temporal and spatial variability response of groundwater level to land use/land cover change in...

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Chinese Science Bulletin 2006 Vol. 51 Supp. I 51—59 DOI: 10.1007/s11434-006-8207-y

Temporal and spatial vari-ability response of groundwa-ter level to land use/land cover change in oases of arid areas YAN Jinfeng1,2, CHEN Xi1, LUO Geping1 & GUO Quanjun3

1. Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China;

2. Graduate School, Chinese Academy of Sciences, Beijing 100039, China;

3. Yijian Hydro-geology Engineering Corporation of XPCC, Urumqi 830009, China

Correspondence should be addressed to Yan Jinfeng (email: yanjf2004@ 163.com) Received July 20, 2005; accepted January 16, 2006

Abstract This paper conducts a case study on the impacts of land use/cover change (LUCC) on the temporal and spatial variability of the groundwater level in an arid oasis in the Sangong River Watershed by using the geographical information system (GIS), remote sensing (RS) and geostatistical methods. The temporal and spatial variability of the groundwater level in the watershed in 1978, 1987 and 1998 is re-gressed by using the semivariogram model and Kriging interpolation. The LUCC classification maps derived from the aerial images in 1978, Landsat TM image in 1987 and Landsat ETM image in 1998 are used to superpose and analyze the conversion rela-tionship of LUCC types in the regions with different isograms of the groundwater depth. The results show that the change of groundwater recharge was not so significant in the whole oasis, but the temporal and spatial LUCC was significant either in the normal flow periods or in the high flow periods during the 20-year period from 1978 to 1998, and there was a close correlation between them. There is generally a mod-erate spatial correlation of groundwater level (33.4%), and the spatial autocorrelation distance is 17.78 km. The regions where the groundwater level is sharply changed are also the regions where the land re-sources are increasingly exploited, which include mainly the exploitation of farmlands, woodlands, and building, industrial and mining lands. The study re-veals that the LUCC affects strongly the temporal and spatial variability of the groundwater level in the arid

oasis. The study results are of direct and practical significance for rationally utilizing shallow groundwa-ter resources and maintaining the stability of the arid oasis.

Keywords: LUCC, groundwater level, semivariogram, Kriging in-terpolation, the Sangong River Watershed.

The oases in arid areas rely strongly on water re-sources[1―4]. The asymmetrical distribution of water resources is a main contradiction in the long-term exploitation and utilization of water and land resources[3―7], in which the drawdown of groundwater level and the deterioration of groundwater quality caused by excessive groundwater exploitation are the most serious problems[8―11], especially in the arid oases. Thus, some ecological environment problems, such as the vegetation degeneration[12―14] and land desertifica-tion[15], occur. Many scholars have researched the en-vironmental problems caused by groundwater exploita-tion and utilization. Most of the studies, however, are qualitative, and the systematically lucubrated achieve-ments about the driving factors resulting in these prob-lems are less[16,17]. For example, the quantitative studies about the processes and results of groundwater change caused by LUCC under artificial driving factors in arid areas are deficient[18,19]. In this paper, a case study is conducted on the spatiotemporal variability of the groundwater level and the spatiotemporal LUCC as well as their affecting processes and internal relations in an arid oasis in the Sangong River Watershed located in north piedmont of the Tianshan Mountains, Xinjiang. In the study, GIS, RS and geostatistical methods are used to research the impacts of increasing human ac-tivities on the aqueous environment in oases. The study results have a certain guiding significance in achieving the sustainable development of oases in arid areas.

1 The study area and data

The Sangong River Watershed (43°09′―45°29′N, 87°47′―88°17′E) is located in the north piedmont of the middle section of the Tianshan Mountains and the south marginal zone of the Junggar Basin in Xinjiang. Its administration is under Fukang City, Changji Hui Autonomous Prefecture, Xinjiang Uygur Autonomous Region. The watershed covers the mountainous region in the south, alluvial-diluvial-fan oasis regions, alluvial plain oasis and desert (Fig. 1), a mountain-oasis-desert landscape pattern form[20], and the terrain slopes from the south to the north. The catchment area of the wa-

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52 Chinese Science Bulletin Vol. 51 Supp. I June 2006

Fig. 1. Divisions of the landforms in the Sangong River Watershed.

tershed is 1 670 km2, in which the oasis area is 870 km2. The annual temperature, precipitation and evaporation are 6.9 , 216 mm and ℃ 1 840 mm respectively, and it belongs to the typical arid continental climate.

The Sangong River Watershed has three main tribu-taries, i.e. the Sangong River, Sigong River and Shuimo River, all of which rise in the Bogda Mountain. Their annual runoff volumes are 5.294×107, 2.538×107 and 2.088×107 m3 respectively. The seasonal distribution of inflow is asymmetrical, the inflow volume in summer occupies 52.80% of the total, but in spring it occupies 13.56% only. However, the water consumption for farming in the oasis is the highest in spring. In order to meet the shortage of water supply in spring, groundwa-ter has to be exploited. Irrigation water in the allu-vial-diluvial-fan oasis in the upper reaches depends mainly on surface water. In Liuyunhu Farm in the up-per part of the alluvial plain in the lower reaches, the volume of irrigation water from surface water equals to that of groundwater. The water consumption for farm-ing in Fubei Farm in the lower part of the alluvial plain depends mainly on groundwater.

The data used in the study include mainly the RS data and hydrological data. The RS data of the study area include 65 aerial images in scale of 1:35000 in August 1978, a TM image in September 1987 and an ETM image in August 1998 in waveband 4, 3 and 2. These images are processed by using ERDAS IMAGE processing software and a topographic map on scale of 1:50000 to derive the images on the scale of 1:50000

in 1978, 1987 and 1998 respectively, which have the same projection coordinates (Transverse Mercator Pro-jection). The perennial hydrological data since the 1960s have been provided by Station for Aqueous En-vironment Experiment and Monitoring in the Sangong River Watershed. The data used in the study also in-clude the average annual groundwater level in the wa-tershed during the period of 1977―2000, runoff vol-umes of the Sangong River, Sigong River and Shuimo River during the period of 1962―2001, and the vol-umes of groundwater recharge in the watershed during the period of 1985―1999.

2 Study methods

2.1 Occurring frequency of runoff volumes

According to the data of the total annual runoff volume in the Sangong River Watershed during the period of 1962―2001, the occurring frequencies of total annual runoff volumes (P) in different years are calculated with equation of P = (n/t + 1)×100%, where, n is the occurring frequency of annual runoff volume, and t is the statistical years. The periods are regarded as the extremely low, low, normal, high and extremely high flow periods if their P>95%, 75%<P≤95%, 50% <P≤75%, 25%<P≤50% and P<25% respectively. Thus, the total runoff volumes in the watershed in 1978, 1987 and 1998 are calculated, which show the situation of water resources in the watershed in corresponding years.

2.2 RS interpretation and GIS spatial analysis

The classification system of land use actuality[21] is used to classify the types of land use/cover in the oasis in the Sangong River Watershed, which is based on the consideration that the study area is an artificial oasis and the features of farming landscapes are obvious. There are specific definitions about the land use/cover types in the classification system in China Technologi-cal Regulations of Land Use Actuality Survey. Accord-ing to the natural features and the social and economic characteristics in the Sangong River Watershed, the land use/cover types are delimited as the farmlands, woodlands, steppes, building, industrial and mining lands, desert steppes, saline or alkaline lands, waters and unused lands[22]. A man-computer mutual interpre-tation of the processed RS images in 1978, 1987 and 1998 is conducted by using the classification system as mentioned above under ERDAS IMGINE and by com-

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bining some thematic information and field survey data. The polygonal topological relations of the vector graphs derived from the interpretation are developed by using CLEAN of ARCTOOS and BUILD command, then the graphical data and attributive data are edited, and the classified maps of land use/cover in 1978, 1987 and 1998 are charted.

2.3 Geostatistical analyzing methods

It is approved that the geostatistical methods can be used to ideally describe the spatial variability of envi-ronment and reveal the spatial heterogeneity and spatial pattern of natural phenomena[23,24], and they have been extensively used in the ecological domain[25―27]. The analysis by using the semivariogram model and the Kriging interpolation are the most common geostatisti-cal analyzing methods[28―31].

(i) Analysis by using the semivariogram model. Semivariogram is a function about a semivariable value (or variability) of data points and the distance between data points, and a figure of the spatial correlation be-tween a data point and an adjacent one that can be de-rived from its graphic expression. It can be defined as

γ (si,sj) = 12

var(Z(si) − Z(sj)),

where, var is the variability coefficient. When the variability features in the study area are

quantitatively described, it needs to develop a theoreti-cal model of the variogram, and then to determine the curve of the theoretical model and select the optimal curve based on the tested variogram values. The spheriform model is one of the regressing models most commonly used, and in this study it is used to develop the semivariogram model of the groundwater level in the study area.

It is determined with crossing validation whether the assumptions of the model and the related parameters are rational. In the crossing validation, the autocorrela-tion model is evaluated by using all the data at first, then all the data points are deleted one by one, and the value of each point is predicted. All the predicted val-ues are compared with the measured data, and an accu-rate evaluation can be derived by using the statistical data. The concrete basis is to let the standard average values be close to 0 and the small values and the stan-dard deviations of average roots be close to 1.

(ii) Kriging interpolation. Kriging interpolation is based on some mathematical models and statistical models. It is to derive the weight coefficients from the

measured values of the around measurement points, and then to predict them. Kriging weight coefficients are calculated by using the semivariogram figure re-flecting the spatial structure of the data. They are de-cided not only by the semivariogram figures and the distances to the prediction points, but also by the spatial relations of the measured values of the around meas-urement points.

Kriging interpolation is an optimal estimating method for the spatial localities. As a main analyzing method of geostatistics, the randomicity and structure of the variables are synthetically considered. The spa-tial variability of the related points in the study area can be estimated by using the monitoring data of the sam-pling sites and the location relations between the sam-pling sites and the variogram model. The estimated values by using Kriging interpolation are linear, impar-tial and optimal. The ordinary Kriging model is used in this study. A spatial interpolation analysis on ground-water level in the study area is conducted based on the semivariogram figure and spatial distribution of the measured values, and the isogram maps of groundwater level are charted.

3 Results and discussion

3.1 Change of the runoff volume and groundwater recharge

The occurring frequencies of different annual runoff volumes in the Sangong River Watershed are calculated based on the measured data (Fig. 2). The results reveal that the occurring frequencies of the annual runoff volumes in 1978, 1987 and 1998 were 34%, 51% and 12%, and these three years are regarded as the high, normal and extremely high flow periods, respectively. The total annual runoff volume in the extremely high flow period was as high as 1.071 3×108 m3, and the volume of surface water diverted to the irrigated area was 9.462×107 m3. However, the surface water cannot satisfy the demands of water resources (about 1.29×108 m3) due to the increasing development of the oasis even in the extremely high flow periods. Thus, groundwater has to be exploited.

The change of groundwater recharge was not so ob-vious in the past 15 years (Fig. 2), and the average an-nual groundwater recharge was about 6.5×107 m3. Moreover, the groundwater recharge is continuously reduced along with the increase of utilization ratio of surface water, but the utilization ratio of groundwater is continuously increased along with the urban expansion

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Fig. 2. Change of the annual runoff volume (a) and of the groundwater recharge (b) in the Sangong River Watershed.

and the enlargement of the area of farmlands.

The increase of the utilization ratio of surface water relies on the sufficient utilization of reservoirs and the decrease of irrigation ration of crops. Combining the groundwater recharge with the groundwater exploita-tion, the process of recharge and discharge of ground-water in the watershed is researched. The results reveal that the volume of excessively pumped groundwater is 9.0×106 m3/a. The volume of pumped groundwater is 4.1×107 m3/a in the upper reaches of the watershed, and the volume of groundwater reserves is −1.0×107 m3/a. Therefore, the groundwater in this region is excessively pumped, a serious drawdown of groundwater level oc-curs, and the LUCC is great. The groundwater evapora-tion occurs mainly in the lower reaches of the water-shed, especially in Fubei Farm close to the margin of the desert. The average annual evaporation in the farm is 1.933×107 m3 and 9 times higher than that in the up-per reaches. The volume of pumped groundwater in the lower reaches of the watershed is 7.41×106 m3/a, and the water consumption for farming in Fubei Farm de-pends mainly on groundwater pumped from the groundwater overflow zone. Thus, a drawdown doline of groundwater level for 6―8 m occurs in this region.

3.2 Analysis on the spatiotemporal LUCC

The classified maps of LUCC in 1978, 1987 and

1998, derived from the man-computer mutual interpretation, are used to derive the information of area change of the various land use types. The derived information is compared and analyzed so as to fully reveal the change of the different land use types in recent 20 years (Table 1).

The area of farmlands was enlarged by 13% and for 28.97 km2 during the 20-year period from 1978 to 1998. The increased farmlands were mainly distributed in the alluvial-diluvial-fan oasis regions, peripheral regions of the water source fields and upper part of the alluvial plain oasis. The area of building, industrial and mining lands dominated by urban building areas and rural residential areas was sharply enlarged by 166% and for 34.83 km2. The increased lands were mainly used to construct the Fukang Petroleum Base (10 km2) and mainly distributed in the alluvial-diluvial-fan oasis re-gions. The area of woodlands was enlarged by 25.01 km2 during the period from 1978 to 1987, but reduced by 15.66 km2 up to 1998, and the woodlands were mainly distributed in the upper part of the alluvial plain oasis. The areas of desert steppes and saline or alkaline lands were sharply reduced by 42.85 and 29.72 km2 respectively. The sharply reduced desert steppes and saline or alkaline lands were directly or indirectly shifted to farmlands and building, industrial and mining lands. Such change reveals a geographical distribution

Table 1 LUCC in the Sangong River Watershed (km2)

Land type 1978 1987 1998 Area change

(1978―1987) Area change

(1987―1998) Area change (1978―1998)

Farmlands 221.92 240.90 250.89 18.98 9.99 28.97 Woodlands 143.03 168.04 152.39 25.01 −15.66 9.36

Steppes 130.06 129.25 128.03 −0.80 −1.22 −2.03 Building, industrial and mining lands 21.04 39.00 55.87 17.96 16.88 34.83

Waters 17.36 14.27 15.57 −3.09 1.29 −1.79 Desert steppes 232.29 174.07 189.45 −58.23 15.38 −42.85

Saline or alkaline lands 65.75 55.82 36.03 −9.93 −19.78 −29.72 Unused lands 110.64 120.74 113.87 10.10 −6.87 3.23

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pattern that the old oasis (the alluvial-diluvial-fan oasis regions, groundwater overflow zones and upper part of the alluvial plain oasis) has been expanded towards its peripheral regions.

3.3 Analysis on the temporal and spatial variability of groundwater level

(i) Development, prediction and validation of the semivariogram model. The spatial variability of groundwater level in the study area in 1987 is predicted and simulated by using the module of geostatistical analysis[32] and the semivariogram model under ArcGIS. The semivariograms are regressed with spherical func-tions. The pace and pace resultant values are adjusted through some group tests so as to determine the optimal semivariogram regressing model, and the regressed results are shown as Fig. 3. Moreover, the derived three-dimensional trend map of groundwater level (Fig. 4) reveals that the groundwater depth becomes deeper gradually from the south to the north and is in an east- west U-shaped distribution.

The ratio between the nugget value and sill value of groundwater level, with a moderate spatial correlation[29], is 33.4%, and the spatial autocorrelation dis-

Fig. 3. Distribution of the spatial groundwater level.

Fig. 4. Three-dimensional trend of the variability of groundwater level.

tance is 17.78 km (Table 2). These reveal that the spatial heterogeneity of groundwater level is mainly caused by the spatial structure, and the groundwater level is jointly affected by the spatial structure and the stochastic factors. The spatial correlation of the groundwater level can be caused by the spatial structure, such as the natural factors including the terrain, landforms, climate, soil types, etc. It, however, can be reduced by human activities, such as the urban construction, land reclamation, growing systems, and irrigation and drainage intensity. The results of crossing validation reveal that the errors are within the allowances, and they validate that the enactments of the model and parameters are rational, and the regressed results are significant. Table 2 also shows that the variability coef-ficient of groundwater depth is 1.07 and in a high vari-ability intensity.

(ii) Spatiotemporal variability of groundwater level. Based on the methods of the semivariogram regressing model as mentioned above and Kriging in-terpolation, the isogram maps of groundwater level in the Sangong River Watershed in 1978, 1987 and 1998 are derived[19,31] (Fig. 5). The area information is de-rived from the isogram regions with groundwater depths of 5, 10, 15, …, 60 m in an interval of 5 m (Ta-ble 3) to analyze the spatial variability of the ground-water level in the Sangong River Watershed.

The compared results of groundwater depth in the study area in 1978, 1987 and 1998 are as follows: (1) Spatially, the groundwater depth in the watershed be-comes generally shallower from the south to the north; temporally, a general drawdown of groundwater level

Table 2 Related parameters of the semivariogram model of groundwater level

Time Variable coefficient

Nugget value

Sill value

Nugget value/sill value (%)

Spatial correlation distance (km)

Standard deviation of average root

Standard average Average

1987 1.07 0.0177 0.053 33.4% 17.78 1.00 −0.006115 0.01298

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Fig. 5. Isogram maps of groundwater depths in the Sangong River Watershed in 1978, 1987 and 1998.

Table 3 Change of areas of the regions with the different isograms of groundwater depth in 1978, 1987 and 1998 (km2)

Groundwater depth (m) Year

5 10 15 20 25 30 35 40 45 50 55 60 1978 261.07 223.57 125.97 48.07 80.60 105.01 53.85 43.96 1987 298.78 152.51 66.82 57.54 74.87 134.29 81.67 55.22 18.95 1.45 1998 267.64 141.04 86.05 52.81 73.57 75.88 69.66 86.18 54.72 23.42 9.44 1.67

occurred. A deeper groundwater depth occurred in a region in the upper reaches. The groundwater depth in the watershed varied in a range of 5―40 m in 1978, but a 20.4-km2 region with groundwater depth of 40―50 m was increased in 1987, and an 11.11-km2 region with groundwater depth of 50―60 m was increased in 1998; (2) from 1978 to 1987 and 1998, the isograms of groundwater depth became denser in the upper part (the alluvial-diluvial fans and the groundwater overflow zones) of the oasis, but they were still sparse in the lower part (the alluvial plain); (3) the spatiotemporal variability of groundwater depth is the most significant in the alluvial-diluvial-fan oasis regions. The region with the deepest groundwater level has been shifted from the central part of the oasis to the eastern part in recent 20 years, where the groundwater depth is dropped from 40 m down to 60 m. The area of the re-gions with 35―50-m isograms of groundwater depth is gradually enlarged and expanded from the east to the west; (4) the groundwater depth in the groundwater overflow zones was dropped from 10―20 m in 1978 down to 15―20 and 15―25 m in 1987 and 1998 re-spectively. The groundwater level is generally in a drawdown trend. The regions with 5―10-m isograms of groundwater depth in the alluvial plain oasis are ba-

sically stable.

3.4 Impacts of LUCC on the spatiotemporal variabil-ity of groundwater level

The information of LUCC and that of the spatio-temporal variability of groundwater level in 1978, 1987 and 1998 are superposed and analyzed to derive the information of change of the different land use types in the regions with different groundwater depths (Table 4) and to develop their correlations.

During the period of 1978―1987, the annual volume of surface water was reduced by 6.31×106 m3, the high flow period shifted to the normal one, and a significant spatiotemporal variability of groundwater level oc-curred. The regions with the deepest groundwater level in the alluvial-diluvial-fan oasis regions were shifted from the central part of the oasis to its eastern part. The regions with 35―40-m isograms of groundwater depth were changed into the ones with 40―50-m isograms. The types of land use/cover in the regions with the deepest groundwater depth were changed from farm-lands and steppes to farmlands and building, industrial and mining lands. The groundwater level beneath the alluvial-diluvial fans was generally in a drawdown in the past 10 years, and it had an obvious spatial variabil-

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Table 4 Situation of LUCC in the regions with different isograms of groundwater depth in 1978, 1987 and 1998 (km2)

Year Isogram of

groundwater depth (m)

Farmlands Woodlands Steppes Building,

industrial and mining lands

Waters Desert steppes

Saline or alkaline

lands

Unused lands Total

5 65.73 33.61 0.00 4.10 0.00 72.79 9.46 75.37 261.0710 43.34 56.29 0.00 2.10 6.96 97.88 2.01 15.00 223.5715 17.20 32.90 19.27 1.15 1.65 25.60 18.52 9.69 125.9720 20.17 1.94 2.14 1.54 0.37 8.09 13.81 0.00 48.07 25 32.67 7.11 5.79 5.13 0.31 9.61 19.20 0.78 80.60 30 22.50 10.11 40.20 4.75 3.14 14.71 2.74 6.84 105.0135 11.41 1.06 32.12 0.55 3.72 3.61 0.00 1.37 53.85

1978

40 8.90 0.00 30.53 1.72 1.22 0.00 0.00 1.59 43.96 5 75.06 48.12 0.00 6.92 0.00 77.73 9.12 81.85 298.78

10 34.66 33.05 0.00 2.24 4.90 52.89 0.52 24.25 152.5115 15.41 28.01 0.00 1.67 0.71 7.92 8.09 5.00 66.82 20 19.51 17.48 0.00 1.79 0.04 6.73 11.99 0.00 57.54 25 20.22 18.65 15.05 4.24 0.87 7.91 7.93 0.00 74.87 30 27.09 10.72 70.22 12.03 2.92 2.90 7.46 0.96 134.2935 26.77 0.69 38.49 3.66 1.38 1.79 7.87 1.02 81.67 40 14.84 10.91 5.49 4.96 3.39 6.93 2.85 5.86 55.22 45 6.95 0.43 0.00 0.63 0.01 9.12 0.00 1.81 18.95

1987

50 0.39 0.00 0.00 0.85 0.05 0.16 0.00 0.01 1.45 5 68.30 57.81 0.00 7.58 0.00 50.22 5.80 77.93 267.64

10 43.73 7.74 0.00 3.34 4.19 53.89 11.62 16.55 141.0415 12.58 32.09 0.00 1.29 3.58 24.11 2.26 10.14 86.05 20 13.52 16.98 0.76 1.05 0.53 16.35 3.61 0.00 52.81 25 14.83 9.91 29.80 2.55 1.29 11.36 3.75 0.08 73.57 30 16.63 15.47 25.61 5.57 0.94 9.57 1.40 0.70 75.88 35 17.74 0.30 27.93 13.86 0.99 8.36 0.09 0.39 69.66 40 27.65 3.72 28.87 10.84 1.75 7.97 4.27 1.12 86.18 45 18.51 3.67 14.78 5.42 2.17 3.03 3.24 3.89 54.72 50 10.08 4.69 0.28 2.51 0.06 3.02 0.01 2.77 23.42 55 6.94 0.00 0.00 0.63 0.03 1.54 0.00 0.30 9.44

1998

60 0.38 0.00 0.00 1.23 0.03 0.03 0.00 0.00 1.67

ity. The groundwater level in the groundwater overflow zones was generally high in the south but low in the north, and the hydraulic gradient was increased. The isograms of groundwater depth became denser, and the land use types in the regions with significant drawdown of groundwater level were the farmlands, woodlands, building, industrial and mining lands, desert steppes and saline or alkaline lands. The groundwater level in the lower part of the alluvial plain oasis was generally risen in the past 10 years, the area of the region with 5-m isogram of groundwater level was enlarged by 37.7 km2, and the land use types were dominated by farm-lands, woodlands, building, industrial and mining lands, and unused lands.

During the period from 1987 to 1998, the annual

volume of surface water was increased by 1.38×107 m3, the normal flow period changed to the extremely high flow period. However, the annual groundwater recharge was reduced by 2.0×106 m3, and a significant spatio-temporal variability of groundwater level occurred. The areas with the deepest groundwater depth in the allu-vial-diluvial-fan oasis regions were still in the eastern region, and the regions with 40―50-m isograms of groundwater depth were changed to the ones with 50―60-m isograms. The area of the region with 50-m iso-gram of the deepest groundwater level was only 1.4 km2 in 1987, but the area of the regions with 50―60-m isograms of groundwater depth was enlarged to 34.5 km2. The main land use/cover types in these regions were farmlands, woodlands, building, industrial and

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mining lands, and desert steppes. The hydraulic gradi-ent in the groundwater overflow zones was increased, and the regions with 15―20-m isograms of groundwa-ter depth were changed to the ones with 15―25-m iso-grams. The LUCC was that the areas of farmlands, woodlands and desert steppes were significantly enlarged, but the area of saline or alkaline lands was significantly reduced. The areas of the regions with 5-m and 10-m isograms of groundwater depth in the alluvial plain oasis were reduced by 31.2 and 11.5 km2 respec-tively, which reveal that the groundwater level was generally in a drawdown. The land use types in the re-gions, where a drawdown of groundwater level oc-curred, were mainly the farmlands, woodlands and de-sert steppes.

4 Conclusions The variability coefficient of groundwater level in

the oasis in the Sangong River Watershed is 1.07, and the variability intensity is high. The analysis by using the semivariogram model reveals that there is a moder-ate spatial correlation of groundwater level (33.4%), and the spatial autocorrelation distance is 17.78 km. These are the joint effects of the spatial structure (such as the terrain, climate, soils and other natural factors) and the stochastic factors (such as the urban construc-tion, land reclamation, growing systems, irrigation and drainage intensity and other human activities).

The isogram maps of groundwater depth are charted by using Kriging interpolation. They reveal that, in general, a sharp drawdown of groundwater level occurs in the upper reaches. During the period of 1978―1998, the groundwater level in the lower reaches was slowly risen in previous years but dropped slowly in recent years in the Sangong River Watershed. The regions with 0―40-m isograms of groundwater depth are changed into the ones with 0―60-m isograms, and the regions with 40―60-m isograms of groundwater level appear in the upper reaches of the watershed.

The RS interpretation and GIS spatial analysis reveal that a significant spatiotemporal land use/cover change occurred in the oasis in the Sangong River Watershed during the 20-year period from 1978 to 1998. The sharp LUCC occurs in the upper part of the oasis, it is mainly the continuous enlargement of the non-farming lands dominated by building, industrial and mining lands and the farming lands dominated by farmlands, and the sharp reduction of the areas of desert steppes and saline

or alkaline lands. There is a close correlation between the spatiotem-

poral LUCC and the spatiotemporal variability of groundwater level in the oasis. The regions, where the land resources including farmlands, woodlands and building, industrial and mining lands are increasingly exploited, are also the regions where the groundwater level is sharply changed. The response of the spatio-temporal variability of groundwater level is caused by the continuous increase of non-farming lands domi-nated by the lands for urban construction, industrial development and mining, and the farming lands domi-nated by farmlands. The study results reveal that it has a certain applicability to analyze the spatiotemporal variability of groundwater level by using geostatistical methods, and it is further approved that the methods can be applied in researching landscape ecology.

Acknowledgements This work was supported by the im-portant directional projects of the Knowledge Innovation Program of Chinese Academy of Sciences (Grant Nos. KZCX3-SW-326-03 & KZCX3-SW-327-01).

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