The impacts of human activities on the water–land...

Hydrological Sciences–Journal–des Sciences Hydrologiques, 49(3) June 2004

Open for discussion until 1 December 2004

413

The impacts of human activities on the water–land environment of the Shiyang River basin, an arid region in northwest China

SHAOZHONG KANG1,2, XIAOLING SU2, LING TONG2,PEIZE SHI3, XIUYING YANG3, YUKUO ABE4, TAISHENG DU1

QINGLIN SHEN3 & JIANHUA ZHANG5

1 Center for Agricultural Water Research in China, China Agricultural University, Beijing 100083, [email protected]

2 Key Lab for Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Ministry of Education, Northwest Sci-Tech University of Agriculture and Forestry, Yangling, Shaanxi 712100, China

3 Bureau of Water Resources Management, Wuwei, Gansu 733000, China 4 Institute of Agricultural and Forest Engineering, University of Tsukuba, Tsukuba 305-8572,

Japan 5 Department of Biology, Hong Kong Baptist University, Hong Kong

[email protected]

Abstract The Shiyang River basin is a typical interior river basin that faces water shortage and environmental deterioration in the arid northwest of China. Due to its arid climate, limited water resources and some inappropriate water-related human activities, the area has developed serious loss of vegetation, and gradual soil saliniza-tion and desertification, which have greatly impeded the sustainable development of agriculture and life in this region. In this paper, the impacts of human activities on the water–soil environment in Shiyang River basin are analysed in terms of precipitation, runoff in branches of the river, inflow into lower reaches, water conveyance efficiency of the canal system and irrigation water use efficiency in the field, replenishment and exploitation of groundwater resources, soil salinization, vegetation cover and the speed of desertification. The results show that human activities and global climate change have no significant influence on the precipitation, but the total annual runoff in eight branch rivers showed a significant decrease over the years. The proportion of water use in the upper and middle reaches compared to the lower reach was increased from 1:0.57 in the 1960s, to 1:0.27 in the 1970s and 1:0.09 in the 1990s. A reduction of about 74% in the river inflow to the lower reaches and a 15-m drop in the groundwater table have occurred during the last four decades. Strategies for improving the water–soil environment of the basin, such as the protection of the water resources of the Qilian Mountains, sustainable use of water resources, maintenance of the balance between land and water resources, development of water-saving agriculture, diverting of water from other rivers and control of soil desertification, are proposed. The objective of this paper is to provide guidelines for reconstruction of the sustainable water management and development of agriculture in this region. Key words human impacts; water–soil environment; salinization; desertification; groundwater table; water-saving; sustainable agriculture; arid region

Les impacts des activités humaines sur l’environnement pédo-hydrologique du bassin de la Rivière Shiyang, une région aride du nord-ouest de la Chine Résumé Le bassin de la Rivière Shiyang est un bassin versant intérieur typique du nord-ouest aride de la Chine, qui est confronté à un déficit en eau et à une détérioration de l’environnement. A cause de son climat aride, de ses ressources en eau limitées et de quelques activités humaines inappropriées, la zone a subi des pertes de végétation sérieuses, ainsi qu’une salinisation des sols et une désertification progressives, qui ont fortement entravé le développement durable de l’agriculture et de la vie dans la région. Nous analysons les impacts des activités humaines sur

Shaozhong Kang et al. 414

l’environnement pédo-hydrologique dans le bassin de la rivière Shiyang, en termes de précipitations, d’écoulement dans les affluents, d’écoulement vers les parties aval, d’efficience de transport de l’eau dans le système de canaux, d’efficience au champ de l’utilisation de l’eau d’irrigation, de renouvellement et d’exploitation des ressources en eau souterraines, de salinisation des sols, de couvert végétal et de vitesse de désertification. Les résultats montrent que les activités humaines et que le changement climatique global n’ont pas d’influence significative sur les précipitations, alors que l’écoulement annuel total a diminué de manière significative au fil des ans, pour huit affluents. La proportion de l’utilisation de l’eau dans les parties amont et intermédiaires par rapport aux parties aval a augmenté de 1:0.57 dans les années 1960, à 1:0.27 dans les années 1970 et 1:0.09 dans les années 1990. Une réduction d’environ 74% de l’écoulement vers les parties aval et une baisse de 15 m du niveau de la nappe ont eu lieu lors des quatre dernières décennies. Des stratégies sont proposées pour améliorer l’environnement pédo-hydrologique du bassin, comme la protection des ressources en eau des Montagnes Qilian, l’utilisation durable des ressources en eau, le maintien de l’équilibre entre les ressources en terres et en eau, le développement d’une agriculture économe en eau, le transfert d’eau depuis d’autres rivières et le contrôle de la désertification. Le but de cet article est de fournir des règles pour reconstruire une gestion de l’eau et un développement de l’agriculture durables dans cette région. Mots clefs impacts anthropiques; environnement pédo-hydrologique; salinisation; désertification; niveau piézométrique; agriculture durable; région aride

INTRODUCTION

Human activities have had serious impacts on the environment in the Shiyang River basin in the Hexi Corridor of northwest China, where water use by agriculture and industry has increased very rapidly in the last five decades. The groundwater table in the lower reaches of the basin has been falling steadily over the years and environ-mental deterioration is intensifying. The desert is spreading to some oases (Chen, 1995). At the same time, water resources are poorly managed, especially for irrigation. Studies have shown that agricultural water use efficiency (WUE) in this area is very low (Kang et al., 1996). Striving towards sustainable development, it is important to analyse the consequences of water-related human activities and to improve the existing water management practice. The Shiyang River is one of the three continental rivers in the Hexi Corridor, lying in the east of the corridor, in Gansu province, north of the Qilian Mountains and between the Badanjilin Desert and the southern part of the Tenggeli Desert (Fig. 1). The river basin covers the area between 101°41′–104°16′Ε and 36°29′–39°27′Ν. It occupies an area of 4.16 × 104 km2 and has the most serious water shortage in the Hexi Corridor. Administrative divisions include seven counties with a total population of 2.2 million (Kang et al., 1996; Shi et al., 1998). The Shiyang River starts in the Qilian Mountains and includes eight tributaries, from east to west: the Dajing, Gulang, Huangyang, Zamu, Jinta, Xiying, Dongda and Xida rivers (Fig. 1). The average annual runoff is about 15.75 × 108 m3. The river is mainly fed by rainfall, snowmelt and glacier melt in the Qilian Mountains. The annual runoff of the rivers is relatively steady (Chang, 1994; Wang, 1998). The Qilian Mountains in the south of the basin (elevation: 2000–5000 m a.m.s.l.) comprise a very frigid, semiarid and humid area, with annual precipitation and potential evaporation at the level of 300–600 mm and 700–1200 mm, respectively, and a drought index (DI—the ratio of potential evaporation to precipitation) of 1–4. The middle part of the basin is the corridor-plain region, cool and arid, with an elevation of 1500–2000 m, annual precipitation and potential evaporation at 150–300 mm and 1200–2000 mm, respectively, and DI of 4–15. The northern part of the basin is warmer

The impacts of human activities on the water–land environment of the Shiyang River basin 415

Fig. 1 Sketch map of the distribution of eight branches in Shiyang River basin, showing isolines of the average annual precipitation.


and more arid, with an elevation of 1300–1500 m, annual precipitation less than 150 mm and potential evaporation about 2000–2600 mm and DI of 15–25. Due to different natural conditions such as climate, soil, hydrology and topography, the Shiyang basin has three distinct natural landscapes. The first is the corridor plain on the south of Badanjilin and Tenggeli deserts. This area has a desert vegetation landscape, apart from some oases, with an agricultural ecological system. Representative plants include Nitraria sibirica, Salsola passerina, Eriocaulon truncatum Ham, Reaumuria soongorica Pall. maxim, Peganum harmala, Artemisiaarenaria and Agriophyllum squarrosum. The second area comprises low mountains and hill belts with a vegetation type between low grassland and dry grassland. Representative plants include Achnatherum, Brylkinia caudate Fr. Schmidt, Artemisiafrigida Willd, Iris tenuifolia Pall., Stipa purpurea cloustouii Edm. and Stipa breviflorn Griseb. The third area comprises the Qilian Mountains with bush and grassland crisscross, and mountain forest and mountain meadow on the slopes. Representative plants include Populus davidiana Dode, Betulaceae, Pinus tabulaeformis Carr., Carex chinensis Retz., Artemisia L., Poa annua L., Aster tataricus L.f., Potentilla chinensis Ser. and Polygonum L. (Shi et al., 1998). The regional soils are: alpine frigid desert soil, alpine meadow soil, subalpine meadow soil, mountain gray-cinnamonic soil, mountain black soil and mountain chestnut soil in the Qilian Mountains area. In the oasis plain and northern desert areas, the zonal soils are sierozem, grey desert soil and grey-brown desert soil. In addition, saline soil, meadow soil and marshy soil are distributed in some small areas. The cultivated land per capita is 0.13 ha. The main planted crops include wheat, melons, cotton, maize, beet, vegetables and fruit trees. The natural ecology in the Shiyang River basin is fragile, easily destroyed and difficult to restore. Agriculture is a major user of water resources in the basin. It is essential to reduce water consumption by agriculture and to provide water for environmental needs. However, a clear picture of the impacts of human activities on the water–soil environment in this region is still lacking. Progress has been made in recent years with respect to arid environment and water resources utilization, hydrological processes and response to water stress (Tennant, 1976; Covich, 1993; Qi & Cheng, 1998; Song et al., 2000; Fan et al., 2001; Laio et al., 2001a,b; Porporato et al., 2001; Qi et al., 2001; Rodriguez-Iturbe et al., 2001; Ruochuan & Mark, 2001; Shen, 2001; George et al., 2002), but these works are not necessarily applicable to the special conditions of the Shiyang River basin. This study reports the analysis of the impacts of water-related human activity on the water–soil environment, based on the data from the Shiyang River basin. Some comprehensive measures are presented that may improve the water–soil environment in this area.

WATER-RELATED HUMAN ACTIVITIES

Water resource use in the Shiyang River basin has been changing due to the development of agriculture. An increase in farmland and traditionally irrigated land both intercepts and consumes a great deal of water. At present, water use in the middle and lower reaches is intensifying. The groundwater table and availability of spring water are continuously declining. The middle and upper reaches of the Shiyang River


basin are adjacent to water sources in the Qilian Mountains, and this is where in the last 50 years 23 water control projects have been built. At present, these projects have a total storage of 4.5 × 108 m3, useful storage of 3.48 × 108 m3 and sediment storage of 0.49 × 108 m3. All of the eight tributaries, except the Zamu River, have reservoirs which have played a role in controlling and regulating the runoff. In the middle reaches, canal systems have a high standard of lining, which has made water diversion efficiency relatively high. Moreover, in the middle reaches the irrigated area has been excessively extended, resulting in drastic reduction of inflow to the lower reaches in the past 50 years. With the continuous increase in population and demand for food and other agricultural products, water-saving irrigation has been promoted in the basin to improve irrigation water conveyance efficiency (WCEI, the ratio of discharge in the last fixed canal reach and the inflow to the first canal) and irrigation water use efficiency (WUEI, the ratio of the outflow from the last fixed canals to the field and the change in storage in soil layer of root depth) and reduce water consumption. From the 1950s to the 1960s, four water-diversion canals were built around the city of Wuwei. The main and branch canals (995.71 km) were lined with cobbles to control seepage and improve the efficiency of irrigation water conveyance. From the 1970s, the irrigation canal system in this region has been rebuilt. Now, there are 22 main canals, lined with cement–cobble and concrete, with a total length of 286.28 km; 271 cement-lined branch canals (total length 730.57 km); 2145 corresponding lateral canals (1683.61 km); and 9058 field well ditches (4582.25 km). An area of 2.576 ×104 ha of farmland supplied by the canals was level-led. The transformation of 6.2 km low-pressure delivery pipeline, rebuilding 1381 old wells and 2337 water-collecting cellars were finished in 1990s. Sprinkle irrigation (1130 ha), drip and subsurface drip irrigation (93 ha), and pipe irrigation (113 ha) were developed. Such new canal systems and field irrigation projects have greatly reduced percolation and improved both the efficiency of water conveyance of the canal system and irrigation water use efficiency in the field. Up to now, 81% of the main canals, 75% of the secondary canals, and 75% of branch canals have been lined. The WCE of the canal system in the irrigated district of the basin has been increased from 0.385 in the 1950s, 0.482 in the 1960s, 0.56 in the 1970s, 0.58 in the 1980s to 0.62 now, while the WUE in the Jinta River irrigation district is now 0.65, in the Xiying River irrigation district 0.632, and in the Huangyang River irrigation district 0.566. Before the secondary canals and field ditches were lined, the water consumption at the check gate was 1410 m3 ha-1; since then it has fallen to 1290 m3 ha-1. Thus, as a result of canal lining in the river water irrigation district, 3350 × 104 m3 surface water was saved every year and could be used to irrigate an extra 1.713 × 104 ha of farmland. However, the replenishment of ground-water was also reduced. The ratio of canal system to well irrigation has been increased from 0.50 to 0.79, a net reduction of 5000 × 104 m3 each year in the well irrigated district. The basin has also developed some water-saving irrigation techniques (e.g. border irrigation, furrow irrigation, drip irrigation, low pressure pipeline irrigation and sprinkler irrigation) to reduce the irrigation amount for crops. For example, at present, in the Xiying River irrigation district, irrigation for sowing is about 1500–1800 m3 ha-1,for the growing season of summer cereal crops it is about 4050 m3 ha-1, for autumn


cereal crops 5175 m3 ha-1 and for industrial crops 3057 m3 ha-1. By adopting water-saving irrigation, these can be reduced by about 15–25%. Nevertheless, the savings in water made by improving the conveyance of irrigation water in canals and efficiency of water use in farmlands are not transferred to the lower reaches to satisfy the ecological and agricultural water requirements there. The irrigated areas of the upper and middle reaches are still expanding.

STUDY METHODS

Several systematic surveys were carried out on the water resources, ecological changes and water use situation in the Shiyang River basin during 1984–2001. The surveys included the investigation and recording of annual rainfall, river inflow and water consumption in all the parts concerned, the changes in available water resources in the lower reaches, the annual irrigated acreages and irrigation schedules, water use effic-iency for the canal systems, the planted acreages for different crops, the tillage and cultivating measures, crop yield, the change of water consumption under the water-saving irrigation, water consumption of different crops, the changes in groundwater tables and quality, the growth conditions of desert plants and ecological groundwater tables in the region. There are 40 rainfall stations, which recorded the rainfall data continuously from 1956 to 1995. The data from these 40 stations were used to calculate the average annual precipitation, P , in the Shiyang River basin as follows:

��==

⋅=40

1

40

1/

iii

ii AAPP (1)

in which Pi is the annual precipitation at station i and Ai is the area controlled by station i. Evaporation was measured by the Φ 20 pan (the diameter at 200 mm). Runoff in the rivers and inflow to Hongyanshan Reservoir (the last one in the lower reach) were recorded by standard hydrological section flow measurements. The groundwater table was measured by test wells distributed throughout the basin. The WCEI in the canal system was estimated as:

�=

=n

iiI QQWCE

10/ (2)

where Qi is discharge in the last fixed canal reach (m3 s-1), Q0 is inflow to the first canal (m3 s-1), both measured by standard weirs of flow measurement, and i is the number of the last fixed canals in an irrigation district. The WUEI was estimated as:

siI WWWUE /= (3)

where Wi is the outflow from the last fixed canals to the field and measured by standard flow measurement weir and Ws is the change in storage in soil layer (of root depth) and estimated by soil water profile measurements before and after each irrigation.


CHANGES IN THE WATER–SOIL ENVIRONMENT

Changes in precipitation and runoff

The average annual rainfall distribution is illustrated in Fig. 1, and its variation during 1956–1995 is illustrated in Fig. 2. The annual precipitation has not changed signifi-cantly in the last 50 years. The surface runoff of eight tributaries at the foot of the Qilian Mountains was affected by fluctuations in the hydrological cycle. Regression analysis of the runoff series has shown that the total annual runoff in the eight tributaries has significantly decreased (Fig. 3). The tendency line is Q = –0.0464x + 106.24, where x is the year, and the correlation coefficient is –0.2339, the significance level is 0.10. The data illustrate that water-related human activity has had a significant impact on some branches of the Shiyang River.

100

150

200

250

300

350

400

1955 1965 1975 1985 1995

Year

Ann

ual p

reci

pita

tion

mm

)

precipitationequalline(281.2)

Fig. 2 The variation in annual average precipitation in the Shiyang River basin in the past 50 years. The average annual precipitation (281.2 mm) is shown as a straight line.

y = -0.0464x + 106.24R2 = 0.0547

9

12

15

18

21

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Year

Annu

al to

tal r

unof

f10

8 m3 /y

ear)

trendline

Fig. 3 The variation in total annual runoff of eight branch rivers in the Shiyang River basin. A linear regression line (Q = –0.0464x + 106.24, R = –0.2339) is shown as a trend line.


y=5E+30e-0.0352xR2=0.8724

0

1

2

3

4

5

6

7

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Year

Annu

al ru

noff

108 m

3 /yea

r)trendline

Fig. 4 The variation of inflow into Hongyanshan reservoir in the past 50 years. The regression equation (equation (4)) is plotted as the trend line (dotted line).

The inflow into Hongyanshan Reservoir in the lower reach of the basin has reduced rapidly. The annual inflow into Hongyanshan Reservoir (108 m3 year-1) (Y)has decreased in time in 1956–2000 (cf. Fig. 4):

xY -0.035230 e105×= (R = –0.8724) (4)

where x is time in years. As a result of such large-scale diversion, the proportion of water use in the upper and middle reaches compared to the lower reach (in the Minqin area) increased from 1:0.57 in the 1960s, to 1:0.27 in the 1970s and 1:0.09 in the 1990s.

Changes in groundwater and water table

Because water consumption in the middle and upper reaches has increased greatly, the inflow to the lower reaches has reduced abruptly. As a result, the groundwater exploitation has increased because farmers want to keep their farmland irrigated. Groundwater is in an over-used condition and the water table has fallen continuously. At the same time, because canals have been lined to a high standard and water-saving technologies used, the groundwater recharge has become marginal. The replenishment and exploitation of groundwater are not balanced in the region. The annual natural replenishment of groundwater in the plain region of the Shiyang River basin has dropped gradually from 15.87 × 108 m3 in the 1950s to 9.24 ×108 m3 in the 1980s. The corresponding annual groundwater exploitation has grown from 1.5 × 108 m3 in the 1950s to 9.8 × 108 m3 in the 1980s, 11.16 × 108 m3 in 1995. In the 1990s the natural replenishment of groundwater was reduced by 53% and exploitation increased by 86% compared with the 1950s, as shown in Fig. 5. Since the 1970s, groundwater was exploited and utilized on a large scale in the Shiyang River basin, especially from the late 1980s, because water consumption in the middle and upper reaches of the basin increased. The water in the lower reach has became scarce, leading to an upsurge in drilling wells. Thus, the groundwater table fell


0

5

10

15

20

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Year

Amou

nt o

f gro

undw

ater

repl

enis

hed

and

expl

oite

d

108 m

3 /yea

r)

Amount replenishedAmount exploited

Fig. 5 The amounts of ground water replenished and exploited in the Shiyhang River basin over the last 40 years.

0

2

4

6

8

10

12

14

16

1960 1965 1970 1975 1980 1985 1990 1995 2000

Year

Dep

th o

f gro

undw

ater

tabl

em

)

Fig. 6 The variation of ground water table in Shajingzi, Minqin over the last 35 years.

continuously. Figure 6 illustrates the depth of the groundwater table in Shajingzi, Minqin, between 1961 and 2000. Up to now, the groundwater table in Wuwei basin (upper reach) has fallen with the rate of 0.25 m year-1, while in Minqin (lower reach), the drop rate was 0.39 m year-1. The rate of fall of the groundwater table has accelerated. According to the well distribution survey, there are 14 200 pump wells in the whole basin. Among them, 8853 wells are in Minqin, with 271 drilled wells of depth exceeding 300 m in Minqin lake district for agriculture, livestock and human water consumption up to October 2000. Water quality analysis for the deep wells shows that 70–73% of them have mineralization below 2.0 g l-1 while 10–12% have more than 3.0 g l-1. On average, the water table has fallen by 0.4–0.8 m per year, and the degree of mineralization of the water has grown by 0.24 g l-1 per year.

Change in soil salinization in the lower reaches

The area of soil salinization in Minqin has considerably grown from 1985 to 1995. Because of lacking water in the lower reach region, much farmland has been


abandoned. As a result of lack of flushing, salinization has become more serious in the Minqin lake district. In the past, the land was flooded once a year to infiltrate salt to deeper layers and make the top layer croppable. With the year-by-year reduction of inflow to the Hongyanshan Reservoir, 1.33 × 104 ha of farmland in the Minqin district has become desert. Soil survey data have shown that the salt content in the 0–0.1 m layer of soil is greater than 10%, 14 times higher than in the 0–0.4 m soil layer in the adjacent and un-deserted irrigated farmland. In recent years, salinization in the Minqin lake district has extended to the south and became more serious. In Shoucheng village, the area of cultivated land that failed to yield due to salinization was 26.67 ha in 1990, and increased to 53.33 ha in 1991.

The accelerated desertification

The reduction of available water in the lower reach region and the fall of the groundwater table have had a serious impact on the desert vegetation. The area of natural bush forest at the edge of the oasis and the front zone of desert decreased from 7.24 × 104 ha in the 1950s, to only 2.373 × 104 ha now. Furthermore, 0.753 × 104 ha of manmade shrub forest has died among the 2.742 × 104 ha afforested over the years. The vegetated area fell drastically from a level of 44.8% in the 1950s to 15% at present. Of the 7.33 × 104 ha plantation Elaeagnus angustifolia and Haloxylon ammodendron forest, which surrounds the existing Minqin oasis, 70% has desertified completely. The area of natural desert shrub forest in Minqin county has been reduced from 13.33 × 104 ha in the 1950s, to present 7.33 × 104 ha, of which 3.6 × 104 ha is degenerating, 1.3 × 104 ha is desertifying and only less than 1.3 × 104 ha is in good condition. The natural grassland near the desert covers 20 × 104 ha and has been reduced from 30% in the 1950s to less than 10% at present. The dune forming, soil desertification and declining of vegetation at the edge of the oasis area have led to the loss of the natural sand-fixation barrier and opened one section after another to the expanding desert. The wind erosion has became more intense. The desert is spreading and the dunes in the main path of sand storms have advanced at a velocity of 10 m per year on average. According to surveys, because of the degeneration of the sand-fixation and wind-break vegetation, the sand damage is very serious in the east of the Minqin lake district. In the last five years, 2.67 ha of cultivated land was covered by sand each year and desertified at Quanshan town. In 1991, 30 ha farmland was buried in sand at Hongshaliang village and another 45.50 ha of cultivated land is under desertification. Due to sand storm disasters and bad water quality, many people who had lived near the desert have migrated, leaving the towns and villages empty.

Relationship between plant growth and groundwater table in Minqin

Plant growth in the arid desert region of Minqin is controlled generally by the groundwater table. Elaeagnus angustifolia is one of the main tree varieties in the desert area of the Shiyang River basin. It is a shallow-rooted tree whose root length is generally less than 4.5 m. Study of Elaeagnus angustifolia in Minqin shows that the


Table 1 The effects of groundwater table on the growth of Elaeagnus angustifolia, Popular diversifolia, and Nitraria tangutorum in Minqin area. Plants were grown for about 5–6 years.

Plants Depth of groundwater table (m)

Growth Degree of land desertification

2–3 Grew normally None 3–5 Grew badly, a few of them degenerated and died Slight 5–6 Many of them degenerated and died Moderate

Elaeagnus angustifolia

>6 Many of them died Severe <4 Grew badly None 4–6 Become bald, leaf wilted, a few of them wilted and died Slight 6–10 Many of them degenerated and died Moderate or

heavy

Popular diversifolia

>10 All of them died Severe <5 Grew normally None 5–7 Growth degenerated, a few of them wilted and died Slight 7–8 Severely degenerated, many of them wilted and died Moderate

Nitraria tangutorum

8–10 All of them died Severe

annual diameter increment was 1.5–2.0 mm when the water table was 5 m deep; when the water table fell to 6–7 m, the trees withered and died after the water around them was used up. In the 1950s and 1960s, the depth of groundwater table in most of Minqin lake district was between 1 and 3 m, and less than 1 m in the northern lake district. The soil was usually under moist and even soaked conditions. Plants grew well. Up to the 1970s, the groundwater table began to fall gradually; herbs, which rely on shallow layer moisture, died first, then the Elaeagnus angustifolia and bush died. Up to 1996, in the Minqin oasis area, the groundwater table fell below 7 m and in the south of the area – below 10 m. The impact of groundwater table depth on plant growth is shown in Table 1. When the groundwater table is too deep, the root system may fail if it cannot reach the moist zone; when the table is too high, the root system may also die gradually because intense evaporation may lead to high salt accumulation in the root zone and soil surface. Because of the differences in plant root systems and their capability to endure salt, each plant species has its own preferred groundwater table for growth, called the “optimally ecological water level”. The primary analysis in the region showed that the optimal groundwater table for growth of Popular diversifolia is 4 m, for Haloxylon ammodendron 3–4 m, for Nitraria sibirica 5 m, for Elaeagnus angustifolia 3 m and for Achnatherum splendens 1–2 m.

STRATEGIES FOR IMPROVING THE WATER–SOIL ENVIRONMENT OF THE BASIN

Protection of the water resources of Qilian Mountains

Protecting the glacier and the water resources in the Qilian Mountains is most urgent. Integrated management and protection of water resources and control of soil erosion are needed as well as measures to curb deforestation. Water resources need to be


allocated according to the proportion of population, agriculture and industry in different regions, while meeting the environmental water needs in the whole basin. The area for cropping should be limited strictly by the available water resources. This calls for curbing the growth of cultivated area, controlling exploitation of groundwater, and licensing of water withdrawal.

Developing water-saving agriculture

Developing water-saving agriculture and improving irrigation water use efficiency are essential for sustainable development of agriculture and environmental protection in the Shiyang River basin. A water-saving crop system and crop structure adjustment are necessary. Regulated deficit irrigation, which is a good water-saving method (Kang et al., 2000a), should be extended. The alternative furrow irrigation has much better effect for water-saving than traditional furrow irrigation according to 4 year experiments in Xiaobakou of Minqin. The seasonal water consumption of maize in alternative furrow irrigation was reduced about 30% when compared to conventional furrow irrigation, while grain yield was largely maintained (Kang et al., 2000b,c), it should be largely extended in this region. There is no potential for a wide spread of the sprinkle irrigation in the Shiyang River basin. However, low-pressure pipeline irrigation in certain area and drip irriga-tion in orchards deserve being considered. The experiments in Wuwei showed that the irrigation WUE for pipeline irrigation method reached 0.944, i. e. by 0.286 more than for open canal irrigation. Moreover, some super absorbent polymers, surface moisture covers and so on, could be adopted, where appropriate to help seeds germinate and establish, and to reduce water loss from soil or plants. The cropping proportion of summer cereals, autumn cereals and industrial crops should be adjusted according to the annual runoff distribution. Drought-resistant varieties should be cultivated. The saved water in this region should be conveyed firstly to the lower reach to satisfy the environmental water requirement in the Minqin lake district, rather than to expand irrigated area in upper reaches. Compensation mechanisms for water-saving by farmers could be devised, such as supporting the canal lining, and using advanced irrigation method in upper and middle reaches.

Diverting water from other rivers

The current water resources in Shiyang River basin are not sufficient for the current demand of industry, agriculture and economic development. For a typical year, when probability of exceedence p = 50% of annual rainfall, the total demand of water is 29.43 × 108 m3, but the total available amount is 26.61 × 108 m3 (including surface and ground water resources). The total water shortage is therefore 2.82 × 108 m3. When p = 75%, the total available can be 24.92 × 108 m3, and the total shortage will be 4.51 × 108 m3. When 4 × 108 m3 of excessively exploited groundwater is added, the shortage amount of water resources reaches 6.82 × 108 m3 (p = 50%) or 8.51 × 108 m3


(p = 75%). If the whole area of 28.52 × 104 ha land in the basin adopts the measures of limiting irrigation quota and water saving irrigation technologies, net irrigation requirement is estimated as 3750 m3ha-1. And if the water conveyance efficiency for canal system is improved as 0.62, the water savings by agriculture in Shiyang River basin can be of the order of 8 × 108 m3. That is, the water demand cannot be met only in drought years. The shortage of water in the region may be relieved by inter-basin diversion and a diversion project from Liutiao River (at Gulang in Fig. 1) to Hongshui River is being built at present. The Jingdian project (at Jingtai in Fig. 1), which is lifting and diverting water from the Yellow River, can transfer 1.0 × 108 m3 water to Hongyashan reservoir in Minqin and solve the problem of water shortage for 1 × 104 ha land, although it cannot solve the water shortage problem in the whole Shiyang River basin. More projects have been envisaged to solve the region’s water shortage, such as diverting water from the Da Tong River in Qinghai province to the Xida River.

Control of soil desertification

Measures to control desertification must be adopted according to local conditions. With the moving sand dunes, barriers made of straw, clay or small branches can be set on the sand surface to fix the sand and stabilize the surface. At present, wheat straw sand barriers are recommended, as they prove effective in sand fixing and can last for many years. Wheat straw is cheap, and the barriers are convenient to build and not easily destroyed by domestic animals. The method of setting such barriers is to press wheat straw into the sand to a depth of 0.1–0.15 m with a shovel making two ends of the straw turn upwards, then step on the sand on both sides to fix it and straighten the straws up. The barriers are formed with a height of 0.15–0.20 m and a thickness of 50 mm. Setting the wheat straw in squares of 1 m × 1 m dimension uses 5–6 t wheat straw and takes 50–60 days work per hectare. The total cost is 450–500 yuan ha-1. Some plants are used to fix the sand, such as Haloxylon ammodendron,Calligonum L., Hedysarum scoparium, Tamarix ramosissma, Atraphaxis bracteata,Zygophyllum xanthoxylom, Artemisia arenaria, which endure drought, sand burying and cutting, wind erosion, and sunshine. The plant density should be estimated according to the moisture balance to fulfil their survival needs. If the plants are too sparse, they cannot provide sand fixation; if too dense, the moisture in the sand is deficient, and their survival and growth are restricted. According to the characteristics of the plants’ root systems and water consumption, the appropriate density of sand-fixing plants can be determined: 25–40 plants per 100 m2 for Hedysarum scoparium,30–50 plants per 100 m2 for Caragana korshinskii, and only three plants per 100 m2

for Haloxylon ammodendron, because each mature plant consumes 4 t of water annually.

CONCLUSION

The fragile environment of the Shiyang River basin, subject to low rainfall, can be easily damaged but it is difficult to restore. Over-expansion of irrigation has produced


negative effects on the water and soil resources in the basin. Excessive diversion in the upper reaches has caused water shortage in the lower reach region and a deterioration of ecosystems. Excessive exploitation of groundwater has led to a drop in the ground-water table, water quality deterioration, vegetation retreat and land desertification. Adoption of water-saving irrigation techniques and measures can reduce the water loss in the canal system and improve the water use efficiency in the field. Measures such as protection of the water resources in the Qilian Mountains, administrative and legal measures to optimally allocate water resources, maintenance of the balance between the land and water resources, development of water-saving agriculture, meeting the environmental water requirement, limiting the expansion of irrigated areas, diverting water from other rivers, and control of desertification, should be urgently adopted in this region.

Acknowledgements The authors are grateful for support from the Chinese National Nature Science Fund (nos 50339030 and 90202001), the Fund of the Chinese Ministry of Education (no. 02075) and the Core University Program of the Japan Society for the Promotion of Science. S. Kang and his colleagues (H. Cai, Y. Chen, T. Du, Y. Pan, Z. Liang, J. Han, W. Sun, Z. Zhang and others) have carried out studies in Wuwei and Minqin since 1984. Yukuo Abe and his colleagues (Kimura Toshinori, Fujii Katsumi, Sasaki Choichi, Nakamura Kouji and others) carried out investigations in the Shiyang River basin in 2001 and 2002. The staff in Wuwei and Minqin Bureaus of Water Resources Management are thanked for their help in the investigations and experiments and for supplying some hydrological data. The authors are also grateful for the comments by Hubert J. Morel-Seytoux and M. Al Bakhri. Jianhua Zhang is grateful for the grant support from the Research Grant Council in Hong Kong (HKBU 2041/01M).

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Received 26 April 2002; accepted 8 March 2004

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