Numerical assessment of the effect of water-saving irrigation on the water cycle at the Manas River Basin oasis, China

Journal Pre-proof Numerical assessment of the effect of water-saving irrigation on the water cycle at the Manas River Basin oasis, China

Guang Yang, Lijun Tian, Xiaolong Li, Xinlin He, Yongli Gao, Fadong Li, Lianqing Xue, Pengfei Li PII:

S0048-9697(19)35582-2

DOI:

https://doi.org/10.1016/j.scitotenv.2019.135587

Reference:

STOTEN 135587

To appear in:

Science of the Total Environment

Received date:

17 August 2019

Revised date:

15 October 2019

Accepted date:

15 November 2019

Please cite this article as: G. Yang, L. Tian, X. Li, et al., Numerical assessment of the effect of water-saving irrigation on the water cycle at the Manas River Basin oasis, China, Science of the Total Environment (2018), https://doi.org/10.1016/j.scitotenv.2019.135587

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Journal Pre-proof

Numerical assessment of the effect of water-saving irrigation on the water cycle at the Manas River Basin oasis, China Guang YANGa,b,c, Lijun TIANc, Xiaolong LIa,b, Xinlin HEa,b*, Yongli GAOc, Fadong LId,e*, Lianqing Xuea,f , Pengfei LIa,b

College of Water and Architectural Engineering, Shihezi University, Shihezi, China; [email protected]

a

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(G.Y.); [email protected](X.L. ); [email protected](X.H.); [email protected](P.L.); Xinjiang Production and Construction Group Key Laboratory of Modern Water-saving Irrigation,

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b

c

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Shihezi, China

Department of Geological Sciences, Center for Water Research, University of Texas at San Antonio,

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Texas, United States; [email protected](L.T.); [email protected](Y.G.); Institute of Geographic Science and Natural Resources Research, Chinese Academy of Sciences, Beijing,

China; [email protected] (F.L.)

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d

University of Chinese Academy of Sciences, Beijing, China

Hydrology and water resources College, Hohai University, Nanjing, China; [email protected](L.X)

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f

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e

* Correspondence: E-mail: [email protected] (X.H.); Tel: +86-153-0993-4677 [email protected] (F.L.); Tel: +86-185-0050-5484

Abstract Mulch drip irrigation is widely used in the arid areas of Northwest China; Consequently, the Manas River Basin has developed into the fourth largest irrigated agricultural area in China. In this study, a groundwater model of the regional water cycle was developed to quantitatively assess the groundwater balance in response to different irrigation schemes, including traditional irrigation, conventional water-saving irrigation, and high-efficiency 1

Journal Pre-proof water-saving irrigation schemes. Our results reveal that 1) The water-saving irrigation technology has affected the water cycle process in farmlands. The higher the degree of water conservation, the lower the infiltration into groundwater, the higher the deficit of the groundwater balance, and the more significant the decline of the groundwater level. 2) The groundwater at the Manas River Basin remains in a negative equilibrium state. To achieve an equilibrium state of the groundwater at the Manas River Basin, the catchment management

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agencies should restrict the scale of oasis development and the utilization of groundwater.

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Keywords: water-saving irrigation; water cycle; groundwater modelling; Manas River Basin.

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1 Introduction

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The Manas River Basin(MRB), a typical arid and semi-arid oasis, is located in Northwest China in the hinterland of the Eurasian continent (Fig. 1). Water scarcity and ecological

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fragility are the main factors restricting economic and environmental development in this

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region (Shen and Lein, 2005). The water in this region mainly come from precipitation and river canals. The MRB is characterized as a typical ‘desert oasis’ in which irrigated agriculture is practiced and the proportion of used agricultural water is 94%. Water-saving irrigation (WSI) agricultural practices are crucial for solving the water scarcity challenges and maximizing agricultural production and farm income (Li, 2001; Pereira et al., 2002; Singh et al., 1978). In China, as the first region to adopt the WSI technology, the MRB has developed into the largest oasis farming area in Xinjiang and the fourth largest irrigated agricultural area in China (Wang et al., 2019). WSI technology has greatly improved the utilisation efficiency of agricultural water resources (Pereira et al., 2002). The total irrigated area showed expanded rapidly with the application of WSI technology. However, the caveat is that the widely employed WSI 2

Journal Pre-proof technology causes direct human interference with the natural hydrological cycle of an irrigated area. As climate change and human activities increasingly alter ecosystems in the MRB, it is essential to ascertain if the large-scale use of the WSI technology in the basin affects the hydrological cycle elements and patterns while promoting oasis development, and further destabilises the fragile ecosystem at the regional scale (Fu et al., 2003; Sepaskhah and Tafteh, 2012). Significant concerns regarding the vulnerability of water resources and the hydrological cycle have been raised by the public, policy-makers, and researchers (Lyu et al.,

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2019).

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To date, most researches have primarily focused on the water cycles in the inland river

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basins of arid Northwest China (Lu et al., 2016; Yang et al., 2017). These studies have focused

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on the impacts of the WSI technology on soil water and salt transport, using the qualitative or semi-quantitative approaches (Autovino et al., 2018; Honari et al., 2017; Phogat et al., 2013;

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Ren et al., 2016). We have assessed the influence of the WSI technology on the hydrological

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cycle of an irrigated area using a groundwater model, to quantitatively analysed the influence of the WSI technology on the groundwater. The main contribution of this study is

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introduction of a gradient theory of different degrees of water-saving to facilitate the analysis of the different impacts of traditional irrigations, conventional WSI, and high-efficiency WSI on the water cycle in the inland river basin. We employed a quantitative method to evaluate the water cycle in the Basin. By analysing the changes in the groundwater level, we identified the influences of water-saving conditions on the hydrological cycle of the MRB. The results of this research can help achieve sustainable utilization of regional water resources and ecological security of the MRB.

2 Materials and Methods

2.1 Overview of the Study Area

3

Journal Pre-proof The MRB (N4327'–N4521', E8501'–E8632') is located in the middle of the northern foot of the Tianshan Mountains in Xinjiang at the southern margin of the Junggar Basin (Fig. 1). The total area of the basin is 3.4104 km2, which mainly includes Shihezi City, Manas County, and Shawan County. Over the past twenty years, various water conservation projects involving the WSI technology have implemented in the MRB, and the WSI area accounts for 94.8% of this area (Guang et al., 2017). As a result, the water use efficiency and agricultural productivity of this region have been greatly improved. However, the rapid expansion of

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cultivated land since 1996 has led to a significant increase in the total amount of water used

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for agricultural water use. The large-scale WSI in the MRB has altered the spatial and

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temporal patterns of the natural variations in its hydrological cycle and the integrity of its

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hydrological system. These changes have disturbed the MRB’s fragile hydrological and ecological equilibrium. For example, the discharge at the downstream of the rivers has

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decreased significantly and might no longer meet the requirements of the ecological

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environments. The fragile balance between the oasis and the desert has been disturbed, which has caused induced effects on the ecological environment, including a decline in the

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groundwater level and the degradation of the local natural vegetation.

2.2 Influence of WSI on MRB

2.2.1 Reduction in man-made canals Since 1976, following the implementation of water conservancy projects, completion of the reservoirs and improvement of the man-made canal system, the volume of artificial water storage has increased over a large area, leading to the replacement of the natural water system with artificial systems. The fastest development period of the man-made canal system occurred from 1976–1997, which resulted in an 89% increase in the total length of the man-made system (Tab. 1). After 1999, due to the large-scale applications of the water-saving drip irrigation technologies, many man-made canals were not maintained. The total length of 4

Journal Pre-proof the man-made canal system had declined by 69% from 1997 to 2015 (Tab. 1). 2.2.2 Agricultural water usage Water-saving agricultural practices are crucial to successful water management in the basin because agriculture is the major consumer of water in the MRB. WSI technologies have greatly improved the utilization efficiency of agricultural water resources. However, the percentage of the total water used by agricultural activities still exceeds 94% as of 2015 (Tab. 2).

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2.2.3 Increase in the irrigation area

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In the MRB, the total irrigation area exhibited a minor change prior to the application of

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the WSI technology. In 1949, the total irrigation area of the basin was 2.47104 ha, which

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increased to 15.5104 ha in 1999, with an average annual increase of 0.25104 ha (Fig. 2). In 1999, the average annual growth of the irrigated area was 0.51104 ha. From 1999–2018, the

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fluctuations in the irrigation areas changed from a small increase to a significant growth trend

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(10.2104 ha).

2.2.4 Reduction in soil evaporation

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Due to utilisation of film mulching in WSI, the soil establishes a relatively independent water cycle system. The water-saving features of the plastic film mulching technology cause the water, which evaporates from the soil under the film, to condense into water droplets on the film and return to the soil. Consequently, the film mulching reduces evaporation from the soil, and the surface soil layer can maintain high water content. Fig. 3 shows the evaporation that occurs in the soil at the site scale, which was obtained by comparing the evaporation data for the soil covered by a plastic film mulch and un-mulched soil (i.e. bare soil). The evaporation trend curve is similar for both practices, but the evaporation from the un-mulched soil is significantly greater (31.8%) than that from the mulched soil. 2.2.5 Reduction in soil infiltration The mulch drip irrigation has distinct effects on soil infiltration (Yang et al., 2017). WSI 5

Journal Pre-proof can reduce the water content in the soil by 30–40%(Tab. 3). Under the condition of drip irrigation, the uniformity of the soil water improved significantly. Under 70cm, the soil water content of the drip irrigation system was apparently lower than that of the flood irrigation system (Tab. 3). 2.2.6 Decline in groundwater levels Since 1999, a large proportion of the irrigated area has been switched from the flood-irrigation system to the WSI technology. Therefore, the recharge of groundwater by

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irrigation processes has significantly decreased, although groundwater extraction has greatly

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increased to meet the needs of the expanded agricultural area and irrigation water shortage.

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The total volume of groundwater resources in the MRB totals 4.2  108 m3. The amount of

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groundwater pumped in the basin has increased from 47% of the total groundwater in 1990 to

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61% in 2015. Groundwater levels show a continuously decreasing trend (Fig. 4).

2.3. Groundwater model of MRB with Mulch Drip Irrigation

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2.3.1 Visual-MODFLOW Model

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Numerical simulations of the groundwater flow in the oasis of the MRB were performed using the Visual-MODFLOW4.2 software (Anderson and M., G., 2005; Mcdonald and Harbaugh, 2003), in which the groundwater recharge sources included rainfall, irrigation, and channel infiltrations (Brunner et al., 2010; Xu et al., 2011; Xu et al., 2012). By changing the recharge and discharge parameters of groundwater, the software could simulate in the groundwater level. We generalised groundwater into heterogeneous homomorphism three-dimensional unsteady flow aquifer systems. The groundwater flow can be described by Equation (1) (Sokol, D.,1963; Li et al., 2016):

  H    H    H  H    k ( x, y, z )  D （1） k    k  W   x  x  y  y  z  z  t 6

Journal Pre-proof H ( x, y, z ) t 0  H 0 ( x, y, z )( x, y, z )  D H

k

B1

H n

= H1 (x, y, z, t)(x, y, z)  B1, t > 0

B2

 q(x, y, z, t)(x, y, z)  B2, t > 0

where D is the seepage area; K is hydraulic conductivity (m/d); H is the head of the water level (m); W is the source item (m/d); µ is the aquifer water storage coefficient ; H0 (x, y, z) is the initial flow head distribution (m); n is the direction of the second boundary; H1 (x, y, z, t) is

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the first type of boundary head distribution (m); B1 is the first type of boundary; q (x, y, z, t) is

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the second type of boundary single-width flow (m3/d); B2 is the second type of boundary; and

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t is time (d).

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2.3.2 Hydrogeology and aquifer characterization

The basin was discretised into 400 columns and 410 rows with a grid size of 360 m × 560

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m (about 0.2 km2). Based on the hydrological and geological profiles, the vertical direction

2.3.3 Boundary conditions

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was discretised into ten layers(Fig.5).

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Both south and north sides of the study area are the lateral recharge boundary, which can be generalised to the second type of the flow boundary (Fig. 6). It is regarded as the constant head boundary in the model calculation, and is assigned by the GHB module; the east side is the Taxi River; the west boundary is located at the outer edge of the sector. The groundwater flow direction is generalised to the second type of the water-blocking boundary. The Wall module is used for the value assignment in the model. The lowest boundary of the groundwater simulation is the bottom of the confined aquifer, which is generalised to the water-blocking boundary. 2.3.4 Model Calibration The initial model parameters were obtained from the literature and field tests, and

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Journal Pre-proof subsequently further calibrated using the model parameter identification module and manual tuning (Mooers et al., 2018; Rajamanickam and Nagan, 2010; Scibek et al., 2007; Zhu et al., 2018). The WHS solver module was used to provide model solutions. The MODFLOW software uses the absolute residual mean (ARM) and the normalised root mean square (NRMS) to test the accuracy of the model (Lyu et al., 2019; Wang et al., 2008), for which the calculation formulas are as follows:

n

R

1

n

i 1

i

n

Ri  i

2

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RMS 

1 n

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ARM 

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R i = Xical - Xiobs

1

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RMS (X obs ) max - (X obs ) min

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NRMS =

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where Ri is the residual value, m; Xical is the calculated value, m; Xiobs is the observation value, m; ARM is the absolute residual mean, m; RMS is the root mean square, m; and NRMS is the

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normalized root mean square, %. The model has a small errors and can substantially represent the movement state of groundwater (Tab. 4).

2.4 Water-saving scenarios

To promote economic and social development that is compatible with the storage capacities of water resources, government proposed clear primary objectives for water resources development and utilisation control and water use efficiency control. In this study, with reference to the water-saving planning index of the MRB, three irrigation schemes were simulated(Tab.5). the changes in the groundwater level were analysed in terms of the traditional irrigation, the conventional WSI, and the high-efficiency WSI. Scenario 1: Traditional irrigation. Flood irrigation adopts traditional practices, with the 8

Journal Pre-proof land irrigation quota set at 6,750 m3/ha, and the groundwater extraction is 5.28 × 108 m3; the drip irrigation and the groundwater extraction period remains April–October. Scenario 2: Conventional WSI. Drip irrigation with a membrane mulch is adopted as the irrigation practice, with a quota of 5,250 m3/ha and the groundwater extraction is 3.58 × 108 m3; the drip irrigation and the groundwater extraction period remains April–October. Scenario 3: High-efficiency WSI. The irrigation quota is reduced to 4,500 m 3/ha; the groundwater extraction is 1.88 × 108 m3, the irrigation and the groundwater extraction period

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remains April–October.

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3. Results and Discussion

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3.1 Results (1) Groundwater balance analyses

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Due to the reduction of the irrigation infiltration, our modelling results indicate that high

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degree of water conservation results in considerably negative groundwater balances. Scenario 2 (conventional WSI) increased the groundwater deficit by 1.90 × 10 8 m3 compared with the

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case of Scenario 1 (traditional irrigation). Scenario 3 (intensified WSI) increased the negative balance of the Scenario 2 groundwater recharge capacity by 0.07 × 108 m3(Fig.8). (2) Groundwater level analysis

Our modelling results show that the higher the degree of water-saving and the lower the groundwater level, the more significant the reduction of the groundwater recharge capacity and the greater the decline in the groundwater level(Fig.8). The results also indicate that the groundwater level declines more under the conventional irrigation scheme (Scenario 2) than under the traditional irrigation scheme (Scenario 1). The groundwater level in the study area under the high-efficiency WSI is lowest of the three irrigation schemes.

3.2 Discussion 9

Journal Pre-proof Through the numerical simulations of the groundwater depth in different scenarios, it is evident that with an increasing in the irrigation water use efficiency (6750 m3/ha, 5250 m3/ha and 4500 m3/ha) and a decreasing groundwater extraction (5.28 ×108m3, 3.58×108 m3 and 1.88×108 m3), the groundwater level in the irrigation area supposedly declines. Jiang (2016) found that the groundwater recharge caused by the drip irrigation reduced to the underground and the contribution ratio of the change in the water level decline was 24.7%; the contribution of increased groundwater exploitation to the change of annual groundwater

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level decline was 75.3%.

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The depth of the groundwater in the irrigation area of the MRB is significantly affected

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by human activities, particularly agricultural irrigation. It is evident that there is a decrease in

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both the groundwater recharge capacity and drainage in the irrigation area; however, the deficiency is gradually increasing under the three scenarios. In addition, the exchange flux

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and groundwater dynamics were significantly altered by the application of WSI technology.

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The exchange flux at the groundwater table is generally downward(310.5mm/year), particularly during the drip irrigation period and the spring flush period (Zhang et al., 2014).

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This phenomenon proves that the decreased extraction of groundwater stabilises the dynamic changes in the groundwater level, whereas the reduction in the infiltration reduction of groundwater recharge is attributed to a significant decline of the groundwater level. The water-saving technology exerted both positive and negative effects. Although WSI technology improves the efficiency of water resource utilisation, it also affects the water circulation process, thus contributing to the decline in the groundwater level. In the process of farmland-scale water circulation, water-saving technology affects the infiltration of soil water to groundwater, modifies the vertical recharge of groundwater, and subsequently controls the change in the groundwater level. This could increase the risk of eco-environmental degradation and require improved governance schemes. Therefore, the scale and scope of the oasis and the utilisation of groundwater in the oasis should be carefully 10

Journal Pre-proof considered (Yang et al., 2019).

4. Conclusions (1) WSI technology has significantly affected the water cycle in farmland. The higher the degree of water conservation, the higher the groundwater imbalance. With the traditional irrigation system, the total recharge capacity of groundwater is 47.04 × 10 8 m3 and the total discharge capacity is 47.17 × 108 m3, with a difference of -0.13 × 108 m3, placing a

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near-equilibrium state for the overall water balance. With conventional WSI practices, the

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groundwater recharge capacity of the study area is 44.94 × 10 8 m3 and the total discharge capacity is 46.97 × 108 m3. Thus, the groundwater is in a negative equilibrium state with a

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deficiency of -2.03 × 108 m3. By practicing highly efficient WSI, the groundwater recharge

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capacity amounts to 44.07 × 108 m3 and the total discharge capacity is 46.17 × 108 m3. Thus, the

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groundwater is still in a negative equilibrium state with a deficiency of -2.1 × 10 8m3. (2) The higher the water conservation level, the more significant the decline in the

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groundwater level. Compared with traditional irrigation, conventional WSI technology

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results in lower groundwater levels, mainly due to the decrease in the infiltration into the groundwater. The spatial extent of this decline in groundwater is generally higher in the south of the basin than in the north. With a highly efficient WSI scheme, the groundwater level declines even further than that with the conventional WSI. (3) To achieve an equilibrium state of groundwater at the MRB, the scale and scope of oasis and the utilisation of groundwater should be controlled by the catchment management agencies. Acknowledgements This work was supported by the National Natural Science Foundation of China [grant numbers U1803244，41601579]; Key Technologies Research and Development Program [grant number 2017YFC0404303]; Xinjiang Production and Construction Corps [grant numbers 11

Journal Pre-proof 2018CB023, CZ027204 ， 2018AB027, 2018BC007], and Shihezi University [grant number CXRC201801, RCZK2018C22]. The work was also supported by the China Scholarship Council. We used the Zenodo general repository to provide direct public access to all data presented in this paper (http://doi.org/10.5281/zenodo.1166252). Conflict of Interest The authors have no conflict of interest to declare. Author Contribution statement

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Guang YANG, Lijun Tian, Xinlin HE, Yongli Gao, Fadong LI and Lianqing Xue wrote the

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main manuscript text. Xiaolong LI and Pengfei LI prepared figures. All authors reviewed the

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manuscript.

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Data availability statement

Reference

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We declare the availability of the data in our manuscript.

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Journal Pre-proof Figure 1. Location of the study area

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Figure 2. Irrigation area changes due to WSI in the MRB, 1949, 1999 and 2016. Figure 3. Evaporation from soil covered by a plastic mulch film and bare soil (non-mulched). Figure 4. Groundwater distributions in the Manas River Basin (1990 left, 2015 right). Figure 5. Hydrogeological profile of the Manas River Basin. Figure 6. Generalization of boundary conditions in the study area. Figure 7. Actual observations and simulated water levels (heads) in typical observation wells. Figure 8. Comparison of the water balances under three irrigation schemes. Figure 9. Groundwater flows in response to traditional irrigation (left panel), water-saving conventional irrigation (middle panel) and high-efficiency WSI (right panel).

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Table 1. Lengths of the artificial canal system in the Manas River Basin (km), 1976–2015. Trunk canal

Branch canal

Lateral canal

Field ditch

Total length

1976

920

722

2469

6328

10439

1989

1013

815

3059

9665

14552

1997

1058

955

3172

14517

19702

2006

1075

1056

3552

9399

15082

2015

1170

1260

3674

—

6104

Jo ur

na

lP

re

-p

ro

of

Year

17

Journal Pre-proof Table 2. Water utilisation in the Manas River Basin (%), 1990–2015. 1995

2000

2005

2010

2015

96.53

96.04

95.65

94.57

94.27

94.03

1.53

1.83

2.28

3.24

3.20

3.53

1.1

1.11

0.82

1.18

1.36

1.42

0.84

1.01

1.25

1.01

1.18

1.02

Jo ur

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of

Agricultural water consumption Industrial water consumption Domestic water consumption Ecological water consumption

1990

18

Journal Pre-proof

Table 3. Soil water content at different depths under mulched drip irrigation and flood irrigation. Drip irrigation

Soil depth

May

June

Flood irrigation July

August

May

June

July

August

（cm)

m /m

m /m

m /m

m /m

m /m

m /m

m /m

m3/m3

30

12%

13.95%

14.5%

13.3%

16.8%

15.3%

14.8%

13.6%

50

12.3%

14.8%

14.6%

14.05%

15.6%

15.55%

15.1%

14.7%

70

11.1%

14.35%

13.65%

14.05%

15%

15.9%

13.9%

15.75%

100

9.2%

11.5%

10.7%

10.1%

14.8%

14.9%

13.2%

12.5%

3

3

3

3

3

3

3

3

3

3

na

lP

re

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of

3

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3

19

3

3

Journal Pre-proof

Table 4. Soil water contents at different depths under mulched drip irrigation

Calibration period NRMS

ARM

RMS

NRMS

1.33 1.877 4.628 3.639

1.81 2.215 5.647 4.387

0.97 1.236 2.943 2.417

0.759 1.699 3.687 2.733

1.188 2.319 4.661 3.496

0.635 1.269 2.519 1.947

lP

re

-p

ro

of

RMS

na

30 days 120 days 240 days 300 days

ARM

Jo ur

Time

Validation period

20

Journal Pre-proof

UNIT: m3/ha

Table 5. Irrigation scheme

Irrigation quantity

540 367.5 315

1620 1155 1035

4117.5 3307.5 2790

472.5 420 360

6750

re

-p

ro

of

Flocking period

lP

Scenario 3

Blossing stages

na

Scenario 2

Bud stage

Jo ur

Scenario 1

seedling stage

21

5250 4500

Journal Pre-proof

HIGHLIGHTS 1. Water-saving irrigation (WSI) technology is crucial to the oasis development at the Manas River Basin (MRB). 2. WSI has altered the process of water circulation in farmland, and there is a significant decline of the groundwater level due to less infiltration

of

to the groundwater. 3. To achieve equilibrium state of groundwater at the MRB, the Water

-p

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lP

re

utilization of groundwater.

ro

Bureau should restrict the scale of oasis development and the

22

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9