07. chapter - v.pdf

CHAPTER 5

DEVELOPMENT OF UNDISTURBED

FLOW REGIME FOR PERIYAR

RIVER

5.1 Introduction

River basin management studies are now focusing on maintaining an

environmentally sustainable flow in rivers. A proper understanding of unaltered

streamflow regime is required as a reference for such management. Hydrologic al-

terations are quantified from this reference flow regime. The major management

objective towards sustainable development of a river is to mimic the undisturbed

flow regime that existed before regulations. Such studies require systematic daily

flow data for a long period. For most of the rivers, historic daily flow data are either

not available or are not reliable.

Streamflow regulations in the Periyar river basin began with the com-

missioning of Mullaperiyar project in 1896. Afterwards, several projects - large and

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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME

FOR PERIYAR RIVER

small - were commissioned in this river basin. The Water Resources Department,

Government of Kerala initiated systematic recording of streamflow data from 1963.

This effort was augmented by the Central Water Commission (CWC) since 1971.

Data on the undisturbed flow regime of Periyar river is not available before the com-

missioning of the regulating structures prior to 1963. An attempt has been made to

develop the undisturbed daily flow time series for the Periyar river before the com-

missioning of the Mullaperiyar project. The period from 1977 to 1987 is considered

for analysis. During this period, except for one year namely 1982, no spill occurred

from the Idukki reservoir. The water yield from the upstream of the Idukki HEP

including Periyar lake catchments were diverted to the adjacent basins. During

this period, the catchments upstream of the Idukki reservoir of 1373 km2 area were

not contributing to the flow measured at the downstream gauging sites. The daily

flow generated for the Periyar lake catchment of the Mullaperiyar project and the

Idukki-Cheruthoni catchments of the Idukki HEP were routed to the downstream

gauging station using Muskingum method to produce an unaltered flow regime.

The spill events in 1982 after the commissioning of Idukki HEP and the diversion at

Bhoothathankettu for irrigation under PVIP were also considered in the analysis.

The effect of small HEPs with reservoir storage capacity ranging from 80-160 MCM

and other interbasin transfers which existed during the period of analysis (1977-87)

were assumed to be negligible. Their effects are not considered in the analysis.

5.2 Continuous daily hydrograph simulation using rainfall

and runoff

The analysis uses Current Precipitation Index (CPI) method proposed

by Smakhtin et al. (2000) to develop undisturbed flow regime of the Periyar river.

This is an extension of spatial interpolation approach developed by Hughes et al.

(1996). The study also uses explicit ratio curve method for generating daily flow

from the monthly flow data. Muskingum routing method is used to route the flow

to stream gauging sites downstream.

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FOR PERIYAR RIVER

5.3 Spatial interpolation algorithm

Spatial interpolation algorithm is a simple method proposed by Hughes

et al. (1996). This algorithm is used for patching and extension of observed time

series (Smakhtin et al.1995). It is based on one-day flow duration curve for each

month of the year. The method assumes the flow occurring simultaneously at the

sites which are reasonably close to each other will correspond to similar percentage

points on their respective duration curves. The model was applied to the South

African catchments and the resulting streamflow simulations compared well with

those obtained using physically based daily time step rainfall- runoff model.

Even in the case of catchments with similar rainfall pattern located

nearby, some nonlinearity in streamflow between stations exist. Because of this,

a daily time step, for each month of the year, was considered for the analysis. Ta-

bles of stream flow values for each site for each month of the year is generated first.

Seventeen percentage points of the flow duration curve (DTQi) where i = 1 to 17

corresponding to 0.01, 0.1, 1, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 95, 99, 99.9, and

99.99%, were considered in the analysis. It is expected that these seventeen points

will give a true representation of the flow regime including the high and low flow

events. Possible source stations are then identified and suitable weights assigned

to the source stations. The degree of similarity between the source station and the

destination station is the base for assigning weights. An estimate of streamflow on

any day at the destination site is got by identifying the percentage point position

(DPj) on the duration table for the relevant month of the streamflow as on the same

day at the source site (QSj) and by reading the flow value (QDj), for the equivalent

percentage points from the duration curve table of destination site. (Figure 5.1) .

Each estimate of the destination site flow value (QDj) is then multiplied

by the source site weight (Wj) and sum of these values divided by sum of weights

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FOR PERIYAR RIVER

(Hughes et al 1996):

QD = (N∑

j=1

QDj ×Wj)÷N∑

j=1

Wj (5.1)

Spatial interpolation approach is based on observed streamflow records.

It is based on the characteristics of flow duration curve (FDC). The FDC at a

site gives a summary of flow variability at a site and is interpreted as a relation-

ship between any discharge value and the percentage of time that this discharge is

equaled or exceeded (Smakhtin et al. 2000). If systematic daily streamflow time

series at a nearby source station is available, it can be used to generate continu-

ous daily streamflow record at the destination site using the above principle. But

in many cases continuous streamflow records are not available near the site where

daily streamflow records are to be generated.

5.3.1 Current Precipitation Index

The Current Precipitation Index (CPI) method suggested by Smakhtin

et al. (2000) is an extension of spatial interpolation approach. This method also

uses daily rainfall records. Rainfall records are used in the spatial interpolation

approach when stream flow records are not available at the source station. The

rainfall duration curves can be constructed similar to the flow duration curves. But

in the humid tropical regions, zero rainy days may occur for about 30-40% time of

the year. This zero rainfall period has to be accounted for in the development of

a continuous rainfall duration curve. A continuous function of daily rainfall that

would abruptly increase on rainy days and gradually decay during the dry periods,

and therefore resemble the general pattern of streamflow variability, is defined for

use in the spatial interpolation approach (Smakhtin et al. 2000).

The daily rainfall information at the source sites is used to generate

continuous daily hydrograph at the destination site. For each raingauge site, a

time series based “Current Precipitation Index ”is developed. It is then used to

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FOR PERIYAR RIVER

*Source: Smakhtin et al. 2000

Figure 5.1: Daily streamflow generation using spatial interpolation algorithmwith source CPI*

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FOR PERIYAR RIVER

establish the duration curve at source raingauge sites. The process of rainfall to

runoff conversion is based on the assumption that daily CPI values at rainfall site(s)

in a catchment and the daily flow at the destination site correspond to similar

probabilities on their respective duration curves (Smakhtin et al. 2000).

*Source:Smakhtin et.al 2000

Figure 5.2: Typical form of annual duration curves of daily rainfall and CPI*

The original concept of the Antecedent Precipitation Index (API) is mod-

ified slightly to be applicable in the context of the spatial interpolation approach.

The CPI reflects not only the antecedent catchment conditions for any day but also

the current daily precipitation input. The CPI for any day is calculated as:

CPIt = CPIt−1K +Rt (5.2)

where CPIt is a current precipitation index (mm) for day t, Rt is the catchment

precipitation for day t and K is the daily recession coefficient. On any day with

no rain (Rt = 0), the CPI is equal to the CPI of the previous day multiplied by

K (similar to API). If it rains on any day, however, the daily rainfall depth is

added to the CPI immediately. Consequently, the index in its current form does not

represent only the antecedent wetness of the catchment, but also reflects the effects

of the current precipitation as given in Figure 5.2 (Smakhtin et al. 2000).

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FOR PERIYAR RIVER

5.3.2 Explicit ratio curve approach

In Kerala State, historic data on streamflow and rainfall are often avail-

able on a monthly time step. Hydrologic models developed for many catchments

are also helpful in simulation of monthly data. This monthly data available would

satisfy data need of most of the large and medium water projects. However an

increased attention to environmental considerations in water resources projects has

led to an increased demand for analysis based on daily time step. The existing

observed daily flow are not suitable for direct use since:

• flow record often contain large data gaps due to missing data

• data may be available for only a short observation period

• flow data are rarely coincident in time with the time series from other sites

within the basin

• deterministic time series models, for generating rainfall-runoff, are rather time

consuming and costly.

Pitman (1993) proposed a method to convert monthly time series to a daily FDC

using daily data at a known representative stream gauging station. The data were

converted to non-dimensional parameters, which were assumed to be representative

for the homogeneous study area or basin considered. This method was refined

by Schultz et al. (1995) to include the effect of anthropogenic developments on

the streamflow regime. A similar approach was proposed by Smakhtin(2000) in

which 1-day annual and 1-day monthly FDC may be derived from monthly flow

volume time series; this approach is used in the present study. The method is

low cost and straightforward. Monthly to daily conversion is done by analyzing

the relationship between daily and monthly streamflow characteristics traced from

observed streamflow records as given below.

The characteristic of FDC is the base for the analysis. The FDC gives a

summary of flow variability at the site and is interpreted as a relationship between

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FOR PERIYAR RIVER

any discharge value and the percentage of time that this discharge is equalled or

exceeded (Smakhtin et al. 2000). This method displays a complete range of river

discharges from low flow to flood events and thus gives a summary of flow vari-

ability at a site. Derivation of FDC representing daily flow regime from synthetic

monthly flow regime and generation of daily flow based on daily FDC for the selected

site make this method more adaptable for assessment of hydrologic alterations of

streamflow.

Smakhtin (2000), Hughes and Smakhtin (1996) and Smakhtin et al.

(1999) outlined several possible methods to produce 1-day flow duration curves

at ungauged catchments. Explicit ratio curve method may be used to prepare daily

flow duration curves for the whole year (1-day annual FDC) or for each month (1-day

monthly FDC). These methods give steps for generating data from 1-month FDC. If

systematic daily streamflow time series at a nearby source station is available, it can

be used to generate continuous daily streamflow record at the destination site using

the above principle. Two sub-basins (Idamalayar in the Periyar basin and Kalam-

pur in the Muvattupuzha basin) were selected for preparation of the ratio curves

(Figure 5.3). Explicit ratio curves for each month were generated for Idamalayar

and Kalampur sub-basins (Figures 5.4 and 5.5). The study assigned equal weights

to these ratios. Historic 1-month FDC available for Idukki-Cheruthoni and Mul-

laperiyar sub-basins were then converted to 1-day FDC using the ratio curves for

each month.

The relationship between two flow duration curves (1-day annual and

1-day monthly) is expressed by ratio curves. The FDCs should be in the similar

unit (ie., either aggregating daily discharge in each month into monthly flow volumes

(MCM), or expressing monthly flow volumes as mean monthly discharge - (m3/sec).

Seventeen fixed percentage points on the curve (0.01, 0.1, 1,5, 10, 20, 30, 40, 50,

60, 70, 80, 90, 95, 99, 99.9 and 99.99% time of exceedence) with corresponding flow

rates represent a Duration Discharge Table (DDT).

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FOR PERIYAR RIVER

Figure 5.3: Location of Idamalayar and Kalampur sub-basins

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Figure 5.4: Explicit ratio curves for the month of July - Idamalayar sub-basin

Figure 5.5: Explicit ratio curves for the month of July - Idamalayar sub-basin

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FOR PERIYAR RIVER

For any site in a river, the variability of daily flow is higher than monthly

flow. In the monsoon months, the monthly average flow is always lower than the

daily peak flow values and in the non-monsoon months, the minimum average daily

discharges will be much lower than the monthly flow average. In other words, 1-

day FDC has a much larger slope compared to 1- month FDC. The ratios of daily

to monthly flow for seventeen fixed points may be calculated and plotted against

the percentage point values to produce “ratio curve” for the site. The ratio curves

may be established for each month of the year or for the whole year.

In a hydrologically homogeneous region, similar sized river sub-basins as

well as different closely located sites in the same river basin are likely to experience

similar variation of daily flow within a month (Smakhtin et al. 2000). Therefore, if

a ratio curve is established at one site on a river (where the observed daily records

are available), it could be applied to other sites of interest in the same river basin,

where synthetic monthly data are available to convert 1- month FDC to 1-day FDC.

5.3.3 Conversion of streamflow data from monthly to daily time step

Historic data on streamflow and rainfall in many cases are available on

monthly time step in Kerala State. Hydrologic models developed for many sub-

basins are also helpful in the simulation of monthly data. This monthly data avail-

able would satisfy most of the large and medium water projects. However, an

increased attention to environmental considerations in water resources projects has

led to an increased demand for analysis based on the daily time step.

The steps followed in the explicit ratio curve method are detailed below:

• In the vicinity of the site of interest, identify a representative stream gauge

with good quality data in the same river basin or in the adjacent basin. The

size of the sub-basin selected should be ideally similar to the size of the basin

under consideration.

• Construct 1- day and 1-month FDC using the data from this gauge (for the

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FOR PERIYAR RIVER

whole year and for each calendar month).

• Calculate ratios of 1- day flow to 1-month flow for 17 fixed percentage points

and using these ratios, the ratio curve for the site may be constructed.

• If several representative flow gauges are identified in the vicinity of the site

under consideration, the exercise should be repeated and average ratio curve

may be produced for each month.

• The established ratio curve represents the conversion function from 1-month

to 1-day FDC. It can be used to generate 1- day FDC for the site under

consideration from the synthetic 1-month FDC.

5.3.4 Muskingum method

The Muskingum method is a simple, approximate method to calculate

the outflow hydrograph at the downstream end of the channel reach given the in-

flow hydrograph at the upstream end. No lateral inflow into the channel reach is

considered. The method is based on the hydrologic storage equation for a channel

reach:

ds/dt = I −Q (5.3)

where S = volume of water in storage in the channel reach, I = upstream inflow

rate, Q = downstream outflow rate, and t = time.

It is also assumed that the volume of water present in the channel reach

consists of a prism storage, KQ, and a wedge storage KX(I-Q),and therefore:

S = K[XI + (1−X)Q] (5.4)

where K = travel time constant, X = weighting factor between 0 and 1.0.

Sometimes, K is vaguely interpreted as the travel time of a flood wave

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FOR PERIYAR RIVER

along the channel reach. However, for the most part, K and X are treated as

calibration parameters into which the channel characteristics are lumped.

To solve equations 5.3 and 5.4 numerically, discretize the time into finite

time increments of △t. For any time increment, Equation 5.3 is written in finite

difference form as:S2 − S1

△t=

I1 + I22

−Q1 +Q2

2(5.5)

where subscript 1 refers to the beginning of the time increment and 2 refers to the

end. Rewriting Equation 5.4 in terms of S1, I1, Q1, S2, I2 and Q2 and substituting

in Equation 5.5 and simplifying, we get:

Q2 = C0I2 + C1I1 + C2Q1 (5.6)

where the Cn values are equal to:

C0 =△t

K− 2X

[2(1−X) +△t/K](5.7)

C1 =△t

K+ 2X

[2(1−X) +△t/K](5.8)

C2 =2(1−X)− △t

K

[2(1−X) +△t/K](5.9)

Various limits are suggested for the parameters used in the Muskingum method.

Cunge (1969) suggested that X should be non-negative for Equation 5.3 to be phys-

ically meaningful, and he also showed that X should be equal to or less than 0.5 for

the Muskingum method to be stable.

The time increment should be equal to or smaller than a fifth of the

time to peak of the inflow hydrograph for accurate representation of the rising limb

(Sturm 2001). C0 (the numerator of Equation 5.8) should have a non-negative value

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(Ponce and Theurer 1982). These two conditions lead to

2KX ≤ △t ≤tp5

(5.10)

where tp = time to peak of the inflow hydrograph.

5.4 Simulation of daily flow for Periyar river basin

Systematic rainfall and streamflow data are available for the Idamalayar

river basin, a sub-basin of the Periyar river basin and Kalampur sub-basin of the

Muvattupuzha basin. As these two basins are near to Idukki-Cheruthoni and Periyar

lake sub-basins, they are considered as source stations (Figure 5.3 ).

The monthly runoff for the Periyar sub-basin upstream of the Periyar lake

was calculated using the empirical equation developed during the design of Idukki

HEP. The monthly runoff from the sub-basin was calculated using the data of two

raingauges located at Vandiperiyar and Kumaly. Monthly inflow information for

the Idukki and Cheruthoni sub-basins were collected from Kerala State Electricity

Board. This data were used for generation of daily runoff using explicit ratio curve

approach as suggested by Smakhtin et al.(2000).

5.4.1 Recession coefficient for the Periyar river basin

Daily streamflow and rainfall records of the Idamalayar sub-basin of the

Periyar river basin and the Kalampur sub-basins of the Muvattupuzha river basin

were used to identify the most suitable recession coefficient (K) to be used in the

CPI method for the region. Daily flow was generated for different K values (0.8,

0.85, 0.9 and 0.95) and was validated using observed streamflow records; 0.85 was

found to be the optimum recession coefficient (K) for simulating daily flow. This

value was selected for further analysis for generating daily flow for the Periyar and

Idukki-Cheruthoni sub-basins (Table 5.1).

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FOR PERIYAR RIVER

Figure 5.6: Simulated vs observed streamflow for a recession coefficient of 0.80- Idamalayar sub-basin


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Table 5.1: Comparison of observed and simulated streamflow for different re-cession coefficients (K) - Idamalayar sub-basin

Paired sample correlation Paired differences

Pair K N Corre- Lower Upper t df Signific.

lation 95% 95% (2 tailed)

confidence confidence

interval interval

1 0.80 1096 0.811 -3.185 0.622 -0.638 1095 0.524

2 0.85 1096 0.794 -2.710 0.221 -0.198 1095 0.843

3 0.90 1096 0.392 -1.197 0.707 -1.238 1095 0.216

4 0.95 1096 0.756 -4.868 0.713 -1.461 1095 0.144

Explicit ratio curves for each month and for the whole year were de-

veloped for the Idamalayar and Kalampur sub-basins. These ratios were used for

conversion of monthly flow to daily flow. The 1- day monthly FDC and the CPI

method was used for the generation of daily flow for the period from 1977 to 1986.

Following are the details of steps followed in the analysis:

• The Idamalayar and Kalampur sub-basins for which continuous daily inflow

and rainfall data are available, have been selected.

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• Streamflow were generated by CPI method using different recession coefficients

(K)

• Optimum recession coefficient for further analysis was selected based on the

validation of generated daily streamflow

• Monthly streamflow at the Mullaperiyar and Idukki sites were used to generate

1- day FDC using the explicit ration curves

• Daily flow was generated using CPI method and daily rainfall data for 1977-

1986 at the above sites

• C0, C1, C2 values were calculated using the Muskingum calculator (IIT Roor-

kee 2012); using the Muskingum method, flow was then routed to the gauging

sites

• Undisturbed flow before the commissioning of Mullaperiayr project and Idukki

HEP was developed taking into consideration the spill from the Idukki HEP in

1982 and the daily water diversion at the Bhoothathankettu barrage upstream.

The simulated undisturbed daily flow values were used for quantification of hydro-

logic alterations for different degrees of regulation as furnished in Chapter 6.

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