Estuaries and Coasts (2012) 35:892–903 DOI 10.1007/s12237-012-9477-z
Objective-Based Method for Environmental Flow Assessment in Estuaries and Its Application to the Yellow River Estuary, China Tao Sun & Jing Xu & Zhifeng Yang
Received: 31 March 2011 / Revised: 4 January 2012 / Accepted: 10 January 2012 / Published online: 7 February 2012 # Coastal and Estuarine Research Federation 2012
Abstract We developed an objective-based method for assessing environmental flows in estuaries; this method consists of two steps: identifying ecological objectives with temporal–spatial variability and establishing a relationship between variations in environmental factors and the alteration of freshwater inflows. Critical salinity and water depth requirements for different species in special seasons in addition to temporal variation in natural river discharge were combined as objectives with spatial and temporal variability. In a case study of the Yellow River Estuary, we determined that 15% and 101% of the natural river discharge should be provided to ensure the minimum and maximum levels of environmental flows, respectively, for successful integration of various objectives. Periods in early April, the end of June, August, and early October were identified as critical for fulfilling reasonable water requirements. Although the recommended environmental flows may not be ideal for certain types of species, they offer a boundary of environmental flows for preserving habitats and biodiversity in estuaries. Keywords Environmental flow assessment . Ecological objective . Temporal-spatial variability . Yellow River Estuary
Introduction Environmental flows, also known as instream flows, represent the quantity, quality, and timing of the water flows required to sustain freshwater and estuarine ecosystems and the human T. Sun (*) : J. Xu : Z. Yang State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China e-mail:
[email protected]
well-being that depend on these ecosystems (The Brisbane Declaration 2007). Various methodologies for environmental flow assessment have been developed that include hydrological, hydraulic, habitat, and holistic methods (Tharme 2003). However, applications of these methodologies have been limited in estuaries (Sun et al. 2009) due to the complexities in estuarine ecosystems under the combined action of river discharge and tidal flows. In an estuary, variations in environmental factors, biological distribution patterns, and habitat losses are threatened easily by upstream freshwater withdrawal or by alteration of the natural flow regime resulting from the construction and regulation of hydraulic projects (Kennish 2002; Sierra et al. 2004; Sun et al. 2008; Więski et al. 2010). Defining environmental flows in an estuarine ecosystem has become a critical issue in global water resource management. Alber (2002) classified management techniques for assessing freshwater inflows in estuaries as inflow-, condition-, and resource-based approaches. To date, most methods to assess environmental flows in estuaries have been established on the basis of correlative empirical or statistical relationships between typical ecological objectives and freshwater inflow alteration (Kimmerer 2000; Doering et al. 2002; Fleenor et al. 2010). In general, goals specific to fisheries were emphasized as priorities in clarifying freshwater flow requirements of estuarine fisheries and ecosystem management (Robins et al. 2005; Eisma-Osorio et al. 2009; Vasconcelos et al. 2010). Jassby et al. (1995) presented a rule known as X2 to identify the freshwater inflows required by various natural resources; this rule is based on the relationship between the locations of the two bottom salinities in San Francisco Bay and the annual measures of different biological estuarine resources. A series of relationships between historic monthly inflow and the harvesting of various fish species was examined by the Texas Estuarine Mathematical Programming (TxEMP) model to achieve an optimized inflow/harvest relationship (Powell et al. 2002).
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However, it was also determined that causality in the relationships between freshwater inflows and biological distribution often remains unproven because of the complex nature of biological responses to hydrological changes (Ardisson and Bourget 1997; Turner 2006; Petts 2009). For example, floods may have short-term negative consequences for oyster harvesting but play vital roles in ensuring the long-term health of oyster populations (Buzan et al. 2009). Close relationships between estuarine fishery production and freshwater flow, which have been established for certain ecosystems, are also limited in their application to other estuaries (Jassby et al. 1995; Robins et al. 2005). Species generally exhibit different and often conflicting life history strategies, which may create more complexities and uncertainties in environmental flow assessments. In recent years, the preservation of biodiversity and ecological services has been suggested as further objectives in these assessments (Arthington et al. 2010). As a result, integrating diverse ecological objectives with temporal and spatial variability has become a critical issue in environmental flow assessments, considering the different requirements for multiple objectives in estuaries. Rather than defining environmental flows in estuaries on the basis of a special objective, temporal and spatial variations in ecological objectives were adopted in an objective-based method developed in the present study to assess environmental flows for ecosystem protection in estuaries. On the basis of the integration of objectives with spatial and temporal variability for species diversity, we defined a boundary of environmental flows for preserving habitats and biodiversity in estuaries instead of an ideal result for certain types of species. In this paper, environmental flows and river discharge in the Yellow River Estuary in China were compared at different temporal scales, and strategies were proposed for sustainable management of water resources in estuaries.
Methodology The objective-based method for environmental flow assessment in estuaries was divided into two steps. Critical objectives for biological processes, ecosystem health, or ecosystem services should first be identified; environmental flows can then be defined on the basis of the relationship between flow alteration and the responses of selected critical objectives. Ecological Objectives for Environmental Flow Assessment in Estuaries In general, typical species and structural characteristics, such as water quality, community composition, and riparian vegetation are used to indicate the status of an ecosystem (Alber 2002; Poff and Zimmerman 2010). To address the link between ecosystems and human welfare, the concept of
893
ecosystem services, rather than ecosystem health, has been developed and used by scientists, managers, and policymakers for conservation and sustainability and is emphasized by the Millennium Ecosystem Assessment (Carpenter et al. 2006; Fisher et al. 2008; Arthington et al. 2010). In the process of assessing environmental flow, ecological objectives should be first identified on the basis of desired status of ecosystem services, biological processes, or habitats. With temporal variations in hydrological and biological processes, environmental flows for maintaining certain functions or services in ecosystems usually exhibit temporal variability at various scales (Richter and Thomas 2007). However, the identification of every specific objective for environmental flows is difficult because of data scarcity for global ecosystems. To maintain a natural flow regime in ecosystems, temporal variation in natural river discharge was chosen as an indicator of the temporal variation objectives of environmental flows, considering the close relationships between hydrological and biological processes in ecosystems. The temporal variation in objectives is expressed as the ratio of the monthly or daily river discharge to the annual discharge, Ri ¼
n X j¼1
Wji
, n X
Wj ;
ð1Þ
j¼1
where Ri is the ratio (%) of the monthly (or daily) river discharge in month i (or day i) to the annual discharge; Wj, the annual river discharge (m3) in year j; and Wji, the river discharge (m3) in month i (or day i) of year j. It is recommended that objectives for ecosystem protection should be combined with critical requirements in a specific season and temporal variation objectives of natural river discharge. Environmental flows can then be defined to satisfy the desired ecological objectives in a critical season in addition to objectives in other seasons that may not be included in initial environmental flows assessments. Relationship Between Freshwater Inflows and Habitat Responses We established the relationship between ecological processes and flow regime using a numerical model that simulates the spatial and temporal distribution of selected environmental factors of habitat as a combined function of river discharge and tidal currents. The depth-integrated equations for conservation of motion and water are written as @z @ @ þ ðHuÞ þ ðHvÞ ¼ 0 @t @x @y
ð2Þ
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pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi @u @uu @vu @z u u2 þ v2 @ @u @ @u þ þ þ þ ¼ fv þ g þg " " HC 2 @t @x @y @x @x @x @y @y
ð3aÞ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi @u @uv @vv @z v u 2 þ v2 @ @v @ @v þ þ þ ¼ fu þ g þg " þ " ; HC 2 @t @x @y @y @x @x @y @y
ð3bÞ where t (s) is time; x and y are the horizontal coordinates perpendicular to and parallel to the shoreline, respectively; u and v are current velocities (m/s) in the x and y directions, respectively; f is the Coriolis factor, defined as 2 sinφ, where
ω is the angular frequency of the Earth’s rotation and φ is the latitude of the studied location; C is the Chezy coefficient (m1/2/s), which is evaluated as C0H1/6/r, where r is the roughness (0.01) and H is the total depth (m) of the water from the water surface to the bottom [H0ζ+d, where d is the local depth (m) of water measured from mean water level to the bottom and ζ is the water surface elevation (m) measured upward from the mean water level]; and g is gravitational acceleration (m/s2). ε is a dispersion coefficient (m2/s). The two-dimensional convection–diffusion equation integrated over water depth, which assumes vertical mixing, is written as
@ ðHS Þ @ ðHuS Þ @ ðHvS Þ @ @S @ @S @ @S @ @S þ þ ¼ Kxx H þ Kxy H þ Kyx H þ Kyy H þ Sm ; @t @x @y @x @x @x @y @y @x @y @y where S is the concentration of dissolved solutes (unit/ volume); Sm, a source term that describes the sources and sinks of the solutes discharged from outfalls and rivers as well as chemical and biological transformations; and K, the depth-averaged dispersion–diffusion coefficient (m2/s) for orientations x and y. The finitedifference method was used to solve the partial differential equations. In the calculation, alternation direction implicit (ADI) schemes were carried out. At the beginning of the calculation, the initial velocities and the water surface elevation are set to zero: z ¼ 0; u ¼ 0; v ¼ 0:
ð5Þ
In subsequent runs during the simulation period, the velocities and elevations are set to the conditions at the end of the previous run, and equilibrium is approached gradually. The closed boundary condition for the solute is @S @2S ¼ 0 and 2 ¼ 0; @n @n
ð6Þ
which indicates that the concentration changes parallel to the closed boundary, and that there is no solute flux across the closed boundary. For the open boundary, we assume that S0S0 when the boundary values for S0 are known. Interpolation is used when the values of S0 are unknown. The finite-difference scheme used in this model is based on the modified ADI scheme (Chen and Falconer 1994), which is third-order accurate for the convection term and second-order accurate for the diffusion term. Water depth, velocity, and distribution of concentrated dissolved solutes in estuaries, such as salinity, can then be simulated under the action of river flows and tide currents using the validated numerical model.
ð4Þ
Results Objectives in Critical Habitats in the Yellow River Estuary The Yellow River is the second-longest river in China and the sixth-longest river in the world. The Yellow River Estuary is located in eastern Shandong province, west of the Bohai Sea (Fig. 1). With abundant freshwater and nutrient inputs, the Yellow River Estuary provides critical habitats for many ecologically and commercially important species of marine wildlife (Deng and Jin 2000; Dong et al. 2007). In recent years, freshwater demand has increased drastically in China due to rapid economic development. As a result, freshwater inflows in the Yellow River Estuary have been decreasing for several decades. The frequency of complete drying or ephemeral flow has increased since the early 1970s. Drying occurred annually in the early 1990s, with an average of 100 days per year without water in the lower reaches. Reduction in freshwater inflow to estuaries causes a reduction in the available aquatic habitat, which in turn can have negative consequences for many aquatic species (Attrill et al. 1996). In the Yellow River Estuary and the Bohai Sea, the number of fish species was 146 in the 1950s, 119 in the 1980s, and 73 in the 1990s. The species number, density, and biomass dropped by 38.7%, 35.5%, and 46.0%, respectively, from 1982/1983 to 1992/1993 (Zhu and Tang 2002; Fan and Huang 2008). To identify the ecological objectives for environmental flow assessment in the Yellow River Estuary, the habitat salinity and water depth requirements of typical biological processes of various species are listed in Table 1. These parameters were determined through examination of ingested nekton, plankton, and benthos. Spawning and fattening seasons for different species were identified as critical periods in the study.
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895
Fig. 1 Locations of measurement stations and critical habitats in the Yellow River Estuary and Bohai Sea in China
37.95 HK10
Jellyfish
37.9
Ridgepail prawn 37.85
TG
Crab HK07 Chinese Shrimp
B1 B2
E14 C14
TJD
37.8
HK05
37.75
0
HK02
LZG
50 km
37.7 119.2
Chinese shrimp (Penaeus chinensis) is a highly valued catch among China’s capture fisheries. In addition, this species plays an important role in estuarine food webs and is an important indicator for assessing ecosystem health in the Yellow River Estuary (Dong et al. 2007). The annual landing of Chinese shrimp fluctuated between 5,200 t and 39,500 t during the period 1962–1985 (Deng et al. 1990). Since the late 1980s, its stock has been largely depleted as a result of overfishing in the 1970s and changes in freshwater inflows (Huang and Su 2002). Beginning in early May, Chinese shrimp spawn mostly in estuaries and coastal waters where water depth is less than 10 m and salinity ranges from 23 to 32 in the Bohai Sea. The larvae of the shrimp become juveniles and move to the habitat around the estuaries in June and July. The energy conversion efficiency of the shrimp declines at salinities below 13 (Zhang and Dong 2002), and the maximum growth rate is maintained in a water salinity range of 8.77 to 25.8 (Hu and Lu 1990; Wang et al. 2006). At the end of July, upon becoming young adults, the shrimp swim toward offshore waters deeper than 10 m (Deng et al. 1990). Regarding the primary productivity of estuaries, phytoplankton communities also serve as indicator species for
119.25
119.3
119.35
119.4
119.45
119.5
ecosystem health assessment (Pasztalenieca and Poniewozik 2010). The salinity in habitats of early jellyfish development range from 12 to 22, and water depth ranges from 5 m to 20 m, which corresponds to the environment of estuaries in May (Lu et al. 1989; Zhao et al. 2006; Chen et al. 1994). An additional important indicator species for the estuary is the Chinese mitten crab (Eriocheir sinensis Milne-Edwards) (Bell and Coull 1978; Sun et al. 2009). This crab migrates from upstream areas to the estuary for spawning from April to May. The salinity and water depth requirements for this crab were also selected as ecological objectives for quantifying environmental flows (Table 1). Figure 2 shows the temporal variation objectives, based on Eq. 1, in the monthly natural river discharge of the Yellow River Estuary. The figure indicates the ratio of the monthly river discharge to the annual total in the 1960s, 1970s, 1980s, and 1990s at Lijin Station. The average ratio of the temporal distribution of natural river discharge was considered to be representative of the temporal variation in water availability. On the basis of the salinity and water depth objectives of the critical habitat during the critical periods in Table 1 and the temporal variation objectives, environmental flows in estuaries can be determined through
Table 1 Habitat requirements for different species in the Yellow River Estuary Indicator species
Salinity
Water depth (m) Ideal
Critical periods
References
Hu and Lu 1990; Zhang et al. 1998; Deng et al. 1990 Wang and Cao 2010 Song et al. 2009; Zhao et al. 2006; Lu et al. 1989 Xue et al. 1997
Minimum
Ideal
Maximum
Minimum
Chinese shrimp
8.77
25.8
29.0
1.0
10.0
June–July
Ridgeptail prawn Jellyfish
9.0 16.0
22.0 21.0
28.0 30.0
1.5 5.0
15.0 20.0
October April–May
Crab
6.0
18.0
27.0
7.0
15.0
October
10.0
Maximum
896
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20% 1960s 1980s Objectives
Ratio
15%
1970s 1990s
10%
5%
0% Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Months
Fig. 2 Temporal variation objectives for environmental flows in the Yellow River Estuary
the established numerical model for the relationship between salinity in habitats and freshwater inflows in the estuary. Relationship Between Salinity and River Flows The numerical model was applied to simulate the distribution of environmental factors under the combined action of river flows and tidal currents. Calculated tidal heights and velocities were first verified using observed data in the Bohai Sea, which is a semi-enclosed interior sea located in northeast China situated between 37°07′–41°00′N and 117° 35′–122°15′E. The sea consists of three bays: Liaodong, Bohai, and Laizhou. The locations of these stations are shown in Fig. 1. In the tidal system of the Bohai Sea, the
M2 tide is the largest component (Sun and Tao, 2006) and was therefore simulated in the calculation. Calculated tidal heights are compared with the values observed at stations TJD, LZW, and TG in Fig. 3. Figure 4 compares the observed velocities and directions with the calculated results. The numerical model was validated by the field data in the Bohai Sea. The numerical results agree well with the field data, which proved that the model is a suitable tool for simulating the distribution of tidal heights and velocities. The boundary condition for simulation of the hydrodynamic processes in the Yellow River Estuary was then determined using the simulated results in the Bohai Sea. Calculated tidal heights and salinity were validated on the basis of data obtained in the Yellow River Estuary from 25 to 26 August 2003; the recorded river discharge was 187 m3/s at the station upstream of the Yellow River Estuary (Shi 2008). The observed and modeled data of tidal heights at various stations, listed in Fig. 1, are compared in Fig. 5. The numerical results generally agree well with the field data, with the exception of station HK05, which is located at the river mouth. In general, there is a complex mixing process between fresh inflows and tidal currents in a river mouth. As for the Yellow River Estuary, a high amount of sediment is transported downstream of the Yellow River, and its deposition and resuspension alters the morphology of the river mouth. Alteration of underwater topography may induce more uncertainties in simulations of hydrological processes in the river mouth of the estuary. The deviations
Fig. 3 Observed and calculated tidal heights at different stations (shown in Fig. 2) in the Bohai Sea
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897
Fig. 4 Observed and calculated velocities and direction in different stations in the Bohai Sea
between the model results and the field data of tidal heights also affected the simulated salinity at this station, and the salinity values are shown in Fig. 6. Salinity at station HK05 varied drastically from 0.5 to 30. To simulate hydrodynamic
processes and salinity variation more accurately near the river mouth, additional research and field data are necessary to refine the numerical model. Because selected habitats for different species are all located outside of the river mouth
Fig. 5 Observed tidal heights and calculated values at different stations in the Yellow River Estuary
898
Estuaries and Coasts (2012) 35:892–903 35
35 30
Calculation
25
25
20
20
Salinity
Salinity
Measurement Calculation
Measurement
30
15
15
10
10
5
5 0
0 0
5
10
15 Time (h)
20
25
0
30
5
10
15
20
25
30
Time (h)
(HK03)
(HK05)
35
35 30
30 25
Salinity
Salinity
25 20 15 Measurement
10
20 15 Measurement Calculation
10
Calculation 5
5
0
0 0
5
10
15 Time (h)
20
25
30
hk0 3
(HK07)
hk05
hk07 Stations
hk02
hk10
(Stations)
Fig. 6 Calculated and observed salinity at different stations in the Yellow River Estuary
(Fig. 1), the negative effects of the deviations of the simulated results and field data can be neglected in the environmental flow assessments in the Yellow River Estuary. On the basis of the validated numerical model, the relationship between salinity in critical habitats and river discharge was established in the Yellow River Estuary (Fig. 7).
Different levels of environmental flow requirements during critical seasons were determined considering different levels of the ecological objectives in this estuary (Table 1). The annual environmental flow requirements, shown in Table 2, were obtained on the basis of the temporal variation objectives described by Eq. 1.
Fig. 7 Relationship between salinity in critical habitats and freshwater inflows in the Yellow River Estuary
Estuaries and Coasts (2012) 35:892–903 Table 2 Environmental flow requirements for typical habitats in the Yellow River Estuary
899
Environmental flows in critical seasons (m3/s)
Annual environmental flow requirements (109 m3)
Minimum
Ideal
Maximum
Minimum
Ideal
Maximum
159 436
286 921
1712 2718
3.92 8.89
7.04 18.76
42.17 55.35
Jellyfish
422
1314
1994
16.38
50.99
77.38
Crab
289
995
2906
5.89
20.26
59.20
Typical species
Chinese shrimp Ridgeptail prawn
Discussion Figure 8 presents the ratios of requirement levels to natural river runoff in the Yellow River basin. Differences exist between the levels of water requirements for various species. The mean square error (MSE) is 0.56 at the ideal level of water requirements with regard to different objectives, which is the largest gap; the minimum and maximum levels are 0.35 and 0.45, respectively. Considering the vast differences in life cycles of various estuarine species, it is reasonable for ideal environmental flow requirements to differ among species. In addition, it is necessary to maintain significant differences in ideal environmental flows for various species to preserve biodiversity in estuaries. As a result, a boundary of environmental flows may be recommended rather than ideal water requirements in water resources management for integrating different ecosystem objectives in environmental flow assessment. For the Yellow River Estuary, the natural river runoff is 58.0×109 m3, according to 1956–2000 hydrological data. We determined a range of environmental flows with a minimum and maximum limitation that included monthly and daily variability. It was concluded that an average 15% of annual natural river discharge should be maintained to protect the minimum flow requirements for various species in estuaries. In addition, 101% of natural river discharge should be considered as the maximum flow limitation to avoid destruction of habitats by over-flushing flows at 140% 120%
Chinese shrimp Jellyfish
Ridgepail prawn Crab
100% Ratio
Fig. 8 Levels of environmental flow requirements according to different ecological objectives in the Yellow River Estuary
critical seasons. Thus, a range of environmental flow requirements, rather than an ideal environmental flow requirement, was defined with consideration of different objectives (Fig. 9). Because human activities were not a major influence on the river discharge in the 1950s, the average ratio of the daily variations in the river discharge during this time was considered as the objective of the temporal variation in the environmental flows at a daily scale. Figure 10 depicts a comparison of river discharge and different levels of environmental flow requirements at an annual scale in the Yellow River Estuary. During the past 40 years, water usage for agriculture, industry, and municipalities has increased by approximately 68%. As a result, river discharge has decreased, actually falling below the calculated minimum environmental flow levels during the past 20 years. In the 1990s, annual river discharge was unable to satisfy the minimum requirements in 25% of years. To alleviate the conflict between water supply and demand and the frequent dry-ups in the lower Yellow River, a unified water resources regulation was implemented for the Yellow River basin in 2001. Since 2003, river discharge in the estuary has increased sufficiently to meet the minimum environmental flow requirements at an annual scale. In addition, the seasonal flow regime has been modified by the usage of water resources for human activities. Figure 11 compares temporal variations in environmental flows and river discharge on a monthly scale. Typical years were
80% 60% 40% 20% 0% M inimum
Ideal Levels
M aximum
900
Estuaries and Coasts (2012) 35:892–903
Fig. 9 Temporal variations in the minimum and maximum environmental flow requirements in the Yellow River Estuary
selected in which annual river discharge were similar to average annual river discharge of the corresponding decade and include 1956, 1962, 1971, 1982, 1995, and 2005. In 1962 and 1971, river discharge satisfied the minimum environmental flow requirements for all months except June. In the Yellow River, dam construction and the corresponding regulation were intended to prevent disastrous floods and to withdraw water for crop irrigation. With the development of agriculture and industry in the Yellow River basin since 1970s, river discharge decreased greatly in the 1980s. In 1982, river discharge was unable to fulfill the minimum requirement levels from April to May, which is the critical period for agricultural water usage in the upstream area of the estuary. Owing to dam regulations for flood control in this area, river discharge increased significantly in August, achieving levels greater than the required maximum. In 1995, river discharge was unable to the meet minimum water requirement levels from March to July. In contrast to the case in 1995, river discharge in June was greater than the minimum water requirements because the water and sediment regulation was enacted in June 2001 in the Yellow River basin. In 1995 and 2005, river discharge satisfied the minimum and achieved levels greater than the maximum required from August to September and from June to July; however, not even the minimum water requirements were fulfilled in other periods. In recent years, it has also been determined that ecological processes in estuaries can respond quickly to changes in Fig. 10 Comparison of river discharge and environmental flow requirements in the Yellow River Estuary.
the river discharge at a daily timescale (Russell et al. 2006; Mead and Wiegner 2010). In the case of the Yellow River Estuary, the period for dam regulation processes, such as those involving the water and sediment regulation implemented downstream of the Yellow River, is generally 10 days, and this necessitates the assessment of the daily variations in the environmental flow. To maintain daily variations in environmental flows in the estuary, four critical periods were identified: early April, the end of June, August, and early October. During early April, a flow of at least 200 m3/s should be maintained, and river discharges should be approximately 300 m3/s, particularly around 7 April. It should be noted that, although we do not identify ecological objectives in early April specifically, this period is generally considered a critical season for reed growing in the Yellow River Delta. Thus, we can determine the environmental flows in this season on the basis of temporal objective analysis following the natural flow regime. In the period from 1 June–15 July, river discharges should be less than 1,200 m3/s and greater than 150 m3/s. In August, flow varying from 650 m3/s to 4,000 m3/s should be provided to ensure that the necessary flooding occurs in the estuary, rather than flooding that has occurred in June since 2001 due to the water and sediment regulation. Early October, when natural river discharges decrease greatly, is also a critical period for environmental flow protection. In this period, the flow should be at least 400 m3/s and not greater
120 River discharge
Water quantity (109m3)
100
Minimum Maximum
80 60 40 20 0 1955
1960
1965
1970
1975
1980
1985 Years
1990
1995
2000
2005
2010
Estuaries and Coasts (2012) 35:892–903
901
Fig. 11 Temporal variations in environmental flows and river discharge in the Yellow River Estuary
than 2,500 m3/s. Although we cannot identify every objective for various species in different seasons, monthly or even daily variation in environmental flows can be defined on the basis of the different levels required in critical seasons and on temporal variation objectives following the natural flow regime. Figure 12 shows a comparison of daily environmental flow requirements and average daily discharges for several decades. In the 1960s and 1970s, daily river discharge varied mainly within the ranges of recommended environmental flows (Fig. 12a). As a result of the water and sediment regulation related to the dam regulation of the Xiaolangdi Reservoir, river discharge greater than the maximum level of environmental flows occurred from 15 June to 5 July in 2005 (Fig. 12d). Although the annual water discharge satisfied the minimum water requirements after 2003 (Fig. 10), it was determined from the comparison shown in Fig. 12 that, in general, the minimum environmental flows are satisfied by
river discharge during the processes of the water and sediment regulation only in June. At the end of June and at the beginning of October, daily river discharge is greater than the maximum requirement levels. Besides the requirements for water quantity and the temporal variation in different seasons, the increased rates of river discharge should be maintained in the range of 31 m3/s to 100 m3/s per day at the end of June. Decreased rates of river discharge should be maintained in the range of 7 m3/s to 28 m3/s per day in early October. Periods in early April, the end of June, August, and early October should be considered critical for fulfilling reasonable water requirements in the estuary. In this study, a method was established and developed to identify different ecological objectives with temporal and spatial variability and to establish relationships between the distribution of environmental factors and the alteration of freshwater inflows. Taking into account river flows for improving habitat conditions is a very complex process and is
Fig. 12 Daily variations in the river discharge and ranges of environmental flow requirements in the Yellow River Estuary
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hampered by numerous uncertainties to support fishes with different and often conflicting life history strategies (Jassby et al. 1995). The results of our study indicate that, although the requirements of salinity, water depth, and life history strategies differ, minimum and maximum limits for recommended environmental flows in estuaries can be defined and accepted for different species in estuaries. The recommended minimum and maximum levels of the environmental flows in the Yellow River Estuary—15% and 101% of annual river flows, respectively—may not be suitable for other estuaries. Moreover, the recommended ranges of environmental flows may vary when additional ecological objectives are considered in the assessments. The proposed objective-based method can be considered as an initial step towards providing a boundary for environmental flows in environmental flow assessment. The recommended environmental flows may not be ideal for certain types of species; they are suitable for preserving habitats and biodiversity in estuaries. We identified fixed locations for different habitats of various species in this study. In future research, ecological adaptation for different species will be considered to establish a more adaptable relationship between ecological responses and freshwater inflow alteration in estuaries. With regard to defining ideal environmental flows, the initial results could provide further interpretation between different stakeholders based on the tradeoff analysis of different water utilization outcomes in terms of the hydrology term or economic term that integrates different objectives for ecosystem protection and human activities. Thereafter, feedback for reasonable environmental flows could be obtained after multiscenario analysis. Thus, it is possible for us to define a recommendation of ideal, more practical environmental flows that could be accepted by various stakeholders. In addition to establishing complex relationships between hydrological alteration and ecosystem responses, identifying reasonable ecological objectives is an important task that needs to be addressed as part of environmental flow assessments. Management as well as science issues should be included in environmental flow assessments. For water resource management in the Yellow River Estuary, much work remains to be undertaken to achieve river discharge that fluctuates between the recommended ranges of environmental flows. Environmental flows must be managed in an integrated manner to support the multiple functions of estuaries. Acknowledgments This work was supported by the Fok Ying Tung Education Foundation (122046), the National Natural Science Foundation of China (51079005 and 50709003), and the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0809).
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