*2.1. Study Area*

The TNR located in the north of China, covers more than 50% of the land area in China, nearly 5.3 million km<sup>2</sup> (Figure 1). More than 551 counties across 13 provinces are contained in this region, including Beijing-Tianjin-Hebei, Harbin-Changchun, and other metropolitan areas, where the population has risen sharply. The population in TNR has increased from 344.10 million in 2000 to 384.06 million in 2016 (these data are available at the website of National Bureau of Statistics of China: http://www.stats.gov.cn/). As a result, the urban areas in TNR have been expanding rapidly. There are five large river basins in this region: The Inland River Basin (IRB), Yellow River Basin (YRB), Hai River Basin (HRB), Liao River Basin (LRB), and Songhua River Basin (SRB), from west to east. In fact, the study area is slightly larger than the area of the TNFS because we assess hydrological responses at close to the basin level.

**Figure 1.** Distribution of land cover types for the year of 2015 in the Three-North region (TNR), where five basins, i.e., the Inland River Basin (IRB), Yellow River Basin (YRB), Hai River Basin (HRB), Liao River Basin (LRB), and Songhua River Basin (SRB), distributed in this region.

The eastern part of this region is mainly influenced by the East Asian Monsoon, and the western part exhibits a typical continental climate [28]. According to the historical datasets from 1959–2009, the climate across the TNR shows grea<sup>t</sup> spatial heterogeneity (Figure 2). The annual precipitation reaches more than 1000 mm/year in the southeast, and gradually declines to less than 50 mm/year in the northwest, with an obvious gradient between these endpoints. Mean annual temperature also presents a wide range of spatial variation, from above 12 ◦C in the south of the HRB and YRB to under 0 ◦C in the north of the IRB, but without any apparent gradient over the area as a whole.

**Figure 2.** Annual mean temperature and annual precipitation from 1959 to 2009 in the TNR, IRB, YRB, HRB, LRB, and SRB of China.

According to the land cover map for the year 2015 (Figure 1), the land cover over this region also exhibits distinct spatial variability [29]. In the SRB, forestlands account for nearly 50% of the land cover, and in the YRB, grasslands cover nearly half of the area. In the SRB and LRB, which are densely populated, the dominant land cover type is cropland, while in the IRB, over 60% of the area is unutilized. Additionally, the proportion of urban lands is minute in all basins. In the entire region, over 35% of the area is unutilized, including bare lands and bodies of water, and the most abundant vegetative land cover type is grassland, at nearly 30%, with forestland and cropland coverage being approximately equal at 15%, and the least abundant land cover type is the urban area, at no more than 2%.
