4.1. The Effects of Climate Change on Runoff
In order to investigate the impact of climate change on runoff variation, regional change of temperature and precipitation in the Juma River Basin was statistically analyzed from different angles. The annual average temperature over the period of 1956–2013 was 10.5 degrees Celsius, which decreased from southeast to northwest and decreased with elevating altitude (
Figure 10). The MK results of measured data from meteorological stations around the study area indicated that the warming trend of temperature in the Juma River Basin was evident at the 0.001 significance level (
Table A1). The change point of annual mean temperature series at four meteorological stations was 1986 or 1987 (at the 0.001 significance level), as shown in
Figure A1, which was later than that of the annual runoff series. Annual temperature anomalies changes [
63] in the period of 1956–1978, 1956–1990, 1956–1997, 1956–2002, 1956–2008, and 1956–2013, were employed to study the regional temperature evolution (
Figure 11). During the baseline period (1956–1978), Yuxian Station, Huailai Station, and Baoding Station showed an increasing trend, while Beijing Station showed a decreasing trend. However, as the research period became longer, the downward trend was gradually replaced by an upward trend, and the increasing trend was becoming more and more obvious. What is more, the slope of the trend lines peaked in 1956–2008 and declined in 1956–2013, especially in Hauilai Station and Baoding Station. To characterize the changes in extreme weather events, the number of days that the temperature exceeded 30 degrees Celsius (hot-day) or dropped below 0 degrees Celsius (frost-day) every year was extracted separately (
Figure A2). Only the hot-day series of Beijing Station passed the MK test, with a significance level of 0.05, and there was a slight uptrend. Conversely, frost days during 1956–2013 at four stations dramatically reduced under the 0.001 significance level, which may be the reason why the annual average temperature of the study basin rose over the past decades.
Figure 10 shows that the south of the Juma River basin was wetter than the north, and the precipitation gradually increased from north to south. The temporal distribution of rainfall throughout the region was extremely uneven (
Figure 12a). The precipitation from November to March accounted for only 5% of the total amount. From April onwards, the rainfall began to increase with the increasing temperature and reached its peak value in July and August. Moreover, some extreme precipitation events occurred in the August of some years. The result of the MK test revealed a slight decrease in regional precipitation, and the annual amount of precipitation time series at half of the stations passed significance inspection (
Table A2). Besides, there was no abrupt change in precipitation series at 14 stations. The mean monthly precipitation data series exhibited a mixed changing trend. Five months showed an increasing trend, while the rest showed a decreasing trend. The greatest reduction was for precipitation in August, with a decrease of 1.33 mm/year.
Given the above, the Juma River Basin became hotter and less humid in past decades. Generally, the temperature rise can increase evaporation and then decrease the runoff in the water circulating processes. Meanwhile, regional precipitation decreases may directly aggravate the reduction of runoff. Hence, to some extent, climate change in the study area resulted in the decline in water of the Juma River. During the two periods of 1998–2002 and 2003–2008, the drought index (
) reached its peak (
Figure 13), which led to the largest relative contribution of climate change to runoff reduction at two stations.
4.2. The Effects of Land Use Change on Runoff
The main land use types of the Juma River Basin were agriculture, forest (dense forest, shrubwood, and open forest), and grass, which accounted for about 98.5% of the entire region, as shown in
Table 4. Overall, there existed some slight changes in land use of the region in the past few decades. From the late 1970s to 2000, land use conversion mainly occurred in the northeastern part, that is, in the middle reaches of the basin. The area of open forest and shrubwood decreased, while the dense forestland and grassland slightly increased. After 2000, the land use of the region basically reverted to its previous state. During the study period, the area of urbanization in the upper reaches of the basin, controlled by the Zijingguan Station, increased gradually, thus making the area of agricultural land smaller.
In order to explore the possible impacts of land use change on hydrological processes, we selected two regions with obvious land use change and a large water yield (
Figure 14). AREA 1 and AREA 2 were located in the middle and upper reaches of the basin, respectively. The hydrological processes of both AREA 1 and AREA 2 were associated with the streamflow of Zhangfang station, and the water yield of AREA 2 had direct effects on the streamflow of Zijingguan station. The area-weighted mean of WYLD (water yield) was extracted under each scenario and then compared with the simulated value of the original underlying surface (the late 1970s). It was found that the change of the underlying surface had a slight effect on the water yield of the sub-basin. In AREA 1, the water yield of different scenarios was about 0.8–1.4 mm smaller than that of the original underlying surface scenario in five stages during the altered period. In AREA 2, the value change is around −0.5 mm to 8.7 mm.
The relationship between forest change and the hydrological process is a worthy topic. A series of studies [
64,
65,
66] in the Dragonja catchment (SW Slovenia) indicated that natural reforestation from 1954 to 2002 directly led to the reduction in discharge (75%) and sedimentation (85%) and narrowing incision of the riverbed. Though the forest area has increased in this region, the difference of water yield under different scenarios was small compared to the total amounts and can be almost negligible, as it was most likely to have resulted from system error. Hence, our conclusion is that land-use types in Juma River basin have changed weakly over the study period, which had less impact on runoff variation.
4.3. The Combined Effects of Climate Change and Human Direct Intervention on Runoff
Human direct intervention influenced the runoff in the Juma River basin, mainly by water diversion for agricultural, domestic, and industrial purposes. The construction of water intake engineering played an important role in local development. Wuyi diversion canal is a project of interbasin water transfer by which part of the water in the Juma River was transported into Angezhuang Reservoir. The annual mean diverted water volume during 1959–2013 was 0.89 billion m
3. The canal mainly provided drinking water for 13 million in Yi County, as well as irrigation water for Yishui Irrigation District and Shengli Irrigation District [
67]. Moreover, it met requirements for landscape water in Western Qing Tombs and hydroelectric power along the way. In addition, Wuyi diversion canal could relieve any excess water from Juma River in the flooding season and supply water to Baiyang Lake through Angezhuang Reservoir in the dry season [
68]. Guandaoling diversion canal is also a project of the interbasin from Juma River to Longwang Reservoir in Yi River. The annual mean diverted water volume during 1977–2005 was 0.3 billion m
3, of which 76 % was used for irrigation and landscape water and 24 % for domestic and industrial use [
67]. Shengtian diversion canal, which is about 21 km long, was a source of drinking and irrigation water along the way, and annual mean diverted water volume was about 0.1 billion m
3. Among the diversion projects mentioned above, the impact of Wuyi diversion canal on the natural runoff process of Juma River is the most serious and the longest. Based on the diverted water volume of the Wuyi diversion canal during 1961–2013 (the data during 1968–1969 and 2002–2005 were missing), the effect of the human direct intervention on the runoff of the Juma River was discussed in this paper.
Since the 1960s, water withdrawal of Wuyi canal has increased gradually and reached a peak value in 1995. Followed by a slight decline, it returned to a stage of positive growth after 2010 (
Figure 15). Extreme precipitation occurred in the Haihe River Basin during the summer of 1995 and 1996 [
69]. In order to save downstream communities from the flood, a large quantity of water was diverted from the river into Angezhuang Reservoir through the Wuyi canal, and the annual diverted water volume of 1995 and 1996 was 2.10 billion m
3 and 2.03 billion m
3, respectively, the highest ever recorded. The contribution of human activities to runoff during 1991–1997 was a little larger than that of the previous period. During 1998–2008, the drought index exceeded long-run averages. Resulting from the violent change of climate, the runoff in Juma River sharply decreased, and there was a corresponding drop in water withdrawal. The annual diverted water volume of 2002 was only 7467 million m
3, which was the smallest amount over a decade [
67]. Meanwhile, the impact of human direct activities was much lower than that of climate change in this period. After 2009, the river was replenished because the climate became more humid and rainy. During this period, human activities during this period played a dominant role in the decreasing runoff at two stations owing to the increasing water withdrawal of Wuyi canal.
Water withdrawal through Wuyi diversion canal was not evenly distributed throughout the year. It was relatively small from April to June and peaked in August and September, which was almost determined by the amount of rainfall and water demand. In winter, the climate was cold and dry, and the water of the Juma River was diverted to meet the domestic and industrial needs of the urban population. Because the Juma river never froze in this season, remarkable temperature rising in this season led not to ice melting and runoff increasing, but to runoff reduction. Although the diverted water in this season was not the smallest amount of the year, the impact of climate change overwhelmed the influence of human activities owing to the violent variation of temperature. In the stormy summer months, large volumes of water were transported into Angezhuang Reservoir in order to reduce the flooding. This meant that the impact of human direct activities on runoff reduction was dominant during this season, except for August, and this exception was due to the obvious trend of climate change that precipitation decreased dramatically in August. Besides, the contribution rate of anthropogenic activity in June was the highest in the year. However, water withdrawal in this month was almost the lowest. This demonstrated that the relative contribution ratio of the two factors was mainly determined by climate change.
Hydrologic connectivity refers to the transport of matter, energy, and organisms from one part of the landscape to another through the hydrologic cycle, which can reflect the continuity of water flow and connectedness of the water system [
70,
71]. Water and sediment dynamics are usually divided into two portions: system phases (structural connectivity) and system fluxes (functional connectivity), and each part can be separately quantified in both temporally and spatially multiple scales through models and tracer experiments [
72,
73,
74]. There are many factors (climate environment, landscape position, delivery pathway, soil type and structure, etc.) that can cause varying degrees of influence on hydrologic connectivity for a watershed scale [
71,
73]. Human alteration of the hydrologic cycle has the characteristics of complexity, but policy about land management and hydropower development was made in most regions, without giving sufficient thought to hydrologic connectivity [
70]. Beguería et al. [
17] found that land use change led to the water yield decreasing by about 30% between 1945 and 1995 in an irrigated district of the Central Spanish Pyrenees. The study, based on paired catchments, stated that vegetable cover is the main factor that affects the frequency, intensity, and timing of floods [
13]. Marhaento et al. [
52] argued that deforestation and urbanization increased the runoff coefficient from 35.7% to 44.6% and decreased the ratio of baseflow to streamflow from 40% to 31.1% in the Samin catchment of Indonesia, which may add to risks of flooding and drought. The comparative research on the abandonment of agricultural land in Mediterranean agriculture fields indicated that bare soils and low vegetation coverage led to higher soil loss and runoff, compared to natural vegetation recovery [
75]. Good water management can not only satisfy the need for water supply and flood control, but also avoid the occurrence of the second disaster. A complex of man-made modifications in the Adda basin (Italy) significantly reduced the flooding risk in the littoral of Lake Como during 1946–2007, without worsening it in the downstream, thus ensuring the safety of urbanized areas [
2]. Different types of soil and water conservation in Minizr catchment of the northwest Ethiopian highlands, like soil bunds, micro-trenches, etc, effectively decreased sediment loads entering Koga reservoir [
76]. Cerdà et al. [
77] suggested that growing catch crops or weeds in orange plantations can reduce water and soil erosion as dense vegetation cover would enhance infiltration and avert surface wash.
Juma in Chinese means the sounds of the stream like the hoofs of ten thousand horses, and Juma River was famous for its large discharge in the past. However, the area is now seriously threatened by water shortages. In the 1970s and 1980s, China carried out land reform, and the Haihe River was diverted in many places for irrigation to meet the crop water requirements and numerous compact towns [
37]. This was particularly true in the Juma River basin. After 2000, a water deficit continuously occurred due to the prolonged, severe drought. Disorder in water resources planning and management finally resulted in a water dispute between Beijing and Hebei. Under the rapid development of industry and tourism up and down the Juma River, increasing water demand will exacerbate the crisis of water shortages in the future [
78]. Agriculture depletes the largest sector of water in Haihe River basin, which makes up above 70% of total water consumption [
79]. In the conditions of a complex and changeable climate, the establishment of strict water resource management measures and water-saving high efficiency crop systems based on adequate information on hydrologic connectivity would have a far-reaching impact on regional water resources of the Juma River basin.