1. Introduction
Climate change could influence regional hydrological cycles by altering precipitation, evapotranspiration, soil moisture, and temperature, and then causing changes in river runoff [
1,
2]. Spatiotemporal changes to precipitation amount and intensity resulting from climate change may lead to increased occurrence of erosive rainfall events and increased sediment yield in watersheds [
3]. In addition, a large number of studies have shown that human activities, such as large-scale vegetation destruction and reconstruction and massive construction of water conservancy facilities (e.g., reservoirs) may lead to major changes in land use patterns, which can greatly change the underlying surface conditions of the basin, thereby producing changes in runoff and sediment processes in a river basin [
4,
5,
6,
7]. Research shows more than half of the world’s large river systems have been impacted by humans to varying degrees. The Chinese Pearl, Yellow and Yangtze River, Yalu Jiang, Liao He, Hai He, and Qiantang Jiang river systems and others have been strongly affected [
8,
9,
10,
11,
12]. As global climate warming increases due to human activities, there is increasing concern in the scientific fields of water conservancy engineering, river geomorphology, and land water cycle regarding trends in water movement and sediment deposition, and regarding the contribution that climate change and human activities make to these processes [
13,
14,
15,
16].
Relevant studies in China and abroad are mainly concentrated in large rivers and their tributaries. For example, an analysis of driving forces affecting long-term changes in sediment yield has been made for large Russian rivers discharging to the Arctic Ocean [
13]. The results of that study showed that changes in sediment yield depended more on human activities than on climate change, and the local increases or decreases in sediment yield were the result of gold mining activities and construction of reservoirs in the upper reaches of the Ob and Yenisei rivers, respectively. The construction of large reservoirs and the resultant flow diversion have resulted in almost complete sediment trapping on several large river basins such as the Colorado and Nile [
8]. Other heavily regulated drainage basins with large reservoirs resulting in large sediment retention rates are found in Europe, North America, Africa, and Australia/Oceania [
8]. A research study identifying driving factors for changes in streamflow of the Xinjiang River basin in Poyang Lake showed that climate (especially precipitation) played a dominant role in changing basin hydrology and streamflow, while the seasonal variations of basin hydrology and water balance were strong functions of land-use change and vegetation distribution in the basin [
17]. That study also found that different land-cover patterns can cause increases or decreases in streamflow in different seasons. Relevant studies on the major Chinese river systems have concluded that human activities have contributed more to changes in annual river and sediment discharge than climate variability, (e.g., 87% in the Wuding River, 83% in the Yellow River, and 71% in the Yangtze River basin), and the impacts of human activities on the whole basin have increased with time [
7,
12,
18,
19,
20].
The Ganjiang River is the most important major river in the Poyang Lake River basin and is also one of the eight major tributaries of the Yangtze River. Variation in runoff and sediment in the Ganjiang River not only affects the water volume and siltation of Poyang Lake, but also affects the variation of runoff and sediment in the lower reaches of the Yangtze River [
18,
21]. Historically, the Ganjiang River Basin also has the most intense area of soil erosion in Poyang Lake. Since the 1980s, a series of soil erosion control projects have been implemented in this area. These projects include the pilot project of comprehensive control of the Tangbei River small watershed, eight key projects of soil and water conservation in China, key construction projects of national soil and water conservation, long-term control project of national agricultural comprehensive development, and key projects of soil and water conservation in the Poyang Lake Basin [
22]. In recent years, relevant research results have been achieved in the study of streamflow and sediment changes in the Ganjiang River. Previous research has demonstrated that, driven by multiple factors, annual runoff in this region has not changed significantly in recent decades, while annual sediment transport has shown a significant downward trend [
21,
23,
24]. Previous studies have identified and quantified the trends of streamflow and sediment discharge in the Ganjiang River basin but have mainly focused on the inter-annual variation tendency at one to three hydrological stations. It remains unclear as to whether changes occur at seasonal or monthly scales, and whether transition years exist in the long time series of hydro-climatic data, and to what extent the driving forces (e.g., climate change and human activities) are responsible for these changes. Therefore, the purposes of this study were to: (a) Identify the seasonal distribution of hydro-meteorological variables; (b) identify trends and abrupt changes in precipitation, streamflow, and sediment discharge in the upper, middle, and lower reaches of the Ganjiang River basin; and (c) estimate the contribution rate of both climate change and human activities to changes in runoff and sediment transport during the past 50 years.
4. Conclusions
In this study, several statistical methods were applied to investigate the changing trends, transition years, and driving forces and their contribution rates to precipitation, streamflow, and sediment discharge changes in the Ganjiang River basin using continuous long time series. The results demonstrated a significant reduction in annual sediment discharge over time (p > 0.05) at all hydrological stations, but no significant trends for either precipitation or streamflow were found. When analyzed by season, significant increasing and decreasing trends were found for both precipitation and streamflow, but only decreasing trends were found for sediment discharge. Moreover, the significant decline of sediment discharge occurred in the 1980s in both Hanlinqiao and Waizhou stations, and the 1990s in Bashang, Xiashan, Ji’an, and Xiajiang stations, and 2002 in Julongtan station. The average degree of decline at stations of midstream and downstream were 30.1%, which were much larger than 18.8% of upstream.
The abrupt changes in the trends of sediment discharge in this region were attributed to both precipitation and human activity, but human activities (especially the vegetation cover change and reservoir construction) were the most plausible factors responsible for sediment discharge changes in the Ganjiang River basin since the 1980s. In the past five decades, the vegetation cover in this area increased by almost 30%. There were 145 large and medium-sized reservoirs almost equally distributed in the upper, middle, and lower reaches of the drainage area, but 52%, 45%, and 3% of the total 214,821.45 km2 water-collecting area were occupied by the reservoirs located at midstream, upstream, and downstream regions of the Ganjiang River basin. Moreover, the ‘before 1980′ period saw construction of numerous medium-sized reservoirs, while only a small number of both large and medium-sized reservoirs were constructed during the 1981–2000 period, and the 2000–2013 period only saw the construction of a few large reservoirs.
In general, this comprehensive analysis based on long-term time series of the hydro-climatic data provides useful insights for watershed management decision-makers to recognize that precipitation and human activities are the main driving factors of runoff and sediment discharge changes of the Ganjiang River during the last five decades, respectively. Comprehensive measures for soil and water conservation by increasing forest coverage and building large and medium-sized reservoirs can effectively reduce sediment discharge in river basins. Moreover, as an example of river evolution under global change environment, study on the changes of streamflow and sediment and its driving force in Ganjiang River basin could provide scientific basis for the control of soil erosion and the management of water resources in Ganjiang River, as well as for the related research of Poyang Lake and the Yangtze River basin. Specifically, the reduced sediment discharge would cause land degradation and river regime evolution like river channel slices downwards, which will influence the ecological balance of terrestrial and river ecosystems, and maybe even exacerbate the seasonal drought and ecological hazards in Poyang Lake Basin.