Spatial and temporal variability of blue/green water flows in typical meteorological years in an inland river basin in China

The influence of climate change on water resource availability has attracted more and more attention around the world (Vörösmarty et al. 2000; Liu et al. 2013a; Evaristo et al. 2015). In recent years, extreme weather events have occurred more frequently due to global warming, resulting in decreases in the water available in many regions in the world and posing serious challenges to the water supply (Vörösmarty et al. 2000, 2010; Alley et al. 2003; Liu et al. 2013b). Particularly in arid and semi-arid regions, competition for available water has been intense due to the relationship between socioeconomic sustainability and ecosystem health; this had led to water crises and ecosystem degradation (Falkenmark & Rockström 2006; Palmer et al. 2015; Sánchez et al. 2015). Therefore, a comprehensive assessment and management of water resources in the context of climate change is a key to understanding the available water endowments and enhancing water management towards sustainable, efficient, and equitable use of scarce fresh water resources. Furthermore, climate change has been a main influencing factor on river basin water resources; there has existed interaction between climate change and water resources (Alley et al. 2003). Water vapor cycling is a major part of the weather system; climate change also influences the spatial and temporal variability of water resources in a watershed (Vörösmarty et al. 2010; Evaristo et al. 2015). The changes of climate factors cause a water shortage or crisis in arid and semi-arid regions (Xiao & Xiao 2004), especially in typical meteorological years (dry, normal, and wet). In typical meteorological years, the green water coefficient (GWC), which is defined as the ratio of green water (water stored in unsaturated soils and used as evapotranspiration by plants) to the total water resources, will have significant variability, changing the availability of water and hydrological processes (Zang & Liu 2013). Therefore, assessments of the impacts of climate change on water resources are especially needed to help understand blue and green water variability and sustainable water use in arid and semi-arid regions.

Traditionally, water resource assessment and management by scientists and policymakers has paid more attention to blue water, while ignoring green water (Falkenmark 1995; Cheng & Zhao 2006; Liu & Yang 2010). The concepts of blue and green water were first proposed by Falkenmark (1995), but in the early years, they were not used to distinguish water resources and flows. Falkenmark & Rockström (2006) redefined the concepts of green and blue water in 2006 to explicitly distinguish water resources and flows. Green water plays a critical role in the crop land system and ecosystem (Cheng & Zhao 2006; Falkenmark & Rockström 2006). Liu et al. (2009), Liu & Yang (2010), and Rost et al. (2008) showed that more than 80% of global crop production was supported by green water. Furthermore, water use in grassland and forest ecosystems is predominantly from green water (Rockstrtöm 1999; Rost et al. 2008).

The Heihe River originates in the Qilian Mountains and ends in Juyanhai Lake. The total basin area is approximately 0.24 million km², with the majority located in the northwest of China and a minor part in Mongolia (Figure 1). The Heihe River can be divided into three sections: upstream, midstream, and downstream. The upstream section runs from the Qilian Mountain to the Yingluo Canyon (Figure 1), the midstream section from the Yingluo Canyon to Zhengyi Canyon (Figure 1), and the downstream section terminates in Juyanhai Lake (Figure 1). The average annual air temperature is 2–3 °C in the upstream area, 6–8 °C in the midstream area, and 8–10 °C in the downstream area. The average annual precipitation is between 200 and 500 mm in the upstream area, between 120 and 200 mm in the midstream area, and below 50 mm in most of the downstream area (Cheng 2003). The potential evaporation ranges from 1,000 mm/year in the upstream region to 4,000 mm/year in the downstream region (Ma et al. 2011). The precipitation in the Heihe River Basin occurs mainly in the summer and autumn (>70% of the annual precipitation falls between May and August) (Ma et al. 2011), whereas the spring is dry with snow and ice melting (4% of total discharge) (Li & Xu 2011).

In this study, the typical meteorological year is a typical wet, dry, and normal year. Drought and flood grades were used to define the typical meteorological year.

The standardized precipitation index (SPI) and precipitation anomaly index (H) indicators were used simultaneously to determine the typical dry, wet, and normal year to avoid the error of a single index calculation in this study.

In the formula, q is the probability when the precipitation equals 0. If m is the number of rainfall time series with a precipitation of 0, then q = m/n。The cumulative probability of H(x) can be determined through the type conversion as a standard normal distribution function.

The parameter values are as follows: c₀ = 2.515517; c₁ = 0.802853; c₂ = 0.010328; d₁ = 1.432788; d₂ = 0.189269; d₃ = 0.001308. The SPI can be obtained according to the above formula, and the drought and flood level values are shown in Table 1 (McKee et al. 1995).

H is the precipitation anomaly percentage, p is the precipitation over a certain period of time, and is the average rainfall over many years and here is 51 years. The drought and flood levels calculated by the precipitation anomaly values are shown in Table 1 (Ju & Yang 1998).

In this study, the Soil and Water Assessment Tool (SWAT) was used to simulate green and blue water flows in the whole Heihe River Basin. The SWAT model is a semi-distributed basin-scale model that has been used around the world under different conditions (Neitsch et al. 2004; Schuol et al. 2008; Faramarzi et al. 2009), and it has been successfully applied to simulate hydrological processes in the Heihe River Basin, including blue and green water flows (Huang & Zhang 2004; Zang et al. 2012).

In SWAT, the Heihe River Basin was divided into 32 sub-basins and 309 hydrological response units by overlaying elevation, land cover, soil attributes, management, and slope class. The details of the SWAT model settings, database and definition information have been published by Zang et al. (2012). The Nash–Sutcliffe coefficient (E_ns) and the coefficient of determination (R²) were selected to evaluate the goodness of the calibration and validation (Nash & Sutcliffe 1970). The SUFI-2 method in the SWAT–CUP interface (Abbaspour 2007) was chosen for parameter optimization.

This study was an expansion of our previous research. SWAT was calibrated and validated to simulate the flows of green and blue water at the whole-basin level by using climate data from 1980 to 2004 and land use data for 2000, as in Zang et al. (2012). The calibration and validation results indicated the high values of E_ns (>0.87) and R² (>0.9). In the present study, the calibration parameters derived in the previous study were used, and the research period was expanded to include 1960 to 2010. The typical meteorological years were then defined based on the precipitation, and analyzed the spatial and temporal distributions of the blue and green water flows and the GWC. Further information on the model simulation, parameters, calibration, and validation can be found in Zang et al. (2012). In previous research, several issues have been discussed including the precipitation, temperature, and green/blue water variability in the Heihe River Basin over a recent 50 year period (Zang & Liu 2013), but the spatial and temporal distributions of green/blue water and the GWC variability in a typical meteorological year were not determined. Therefore, this study combined the previous research (Zang et al. 2012, 2015; Zang & Liu 2013) and expanded its content to a typical meteorological year to explore the variability of green/blue water flows and the GWC in typical years.

The green water flow refers to the actual evapotranspiration, whereas the flow of blue water is the sum of surface runoff, lateral flows, and groundwater recharge (Schuol et al. 2008). To account for the relative importance of the two flows, the GWC is defined as the ratio of the green water flow to the total flows of green and blue water (Liu & Yang 2010).

Through the calculation of the standard precipitation index (SPI) and precipitation anomaly percentage (H) of the up-, mid-, and downstream areas of the Heihe River Basin, it was found that the typical meteorological years in the three reaches are disparate (Table 2). For example, the midstream region suffered from extreme drought in 1965, but this was a normal year in the up and downstream regions. The whole river basin was moderately moist in 1983, but there was drought downstream. Similar situations occurred in 1970 and 2009 (Table 2). According to the detailed analysis, the Heihe River Basin was typical arid only in 1978 and typical wet only in 1998, within the period from 1960 to 2010 (Table 2). The year of 1984 was selected for a comparative analysis, as the two indicators are normal in this year, and the 1980s was between two typical meteorological years. Table A1 (available with the online version of this paper) shows that in a drought year (1978), the precipitation was low, not only significantly below that of a wet year (in 1998) but also significantly lower than that in a normal year (1984) and the average of the 51 years (Table A1), which indicated that the SPI and H values calculated were reliable. Therefore, the selected typical dry, wet, and normal years were 1978, 1998, and 1984, respectively (Table 2, shown in bold font).

The blue water depth was 1.3 mm in 1978 and 9.1 mm in 1998 (calculated by Figure 2(a)). The blue water depth was higher in the upstream than in the middle and down reaches in the Heihe River Basin due to the greater rainfall from upstream to downstream; the precipitation was higher in wet years than in dry years (Figure A1, available with the online version of this paper). The green water depth (green water in per unit area) of the Heihe River Basin in wet years (1998) was significantly higher than that in drought years (1978) in the upstream and middle reaches, although the downstream was less (Figure 2(b)). The green water depth of the entire river basin was 120.6 mm in 1978 and 175.4 mm in 1998 (calculated by Figure 2(b)). However, the overall green water depth exhibited a trend of increasing from a dry year (1978) to a wet year (1998). Over the space of the whole river basin, the green water depth still presented a decreasing trend from the upstream to downstream (Figure 2(b)).

In the past 51 years, the blue and green water depth and GWC exhibited spatial and temporal differences from the upstream to the downstream in Heihe River Basin, especially in typical meteorological years. As the cross section was larger (more than 800 km) from the upstream to the downstream of the Heihe River Basin and the terrain varies, the sources of precipitation and regional climates showed significant differences. The regional climates were most strongly affected by different forms of atmospheric circulation (Wu et al. 2010), namely, precipitation in the upstream, oceanic evaporation in the midstream basins, and terrestrial evapotranspiration in the downstream basin (Lan et al. 2004; Jia et al. 2008). This difference led to the dry and wet year inconsistency from the upstream to the downstream. The blue/green water depths of Heihe River Basin presented a decreasing trend and spatial difference from the typical dry year to wet year, mainly due to the spatial distribution characteristics of precipitation in typical meteorological years (Zang & Liu 2013; Figure A1). The water amount of some sub-basins in the up- and midstream was larger, especially in the southeast basin, mainly due to the distribution of some ice, snow melt water and lake groups in typical wet years (Zang et al. 2012).

The GWC of a drought year in the downstream of the Heihe River Basin was higher; in the wet year, it was relatively low in the up- and midstream regions (Figures 4 and 5). This showed that there was a negative correlation between the GWC and the degree of dry and wet in a typical meteorological year. The precipitation and the GWC were inversely related; the variation amplitude decreases according to the annual precipitation (Figure 5). However, the GWC variation increases with the precipitation (Figures 5 and A1). This showed that the GWC variability was more sensitive in wet than in dry climate environments.

The average GWC was 88% for the entire Heihe River Basin (Zang et al. 2012); in this study, the GWC in the past decade was 87%, 90% in the drought year, and 83% in the wet year. Based on the GWC in a normal year, it was inferred that approximately 87.5% of the available water resources in the Heihe River Basin participated in the hydrologic cycle in the form of green water. This result can help develop a scientific understanding of the evolution of the blue and green water resources under the climate change background in continental inland river basins in arid and semi-arid regions and enable reasonable management of the available water resources, especially green water resources.

Although the spatial distribution of blue/green water was studied in different typical years, the green and blue water transformation mechanism and the laws of the Heihe River Basin are still not clear, especially the human activity influence, so further research is still needed (Zhao et al. 2016). Further typical meteorological year blue/green water research will help to prevent and control drought and flooding in a typical year in some regions and more efficiently manage water resources, especially in continental inner river basins in arid and semi-arid areas. Strengthening green water research in northwestern China and other arid regions will help to alleviate water shortage crises and the eco-system pressure in these areas. In our research, human activities were not included, although they are very important to water resources. Therefore, further exploring the spatial and temporal distribution patterns and the transformation law of green and blue water in the arid and semi-arid inland river basins in China will be helpful to solve the problems of water cycle and ecological system science and developing protocols for sustainable water resource utilization and management. This has important theoretical and realistic significance for socioeconomic sustainable development in the Heihe River Basin.

In our research, the spatial and temporal variations of the blue/green water depth and GWC variability were analyzed in typical years in the up-, mid-, and downstream and the entire river basin over the past 51 years. The main conclusions are as follows:

1. The total blue/green water depth of the Heihe River Basin showed a gradually decreasing tendency from the upstream to downstream, and the blue and green water depths also showed the same decreasing trend from the upstream to the downstream due to the geographical environment differences in the Heihe River Basin.

2. The green and blue water depths of the Heihe River Basin were significantly lower in a typical drought year than in a typical wet year; the total blue/green water depth changed from 150.1 mm in drought years to 231.3 mm in wet years due to the differences in precipitation.

3. The GWC in the Heihe River Basin presented an increasing trend from the upstream to downstream, changing from 77% in the upstream to 94% in the downstream. Furthermore, the GWC in the typical drought years was higher than in the typical wet years. Therefore, evapotranspiration (green water) accounted for a proportion of water consumption in typical dry years that was significantly higher than that in normal and wet years, while runoff (blue water) accounts for a proportion of water in wet years that was significantly higher than in dry years.

4. The GWC had a regressive relationship with the drought index; wetter weather caused more significant variability in the GWC.

This study was supported by the National Natural Science Foundation of China (91325302, 91425303), Excellent Young Scientist Foundation of Inner Mongolia Agricultural University of China (2014XYQ-5), Natural Science Foundation of Inner Mongolia Autonomous Region of China (2015BS0401), the National Program for Support of Top-notch Young Professionals. I thank the wetland hydrology team from the Beijing Forestry University, Heihe data team from the Cold Area and Drought Zone Environmental Conditions and Engineering Institute of the Chinese Academy of Sciences for their support during our research.