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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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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
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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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
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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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
FOR PERIYAR RIVER
*Source: Smakhtin et al. 2000
Figure 5.1: Daily streamflow generation using spatial interpolation algorithmwith source CPI*
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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
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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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
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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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
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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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
FOR PERIYAR RIVER
Figure 5.3: Location of Idamalayar and Kalampur sub-basins
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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
FOR PERIYAR RIVER
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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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
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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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
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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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
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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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
FOR PERIYAR RIVER
(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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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
FOR PERIYAR RIVER
Figure 5.6: Simulated vs observed streamflow for a recession coefficient of 0.80- Idamalayar sub-basin
Figure 5.7: Simulated vs observed streamflow for a recession coefficient of 0.85- Idamalayar sub-basin
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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
FOR PERIYAR RIVER
Figure 5.8: Simulated vs observed streamflow for a recession coefficient of 0.90- Idamalayar sub-basin
Figure 5.9: Simulated vs observed streamflow for a recession coefficient of 0.95- Idamalayar sub-basin
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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
FOR PERIYAR RIVER
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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Chapter 5. DEVELOPMENT OF UNDISTURBED FLOW REGIME
FOR PERIYAR RIVER
• 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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