Runoff projections of the Qinling Mountains and their impact on water demand of Guanzhong region in Northwest China

The Qinling Mountains are the main source of water supply to the Weihe River and Hanjiang River, but systematic studies on runoff changes in this region are lacking. This study simulated the annual runoff in the Qinling Mountains from 1970 to 2015 using the Variable Infiltration Capacity (VIC) hydrological model and projected runoff changes from 2015 to 2100 under different Shared Socio-economic Pathways (SSPs). Moreover, the water demand in the north and south of the Qinling Mountains from 2019 to 2100 was analyzed by constructing a system dynamic method. The results show that the mean annual runoff in the southern and northern slopes of the Qinling Mountains from 1970 to 2015 were 21.5× 10⁹ and 3.52 × 10⁹ m³, respectively. Under the SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios from 2015 to 2100, the mean annual runoff in the southern (northern) slope will increase by 2.9 (1.5), 2.0 (1.4), 2.7 (1.6), and -2.1 (0.4) × 10⁹ m³, respectively. The mean annual water consumption in the northern foothills of the Qinling Mountains for Baoji, Xi'an, and Weinan cities is projected to increase from 4.1× 10⁹ m³ in 2015 to 7.6 × 10⁹ m³ in 2100, while that in the southern foothills for Hanzhong, Ankang, and Shangluo cites is projected to increase from 2.7 × 10⁹ m³ in 2015 to 4.8 × 10⁹ m³ in 2100. Thus, runoff in the southern slope is much larger than the water demand in the southern foothill. The increase of runoff in the northern slope cannot satisfy the water demand in the northern foothill. Therefore, the South-to-North Water Transfer project is proposed for meeting the water demand in the northern foothill. Under the premise of ensuring downstream water demand, approximately 10.5 × 10⁹ m³ of water resources can be transferred from the southern slope to the Weihe River and the Beijing-Tianjin-Hebei-Henan regions under SSP1-2.6, but only 3.9 × 10⁹ m³ can be transferred under SSP5-8.5.