1. Introduction
Climate change and human activities are two major factors affecting the hydrological cycle and driving the evolution of water resources. They are also important driving forces for the evolution of regional drought [
1,
2,
3]. With the changing environment in recent decades, drought has shown a trend of widespread and frequent occurrence. The resulting water shortage, food crisis, environmental deterioration, and other problems, have seriously threatened human survival and sustainable economic development [
4,
5,
6]. Currently, drought is classified into four categories: meteorological drought, socioeconomic drought, hydrological and agricultural drought [
7]. Hydrological drought is affected by precipitation, infiltration, and runoff, involving the transformation of surface water, soil water and groundwater, and is related to the hydrological cycle and water balance [
8]. The processes of its formation and completion are relatively slow, and the mechanism is more complicated than the other three categories of drought [
9,
10]. With the profound impact of high-intensity human activities on the hydrological cycle, hydrological drought is no longer a natural disaster in a strict sense, but rather a natural-anthropogenic composite disaster [
11,
12]. To accurately predict the occurrence and development of the hydrological drought and effectively manage it, the formation and development of drought cannot be considered solely from the natural perspective. The role of human activities in the formation and development of hydrological drought needs to be fully considered [
13,
14,
15].
Human activities alter hydraulic connections in the water cycle by changing the underlying surface conditions of the basin, resulting in changes in surface runoff, consumption, discharge conditions, river regulation, and storage functions. These can further affect the occurrence and development of the hydrological drought [
16,
17,
18,
19,
20]. Scientific evaluation and identification of the hydrological drought events and determination of the main driving factors in their evolution have become the focus of international drought research [
21,
22,
23,
24,
25]. Based on the threshold method and standardized index method, Rangecroft et al. [
26] evaluated the characteristics of drought before and after the completion of the San Juan Dam in northern Chile. They reported that reservoir regulation reduced the frequency, duration, and intensity of drought events but had no significant effect on multi-year drought. By applying the VIC (Variable Infiltration Capacity) model to analyze the attribution of runoff attenuation in the Weihe River Basin, Ren et al. [
27] suggested that both climate change and human activities were important factors affecting the evolution of drought and flood in the basin. In particular, human activities were the dominant factor in the occurrence of short-term drought. Su et al. [
28] used the SWAT model to reconstruct the historical hydrology of the Daling River Basin under natural conditions. Based on a multi-time scale standardized runoff index, that study compared the evolution of the basin’s drought under changing environments and concluded that human activities have intensified the duration and intensity of seasonal drought and increased the uncertainty of drought events in the basin. By discussing the occurrence and evolution of hydrological drought in the basin from the perspective of water cycle, we can further improve the accuracy and objectivity of drought evaluation.
The Xilin River Basin, a typical grassland inland river basin, was selected as the study area. In recent years, the rapid development of regional animal husbandry has doubled the total number of livestock in the study area, and the amount of livestock has far exceeded the carrying capacity of the natural grassland. Meanwhile, with the depletion of energy resources in central and eastern China, China’s coal power development strategy has gradually moved westward. The Xilingol League region (where the study area is located) is rich in energy resources, and is gradually becoming a main source of coal and a national energy strategic base supporting the long-term socioeconomic development of China. Intensive human activities have accelerated soil erosion and aggravated drought in the basin during the past two decades [
29,
30,
31]. In this context, the SWAT model was used to reconstruct the natural runoff of the basin, and on this basis the temporal characteristics of drought and the influence of environmental change on drought evolution were examined. This study should provide a scientific basis for water resources management, drought early warning, and forecasting under a changing environment.
4. Discussion
Previous studies on grassland inland river basins in semi-arid regions mostly focused on the impacts of environmental change on precipitation or runoff [
51,
52], while there have been few studies on the impact of human activities on hydrological drought. In this study, we found the precipitation and runoff in the study area has trended downward over the past 45 years, which is consistent with the findings of many previous studies [
29,
31,
53]. Some studies have shown that in recent decades, a high latitude circulation anomaly has increased the air pressure and weakened the east Asian monsoon, which in turn has reduced the northward transport of water vapor [
54,
55]. This is one of the main reasons for the decrease in precipitation in the northern China.
Since the 21st century, the development of animal husbandry and the coal industry has driven the rapid development of the economy in the study area. With the growth of GDP, the underlying surface conditions (e.g., land use and land cover) of the basin have also changed. Although climate change and human activities have combined to cause a decrease of runoff in the study area, human activities have played a dominant role; this conclusion is similar to that of Gao et al. [
56].
Human activities have a significant impact on short-term hydrological drought, especially in summer and fall. During this period, the study area is in the peak production and peak water use period, and intensive human activities lead to an extension of drought duration and increase of drought intensity [
27,
28]. The impact of human activities on long-term hydrological drought is not obvious. This phenomenon is determined by the occurrence mechanism of hydrological drought. As we know, meteorological drought is caused by a reduction of precipitation, and the resulting hydrological cycle anomaly may lead to the occurrence of hydrological drought at longer time scales [
57]. Since climate change is a long-term process, the propagation process from meteorological drought to hydrological drought is mostly affected by climate change at longer time scales [
58].
Quantifying the impact of human activities on hydrological drought was carried out on the basis of the hydrological model simulation in this study, and accurate simulation of the hydrological process directly affected the results of hydrological drought assessment. Due to the limitations of the observation stations, the uncertainty of the model is relatively high; therefore, intensive observations are necessary to obtain more data for calibrating the model. In addition, we can compare the simulation results of multiple hydrological models and use optimization method to reduce uncertainties. It should be pointed out that this study conducted only a generalized evaluation of the impact of various potential human activities (e.g., water withdrawal and changes in underlying surface conditions) on drought. There are still some deficiencies in the fine evaluation of the driving effect of various activities on drought. In the future, the contribution of different human activities to drought should be more accurately and precisely distinguished based on water use and water conservation project monitoring data from human activities.
5. Conclusions
The SWAT model was applied to the Xilin River Basin to reconstruct the historical hydrological series under natural conditions. The multi-time scale standardized runoff index (SRI) was used to compare and evaluate the characteristics of the hydrological drought in the basin under natural conditions and the influence of human activities. This study revealed the impact of human activities on the hydrological drought at different time scales. The main conclusions are as follows:
(1) The runoff and precipitation series in the Xilin River Basin showed a downward trend, and the change in the runoff demonstrated a significant downward trend. The mutation test and the double accumulation curve of precipitation and runoff indicated that runoff in the Xilin River Basin showed a significant change in 1998.
(2) The SWAT model was used to reconstruct the natural runoff of the basin during a period of human influence. We found that human activities caused a significant reduction in runoff, contributing 68% of the reduction in basin runoff.
(3) Using SRI as the drought index, we evaluated the characteristics of the hydrological drought over multiple time scales. The results suggest that human activities had an obvious effect on the short-term drought and certain delaying effect on the onset of drought during the same period. In the case of long-term drought, human activities changed the drought level only slightly.
(4) Analysis of the frequency and characteristic variables of the hydrological drought showed that human activities have increased the frequencies of severe and extreme droughts in the basin over a short time scale. At the same time, human activities have significantly modified the seasonal distribution characteristics of short-term drought under natural conditions, resulting in severe and extreme drought events in autumn. In addition, human activities have increased drought duration and drought intensity at different time scales (i.e., prolonged drought duration and increased drought intensity). Human activities have also increased the uncertainty of the hydrological drought characteristics and expanded the distribution range of drought duration and drought intensity.