed of Daihai Lake level post-2006 (-0.46m/a) was 3.22 times that of pre-2006 (-0.14m/a). The centroid of Daihai Lake moved 1365.18 m from southwest to northeast during 1989 ~ 2018 with an average speed of 47.08 m/a. The results of Mann-Kendall trend test revealed that the annual evaporation showed a significant downward trend with a rate of approximately -5.33 mm/a, while no significant trend was found in precipitation. Daihai lake water level showed a very weak relationship with evaporation (r = 0.078, p < 0.01) and precipitation (p>0.05) respectively. Daihai Lake was influenced by human activities mainly from land use/ land cover, building reservoirs, pumping groundwater and directly consuming Daihai Lake water by Daihai power plant (DHPP). It was thought-provoking that DHPP began to consume Daihai lake water in 2006, which was consistent with abrupt change of Daihai lake level. The proportion of human impact was fluctuating upward. Human factors were the main factor of lake water reduction in last 10 years and the 5-year average contribution of human activities to Daihai Lake shrinkage was more than 61.99%. More attention and economic support should be given to prevent the continuous shrinkage of Daihai Lake.

Karst leakage is the key problem that restricts the construction of reservoir areas. In this article, the hydrogeochemical origin and hydraulic connection of the river water, pore water, fissure water, and karst water in Jinghe Dongzhuang Reservoir, which is located in a karst area, are analyzed to determine the possibility of karst leakage in the reservoir area. Piper diagram, Gibbs diagram, ion proportion coefficient, and cluster analysis were comprehensively used to systematically study the hydrogeochemical characteristics and formation mechanism of the study area. The research results show that the water in the study area is weakly alkaline, with complex hydrogeochemical types, including SO₄₋Na, HCO₃·SO₄₋Na, and HCO₃·SO₄₋Na·Mg. Affected by evaporation and concentration, Jinghe River and shallow pore water have high TDS content, and the content of Na⁺(including K⁺), Cl⁻ and SO₄²⁻ is significantly higher than that of fissure water and karst water. Fissure water and karst water are significantly weathered by rocks, and their Ca²⁺ and Mg²⁺ mainly come from carbonate rock dissolution. In the process of groundwater evolution, cation exchange occurs more or less in the three groundwater bodies, resulting in different cation contents in different water bodies. In general, Jinghe River is similar to most of the pore water, but its hydrogeochemical characteristics are obviously different from those of fissure water and karst water, so it has little hydraulic connection with fissure water and karst water, indicating that the leakage in the reservoir area is not significant.

Expansion screen pipe is a new sand control completion technology. In this paper, the optimization design of the slotted structure of the screen pipe is studied. Mechanical models for both seamless base pipes and slotted base pipes have been established based on elastic-plastic mechanics. The finite element simulation analysis of the expansion process of screen pipes with different slotted structures is carried out, and the relationship among the length of the slot L, the number of slots N, the internal pressure P and the maximum stress at the slot end σ is obtained. The principle and optimization process of the NSGA-II algorithm are introduced. Furthermore, a multi-objective optimization model is formulated, taking P and σ as the objective functions, and the NSGA-II algorithm is applied to optimize this model. By analyzing the obtained optimization results for the screen pipe with an outer diameter of 68 mm, the optimal slotting parameters are L = 137 mm and n = 12. For the optimal slotted structure obtained, P is greater than the minimum expansion initiation pressure, and σ is less than the ultimate tensile strength, which indicates that the optimal solution meets the requirements of both theoretical analysis and engineering practice. In this paper, the NSGA-II multi-objective optimization algorithm is introduced into screen pipe structure optimization for the first time. The structural optimization method of expandable screen pipe proposed in this paper improves the design level and efficiency of the screen pipe. At the same time, it provides a new means for research on screen pipe structures in the fields of petroleum and exploration.