Environ Earth Sci (2016) 75:984 DOI 10.1007/s12665-016-5772-5

THEMATIC ISSUE

Study of suitable oasis scales based on water resource availability in an arid region of China: a case study of Hotan River Basin

1,2 1 1 3 Hongwei Guo • Hongbo Ling • Hailiang Xu • Bin Guo

Received: 31 January 2016 / Accepted: 23 May 2016 Ó Springer-Verlag Berlin Heidelberg 2016

Abstract The object of this study was to answer a scien- vegetation distribution ranges for the west and east banks tific question, i.e., how to determine the suitable scale of of the Karakax River are 31–45 and 2–9 km from the river, natural and artificial oases management based on water respectively. In the lower reaches, the average vegetation resource availability in an arid region. For this purpose, the distribution range is 3–10 km from the river. (3) During Hotan River Basin was chosen as the study area, we ana- high-, normal-, and low-flow periods, the total available lyzed the distribution characteristics of natural vegetation water for the river basin is 54.48 9 108, 42.67 9 108 and in the river basin using remote sensing images, water 34.00 9 108 m3, respectively. (4) The ecological water resources, weather, socioeconomic data, and field survey demand of the Hotan River is 8.85 9 108 m3, and the data. We also divided the natural vegetation into two ecological water demands of the middle and lower reaches ecological protection targets: the major and the basic pro- are 5.13 9 108 and 3.72 9 108 m3, respectively. (5) The tection zones. Suitable scales of oasis management were optimal area for the natural oasis in the middle reaches is calculated under different inflow variations based on the 3145–3293 km2. This study provides a theoretical basis for above analysis. We had five major findings. (1) The total planning rational development of similar river basins in area of natural vegetation of the river basin is 4968.87 km2 arid regions. with forestland, open forestland, high-covered grassland, and low-covered land accounting for 14.08, 14.98, 13.16, Keywords Ecological protection target Á Eco- and 57.78 % of the total area, respectively. (2) In the environmental water demand Á Arid regions of Central middle reaches, the average vegetation distribution ranges Asia Á Suitable scales of oases Á Hotan River Basin for the west and east banks of the Yurunkax River are 5–8 and 34–51 km from the river, respectively. The average Introduction

This article is part of a Topical Collection in Environmental Earth Oases are a landscape feature specific to arid and semiarid Sciences on ‘‘Water in Central Asia,’’ guest edited by Daniel Karthe, regions (Hu et al. 2007; Su et al. 2007) and are essential Iskandar Abdullaev, Bazartseren Boldgiv, Dietrich Borchardt, Sergey Chalov, Jerker Jarsjo¨, Lanhai Li, and Jeff Nittrouer. components of arid ecosystems (Cheng et al. 2006; Wang et al. 2011). Oases can be divided into natural oases and & Hongbo Ling artificial oases according to their specific formation [email protected] mechanism. Artificial oases develop from desert or natural oases and are influenced by long-term human activities, 1 State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of including irrigation, cultivation, artificial landscaping, and Sciences (CAS), U¨ ru¨mqi 830011, Xinjiang, China city or town development (Fan et al. 2004; Liu et al. 2010). 2 University of Chinese Academy of Sciences, Beijing 100049, Artificial oases are also a key to sustaining human survival China and development in arid regions. Natural oases can contain 3 College of Geomatics, Shandong University of Science and desert riparian forests, valley meadows, and shrubs. They Technology, Qingdao 266510, China lie between artificial oases and desert in most cases and 123 984 Page 2 of 14 Environ Earth Sci (2016) 75:984 thus form an important barrier guaranteeing the stability of oases management using the ‘‘certain oases planning the artificial oasis ecosystems (Li et al. 2008). However, mode’’ (Hu et al. 2007; Huang et al. 2008) however, they due to rapid socioeconomic development, large-scale did not consider variations in demand of oases water expansion of artificial oasis has resulted in severe water resources and the impact of social economic development resource shortages and a reduction in the size of natural on artificial oases. This has resulted in a discrepancy oases (Wang et al. 2010; Ling et al. 2011a, b). Government between the study findings and the actual situation within planning requires knowledge of the suitable scale for oasis the study region. In most cases, the results overestimated development. Therefore, it is essential to calculate the most the appropriate size of individual oasis, and the social and suitable scale for an oasis based on the carrying capacity of economic water consumption should have been deducted water resources. Thus, ensuring the ecological security of from total water resources in the calculations of the suit- an oasis through understanding of suitable scales and car- able scale of the oasis. Another set of studies have focused rying capacity is a practical question, but also involves on the suitable scale of oases using water-heat balance important scientific questions, which researchers are cur- methods (Li et al. 2011; Lei et al. 2014). They used the rently working to address in arid and semiarid regions. quota-area method to calculate the ecological water The Hotan River Basin has an extremely fragile eco- demand of the natural vegetation, but neglected to include environment with one of the highest incidents of severe the different water demand quotas of different vegetation sandstorms in northern China. Sandstorms can occur on up types. Therefore, their results did not take into account the to 200 days annually, and levels of inhalable particles are water resource allocation and protection of natural vege- 2.5 times that of the rest of the Xinjiang Autonomous tation for the entire oasis. Region. Natural vegetation on both sides of the river has This study analyzed the high- and low-flow variations of been dramatically reduced—the vegetation richness index surface runoff in the Hotan River using water resource data and coverage index decreased by 1.4 and 3.7 %, respec- (1957–2010), meteorological data, socioeconomic data tively, during 1990–2010 (Zhao et al. 2013). (2010), and ALOS satellite remote sensing images (2010) The oasis and desert form a delicate mosaic landscape (resolution ratio of 2.5 m). The required quantities of water pattern (Tureniguli and Li 2012), and disruptions can easily resources were calculated for maintaining the socioeco- cause land desertification. Improper economic develop- nomic development of artificial oases in the middle ment approaches have caused a series of environmental reaches. We first divided the natural vegetation in the problems as Hotan Prefecture represents the key region in Hotan River Basin into two ecological protection targets the construction of the Silk Road Economic Belt. For according to distribution characteristics, and the ecological example, rapid increases in population and agricultural water demands of the two ecological protection targets cultivation have depleted water resources needed by natu- were estimated. Different ecological protection targets ral oasis ecosystems. In this region, rapid and sustainable were established in different high- and low-flow variations economic development is difficult to achieve without for the river. On the basis of the above analysis, we also understanding and planning for the suitable scale of oasis discuss the suitable scale of natural and artificial oasis development. Therefore, studies of the suitable size of management in the region. The study aimed to answer a oases will benefit the sustainable utilization of water scientific question, i.e., how to determine the suit- resources, as well as ecological protection and stable so- able scales of oases management in inland river basins of cioeconomic development in the region. extremely arid regions with the objective of supporting Previous studies of the suitable scale of oases manage- sustainable oasis development. ment in the inland river basins of arid regions provided background for the study presented here, in terms of cal- culation methods for the suitable proportions of forestland, Study region grasslands, and cultivated land in oases (Eagleson 1982; Eagleson and Tellers 1982); the water consumption of The Hotan River Basin is located in southern Xinjiang natural oasis (Kovda 1987); the water consumption of Uygur Autonomous Region, China, in the southwest por- crops in artificial oasis (Bandyopadhyay 1987; Neilson tion of the Tarim Basin (77°250–81°430E, 34°520–40°280N). 1995); the definition of ecological landscape types in nat- The total area of the Hotan River Basin is 48,870 km2. The ural and artificial oases (Richter et al. 2003); models for Hotan River originates in the Kunlun Mountains and the calculating the most suitable oasis size in arid regions Karakorum Mountains and flows northward into the Tarim (Maneta et al. 2009); and analysis of oasis vegetation River. The upper and middle reaches of the Hotan River ecological water demand and suitable area for oasis farm- consist of two tributaries. The east tributary is the Yur- ing (Contreras et al. 2011). In recent years, a few ungkash River with a length of 513 km, and an average researchers have attempted to describe the suitable scale of annual runoff of 21.95 9 108 m3. The west tributary is the 123 Environ Earth Sci (2016) 75:984 Page 3 of 14 984

Karakash River with a length of 808 km, and an average The average annual temperature is 12.2 °C. The average annual runoff of 21.51 9 108 m3. The upper reaches annual precipitation is 39.6 mm, and the average annual encompass the river source to the mountain pass, and the evaporation capacity is 2882.6 mm. Sandstorms can occur middle reaches extend from the mountain pass to the on up to 200 days annually. intersection (i.e., Koxlax) of the two tributaries. The lower reach includes the river section between the Koxlax and Xiaota Hydrologic Stations with a length of 319 km. Data resources (Fig. 1). The Hotan River Basin is situated in the hinterland of The resolution of ALOS satellite remote sensing images is Eurasia. Since the Kunlun Mountains and the Pamirs Pla- higher than that of other available imagery, so the inter- teau block the warm and humid airflow of the Indian pretation results are more accurate. Therefore, we obtained Ocean, monsoons rarely reach the river basin. Thus, it has ALOS satellite remote sensing images of the Hotan River an extremely dry and warm temperate continental climate. Basin for July 19, 2010 (day with no cloud cover during

Fig. 1 Sketch map of the Hotan River Basin

123 984 Page 4 of 14 Environ Earth Sci (2016) 75:984 peak vegetation growing season) with a resolution ratio of registered using DRG. Thirty interpretation symbols, such 2.5 m. The images were digitized to obtain land cover/land as the crossing points of roads and canal systems, and use data and data regarding the natural vegetation distri- residential centers, were located evenly on the entire image bution in the river basin (Figs. 1, 2). For classification of plane. The error precision was below 0.5 pixels in image land cover/land use data, irrigation areas, cultivated land, registration. In addition, the 2010 images were further artificial landscapes, and cities or towns located in oasis corrected based on field surveys and sampling. The land were extracted as the artificial oases. Likewise, desert use types were classified based on the national criteria from riparian forests, valley meadows, and shrubs were extracted the book ‘‘Current Land Use Classification,’’ which was as the natural oases. For image processing, relief maps of published by the Chinese Government in 2007. According two amplitudes with a ratio of 1:100,000 for the research to the classification accuracy evaluation for the land cover/ regions were scanned and registered by ArcGIS 9.3 (error use of the Hotan River in 2010, the Kappa coefficient precision was below 0.5 pixels), and the digital raster (classification accuracy evaluation index) (Congelton and graphic (DRG) was created. Next, the 2010 images were Green 1999) was 95.8 %.

Fig. 2 Spatial distribution characteristics of natural vegetation in the Hotan River Basin

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Socioeconomic data were obtained from the compre- sparse natural grasslands with coverage between 10 and hensive planning report of water resource utilization in the 20 % (Fig. 2), poor moisture conditions, and poor suit- river basin and included the population, crop planting ability for livestock production. structure, GDP, and livestock inventory statistics for 2010. The water resources included measured runoff from the Z index method mountain pass of the Hotan River (Wuluwati and Tong- guziluoke Hydrometric Stations) between 1957 and 2010, The Z index can reflect the high- and low-flow variations underground water, and other surface water resources within a certain period (Ling et al. 2013). The annual between 2000 and 2010. The yearly precipitation and air runoff of the Hotan River for 1957–2010 follows a Person- temperature were acquired for Hotan Prefecture and III distribution, and the formula for calculating the Z index Wuluwati and Tongguziluoke Hydrometric Stations for high- and low-flow variation of the runoff was:  between 1971 and 2010. 1=3 6 Cs 6 Cs Z ¼   ui þ 1 À þ ð1Þ Cs 2 Cs 6 P Study methods n ðRi À RÞ3 Cs ¼ i¼1 ð2Þ n  r3 This paper divided the Hotan River into six river sections, Ri À R and the natural vegetation in the river basin was classified ui ¼ ð3Þ r into four types. Two ecological protection targets were sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi established according to the distribution characteristics and 1 Xn r ¼ ðRi À RÞ2 ð4Þ ecological functions of the natural vegetation. The Z index n À 1 i¼1 was used to analyze the high- and low-flow variations of Xn surface runoff. The phreatic evaporation model was used to 1 R ¼  Ri ð5Þ estimate the ecological water demands of the two ecolog- n i¼1 ical protection targets. Suitable scales of natural and arti- ficial oases were calculated using a model for calculation of where Z is the Z index; Cs is the skewness coefficient; ui is the most suitable oasis size in arid regions. the standard variable; Ri is the annual runoff; n is the sample number; and R is the mean value of the annual River sections and natural vegetation distribution runoff sequence. According to the normal distribution curve of the The Hotan River Basin has an extremely dry and warm Z variable, the limiting value of the Z index is divided into climate, so precipitation has almost no influence on plant three grades and into high- and low-flow types (Table 1). growth. Natural vegetation distribution is mainly affected by surface runoff in the main stream and is characterized Model for calculating ecological water demand by zonal distribution centered on the river. Therefore, analysis of natural vegetation distribution characteristics in Precipitation is extremely low in the plains area of the the Hotan River Basin was premised on a set of different Hotan River Basin, so most of the natural vegetation on ecological protection targets for calculation of ecological both sides of the river is mid-xeric and non-zonal, and the water demand. This paper divided the Hotan River into six species mainly rely on groundwater for survival. Therefore, river sections. Two sections were in the middle reaches: we chose a phreatic evaporation method to estimate natural section 1 from the Yurungkash River Canal Head to vegetation water demand (Wang et al. 2001; Fan et al. 2004; Guan et al. 2005). The calculation formula was: Koxlax and section 2 from the Karakash River Canal Head X to Koxlax. The lower reach was divided into four equal 3 W ¼ 10 AiWgikp ð6Þ river sections (sections 3–6) by length. Based on the nat- ural vegetation data for the river basin, the different natural where W is the ecological water demand of vegetation 3 2 vegetation types were divided into the following cate- (m ); Ai is the area of vegetation type I (km ); Wgi is the gories: open forestland: various trees and shrubs with phreatic evaporation capacity of the vegetation type i at a canopy density between 10 and 20 %; forestland: The certain groundwater depth (mm); and kp is the vegetation primary species were dense stands of Populus pruinosas, influence coefficient, defined in Table 2 (Hu et al. 2007). Populus euphraticas, and Tamarix chinensis, with a canopy Wgi is the key to calculating vegetation ecological water density of more than 20 %; high-covered grassland: dense demand using the phreatic evaporation method, and it is natural grassland with a coverage of more than 20 % and usually calculated using Averyanov’s phreatic evaporation better moisture conditions; and low-covered grassland: equations (Ye et al. 2007): 123 984 Page 6 of 14 Environ Earth Sci (2016) 75:984

Table 1 Grade of high- and Grade Cumulative frequency High- and low-flow index (Z) High- and low-flow type low-flow index 1 [70 % Z [ 0.54 High flow 2 30–70 % -0.54 \ Z B 0.54 Normal flow 3 \30 % Z B-0.54 Low flow

Table 2 k of different p Groundwater depth(m) \1.25 1.25–1.75 1.75–2.25 2.25–2.75 2.75–3.25 3.25–4 [4 groundwater depth

kp 1.98 1.63 1.56 1.45 1.38 1.29 1.00

b conversion coefficient of surface evaporation, which has a Wgi ¼ að1 À hi=hmaxÞEu ð7Þ 20 value of 0.61 (Xu et al. 2010). where a and b are empirical coefficients (0.52 and 2.51 in the Hotan River Basin) (Guan et al. 2005), and hi is the groundwater depth of vegetation type i (m), determined by Results and analysis field monitoring data and related research results (Hu et al. 2007). In the Hotan River Basin, the minimum ecological Distribution characteristics of natural vegetation water demands of the major ecological protection zones are in the Hotan River Basin the evaporation capacities when the groundwater depth is

2.0 and 3.5 m, respectively. hmax is the maximum depth of We analyzed the spatial distribution characteristics of phreatic evaporation (m), which is 4.5 m (Hu et al. 2007); natural vegetation (Fig. 3; Table 3). Figure 3 shows that 2 and E/20 is the surface water evaporation capacity of a natural vegetation covers an area of 4968.87 km in the 20 cm general evaporation dish (mm) (2882.6 mm, Hotan River Basin. Forestland, open forestland, high-cov- according to long-term data from the Wuluwati hydrologic ered grassland, and low-covered grassland account for station). 14.08, 14.98, 13.16, and 57.78 % of the total area, respectively. The area distribution of the four natural Model for calculating the most suitable oasis size vegetation types had clear spatial differences in the dif- ferent river sections. In sections 1–6, forestland accounted Based on the water balance in the Hotan River Basin, the for 2.13, 5.68, 29.30, 21.03, 75.76, and 49.69 % of the total following model was used for calculating the most suit- area, respectively, and high-covered grasslands in sec- able oasis size in arid regions (Lei et al. 2014): tions 1–6 accounted for 19.42, 17.78, 3.19, 0.64, 0.45, and W À W 5.99 % of the total area, respectively. 0 1 8 À5 ¼ ð Þ Areas of natural vegetation in the six river sections ðaAN þ bAA þ AW E/ c þ AOEpÞ10 20 accounted for 34.75, 31.52, 13.24, 12.07, 3.64, and 4.78 % 2 where AN is the area of a natural oasis (km ); AA is the of the total area in the river basin (Table 3). Due to higher irrigated area of an artificial oasis, including farmland, availability of water resources, the middle reaches of the forestland, and grassland under artificial irrigation (km2); river (sections 1 and 2) had the most natural vegetation, 2 AW is the area of artificial water (km ); and AO is the area 66.27 % of the total area. The lower reaches (sections 3–6) of other land use types in the artificial oasis region, accounted for 33.73 %. Due to topography, there is a large including bare land and construction land (km2). W is the difference in the area of natural vegetation on the east and 8 3 total quantity of available water resources (10 m ); WO is west banks in section 1 and 2. In section 1, the natural non-vegetation water consumption, including industrial vegetation area of the east bank was 1377.14 km2 more and domestic water uses, as well as surface evaporation and than the natural vegetation area of west bank. In section 2, the minimum ecological water demand in the river channel the natural vegetation area of the east bank was (108 m3); a and b are the water demand quotas of the 1299.41 km2 less than that of the west bank. In section 3, natural and artificial oases, which are 400 and 650 mm in the natural vegetation area of the east bank was 45.62 km2 the Hotan River Basin, respectively (Lei et al. 2006). E/20 more than that of the west bank. In sections 4–6, areas of is the surface water evaporation capacity of a 20 cm gen- natural vegetation were 89.87, 38.96, and 52.79 km2 less eral evaporation dish (mm); Ep is the phreatic evaporation than that of the west bank, respectively. In sections 3–6, (mm) from bare land and construction land in an artificial the natural vegetation area on the west bank was 136.0 km2 oasis, calculated by formula (7). In the formula, the value more than the east bank, and the natural vegetation in the of H is 4.0 m in this river basin (Xu et al. 2010). c is the west bank was in better condition than that of the east bank.

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Fig. 3 Area of different natural vegetation types in 6 river sections

Table 3 Area of natural Area (km2) Section 1 Section 2 Section 3 Section 4 Section 5 Section 6 Sum vegetation in different river sections of the Hotan River East bank 1551.94 133.48 351.72 254.87 70.97 92.33 2455.30 West bank 174.80 1432.88 306.10 344.73 109.93 145.12 2513.57 River section 1726.74 1566.36 657.82 599.60 180.91 237.45 4968.87

In the Hotan River Basin, natural vegetation distribution vegetation on the west bank of section 3 was distributed at is determined by the temporal and spatial distribution of 5–6 and 8–9 km from the river, respectively. On the east water resources. Accordingly, analyzing natural vegetation bank, the distribution ranges were 6–9 and 15–19 km, distribution at different distances from the river is crucial in respectively. More than 90 % of the high- and low-covered setting corresponding ecological protection targets. Based vegetation on the west bank of section 4 was distributed on the natural vegetation coverage, we combined forestland 3–6 and 9–10 km from the river, respectively. On the east and high-covered grassland into the high-covered vegeta- bank, the distribution ranges were 5–7 and 8–10 km, tion classification and combined open forestland and low- respectively. More than 90 % of the high- and low-covered covered grassland into the low-covered vegetation classi- vegetation on the west bank of section 5 was distributed fication. Using the spatial analyst tools of ArcGIS 9.3, we 3–4 and 4–5 km from the river, respectively. On the east extracted the area of high- and low-covered vegetation at bank, the distribution ranges were 3–4 and 5–6 km, 1 km intervals from the river and calculated the distribu- respectively. More than 90 % of the high- and low-covered tion rates of high- and low-covered vegetation in every vegetation on the west bank of section 6 was distributed interval by calculating the proportional area of high- and 2–3 and 3–4 km from the river, respectively. On the east low-covered vegetation in each interval compared to the bank, the distribution ranges were 1–2 and 3–5 km, total area in the interval. Then, we developed the cumu- respectively. lative frequency distribution curve of the high- and low- The natural vegetation distribution generally follows a covered vegetation using the distribution frequency per regular pattern: High-covered vegetation is located close to kilometer (Fig. 4). the river, while the low-covered vegetation is far from the By solving the cumulative frequency distribution curve river. In the lower reaches, the average distribution ranges fitting equations (Table 4), we obtained the distribution of high- and low-covered vegetation were 3–6 and range of the high- and low-covered vegetation on both 6–10 km, respectively. sides of each river section. More than 90 % of the high- and low-covered vegeta- Different ecological protection targets in the Hotan tion on the west bank of section was distributed 5–7 km River Basin and 7–8 km from the river, respectively. On the east bank, the distribution ranges were 48–51 and 34–40 km, Ecological protection targets are determined by ecosystem respectively. More than 90 % of the high- and low-covered and social needs, and the setting of targets is an important vegetation on the west bank of section was distributed part of ecosystem management. Over the past 50 years, the 31–41 and 42–49 km from the river, respectively. On the inflow recorded by hydrologic stations in the lower reaches east bank, the distribution ranges were 2–3 and 4–5 km, has shown a significant declining trend due to the rapid respectively. More than 90 % of the high- and low-covered increase in water consumption of industrial and domestic

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Fig. 4 Cumulative frequency distribution of natural vegetation in the Hotan River Basin

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ÂÃ Table 4 Fitting equation and 2 River section River bank Natural vegetation À0:5 xÀx0 parameters Equation: fxðÞ¼ae ðÞb abx0 R2 P

Section 1 West bank High covered 100.24 5.28 6.95 0.92 \0.01 Low covered 522.45 17.93 40.47 0.97 \0.01 East bank High covered 2.41 9 1010 186.56 1210.49 0.97 \0.01 Low covered 102.64 19.87 44.78 0.99 \0.01 Section 2 West bank High covered 99.77 22.89 41.52 0.96 \0.01 Low covered 101.40 22.61 52.79 0.99 \0.01 East bank High covered 103.92 4.00 4.54 0.92 0.02 Low covered 100.57 3.59 5.66 0.99 \0.01 Section 3 West bank High covered 104.16 10.96 9.24 0.52 0.02 Low covered 104.56 4.01 10.24 0.98 \0.01 East bank High covered 105.68 13.70 13.79 0.59 \0.01 Low covered 100.02 7.96 18.58 0.99 \0.01 Section 4 West bank High covered 104.16 10.96 9.24 0.52 0.02 Low covered 103.84 5.13 11.59 0.98 \0.01 East bank High covered 105.27 8.97 9.61 0.78 \0.01 Low covered 102.73 5.89 10.99 0.92 \0.01 Section 5 West bank High covered 105.70 3.05 4.62 0.88 0.04 Low covered 106.61 1.85 5.16 0.97 \0.01 East bank High covered 104.72 3.34 4.65 0.93 0.02 Low covered 104.96 2.84 6.76 0.99 \0.01 Section 6 West bank High covered 104.88 2.47 4.05 0.96 0.04 Low covered 105.64 1.90 4.26 0.99 0.01 East bank High covered 103.55 2.80 4.02 0.93 0.07 Low covered 103.81 3.72 3.72 0.83 0.17 uses in the middle reaches of the river. Moreover, surface and the upper boundary of flood plain during normal runoff usually shows high- and low-flow variation in inflow periods. The main vegetation includes sparse trees, relation to hydrologic cycles. Therefore, we divided the shrubs, and natural grasslands with coverage between 10 natural vegetation in the lower reaches into different eco- and 20 %. The protection targets for this zone include logical protection targets according to the different high- halting the degradation and desertification of natural and low-flow variations. vegetation, and ensuring that the groundwater depth is On the basis of the cumulative frequency distribution suitable for Tamarix chinensis (i.e., generally B6 m). In curves and ecological functions of plant communities, we addition, we should improve the growth conditions for set two ecological protection targets for different levels natural vegetation by strictly ensuring the ecological centered on the river. The major protection zone is the water requirements of the major protection zone in low- high-covered vegetation region on both sides of the river, flow periods. In the high- and normal-flow periods, the which can be influenced by the main channel of the river. ecological water requirements of both the major and basic This zone is located within the range of 6 km from the protection zones should be ensured. river channel. The main plants are Populus pruinosa, Populus euphratica, and Tamarix chinensis with canopy High- and low-flow variations for surface runoff densities of more than 20 %. The ecological protection in the Hotan River Basin targets for the high-covered vegetation include maintain- ing the ecological water demand for this zone, and also The high- and low-flow variations for runoff were analyzed maintaining the existing vegetation area and coverage. using the Z index method (Fig. 5), based on the annual The basic protection zone is the low-covered vegetation runoff of the Hotan River between 1957 and 2010. Figure 5 region on both sides of the river. This zone is located shows that the runoff in the Hotan River mountain pass within the range of 6–10 km from the river channel, headstreams changed from a normal-flow period to a low- between the outer boundary of the major protection zone flow period between 1957 and 1959. Three high-flow years

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Fig. 5 High- and low-flow variations of the Hotan River Basin

(1961, 1966, and 1967) and three normal-flow years (1960, Table 5 shows that the ecological water demands of the 1968, and 1969) occurred between 1960 and 1969, and the major and basic protection zones in the Hotan River are rest were low-flow periods. The flow alternated three times 7.25 9 108 and 1.60 9 108 m3, respectively, and the total between high- and low-flow during the 1970s. In the 1980s, ecological water demand is 8.85 9 108 m3. The ecological flow was high in 1981 and 1988; low in 1980, 1987, and water demands of the major and basic protection zones in 1989; and normal between 1982 and 1986. The flows were the middle reaches (section 1 and 2) are 3.96 9 108 and mostly normal during the 1990s, except for one high-flow 1.17 9 108 m3, respectively, and the total ecological water year (1994) and three low-flow years (1992, 1993, and demand is 5.13 9 108 m3. The ecological water demands 1995). During 2000–2009, high-flow years (2001, 2003, of the major and basic protection zones in the lower 2005, and 2006) and normal-flow years (2000, 2002, 2007, reaches (section 3–6) are 3.27 9 108 and 0.45 9 108 m3, and 2008) occurred four times, respectively, while flow respectively, and the total ecological water demand is was low in 2004 and 2009 and runoff rose in a high-flow 3.72 9 108 m3. period in 2010. In addition, the area covered by natural lakes and the With the clear high- and low-flow variations in the river channel is 468.10 km2 and the water surface evapo- Hotan River, it is possible to determine the suitable oasis ration capacity is 2,882.6 mm. Therefore, total water con- sizes for the river based on variations in surface runoff. The sumption is 7.69 9 108 m3 if the water surface conversion mean values of annual runoff in the three flow categories coefficient (0.57) is used. Therefore, the available water (high, normal, and low) between 1957 and 2010 were resources for oases in the middle reaches in the high-flow, 55.17 9 108, 43.36 9 108 and 34.69 9 108 m3, respec- normal-flow, and low-flow periods are 39.77 9 108, tively. In addition, annual groundwater exploitation was 27.96 9 108 and 19.74 9 108 m3, respectively. 7.98 9 108 m3, and available spring water was 1.66 9 108 m3. The average runoff of the river was Water demand of artificial oases in the middle 44.29 9 108 m3; thus, its minimum ecological base flow reaches of Hotan River Basin was 4.43 9 108 m3, according to the Montana method (Ye et al. 2014). Based on the planning requirements of local Land cover/land use data for 2010 show that the irrigated authorities, the minimum annual released water volume area supplied by artificial oases in the middle reaches of the into the Tarim River is 9.2 9 108 m3. Accordingly, the Hotan River Basin was 2029.8 km2, and the total water total available water in the river basin over the three consumption was 13.193 9 108 m3. Furthermore, the periods was 51.18 9 108, 39.37 9 108 and water consumption of the irrigated areas was 30.70 9 108 m3, respectively. 1.130 9 108 m3, and the water consumption of bare land and construction land was 0.039 9 108 m3. Therefore, the Ecological water demand of a natural oasis total water consumption for irrigation and the other land in the lower reaches of the Hotan River use types was 14.363 9 108 m3. and available water resources for oases The populations of the cities and villages located in the in the middle reaches river basin in 2010 were 271,400 and 1,081,600, respec- tively, and the water consumption quotas were 0.096 and The ecological water demands of the major and basic 0.08 m3 per person per day (Xu 1993), respectively. protection zones in each river section of the Hotan River Therefore, the population water consumption of the river Basin were calculated using the phreatic evaporation model basin was 0.457 9 108 m3 if the water resource utilization (Table 5). coefficient was 0.9. In addition, the regional industrial

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Table 5 Ecological water demands of the major and basic protection zones in different sections of the Hotan River (108 m3) Protection Section 1 Section 2 Section 3 Section 4 Section 5 Section 6 zone West East West East West East West East West East West East bank bank bank bank bank bank bank bank bank bank bank bank

Major 0.67 1.33 1.55 0.42 0.49 0.65 0.35 0.34 0.43 0.31 0.38 0.32 Basic 0.02 0.58 0.51 0.02 0.09 0.10 0.12 0.08 0.01 0.01 0.03 0.01 Sum 0.69 1.91 2.06 0.45 0.59 0.75 0.48 0.43 0.44 0.31 0.42 0.34 output in 2010 was 59,435 9 104 yuan. The water con- This is significant for the sustainable development of the sumption quota for the industrial output of one ten thou- river basin. sand yuan was 149 m3 (Xu 1993); thus, the total industrial Table 6 shows that the optimal size of the oases in the water consumption was 0.098 9 108 m3 based on a water middle reaches should not be below 1089 km2 or over resource utilization coefficient of 0.9. Moreover, the study 6096 km2. The area of natural oasis in the middle reaches region had 2,974,441 adult livestock and 1,411,159 young was 3293 km2 in 2010. If normal-flow (22 years) and low- livestock at the end of 2010, and the daily consumption flow (17 years) periods comprise 72.2 % of the annual quotas of the two livestock categories were 0.04 and flows over the last 54 years, then the optimal area of the 0.006 m3 (Xu 1993). So the total water consumption was natural oasis in the middle reaches is 3145–3293 km2, and 0.465 9 108 m3. In summary, the socioeconomic water the optimal area covered by all the oases should be consumption of the artificial oasis in the river basin was 6450–6598 km2. 1.020 9 108 m3, and the total water consumption of the artificial oasis was 15.383 9 108 m3. Therefore, the available water resource quantity for the natural oasis in the Discussions middle reaches was 24.384 9 108, 12.580 9 108 and 4.357 9 108 m3, respectively, for the high-, normal-, and Distribution characteristics of natural vegetation low-flow periods. in the Hotan River Basin

Optimal size for an oasis in the middle reaches Remote sensing images and field survey data indicated that of Hotan River Basin under different inflow vast stretches of Populus pruinosas, Populus euphraticas, variations Tamarix chinensis, and other species grow on both sides of the river. Due to higher availability of water resources, the If the scale of the oasis is set too small, it would not make middle reaches of the river (sections 1 and 2) had the most full use of the allocated water and soil resources, limiting natural vegetation, accounting for 66.27 % of the total uses for human production and livelihoods. However, if the area. Natural vegetation in the lower reaches follows a scale of the oasis is set too large, it would lead to a con- clear zonal distribution centered on the river, and the typ- tinuous deterioration of the oasis environment, eventually ical vegetation profiles along the river are: meadow zone– threatening the existence and development of the oasis. forest zone–fixed dune zone–quicksand zone. The meadow Consequently, development of the oases should be con- zone is mainly distributed in the higher parts of the flood trolled within a certain range (Lei et al. 2014). According plain, sometimes reaching the edge of the river banks. The to statistical data for Hotan Prefecture, the local population main plants are Calamagrostis pseudophragmites and was 1,400,550 in 1990 but had increased by 1.62 times by Phragmites australis. The forestland zone is primarily 2010. The area under cultivation had increased by distributed on the first terraces, and the main plants are 298.9 km2 during the same period (Yang et al. 2006). Due Populus pruinosas, Populus euphraticas, Tamarix chinen- to the increase in population and the area under cultivation, sis, and Phragmites australis. The fixed dune zone is dis- the demands for irrigation water in the Hotan River Basin tributed outside the forest zone, and the primary plant had significantly increased. As a result, the river has been species is Tamarix chinensis. The quicksand zone is loca- able to supply water to the natural oasis in the lower ted farthest from the river, and the main plants are sparse reaches only during flood periods since the 1980s. The stands of Tamarix chinensis, as well as occasional Populus suitable scale for natural oases under certain water pruinosas, Populus euphraticas, and Poacynum hender- resources was calculated, based on the stability of the sonii (Liu 1985). natural oasis in the lower reaches and the guarantee of The water supply of natural vegetation in this region normal artificial oasis water demand in the middle reaches. mainly depends on flooding, and the flood period (from

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Table 6 Areas of natural oasis and total oases in the middle reaches of the Hotan River Basin Grade Water resource Water demand Natural oases (km2) Total oases (km2) quantity (108 m3) quota (mm) Suitable scale Actual scale Suitable scale Actual scale

High-flow period 24.384 400 6096 3293 9401 6598 Normal-flow period 12.580 3145 6450 Low-flow period 4.357 1089 4395

July to September) is the maturation period for Populus addition, vegetation provides habitat for local wildlife and pruinosas seeds. Floods desalt the groundwater on both provides the necessary conditions for humans (Lu 1994). sides of the river, which results in an increase in Populus Therefore, the major protection zone must be given suffi- pruinosas seedlings, but the flood period is not long, and cient attention in planning the scale of oasis development. the desalinization is limited in the range, which is not far In addition, to guarantee water availability for the eco- from the river, so the vegetation distribution on both sides logical water demand and reasonable control of the of the river is narrow (Lu 1994). underground water levels (C4 m), managers should restore Though the distribution of natural vegetation on both natural flooding cycles. According to previous research, the sides of the lower reaches is narrow and sparse, the plant most suitable frequency of flooding disturbance is 1–2 species are not very rich. Natural vegetation can greatly times per year, which will not only maintain a high bio- slow the movement speed of barchan chains outside the logical diversity, but also be conducive to the growth of fixed dune zone. The dune movement speed in the study shrubs and the maintenance of their dominance in the region is 2 m per year, while the speed of non-vegetation community (Xu et al. 2007; Fu et al. 2013). The natural dunes of the same height is 5 m per year. The natural vegetation of the basic protection zone is sparse in this vegetation effectively prevents the expansion of the desert area, but it can effectively slow down the movement speed and protects the eco-environment of the oasis. However, of the barchan chains. The main plants in the basic pro- the expansion of artificial oasis in the middle reaches of the tection zone have high drought resistance; so, flooding river has consumed excessive water resources in recent once or twice every 5 years can ensure that the vegetation years and has resulted in continuous decreases in the is not degraded (Xu et al. 2007). Moreover, in the high- and amount of water released and the groundwater depth in the normal-flow period, an appropriate increase in the fre- lower reaches. Precipitation in the study area is very low, quency of flooding disturbance can promote the restoration and the depth of groundwater is an important factor in the of natural vegetation in the basic protection zone. development and succession of plant communities. The declining groundwater levels directly affect the natural Study of the optimal size of oases in arid regions regeneration of Populus pruinosas and Populus euphrati- cas and gradually promote their evolution to sparse desert According to related research, 90 % of oases have no riparian forests and quicksand zones. Moreover, unrea- established targets for appropriate proportions of artificial sonable human activities in the lower reaches also accel- and natural oases in arid regions of China. Currently, erate the above-mentioned process, e.g., over grazing, artificial oases account for 59 % of the total oasis area, and massive deforestation, and over excavation of medicinal this cannot be maintained. For this reason, suitable oases materials (Lu 1994). Therefore, planning of the appropriate sizes have been investigated (Li et al. 2008). There are two development scale of oases in the river basin should be factors that determine the appropriate scale of development based on the principle of ‘‘ecological priority.’’ of oasis in arid regions (Huang et al. 2008): The first is the ecological water demands of natural vegetation in oases, Different ecological protection targets in the Hotan and the second is the temporal and spatial distribution of River Basin water resources. Studies of the suitable scales of oases in arid regions should be based on these two premises. The natural vegetation of the major protection zone is the Therefore, based on remote sensing images, water resour- main component of the green corridor of the Hotan River. ces, weather, and field survey data, we first divided the As most of the vegetation types are ancient species that natural vegetation into two ecological protection targets have evolved and adapted to the extreme natural environ- according to its distribution characteristics, and the eco- ment over the long term. Their existence helps reduce wind logical water demands of the two ecological protection speeds, decrease temperatures, increase air humidity, and targets were more accurately estimated. Additionally, in assists in the formation of special microclimates. In order to avoid determination of the oasis scale from only

123 Environ Earth Sci (2016) 75:984 Page 13 of 14 984 one year of study results, the high- and low-flow variations high-covered grassland, and low-covered land account of surface runoff from 1957 to 2010 were also analyzed, for 14.08, 14.98, 13.16, and 57.78 % of the total area, and the annual mean runoff of high-, normal- and low-flow respectively. The middle reaches of the river contain years was calculated separately. Calculation of available most of the natural vegetation, accounting for 66.27 % water resources was obtained from the annual mean runoff. of the total area. On the basis of the above analyses, the suitable scale of 2. In the middle reaches, the average distribution range natural and artificial oases was estimated. for the west and east banks of the Yurunkax River is Socioeconomic data included the population, crop 5–8 and 34–51 km from the river, respectively. The planting structure, GDP, and livestock inventory statistics average distribution range of the west and east banks came from the ‘‘Planning report of comprehensive uti- of the Karakax River is 31–45 and 2–9 km from the lization of water resources in the Hotan River Basin,’’ river, respectively. In the lower reaches, the average completed in 2015 by the Hotan regional government. The distribution range is 3–10 km from the river. data are the most current and most authoritative available 3. During the high-, normal-, and low-flow periods, the total at present. The population, cultivated land area, and live- available water for the river basin was 54.48 9 108, stock inventory are strictly restricted in the future in the 42.67 9 108 and 34.00 9 108 m3, respectively. book. Though the data were collected in 2010, it likely 4. The ecological water demands of the Hotan River represents the current social and economic water con- Basin are 8.85 9 108 m3. The ecological water sumption conditions in the Hotan River Basin. Plant demands of the middle and lower reaches are evaporation and surface evapotranspiration represent 5.13 9 108 and 3.72 9 108 m3, respectively. averages for records going back many years and were 5. The optimal area of the natural oasis in the middle obtained from related research results (Chen 2010; Ye et al. reaches is 3145–3293 km2. 2014). Accordingly, the data used in this paper are rela- tively accurate and the research results are reasonable. Acknowledgments This work was supported by the West Light Most of the arid inland rivers originate in the mountains Foundation of The Chinese Academy of Sciences (XBBS-2014-13), and flow into the deserts. Eventually, they disappear or the Main Service Project of Characteristic Institute of Chinese collect in a depression and form terminal lakes. The water Academy of Sciences (TSS-2015-014-FW-2-2), and the National Natural Science Foundation of China (41471099, 41171427, flows gently in the middle reaches of the river and is 31370551, and 31400466). conducive to diversion for irrigation. Artificial oases are usually located in the middle reaches of the river, while natural oases are distributed on the periphery of artificial References oasis, on both sides of the river in the lower reaches, and around terminal lakes. For example, in the Keriya River Bandyopadhyay J (1987) Political ecology of drought and water Basin and the Qarqan River Basin, the development of scarcity. Econ Political Wkly 22(50):2159–2167 artificial oasis is mainly dependent on irrigation agricul- Chen YN (2010) Study on the eco-hydrology of Tarim River Basin in ture, and the remaining water flows to the desert. The Xinjiang. 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