September 25, 2008
Protection and Sustainable Utilisation of Water Resources in Lake Erhai Basin
By Yan, Changzhou Lu, Xin; Zhao, Xiaolin
Key words: Lake Erhai basin, water resources, eutrophication, ecological protection, sustainable utilisation, counter measures SUMMARYLake Erhai is the main water source for Dali City and the surrounding area. Economic development and population growth have caused serious environmental problems in the Lake Erhai basin. The annual water shortage in the basin and surrounding areas is about 324.95 million m^sup 3^. It is estimated that, by 2020, water consumption in the region will increase by 172 million m^sup 3^, of which 96 million m^sup 3^ will be used in Dali City alone. It is clear that the imbalance between the water supply and demand in the Lake Erhai basin will rapidly increase. In addition to water shortage, Lake Erhai also faces serious eutrophication and water quality deterioration. According to a survey in 2002, the pollutant loads of Lake Erhai were: COD^sub Cr^ 3008 t/a, TP 137.31 t/a and TN 1426.35 t/a. The nitrogen (TN) and phosphorus (TP) loads of the lake were far above its environmental capacity. Lake Erhai is now in a pivotal, critical and hypersensitive period of trophic state change. In order to address these problems, sustainable utilisation countermeasures, such as reducing industrial and agricultural water consumption, optimising the distribution and allocation of water resources, and ecological protection measures to control eutrophication are suggested.
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As the main water source for Dali City and the surrounding areas, Lake Erhai in Yunnan Province is facing a serious threat of eutrophication (Li 2001; Zheng et al. 2004). During the past 20 years, with rapid economic development and a rapid increase in population, the intensity of exploitation and utilisation of water resources has extensively increased in the Lake Erhai basin; therefore, the gap between water supply and demand has gradually grown and the quality of water has deteriorated (Zheng et al. 2004; Du 1994). Since the environment of Lake Erhai has a direct relationship with the ecosystem stability of the basin (He 2004), some measures to protect water resources and the eco-environment are required for the sustainable development of the basin.
Lake Erhai is in the Bai Autonomous Prefecture of Dali, Yunnan Province (25[degrees]25' to 26[degrees]16'N, 99[degrees]32' to 100[degrees]27'E), extending over Dali City and Eryuan County, with a catchment area of 2565 km^sup 2^ (Jin 1995). The Lake Erhai basin and its surrounding areas, such as the counties of Xiangyun, Binchuan and Weishan, have a serious shortage of fresh water (Luo and Fu 2003; Zhang et al. 2006). Although there are several lakes in the investigated area, available water resources are not abundant, and the imbalance between water supply and demand is serious due to geographical factors and uneven rainfall distribution. The current water resources utilisation rate of Lake Erhai has reached 90.7% (He 2004). The survey also showed that the shortage of water and uneven rainfall distribution are severely restricting economic development in the area, and many people live in poverty. Apart from Dali City, other areas in the watershed are all underdeveloped (Luo and Fu 2003).
Information on the water resources of Erhai Lake was compiled using various secondary sources including books, research papers, articles, databases and existing literature (Du 1994; Jin 1995; Luo and Fu 2003; He 2004; Zheng et al. 2004; Li 2001; SBDAP 2006; Zhang et al. 2006).
Samples for water quality analyses were collected each month at regular monitoring sites in Lake Erhai. Secci depth (SD) was measured in situ, with a 20 cm diameter Secchi disk to estimate water transparency. Biological oxygen demand (BOD), chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) were analysed according to methods outlined by the APHA (1992). Pollutant loads of Lake Erhai were investigated by the Dali Prefecture Environmental Monitoring Station in 2002, and calculated from the formula:
where P^sub i^ is the amount of pollutant i flowing into the lake; W^sub i^ is the amount of pollutant i from precipitation; q^sub k^ is the average flow of river k in the low water seasons; c^sub ik^ is the concentration of pollutant i of river k in the low water seasons; Q^sub k^ is the average flow of river k in the flood seasons; C^sub ik^ the concentration of pollutant i of river k in the flood seasons.
RESULTS AND DISCUSSION
The conflict between water supply and demand
According to statistics, the annual average water intake to the lake was 825 million m^sup 3^ and the outtake was 863 million m^sup 3^ in the past 35 years (He 2004), giving a 38 million m^sup 3^ shortfall. The current water resource utilisation in the Lake Erhai basin is: 700 million m^sup 3^ for generating electricity, 130 million m^sup 3^ for agricultural irrigation, 40 million m^sup 3^ used by industry, and 13 million m^sup 3^ for domestic purposes. A further 50 million m^sup 3^ of water were diverted from Lake Erhai to Binchuan County. The annual total water consumption was 933 million m^sup 3^, which exceeds the inflow water to Lake Erhai (He 2004) (Table 1).
Based on the rate of increase in population and economic development plans in the studied areas, water demands can be forecasted. Even without any increase in water use for agriculture, by 2020 industrial and domestic water requirements will increase by 172 million m^sup 3^, of which urban water demand will increase by 96 million m^sup 3^ in Dali City at a rate of 6 million m^sup 3^/a. Furthermore, the water going into Lake Erhai will decrease by 120 million m^sup 3^ following the establishment of the Meihe irrigation area around the Haixi Reservoir in Eryuan County, and the amount of water diverted from Lake Erhai to Binchuan County will increase from 50 million m^sup 3^ to 150 million m^sup 3^. The gap between water supply and demand will become larger; therefore, protecting and using water resources rationally will be key for economic development of the Lake Erhai basin.
Water pollution of Lake Erhai
Before the 1970s, water in Lake Erhai was abundant, the water quality was very good, and the ecosystem was healthy (Du 1994). However, over-exploitation and over-utilisation of water resources not only caused the water level and catchment area to decrease, but also accelerated eutrophication. Before 1976, Lake Erhai was 1973.53- 1974.30 m deep, the lake covered about 255 km^sup 2^ and contained 2.9 billion m^sup 3^ of water (Li 2001). After the construction of the Xi'er River Power Station in 1977, water was used for generating electricity, and the average water level of Lake Erhai had decreased to 1972.30 m by the beginning of the 1990s; the surface area was 242 km^sup 2^ and the volume was 2.49 billion m^sup 3^, and the water quality also gradually worsened (Du 1994; Li 2001). In 2003, algal blooms covered the surface of Lake Erhai, SD decreased from 3.0-5.0 m to 0.4-1.5 m, and the trophic status changed from mesotrophic to eutrophic. Even if the amount of nutrient inflow remains stable, the reduced water level is one of the main reasons for Lake Erhai eutrophication and the water blooms in 1996, 1998 and 2003 (Table 2).
The community structure of the phytoplankton in Lake Erhai has changed greatly with the progress of eutrophication and the number of species has decreased but the density and biomass has increased rapidly (Wu and Wang 1999). The dominant species changed from Cryptophyta and Bacillariophyta to Cyanophyta (e.g. Microcystus, Chroococcus, Aphanizomenon flos-aquae), indicating that Lake Erhai had become eutrophic (Figure 1). Between 1995 and 2003, Cyanophyta increased from 2.124 to 20.201 million/L, Chlorophyta decreased from 1.313 to 1.039 million/L, and Bacillariophyta increased from 0.764 to 4.941 million/L. In July of 2003, phytoplankton blooms in Lake Erhai reached 60 million/L, showing that the ecosystem was already damaged and was still undergoing rapid degradation.
The pollutant loads of Lake Erhai mostly came from non-point sources. The main sources of N and P were village sewage, farmland runoff, water and soil loss and livestock manure, contributing over 80% of the N and P loads (Pan et al. 1999). Pollutants came into Lake Erhai through rivers and airborne dust via precipitation. The total COD was 3007 t/a, TP was 137.31 t/a and TN was 1426.35 t/a, and these loads from the Miju, Luoshijiang, Yong'anjiang, Boluojiang and Wanhuaxi Rivers accounted for 75.4%, 38.6% and 54.1% of the total loads, respectively (Table 3). The total loads of N and P exceed the environmental capacity of Lake Erhai (Pan et al. 1999).
Protection and sustainable utilisation of water resources
Defining the function and implementing sustainable development plans in drainage areas
Lake Erhai is not only the centre of a State Nature Reserve in the Great Cangshan Mountain area, but also the mother lake of the Dali people. Based on the social and economic development condition of the Lake Erhai basin and the fact that Dali City is a tourist and commercial city, the main lake functions are sightseeing and water supply. At the same time, the balance between the main functions and other functions should be scientifically regulated, and conflicts between development and protection also should be addressed through policies and laws. In the course of urbanisation, Dali City, as a key city in the west of Yunnan Province, should focus on development of the lower reaches of Lake Erhai such as Xiaguan District, Fengyi Town and a new city zone, rigorously restrict the size of townships and industrial structures in Eryuan County (north of Lake Erhai), and encourage development of biological resource-oriented industries that have high economic benefits and low pollution. Furthermore, the land-use plan for the Lake Erhai basin must be enforced, and the construction of holiday villages, hotels and restaurants should be strictly prohibited in the lakeside zone. At the same time, bottomlands and swamps should be restored through returning farmland and fishponds to the lake, as well as moving people from the lakeside.
Water consumption of industry and agriculture
Due to unreasonable industrial structures and out-dated equipment and techniques, the rate of water reuse is low, and water consumption per 10,000 Yuan output value is 106 m^sup 3^ (Luo and Fu 2003), higher than the national average of 58.3 m^sup 3^. Therefore, the industrial structure should be adjusted, with emphasis on industries with less water consumption. Furthermore, water saving policies, new equipment and techniques should be adopted to improve water reuse and to reduce water consumption.
Analysis of water resource condition showed that Eryuan County requires more irrigation projects, and Dali City, Binchuan, Weishan and Xiangyun Counties are short of water. Water saving measures for agricultural water should be adopted on the basis of the characteristics of irrigation areas. In those that have few irrigation projects, multiple crop indexes need to be improved. Irrigation projects should be built in these areas, along with comprehensive water saving polices. In irrigation areas that are short of water, multiple crop indexes should not be further increased, but measures to save agricultural water should be implemented and the management of water use should be strengthened. Irrigation systems should be upgraded for better water usage. The rate of water use needs to be improved from 0.62 now to 0.68 by 2010 and 0.74 by 2020.
Ecological protection measures
In order to slow down the eutrophication of Lake Erhai, ecological protection measures should be taken by treating urban sewage, rehabilitating riparian vegetation of the main inflow rivers (Gregory et al. 1991), constructing eco-agriculture, improving the rural environment, restoring aquatic ecosystems (Moss 1990; Jin et al. 2006), and conserving soil and water.
Adjusting the water level and ensuring ecological water demand
Water level plays an important role in a lake ecosystem (Medeiros and Esteves 2002; Jin et al. 2006). In order to alleviate the negative environmental impacts of hydropower stations, the low and high water levels of Lake Erhai should be adjusted to 1972.61 m and 1974.31 m, respectively, to meet the ecological water demand.
Optimising water resources distribution of the Lake Erhai basin and surrounding areas
After pollution control and ecological restoration, there will be a great improvement in the ecological environment of Lake Erhai; however, shortage of water resources is still the primary problem. To solve this problem in Dali City, Xiangyun, Binchuan and Weishan Counties, it is necessary to divert clean water to Lake Erhai from other rivers. At present, the rate of water exploitation and utilisation is very low in Jianchuan and Yangbi Counties, where water resources are plentiful. By analysing and calculating water requirements, a regulating reservoir could be constructed on the Taoyuan River in Jianchuan County without causing significant impacts to the ecological water demand in Yangbi County, and an annual average of 150 million m^sup 3^ of water could be diverted to Lake Erhai.
This study was supported by the Chinese Academy of Sciences (KZCX2-YW-422). The authors thank Dali Prefecture Environmental Protection Bureau and Prefecture Environmental Monitoring Station for providing essential data. Additionally, the authors are grateful to Dr. Dali Wang of the Southeastern Universities Research Association (SURA) for his review and comments.
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Changzhou Yan1, Xin Lu1 and Xiaolin Zhao2
1 Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, China
2 Freshwater Fisheries Sciences Institute, Liaoning, China
Correspondence: Changzhou Yan, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361003, China. Email: [email protected]
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