Distributed hydrological simulation in karst regions has always been a challenging task because of their unique hydrogeological characteristics. The karst mountain region of southwest China (KMRSC), one of the largest continuous karst areas in the world, contributes to about 54 percent of water supply in the basins. In spite of its importance, we have a poor understanding of the evolution laws of hydrological cycle and water resources in KMRSC. We developed a physically-based, distributed hydrological model, called Water and Energy transfer Processes (WEP)-karst model, for KMRSC by introducing the equivalent porous medium approach to the WEP-L model, and dividing the modelling domain into 2021 sub-watersheds. The area of sub-watersheds ranges from 55 to 920 km², with an average value of 170 km². The model showed a good performance in simulating the monthly discharge at 18 representative hydrological stations, with the Nash–Sutcliffe efficiency (NSE) values ranging from 0.71 to 0.94, and the relative error (RE) values from −9.8% to 8.3% during the validation period (1980–2000). Then, we employed an in-depth analysis of the temporal and spatial variation of main water cycle fluxes, including precipitation, infiltration, evapotranspiration, blue water (i.e., river runoff), and green water (i.e., vegetation transpiration) over 1956–2015. In addition, the impact of climate change on these fluxes was evaluated under the median emission scenario (RCP4.5). The results showed that: (1) annual average precipitation of KMRSC reached 1506 mm, which is 2.4 times of the national average level, and about 47% (701 mm) of it contributed to river runoff. The infiltration and evapotranspiration were 862 and 870 mm, respectively. The transpiration from plants and trees accounted for 51% of the evapotranspiration. (2) Except for the green water, other fluxes experienced a significant decrease over the past 60 years. Blue water showed the largest interannual fluctuation and the strongest sensitivity to climate change. (3) Both precipitation and infiltration concentrated from May to August, and blue water increased notably from May to June and peaked in June. Blue water and precipitation were more likely to decrease in the future over 2021–2050 due to the climate change.

Pulse flow, which includes base flow, peak flow, flow duration, occurrence time, and frequency, is a manifestation of environmental flow in rivers. This paper describes a methodological framework for determining pulse flow based on an analysis of fish spawning behavior and presents its application to the second Songhua River in northeastern China. Peak flow was determined based on the hydrographic-habitat relationship for fish spawning in conjunction with physical habitat simulation. The flow duration was determined based on the incubation period of fish eggs. The occurrence time and frequency were determined based on the suitable temperature for fish spawning. This application differs from conventional methods for dividing fish spawning periods and selecting target species in the corresponding period. Fish were divided into cold-water fish, hypothermal-water fish, and warm-water fish according to spawning temperature, and the target species in each month of the spawning period (April to July) were identified. For the same spawning period, the hydrographic-habitat relationships of target species with adhesive and drifting eggs were weighted to determine the peak flow. The most suitable peak flow for fish spawning from April to May in the research region is 900 m³/s and 1200 m³/s for June to July. Using the proposed framework, an ecological hydrograph from April to July was obtained by combining the method of pulse flow with habitat simulation. Fifteen days, eight days, and five days were chosen as the flow durations for April, May, and June to July, respectively. We recommend scheduling two high pulse flows each month from May to July while limiting the occurrence to only one in April. The results show that this framework offers a promising approach for developing environmental flows in rivers with a variety of fish species.