No abstract

The headwater of Yellow River Basin (HYRB) is crucial for the water resources of the whole basin in Northwest China. Based on the semi-distributed hydrological model "Soil and Water Assessment Tool" (SWAT), the spatiotemporal change trends of blue water and green water resources in the HYRB were analyzed quantificationally. By using the Sequential Uncertainty Fitting program (SUFI-2), the model was calibrated at Tangnaihai hydrological station and uncertainty analysis was performed. The results showed that the total water resources decreased by 1.08 billion m 3 over the past five decades in the HYRB. Blue water and green water storage (soil water) presented the downtrend, while green water flow (actual evapotranspiration) increased between 1961 and 2010. The decrease in blue water resources were mainly attributed to the decrease in precipitation in the southwest parts of the study area while the increase in actual evapotranspiration and the decrease in soil water were the results of the uptrend of air temperature. In 1990s, an enormous transition occurred between the blue water (24.86 %) and green water flow (63.46 %). At seasonal scale, the largest down trend of blue water and uptrend of actual evapotranspiration all occurred in autumn. The decrease ratios of them were 88.3 and 83.1 % in inter-annual variation, respectively. The study can provided a scientific basis for integrated water resources management under the background of global climate change and human activity.

Deep insight into the forces driving chloramine decay in different material of pipes is the key to take sound actions to cope with the pipe water quality deterioration. By using the newly developed RTCDM (refined Total Chloramine Decay Model) and pipe section reactor, the role of four typical pipes in disinfection chemistry was qualitatively and quantitatively compared, and the mechanism of pipe wall mediated chloramine decay was further described. As for the four typical pipes studied, the ability of deteriorating water quality, especially for accelerating TCR decay was in the order of cast iron pipe > steel pipe > cement lined ductile iron pipe > polypropylene-random pipe. Cast iron pipe, cement lined ductile iron pipe, and steel pipe with long serving age are characterized by one or two driven forces leading to TCR decay. Aged cast iron pipe could consume chloramine by Fe(0) and microbe (especially for nitrifier) spreading over the inner wall. Aged steel pipe is characterized by aggressive electrochemical corrosion and weak nitrification. Lime and gypsum leaching is the main cause, and nitrification/denitrification may also occur in aged cement lined ductile iron pipe. Polypropylene-random pipe has minimum effect on disinfection chemistry. This knowledge is of value in speculating the reasons leading to TCR loss in the full scale distribution system.

In this study, a pilot-scale pipeline reactor and refined total chloramine decay model were employed to study the effects of the highly corrosive pipe wall and three kinds of disinfectant constituents on the chlorine decay behavior. The bulk decay coefficient, kbulk for NaClO, NaClO + NH3·H2O, and NaClO + (NH4)2SO4 were 0.011, 0.004, and 0.004 h−1, respectively. By resorting to the refined total chloramine decay model, the comprehensive wall decay coefficient kwall appeared in the ascending order of NaClO, NaClO + NH3·H2O, and NaClO + (NH4)2SO4. The remarkable contribution of the aged cast iron pipe wall to overall total chlorine residual decay was manifested by the ratio, kwall/kbulk. The pipe wall-induced decay was related to microbe consumption and electrochemical corrosion as indicated by variations in TOC, NO2−-N, NO3−-N, and Fe residual. The larger kwall for NaClO + NH3·H2O (0.720 h−1) relative to NaClO (0.465 h−1) was mainly attributed to enhanced nitrifier-mediated microbe consumption. The largest kwall for NaClO + (NH4)2SO4 (0.864 h−1) was due to the further promoted microorganism regrowth and metabolization as evidenced by the SO42- declining behaviors. On this basis, it was suggested to minimize extra inorganic salt introduction into treated water to constrain microbial development in DWDS. Temporal-free chlorine disinfection was also recommended for the chloraminated DWDS before the critical temperature.