Journal of Geographical Sciences 15, 4 (2005) 405-414 ISSN: 1009-637X www.geog.cn
Impacts of land use changes on groundwater resources in the Heihe River B a s i n W A N G Genxu ~'2, Y A N G L i n g y u a n 2, C H E N L i n ~ ,
Jumpei K u b o t a 3
(1. Cold and Arid Regions Environmental and Engineering Research Institute, CAS, Lanzhou 730000, China; 2. College o f Earth and Environment Sciences, Lanzhou University, Lanzhou 730000, China; 3. Research Institute for Humanity and Nature, Kyoto 602-0878, Japan) Abstract: Land use and land cover changes have a great impact on the regional hydrological process.
Based on three periods of remote sensing data from the 1960s and the long-term observed data of groundwater from the 1980s, the impacts of land use changes on the groundwater system in the middle reach of Heihe River Basin in recent three decades are analyzed by the perspective of groundwater recharge and discharge system. The results indicate that with the different intensities of land use changes, the impacts on the groundwater recharge were 2.602 • 108 m3/a in the former 15 years (1969-1985) and 0.218 >(108 m3/a in the latter 15 years (1986-2000), and the impacts on the groundwater discharge were 2.035 • 108 m3/a and 4.91 • 108 m3/a respectively. When the groundwater exploitation was in a reasonable range less than 3.0 • 108 m3/a, the land use changes could control the changes of regional groundwater resources. Influenced by the land use changes and the large-scale exploitation in the recent decade, the groundwater resources present apparently regional differences in Zhangye region. Realizing the impact of land use changes on groundwater system and the characteristics of spatial-temporal variations of regional groundwater resources would be very important for reasonably utilizing and managing water and soil resources. Key words: land use change; inland river; groundwater system; Heihe River Basin doi: 10.1360/gs050403 The impact o f land use and land cover changes on the regional water balance is the most vigorous research in the international hydrological fields, and lots of research indicate that large-scale land use and land cover changes are the important factors resulting in the regional climate and hydrological cycle changes (Hutjes et al., 1998; Zhang L e t al., 2001). Therefore, IGBP, IHDP, WCRP, DIVERSITAS, etc. take the relationship between Biosphere Aspects o f the Hydrological Cycle (BAHC) and the land use and land cover changes, as well as its climate frangibility, as the core plans (Hoff, 2002; Lambin eta!., 2002). Moreover, in LUCC established by IGBP and IHDP, one core problem is to understand the impact o f the regional land use and land cover changes on hydrological process and water resources (Suzanne Serneels, 2001; Nunes et al., 1999; Huang et al., 2004). Much work indicates that the regional vegetation ecosystem changes caused by land use and land cover changes remarkably affect the regional hydrological cycle (Zhang L e t al., 2001; Deng et al., 2003; Fu eta/., 2002). Therefore, the mechanism o f land use and land cover changes in the catchment impacting on hydrological process become important fields in the development o f hydrology (Hoff, 2002; Huang et al., 2004). The characteristics o f water resources in arid region determine that groundwater is usually the most important source and the best choice o f water supply, at the same time, groundwater is also the primary factor to maintain arid oasis ecosystem, especially the socio-economic developments in arid area (Mtembezeka et al., 1997; Alley et al., 1999). Globally, groundwater resources are actually irreplaceable in resolving water scarcity. Precise evaluation and effective management are the important guarantees and necessary preconditions to explore groundwater resources in Received: 2005-03-05 Accepted: 2005-06-10 Foundation: National Natural Science Foundation of China, No.40171002; China-Japan CooperationProject "Estimation of
oasis adaptabilityto water resourceunder changingenvironment" Author: Wang Genxu (1965-), Ph.D. and Professor, specializedin land use/cover changes, hydrologyand water resources in arid and cold regions. E-mail:
[email protected]
406
Wang Genxu, Yang Lingyuan, Chen Ling et ol.
arid region. Thereinto, actual estimation of the impact of human activities on groundwater system is critical to establish reasonable utilization program of regional groundwater resources (Schwarts et 01., 2003; Sato et 01, 2003). Previous research of the impact of human activities on groundwater system mainly focused on the aspects of the intensity and reasonability of groundwater utilization, while ignoring the impact of land use changes on the groundwater system in the basin. Actually, as the important part of regional hydrological cycle, groundwater system has strong response to land use and land cover changes (Mtembezeka et 01., 1997; Alley et 01., 1999). Taking Zhangye region of the middle reach of the Heihe River in Hexi Corridor as the study area, the response of arid groundwater system to land use changes is analyzed in this paper.
1 Study area a n d m e t h o d o l o g y 1.1 Study area This paper takes Zhangye region in the middle reach of the Heihe River as the study area, which is located in the middle part of the Hexi Corridor, lying between 97~176 and 37~176 (Figure 1). The total area is 421.4 • 104 ha. It belongs to the typical temperate continental climate with scarce precipitation and strong evaporation. The average annual precipitation is 62-280 mm and the average annual evaporation is 1000-2000 ram. The mountainous river of the Heihe is the only surface runoff in Zhangye region, where the runoff and precipitation have been stable since 1951 (Zhang Jishi et 01., 2003). The topography of the study area declines from southeast to northwest, and the slope gradient is 25%0-4%0. The landscape is composed of piedmont alluvial and diluvial gravel plain in the south and f'me-grained soil plain in the basin center. Restricted by the landscape, deposit and tectonic conditions, groundwater in this area is mainly the Quaternary unconsolidated sand and gravel aquifers. Based on the topographic units, the study area is divided into four hydro-geological units from south to north: the upper-middle part of the alluvial-diluvial fan in the south, the lower part of the alluvial-diluvial fan, the fine-grained soil plain in the basin center, and the river valley plain in the lower reach of the basin (Figure 1). In the past four decades, land use and land cover in the Heihe River Basin changed intensively, which was indicated by the shrinkage of the natural oasis system, namely, the expansion of man-made oasis system and the abandonment of the original riverway caused by
Figure 1 The hydrogeological sub-zones and observation sites in the Zhangye region
Impacts of land use changes on groundwater resources in the Heihe River Basin
407
the increase of the irrigated fields (Wang eta/., 2003). All these changes would lead to the thorough spatial-temporal variations of water resources system, especially the core factors such as recharge, runoff and discharge of groundwater system. The study of the strong influence of land use and land cover changes on water resources system is not only critical to reasonable utilization and management of water resources in the basin, but in return, is critical to the sustainability of land use itself. 1.2 Methodology and data The elementary formula of composing change and management of groundwater resources is: QR - Qo = Ds (1) where QR is groundwater recharge, such as riverway infiltration, precipitation infiltration, irrigating water infiltration and groundwater lateral inflow; Qo is groundwater discharge, mainly the spring outflow, evaporation of unconfined groundwater, exploitation and groundwater lateral outflow; and Ds is the groundwater change of the entire system. Changes of all these composing factors reflect the response of groundwater system to land use and land cover changes. 1.2.1 Method for analyzing the changes of recharging and discharging factors In recharging factors, the precipitation infiltration and lateral inflow recharge were ignored because precipitation infiltration in the arid inland plain mainly as storm flood recharging groundwater system and together with lateral inflow recharge, were little influenced by land use and land cover changes. The analytic method of the main factors--riverway and irrigating water infiltration is as follows: For the recharge of riverway water: AQr = 2 q~~ i ~'~
(2)
i=1
While for the main stream, the riverway is divided into two parts by diversion dam--the above water division dam part (upper part) and down to spring outflow part (lower part) when calculating the infiltration. The upper part infiltration is closely related to the stable mountainous runoff for years but not to the regional land use, so its measurement is ignored here. For the recharge of the lower part: AQ~, = (Qo - Qz - Aqc)hm (3) In the formulas (2) and (3), qi is the mountainous runoff of each river; AL~ represents length change of each river; ,~i is infiltration coefficient; Q0 and Qz represent the mountainous runoff of main stream and upper part river seepage, respectively; Aqc is the change of water diversion of upper part river, which is close to ploughed field; and Am is the riverway infiltration rate of the part of lower part. Irrigating water recharge is farther divided into canal infiltration and field infiltration, and the analytic formulas are shown as follows respectively: For the recharge of canal: AQc = qc (1 - Ao0fl (4) For the recharge of field: AQ~ = 2 A~q0r/j
(5)
j=l
where qc and qo represent the water diversion of the upper part river and the net irrigation rating, respectively; Aa is the change of canal utilizing coefficient along with canal length; fl and ~b represent, respectively, the canal infiltration coefficient and irrigating infiltration coefficient; and AFj is the area change of each irrigating field. In the discharging factors of groundwater, the increase of the area of well irrigating field leads to the increase of exploitation that is closely related to the land use changes. Additionally, spring outflow is determined by the change of groundwater level, which is indirectly affected by the land use changes. Therefore, we use the statistical data of the changes of exploitation and spring outflow to perform direct calculation, and the data of evaporation of unconfined groundwater is calculated by the field areas of various groundwater levels and the evaporation intensity. The field with different types of land use and land cover has different evaporation capacities, so the evaporation is the coupling result of groundwater level and land use changes,
408
Wang Genxu, Yang Lingyuan, Chen Ling et ol.
as shown in the following formula: K
AQ~ = ~ p=l
L
~F.~/p,~
(6)
v=l
where F,.~ and %,~ represent the area and evaporation of the v-th sub-area divided according to the land cover in the p-th area which is divided according to the groundwater level. 1.2.2 Method for analyzing the groundwater storage changes Based on the hydrogeologic division of the entire region, we analyze the observing data of groundwater table and calculate the annual variations of groundwater storage in the last two decades (formula 7), which comes from Darcy Law. A W o = txiFi(H~ - Hio) (7) where A W o is the relative groundwater storage change of the j-th year compared to the groundwater storage of the year 1981 (1984) in the i-th area, 106 m3; /zl is the storage coefficient, dimensionless; Fi is the area of the i-th area, mZ; H~, the average observing groundwater level of the j-th year in the i-th area, m; and H~0 is the average observing groundwater level of the year of 1981 in the i-th area, m. 1.2.3 Data sources and parameters (1) Data of land use changes. With the regional aerial data of 1969 and TM data for regional satellite remote sensing research of 1986 and 2000, we mark the plots of diverse land use on the topographic map of 1:100,000 according to the national standard of the division of land use, and pick up the areas of irrigating field, grassland, forestland, desert (bare rock, soil, sand desert, Gobi, etc.), river and canal system. And then, based on the law of population changes we interpolate the area of every land use type per five years, and obtain the data of land use changes that match five-year-scale variations of groundwater level. (2) The observation of groundwater level. There are 54 observing points along the Heihe River in Zhangye region, in which, the observation of 28 points began" in 1980 and the effective data concerning the average annual and mensal values of the two decades, and the observation of the other 26 points began in late 1983 and the effective data concerning the average annual and mensal values of 17 years. Data of surface water resource and its utilization come from the research results of the Heihe River special topic of the National Ninth Five-Year-Plan project (1996-2000)(Pan et o1., 2001). (3) Parameters. In 1986-1989, lots of large-scale investigations of groundwater resources were conducted successively, and great pumping fields, dynamic observing fields and the study fields of parameters of groundwater movement were established in Zhangye region. After lots of observations, testing and analyses, a serial of parameters were obtained. According to such testing results, the uniform parameters gradually came out in the study area. The storage coefficient, percolation coefficient, irrigating infiltration coefficient and evaporation coefficient used in this paper come from these broadly used testing results @(Table 1).
2 Results and discussion 2.1 Characteristics of land use changes in recent three decades in Zhangye region In 1967-2000, the remarkable characteristics of land use changes are the extension of plantation and the decrease of natural forestland and grassland (Figure 2). In the three decades, in Zhangye region the high cover grassland (>70%) decreased by 93.8%, the medium cover grassland decreased by 75.1%, the natural riverway decreased by 54.8%, and the natural forestland decreased by 16.6%. Thereinto, the changes of forestland are different in the former and latter 15 years. Including natural and man-made, the forestland increased by 163.1% in 1967-1986, while decreased by 2 0 . 2 % in 1986-2000. In contrast to the decrease of grassland and forestland, the @ Gansu Hydrogeological Investigation Team: "Groundwater resources and its reasonable exploitation and utilization in the middle reach of Heihe mainstream"(1990), "Report of regional hydrogeologicalinvestigation in Zhangye"(1998)
Impacts of land use changes on groundwater resources in the Heihe River Basin
409
Table 1 The measurement of some parameters Groundwater table/m Irrigating infiltration coefficient/% Evaporation of unconfined groundwater /mm/a
Irrigating area
Grassland
<1
1-3
3-5
5-10
Subarea
Upper-middle part of
Fine-gained
alluvial-diluvial fan
soil plain
28.1
34.9
28.4
18,5
Storage coefficient
0.15
0.1
327.79
55.3
41.0
12,0
Riverway infiltration rate/% Consuming rate of unconfined zone/% Canal leak recharging rate/%
0.32-14.7
0.67-1.52
30
25
0.7-0.8
0.8-0.9
191.43 3 4 , 6 9
23.94
7.01
Figure 2 Changes of land use pattern from 1967 to 2000 in the Zhangye region field increased by 31.03%, including 18% of the irrigating field of plain and 89.8% of drought field that was converted from grassland in the upper-middle part of the alluvial-diluvial fan. Additionally, the constructing field of the urban and rural areas and plants increased 87.6%, 148.8%, and 72%, respectively. In recent three decades, the spatial distribution of land use change pattern in Zhangye region is: (1) Irrigating fields extended in the middle-lower part of the alluvial-diluvial fan and the valley along the fiver banks, which extended toward peripheries on the basis of old oasis. While the development of drought field was focused on the upper-middle part of the alluvial-diluvial fan. (2) The changes of natural forestland and high cover grassland mainly happened in mountainous areas and the upper part of the alluvial-diluvial fan; and the man-made forestland changed in the middle-lower part of the alluvial-diluvial fan and river valley plain. (3) Many riverways in the upper and lower parts of the alluvial-diluvial fan were transformed into fields and barefields. Such land use changes, especially changes of the spatial distribution of irrigating oasis, will impel the surface water cycle and surface water system to change in spatial distribution, which definitely have great impact on groundwater system in the basin. 2.2 Impact of land use changes on the groundwater recharging system Before 1985, 5.186 • m3/a out of the total mountainous runoff (8.395 x l08 m3/a) of the tributaries of the Heihe River in Zhangye region was diverted into canals and reservoirs (Pan et a/., 2001; Gao eta]., 1990), which caused the intensive changes of riverway distribution and 30% of the rivers leaked into the unsaturated zone. Based on such information, the infiltration recharge decreased by 1.608 x 10s m3/a in 1985 and 0.266 x 108 m3/a in 1986-2000 due to the
Wang Genxu, Yang Lingyuan, Chen Ling et al.
410
changes of riverway distribution. Moreover, infiltration recharge decreased by 0.529 • 108 m3/a due to the power generation water diversion (7.2 x l08 m3/a) (Pan et al., 2001). After the diversion, the infiltration decreased 1.156 • 108 m3/a in the riverway between the canal head and spring outflow zone in 1985 and 0.313x 108 m3/a in 1986-2000 (Table 2). During 1970-1985, the length of canal system and the high standard lining length increased 2/3 and 75%, respectively, which directly led to more canal water diversion. Additionally, the average water conveyance efficiency was 0.5 before 1985, but after 1986, especially in the 1990s, it increased to 0.65 averagely. Based on the formulas presented in section 1, the changes of canal infiltration recharge were calculated with results shown in Table 2. In the 15 years before 1985, water diversion enhancing the canal recharge to groundwater increased 0.908 • 108 m3/a, and 0.537 x 108 m3/a in 1986-2000. The changes of river infiltration mainly happen in the upper-middle part of the alluvial-diluvial fan because a large number of rivers were diverted into canals or reservoirs, while the changes of canal infiltration mainly happen in the center of the man-made oasis located in the middle-lower part of the alluvial-diluvial fan and fine-grained soil plain. Groundwater recharging changes caused by the changes of irrigation land use are due to the changes of irrigating infiltration, the core problem of which is to make the region related to the irrigating area changes match that of the spatial-temporal distribution of groundwater. Assisted by GIS, comparing and analyzing the groundwater table isolines in 1980 and 1990, with the land use plots in 1969-1986 and 1986-2000, the two periods of irrigating area changes with different groundwater table depths are obtained (Table 3). In the arid inland region, where irrigating infiltration can recharge to groundwater, the groundwater table depth is generally lower than 10 m. Different regions have different irrigating ratios, for example, the nominal irrigating ratio in Zhangye is 7872-14,191.5 m3/ha, in Linzhe is 10,092-10,957.5 m3/ha, and in Gaotai is 9618-17,356.5 m3/ha. Since the mid 1990s, because the vegetation structure was transformed from merely wheat in domination to corn and wheat in domination, the irrigating ratio became a little higher than that of the former period. Some 50% of the nominal irrigating rate is used as the net rate before 1985 and 70% after 1986. The increase of irrigating area enhanced recharge by 2169.69x 104 m3/a in 1970-1985, and 1767.09x 104 m3/a in 1986-2000. 2.3 Impact of land use and land cover changes on the groundwater discharging system Land use changes affect the groundwater discharge system directly through evaporation. Since evaporation mainly appears in the zone with groundwater table less than 10 m, where low cover grassland, semi-fixed and fixed dunes, and desert all turned to irrigating fields. Therefore the turns of irrigating fields to the original grassland are used to calculate the impact of land use changes on evaporation. It is worth pointing out that evaporation of high cover grassland (>70%) Table 2 The impact of the changes in riverway and water channels on the groundwater recharge in Zhangye region Period
1970-1985 1986-2000
Changes of river length /kra
Changes of diversion /108m3/a
Changes of river infiltration /108ra3/a
Changes of canal diversion /10Sm3/a
Water conveyance efficiency/%
Changes of canal infiltration /10Sm3/a
Changes of canal system /10Sm3/a
-59.07 -17.63
6.02 1.49
- 3.293 -0.579
2.27 1.92
0.5 0.65
0.908 0.537
- 2.385 -0.042
Table 3 The impact of the changes of irrigating fields on groundwater recharge in Zhangye region Groundwater table/ra 1970-1985
1986-2000
Area changes of irrigating fields/krn 2 Average irrigating rate/m3/ha Infiltration/104 m 3 Area changes of irrigating fields/kin 2 Average irrigating rate/m3/ha Infiltration/104 m 3
< 1
1-3
3-5
5-10
Total
5.5 4950 76.5 6.5 5250 95.89
41.35 5400 779.28 15.41 7035 378.34
40.67 6150 710.34 30.03 7560 644.75
53.05 6150 603.57 46.34 7560 648.11
140.57 2169.69 98.28 1767.09
Impacts of land use changes on groundwater resources in the Heihe River Basin
411
Table 4 Comparison of groundwater evaporation under different land use in Zhangye region Groundwa~r table/rn 1970-1985 Area changes of irrigating fields/km2 Vaporizing intensity/m3/ha Evaporation of unconfined groundwater / 104ms 1986-2000 Area changes of irrigating fields/kin2 Vaporizing intensity/mS/ha Eval~orationof unconfined ~roundwater / 104ms
<1 1-3 2.3 31.15 3277.9 553 75.39 172.25 3.5 12.74 3277.9 553 114.72 70.45
5-10 3-5 38.54 53.05 410 120 158.02 63.66 30.03 46.34 410 120 123.12 55.6
Forestland chanses 88.45 2700 2388.15 -33.03 2700 -891.81
Total 213.49 2857.47 48,58 -527.92
Table 5 The changes of groundwater discharge in spring outflow and exploitation Period Spring outflow/10s m3/a Exploitation/10s mS/a
1970 10.190 0.186
1985 7.23 0.83
1995 5.340 1.086
2000 5.17 3.62
1970-1985 -2.96 0.644
1986-2000 -2.06 2.79
in the plain is thought to be equal to that of crops, so that the impact of the transformation of the high cover grassland to crop on groundwater is ignored. The impact of forestland on groundwater evaporation is intensive, and one of the important plans for developing man-made oasis in the arid river basin is to construct the various nets of shelter belt so as to extend the forestland area. According to relevant experimental research in this region r the evaporation of woodland is about 2700 m3/ha. All the evaporation changes are calculated (Table 4). The other discharging items are spring outflow and exploitation, which are indirectly related to land use. Because the numerical relationship between them has not been obtained yet, the statistical and metrical data is used to measure the changes of spring outflow and groundwater exploitation ~ | (Table 5), in which the spring outflow in 2000 is the forecast result | The change of groundwater discharging caused by land use changes in Zhangye decreased by 2.035 x 108 m3/a averagely in the 15 years before 1985 (Tables 4 and 5), mainly related to sharp decrease of spring outflow; while in the 15 years after 1986 it increased by 0.6785 x 108 m3/a, mainly related to the slower decrease of outflow and the intensive increase of exploitation.
3 Balance analysis of groundwater resources variations 3.1 Changes of groundwater storage Based on the observation of groundwater level variations in every hydrogeologic unit, the changes of groundwater storage are calculated by formula (7) (Table 6). It indicates that the groundwater storage of the upper-middle part of the alluvial-diluvial fan presents an apparent declining trend, thereinto, the storage declined by 6.27 x 106 m 3 y r 1 in the 1980s, and 15.08 x 106 m 3 y r t in the 1990s, totally 220.7 x 106 m 3 in the two decades and averagely l l . 0 x 106 m 3 per year. In the lower part of the alluvial-diluvial fan, groundwater storage tended to decline generally, but the differences in spatial-temporal distribution existed. In the 1980s the storage declined slowly, thereinto, it decreased by 11.80x 106 m 3 totally in 1980-1989, or 1.2 x 106 m 3 per year, however, in 1981-1983 the storage did not decline but increased. In the 1990s, the storage in this area declined rapidly, totally 77.50 x 106 m 3 in the decade and averagely 7.8 x 106 m 3 per year. In the fine-grained soil plain, groundwater resource kept relatively stable, and the decline range was only 0.02 x 106 m3-0.33 x 106 m 3. In the river valley plain, the range of storage variations was small, and since the late 1980s, the storage showed an increasing trend.
3.2 Balance analysis of recharge and discharge of groundwater system Changes of regional groundwater recharging system: AQR = AQr + AQ,, + AQc + AQ~, based on this formula and the former results, the changes of recharge in different periods in Zhangye (L) Gansu Hydrogeologic Investigation Team: "Report of regional hydrogeologic investigation in Zhangye" (1998) (~)(~) Gansu Hydrogeologic Investigation Team: "Manual of Synthesized Hydrogeologic Maps of P. R. China, Zhangye: 1:200000" (1981)
Wang Genxu, Yang Lingyuan, Chen Ling et al.
412
Table 6 The annual changes of regional groundwater table and storage Year Subarea lApper-middle of alluvial-diluvial fan Lower of alluvial-diluvial fan Fine-grained soil plain River valley Zhang'~cere~ion
1981 (1984)-1985 Table Storage change change /m /10am3 -0.60 -0.32 0.03 -0.03 ~
-15.82" -3.42 0.11 -0.40 -19.53
1986-1990 Table Storage change change /m /106m3 -1.11 -1.06 -0.05 -0.22
-29.39 -11.21 -0.17 -2.95 -43.72
1991-1995 Table Storage change change /m /106m 3 -1.96 -1.89 -0.04 0.49
-52.04 -19.97 -0.15 6.77 -65.39
1996-2000(2001) Table Storage change change /m /106m3 -2.47 -2.43 -0.18 0.7
-65.60 -25.68 -0.66 0.93 -91.01
region are analyzed as follows: 1970-1985: AQR = -2.385 + 0.217 = -2.168x 108 m3/a; 1986-2000: AQR = -0.042 + 0.176 = 0.134x 108 m3/a. Changes of regional groundwater discharging system: AQo = AQE + AQM + AQs, where AQM and AQs represent the changes of groundwater exploitation and spring outflow, respectively. 1970-1985: AQo = -2.035 x 108 m3/a; 1986-2000: AQo = 0.6785 x 108 m3/a. The result: 1970-1985: AQR - AQD = -0.133 x 108 m3/a; 1986-2000: ~QR - AQo =-0.545 x l0 s mS/a. The above analysis indicates that the changes of groundwater recharging and discharging system caused by land use changes resulted in a groundwater storage decline by 0.133 x 108 m3/a before 1985, and 0.545 x l08 m3/a in 1986-2000. While the groundwater storage change calculated by the groundwater table variance (Table 6), it declined by 0.195 x l08 m3/a in 1981-1985 and 0.667 x l08 m3/a in 1986-2000. Comparing the results of the two calculating methods that the deviation of the results before 1985 is 0.062 x 108 m3/a and 0.126 x 108 m3/a in 1986-2000, and their relative errors are both less than 20%. All parameters are the testing results of the 1980s, which are unavoidably deviated from the real values, and the investigation of spring outflow after 1990 is absent, which also leads to some errors. Therefore, the analysis results have good veracity under such precision.
3.3 Analysis of groundwater table dynamics The different impacts of land use on groundwater storage lead to various characteristics of groundwater table dynamics (Figure 3a). In the middle-upper part of the alluvial-diluvial fan in the south of Zhangye Basin, the groundwater table is up to 30 m, mostly up to 50 m. Influenced by the decrease of riverways and increase of irrigation fields, the groundwater presented a continuous decreasing trend with an annual average decreasing range of 0.23-0.26 m. From the 1990s, it decreased more intensively with a total of 4.6-5.3 m in the two decades. In the lower part of the alluvial-diluvial fan located in the center of the basin, the aquifer is composed of multi-layer and confined groundwater. The groundwater table is 5-20 m. Since the 1980s, the irrigating fields were irrigated by the river water and groundwater together, partly irrigated only by wells. Before the 1990s, the groundwater table dynamics kept relatively stable or decreased slowly, while after the 1990s, however, it decreased rapidly (Figure 3a). In the fine-grained soil plain in the center of the basin, the complicated aquifer is composed of multi-layer, which is the main area where the spring and confined groundwater emerged. The area is the traditional spring and groundwater water irrigating area. Influenced by the decrease of spring water, large amount of river water were diverted for irrigation in the recent decade, the groundwater table kept stable and had no apparent variation in recent two decades (Figure 3b) although the groundwater storage had a slight decrease. In the river valley plain located in the northwest of the Zhangye region, irrigation was done mainly by the river water, thus the groundwater table increased slightly with the increase of irrigating field (Figure 3b). Figure 4 shows the groundwater table isolines in 1985 and 2000. For the lower part of the diluvial fan in the central and northwestem parts of Zhangye region, the groundwater table kept stable in most areas in recent 15 years, and distribution of regional groundwater table did not
Impacts of land use changes on groundwater resources in the Heihe River Basin
413
Figure 3 The groundwater table dynamic changes in different sub-zones
Figure 4 The groundwater table isoline in 1985 and 2000 have apparent variation. In the southeast edge of the region and the upper-middle part of the alluvial-diluvial fan, the groundwater table changed intensively, but the change was not indicated in the figure because of the lack of observing points. If the increase of exploitation is taken into consideration (rising to 1.62 • 108 m3/a), the balanced relationship between recharge and discharge in 1986-2000 should be: AQR - AQo = 1.457 x 108 m3/a, followed by the increase of regional groundwater storage. As a whole, land use changes would control the changes of groundwater resources as long as a reasonable amount of exploitation (less than 3.0x 108 m3/a in the study area) is kept. In recent 15 years, a large amount of groundwater has been exploited in the alluvial-diluvial fan, but the Changes of recharging system caused by the land use changes primarily happened in the fine-grained soil plain in the center of the basin and the river valley plain in the western part of the region, therefore, the apparent diversity of regional changes of groundwater resources appeared in the entire region. Groundwater storage in the upper-middle and lower parts of the alluvial-diluvial fan decreased continuously, but in the fine soil plain and river valley plain it increased apparently (Table 6).
4 Conclusions and discussion From the 1970s, when the large-scale development in Zhangye region of the Heihe River started, up to 1985, the groundwater recharge decreased by 2.168 x 108 m3/a due to the land use changes and the total impact of land use changes on the whole groundwater recharging system was
414
Wang Genxu, Yang Lingyuan, Chen Ling et al.
2.602 • 108 m3/a; while the discharge decreased 2.035 x 108 m3/a and the total variation range was 3.889 x 108 m3/a. All these changes led to 0.133 • 108 m3/a decrease of groundwater storage. In the latter 15 years, regional land use had further changed, which resulted in an increase of 0.1342 x 108 mVa of groundwater recharge and 0.6785 • 108 m3/a of groundwater discharge. All these changes made groundwater storage increased by 0.545 x 108 mVa. In recent three decades since 1970, land use had changed intensively in the former 15 years and the impact on groundwater recharging system occupied 92.27% of that of 30 years. Thereinto, the most important factors impacting the groundwater resources are the changes of riverway and irrigation canal areas, the impact of which on the groundwater recharge occupied 91.6% of the total change of recharging system. The remaining 8.4% came from the changes of irrigating areas. In all the discharging factors, spring outflow changed most intensively, and the evaporation, which was directly related to land use changes, occupied 7.33% of the total discharge. In the latter 15 years, the land use changes were relatively small, followed by smaller influence on groundwater, especially the impact of change of river and canal areas on groundwater recharge. In contrast to the former 15 years, the impact of change of irrigating area occupied 80.7% of the total recharge in the latter 15 years. The impact of land use and land cover changes on water resources system is intensive in arid inland river basin, and that the spatial-temporal distribution of surface water system changes continuously with the changes of land use pattern. Based on the above analysis, it is suggested that the exploitation and abandonment of natural riverways in the upper-middle part of the alluvial-diluvial fans should be reduced, and the development of irrigation cropland should be avoided in the fine-grained soil plain and river valley plain, however, the well irrigated cropland should be strictly controlled in the river valley plain and the t'me-grained soil plain.
References Alley W M, Reilley T E, 1999. Sustainabilityof groundwater resources. U.S. GeoL Surv. Circular, 11-86. Asmuth J R, Mass K, 2001. The method of impulse response moments: a new method integrating groundwater and eco-hydrologicalmodeling. IAHS Pub1., No.269: 51-58. Deng Huiping, Li Xiubin, Chert Junfeng et al., 2003. Simulation of hydrological response to land cover changes in the Suomo Basin. Acta Geographica Sinica, 58(1): 53-62. (in Chinese) Fu Bojie, Qiu Yang, Wang Jun et all., 2002. Effect simulations of land use change on the runoff and erosion for a gully catchment of the Loess Plateau, China. Acta Geographica Sinica, 57(6): 717-722. (in Chinese) Gao Qianzhao, Li Fuxing, 1990. Reasonable Development and Utilization of Water Resources in Heihe River Basin. Lanzhou: Gansu Science and Technology Press, 11-69. (in Chinese) Hoff H, 2002. The water challenge: Joint Water Project. Global Change Newsletter, No.50, 46-48. Huang Mingbin, Zhang Lu, 2004. Hydrological responses to conservation practices in a catchment of the Loess Plateau, China. Hydrological Process, 18: 1885-1898. Hutjes R W A, 1998. Biospheric aspects of the hydrological cycle. Journal of Hydrology, 212-213: 1-21. Lambin E F, Baulies X, Boekstael N E eta/., 2002. Land-use and land-cover change implementation strategy. Stockholm: IGBP Report No. 48 and IHDP Report No.10: 21-66. Mtembezeka P, Andrews A J, Appiah S O, 1997. Groundwater management in drought-prone areas of Afiica. Water Resources Development, 13(2): 241-261. Nunes, Auge J I, 1999. Land-use and land-cover change implementationstrategy. IHDP Report 10: 7-21. Pan Qimin, Tian Shuili, 2001. The Water Resources of Heihe River Basin. Zhengzhou: Yellow River Science Press, 39-128. (ln Chinese) Sato K, Iwasa Y, 2003. Groundwater Hydraulics. Tokyo: Springer-Verlag. Schwarts W F, Hubao Z, 2003. Fundamentals of Groundwater. New York: John Wiley & Sons. Suzanne Semeels, 2001. Priority questions for land use/cover change research in the next couple of years. LUCC Newsletter, 1-9. Wang Genxu, Qian Ju, Wang Yibo eta/., 2003. Impacts of land use change on environment in the middle reaches of the Heihe River. Journal of Glacialogy and Geocryology, 25(4): 359-367. (in Chinese) Zhang Jishi, Kang Ersi, Lan Yongchao et a!., 2003. Impact of climate change and variability on water resources in Heihe River Basin. Journal of Geographical Sciences, 13(3): 286-292. Zhang L, Dawas W R, Reece P H, 2001. Response of mean annual evapotranspiration to vegetation changes at catchment scale. Water Resour. Res., 37(3): 701-708.