Identifying the evolution of water balance and influencing factors in the mountainous area of Beijing-Tianjin-Hebei

Quantitatively identifying the effects and contributions on the water balance evolution is the premise of ecological environmental protection and scientific management of water resources. However, most of the previous studies focused on the effects of climate change and human activities on runoff attenuation; few studies paid attention to the attribution and quantitative identification of different factors in human activities. On the basis of identifying the evolution process of water balance in the study area, the effects of precipitation change and human activities on the evolution of water balance at different time periods were analyzed by using the double cumulative curve method and the quantitative separation method of runoff change. The effects of the increase of water resource exploitation and forestry water consumption on water balance evolution were analyzed in the Laiyuan Basin and Qingshuihe Basin, respectively. The results show that: (1) The annual precipitation presented a slightly decreasing trend with a decreasing rate of 0.97 mm/a in the past 65 years; since the 21st century, precipitation has increased slightly. (2) Compared with the period of 1956−1979, the natural runoff attenuation during 1980−2000 was affected by precipitation change and human activities, with corresponding contributions of 49.25% and 50.75%, respectively. Soil and water conservation and afforestation are the main factors of human activity affecting the change of water balance. the decrease in natural runoff was mainly caused by human activities, with the influence contribution of 68.2%. The increase of groundwater exploitation was the main factor of human activity, and the increase of water consumption was the second factor of human activity. (3) The increase in reservoir storage and water consumption in mountainous areas led to the actual runoff attenuation directly, which was the main reason for the intensification of water balance in the plain areas. (4) In the Qingshuihe River Basin, there was a positive correlation between the increase of forest water consumption and the decline of mountain runoff at a certain scale. The effect of runoff attenuation on the increase of forest area was not immediate, with a lag of 15−20 a. As the forest area reached 23.48%, the increase in forest water consumption had a significant negative effect on the natural runoff. An average increase of 1 km² of forest land led to water consumption being increased by 37.25×10⁴ m³/a, and the natural runoff being decreased by 59×10⁴ m³/a. The increase in annual water consumption of forest land accounted for 63.22% of the natural runoff attenuation. It indicated that the increase in forest area played an important role in water conservation, resulting in a decrease in effective runoff. The results of the study are of great significance for the ecological and environmental protection, scientific management of water resources and regulation of water balance in the Beijing-Tianjin-Hebei mountain area.