Research papersExtended growing season reduced river runoff in Luanhe River basin
Introduction
In temperate and boreal regions, vegetation phenology is particularly sensitive to climate change (Chuine et al., 2004, Schwartz, 2013). Recent climate warming has extended the growing season mainly due to the advanced start of season (SOS) and the delayed end of season (EOS) in the Northern Hemisphere (Menzel et al., 2006, Peñuelas and Filella, 2001, Piao et al., 2019, Fu et al., 2015), and has subsequently affected the ecosystem structure and function (Piao et al., 2017, Thackeray et al., 2016, Zohner et al., 2018). Extended growing season results in higher carbon assimilation and increases the water retention of terrestrial ecosystems (Lu et al., 2013, Piao et al., 2007a, Piao et al., 2007b, Williams et al., 2012). However, to our knowledge, how the phenological shift affects the regional water cycle, especially at the watershed scale, has not been well investigated. Hence, it is essential to investigate the water cycle response to climate warming by analyzing the interaction between phenological shift and river runoff at the watershed scale (Peñuelas et al., 2009, Zeng et al., 2018a, Zeng et al., 2018b).
It is widely accepted that water cycle processes are mainly controlled by changes of climatic variables, such as precipitation and radiation, as well as land use and land cover change (Chiew et al., 2009, Hundecha and Bárdossy, 2004, Singh et al., 2014, Williams et al., 2012). Climate warming may directly cause the frequency and intensity changes in precipitation, accelerate the transfer of water from the earth's surface to the atmosphere, and lead to higher drought stress, and thus reduce river runoffs (Hao et al., 2018, Prudhomme et al., 2014). Except for climate variables, recent studies have found that warming-induced vegetation growth has affected evapotranspiration in river basins, and thus has a great impact on water yield and river runoff (Goulden and Bales, 2014, Li et al., 2018, Richardson et al., 2013, Thompson et al., 2011, Yang et al., 2019a, Yang et al., 2019b, Zeng et al., 2018a, Zeng et al., 2018b). For example, Lemordant et al. (2018) found that compared with the changes of precipitation or radiation, the physiological response to CO2 and water use efficiency of vegetation growth played a more pivotal role affecting terrestrial water cycles based on earth system models. In general, few studies have investigated the influence of vegetation growth on river runoff, especially using in situ records, at the watershed scale.
Vegetation phenology also contributes to the shift of river runoff. Recent studies found that in a temperate deciduous forest, advanced spring green-up dates caused higher evapotranspiration and reduced river runoff (Kim et al., 2018) where the spatial resolution was small, i.e. based only on a small catchment scale (0.8 km2). How phenological shifts affect river runoff at a large scale, especially at the large watershed scale, is still unclear. Furthermore, climate warming has extended the vegetation growing season, but the contribution of spring and autumn phenology has been reported inconsistently among studies. Some studies found that the spring phenology contributed more to the length of growing season (Chen et al., 2000, Linderholm, 2006, Menzel and Fabian, 1999), however, other studies reported that autumn phenology played a more decisive role than spring phenology (Wu et al., 2013). Overall, how spring and autumn phenology affect the length of growing season (GSL) and subsequently hydrological processes of the watershed ecosystem remains unclear. One possible reason is that large uncertainty exists when extracting remote sensing-based phenology dates using a single method (Cong et al., 2012, White et al., 2009). Using multiple methods to accurately extract phenology dates are thus necessary to apply in the phenology-river runoff analysis.
We, therefore, applied five methods to determine the start and the end of growing season (SOS and EOS) using the normalized difference vegetation index (NDVI) over the period 1982–2015 in Luanhe River basin, a typical semi-arid watershed in temperate China. Grey relational analysis (GRA) was applied to detect the relationships among phenology, vegetation growth, climatic variables, and river runoff. The objectives of this study were to (a) determine the temporal shifts of vegetation phenology and growth and river runoff, (b) investigate the relative contribution of spring and autumn phenology to vegetation growth, and (c) explore the relationships between trends in phenology, vegetation growth, and river runoff over the period 1982–2015.
Section snippets
Study area
Luanhe River basin, with a total area of 38,000 km2, is located in arid and semi-arid areas of North China (115°27′E-118°56′E, 40°N-42°41′N) (Fig. 1a). The Luanhe River flows from northwest to southeast with 12 main sub-basins (Fig. 1b and Figure A.1). The annual average temperature and precipitation during period of 1982–2015 were 7.0 ± 2.6℃ and 488.4 ± 80.7 mm, respectively, but a large spatial heterogeneity exists. In recent decades, the land use types in the area have not changed much, and
Changes of climatic variables
During the period 1982–2015, the whole region of Luanhe River basin (WR) was getting warming and drying, with temperature significantly increasing by 0.03 ± 0.002 ℃ per year (℃/y) and precipitation decreasing by 1.35 ± 0.19 mm per year (mm/y), and the radiation was increasing by 0.19 ± 0.03 W·m−2 per year (Fig. 2). These trends were consistent in all sub-basins (Figure A.2), but the warming trend was stronger in UR than in MLR (0.04 ± 0.002 and 0.02 ± 0.001 ℃/y, UR and MLR, respectively), while
Phenological shift and spatial heterogeneity
Earlier SOS and later EOS, and extended growing season were found in the Luanhe River basin over the period 1982–2015 when significant warming trend was detected. It was further found that the extended growing season promoted the vegetation growth in the basin. Such results are consistent with previous studies in temperate areas in the Northern Hemisphere (Cong et al., 2012, Gong et al., 2015, Linderholm, 2006), whereas higher vegetation growth largely relied on the earlier SOS rather than EOS
Conclusion and implications
Our results confirmed that river runoff was regulated by both climatic and phenological variables at the watershed scale (Hwang et al., 2018). The prominent effects of vegetation growth and phenological shift on river runoff were revealed in Luanhe River basin. However, research at the watershed scale is still lacking, considering that Luanhe River basin is located in a typical semi-arid area. In addition, large-scale research cannot effectively reveal the underlying mechanisms of the impacts
Author contribution
Y.H.F. designed the research and drafted the paper; X.J.G and X.C.Z performed the analysis and all authors contributed to the interpretation of the results and to the text.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was supported by the General program of National Nature science foundation of China (Grant No. 31770516), the National Key Research and Development Program of China (2017YFA06036001), and the 111 Project (B18006) and Fundamental Research Funds for the Central Universities (2018EYT05). The authors gratefully acknowledge all members of the Hydrological Yearbook of the People’s Republic of China for providing the in-situ runoff data. Y.H.F. designed the research and drafted the paper;
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