Study On the Ecological Flow in the Middle Reaches of Irtysh Rivers

Guanghui Ye1, Yuqi Bai1*

1Ministry of Education Key Laboratory for Earth System Modeling, Center for Earth System Science, Tsinghua University Beijing, China

yegh09@163.com, yuqibai@tsinghua.edu.cn *Corresponding author (Tell Fax: +8610 62795269)

Abstract—Quantitative measure of the ecological flow of the river is the key to meet its ecological objectives. In this study, data captured at 47 hydrological control transects in the middle reaches of Irtysh River were examined to know the ecological flow , using the simplified 7Q10 method, the slop method, the curvature method and several multi-objective evaluation methods including the weighted sum method, the ideal point method with the scaling coefficient of 1 and the multiply-divide method . These calculation results were evaluated using Tennant method. After having water resources scheduling, ecological protection and other factors considered, the ecological flow isdetermined as 23.97 m3/s, which is about 21.6% of the nature average discharge of Irtysh River.

Keywords—Ecological flow; Tennant method; Wetted perimeter method; Irtysh River; Cross section

I. INTRODUCTION

Water is the most active and irreplaceable element of the ecosystem. Only when the essential water requirement is met, the ecosystem can play its normal function and maintain the ecological balance. Accurate estimation of the water requirements of natural ecosystems are critical in preventing serious ecological problems, such as the drying up of the river channel, degradation of natural vegetation, and sharply reduction of the aquatic biology, from being present, especially in some arid regions.

To protect the river ecosystem, research on Ecological flow (EF) become very important in recent years. EF can be interpreted as ecological water requirement or ecological system water requirement. Quantitative measure of the EF is significant in river management, protecting aquatic ecology environment and maintaining ecological function of rivers [1]. Currently, there are around 200 kinds of calculation methods that have been utilized in 44 different countries and regions [2]. these methods can be divided into hydrologic method, such as Tennant method [3], 7Q10 method [4], RVA method [5]; Hydraulic research method, such as wetted perimeter method [6], R2-Cross method [7]; Physical Habitat Simulation System (PHABSIM) method [8], such as IFIM method [9]; comprehensive analysis method, such as BBM method [10].

Xinjiang province is in the northwest of China. It features arid and semiarid continental climate, large area and complex terrestrial. The ecological system in Xinjiang is very fragile and changeable, Determining the requirement of water for the development of both human society and ecosystems in Xinjiang is crucial [11].

Irtysh River is the second largest river in Xinjiang, and it is the only one river in Xinjiang that flows into the Arctic Ocean. Irtysh River stretches approximately 546 km in the territory of China. The basin area is 57000 km2. Its annual average sediment concentration is 0.076 kg/m3. The annual average sediment discharge is 185kt. Irtysh River’s upstream flood season normally begins in April, while the downstream flood season is from the end of May to October, and the discharge in June is the biggest occupying 50% of nature discharge all the year round.

The purpose of this paper is to introduce the research on determining the ecological flow of Irtysh River, especially in its middle reaches. The structure of the paper goes as follows. Section two introduces the study area. Section three presents the calculation of the EF using several different methods. The calculation results are analyzed in section four, which leads to the conclusion.

II. GENERAL SITUATION OF STUDY REGION

The research area is the middle reaches of the Irtysh River. . This research examined the survey data of 47 cross sections measured in 1987 and 1988 and the hydrological data at thattime to calculate the ecological flow.

According to the schematic diagram of the 47 river cross sections, we can see that the river morphology changes obviously in the middle reaches of Irtysh Rivers. As shown in Figure 1 below, the shape of the river cross section (CS) can be divided into three categories, 1) rectangle CS, such as CS 24 and CS 44, 2) parabola CS, such as CS 15 and CS 35, and 3) triangular CS, which can be further divided into broad-shallow triangular CS and narrow-deep triangular CS, such as CS 17 and CS 29. The three forms of the CS are shown in Figure 1.

Fig. 1. Schematic diagrams of typical river cross section

III. CALCULATION OF THE ECOLOGICAL FLOW

Based on the data that we could collect and use, we selected hydrology method and hydraulics method to calculate the EF.

A. Tennant Method Tennant method, also known as “Montana method”, was

proposed by Tennant et al. and USFWS (United States Fish and Wildlife Service) in 1976. The method firstly divides one year into two calculation times, determines the recommended flow as a certain percentage of the nature average discharge (NAD) of the river, then makes fine adjustment based on the actual situation of the river and fish spawning period. Asshown in Table I, this method takes the relationship between the discharge and the habitat parameters, including rivers flow velocity, water depth and channel width.

TABLE I. THE RELATIONSHIP OF HABITAT QUALITY AND FLOW DESCRIPTED BY TENNANT METHOD[3]

The Qualitative Description Of Flow

And Habitats

Recommended Flow (%)Fish Spawning

Period(Apr.-Sep.)Ordinary

Period(May.-Aug.)

Maximum 200 200

Optimum Range 60-100 60-100

Very Good 60 40

Good 50 30

Better 40 20

General or Poor 30 10

Minimum 10 10

Serious Degradation <10 <10

In this study, we examined the local climate conditions and fish breeding season. We made an adjustment to the calculation times of Tennant method by dividing the whole year into May to August and September to April. Based on the historical hydrologic data, the NAD is 111 m3/s, the minimum and the most appropriate ecological flow of research region were 11 and 38.5 m3/s,, which occupies 10%, 34.7% of NADrespectively.

B. Simplified 7Q10 Method 7Q10 method uses the averaged discharge in the continuous

7 days that are in the most drought month with 90% assurance ratio as the designed EF. The simplified 7Q10 method was further proposed using the average monthly discharge of the most drought month with 90% assurance ratio, or the averaged discharge of the most drought month in last 10 years, as the designed EF.

In this study, we adopted the simplified 7Q10 method. We analyzed the historical discharge data of Irtysh Rivers to determine the averaged discharge and its logarithmic value during the most drought month, calculated the statistical parameters E (X), CV and CS, and finished parameter calibration. Here E(X) standards for the expected average value, CV is the coefficient of variation and CS is the standard deviation. These parameters are shown in Table II.

TABLE II. STATISTICAL PARAMETERS OF THE HISTORICAL DISCHARGE DATA

Variables E(X) CS CV

Q 19.96 3.26 0.16

logQ 2.99 0.19 0.06

The EFs calculated from the Pearson Curve III and the Logarithmic Pearson Curve III are 23.64m3/s and 24.29m3/srespectively. The average of these two numbers, 23.97m3/s,was derived as the calculation results of the EF using the simplified 7Q10 method.

C. Wetted Perimeter Method Wetted perimeter method is a commonly used hydraulic

method. It uses the wetted perimeter as the index of ecosystem habitat, and it takes the flow that corresponds to a certain critical point in the wetted perimeter-flow curve as the EF. There are two ways to determine the critical point: curvature method and slope method. The relation between wetted perimeter and flow is a function of river CS’s geometrical morphology, flow and the depth of the water.

According to the Manning formula and Chezy formula (Assuming that rivers as constant uniform flow in open channel). The relation between wetted perimeter and flow is defined as in (1):

1 5 3 2 3 1 2Q n A P S

Where n is the roughness coefficient (m-1/3s), P is the wetted perimeter (m), S is the water surface slope (dimensionless), and A is Chezy coefficient of the CS (m2).

Natural rivers have a variety of forms, but it basically can be divided into rectangle, triangle, trapezoid and parabolic profile according to their shape. The relation between wetted perimeter and the flow of the channel with different geometrical morphologies, as in Table III.

TABLE III. THE RELATION BETWEEN WETTED PERIMETER AND FLOWOF DIFFERENT CS [11]

Shape Of CS Formula Description

Rectangle 5 3 2 3[( ) ]3.17

SQ P B B Pn

Q is flow, n, P ,S is defined in (1), B is width

Trapezoid2

5 3 2 322

( ) ( )[ ]4(1 )2 1

S b P b m P bQ Pn mm

m is slope coefficient,parameters

defined in (1)

Triangle 5 3 8 3(sin )32

SQ Pn

θ is section angle, other parameters

defined in (1)

Parabolic Profile 5 3 2 32 3 ( )( )3 8

S B P BQ B Pn

parametersdefined in (1)

According to the research of Gippel [6], wetted perimeter and flow are standardized to eliminate the influence of the coordinate scales, so they transform to the range 0 to 1, the formula of fitting with power function is as (2).

p=qb

Where p is standardized wetted perimeter, q is standardized flow, b is relevant parameters of the section shape.

Then we calculated the parameters b, and EF using the value of b by means of the slop method(SM), the curvature method (CM), and several multi-objective evaluation methods including the weighted sum method (WSM), the ideal point method with the scaling coefficient of 1(IPM1) and the multiply-divide method (MDM) that were put forward by Shang in 2008 [12].

We used power function to fitting the curves, and the results of the typical CSs are shown in figure 2:

Fig. 2. Power function fitting of typical CSs

EF values derived from these methods are shown in figure 3. Because the results of SM (k=1, k is the value of slop), WSM (ω1=ω2, ω is the proportion of the importance of ecological water requirement and human requirement), IPM1 are the same with attachment conditions, SM can represent the other two methods.

Fig. 3. Results of different wetted perimeter methods

IV. THE RESULTS AND ANALYSIS

Hydrological method and hydraulic method are adopted in this study as the two main methods to calculate the EF of Irtysh River. The results derived from these methods are shown in Table IV.

TABLE IV. RESULTS CALCULATED USING DIFFERENT METHODS

Calculation Method Formula Condition EF(m3/s) Proportion

of NAFSM

11 bq b

k=1

18.91 17.03IPM ω1=ω2

WSM r=1

CM

12( 1)

22

(2 1)

bbqb b 0<b<0.5 6.6 6

MDM1

bqb

The productof objective function is Maximum

27.02 24.34

Tennant Method /

Minimum 11 10

Optimum 38.5 34.7Simplified

7Q10 Method / / 23.97 21.6

From the perspective of protecting the local ecological environment, and having the conditions of Irtysh Rivers’ water resources scheduling considerate, we took results calculated using Tennant method as the referenced range of EF. We then took the bigger one of the results calculated using simplified 7Q10 method and wetted perimeter method. The final conclusion come out that the EF of Irtysh River is 23.97m3/s.

V. CONCLUSIONS AND FURTHER WORK

This study performed quantitative measurements of the ecological flow of the Irtysh River using both of the hydrology method and the wetted perimeter method. The conclusion is that the EF of the middle reaches of Irtysh River is 23.97 m3/s.

This finding is very useful in the scheduling of water resources and protecting the local ecological environment.

Further work lies in considering the ecological situation of the CS of the Irtysh River, such as the structure of aquatic organisms, life cycle of herbs, the intensity of the interference of human activities, to get a more complete and comprehensive evaluation of the EF.

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