Runoff in the Yellow River (YR) of China is steadily declining due to climate change and human activities. In this study, the basic trend and abrupt changes of precipitation at 63 meteorological stations and runoff as measured at six hydrological stations from 1956 to 2010 are analyzed. Results indicate that 38 stations exhibit negative precipitation trends. These stations are mainly located in the lower reaches. All six hydrological stations exhibit declining runoff trends. Abrupt runoff changes were mainly noted in the downstream portion of the basin. These variations then expanded to the middle and upper reaches. A precipitation–runoff double cumulative curve was used to detect the breakpoint of the precipitation–runoff relationship and to identify the impacts of human activities on runoff in the YR. Results show that the relatively uniform precipitation–runoff relationship has changed since 1993 in the upstream reaches and since 1970 in the middle and downstream reaches. Additionally, the relationship was more sensitive in the Lanzhou section. Human activities have become the dominant influencing factor on runoff variation since the 1970s. After the 1990s, the percentages of runoff variations due to human activities were 74.87%, 82.2%, 80.63%, and 88.71% at the Lanzhou, Toudaoguai, Huayuankou, and Lijin stations, respectively.
INTRODUCTION
Recent climate research suggests that the world is warming and the climate is changing. These changes will impact water resources. Therefore, runoff generation and environmental variations have become important hydrologic issues over the past 20 years. Environmental changes can be grouped into two categories: climate-driven changes and human activities (Franczyk & Heejun 2009; Zhang et al. 2010, 2014a, 2014b; Raghavan et al. 2014). The former includes temperature and precipitation changes. The latter includes population, water use, fertilizer consumption, river damming, domestic water consumption and industrial water consumption increases. In semi-arid and arid regions, water resources, primarily runoff, are highly sensitive to climate change. Changes in temperature and precipitation can affect evapotranspiration rates, soil moisture, and runoff regimes. Small changes in climate variables may result in significant variations in hydrological cycles and subsequent changes to regional water resources (Lioubimtseva & Henebry 2009; Siliverstovs et al. 2009; Wang & Hejazi 2011; Istanbulluoglu et al. 2012; Van Vliet et al. 2013). Runoff, as an important hydrological variable, is a combination of precipitation, evaporation, and other hydrological cycle processes on a large basin scale (≥100,000 km2). As such, runoff can be used as an indicator of the hydrological response to climate change (Hao et al. 2008; Yu et al. 2014; Zhang et al. 2014a, 2014b).
Precipitation and runoff are two critical processes in the hydrological cycle. The main objectives of this study are as follows: (1) to explore the temporal and spatial patterns of precipitation and runoff in the YR; (2) to determine abrupt runoff changes via the sequential Mann–Kendall test; (3) to analyze the relationships between runoff and precipitation in different sections of the YR basin; and (4) to explore the effects of human activities and precipitation variations on runoff change.
YRB
The mean annual precipitation and temperature of the YR are highly variable across the river basin and correspond with changes to the geology of the river. The upper reaches are located in arid regions with low annual precipitation (368 mm/year). The semi-arid middle reaches have an annual precipitation of 562 mm/year, while the lower reaches are more humid, with an annual precipitation of more than 600 mm/year. The annual mean temperature varies from 1 to 4 °C in the upper reaches, 8 to 14 °C in the middle reaches, and 12 to 14 °C in the lower reaches, with the highest temperatures occurring in July and the lowest in January. Table 1 lists the mean precipitation and runoff values in different reaches of the YRB. These variables display large variabilities between reaches.
Summary of major variables for different reaches of the YRB
Reach . | Precipitation mean (mm/yr) . | Length km . | Area km2 . | Streamflow mean (106 m3) . | Temperature mean (°C) . |
---|---|---|---|---|---|
Upper reaches | 368 | 3,471 | 385,996 | 33,896.6 | 1–4 |
Middle reaches | 562 | 1,206 | 343,751 | 56,166.22 | 8–14 |
Lower reaches | 648 | 786 | 22,726 | 56,429.22 | 12–14 |
Reach . | Precipitation mean (mm/yr) . | Length km . | Area km2 . | Streamflow mean (106 m3) . | Temperature mean (°C) . |
---|---|---|---|---|---|
Upper reaches | 368 | 3,471 | 385,996 | 33,896.6 | 1–4 |
Middle reaches | 562 | 1,206 | 343,751 | 56,166.22 | 8–14 |
Lower reaches | 648 | 786 | 22,726 | 56,429.22 | 12–14 |
DATA AND METHODS
Data preparation
Daily precipitation data in the YRB were collected from January 1956 to December 2010 by the China Meteorological Administration. Monthly and annual precipitation were established from the collected data. Monthly precipitation data sets from 63 meteorological stations are used in this study. These stations are shown in Figure 2. The observed runoff is used to investigate stream flow changes at the Guide, Lanzhou, Toudaoguai, Sanmenxia, Huayuankou, and Lijin hydrological stations (Figure 2). Furthermore, to analyze the sensitivity of runoff to precipitation, the ‘naturalized runoff’ was used instead of the observed runoff, reducing the influence of human activities, such as domestic, industrial, irrigation, and dam-control water use. These data were provided by the Yellow River Conservancy Commission (YRCC).
To explore the runoff response to precipitation in different sections of the YRB, the basin has been divided into four sections: (1) the region above Lanzhou station, denoted the Lanzhou section, includes 11 meteorological stations, which were used to analyze the precipitation variation in this section; (2) the region from Lanzhou to Toudaoguai station, called the Toudaoguai section, has 19 meteorological stations; (3) the region from Toudaoguai to Huayuankou, denoted the Huayuankou section, encompasses several tributaries that flow into the YR as well as 23 meteorological stations; and (4) the region from Huayuankou to Lijin station, denoted the Lijin section, has eight meteorological stations. The annual mean precipitation for each section was derived using the Thiessen polygon method. The annual runoff for each section refers to the interval runoff. For instance, the runoff in the Toudaoguai section equals the measured runoff at Toudaoguai station minus that at Lanzhou station. The upper two sections (the Lanzhou and Toudaoguai sections) belong to the upper reaches of the YR, which are located in the Tibetan Plateau (>3,000 m above sea level (a.s.l.)) and in a transitional zone from the Tibetan Plateau to the Loess Plateau (>1,500 m a.s.l.), which is mainly characterized by grassland and a cold, semi-arid climate. The Toudaoguai section is located in the northwestern margin of the Loess Plateau, which is characterized by a marginal, arid climate zone with portions of irrigated farmland (>1,000 m a.s.l.). The Huayuankou section belongs to the middle reaches and is located in the Loess Plateau. This section is characterized by dry farmland with both semi-arid and semi-humid climates. The Lijin section belongs to the lower reaches and is located in an alluvial plain with a humid climate.
Methodology
The Mann–Kendall trend test and regression analysis methods were used to detect the time-series trends for precipitation and runoff and to identify periods of abrupt precipitation and runoff changes in the YRB. A double mass curve (DMC) is used to assess the effects of precipitation and human activities on runoff change.
The Mann–Kendall trend test is a non-parametric assessment of the significance of monotonic trends (Mann 1945; Taxak et al. 2014). This test has the advantage of not assuming any special form for the distribution function of the data, while having predictive power nearly as high as parametric competitors. Thus, the test has been widely used and tested as an effective method to evaluate the presence of a statistically significant trend in hydrological and climatological time series (Novotny & Stefan 2007; Jones et al. 2015).
To analyze the precipitation–runoff relationship and address the impacts of precipitation and human activities on runoff, DMCs and linear regression relationships were used to show the correlation between cumulative annual runoff and precipitation. Regression analyses can effectively analyze the relationship between precipitation and runoff. The least-squares linear model is the most common method used to detect trends. In addition, the method is commonly used for statistical diagnoses and forecasting in modern climatology (Hameed et al. 1997; DaSilva 2004). The linear trend of the annual precipitation series, which was calculated using the least-squares regression, was chosen in this paper because it provides the simplest model of the unknown trend. The estimated slopes were tested against the null slope hypothesis using a two-tailed T-test at a confidence level of 95%.
RESULTS
Annual precipitation trend
In the region above Lanzhou station (Lanzhou section), the annual precipitation trends for all 11 meteorological stations are statistically insignificant. There are six stations (Maduo, Zeku, Guide, Xining, Minhe, and Lanzhou) with increasing trends (indicating precipitation increases). In the region from Lanzhou to Toudaoguai (Toudaoguai section), eight of the 19 stations exhibit increasing trends; however, most of the trends are not significant. Only the Linhe station has a significant increasing trend (Z-value of 2.23). There are 11 stations with decreasing trends, but the trends are not significant. In the region from Toudaoguai to Huayuankou (Huayuankou section), 20 of the 27 stations exhibit decreasing trends in annual precipitation. Additionally, the Wu Zhai, Lishi, Jiexiu, and Hengshan stations display significant declines, with Z-values of −2.01, −2.34, −2.02, −2.35, and −2.10, respectively. In the region from Huayuankou to Lijin (Lijin section), five of the six stations exhibit decreasing trends. These trends are statistically insignificant except for the trend at Taian station (Z-value of −2.02).
The spatial distribution of precipitation indicates that the stations displaying increasing trends mainly lie in the upper region (Lanzhou section and Toudaoguai section), and most stations display insignificant trends. However, the stations that display decreasing trends for precipitation are mainly located in the middle and lower regions of the YRB (Huayuankou section and Lijin section). Moreover, only one significant increasing trend change occurred in the upward reaches of the YR, so climate in these regions has not significantly changed in the study period. Five significant downward trends were concentrated in the middle reaches.
Precipitation characteristics in the YRB during different decades
. | Lanzhou section . | Toudaoguai section . | Huayuankou section . | Lijin section . | ||||
---|---|---|---|---|---|---|---|---|
Year . | Average precipitation . | Cv . | Average precipitation . | Cv . | Average precipitation . | Cv . | Average precipitation . | Cv . |
1960s | 486.00 | 0.15 | 276.87 | 0.26 | 601.07 | 0.16 | 689.29 | 0.27 |
1970s | 484.39 | 0.08 | 266.40 | 0.19 | 550.65 | 0.11 | 649.40 | 0.10 |
1980s | 496.47 | 0.10 | 239.59 | 0.20 | 572.65 | 0.14 | 568.30 | 0.21 |
1990s | 471.27 | 0.07 | 268.52 | 0.12 | 512.51 | 0.15 | 668.07 | 0.19 |
2000s | 477.31 | 0.11 | 246.59 | 0.15 | 543.83 | 0.11 | 634.24 | 0.12 |
. | Lanzhou section . | Toudaoguai section . | Huayuankou section . | Lijin section . | ||||
---|---|---|---|---|---|---|---|---|
Year . | Average precipitation . | Cv . | Average precipitation . | Cv . | Average precipitation . | Cv . | Average precipitation . | Cv . |
1960s | 486.00 | 0.15 | 276.87 | 0.26 | 601.07 | 0.16 | 689.29 | 0.27 |
1970s | 484.39 | 0.08 | 266.40 | 0.19 | 550.65 | 0.11 | 649.40 | 0.10 |
1980s | 496.47 | 0.10 | 239.59 | 0.20 | 572.65 | 0.14 | 568.30 | 0.21 |
1990s | 471.27 | 0.07 | 268.52 | 0.12 | 512.51 | 0.15 | 668.07 | 0.19 |
2000s | 477.31 | 0.11 | 246.59 | 0.15 | 543.83 | 0.11 | 634.24 | 0.12 |
Variations in the decadal mean precipitation in different sections of the YRB.
Abrupt precipitation change
Abrupt precipitation changes in the YRB during different decades
Year . | Lanzhou section . | Toudaoguai section . | Huayuankou section . | Lijin section . | Abrupt changes . |
---|---|---|---|---|---|
1960–1969 | 1 | – | 6 | 2 | 9 |
1970–1979 | – | 3 | 8 | 2 | 13 |
1980–1989 | 1 | 3 | 1 | – | 5 |
1990–2000 | 2 | 6 | 9 | 2 | 19 |
Year . | Lanzhou section . | Toudaoguai section . | Huayuankou section . | Lijin section . | Abrupt changes . |
---|---|---|---|---|---|
1960–1969 | 1 | – | 6 | 2 | 9 |
1970–1979 | – | 3 | 8 | 2 | 13 |
1980–1989 | 1 | 3 | 1 | – | 5 |
1990–2000 | 2 | 6 | 9 | 2 | 19 |
Abrupt changes detected by the Mann–Kendall method at the YRB meteorological stations.
Abrupt changes detected by the Mann–Kendall method at the YRB meteorological stations.
The results indicate that abrupt precipitation changes occurred over the past 55 years. There were 38 stations with abrupt changes among the 63 meteorological stations. The stations with abrupt changes are mainly located in the middle Huayuankou section and downstream Lijin section of the YRB. For example, two or three abrupt changes took place at nine stations. Seven of these stations are located in the above-mentioned area, indicating that precipitation variations in this region are more frequent. Furthermore, abrupt changes in recent decades were different. There were 9, 13, 5, and 19 abrupt changes in the 1960s, 1970s, 1980s, and 1990s, respectively. The stations with abrupt changes in the 1960s and 1970s are mainly in the southeastern YRB, while the stations with abrupt changes in the 1980s and 1990s mainly lie in the central YRB and expand to the west in the upper reaches of the YRB. When comparing the El Niño–Southern Oscillation (ENSO) events to the regional precipitation from 1950 to 2000, it is clear that the ENSO events correspond to low precipitation in the YRB. Wang et al. (2006) confirmed that the regional precipitation was strongly affected by the global climate system; since the mid-1980s, global ENSO events have become stronger and more frequent, most of the low-level precipitation years in the YRB were closely associated with moderate and strong ENSO events. Other studies have also detected significant linkages between ENSO and hydro-climatic variables (Trenberth 1997; Chiew & McMahon 2002). Zhao et al. (2014a, 2014b) concluded that strong ENSO events and monsoon activities, which primarily affect regional precipitation, together with global climate change influence the hydrological cycle in the MRYRB.
Temperature variations
The temperature trend in the YRB has increased over the past 60 years. This trend was particularly significant after the 1990s. Among the 63 stations, only two displayed decreasing temperature trends. The temperature increased at the other 61 meteorological stations, with an average increase of 0.6 °C. The increasing trend was most obvious during winter and weakest during summer. In addition, the increase was higher in the north than in the south, which is consistent with the zonal distribution of global warming. Overall, the annual average temperature increased by 0.6 °C. The correlation coefficients between the temperature and runoff and between the precipitation and runoff were −0.4 and 0.15 in the source region of the YR and −0.31 and 0.42 below the Lanzhou section, respectively. This result indicates that precipitation is the main factor that influences runoff variations in the majority of the YR.
Annual runoff trend
Mean observed runoffs in different decades at the six stations
. | Guide . | Lanzhou . | Toudaoguai . | Sanmenxia . | Huayuankou . | Lijin . | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Year . | 108 m3 . | % . | 108 m3 . | % . | 108 m3 . | % . | 108 m3 . | % . | 108 m3 . | % . | 108 m3 . | % . |
1956–1969 | 209.96 | 336.89 | 254.4 | 444.91 | 493.68 | 483.09 | ||||||
1970–1979 | 209.76 | 0.10 | 317.96 | 5.62 | 233.12 | 8.36 | 358.16 | 19.50 | 381.57 | 22.71 | 311.05 | 35.61 |
1980–1989 | 230.86 | −9.95 | 333.52 | 1.00 | 239.03 | 6.04 | 370.91 | 16.63 | 411.74 | 16.60 | 285.82 | 40.84 |
1990–1999 | 179.75 | 14.39 | 259.74 | 22.90 | 155.22 | 38.99 | 235.07 | 47.16 | 248.56 | 49.65 | 132.37 | 72.60 |
2000–2010 | 173.82 | 17.21 | 271.81 | 19.32 | 150.82 | 40.71 | 200.47 | 54.94 | 235.66 | 52.27 | 145.71 | 69.84 |
. | Guide . | Lanzhou . | Toudaoguai . | Sanmenxia . | Huayuankou . | Lijin . | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Year . | 108 m3 . | % . | 108 m3 . | % . | 108 m3 . | % . | 108 m3 . | % . | 108 m3 . | % . | 108 m3 . | % . |
1956–1969 | 209.96 | 336.89 | 254.4 | 444.91 | 493.68 | 483.09 | ||||||
1970–1979 | 209.76 | 0.10 | 317.96 | 5.62 | 233.12 | 8.36 | 358.16 | 19.50 | 381.57 | 22.71 | 311.05 | 35.61 |
1980–1989 | 230.86 | −9.95 | 333.52 | 1.00 | 239.03 | 6.04 | 370.91 | 16.63 | 411.74 | 16.60 | 285.82 | 40.84 |
1990–1999 | 179.75 | 14.39 | 259.74 | 22.90 | 155.22 | 38.99 | 235.07 | 47.16 | 248.56 | 49.65 | 132.37 | 72.60 |
2000–2010 | 173.82 | 17.21 | 271.81 | 19.32 | 150.82 | 40.71 | 200.47 | 54.94 | 235.66 | 52.27 | 145.71 | 69.84 |
Annual observed runoff variations (1956–2010) at six hydrological stations in the YRB.
Annual observed runoff variations (1956–2010) at six hydrological stations in the YRB.
Mann–Kendall test for annual runoff at six hydrological stations. Horizontal lines represent critical values (−1.96) corresponding to the 95% confidence interval.
Mann–Kendall test for annual runoff at six hydrological stations. Horizontal lines represent critical values (−1.96) corresponding to the 95% confidence interval.
Abrupt runoff changes
Mann–Kendall test for annual observed runoff. Dotted horizontal lines represent critical values corresponding to the 95% confidence interval.
Mann–Kendall test for annual observed runoff. Dotted horizontal lines represent critical values corresponding to the 95% confidence interval.
In general, 1990–2010 is the period with abrupt runoff declines in the YRB. Lijin station exhibits the earliest runoff decline. This decrease starts as early as 1981 and accelerates drastically in 1986. At Huayuankou station and Sanmenxia station, abrupt changes began in 1989 and 1990, respectively, and became significant in 1993. Runoff declines at Toudaoguai station and Lanzhou station began in 1990 and accelerated drastically in 1993. At Guide station, abrupt change occurred in 1993. Runoff decline there began later (starting in 1997), but the trend is not significant. Therefore, abrupt runoff changes initially occurred in the downstream portions of the YRB and then expanded to the middle and upper reaches of the basin.
Relationships between runoff and precipitation
Contributions of precipitation and human activities to the runoff change
The DMCs indicate that human activities clearly impact runoff change. The variations in hydrological processes were not only affected by precipitation, but also by human activities in the catchment. To quantitatively analyze the impacts of human activities and precipitation variations on runoff change, we use the period less affected by human activities (before the breakpoint) as a reference period based on the characteristics of the DMCs. The reference period of the Lanzhou and Toudaoguai sections is 1956 to 1992. The reference period is 1956 to 1970 for the Huayuankou and Lijin sections. The period after the breakpoint is the affected period, in which human activities impact runoff variations. The regression equations for the reference period were used to represent the effects of precipitation variations on the runoff variables. By extending the regression equations between observed runoff and precipitation during the reference period to the precipitation in the affected period, runoff variations due to precipitation variations can be estimated. By subtracting the observed values of runoff variables from estimated values based on precipitation, the changes caused by human activities can be further estimated. The estimated contributions of precipitation variations and human activities to runoff changes in different sections are summarized in Table 5 for the affected periods. Runoff in the Lanzhou section decreased by 18.70% compared with the reference period. Additionally, the contributions of human activities and precipitation variations are 74.87% and 25.13%, respectively. In the Toudaoguai section, the runoff is more negative compared with the reference period. The impacts of human activities and precipitation are 82.20% and 17.80%, respectively. In the middle reaches of the YRB (Huayuankou section), runoff consistently decreased throughout the entire period after 1969. Human activities played a role in decreasing runoff, with contributions of 80.36% and 80.63% in 1970–1989 and 1990–2010, respectively. In the lower reaches, the impact of human activities gradually increased, dominating the runoff change with contributions of 83.98% and 88.71% in the two affected periods. These results provide the quantitative contributions of human activities and precipitation to the runoff change based on the reference period. Generally, human activities increasingly contribute to temporal and spatial runoff change in the YRB.
Quantification of the impact of precipitation change and human activities on runoff
. | . | . | . | Measured runoff change (ΔQo) . | Effect of precipitation (ΔQp) . | Effect of human activity (ΔQh) . | |||
---|---|---|---|---|---|---|---|---|---|
Section . | Period . | Observed runoff . | Calculated runoff . | 108 m3 . | % . | 108 m3 . | % . | 108 m3 . | % . |
Lanzhou | 1956–1992 | 325.73 | 327.99 | ||||||
1993–2010 | 264.81 | 310.42 | 60.92 | 18.70 | 15.31 | 25.13 | 45.62 | 74.87 | |
Toudaoguai | 1956–1992 | − 88.46 | − 87.04 | ||||||
1993–2010 | − 113.11 | − 92.85 | 24.64 | 27.85 | 4.39 | 17.80 | 20.26 | 82.20 | |
Huayuankou | 1956–1969 | 239.27 | 237.70 | ||||||
1970–1989 | 160.58 | 223.82 | 78.69 | 32.89 | 15.45 | 19.64 | 63.24 | 80.36 | |
1990–2010 | 92.13 | 210.77 | 147.14 | 61.49 | 28.50 | 19.37 | 118.64 | 80.63 | |
Lijin | 1956–1969 | 483.09 | 462.87 | ||||||
1970–1989 | 298.44 | 453.50 | 184.65 | 38.22 | 29.59 | 16.02 | 155.07 | 83.98 | |
1990–2010 | 143.35 | 444.74 | 339.74 | 70.33 | 38.35 | 11.29 | 301.39 | 88.71 |
. | . | . | . | Measured runoff change (ΔQo) . | Effect of precipitation (ΔQp) . | Effect of human activity (ΔQh) . | |||
---|---|---|---|---|---|---|---|---|---|
Section . | Period . | Observed runoff . | Calculated runoff . | 108 m3 . | % . | 108 m3 . | % . | 108 m3 . | % . |
Lanzhou | 1956–1992 | 325.73 | 327.99 | ||||||
1993–2010 | 264.81 | 310.42 | 60.92 | 18.70 | 15.31 | 25.13 | 45.62 | 74.87 | |
Toudaoguai | 1956–1992 | − 88.46 | − 87.04 | ||||||
1993–2010 | − 113.11 | − 92.85 | 24.64 | 27.85 | 4.39 | 17.80 | 20.26 | 82.20 | |
Huayuankou | 1956–1969 | 239.27 | 237.70 | ||||||
1970–1989 | 160.58 | 223.82 | 78.69 | 32.89 | 15.45 | 19.64 | 63.24 | 80.36 | |
1990–2010 | 92.13 | 210.77 | 147.14 | 61.49 | 28.50 | 19.37 | 118.64 | 80.63 | |
Lijin | 1956–1969 | 483.09 | 462.87 | ||||||
1970–1989 | 298.44 | 453.50 | 184.65 | 38.22 | 29.59 | 16.02 | 155.07 | 83.98 | |
1990–2010 | 143.35 | 444.74 | 339.74 | 70.33 | 38.35 | 11.29 | 301.39 | 88.71 |
DISCUSSION
Sensitivity of runoff to precipitation
In this section, the naturalized runoff was used to analyze the sensitivity of runoff to precipitation. The linear correlation between annual precipitation and naturalized runoff was calculated using the least squares regression. The estimated slopes were tested against the null slope hypothesis using a two-tailed T-test at a confidence level of 95% (DaSilva 2004). The slopes indicate that runoff is positively related to precipitation in the Lanzhou, Toudaoguai, Huayuankou, and Lijin sections. The sensitivity of the runoff response to precipitation change varies in different sections, with R2 values of 0.73, 0.02, 0.67, and 0.59 in the Lanzhou, Toudaoguai, Huayuankou, and Lijin sections, respectively. Therefore, the relationship between runoff and precipitation is most sensitive in the Lanzhou section, followed by the Huayuankou, Lijin, and Toudaoguai sections. This result indicates that watershed characteristics, such as vegetation conditions, soil moisture, and saturated hydraulic conductivity, influence the response of runoff to precipitation. The YRB has a very broad range of climate and land surface conditions. Compared with other regions, the Lanzhou section has good vegetation cover and relatively less human activity. Thus, the area has relatively saturated soil conditions that promote runoff. Runoff is more sensitive to precipitation changes in the Lanzhou region than in other regions. Furthermore, the spatial precipitation distribution and trends reveal that no significant variations occurred in the upper reaches of the Lanzhou section. Additionally, the declining annual runoff trend in Lanzhou is insignificant because of the strong relationship between runoff and precipitation. In the Toudaoguai section, the river flows across the Loess Plateau and less water enters the river from fewer tributaries. As a result, more water is consumed in this region due to a high evaporation rate and drought soil conditions on the Loess Plateau. Therefore, runoff in the Toudaoguai section is less sensitive to precipitation than in the Lanzhou section.
Average runoff change minus precipitation change as a function of precipitation change for the Lanzhou, Toudaoguai, Huayuankou, and Lijin sections in the YRB.
Average runoff change minus precipitation change as a function of precipitation change for the Lanzhou, Toudaoguai, Huayuankou, and Lijin sections in the YRB.
In addition to the significant impact of precipitation on the regional runoff in the YRB, other global climate changes also influence the hydrological cycle. In particular, global warming has increased air temperatures in the river basin, causing increased evapotranspiration (Zhang et al. 2004). Since 1970, the average annual air temperature over the river basin has increased from 16.5 °C to 17.5 °C (Xu 2005). As a result, the evapotranspiration from agriculture and reservoir evaporation have increased. Thus, the climate of the YRB has become warmer and drier, while the regional runoff has decreased.
Different abrupt runoff change years among different sections
This study demonstrates that the abrupt runoff change years vary in different sections of the YRB. The abrupt runoff changes in the upper reaches of the Lanzhou section and Toudaoguai section occur in 1993, suggesting simultaneous inflexion points in the runoff change of these two sections. The abrupt runoff changes in the middle and lower reaches of the Huayuankou and Lijin sections occurred in 1970 and 1990, respectively. The abrupt runoff change years reflect the shift in the relative intensities of influencing factors, including both climate-driven factors and human activities. The Huayuankou and Lijin sections are located in the middle and lower reaches of the YR, which experienced more intense and earlier human influences than other sections (Yang et al. 2004). In contrast, the Lanzhou section is located in the headwater region of the river, which is subject to only limited human influences. Thus, the abrupt runoff change years in the downstream reaches were earlier than those for the upstream reaches. Other studies of different tributaries of the middle YR have suggested different abrupt runoff change years (Chang et al. 2015), such as 1971 for the Wuding River (Xu 2011) and 1992 for the Weihe River (Wang et al. 2008). However, the results in the tributaries correspond with the abrupt change points in the middle reaches of the YR.
Impacts of human activities on the runoff
According to the results of Table 5, human activities played a dominant role in runoff reduction in the YRB after each breakpoint year. Additionally, the impacts of human activities on the runoff changes vary in the different sections. The two most important aspects of human activities in the YRB are basin-wide erosion control practices and water diversion for irrigation and other human uses. Runoff has significantly decreased due to the effects of these factors.
In the upper reaches of the Lanzhou and Toudaoguai sections of the YRB, water resources are abundant; however, human activities have inevitably influenced the runoff. Large-scale human activities include hyper-irrigation and large hydroelectric projects. There are 24 reservoirs scattered throughout the YRB, with storage capacities exceeding 0.10 × 109 m3. These reservoirs redistribute the seasonal and annual water discharges and sediment loads. Large hydroelectric projects (including reservoirs at Longyangxia, Liujiaxia, Qingtongxia, and Sanshenggong) are located in the upper reaches (Figure 1). The slope of the DMC in the Lanzhou section (Figure 10) decreases in 1993, indicating that the impact of human activities increased, reaching 74.87% (Table 5). This increase is mainly due to the Liujiaxia and Longyangxia reservoirs. The Liujiaxia and Longyangxia reservoirs were constructed above Lanzhou in 1968 and 1985, respectively. The Liujiaxia reservoir has a storage capacity of 5.7 km3 and a 147 m high dam wall. The Longyangxia reservoir (27.6 km3 storage capacity and a 178 m high dam wall) is a multi-year reservoir. The combined operation of these two reservoirs regulates the seasonal water discharge from the upper reaches to meet demands for agricultural irrigation; thus, these operations affect the middle and lower reaches (Chen 1997). Moreover, Figure 10 shows that the precipitation and runoff DMC in the Toudaoguai section is negatively correlated because runoff at Toudaoguai station is less than that at Lanzhou station. Human activities and underlying surface conditions directly reduce the runoff in this section. The hyper-irrigation area of the upper reaches of the YRB is approximately 1,230 km2 according to statistical data from the Chinese Ministry of Water Resources. More than 94% of this area is located in the Toudaoguai section (Table 6). Hyper-irrigation directly reduces the regional water discharge due to water consumption by irrigation fields. The runoff decrease in the upper reaches has been affected by increasing water diversion from the main stream for irrigation and industrial utilization. Therefore, the impacts of human activities on runoff are most significant in the upper reaches. As a result, the runoff trends at three stations (Guide, Lanzhou, and Toudaoguai) in the upper reaches have declined. However, the annual precipitation trend at one station displayed a significant increase (Figure 4).
Hyper-irrigation areas in the YRB
Section . | Area (km2) . | Hyper-irrigation area (km2) . |
---|---|---|
Lanzhou | 222,551 | 73 |
Toudaoguai | 145,347 | 1,161 |
Huayuankou | 362,138 | 2,316 |
Lijin | 21,833 | 3,580 |
Section . | Area (km2) . | Hyper-irrigation area (km2) . |
---|---|---|
Lanzhou | 222,551 | 73 |
Toudaoguai | 145,347 | 1,161 |
Huayuankou | 362,138 | 2,316 |
Lijin | 21,833 | 3,580 |
Source:Yang et al. (2004).
In the middle reaches of the YRB, soil conservation practices, including biological measures (e.g., trees and pasture) and structural projects (e.g., terraces and dams), were implemented in the 1970s and 1980s because of severe soil losses that contributed to >90% of the total river sediment load. Thus, the DMC exhibits breakpoints in 1970 and 1990 (Figure 10). The effects of human activities on runoff increased gradually, reaching 80.36% and 80.63% in the two affected periods. As shown in Table 7, the soil conservation area has expanded with time. Human activities have changed the local micro-topography, increased the ability of intercepting precipitation, and consequently, delayed and reduced the runoff (Miao et al. 2011). Tang indicated that the surface runoff decrease in the upper and middle reaches due to soil conservation measures was approximately 0.63 × 109 m3/year over the period of 1980–1997 (Tang 2004). In addition, the Sanmenxia reservoir, the first large reservoir built in the river basin, is located in the middle reaches (Figure 1). The Xiaolangdi reservoir, which has a storage capacity of 12.7 km3 and a 160 m high dam wall, is also located in the middle reaches between the Sanmenxia reservoir and Huayuankou station. In 2002, the YRCC initiated the Water-Sediment Regulation Scheme, which uses the controlled release of floodwaters from the Xiaolangdi reservoir and other small reservoirs to distribute the sediment retained within the Xiaolangdi across the lower reaches (Wang et al. 2005). These major reservoirs are situated on the main stream, and make the largest contributions to water regulation and sediment retention (Wang et al. 2006).
Area of soil conservation practices in the Hekou-Longmen sub-catchment
. | Level terrace . | Afforestation . | Grass-planting . | Check dam . | Total . |
---|---|---|---|---|---|
1959 | 331 | 1,513 | 357 | 28 | 2,229 |
1969 | 1,158 | 3,423 | 383 | 154 | 5,118 |
1979 | 2,305 | 8,818 | 1,045 | 395 | 12,563 |
1989 | 3,448 | 19,862 | 2,114 | 563 | 25,987 |
1996 | 4,859 | 25,373 | 2,408 | 682 | 33,322 |
. | Level terrace . | Afforestation . | Grass-planting . | Check dam . | Total . |
---|---|---|---|---|---|
1959 | 331 | 1,513 | 357 | 28 | 2,229 |
1969 | 1,158 | 3,423 | 383 | 154 | 5,118 |
1979 | 2,305 | 8,818 | 1,045 | 395 | 12,563 |
1989 | 3,448 | 19,862 | 2,114 | 563 | 25,987 |
1996 | 4,859 | 25,373 | 2,408 | 682 | 33,322 |
In the lower reaches of the Lijin section, hydrological change results from the combination of contributory effects from the local region, and the upper and middle reaches of the YRB. Similar to the Huayuankou section, the DMC in this section occurs in the years 1970 and 1990, and human activities have an increasing influence on runoff change. Due to the flat topography of the lower YR sub-basin, the irrigation areas were extended outside the basin in the 1970s and 1980s (Yang et al. 2004). The water diversion for irrigation resulted in a remarkable decrease in the runoff in this area, and the abrupt change year (1971) is the earliest among all the surveyed areas and stations.
CONCLUSIONS
Using the runoff records from six major hydrological stations and precipitation measurements from 63 meteorological stations in the YRB, this study identified the basic trends and abrupt changes associated with precipitation and runoff in four sections of the basin. DMCs were used to analyze the relationship between precipitation and runoff, and to detect the impact of human activities on runoff in the YRB.
Different trends and spatial patterns were obtained for precipitation and runoff in the YRB. Only seven of the 63 stations displayed a significant precipitation trend, with one station displaying an increasing trend and six stations displaying decreasing trends. Thirty-eight stations exhibited negative trends. These stations are mainly located in the Huayuankou and Lijin sections of the basin. Annual runoffs at the six hydrological stations all display declining trends, although the trend is insignificant at Guide station.
There were 49 abrupt precipitation changes at 63 meteorological stations. Most of these were located in the middle Huayuankou section and downstream Lijin section of the YRB. In addition, abrupt changes were more common from 1991 to 1998 than in other years due to the impacts of land use and human activities. The most abrupt runoff decline in the YRB was observed between 1990 and 2010. Additionally, abrupt runoff changes were mainly noted in the downstream reaches of the YRB. These changes then expanded to the middle and the upper reaches of the basin.
Precipitation–runoff double cumulative curves were used to detect the breakpoints of the precipitation–runoff relationships in different sections for the period of 1956–2010. In the upper reaches of the YRB, precipitation and runoff were relatively uniform. However, precipitation or runoff characteristics changed after 1993. In the middle and downstream reaches, the breakpoints were 1970 and 1990. Furthermore, the relationship between runoff and precipitation is most sensitive in the Lanzhou section, followed by the Huayuankou, Lijin, and Toudaoguai sections.
Human activities have become the dominant factors influencing runoff variations since the 1970s. After the 1990s, the percentages of runoff variations due to human activities were 74.87%, 82.2%, 80.63%, and 88.71% at the Lanzhou, Toudaoguai, Huayuankou, and Lijin stations, respectively. The two most important human activities in the YRB are erosion control practices and water diversion for irrigation and other human uses.
This study illustrated the runoff changes in the YRB and analyzed the influences of precipitation and human activities on runoff variations. The influences of evaporation, air temperature, and land use change on runoff are not considered due to data limitations. Future studies will consider these factors in an attempt to fully reveal the impacts of climate change and human activities on runoff variations in the YRB.
ACKNOWLEDGEMENTS
This research was supported by the Natural Science Foundation of China (51190093) and Key Innovation Group of Science and Technology of Shaanxi (2012KCT-10). Sincere gratitude is extended to the editor and the anonymous reviewers for their professional comments and corrections.