A new approach to separating the impacts of climate change and multiple human activities on water cycle processes based on a distributed hydrological model

The impacts of human activities on the natural water cycle have become increasingly severe due to population growth and economic development. Therefore, it is important to evaluate the contributions of different human activities (e.g., land-use change, industrial water use, and domestic water use) on different hydrological variables. However, existing attribution methods have shortcomings when attributing the impacts of different human activities; for instance, human activities may only be considered as one impact factor, and the total impact of all factors may not be guaranteed to equal 100% of the total change. A new approach, using multiple scenario simulations from a distributed model, was developed in this study to overcome these shortcomings. All potential contributions for one impact factor were calculated by fixing the other impact factors at different statuses, and the arithmetic mean of all potential contributions was presented as the final result. This method ensures that the sum of the contributions of all impact factors equals the total change in hydrological variables between two periods, which was demonstrated mathematically. A case study of the Wei River Basin was used to evaluate the proposed methodology, which attributed four impact factors (climate change, land-use change, industrial and domestic water use, and agricultural irrigation water use) to three hydrological variables (evapotranspiration, rainfall infiltration, and streamflow drainage into the Yellow River). The results showed that the new method could attribute the impacts of the four factors to the changes of the three annual average hydrological variables between the period 1956-1980 and the period 1981-2005; these four factors accounted for 373.3%, - 33.3%, -133.3%, and -106.7% of the change in evapotranspiration, 129.8%, - 2.2%, - 3.5%, and - 24.1% of the change in rainfall infiltration, and 75.2%, 4.4%, 13.4%, and 7.0% of the change in streamflow drainage into the Yellow River, respectively. Moreover, the sum of the relative contributions of the four impact factors to the three hydrological variables was equal to 100% at an annual time scale. To avoid the limitations of existing methods, we recommend adopting the newly proposed method for evaluating the impacts of climate change and multiple human activities on water cycle processes.