Water allocation and water consumption of irrigated agriculture and natural vegetation in the Aksu-Tarim river basin, Xinjiang, China

Journal of Arid Environments 112 (2015) 87e97

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Water allocation and water consumption of irrigated agriculture and natural vegetation in the Aksu-Tarim river basin, Xinjiang, China Niels Thevs a, *, Haiyan Peng a, Ahmedjan Rozi a, Stefan Zerbe b, Nurbay Abdusalih c a

Institute of Botany and Landscape Ecology, University of Greifswald, Grimmer Strasse 88, 17487 Greifswald, Germany
5, 39100 Bolzano, Italy Faculty of Science and Technology, Free University of Bozen-Bolzano, Piazza Universita c Institute of Resource and Environmental Sciences, Xinjiang University, Shengli Lu 14, 830046 Urumqi, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 January 2013 Received in revised form 23 May 2014 Accepted 26 May 2014 Available online 9 July 2014

A significant part of the world's largest river basins are located in areas of arid and semi-arid climate, such as the Amu Darya, Jordan, Murray-Darling, Yellow River, and Aksu-Tarim river basin. These river basins are experiencing water scarcity resulting in conflicts between upstream and downstream, conflicts between water users, and degradation of the natural ecosystems. Therefore, in many river basins, including the Aksu-Tarim river basin, water quota systems have been established, in order to allocate water under scarcity. The Aksu-Tarim river basin (NW China) has developed into one of the most important cotton production areas worldwide. In this paper, we aim at assessing the water consumption through irrigated agriculture, mainly cotton, and natural vegetation in the Aksu-Tarim river basin against the background of this water quota system. Firstly, we map the evapotranspiration (ETa) as water consumption of irrigated agriculture and natural vegetation in the Aksu-Tarim river basin. Secondly, we calculate water balances and relate them to the water quota system. We employed the remote sensing method Simplified Surface Energy Balance Index (S-SEBI), in order to map ETa based on MODIS satellite images for the growing seasons 2009, 2010, and 2011. Thereby, the MODIS products 8-day land surface temperature (MOD11A2), 16-day albedo (MCD43A3), and 16-day NDVI (MOD13A1) were used. The ETa of cotton ranges from 884 to 1198 mm. The ETa of the natural vegetation of a total coverage ranges from 715 in 2009 to 960 mm in 2011, clearly following the annual runoff of the Aksu and Tarim River. The water balance of the Aksu-Tarim river basin is
3.25 to
3.73 km3, 0.1e0.53 km3, and
3.55 to
4.12 km3 in 2009, 2010, and 2011, respectively. The water quotas along the Aksu River and the upper reaches of the Tarim are exceeded by water consumption, while the quotas along the middle and lower reaches are not met. Considerable amounts of groundwater, including fossil groundwater, are exploited for irrigation along the Aksu and Tarim River, which must be regarded as exploitation of a non-renewable resource. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Central Asia Cotton Evapotranspiration Remote sensing Riparian vegetation Water resource management

1. Introduction A significant part of the world's largest river basins are located in areas of arid and semi-arid climate, such as the Amu Darya, Jordan, Murray-Darling, Yellow River, and Tarim river basin (Central Asia Atlas, 2012; GLOWA Jordan, 2008; Glantz, 2010; ICWC, 1992; Murray Darling Basin Commission, 2006; Song et al., 2000; Tang and Deng, 2010; Zhu et al., 2003). These river basins are experiencing water scarcity resulting in conflicts between

* Corresponding author. Tel.: þ49 3834 864131. E-mail address: [email protected] (N. Thevs). http://dx.doi.org/10.1016/j.jaridenv.2014.05.028 0140-1963/© 2014 Elsevier Ltd. All rights reserved.

upstream and downstream regions, conflicts between water users, and degradation of the natural ecosystems. In these areas, agriculture, mostly irrigated agriculture, is the largest consumer of water (Howell, 2001). Given the background of increasing populations, increasing demand for agricultural products, and thus increasing water demands, water distribution in river basins between upstream and downstream as well as between water users plays and will play an important role for societies within such river basins (FAO, 2012). In many river basins, water quota systems have been established in order to allocate distinct amounts of water to different users and different sections of a river as reviewed by Molle (2009). In Central Asia, quota systems have been introduced between countries for the Amu Darya and Syr

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Darya, which are the two tributaries of the Aral Sea (ICWC, 1992), on provincial level for the Heihe River, China (Chen et al., 2006), and for the Tarim River according to tributaries and river sections (Tang and Deng, 2010). The Tarim Basin, which includes the Tarim and its tributaries, located in Xinjiang, Northwest China (Fig. 1), harbors 54% (352,200 ha) of the world's riparian Populus euphratica Oliv. forests (http://whc.unesco.org/en/tentativelists/5532/). Those forests form a mosaic of riparian forests, wetlands, shrub vegetation, and small stands of herbaceous vegetation (Thevs et al., 2008; Zhang et al., 2005) and provide habitat for wildlife (http://whc.unesco.org/en/ tentativelists/5532/). The P. euphratica forests are the only forests in the Tarim Basin. Those forests and the wetlands are the most productive ecosystems of the drylands in the Tarim Basin (Thevs et al., 2007, 2012). Furthermore, the Tarim Basin has become the world's most important cotton production region with a total annual cotton lint production of 2.1 million t, i.e. 8.85% of the world production in 2010 (Xinjiang Statistics Bureau, 2011; http://faostat. fao.org/). In 2011, the share of the cotton lint production in Xinjiang of the worldwide production climbed to 11% (USDA, 2013; http:// faostat.fao.org/). Half of the cotton in the Tarim Basin is produced along the Aksu and Tarim River (Feike et al., 2014; Xinjiang Statistics Bureau, 2011). Therefore, it is relevant to show how the water quota system under intensive and increasing agriculture works.

The Tarim Basin covers an area of 1.02 million km2, and is home to a population of 9.02 million people (Tan and Zhou, 2007). The area of irrigated land has increased all over the Tarim Basin, from 706,000 ha in 1949, over 1,330,000 ha in 1980 and 1,412,000 ha in 1990 (Xia, 1998), in 2010 to 1,600,000 ha (Xinjiang Statistics Bureau, 2011). Due to the arid climate with an annual precipitation of 30e70 mm (Liu, 1997; Tang and Deng, 2010), all agriculture depends on irrigation with river water being the most important source for water. However, during the past five decades there has been a slight increase of runoff from the headwaters into the Aksu River, but due to land reclamation along the Aksu, the inflow from the Aksu into the Tarim at Aral shows a decreasing trend (Tang and Deng, 2010; Xu et al., 2005, 2010). Population growth and agricultural development, partly driven by resettlement of people from other Chinese provinces to Xinjiang, resulted in degradation of the natural ecosystems and desiccation of the two terminal lakes of the Tarim river basin, Lop Nor and Taitema, by beginning of the 1970s (Feng et al., 2005; Hao et al., 2009; Song et al., 2000; Zhang, 2006; Zhang et al., 2003). In order to balance the water use between economic development (mainly irrigated agriculture) and environmental flow along the Tarim, the Xinjiang Government developed a water distribution program and water quota system, which will be introduced in section 3 (Peng et al., 2014; Song et al., 2000; Tang and Deng, 2010; Zhang, 2006). The aim of this paper is to assess the water consumption through

Fig. 1. Map of the Tarim Basin with its administrative units.

N. Thevs et al. / Journal of Arid Environments 112 (2015) 87e97

irrigated agriculture and natural vegetation in the Aksu-Tarim river basin against the background of this water quota system. Firstly, we map the evapotranspiration (ETa) as water consumption and calculate the net water loss of irrigated agriculture and natural vegetation in the Aksu-Tarim river basin. Secondly, we relate these results to the water distribution program and water quota system. We employed the remote sensing method Simplified Surface Energy Balance Index (S-SEBI) after Roerink et al. (2000) and Gowda et al. (2008), in order to map ETa based on MODIS satellite data for the vegetation seasons 2009, 2010, and 2011. 2. Study area The study area is the Aksu-Tarim river basin (Figs. 1 and 3). Until the 1970s, the three tributaries Aksu, Hotan, and Yarkant flowed together at Aral and formed the Tarim River. Today, only the Aksu River discharges perennially into the Tarim River as shown in Fig. 1 (Song et al., 2000). Therefore, in this paper we consider the AksuTarim river basin as one river basin with the two sub-basins of the Aksu and the Tarim. The headwaters of the Aksu are located north of Aksu City in the Central Tianshan (Fig. 1) where snow and glacier melt, as well as summer rainfall, deliver water into the Aksu River's headwaters (Chou, 1960; Giese et al., 2005; Song et al., 2000). The long-term average of water discharge from the headwaters into the Aksu River just upstream of Aksu City is 8.06 km3/a (Fig. 3). Within each year, about 75% of the annual runoff is discharged during July, August, and September, which results in annual summer floods (Song et al., 2000; Tang and Deng, 2010). In 2009, this was the driest year within the three consecutive years 2007, 2008, and 2009, during which the Tarim ceased to flow downstream of Yingbaza in June (pers. observation of the authors). In 2009, the discharge of the Tarim reached an extreme low that the Tarim ceased to flow downstream of Xayar during spring and early summer. During summer, which is the flood season, the discharge in the middle and downstream section was lower than the average discharge during spring, which is the low discharge time. In 2010, the Tarim experienced one of the highest summer floods since the 1950s. Therefore, river branches and large areas under natural vegetation were flooded. River branches and depressions remained submerged until May/June 2011 (Fig. 2). The climate is extremely arid and continental, as shown in Table 1. From the foothills of the Tianshan Mountains over the Tarim River toward the Taklamakan Desert, the annual precipitation decreases from about 130 mm to less than 30 mm. Within the study area, the precipitation ranges from about 70 mm north of the Tarim River to 30 mm south of the Tarim (Liu, 1997). This is reflected by the climate data given in Table 1. The stations Kuqa and Korla are located close to the foothills of the Tianshan, (i.e. north of the Tarim River), while Aral and Tikanlik are located at the Tarim River.

89

Table 1 Mean Air Temperature and precipitation at the climate stations Aral, Kuqa, Korla, and Tikanlik (http://www.weatherbase.com). Aral, Kuqa, and Korla are indicated in Fig. 1. Tikanlik is located 120 km southeast of Yuli (Fig. 2). Climate station Time of record [a] Position

Aral

32 40.30 N, 081.03 E Elevation [m a.s.l.] 1013 Annual average air temperature [ C] 10 Average January air temperature [ C]
7 Average July air temperature [ C] 25 Annual average precipitation [mm] 42

Kuqa

Korla

Tikanlik

39 41.43 N, 082.57 E 1100 11
7 25 62

32 41.45 N, 086.07 E 933 11
7 26 52

33 40.37 N, 087.42 E 847 10
8 26 34

The natural vegetation is a mosaic composed of riparian forests, wetlands, and shrub vegetation with P. euphratica Oliv, Phragmites australis Trin. (ex Steud)., and Tamarix ramossisima Ledeb., or halophytes as key-stone species (Thevs et al., 2008). P. euphratica and P. australis are obligate phreatophytes. These plants continuously exploit the groundwater (Gries et al., 2003). Tamarix ramosissima is a facultative phreatophyte, i.e. it strives to continuously connect with the groundwater, but also can survive on moist soil without tapping the groundwater directly (Cleverly et al., 2002; Gries et al., 2003). 3. The water quota system in the Aksu-Tarim river basin The water quota system for the Tarim River and its tributaries was developed by 2005 and shall be enforced at the latest by 2020 (Peng et al., 2014; Thevs, 2011). Under average conditions, the Tarim shall receive the following amounts of water: 3.42 km3/a from the Aksu River, 0.90 km3/a from the Hotan River, and 0.33 km3/a from the Yerkant River, which total to 4.65 km3/a at Aral, where the three tributaries of the Tarim flow together (Table 2 and Fig. 3). Additionally, 0.45 km3/a have to be released from the Kaidu-Konqi River into the Tarim lower reaches at Qala (Table 3 and Fig. 3). Under average conditions, which means an annual inflow of 8.06 km3 into the Aksu River (Table 2), the net water loss along the

Table 2 Annual inflows into the Aksu River and into the Tarim River starting in Aral from the three tributaries Aksu, Yarkant, and Hotan [km3] (Tang and Deng, 2010). 50% probability in this table refers to average conditions, as found back in Table 3. Probability

90% 75% 50% 25%

Inflow into Aksu River upstream of irrigated land Inflow into the Tarim at Aral from Aksu Yarkant Hotan Total inflow into the Tarim at Aral Total inflow into the river basin of the Aksu and Tarim

6.68 7.25 8.06 2.52 0 0.2 2.72 6.88

2.64 0 0.64 3.28 7.89

8.85

3.42 4.2 0.33 0.56 0.9 1.55 4.65 6.31 9.29 10.96

Fig. 2. River stretch of the Tarim from Yingbaza to Iminqak. NDVIs from Landsat TM (path 144 row 31) from 2009, 2010, and 2011 (dates are written in the images). a: Dry river bed, b: natural riparian vegetation, c: cotton fields, d: river bed during summer flood 2010 with submerged river banks, e: submerged depressions during flood season 2010, f: depressions remaining submerged until spring 2011.

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Fig. 3. Water inflow from the tributaries Aksu, Yarkant, and Hotan into the Tarim mainstream and water quotas along the Tarim mainstream under average conditions [km3/a], after Thevs (2011) with I: irrigation and industry, E: environmental flow, O: oil exploitation. AeB: upper reaches, BeC: middle reaches, CeD: upper section of lower reaches, D and below: lower reaches.

Aksu River must not exceed 4.64 km3/a (TMB, 2005) so that 3.42 km3/a flows into the Tarim (Fig. 3). Under low water inflow conditions, i.e. 90% probability in Table 2 (6.68 km3/a), only 2.52 km3/a must be released into the Tarim River. Similar water quota regulations are also applied for the Yarkant and Hotan River (TMB, 2005). Water quotas for the Tarim River are more detailed regarding the water use of different sectors. Based on the hydrologic data of the period from 1957 to 2000, the maximum annual water inflow at Aral Station was 6.96 km3/a while the minimum was 2.56 km3/a. Therefore, ten scenarios varying from 2.5 to 7.0 km3/a are made for water quotas along the Tarim according to the inflow at Aral. For the Tarim River, no probabilities as for the tributaries (Table 2) are used. In Table 3, only the water allocation for the minimum, average, and maximum inflow scenario are shown. When an average annual inflow at the Aral Station is given (4.65 km3/a), the annual water quota for the upper reaches of the Tarim is 2.01 km3/a in total with

Table 3 Annual water quotas [km3/a] fixed within the Scheme of Surface Water Distribution for water withdrawal for economic activities and environmental flow along the upper, middle, and lower reaches of the Tarim River starting in Aral. Aral is located at the confluence of the three tributaries Aksu, Yarkant, and Hotan. The three columns refer to minimum, average, and maximum water inflow at Aral and respective water allocation (Tang and Deng, 2010). River section and water user

4. Methods 4.1. Evapotranspiration mapping

Inflow at Aral and respective water allocation Minimum

Average

Maximum

Upper reaches Inflow at Aral Agriculture Environmental flow Runoff released into middle reaches

2.50 0.37 0.72 1.42

4.65 0.41 1.60 2.64

7.00 0.41 2.62 3.97

Middle reaches Runoff at Yingbaza Oil exploitation Agriculture Environmental flow Runoff released into lower reaches

1.42 0.12 0.30 0.73 0.27

2.64 0.12 0.35 1.66 0.51

3.97 0.12 0.35 2.74 0.76

0.45 0.72 0.41 0.09 0.23

0.45 0.96 0.46 0.15 0.35

0.45 1.21 0.46 0.31 0.44

Lower reaches Water transferred from Kenqi river Runoff at Qala Station Agriculture Environmental flow Runoff released into the lower reaches below Daxihaizi

0.41 km3/a allocated for irrigated agriculture, and 1.60 km3/a for environmental flow. Under these average conditions, 2.64 km3/a water must be released into the middle reaches (Table 2 and Fig. 3), The annual water quota for the middle reaches, i.e. from Yingbaza until Qala, is 2.14 km3 in total, with 0.35, 0.12, and 1.66 km3 allocated for irrigated agriculture, oil industry, and environmental flow, respectively. The remaining 0.51 km3/a, together with 0.45 km3/a carried through channels from the Konqi River, which amount to 0.96 km3/ a, must be released from the Qala Gauging Station to the lower reaches of the Tarim. Along the river stretch from the Qala Gauging Station to the Daxihaizi Reservoir, 0.46 km3/a and 0.15 km3/a are allocated for irrigation and environmental flow, respectively. Finally, 0.35 km3/a water, as environmental flow, shall be released from the Daxihaizi Reservoir into the lower reaches toward the Taitema Lake which is the current terminal lake of Tarim river (TMB, 2005; Tang and Deng, 2010). The water quota for irrigation is kept constant from average inflow at Aral to the maximum inflow at Aral, while the water quota for environmental flow changes according to the inflow at Aral (Table 2; Tang and Deng, 2010).

In this paper, we mapped the actual evapotranspiration (ETa) of the Aksu-Tarim river basin for the growing seasons 2009, 2010, and 2011, in order to retrieve the amounts of water consumed by irrigated agriculture and natural vegetation for representative sites and on a river basin scale. We used the following MODIS satellite data products, in order to cover the whole Aksu-Tarim river basin: 8-day land surface temperature (MOD11A2), 16-day albedo (MCD43A3), and 16-day NDVI (MOD13A1). The growing season was defined to last from 1st of April to 31st of October in each year. Cotton as the major crop in the study area is planted at the beginning of April and harvested until the end of October (Protze, 2011). P. euphratica, as a keystone species of the natural vegetation, flowers in April, and the leaves shoot in May and fall until the end of October (Xinjiang Linkeyuan Zhisha Yanjiusuo, 1989). ETa can be determined through: i) climate station data, either by calculating a reference ET applying crop coefficients (Allen et al., 1998) or through the Bowen Ratio method (e.g. Malek and

N. Thevs et al. / Journal of Arid Environments 112 (2015) 87e97

Bingham, 1993) as used by Hou et al. (2010) to determine evapotranspiration of P. euphratica at the Heihe River in Inner Mongolia, China, ii) lysimeters (e.g. Sammis, 1981), iii) eddy co-variance measurement devices (e.g. Cleverly et al., 2002; Scott et al., 2008), or iv) using the residual of the water balance equation (e.g. Reddy et al., 2012). These methods measure ETa at a specific point but cannot cover large, diverse, and remote areas. Therefore algorithms have been developed, in order to map ETa on the basis of satellite images (Landsat, MODIS, or ASTER), e.g. SEBAL (Bastiaanssen, 1995; Bastiaanssen et al., 2002, 2005), METRIC (Allen et al., 2005), SEBS (Su, 2002), and S-SEBI (Roerink et al., 2000; Sobrino et al., 2005, 2007), as reviewed by Gowda et al. (2007, 2008) and Senay et al. (2011). On the basis of the Simplified Surface Energy Balance Index (SSEBI), developed by Roerink et al. (2000), the latent heat flux can be calculated as:

LE ¼ LðRn
GÞ

(1)

with LE, L, Rn, and G being latent heat flux [W/m2], evaporative fraction, net radiation [W/m2], and soil heat flux [W/m2], respectively. When calculating daily values instead of instantaneous values, G ¼ 0 (Sobrino et al., 2005, 2007) so that Eq. (1) is simplified:

LEd ¼ Ld Rnd

(2)

with LEd, Ld, and Rnd referring to daily latent heat flux sum [MJ], daily evaporative fraction, and daily net radiation sum [MJ]. In the S-SEBI (Roerink et al., 2000; Sobrino et al., 2007) it is assumed that net radiation (Rnd) is converted into evapotranspiration, i.e. potential evapotranspiration (ETpot). Following this assumption, ETa, can be calculated as follows:

ETa ¼ LETpot

(3)

Thereby, we estimated Rnd, and thus also ETpot, on the basis of the date, geographical position, transmissivity of the atmosphere, land surface temperature (MOD11A2), and albedo (MCD43A3), applying the module i.evapo.potrad (http://grasswiki.osgeo.org/wiki/ AddOns/GRASS_6#GIPE) of the software package GRASS GIS 6.4 (http://grass.fbk.eu/). The transmissivity was estimated from the data of a climate station set up in Iminqak (Fig. 1). The evaporative fraction (L) is the part of the ETpot, which is realized as ETa. Thus, over well-watered vegetation, e.g. wetland vegetation, we can assume L z 1 and ETa z ETpot. The land surface temperature is low, because the net radiation is consumed by evapotranspiration. In contrast, at places without any vegetation or other moisture, L and ETa are zero. Here, the land surface temperature is high, because the net radiation goes into the sensible heat flux. Between these two extremes it is assumed that L and land surface temperature have a linear relationship (Roerink et al., 2000). The evaporative fraction is calculated as follows:

L¼

TH
Tx TH
TC

(4)

whereby TH, TC, and Tx refer to the land surface temperatures (MOD11A2) at the hot pixel, cold pixel, and pixel, for which L is calculated, respectively. The satellite records TH, TC, and Tx at its overpass time, which is between 10:50 and 11:10 local time for MOD11A2 according to the day view time channel. So, L is calculated for the overpass time. According to Bastiaanssen (2000) and Farah et al. (2004), L remains constant during daytime so that L calculated for satellite overpass time can be used to calculate daily ETa.

91

Cold pixels were chosen from wetlands with dense reed vegetation interrupted by small open waters, which do not fall dry during the growing season. Hot pixels were selected from areas free of vegetation but lying adjacent to the riparian vegetation along the Tarim River. Hot pixels could be selected from fallow fields as suggested by Waters et al. (2002), because there were none available, which fitted into one of the MODIS pixels. At the hot pixels, L ¼ 0 and ETa ¼ 0. Thus, all the radiation is converted into sensible heat flux so that the land surface temperature is high. Following this approach, 8-day ETa maps were produced according to the 8-day MOD11A2 satellite images. These ETa maps were aggregated to monthly ETa maps and finally summed up to ETa maps for the growing seasons 2009, 2010, and 2011. The MOD11A2 product has a resolution of 1 km by 1 km so that all ETa maps were produced in this resolution. In Iminqak, in 2009 and 2011 a climate station was operated, in order to measure ETa for a validation of the MODIS ETa. The climate station was installed on the ground of the ranger station in ImInqak due to security reasons. This station is located near the edge of the riparian forest. The climate station was equipped with sensors for incoming and outgoing radiation (pyranometers CMP3, Kipp & Zonen) and ventilated air temperature/humidity sensors in two different heights so that ETa was calculated with the Bowen Ratio method (Malek and Bingham, 1993). One air temperature/humidity sensor was mounted 2 m above soil surface, while the other sensors were mounted 10 m above soil surface level. Electricity supply came from two 12 V car accumulators, which were charged by the nearby ranger station. Data were recorded every 0.1 s and stored by a data logger at 15 min intervals. 4.2. Retrieve ETa data from irrigated agriculture and natural vegetation The ETa maps of each year's growing season were laid over a map of irrigated land in order to retrieve the ETa of the whole irrigated land area. The average annual precipitation (Liu, 1997) was subtracted from ETa, in order to calculate the net water loss of the irrigated land (Scott et al., 2008). The map of the irrigated land was manually digitized from Landsat satellite images from 2009 (cf. Thevs, 2011). ETa values for cotton, as the dominant crop, were retrieved from MODIS pixels (1 km  1 km), which were completely covered by irrigated fields. Thus, places listed in Table 4 between 3 and 39 pixels of the MODIS ETa maps were selected manually. Riverbed pixels were selected manually from MODIS pixels, as well. Pixels, which were completely covered by the riverbed, qualified as riverbed pixels. MODIS ETa pixels, from which we read the ETa of the natural vegetation, were selected from two Quickbird satellite images. These Quickbird images each cover a cross section through the Tarim River and adjacent natural vegetation south of Xayar and at Iminqak (Fig. 3). The two Quickbird images date from 2009/07/15 and 2009/06/09, correspondingly. As Quickbird has a ground resolution of 0.6 m  0.6 m, only single trees and shrubs are visible. For each MODIS pixel, the total vegetation coverage in percent was estimated visually. Table 4 Sum of ETa during the growing seasons (2009e2011) measured with the climate station Iminqak and detected through remote sensing. Year

2009 2010 2011

Sum of ETa during growing season [mm] Climate station

Remote sensing

612

611 794 929

836

Deviation [%] 0.2 10.8

92

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Table 5 ETa [mm] of the agricultural land and natural vegetation along the Aksu and Tarim River from 2009, 2010 and 2011. The names of the different agricultural fields are displayed in Fig. 1. n e number of MODIS pixels, Std. Dev. e standard deviation, Sign. a ¼ 0.05 e letters indicate significant differences of mean of ETa at a ¼ 0.05 (Tukey post hoc test). Land cover

2009 N

2010

ETa mean [mm] Std. Dev. Sign. a ¼ 0.05 N

2011

ETa mean [mm] Std. Dev. Sign. a ¼ 0.05 N

ETa mean [mm] Std. Dev. Sign. a ¼ 0.05

Cold Pixels Irrigated land Karatal Bexerik Aral Bingtuan Xayar Yingbaza Dongkutan Puhui Lopnor Tikanlik

10 1687

373

a

10 1660

298

a

10 1790

248

a

19 22 33 23 18 4 3 39 20 19

1145 1164 1198 1068 1115 644 560 1108 1063 580

22 103 119 107 81 172 126 187 74 199

b b b bc b de de b bc de

19 22 33 23 18 4 3 39 20 16

1133 1078 1043 1004 884 641 504 910 947 501

32 41 84 83 69 189 98 154 59 212

bcd bcde bcde bcdef efg ghij jk def cdef jk

19 22 33 23 19 4 3 39 20 19

1143 1124 1164 1130 1027 614 756 963 959 666

37 58 96 59 33 276 127 139 66 159

bc bc bc bc bc ef de bcd cd e

River bed pixels

41

753

142

d

41

882

129

efg

41 1167

132

bc

73 113 149 204 296 264 238 153

cd d de de def ef fg g

3 16 26 39 42 39 32 77

889 774 780 604 553 470 284 51

116 127 166 234 314 285 221 136

def fghi fghi hij ij jk kl lm

3 1115 16 978 26 960 39 756 42 688 39 603 32 365 77 71

66 132 236 286 356 334 297 174

bc bcd bcd de e ef f g


5

54


1

54

Pixels natural vegetation coverage 70% 3 798 60e70% 16 760 50e60% 26 715 40e50% 39 556 30e40% 42 514 20e30% 39 426 10e20% 32 245 <10% 77 8 Hot pixels (n ¼ 124)


1

63

g

4.3. Calculating water balances for the Aksu-Tarim river basin Water balances were calculated after Wu et al. (2013), in order to estimate how much the water quotas were met or exceeded:

P
ET
O þ I ¼ Dgw þ Ds

(5)

with P being precipitation, ET referring to total evapotranspiration, O being basin outflow, I being inflow in the basin, and Dgw and Ds referring to change in groundwater and soil moisture storage. Basin inflows are given Table 2. Thereby, we used 90% probability (minimum inflow), 25% probability (maximum inflow), and 50% probability (average inflow), in order to calculate water balances for 2009, 2010, and 2011, respectively, because measured inflow and runoff data are not available to the public after 2005. Basin outflow for the whole Aksu-Tarim river basin is zero, while outflow for the Aksu river basin is given in Table 2. Precipitation has been taken from Liu's (1997) precipitation map and subtracted from the ETa maps for the growing seasons 2009, 2010, and 2011, respectively, in order to yield net water loss (NWL) of the Aksu-Tarim river basin. Eq. (5) was simplified as follows:

I
NWL ¼ Dgw þ Ds

(6)

A negative term Dgw þ Ds would indicate that the water quota was exceeded and groundwater and soil moisture was depleted, in order to meet the water consumption of irrigation and natural vegetation. 5. Results The sum of the daily ETa values over the vegetation season 2009 measured at the climate station Iminqak nearly equals the ETa detected from the MODIS satellite images (Table 4). In 2011, the MODIS ETa is 10.8% higher than the ETa measured at the climate station. The mean ETa of agricultural land used for cotton along the

m

g

Aksu River, Tarim upper reaches, Puhui, and Lopnor, summed up over the growing seasons 2009, 2010, and 2011, respectively, ranged from 884 to 1198 mm (Table 5). In contrast, the mean ETa of agricultural land used for cotton in Yingbaza, Dongkutan, and Tikanlik ranged between 501 and 666 mm within the period of 2009e2011. Correspondingly, the mean ETa of other agricultural land ranged from 884 to 1226 mm along the Aksu River and Tarim upper reaches, while it only was between 656 and 674 mm in Tikanlik. In most of the locations listed in Table 5, the ETa of the vegetation season dropped slightly from 2009 to 2010 and increased again from 2010 to 2011. This trend is most pronounced for Dongkutan, Lopnor, and Tikanlik. In contrast, the ETa of the natural vegetation increased slightly from 2009 to 2010 and sharply from 2010 to 2011. For example, the mean ETa of natural vegetation of a total coverage of 70% and more was determined with 798, 889, and 1115 mm in 2009, 2010, and 2011, respectively. Fig. 4 also reflects these two trends. The mean ETa at the hot anchor pixels did not change over the study period and was
1,
5, and
1 mm in 2009, 2010, and 2011, correspondingly. The mean ETa of the cold pixels was 1,687, 1,660, and 1790 mm in 2009, 2010, and 2011, correspondingly, thus showing an increase only from 2010 to 2011. The net water loss from water reservoirs, agricultural land, and natural vegetation along the whole Aksu and Tarim River ranges from 10.13 to 10.61 km3/a in 2009, to 12.84e13.41 km3/a in 2011 as shown in Table 6. The groundwater and soil moisture storage (Dgw þ Ds) is
3.25 to
3.73 km3, 0.1e0.53 km3, and
3.55 to
4.12 km3 in 2009, 2010, and 2011, respectively for the whole AksuTarim river basin, indicating a decreasing groundwater and soil moisture storage in 2009 and 2011 (Table 6). For the Tarim River alone, the corresponding Dgw þ Ds are
0.22 to
0.71 km3, 2.55e2.97 km3, and
0.18 to
0.76 km3. This corresponds to an increase of the groundwater and soil moisture storage in 2010 and slight decreases in 2009 and 2011. At the Aksu River, Dgw þ Ds is negative during all years studied (Table 6). In 2009 and 2011, the net

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93

Fig. 4. Net water loss from water reservoirs, irrigated land, and natural vegetation (ETa e P) along the Aksu River and Tarim River (upstream, middle, and downstream section) during the growing seasons 2009, 2010, and 2011.

water loss on the irrigated land alone slightly exceeded the inflow available for the Aksu River (Table 6). Along the Aksu River, the net water loss of the irrigated agriculture was more than twice the net water loss of the natural vegetation in 2009 and 2010, and almost twice the net water loss of the natural vegetation in 2011. In contrast, along the Tarim River's upstream and midstream section, the net water loss of the natural vegetation was higher than the net water loss of the irrigated agriculture (Fig. 4). The net water loss through agricultural land along the upstream section of the Tarim River (Table 6) was in the range of the corresponding quota of 0.41 km3/a under average to high runoff conditions and 0.37 km3/a under low runoff conditions, respectively (Table 3). In 2009, the net water loss of the natural vegetation (Table 6, Fig. 4) exceeded the corresponding quota fixed for low inflow conditions of 0.72 km3/a. In 2011, the net water loss of the natural vegetation (2.34e2.53 km3 during the growing season) was close to the corresponding water quota established for maximum water inflow conditions (2.62 km3/a). Along the middle and lower reaches, the water quotas for irrigation (Table 3) were not reached by the net water loss of irrigated land (Table 6, Fig. 4) during the three years studied. The net water loss of the natural vegetation along the mid-stream section of the Tarim lagged far behind the respective water quotas for environmental flow during 2009, 2010, and 2011. In 2009 and 2010, the net water loss of the natural vegetation during the growing season was 0.53e0.55 km3 and 0.77e0.79 km3, respectively (Table 6), while the water quotas for environmental flow were 0.73 km3/a (low inflow at Aral reflecting 2009) and possibly 2.74 km3/a (maximum inflow at Aral reflecting 2010). In 2011, the net water loss of the natural vegetation was 1.15e1.19 km3 during the growing season (Table 6), while the corresponding water quota was 1.66 km3/a (Table 3). Along the lower reaches of the Tarim River, the net water loss of the natural vegetation was 0.11 km3, 0.09 km3, and 0.27 km3 during the growing seasons 2009, 2010, and 2011, respectively. The corresponding water quotas were 0.09 km3/a (low inflow at Aral reflecting 2009), possibly 0.31 km3/a (maximum inflow at Aral reflecting 2010), and 0.15 km3/a (average inflow at Aral reflecting 2011). Thus, in 2009 and 2011, the net water loss of the natural vegetation was in the range of, or exceeded the water quota fixed for environmental flow at the Tarim down-stream section.

In Fig. 4, it is visible that the net water loss of the irrigated land drops from 2009 to 2010 and increases from 2010 to 2011, while the net water loss of the natural vegetation increases from 2009 over 2010 to 2011. This trend is also visible when referring to land cover classes in Table 5. The evaporation from water reservoirs is constant during the three years, except for the lower reaches. There, the evaporation from reservoirs triples from 2009 to 2011 (Table 6, Fig. 4). 6. Discussion 6.1. MODIS ETa and ETa measured by climate station The deviation of the MODIS ETa from the climate station ETa, i.e. 0.1% in 2009 and 10.8% in 2011, is within the range of such deviations reviewed by Bastiaanssen et al. (2005). The higher deviation in 2011 can be explained by the water, which submerged river branches and depressions along the Tarim until spring 2011 (Fig. 2). Periodical open waters are also located within the MODIS pixel, in which the climate station is located. Thus, in 2011 these periodically submerged depressions and river branches increase the evapotranspiration of the MODIS pixel at the climate station. Furthermore, we also have to recall on the discharge pattern of the three years 2009e2011 (pers. observation by the authors, cf. Fig. 2). In 2009, the Tarim River fell dry during spring and early summer downstream of Xayar and did not carry a summer flood. The summer flood in 2010 was one of the highest floods since the 1950s. River branches and depressions remained filled with water until spring 2011. Thus, the groundwater layer was refilled all over the Tarim flood plain. The summer flood in 2011 was an average flood event. The surface water runoff is reflected in groundwater level fluctuations (Chen et al., 2006; Ye et al., 2009). 6.2. ETa of cotton The ETa of cotton along the Aksu River, Tarim upper reaches, in Puhui, and in Lopnor, is 884 to 1198 mm, which is in the range of other studies in Central Asia (Chapagain et al., 2006; Conrad et al., 2007; Steduto et al., 2012). The low ETa for agricultural land used for cotton and other crops in Yingbaza, Dongkutan, and Tikanlik may be explained as follows. Along the middle and lower reaches of the

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Table 6 Net water loss [km3/a] of water reservoirs, irrigated land, and natural vegetation along the Aksu and Tarim River and water balances for the Aksu-Tarim river basin during the growing seasons 2009, 2010, and 2011. For some parts of the Aksu and Tarim river basin it is unclear whether they hydrologically belong to this river basin or neighboring € € basins, e.g. in Xayar water can be diverted from the Tarim River or from the Og an River. Therefore, ranges are given. Inflow data is taken from Table 2. Inflow for the Aksu River is the difference between inflow upstream of the irrigated land and outflow to the Tarim River (Table 2). River stretch

Aksu

Tarim

Total Aksu and Tarim

Upstream

Midstream

Downstream

Total

0.20 4.83 2.15 7.18 4.16
3.02

0.05 0.34e0.69 1.58e1.70 1.97e2.44

0.00 0.14 0.53e0.55 0.68e0.69

0.05 0.13 0.11 0.30

0.10 0.61e0.96 2.23e2.36 2.94e3.43 2.72
0.22 to
0.71

0.30 5.44e5.79 4.38e4.52 10.13e10.61 10.13
3.25 to
3.73

Growing season 2010 Water Reservoirs Irrigated land Natural vegetation Total Inflow (IeO for the Aksu) Dgw þ Ds

0.21 4.58 2.30 7.09 4.65
2.44

0.06 0.32e0.59 1.78e1.91 2.16e2.57

0.00 0.17 0.77e0.79 0.94e0.96

0.06 0.09 0.09 0.24

0.12 0.58e0.85 2.65e2.80 3.34e3.76 6.31 2.55e2.97

0.33 5.16e5.43 4.95e5.10 10.43e10.86 10.96 0.1e0.53

Growing season 2011 Water Reservoirs Irrigated land Natural vegetation Total Inflow (IeO for the Aksu) Dgw þ Ds

0.21 5.04 2.75 8.00 4.64
3.36

0.06 0.41e0.76 2.34e2.53 2.81e3.35

0.00 0.25 1.15e1.19 1.40e1.44

0.15 0.20 0.27 0.62

0.21 0.85e1.21 3.76e3.99 4.83e5.41 4.65
0.18 to
0.76

0.42 5.90e6.25 6.51e6.74 12.84e13.41 9.29
3.55 to
4.12

Growing season 2009 Water Reservoirs Irrigated land Natural vegetation Total NWL Inflow (IeO for the Aksu) Dgw þ Ds

Tarim River, large parts of the agricultural land is fallow, because during 2007, 2008, and 2009 the Tarim River ceased to flow during early summer, when irrigation demand increases (Bothe, 2010; Thevs, 2011). Farmers thus exploit groundwater in order to overcome the season during which the Tarim runs dry. The groundwater supply can only satisfy the irrigation demand for a part of the agricultural land, therefore nothing was planted or the crop was lost on part of the agricultural land. As MODIS has a spatial resolution of 1 km by 1 km, the pixels covering the agricultural land along the middle and lower reaches contain land planted with cotton and fallow land, which results in mixed pixels (Settle and Drake, 1993). Therefore, the ETa of mixed pixels detected with MODIS is an average of the ETa of cotton and fallow land. Along the Aksu River and Tarim upstream there was no pronounced water shortage during 2007e2009, because these regions are located upstream from other water users. Additionally, in Xayar € € water also can be diverted from the Og an River into the irrigated land along the Tarim River. In Puhui and Lopnor, though located in the vicinity of the Tarim downstream section, all agricultural land is planted with cotton. The ETa is similar to the Aksu River and the Tarim upstream section, because these two areas receive water from the Kenqi River rather than the Tarim (Song et al., 2000). Due to the low runoff during 2009, more land remained fallow along the Aksu and Tarim River than during the years before. This may explain that the ETa of cotton decreased from 2009 to 2010. In 2010/2011 more water was available in reservoirs and natural depressions so that more land was allocated for cotton than before. 6.3. ETa of the natural vegetation The ETa of the natural vegetation strongly depends on the total coverage. In Table 7, values of water consumption of P. euphratica, T. ramosissima, and Elaeagnus angustifolia L., the keystone species dominating the natural riparian vegetation in Central Asia, are displayed. The water consumptions reported by Khamzina et al. (2009) are considerably higher than the corresponding MODIS ETa for natural

vegetation with a total coverage of 70% or more (Tables 5 and 7). This might be explained through the high groundwater level under the sites Khamzina et al. (2009) investigated and the presumably high tree density, where the trees had been planted. The water consumptions of T. ramosissima (Cleverly et al., 2002) correspond with the MODIS ETa of this study (Tables 5 and 7). The water consumption measured by Thomas et al. (2006) in Table 7 is lower compared to the MODIS ETa (Table 5), due to the P. euphratica stand measured by Thomas et al. (2006) which had a greater coverage than 20%. The water consumption of the natural vegetation along the Tarim River increased from 2009 to 2010 and again from 2010 to 2011 (Tables 5 and 6, Fig. 4). This also is explained with the different discharge into the Tarim in 2009, 2010, and 2011. During the summer flood in 2010, which was among the highest since the 1950s, large areas and many river branches were flooded. The evaporation from these surface waters increased the ETa detected. Furthermore, the groundwater recharge was strongly enhanced through this flood. Therefore, the water supply for part of the natural vegetation might have increased, which results in higher transpiration. This may apply for T. ramosissima, which being a facultative phreatophyte mainly uses groundwater as water source, but also can survive on moist soils (Busch et al., 1992). Thus, the transpiration might be decreased, when the groundwater drops and only soil moisture is available. However, the transpiration rate might increase, when the groundwater level increases again. Hao et al. (2010) assumed that hydraulic lift from the groundwater toward the soil surface by P. euphratica contributes to the water supply of herbs in the undergrowth of riparian forests. So, during 2009, when the groundwater level was lower than in 2010 and 2011, this hydraulic lift probably was lower, too, because P. euphratica needed a larger share of the water uptaken from the groundwater for itself. During 2010 and 2011, the hydraulic lift increased so that the water consumption of the undergrowth increased, too. After the high flood in 2010, many river branches and depressions carried water until spring 2011 (cf. Fig. 3). Also,

N. Thevs et al. / Journal of Arid Environments 112 (2015) 87e97

95

Table 7 Water consumption (ETa) of riparian vegetation types in Central Asia over the growing season. Vegetation type

ETa [mm]

Source

Corresponding natural vegetation of this study (cf. Table 3)

Populus euphratica, Tarim Basin (China), groundwater 4 m, tree density 2300e3400 trees/ha Tamarix ramosissima, Tarim Basin (China) Populus euphratica, Ejina (Heihe Basin, Inner Mongolia, China) Elaeagnus angustifolia, Khorezm, Uzbekistan, planted on shallow (0.9e2 m) groundwater

192e392

Thomas et al. (2006)

10%e20% total coverage

92e180 447 1250

Thomas et al. (2006) Hou et al. (2010) Khamzina et al. (2009)

Populus euphratica, Khorezm, Uzbekistan, planted on shallow (0.9e2 m) groundwater

1030

Khamzina et al. (2009)

Tamarix ramosissima, Rio Grande (USA), LAI ¼ 2.5, not flooded

740

Cleverly et al. (2002)

Tamarix ramosissima, Rio Grande (USA), LAI ¼ 3.5, flooded during spring

1220

Cleverly et al. (2002)

10%e20% total coverage 20%e30% total coverage Exceeds MODIS ETa of natural vegetation with total coverage of 70% and more Exceeds MODIS ETa of natural vegetation with total coverage of 70% and more 50%e70% total coverage in 2009, i.e. no summer flood Total coverage 70% and more in 2011, i.e. vegetation partly submerged.

groundwater levels and soil moisture contents were higher compared to 2010 and 2009. Therefore, such surface waters already contributed to ETa from the beginning of the vegetation season in 2011. Additionally, more water was available for the vegetation from the beginning of the vegetation season in 2011, which presumably resulted in an increased ETa. As the summer flood in 2011 reached average levels, no water shortage occurred during the vegetation season, therefore, the ETa of the natural vegetation increased again from 2010 to 2011. 6.4. Water balance and water quotas The net water loss along the upper, middle, and lower reaches of the Tarim during the growing seasons 2009 to 2011 ranged between 1.97 and 3.35 km3, 0.68 and 1.44 km3, and 0.3 and 0.62 km3, respectively. Xu et al. (2005) give values for water consumption, which perhaps refer to the net water loss of 1.7 km3, 2.3 km3, and 0.61 km3 for the upper, middle, and lower reaches of the Tarim. The differences between the results of Xu et al. (2005) and the results of this study can be explained as follows. The area under irrigation along the Tarim has increased until now with the largest increases along the upper reaches. Therefore, the water consumption and thus net water loss along the upper reaches increased from the time span of the data of Xu et al. (2005) to the data from 2009 to 2011 of this study. Along the middle reaches dykes were built until 2004 (Tang and Deng, 2010), which cut off part of the inland delta along the middle reaches so that the net water loss decreased from Xu et al. (2005) toward the results of our study. The total amount of water consumed in the Aksu-Tarim river basin exceeded the inflow during 2009 and 2011 (Table 6). Considering only the Aksu, consumption exceeded inflow during all three years. This finding agrees with Xu et al. (2010), who found that there was an increasing trend of the inflow into the Aksu River, while the runoff from the Aksu into the Tarim at Aral (Fig. 1) showed a decreasing trend. The gap between inflow and water consumption partly can be explained through exploitation of groundwater, as Siebert et al. (2006) indicate that 10%e20% of the irrigated area in the Tarim Basin is equipped for irrigation with groundwater. Groundwater exploitation for irrigation from fossil groundwater aquifers also was documented around Yingbaza (Thevs, 2011). Furthermore, the MODIS ETa of the sparse vegetation may have been over-estimated (cf. Table 7). The water consumption for agriculture and environmental flow along the Tarim River downstream of Yingbaza, i.e. middle

and lower reaches of the Tarim River, falls behind the respective quotas, while the quotas for the Tarim upstream of Yingbaza are met or exceeded. This finding agrees with Peng et al. (2014), who investigated the perspective of land users and other stakeholders with regard to the implementation of the water quota system of the Aksu-Tarim river basin. During 2010, there was a surplus in groundwater and soil moisture storage of 2.55e2.97 km3 along the Tarim. This surplus is due to the extremely high summer flood of that year as discussed in section 6.3. In 2010, there was increase in groundwater storage, but also in soil moisture, at least in areas close to river branches and depressions, which carried water until spring 2011 (Fig. 2). Therefore, in this paper we refer to Dgw þ Ds rather than assuming Ds ¼ 0 as suggested by Wu et al. (2013). In 2011, the surplus groundwater and soil moisture from 2010 was partly consumed as indicated by the negative Dgw þ Ds of
0.18 to
0.76 km3. However, as the surplus of 2010 was much larger than Dgw þ Ds in 2011, the flood in 2010 built up groundwater and soil moisture storage for water users for longer than 2011. Similar water balance studies at Basin level under arid climate conditions were carried out by Guerschmann et al. (2008) for the Murray-Darling Basin, where irrigated crops and floodplain vegetation showed a negative water balance, too, Barnett and Pierce (2008) for the Colorado River, Wu et al. (2013) for the Hai River Basin in Northern China, and by Hochmuth et al. (submitted for publication) for the Heihe river basin in Northwestern China. In the Hai Basin, the target water consumption as defined after scientific assessments was exceeded by 6.73 km3/a during 2002e2007 (Wu et al., 2013). As, according to Wu et al. (2013), this excess water consumption cannot be avoided by only introducing water saving measures, the South to North Water Transfer Project is needed, in order to carry water from the Yangze river basin into the Hai River Basin. For the Aksu-Tarim river basin such water transfer is not an option, because the neighboring large river basins (Indus, Syr Darya, Amu Darya, or Ili) are located outside of China, are far away, and water stressed themselves (Shen and Chen, 2010) so that a water transfer would transfer water stress from one basin to the other, too. For the Colorado River a reduction of inflow of 10%e30% is expected due to climate change. Now already all water is distributed among water consumers. Therefore, the question of who has to reduce its water consumption by how much must be solved (Barnett and Pierce, 2008). This is similar to the situation in the Aksu-Tarim river basin. If the water consumption upstream is reduced to a level, which does not require exploitation of fossil groundwater and which is within the stipulated water quota, the

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issue must be tackled, how the resulting reduction of water consumption will be distributed. Hochmuth et al. (submitted for publication) found a slightly positive water balance for the Heihe River, including its downstream, so that it was concluded that the water quotas there were fulfilled. This might be explained, as the water allocation along the Heihe has received more attention than the water allocation along the Tarim. In the case of the Heihe river basin the Central Government of China urged the two provinces Gansu and Inner Mongolia, which share that river basin, to enforce the water quotas (Zhang, 2005), while in the Tarim Basin the Province Government has to enforce the water quotas so that the political pressure is lower. 7. Conclusion We mapped the evapotranspiration (ETa) for irrigated agriculture, natural vegetation, and water reservoirs in the Aksu-Tarim river basin, Xinjiang, China for the growing seasons 2009, 2010, and 2011. Water balances were calculated and compared with the water quotas fixed for the Aksu and Tarim River. We used the S-SEBI approach to map ETa. This approach yielded accurate results for irrigated agriculture and dense natural vegetation, but presumably over-estimated the ETa of sparse vegetation. The water balance for the Aksu river basin was negative during 2009, 2010, and 2011. The water quota fixed for the Aksu River does not differentiate between water consumption of irrigated agriculture and natural vegetation. The water consumption of the natural vegetation, in addition to the irrigated agriculture, resulted in a negative water balance. Further research works should investigate more closely the ETa of the natural vegetation in the Aksu river basin, and the water quotas for the Aksu River should be refined regarding the natural vegetation, i.e. environmental flow. In the Tarim river basin, the water quota allocated for irrigated agriculture and environmental flow along the mid and downstream area of the Tarim River are not met, while the water consumption along the Tarim upstream often exceeds the quotas given. Thus, the limits specified for the Tarim upstream and the Aksu River must be enforced in order to ensure sufficient water supply to mid and downstream areas of the Tarim. Considerable amounts of groundwater, including fossil groundwater, are exploited for irrigation along the Aksu and Tarim River, which must be regarded as exploitation of a non-renewable resource. The groundwater exploitation must be limited to a renewable amount without an impact on groundwater supply to the natural vegetation, in order to maintain the natural vegetation as stipulated in the water quota system in the Aksu-Tarim river basin. Water users along the mid and downstream river stretches of the Tarim depend on the enforcement of rules and to some extent, on good will of the water users upstream, in order to secure the water amount needed for irrigation and environmental flow. A situation of non-secure water supply downstream poses risks to irrigated agriculture. Therefore, we propose that, in addition to the enforcement of rules, land use systems should be based on indigenous plants. These may include Apocynum venetum as a textile plant or Glycyrrhiza spec. and Alhagi sparsifolia as medicinal plants. These should be proposed because they are species that exploit the groundwater and remain productive in water scarce years, as seen in 2009. Acknowledgements We thank the Bauer-Hollmann Foundation and the Rudolf and Helene Glaser-Foundation for the funding of this study within the Junior Research Group Adaptation Strategies to Climate Change and

Sustainable Land Use in Central Asia (Turkmenistan and Xinjiang, China). References Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop Evapotranspiration e Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56. FAO, Rome. Allen, R.G., Tasumi, M., Morse, A., Trezza, R., 2005. A Landsat-based energy balance and evapotranspiration model in Western US water rights regulation and planning. Irrigat. Drain. Syst. 19, 251e268. Barnett, T.P., Pierce, D.W., 2008. When will Lake Mead go dry? Water Resour. Res. 44, W3201. Bastiaanssen, W.G.M., 1995. Regionalization of Surface Flux Densities and Moisture Indicators in Composite Terrain. Ph.D. Thesis. Wageningen University, Wageningen. Bastiaanssen, W.G.M., 2000. SEBAL-based sensible and latent heat fluxes in the irrigated Gediz Basin, Turkey. J. Hydrol. 229, 87e100. Bastiaanssen, W.G.M., Ahmad, M., Yuan, C.M., 2002. Satellite surveillance of evaporative depletion across the Indus Basin. Water Resour. Res. 38, 1273e1282. Bastiaanssen, W.G.M., Noordman, E.J.M., Pelgrum, H., Davids, G., Thoreson, B.P., Allen, R.G., 2005. SEBAL Model with remotely sensed data to improve water resources management under actual field conditions. J. Irrigat. Drain. Eng., 85e93. Jan./Feb. Bothe, J., 2010. The Water Use of Cotton Cultivation in the Tarim River Basin in Northwest China. Bachelor Thesis. University of Osnabrück, Osnabrück, Germany. Busch, D.E., Ingraham, N.L., Smith, S.D., 1992. Water uptake in woody Phreatophytes of the southwestern United States: a stable isotope study. Ecol. Appl. 2, 450e459. Central Asia Atlas, 2012. http://www.caatlas.org/ (assessed 28.12.12). Chapagain, A.K., Hoekstra, A.Y., Savenje, H.H.G., Gautam, R., 2006. The water footprint of cotton consumption: an assessment of the impact of worldwide consumption of cotton products on the water resources in the cotton producing countries. Ecol. Econ. 60, 186e203. Chen, Y.N., Ziliacus, H., Li, W.H., Zhang, H.F., Chen, Y.P., 2006. Ground-water level affects plant species diversity along the lower reaches of the Tarim river, Western China. J. Arid Environ. 66 (2), 231e246. Chou, T., 1960. The problem of channel shifting of the middle reaches of the Tarim River in Southern-Sinkiang. In: Murzayev, E.M., Chou, L. (Eds.), Natural Conditions of Sinkiang. Chinese Academy of Sciences, Peking, pp. 87e133. Prirodnyye usloviya Sin'tsziana NAUK. Peking (in English). Cleverly, J.R., Dahm, C.N., Thibault, J.R., Gilroy, D.J., Allred Coonrod, J.E., 2002. Seasonal estimates of actual evapotranspiration from Tamarix ramosissima stands using three-dimensional eddy covariance. J. Arid Environ. 52, 181e197. Conrad, C., Dech, S.W., Hafeez, M., Lamers, J.P.A., Martius, C., Strunz, Gh, 2007. Mapping and assessing water use in a Central Asian irrigation system by utilizing MODIS remote sensing products. Irrigat. Drain. Syst. http://dx.doi.org/ 10.1007/s10795-007-9029-z. FAO, 2012. Coping with Water Scarcity. FAO Water Reports 38. FAO, Rome. Farah, H.O., Bastiaanssen, W.G.M., Feddes, R.A., 2004. Evaluation of the temporal variability of the evaporative fraction in a tropical watershed. Int. J. Appl. Earth Observ. Geoinform. 5, 129e140. Feike, T., Mamitimin, Y., Li, L., Abdusalih, N., Doluschitz, R., 2014. Development of agricultural land and water use and its driving forces in the Aksu-Tarim Basin, P.R. China. Environ. Earth Sci. http://dx.doi.org/10.1007/s12665-014-3108-x. Feng, Q., Liu, W., Si, J.H., Su, Y.H., Zhang, Y.W., Cang, Z.Q., Xi, H.Y., 2005. Environmental effects of water resource development and use in the Tarim river basin of Northwestern China. Environ. Geol. 48, 202e210. Giese, E., Mamatkanov, D.M., Wang, R., 2005. Wasserressourcen und deren Nutzung im Flussbecken des Tarim (Autonome Region Xinjiang/VR China). Zentrum für €t Giessen, Internationale Entwicklungs- und Umweltforschung/ZEU), Universita Giessen, Germany. Glantz, M.H., 2010. Creeping environmental disasters: Central Asia's aral seas. In: Kostianoy, A.G., Kosarev, A.N. (Eds.), The Aral Sea Environment, The Handbook of Environmental Chemistry, vol. 7. Springer, Heidelberg, pp. 305e316. Gowda, P.H., Chavez, J.L., Colaizzi, P.D., Evett, S.R., Howell, T.A., Tolk, J.A., 2007. Remote sensing based energy balance algorithms for mapping ET: current status and future challenges. Am. Soc. Agric. Biol. Eng. 50, 1639e1644. Gowda, P.H., Chavez, J.L., Colaizzi, P.D., Evett, S.R., Howell, T.A., Tolk, J.A., 2008. ET mapping for agricultural water management: present status and challenges. Irrigat. Sci. 26, 223e237. Gries, D., Zeng, F., Foetzki, A., Arndt, S.K., Bruelheide, H., Thomas, F.M., Zhang, X.M., Runge, M., 2003. Growth and water relation of Tamarix ramosissima and Populus euphratica on Taklamakan desert dunes in relation to depth to a permanent water table. Plant Cell Environ. 26, 725e736. GLOWA Jordan, 2008. GLOWA Jordan River: an integrated approach to sustainable management of water resources under global change. http://www.glowajordan-river.de/ (accessed 20.04.12). ~ aGuerschmann, J.-P., Van Dijk, A.I.J.M., McVicar, T.R., Van Neil, T.G., Li, L., Liu, Y., Pen Arancibia, J., 2008. Water balance estimates from satellite observations over the Murray-Darling Basin. A report to the Australian Government from the CSIRO Murray-Darling Basin Sustainable Yields project, CSIRO Australia. http://www.

N. Thevs et al. / Journal of Arid Environments 112 (2015) 87e97 clw.csiro.au/publications/waterforahealthycountry/mdbsy/technical/VWaterBalanceEstimates.pdf. Hochmuth, H., Thevs, N., He, P., Water allocation and water consumption of irrigation agriculture and natural vegetation in the Heihe River watershed, NW China, Environ. Earth Sci. (Submitted for publication). Hao, X.M., Chen, Y.N., Li, W.H., 2009. Impact of anthropogenic activities on the hydrologic characters of the mainstream of the Tarim River in Xinjiang during the last past 50 years. Environ. Geol. 57, 435e445. Hao, X.M., Chen, Y.N., Li, W.H., Guo, B., Zhao, R.F., 2010. Hydraulic lift in Populus euphratica Oliv. from the desert riparian vegetation of the Tarim River Basin. J. Arid Environ. 74 (8), 905e911. Hou, L.G., Xiao, H.L., Si, J.H., Xiao, S.C., Zhou, M.X., Yang, Y.G., 2010. Evapotranspiration and crop coefficient of Populus euphratica Oliv forest during the growing season in the extreme arid region northwest China. Agric. Water Manag. 97, 351e356. Howell, T.A., 2001. Enhancing water use efficiency in irrigated agriculture. Agron. J. 93, 281e289. ICWC (Interstate Commission for Water Coordination of Central Asia), 1992. Agreement of five Central Asian States. http://www.icwc-aral.uz/statute1.htm (accessed 02.12.12). Khamzina, A., Sommer, R., Lamers, J.P.A., Vlek, P.L.G., 2009. Transpiration and early growth of tree plantations established on degraded cropland over shallow saline groundwater table in northwest Uzbekistan. Agric. For. Meteorol. 149, 1865e1874. Liu, M.G., 1997. Atlas of Physical Geography of China. China Map Press, Beijing. Malek, E., Bingham, G.E., 1993. Comparison of the Bowen ratio-energy balance and the water balance methods for the measurement of evapotranspiration. J. Hydrol. 146, 209e220. Molle, F., 2009. Water scarcity, prices and quotas: a review of evidence on irrigation volumetric pricing. Irrigat. Drain. Syst. 23, 43e58. Murray Darling Basin Commission, 2006. Murray Darling Basin Agreement. http:// www.comlaw.gov.au/Details/C2011C00160/Html/Text#_Toc289261269 (accessed 03.12.12). Peng, H.Y., Thevs, N., Ott, K., 2014. Water distribution in the perspectives of stakeholders and water users in the Tarim River Catchment, Xinjiang, China. J. Water Resour. Protect. http://dx.doi.org/10.4236/jwarp.2014.66053. € Protze, M., 2011. Baumwollanbau im Tarim-Becken (China) e Okonomie und Wasserrechte. Diploma Thesis. University of Greifswald, Greifswald, Germany. Reddy, J.M., Muhammedjanov, S., Jumaboev, K., Eshmuratov, D., 2012. Analysis of cotton water productivity in Fergana Valley of Central Asia. Agric. Sci. 3, 822e834. Roerink, G.J., Su, B., Menenti, M., 2000. S-SEBI A simple remote sensing algorithm to estimate the surface energy balance. Phys. Clim. Earth B 25, 147e157. Sammis, T.W., 1981. Yield of alfalfa and cotton as influenced by irrigation. Agron. J. 73, 323e329. Scott, R.L., Cable, W.L., Huxman, T.E., Nagler, P.L., Hernandez, M., Goodrich, D.C., 2008. Multiyear riparian evapotranspiration and groundwater use for a semiarid watershed. J. Arid Environ. 72, 1232e1246. Senay, G.B., Leake, S., Nagler, P.L., Artan, G., Dickinson, J., Cordova, J.T., Glenn, E.P., 2011. Estimating basin scale evapotranspiration (ET) by water balance and remote sensing methods. Hydrol. Process. 25, 4037e4049. Settle, J.J., Drake, N.A., 1993. Linear mixing and the estimation of ground cover proportions. Int. J. Remote Sens. 14, 1159e1177. Shen, Y.J., Chen, Y.N., 2010. Global perspective on hydrology, water balance, and water resources management in arid basins. Hydrol. Process. 24, 129e135. Siebert, S., Hoogeveen, J., Frenken, K., 2006. Irrigation in Africa, Europe and Latin America. Update of the Digital Global Map of Irrigation Areas to version 4, Frankfurt Hydrology Paper 5. Institute of Physical Geography, University of Frankfurt, Frankfurt am Main, Germany. mez, M., Jime nez-Mun ~ oz, J.C., Olioso, A., Chehbouni, G., 2005. A Sobrino, J.A., Go simple algorithm to estimate evapotranspiration from DAIS data: application to the DAISEX campaigns. J. Hydrol. 315, 117e125. mez, M., Jime nez-Mun ~ oz, J.C., Olioso, A., 2007. Application of a Sobrino, J.A., Go simple algorithm to estimate daily evapotranspiration from NOAAeAVHRR images for the Iberian Peninsula. Rem. Sens. Environ. 110, 139e148. Song, Y.D., Fan, Z.L., Lei, Z.D., Zhang, F.W., 2000. Research on Water Resources and Ecology of Tarim River, China. Xinjiang Peoples Press, Urumqi, China (in Chinese).

97

Steduto, P., Hsiao, T.C., Fereres, E., Raes, D., 2012. Crop Yield Response to Water. FAO Irrigation and Drainage Paper 66. FAO, Rome. Su, Z., 2002. The Surface Energy Balance System (SEBS) for estimation of turbulent heat fluxes. Hydrol. Earth Syst. Sci. 6, 85e99. Tan, X.W., Zhou, H.Y., 2007. Integrated management and hydroelectric development in the Tarim river basin. Collection of Papers for the First Xinhua Forum of Hydroelectric Development and Green Future. Beijing, February 2007 (in Chinese). Tang, D.S., Deng, M.J., 2010. On the Management of Water Rights in the Tarim River Basin. China Water Power Press, Beijing (in Chinese). Thevs, N., 2011. Water scarcity and allocation in the Tarim Basin: decision structures and adaptations on the local level. J. Curr. Chin. Aff. 3, 113e137. Thevs, N., Zerbe, S., Gahlert, F., Mijit, M., Succow, M., 2007. Productivity of reed (Phragmites australis Trin. ex. Staud.) in continental-arid NW China in relation to soil, groundwater, and land-use. J. Appl. Bot. Food Qual. 81, 62e68. Thevs, N., Zerbe, S., Peper, J., Succow, M., 2008. Vegetation and vegetation dynamics in the Tarim River floodplain of continental-arid Xinjiang, NW China. Phytocoenologia 38, 65e84. Thevs, N., Buras, A., Zerbe, S., Kühnel, E., Abdusalih, N., Ovezberdyyeva, A., 2012. Structure and wood biomass of near-natural floodplain forests along the Central Asian rivers Tarim and Amu Darya. Forestry 81, 193e202. Thomas, F.M., Foetzki, A., Arndt, S.K., Bruelheide, H., Gries, D., Zeng, F.J., Zhang, X.M., Runge, M., 2006. Water use by perennial plants in the transition zone between river oasis and desert in NW China. Basic Appl. Ecol. 7, 253e267. TMB (Tarim Management Bureau), 2005. Scheme of surface water distribution in the tarim river basin (four source streams and one mainstream). http://www. tahe.gov.cn/e/action/ShowInfo.php?classid¼106&id¼7375 (accessed 12.08.12). USDA, 2012. China e Peoples Republic of cotton and products annual. GAIN Report Number: CH12031. http://www.thefarmsite.com/reports/contents/ chinacotmay12.pdf (accessed 30.12.13). Waters, R., Allen, R.G., Tasumi, M., Trezza, R., Bastiaanssen, W., 2002. SEBAL Surface Energy Balance Algorithms for Land e Idaho Implementation e Advanced Training and Users Manual. University of Idaho, USA. Wu, B.F., Jiang, L.P., Yan, N., Perry, C., Zeng, H.W., 2013. Basin-wide evapotranspiration management: concept and practical application in Hai Basin, China. Agricultural Water Management. http://dx.doi.org/10.1016/j.agwat.2013.09.021. Xia, D.K., 1998. Changing and water resource of the Tarim River in Xinjiang. J. Arid Land Resour. Environ. 12, 7e14 (in Chinese). Xinjiang Linkeyuan Zaolin Zhisha Yanjiusuo, 1989. Huyanglin Gengxin Fuzhuang Jishu Yanjiu. Xinjiang Linkeyuan Zaolin Zhisha Yanjiusuo, Urumqi (in Chinese). Xinjiang Statistics Bureau, 2011. Xinjiang Statistical Yearbook of 2010. China Statistics Press, Beijing. Xu, H.L., Ye, M., Song, Y.D., 2005. The dynamic variation of water resources and its tendency in the Tarim River Basin. J. Geogr. Sci. 15, 467e474. Xu, Z.X., Liu, Z.F., Fu, G.B., Chen, Y.N., 2010. Trends of major hydroclimatic variables in the Tarim River basin during the past 50 years. J. Arid Environ. 74, 256e267. Ye, Z.X., Chen, Y.N., Li, W.H., Yan, Y., Wan, J.H., 2009. Groundwater fluctuations induced by ecological water conveyance in the lower Tarim River, Xinjiang, China. J. Arid Environ. 73 (8), 726e732. Zhang, J., 2005. Barriers to water markets in Chinas Heihe River basin. Res. Rep. Econ. Environ. Progr. Southeast Asia. No. 2005-RR8. Zhang, J.B., 2006. Water management issues and legal framework development of the Tarim river basin. In: Wallance, J., Wouter, P. (Eds.), Hydrology and Water Law e Bridging the Gap: a Case Study of HELP Basins. IWA Publishing, London, pp. 108e142. Zhang, H., Wu, J.W., Zheng, Q.H., Yu, Y.J., 2003. A preliminary study of oasis evolution in the Tarim Basin, Xinjiang, China. J. Arid Environ. 55, 545e553. Zhang, Y.M., Chen, Y.N., Pan, B.R., 2005. Distribution and floristics of desert plant communities in the lower reaches of Tarim River, southern Xinjiang, People's Republic of China. J. Arid Environ. 63 (4), 772e784. Zhu, Z.P., Giordano, M., Cai, X.M., Molden, D., Hong, S.C., Zhang, H.Y., Lian, Y., Li, H., Zhang, X.C., Zhang, X.H., Xue, Y.P., 2003. Yellow River comprehensive assessment: basin features and issues. IWMI and YRCC. http://www.iwmi.cgiar.org/ Publications/Working_Papers/working/WOR57.pdf (accessed 02.12.12).