Climate and landuse change enhance spatio-temporal variability of Dongjiang river flow and ammonia nitrogen.

The adverse impacts of climate and landuse change are threatening the availability of water quantity and its quality, yet there are limited understandings in the response of water availability to changing environment at different spatio-temporal scales. Aimed at quantifying the individual and superimposed effects of climate and landuse change on streamflow and ammonia nitrogen (NH3-N) load in the Dongjiang River Basin (DRB), we dynamically simulated the historical (1981-2010) and future (2030-2070) variation of runoff depth and NH3-N load coupling multiple regional climate model and landuse data. The increase in runoff depth (avg. +233.9 mm) due to climate change was about 33 times greater than that caused by landuse change (avg. -7.2 mm). Especially in the downstream of DRB (Hong Kong, Shenzhen and Dongguan cities, etc.), the maximum rise of runoff depth under climate change was near twice compared with baseline period, indicating the dominant control of climate change on runoff. Also there existed higher coefficient of variation (Cv) value of runoff in the dry season of downstream DRB, contributing potential great fluctuation in runoff. Besides, the variation of NH3-N load was jointly influenced by climate and landuse change, revealing an offset or amplification effect. Moreover, the variability of NH3-N load (Cv value as the metric) increased from 2030, reached a maximum in 2050, following decreased to 2070. The spatial distribution of NH3-N load, in general, presented a downward trend and concentrated near the water body, while the monthly average NH3-N load showed distinct peaks in spring and late summer temporally. Overall, the results highlight the significance of investigating the water availability under changing environment and more adaptive strategies should be proposed for better basin water management.

Effects of different cropping systems on ammonia nitrogen load in a typical agricultural watershed of South China.

The excessive application of agricultural irrigation water and chemical fertilizer has increased crop yields to help meet the demand for food, but it has also led to major water environment problem, i.e. non-point source (NPS) pollution, which needs to be addressed to achieve sustainable development targets. Although numerous studies have focused on the control and reduction of agricultural NPS pollution from the perspective of irrigation and fertilizer, the effects of different cropping systems on NPS pollution (ammonia nitrogen (NH3-N)) in the Dongjiang River Basin (DRB) were seldom assessed. Specifically, variation in the NH3-N load was simulated and analyzed at the annual and semi-annual scales under ten different cropping systems using the Soil and Water Assessment Tool (SWAT) model, which was calibrated and validated with satisfactory statistical index values in the DRB. The results indicated that the NH3-N load decreased, distinctly increased, slightly decreased when sweet potato, peanut, and rice were planted, respectively. Compared with mono-cropping, crop rotation could reduce the NH3-N load, and the planting sequence of crops could affect the NH3-N load to a certain extent. Planting peanuts in spring would dramatically increase NH3-N load. To evaluate NH3-N pollution, a critical threshold of NH3-N emission (5.1 kg·ha-1·year-1) was proposed. Meeting the NH3-N emission threshold cannot be achieved by altering the cropping system alone; additional measures are needed to reduce agricultural NPS pollution. This study facilitates the development of cropping systems and provides relevant information to aid the sustainable development of agriculture in the DRB.

A novel spatial optimization approach for the cost-effectiveness improvement of LID practices based on SWMM-FTC.

Due to the increasingly frequent occurrence of urban waterlogging, the spatial optimization of low impact development (LID) practices has been commonly used to detain and reduce storm water runoff in the most cost-effective way. In this study, the flow transmission chain (FTC) was proposed to replace the routing portion of the Storm Water Management Model (SWMM) and was combined with the runoff component of the SWMM to simulate LID practices (SWMM-FTC). In the SWMM-FTC, the third Evolution Step of Generalized Differential Evolution (GDE3) was employed to optimize the LID layout design. The results showed that the relative error between the modified SWMM-FTC and the calibrated SWMM was less than 0.25% under various LID scenarios, and the computational efficiency of the SWMM-FTC was improved by 19.3 times. Moreover, the GDE3 outperformed the commonly used non-dominated sorting genetic algorithm (NSGA-II), the strength Pareto evolutionary algorithm (SPEA2), and the multi-objective shuffled frog leaping algorithm (MOSFLA) due to its ability to find the most cost-effective solution. The LID layout obtained from the SWMM-FTC with the GDE3 saved $210-1067 to achieve a 1% reduction in storm water runoff. This result demonstrates that the SWMM-FTC with the GDE3 can achieve higher environmental benefits than comparable models, providing better guidance for managers and stakeholders.

Flash droughts in the Pearl River Basin, China: Observed characteristics and future changes.

Heat wave flash drought or precipitation deficit flash drought has devastating impacts on society and the environment. This study explored the historical changes (1960-2015) of the two categories of flash drought over the Pearl River Basin (PRB) in China, and revealed how they would change in the future (2016-2100), by coupling the variable infiltration capacity mode with the global climate model under representative concentration pathway (RCP) 2.6, 4.5, and 8.5 scenarios. Our results indicate that during 1960-2015, the mid-northern PRB has experienced heat wave flash drought frequently while the western PRB suffered from precipitation deficit flash drought. In future, heat wave flash drought under RCP2.6 and 4.5 would occur mostly in the western and eastern PRB. Specifically, heat wave flash drought would become severe under RCP8.5, especially for the eastern PRB. However, precipitation deficit flash drought would be concentrated in the western PRB. Except for the central regions, PRB generally exhibits a significant upward trend in heat wave flash drought under RCP4.5. Under RCP8.5, distinct increases in both categories of flash drought across almost the whole PRB are expected. For precipitation deficit flash drought, only a few regions show significant upward trends under RCP2.6 and 4.5.