Ecological
deterioration in arid regions caused by agricultural
development has become a global issue. Understanding water requirements of
the oasis ecosystems and the influences of human agricultural activities and
climate change is important for the sustainable development of oasis
ecosystems and water resource management in arid regions. In this study,
water requirements of the main oasis in Heihe River basin during 1986–2013
were analyzed and the amount showed a sharp increase from 10.8
Inland river basins take up about 11.4 % of the land area in the world and most of them are distributed over arid regions (Li et al., 2013). Water resources in arid regions are scarce and critical to ecosystems and societies. For the inland river basins in arid regions, water resources mainly originate from the precipitation and snow/glacier melting in the upstream mountainous areas, and are consumed mainly by agriculture and human society in oases of the piedmont plains in the midstream area, and then finally are discharged and dispersed in the tail lakes downstream (Kang et al., 1999; Shen and Chen, 2010). The precipitation in plain areas or in major economic centers of arid basins has nearly no significant meaning for generating runoff (Shen and Chen, 2010).
Owing to scarce water resources in arid regions, ecosystems and societies are vulnerable to hydrologic changes. With the rapid growth of population in arid regions of the world (Shen and Chen, 2010), the utilization of surface and groundwater for irrigation increased without enough consideration for ecological conservation, which caused severe deterioration of water and ecosystems in most arid river basins. For example, the Amu Darya and the Syr Darya are two main rivers in Central Asia, which flow towards the Aral Sea. More than 90 % of the water withdrawal in the region was used for agricultural irrigation (Sorg et al., 2014). With the increase in irrigated area in the past decades, irrigation withdrawals have measurably reduced inflow to the Aral Sea since 1960s, which caused significant shrinking of the water surface of the Aral Sea and land desertification, and even the fishery in the Aral Sea has almost been destroyed because of salinization (Micklin, 1988; Sorg et al., 2014; Shen and Chen, 2010; Karimov and Matthies, 2013). Similarly, the disappearance of Lop Nor in western China, the dying of the Dead Sea in the Middle East, and the shrinking of Lake Chad in Africa are all notable examples. Ecological deterioration in arid regions caused by agricultural development has become a global issue and has become the main obstacle to the sustainable development of oasis ecosystems.
Despite human exploitation, climate change can also influence the water resources in arid regions. It is reported that the climate in arid regions has become drier in the past century (Narisma et al., 2007; Dai et al., 2004), showing increasing temperature, variability of precipitation, and reduction of glaciers and snow areas (Wang and Qin, 2017) and will be more arid in the future (Bates et al., 2008). However, huge amount of studies suggested that the water and ecological degradation in arid regions was largely affected much more by irrational human exploitation than climate change (Jarsjö et al., 2008; Aus der Beek et al., 2011; Huo et al., 2008; Dong et al., 2014; Ma et al., 2014).
The ecological degradation and water shortages have heightened the importance of water allocated to the agriculture in the oasis ecosystems. Water requirement is an important parameter for irrigation scheduling and regional water allocation. Studies on water requirements are theoretically and practically indispensable for the sustainable development of oasis ecosystems in arid regions. Scientists have obtained some research results about water requirements of oasis ecosystems, including the crop water requirements (Kawy and El-Magd, 2013; Liu et al., 2010; Siebert et al., 2007; Zhao et al., 2005, 2010; De Silva et al., 2007; Kawy and Darwish, 2014), and ecological water requirements (Guo et al., 2016; Ye et al., 2010; Zhao et al., 2007). Studies have shown that the water requirement will increase if the climate becomes drier and warmer (Döll, 2002; Nkomozepi and Chung, 2012; Fu et al., 2014), and human activities have gradually become the predominant factor increasing the water requirement in the past decades (Bai et al., 2014; Coe and Foley, 2001; Zou et al., 2017). However, there are few studies that separately quantify the contributions of climate change and human agricultural activities to changes in water requirement.
Approximately one-quarter of land area in China is located in arid regions. As
the second-largest inland river in China, the Heihe River basin has also suffered
water conflict between agricultural development and ecological health, and
was chosen as the target basin for a key national research program on
ecohydrology and integrated basin water management by the Natural Scientific
Foundation of China in 2012, and the program is still ongoing. Thus, the
oasis in the middle Heihe River basin, where more than 90 % of the arable
land is concentrated was taken as the study area. The main objectives of
this study are to make clear the changes in water requirements in the oasis
under climate change and human agricultural activities and identify the main
factor that influences the changes in water requirement based on the
clarification of the contributions of climate and human activities,
including land structure and area, to the changes in water requirement. The research questions addressed were as follows: (1) how have the water
requirements of the oasis changed in the past
Details of the study area.
Heihe River originates in the Qilian Mountains, and flows to the oases in
the piedmont plain after reaching the mountain outlet at the Yingluo Gorge,
then finally terminating at the East and West Juyan lakes. It contains an
ecosystem which consists of ice–snow, frozen soil, and mountain vegetation
zones at the upper reach, and oasis zone and desert zone at the middle and down
reaches (Ersi et al., 1999; Kang et al., 2005; Zhao et al., 2007). The study
was conducted in the oasis in the middle Heihe River basin (between
38
Situated in the interior of the Eurasian continent, the study area possesses a
temperate continental arid climate with sufficient sunlight, wide
temperature variations, and scarce precipitation. According to the observed
data by Gaotai and Zhangye meteorological stations in the study region
during 1953–2014, the annual average temperature is about 6.0–9.4
The study area has an agricultural development history of over 2000 years
owing to its flat land, adequate sunlight, and convenient water resource
from Qilian Mountains. The oasis in the middle Heihe River basin has thus
become an important commodity grain base in China. Combined together, the
cultivated land, forest, grass, swampland, and waters make up the oasis. The oasis area has been expanding in the past
The areas of different land use types in the oasis in the middle Heihe River basin.
Lacking in precipitation, surface runoff has become the main surface water
resources for irrigation. The middle Heihe River flows from Yingluo Gorge to
Zhengyi Gorge, supplying water for oasis in the middle river basin. Annual
discharge observed at Yingluo Gorge increased from around 14.4
Daily meteorological observations were collected from China Meteorological Administration (CMA), mainly including the maximum, minimum, and average air temperatures, wind speed, relative humidity, and sunshine duration. Ten meteorological stations – which covered the Gaotai and Zhangye stations inside the study region and Dingxin, Jinta, Jiuquan, Tuole, Yeniugou, Qilian, Shandan, and Alxa Youqi stations outside the study region – were selected to get the spatial distribution of meteorological elements (Fig. 1). Observations on crop growth and phenology were collected from the agricultural meteorological stations in Gansu Province, especially from the station in Zhangye. However, the data on crop growth and phenology were only basically recorded completely for the maize (1993–2013) and spring wheat (1992–2013), so the growth and phenology data for other vegetation were obtained by references (Liu, 2014; Allen et al., 1998; Pu et al., 2004; Li et al., 2009; Zhou et al., 2015), combining practical investigation. The growth and phenology data for maize before 1993 were set as that in 1993, and for spring wheat before 1992 as that in 1992.
Land use data for years 1986, 1995, 2000, and 2011 at a spatial resolution of 30 m (Wang et al., 2011a, b; Liu et al., 2003; Wang et al., 2014), which were developed by CAS, were used in this study. The same classifying system for land cover was applied to the 4 years of land use data. The land use patterns in the basin have been divided into six types: cultivated land; forest land, which includes closed forest land, sparse woodland, shrubs, and other woodland; grassland, which contains high-coverage grassland, moderate-coverage grassland, and low-coverage grassland; waters which comprise rivers, lakes, reservoirs, and beach land; construction land; and unused land, which contains sand, gobi, saline–alkali land, swampland, bare land, bare rock, and gravel. To obtain the continuous land use maps, the land use data at the spatial resolution of 30 m were transformed into the land use data at the spatial resolution of 1 km in the form of percentage. Then the spatial distribution of the land use data between the four discrete years could be obtained by linear interpolation.
The planting proportion of the crops in
To obtain the spatial distribution of specific crops in the cultivated land,
the socioeconomic statistical data were collected from the
The water requirements estimated in this study were compared with two
evapotranspiration (ET) data sets provided by Cold and Arid Regions Science
Data Center at Lanzhou (
In this study, the water requirements of the cultivated land, forest land, high-coverage grassland, moderate-coverage grassland, waters except the beach land, and the swampland in the unused land were considered. The water requirements of the low-coverage grassland, beach land, construction land, and unused land except the swampland were taken as zero.
Water requirements of the crops and grass in the oasis refer to the
evapotranspiration from disease-free, well-fertilized crops, grown in large
fields, under optimum soil water conditions and achieving full production
under the given climatic conditions. This can be calculated using crop
coefficient approach as following:
ET
Crop coefficients of the different crops in different growth stages in the oasis of the middle Heihe River basin.
Different vegetation types have different
Vegetation coefficient in different depths of groundwater level.
For the forest land, the water requirements of closed forest land, sparse
woodland and shrubs were estimated by phreatic evaporation. It can be
calculated as below:
The other woodland in the study area was mainly orchard, so the water requirement of other woodland was calculated by the crop coefficient approach (Table 1).
The water requirement of waters can be taken as the evaporation from water
surfaces, which can be calculated according to Shuttleworth (1993):
The water requirement for the swampland was calculated by crop coefficient
approach. The
According to the methods to estimate water requirements of the oasis in the
middle Heihe River basin, the value of the water requirements (
The individual proportional contribution (
The water requirement
There are 15 specific land use types in the oasis of the middle Heihe River basin, which are cultivated land (maize, spring wheat, cotton, oilseed, sugar beet, potato, vegetables), grassland (high-coverage grassland, moderate-coverage grassland), forest land (closed forest land, sparse woodland, shrubs, other woodland), waters, and swampland. Different land use types may have different water requirements. To understand the water requirements in the oasis, the spatial and temporal variations of the total water requirement and the water requirement per unit area were analyzed. In the study, the water requirement per unit area for each land use type was calculated by dividing the total water requirement of each land use type by the corresponding land area. After validation to ensure the accuracy of the results, the water balance and determinants to the variation of the water requirement of the oasis in the middle Heihe River basin were analyzed.
The water requirement of the total oasis increased from
10.8
The water requirement of the cultivated land per unit area increased from 519.2 to 624.9 mm during 1986–2013, while the water requirement of the oasis per unit area increased from 527.1 to 642.0 mm during 1986–2013 (Fig. 4b). The mean annual water requirements of the cultivated land and the oasis per unit area were 545.2 and 557.9 mm, respectively. Maize, spring wheat, and vegetables are the main crops in the middle Heihe River basin. The mean annual water requirements of the maize, spring wheat, and vegetables per unit area were 571.5, 413.7, and 728.8 mm, respectively. Waters required the most water per unit area, the mean annual water requirement of which reached 1323.9 mm. The swampland covered with reeds also needed a large amount of water per unit area, the mean annual water requirement of which reaching 968.6 mm. Different land surface coverage for grassland and forest land had different water requirements. The mean annual water requirements of the closed forest land, sparse woodland, shrubs, other woodland, high-coverage grassland, and moderate-coverage grassland per unit area were 477.5, 128.9, 264.0, 705.1, 663.6, and 340.0 mm, respectively.
The spatial distribution of the water requirement in the cultivated land and oasis at the spatial resolution of 1 km in the middle Heihe River basin in 1986, 1995, 2000, 2011.
The oasis in the middle Heihe River basin was scattered along the rivers. Most of the water requirements in the oasis were below 500 mm per square kilometer in 1986 considering the mixed pixel and area weight, but with the climate change and human agricultural activities, the water requirement in large area of the oasis exceed 500 mm per square kilometer in 2011 (Fig. 5). In addition, the area of high water requirement in the oasis agreed with the location of the cultivated land (Fig. 5). Additionally, the ecological vegetation in the oasis except northwest Gaotai county did not show significant increase in water requirement (Fig. 5).
The difference of
The cultivated land in most area of the oasis expanded during the past
Water requirement is defined as a theoretical value. For the crops, it can be taken as the potential crop ET. However, there were no available data observed or calculated by others for the potential crop ET, so the actual ET data were adopted to validate the water requirement in the oasis to see if the results were acceptable.
The comparison between the yearly and monthly (May, June, July) water requirements and ET data, which included the ET data estimated by ETWatch model from 2000 to 2013 for the cultivated land and the oasis, and the ET data estimated by ETMonitor model from 2009 to 2011 for the oasis.
According to the yearly and monthly ET estimated by ETWatch and ETMonitor,
the total ET was well correlated with the estimated water requirement
with the determination coefficient (
Compared with the ET data sets (2009–2011) estimated by ETMonitor at 1 km
spatial resolution in the middle Heihe River basin, the yearly and monthly
water requirements were all larger than the corresponding ET data
(Fig. 7), and the RMSE for the monthly data in May, June, and July was
1.27
Yingluo Gorge divides the upper and middle Heihe River, and Zhengyi
Gorge divides the middle and lower Heihe River. The two hydrologic
stations recorded the inflow and outflow of the mainstream of the middle
Heihe River. Thus, the surface runoff of the mainstream of the middle Heihe
River consumed in the middle Heihe River basin can be considered as the
difference between Yingluo Gorge and Zhengyi Gorge. Additionally, there are some
small rivers that also flow into the middle Heihe River basin, like Shandan River
and Liyuan River. The mean annual runoff of the Liyuan River and Shandan
River is 2.36
The water requirement and the surface water supply including the precipitation and runoff data for the oasis in the middle Heihe River basin. The runoff data included the runoff from the mainstream of middle Heihe River, Shandan River, and Liyuan River.
Water balance items in the middle Heihe River basin during 1986–2013.
The vegetation in the oasis can be divided into two categories: one is
agricultural vegetation, which includes the crops and orchard, and the other
is the ecological vegetation. The precipitation in the middle Heihe River
basin is too little to supply enough water for the ecological vegetation
(Table 3). The ecological vegetation usually grows around the cultivated
land, so they can absorb the water of infiltration. In addition, the shelter
forest often needs irrigation, and the shrubs like
Both climate change and human agricultural activities can influence the
water requirement of the oasis. In this study, the land expansion, which
influences the total oasis area, and the land structure, which influences
the area proportion of each land use type in the oasis, were considered for
the human agricultural activities. Because the oasis is dominated by the
cultivated land, both the contributions of the influencing factors to the
changes in water requirement of the oasis and of the cultivated land
were analyzed. For the cultivated land, the three influencing factors
considered were climate change, the expansion of the cultivated land,
and the crop structure in the cultivated land. The area of the oasis in
1986, 1995, 2000, and 2011 was 2048.96, 2091.13, 2216.97, and
2954.85 km
The long-term trend of the water requirement for the cultivated land
and oasis under different scenarios during 1986–2013.
The water requirements of the oasis and cultivated land increased by
0.3455
Considering the impact of land expansion on the water requirement,
the climate and the land structure were set stable, and only the total land
area changed with time during 1986–2013. In this scenario, the water
requirement increased rapidly at the rates of 0.2008
Considering the impact of land structure on the water requirement,
the climate and total land area were set stable, and only the land structure
changed as usual during 1986–2013. In this scenario, the water requirement
increased at the rates of 0.0874
The three influencing factors explained approximately 91 % of the
increase in the water requirements of the oasis and cultivated land
during 1986–2013. In the past
Based on the land use and meteorological data, the impact of climate change and human agricultural activities, including land expansion and changes in land structure, on the water requirements of the oasis and the cultivated land which is the main part of the oasis in the middle Heihe River basin were calculated and analyzed. The results suggest that both climate change and human agricultural activities can lead to an increase in water requirements, and the contribution of human agricultural activities to the increase was significantly greater than the climate change. In addition, the land expansion was the dominant factor contributing to the increase in water requirements.
Crop water requirement is the ET from disease-free, well-fertilized crops
under optimum soil water conditions and achieving full production. There is
no available observed crop water requirement to validate the results. Only
actual ET data can be obtained. There are 18 field stations in the oasis
that all located in Ganzhou district in the middle Heihe River basin for
conducting meteorological observations and flux measurements from around
June 2012. However, due to the incomplete daily data and short time series, we
used the ET data sets provided by Cold and Arid Regions Science Data Center
at Lanzhou (
Crop coefficient is an important parameter to estimate the water requirement, and it is related to many factors, such as the biological characters of crops, cultivation, and soil conditions, so the crop coefficients for different crop varieties of the same crop could be different. Some researchers (Nader et al., 2013; Mu et al., 2005) have studied the crop coefficients affected by different crop varieties, and found that there were differences in every growth stage between different varieties, and the differences were almost less than 0.3. However, it is difficult to get the crop coefficients for every specific crop variety because there are too many varieties. Additionally, the water requirement is not only related to the crop coefficient; it is also related to the crop growth period. Many factors influencing the crop coefficient also have an effect on the growth stages. Like the study by Nader et al. (2013), the water requirement variation was much smaller than the variation of crop coefficients for different varieties. Therefore, though we did not distinguish the crop coefficients among different varieties, the estimated water requirements in the study were still reliable.
There are many factors influencing the water requirement. This study only analyzed the major factors which influence it (climate change and human agricultural activities). Climate change (including factors for air temperature, wind speed, relative humidity, and sunshine duration) and human agricultural activities (including land expansion and changes in land structure) in all constitute about 91 % of the increase in water requirement of the oasis. Other influential factors, such as changes in location of different land types, are difficult to quantify and were not considered in the study. Additionally, some driving factors are not independent, and changes in one factor can cause changes in other factors, such as the climate change and changes in crop phenology. Thus, in the contribution analysis, when the climate was set stable, the crop phenology also kept stable, and when the climate changed, the crop phenology varied according to the statistical data.
As an oasis located in an ecologically vulnerable area and dominated by agriculture, the development of agriculture should match up with the climate and ecological capacity. The amount of water consumed in the oasis ecosystem concerns the ecological security of the whole basin. To promote harmonious development among the upper, middle, and lower reaches, the amount of water consumed in the agricultural oasis must be controlled and a series of water-saving measures should be carry on. Because the oasis area and the land structure are the main reason why the water requirement of the oasis increased so fast, additional efforts will be made to determine the appropriate oasis area and crop structure in the oasis.
Affected by the climate change and human agricultural activities, the water
requirement of the oasis increased significantly during 1986–2013,
from 10.8
The meteorological data (CMA, 2017) and observations on crop growth and phenology (CMA, 2013) are available at
The authors declare that they have no conflict of interest.
This study was supported by the National Natural Science Foundation of China (no. 91425302). We would like to extend our thanks to Yulu Zhang from Qinghai Normal University, who helped process the wind speed data in three meteorological stations. Edited by: Zhenghui Xie Reviewed by: two anonymous referees