1. Introduction
The problem of water resources will become the most important natural resource problem facing mankind in the 21st century, and the exploitation of water resources in northern China has exceeded the carrying capacity of the resource environment, indicating that the situation facing water resources is very serious [
1]. In recent years, the combined effects of climate change and human activities have led to significant changes in the river runoff of many rivers and further intensification of water scarcity, which seriously threatens social development and human life [
2]. Exploring trends and turning points in runoff change and revealing the main drivers of runoff change play a key role in future water resources prediction [
3].
Runoff change is a complex dynamic process as an integrated response to climate change and human activities in a watershed. The effects of climate change and human activities on hydrological processes have become a hot research topic. Currently, statistical analysis methods [
4], hydrological modeling methods [
5], and elasticity coefficient methods based on the Budyko framework [
6] are the main methods to study the impact of climate change and human activities on hydrological water resources. The elasticity coefficient method based on the Budyko framework integrates the coupled hydrothermal equilibrium of the watershed and establishes the relationship between watershed runoff and precipitation, evaporation, and underlying surface characteristics, which is easy to calculate and has been validated in many watersheds [
7,
8,
9].
In recent years, significant changes in runoff and other hydrometeorological elements have occurred in the Weihe River basin, causing widespread concern. Zuo et al., used a sensitivity coefficient approach based on the Budyko framework and a hydrological modeling approach to estimate the effects of climate change and human activities on runoff in the Weihe River basin. They found that the impact of human activities on the control basins of the upper and middle reaches of the Weihe River at Linjiacun, Weijiabao, and Xianyang hydrological stations, and the control basins of the lower reaches of the Jinghe River at Zhangjiashan station, accounted for greater than 50% of the runoff changes [
10]. Sun et al., found that the intensification of potential evapotranspiration due to climate warming contributed negatively to runoff changes by more than 60%, which was higher in absolute value than the positive contribution of precipitation [
11]. Shi et al., found that the contribution of human activities to runoff changes in the Weihe River source area was close to 50% [
12]. Zhang et al., found that intense human activities were the main cause of runoff reduction, and their contribution to the reduction in runoff was over 60% [
13]. Although many previous studies have been conducted to analyze runoff changes in the Weihe River basin, the results are not entirely consistent (
Table 1). The contribution of potential evapotranspiration, precipitation, and human activities to runoff changes in the Weihe River basin varies widely among the results obtained in each article due to different study periods, hydrological stations, and methods. However, generally, they indicate that the modification of the underlying surface by human activities has gradually become a major factor affecting runoff changes.
Most of the previous studies were based on the period before the 2010s and did not explore the continuous changes of runoff in the Weihe River in the last 10 years. In order to deeply analyze the characteristics and causes of runoff changes in the Weihe River basin in recent years, this paper conducted a trend and abrupt change point test for each hydrological element in the Weihe River basin and selected the base period and change periods based on abrupt change points. This paper applies the Budyko framework to analyze the contributions of precipitation, potential evapotranspiration, and underlying surface characteristics to runoff variability and conducts an attribution analysis of runoff variability to provide a theoretical basis for the integrated management and sustainable use of water resources in the Weihe River basin and similar areas.
2. Study Area
The Weihe River is the largest tributary of the Yellow River, located in the Yellow River hinterland (103°57′–110°17′ E, 33°42′–37°24′ N), originating in the Wushu Mountain in Weiyuan County, Dingxi City, Gansu Province, and flowing through three provinces, Gansu, Ningxia, and Shaanxi, east to Tongguan County, Shaanxi Province, where it joins the Yellow River, with a main stream length of 818 km and a basin area of 134,800 km
2. The Weihe River has many tributaries, and the tributaries on both sides of the river are asymmetrically distributed. The water system on the south bank originates from the Qinling Mountains and flows through the rocky mountainous areas, which are mostly tributaries with a short course and more water and less sand. The water system on the north bank is developed on the Loess Plateau, with a large water catchment area and serious soil erosion, and is the main sand-producing area in the watershed. The largest tributary is the Jing River, with a length of 455.1 km and a basin area of 45,400 km
2; the second largest tributary is the Bei Luo River, with a length of 680 km and a basin area of 26,900 km
2. The Weihe River basin is located in the transition zone between arid and humid and has a temperate monsoon climate with an average annual temperature of 7.8 °C~13.5 °C, annual precipitation of 300~800 mm, annual potential evaporation of 700~1400 mm, and annual evaporation of 400~700 mm. Combining the runoff information from the hydrological stations of Huaxian and Zhuangtou, the average multi-year runoff of the Wei River is 6.385 billion m
3 (
Table 2).
5. Discussion
The results obtained in this paper, where elevated
n is the main cause of the sharp decrease in
Q, are consistent with previous studies [
10,
13,
19], but the contribution of
P to the change in
Q is significantly different from previous studies. In this paper, we conclude that
P increases during the PII period and contributes to an increase in
Q. Zuo et al., conclude that climate change (
P and
ET) contributes 29% to 65% to the decrease in runoff at each hydrological station in the Weihe River basin [
10]. Bai et al., conclude that the combination of both climate change and human activities leads to a significant decrease in runoff in the Weihe River main stream [
20]. Bi et al., and Liu et al., also reached similar conclusions [
21,
22]. The differences in the above findings are most likely related to the different study periods, with
P elevated in the 2010s compared with the 2000s and 1990s, and an increasing effect on
Q. The former study period was probably in the dry phase of the hydrological cycle, and the decrease in precipitation had a significant decrease in runoff.
n is an important factor influencing runoff variation. In this paper, the variation of
n and its effect on
Q are analyzed in three periods. To further reflect the changing state of
n, the meteorological and hydrological data for the whole time period were subjected to a 10-year sliding average, and the corresponding
n was obtained by back-calculating Equation (4). The change of
n actually shows the influence of other factors (underlying surface) on runoff change after excluding
P and
ET. The increase in
n indicates that the influence of underlying surface change on
Q increases, and in this paper, the value of
n is negatively correlated with
Q. The increase in its value indicates a stronger effect on
Q reduction. As can be seen from
Figure 5,
n is in a fluctuating rising state throughout the period, with a significant continuous rising phase after 1995, followed by a gradually declining phase in the 2000s, and finally, a significant rising trend starting around 2008. This indicates that the underlying surface began to change more drastically around 1995 and 2008 than before due to human activities. The overall upward trend in
n is likely related to increased artificial water withdrawal activity, and a study using reduced natural runoff would likely remove this trend.
The construction of soil and water conservation projects, the expansion of forest area, the intensification of human water extraction activities, the change of watershed water storage, and the construction of various water conservancy projects to a certain extent make the process of converting rainfall into runoff more complicated, and the water brought by rainfall is kept in the watershed for a longer period of time, increasing the degree of wetness in the watershed, so the runoff in the watershed will show a decreasing trend for a period of time, which is reflected in the
n a significant increase. After a period of time, the vegetation coverage and wetness of the watershed reach a certain stage, and
n is likely to remain more stable or even decline. In 1995, the middle reaches of the Yellow River protection forest creation project began to be implemented, and the first trials were carried out in Shaanxi and other forest areas. The Weihe River basin has since been subsumed under the project of protection forest construction [
23,
24]. The change in
n from 1995 until the end of the 2000s is consistent with the pattern of change described above. In 2008, the Shaanxi provincial government launched the comprehensive management project of the Weihe River basin in Shaanxi Province, accelerating the construction of watershed water conservation and ecological projects, forest ecosystem protection and restoration projects, green ecological projects of the Weihe River channel and inter-basin water transfer, etc. From around 2008,
n again showed an increase, but this state is likely not to last for a long time and is more likely to remain stable or decline in the future.
The impact of future global climate change on the Weihe River basin cannot be accurately simulated, but the increase in temperature and precipitation is recognized and determined at present, which will lead to the change of hydrothermal conditions in the Weihe River basin. The increase in atmospheric temperature and the humidity of the watershed will change the potential evapotranspiration. The increase in precipitation and potential evapotranspiration provides basic conditions for the increase in evaporation. At the same time, the vegetation coverage of the Weihe River basin is also growing rapidly, which makes the change of evaporation more rapid. The increase in precipitation has a positive effect on runoff, but evaporation has the opposite. A very important point is that in the future, Hanjiang to Weihe River Project will add 1.5 billion·m3 per year of water to the Weihe River. Therefore, there will be many uncertainties in future runoff changes in the Weihe River basin, which require more research on future climate change and human activities.