Biochar modification can enhance the properties associated with porosity and functional groups and has been identified as an effective way to improve adsorption capacity. Modified corncob biochars pretreated by different contents of MgCl₂ are obtained through slow pyrolysis; then, this work explores the adsorption abilities towards heavy metals. After modification, biochars mainly impregnate carbon surface with Mg(OH)₂ and MgO particles. The best mass ratio of magnesium (Mg)/feedback is 15% for modified biochar, and 15%Mg-BC has much higher (2.36–9.34 times) metal sorption capacity than pristine biochar. Batch adsorption experiments show that copper (Cu(II)) and lead (Pb(II)) on adsorbents follow a pseudo-second-order model and cadmium (Cd(II)) follows an intraparticle diffusion model. The adsorption isotherm of Cu(II) fits well with the Langmuir model, and Pb(II) and Cd(II) fit with the Freundlich model. In the binary system, modified biochar still effectively removes metals, but the sorption capacity of metals decreases rapidly because of competitive sorption. The main adsorption mechanisms of metals include surface precipitation, cation π-banding, complexation, and ion exchange. The results show that Mg-modified biochar may be an environmentally friendly material for reducing the water pollution of heavy metals.

Based on the daily temperature data from 26 meteorological stations in the Minjiang River Basin, for 1961 to 2016, the temporal trend and spatial distribution of extreme temperature in this region were analyzed using 16 extreme temperature indices. The results show that in terms of time variation, determined using linear trend analysis and a Mann–Kendall trend test, the warm and day indices mainly show an upward trend while the cold and night indices mainly show a downward trend in the entire basin. Among them, FD0, TN10, and CSDI significantly decrease at −1.3, −2.9, and −1.1 d/10a, respectively. TN90, TX90, SU25, and TR20 significantly increase at 3.0, 2.6, 2.1, and 2.2 d/10a, respectively. The crop growth period in the basin showed a significant increasing trend at 1.4 days/decade, while the diurnal temperature range showed a nonsignificant increasing trend at 0.03 days/decade. On comparing the change range of each index, it was found that the change range of the warm index is greater than that of the cold index, while the change range of the day index is smaller than that of the night index; thus, the change trends of the maximum and minimum temperatures in the whole basin are not obvious. Analysis of the changing trend at each station showed that the relative index of the extreme temperature in the basin has good climatic consistency in terms of spatial distribution. The distributions of the absolute and extreme indices are not uniform, which is consistent with the change in elevation in the basin. Further, the diurnal temperature range in the upper reaches of the basin is greater than that in the middle and lower reaches. However, because of the more obvious upward trends of the day and warm indices in the middle and lower reaches, and the more obvious downward trends of the night and cold indices in the upper reaches, the daily temperature differences in the upper, middle, and lower reaches of the Minjiang River Basin tend to be consistent. Therefore, the precipitation in the Minjiang River Basin shows a significant decreasing trend; thus, the basin shows a trend of drying and warming.

Hydrological frequency analysis plays an indispensable role in the construction of national flood control projects. This study selects the stations with the smallest and largest discordances in the nine homogeneous regions of Sichuan Province as the representative stations, and results obtained by regional frequency analysis are compared with those obtained by traditional at-site hydrological frequency analysis. The results showed that the optimal frequency distribution of each representative station obtained by traditional at-site hydrological frequency analysis and the ones of corresponding homogeneous regions obtained by regional frequency analysis were not necessarily consistent, which was related to the site and homogeneous regions. At the same time, there were also differences between the fitting of the theoretical rainstorm frequency curve obtained by the two methods and the observation. In general, in each homogeneous region, the results obtained by regional frequency analysis and traditional at-site hydrological frequency analysis at the stations with the largest frequency analysis were quite different. The design values obtained by the two methods were also increasingly different with the increase of the return period. The study has specific reflections on the differences between regional frequency analysis and traditional at-site hydrological frequency analysis.