Changes in runoff in a typical temperate continental-monsoon transitional zone in the last four centuries

Highlights

• We reconstructed the runoff in the low-latitude region of the transition zone over the 351 years.

• The annual runoff has gradually decreased with dramatic variation.

• Most of the areas have simultaneous variation in wetting and drying in the same time periods.

• Most of the areas experienced a significant drying trend in the first half of the 20th century.

Abstract

Temperate continental-monsoon transitional zones are sensitive to variations in the global hydrological climate, and the long-term patterns of hydrological variability in this zone should be explored urgently. In this study, based on the tree-ring width of Platycladus orientalis, we reconstructed the annual runoff sequence during 1666–2016 (351 years) on Wula Mountain, which lies within the low-latitude temperate continental-monsoon transitional zone. Our reconstruction model is stable and reliable because our reconstruction results overlap well with the arid and flood years and periods found in local historical records. Our analysis results show that over the last four centuries, the annual runoff in the low-latitude transitional zone is characterized by an overall decrease, with heightened fluctuations. Specifically, in the 18th century, wet and extremely wet years appeared frequently, while continuous wet periods grew progressively shorter. Later, this area experienced a mean-flow period of approximately 100 years ending in the 1930 s, when progressively longer dry periods began to occur. In the last 30 years, this area has suffered from its longest dry period in 351 years. Regarding runoff variability, over two centuries in the temperate continental-monsoon transitional zone, high-latitude areas had more wet periods with significant variation in large-scale periodicities; the majority of the zone had simultaneous variation in wetting and drying and a significant drying trend in the first half of the 20th century. High- and low-latitude areas had long and slowly changing arid periods, while mid-latitude areas had short and quickly changing arid periods. This study enriches the understanding of the dendrohydrology of the low and middle latitudes of the temperate continental-monsoon transition zone, analyzes the runoff variations throughout the transition zone area, and helps reconstruct the long-term historical runoff sequence over four centuries in the transitional zone.

Introduction

Water resource is an irreplaceable, precious natural resource that is critical for humans and all other biological species (Wada and Bierkens, 2014, Konapala et al., 2020). Since the 1980s, shortages of water resources have become more and more severe around the globe (Wada and Bierkens, 2014), and uncertainties about climate change have further intensified the water-shortage problem (Konapala et al., 2020). The temperate continental-monsoon transitional zone in east Eurasia is sensitive to variations in global hydrological climate. However, the pattern and trend of long-term historical hydrological variability in the entire transitional zone has remained unclear.

Since Hardman’s first reconstruction of hydrological factors of the Truckee River in Nevada, USA, in 1936 (Hardman, 1936), tree-ring width has become a main tool for reconstructing long-term historical hydrological variability (Woodhouse, 2000, Chen et al., 2019, Liu et al., 2010). Scholars have conducted related research in a number of countries and regions on all continents except Antarctic (Liu et al., 2004). Early on, most dendrohydrological studies were done in America and Europe, which produced fruitful scientific achievements. For instance, a number of scholars studied Pinus sylvestris (Agafonov et al., 2016, Björklund et al., 2020), oaks (Loader et al., 2019), and redwoods (Matskovsky et al., 2010, Cleaveland, 2000, Bégin, 2001) in the Colorado Basin (Woodhouse and Pederson, 2018), the Yellowstone River Basin, Lake Baikal (Dabaeva et al., 2016, Magda, 2001), and the OB River (Agafonov et al., 2016) to investigate hydrological factors, including runoff (Woodhouse and Pederson, 2018, Graumlich et al., 2003), extreme hydrological events (Cook et al., 1997, Cook et al., 1999, Bégin, 2001), and lake levels (Dabaeva et al., 2016, Magda, 2001). The findings included that the “Dust Bowl” drought in 1930 s U.S. was the most severe drought event since 1700 (Cook et al., 1999), the magnetic periodicity of the sun and the tidal periodicity of the moon are likely the driving forces for continuous drought events (Cook et al., 1997), there are no significant long-term trends of increasing inputs from Eurasia to the Arctic Ocean through six main rivers in the last two centuries (MacDonald et al., 2007), 1978–1982 was the most severe drought period in the last three centuries in the south Lake Baikal region in Siberia (Magda, 2001), and the dramatic increase in extent and frequency of ice-jam floods in the 1930 s indicated by the levels of two lakes in northern Quebec, Canada, recorded by the ice marks in coastal tree rings (Bégin, 2001).

Eurasia is the largest continental plate, spanning widely in longitude and latitude, with diverse geographical environments. Its hydrological climate environment is quite complex and belongs to one of the typical vulnerable hydrological climate regions in the Northern Hemisphere (Tar, 2001, Ji et al., 2012). Eurasia has a variety of climate types, the most widely distributed of which are the temperate continental climate and the temperature monsoon climate. The junction of these two main climate regions forms a unique and sensitive transitional zone, known as the temperate continental-monsoon transitional zone. This transitional zone starts from eastern West Siberia, extends southwest, and finally ends at the hinterland in northwest China in the Qilian Mountains. It obliquely crosses through east Eurasia at latitudes 33–63°N and longitudes 102–164°E, and the coupling effect of numerous geographic and hydrological climate factors leads to the sensitive and vulnerable characteristics of the hydrological environment in the transitional zone.

Current dendrohydrological research in the transitional zone mainly utilizes P. sylvestris (Bao et al., 2012), Pinus tabulaeformis (Liu et al., 2005, Liu et al., 2017, Liu et al., 2019) and has focused on investigating runoff (MacDonald et al., 2007, Bao et al., 2012) and the Palmer Drought Severity Index (PDSI) (Bao et al., 2012), based on which 130-220-year hydrological histories have been reconstructed in the Kolyma River (KLR) and Lena River (LNR) basin (MacDonald et al., 2007), northeast China (Bao et al., 2012), and Helan Mountain (HLM) (Liu et al., 2005, Liu et al., 2019). Little research has utilized Thuja sutchuenensis, Platycladus orientalis, or Chamaecyparis obtusa (Sieb. et Zucc.) in the transitional zone, and some of the previous studies focusing on short sequences only investigated runoff variability in specific areas without considering the whole picture across the entire transitional zone. Thus, it is urgent to reconstruct longer historical runoff variability in the temperate continental-monsoon transitional zone and to analyze the associated variability trends and patterns. A better understanding of historical runoff variability in the entire transitional zone will be valuable to general long-term hydrological research on this special transitional zone.

On this basis, we chose a typical area in the temperature continental-monsoon transitional zone at low latitudes, Wula Mountain in China, as our sampling area. No long-term historical runoff sequence has been constructed there, so research on this topic is needed. Many P. orientalis aged hundreds of years are densely distributed in this area and are an ideal proxy for reconstructing runoff over long historical periods. In this study, based on the annual tree-ring width table for P. orientalis, we reconstruct the runoff historical sequence over the last four centuries in the typical temperate continental-monsoon transitional zone at low latitudes with the goal of fully revealing the trend and pattern of its variability. Then, to analyze hydrological variability in the entire transitional zone, we compare our results against hydrological events found in the local records and against the literature results for other areas in the transitional zone.