Regional hydrology heterogeneity and the response to climate and land surface changes in arid alpine basin, northwest China

Highlights

• The hydrology changes is sensitive to energy in high-altitude.

• The hydrology changes is controlled by water limitation in low-altitude area.

• The water-energy coupling relation highly relies on altitudinal gradient.

• Climate change dominate the hydrological shifts in alpine region.

Abstract

Hydrology heterogeneity refers to the different water regimes or hydrological processes in response to heterogeneous in topography, landscapes, land cover, soil properties, geology and meteorology reflecting as the spatial variation of precipitation, evapotranspiration and water yield along with landscape, elevation, terrain and climate variation. However, the hydrology heterogeneity in the alpine region of inland river basin has remained unclear to date, which limits the understanding of the hydrology and landscape interaction mechanism and affects the rational utilization of regional water resources. In this study, we combined the soil and water assessment tool (SWAT) and water-energy coupling framework to detect the hydrology heterogeneity in the arid alpine region of Heihe River Basin (HRB). We applied SWAT model to present the hydrological processes and regimes at different landscapes and elevation bands by using field measured parameters, and investigate the hydrology heterogeneity as well as the response to the changes in recent climate and land surface in the perspective of water-energy coupling framework. The results indicate that SWAT is indeed a robust tool in representing the streamflow process in alpine region with accurate performance. The distribution of landscapes reflect the comprehensive effect of regional water-energy coupling, which highly rely on the altitudinal gradient. The hydrology heterogeneity in high-altitude area is sensitive to energy and in low-altitude area is sensitive to water limitation (drought stress). Climate change is the dominate factor that driving the hydrology heterogeneity shifts in alpine region of HRB, while, the effect of land surface change is gradually strengthening over the past 50 years, reflecting with the contribution of land surface change on hydrology shifts is gradually increasing.

Introduction

Regional or watershed hydrology is the comprehensive reflection of the regional climate, landscapes, soil properties, topography, land use practices and the spatial overlaying patterns among these factors (Cheng et al., 2014). Hydrology heterogeneity refers to the different water regimes or hydrological processes in response to heterogeneous land surfaces and climate conditions, reflecting as the heterogeneity of precipitation, evapotranspiration and water yield along with landscape, elevation, terrain and climate variation (Sun et al., 2017) However, few researches focus on the hydrology heterogeneity at regional or watershed scales, especially in the arid alpine region, where the spatial heterogeneous traits of landscapes, topography, and climate are particularly evident (Gebrehiwot et al., 2019, Sivapalan, 2018, Viola et al., 2019). Which limits the understanding of the interaction mechanism of hydrology and landscape. Moreover, the water-energy coupling relationships at different landscapes and elevation zones remain to resolve and the hydrological response to the changes of recent climate and land surface need to be investigated. In the arid land, meanwhile, the socio-economic development and ecosystem sustainability of mid-downstream significantly rely on the discharge from the upstream alpine region (Feng et al., 2015, Feng et al., 2019). Therefore, understanding the hydrology heterogeneity and its response to climate and land surface changes in the perspective of water-energy coupling framework is of great importance in terms of sustainable ecology conservation and integrative water resources management.

For a single watershed, the hydrology heterogeneity could shape the redistribution of vegetation structure and adjustment of regional management practices (Gao et al., 2018). In addition, the global climatic changes are likely to significant increases in temperatures, in combining with changes in regional precipitation patterns historically and within the next few decades (Fang et al., 2017). There is no doubt that the hydrology heterogeneity has accordingly changed (Shi et al., 2007, Sun et al., 2017, Wang et al., 2013, Zhang et al., 2017). These studies focused on the hydrology changes of basin or region as a whole, but seldom attention to the change in hydrology heterogeneity in the single region, especially in the perspective of water and energy coupling framework.

Tomer and Schilling (2009) utilized two non-dimensional hydro-climatic state variables representing excess water ${(P}_{\mathit{ex}}=(P-{\mathit{ET}}_a)/P)$ and excess energy $(E_{\mathit{ex}}=({\mathit{ET}}_0-{\mathit{ET}}_a)/{\mathit{ET}}_0)$ of a basin, which refer to the efficiency of water and energy available used by ecosystem and carrying information of how water and energy available fluxes are partitioned at the catchment scale. Changes in an ecosystem or its environment, whether these changes are in space and time, are likely to result in shift in $P_{\mathit{ex}}$ and $E_{\mathit{ex}}$. The plots of P_ex versus E_ex provide an effective way to empirically distinguish the relative effects of climate and land surface changes on watershed hydrology for any two different periods (Tomer and Schilling, 2009). Renner et al. (2012) modified this method by introducing the aridity index (ET₀/P) to determine climatic state. As a result, a change in P_ex and E_ex for the same aridity index is considered as a land surface change impact and a change of P_ex and E_ex moving away from a constant aridity index is considered as a climate change impact (Marhaento et al., 2017). However, the water and energy coupling traits for different landscapes and elevation band are never investigated, especially in data scarce region where distributed observation is limited.

Despite the importance of alpine region in freshwater supply and ecological environment preservation in arid and semi-arid area, advance in hydrology research in this region has been limited by data scarcity. For instance, there are only six stations established in and around the upstream of Heihe River Basin (HRB) by the Chinese Meteorological Administration (CMA) for long term meteorological observation. Besides, these stations are generally sparse and unevenly distributed at relatively low elevation regions and lacking of basic land surface observations such as evapotranspiration, radiation flux and water budget at different landscapes. Therefore, the hydrological processes and regimes and their interactions with land surface changes are scarcely understood (Yang et al., 2017b). Thanks to the launch of the integrated study projection of the eco-hydrology in Heihe River, the in-site or small watershed observation systems were installed (Chen et al., 2014, Cheng et al., 2014), which focus on understanding the interaction of ecology and hydrology at site and providing observation parameters for regional simulations (Chen et al., 2015). These field observations and experiments provide abundant data support for understanding hydrological processes, as well as provide the necessary input parameters for hydrological process simulation. However, due to the heterogeneity of vertical distribution of water-energy differences along the altitude, the landscape patterns are accordingly different. Hydrological in-sites experiments and small-scale observations provide limited information for the spatial patterns of hydrological heterogeneity at a basin scale.

Hydrological models, such as the Soil and Water Assessment Tool (Arnold et al., 1998), the System Hydrologic Europe(Storm, 1984) and the Variable Infiltration Capacity(Liang et al., 1994), are comprehensive and based on physical mechanisms which offer a framework for conceptualizing and investigating the relationships between climate, underlying surface and hydrological processes in various categories of time and space (Smith, 1978, Tian et al., 2017a, Zhang et al., 2015). These models provide a variety of spatial discretion methods and can fully utilize the spatial information provided by Geographic Information System (GIS) and Remote Sensing (RS) technology. By using the parameters that from hydrological in-sites experiments, small-scale observations and field investigation, the hydrological model could present the spatial hydrological details. The resulting data is used to determine parameters controlling hydrological processes in a given watershed and making it possible to analysis the heterogeneity of regional hydrology (Marshall et al., 2019). For example, Yin et al. (2016) applied SWAT analyzing water balance components in HRB and found the main runoff generated area is the region above 3500 m. Gao et al. (2016) applied geomorphology-based eco-hydrological model (GBEHM) modeling eco-hydrological processes and suggested that vegetation distribution enlarged the spatial variation of actual evapotranspiration. Zhang et al. (2018a) applied distributed hydrology soil vegetation model (DHSVM) to simulate climate and land use contribution to streamflow variability.

Thus, in this paper, we combined the hydrological model and water-energy coupling framework to detect the hydrology heterogeneity in the arid alpine region of HRB. We applied SWAT model to present the hydrological processes and regimes at different landscapes and elevation bands by using field measured parameters, and investigate the hydrology heterogeneity as well as the response to the changes in recent climate and land surface in the perspective of water-energy coupling framework. The objective of this study is to detect the heterogeneity of hydrological dynamics and its water-energy coupling mechanism at different landscapes and elevation bands, and to identify the hydrological response to climate and land surface changes for the last 50 years. The results can be utilized for understanding the eco-hydrology interaction mechanism and contribution to the sustainable watershed management in arid inland region.