Sustainable Water Resources Management
https://doi.org/10.1007/s40899-022-00643-y
(2022) 8:53
ORIGINAL ARTICLE
Achieving sustainable low flow using hydropower reservoir
for ecological water management in Glomma River Norway
Folakemi Ope Olabiwonnu1 · Tor Haakon Bakken1 · Bokolo Anthony Jr.2
Received: 3 September 2021 / Accepted: 9 February 2022
© The Author(s) 2022
Abstract
Generally, low flow in rivers occur as a result of extended period of dryness which is likely attributed to drought. Drought
is a natural occurrence as an outcome of reduction in precipitation in a region for a long time. Hence, low flow is a growing
concern as there are possibilities of more reduced flows in rivers. However, reservoirs can be utilized to mitigate negative
effects on the supply of water in dry periods and supply water for other purposes. This study aims at verifying how the low
flow condition of Glomma River in Norway has been progressively sustained by hydropower reservoirs. Water Evaluation
and Planning Systems (WEAP) software was used for modelling the natural streamflow condition of Glomma River, which
is the longest river in Norway, using two unregulated sub-basins within the Glomma catchment. Findings from this study
presents that the period between January and March are critical periods in Glomma River. Results show that the values
the annual minimum low flow gotten from the three gauges suggest the flow after regulation has increased significantly as
opposed to before regulation. The daily average flow is simulated by WEAP to be an average of 100 m3/s during the low flow
periods and an average discharge of 350 m3/s during the summer. However, the result indicates that the flow in the summer
has reduced by 80% in majority of the years. In addition, Nash Sutcliffe efficiency (NSE) for the two sub basins used for
this study was 0.9 and 0.76, respectively. Also, the calculation of the coefficient of determination (R2) resulted in 0.85 and
0.78 respectively for the two sub basins. In particular, findings from this study presents evidence on the low flow condition
in Glomma River prior to its regulation and how the regulation has sustained the flow.
Keywords Low flow · Pre-regulation · Hydropower reservoir · WEAP · Glomma River · Norway
Abbreviations
WEAP Water evaluation and planning systems
IHA
International Hydropower Association
IRENA The International Renewable Energy Agency
NVE
Norwegian Water Resources and Energy
Directorate
TNC
The nature conservancy
* Folakemi Ope Olabiwonnu
olabiwonnufolakemi@gmail.com
* Bokolo Anthony Jr.
anthony.j.bokolo@ntnu.no
Tor Haakon Bakken
tor.h.bakken@ntnu.no
1
Department of Civil and Environmental Engineering,
Norwegian University of Science and Technology, S.P.
Andersens veg 5, 7491 Trondheim, Norway
2
Department of Computer Science, Norwegian University
of Science and Technology, NTNU, NO-7491 Trondheim,
Norway
PEST
SEI
NSE
Parameter estimation tool
Stockholm Environment Institute
Nash Sutcliffe efficiency
Introduction
Over several centuries the need for water has increased
for more purposes than sanitation. As time went by, it
began to be seen as an economic good as it can be used
for hydropower production (Barbier 2019). But with the
increasing need for water comes different challenges arising from day to day of which one of them is the changing climate (Dai 2011). Drought is seen as one of the
most damaging weather-related challenges as regards
economic cost (Van Loon and Laaha 2015). Even though
drought occurs naturally, due to climate change, its effect
on hydrological processes has become more intense
(Mukherjee et al. 2018). Moreover, drought is a temporary dry period and can be termed as a disaster which
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occurs periodically. It has environmental, social, and
economic effect in any region where it occurs (Wen et al.
2011; Kumar 2020). Thus, the damaging effect it has on
the eco-system cannot be over emphasized (Van Loon and
Laaha 2015) and consequently, low flow periods can be
experienced in rivers (Vicente-Serrano et al. 2014).
As regarded to morphology of the valleys and the type
of river (braided, meander, anastosomado) the behavior
of the Glomma River with respect to its flow and use
as energy hydroelectric. The Glomma River is regulated
and have regulation capabilities of 16 percent and have
experienced fewer floods in the 1900s as compared to
1800s. The size of the floods has also reduced. This is
because the regulation reservoirs, which were built within
the 1900s for hydroelectric production have had a moderating impact on floods. The regulation reservoirs in the
Glomma River Basin can retain approximately 16 percent of all the water that the river basin brings to the
sea annually. Corresponding results from other regulated
river basins in Southern and Eastern Norway are 30–55
percent. River basins with higher percentage of reservoirs
hardly experience damage throughout spring floods (NVE
2022). It meanders southward through the Østerdal valley,
sometimes breaking up into braids and then flows westward into Lake Øyeren, hence forming Europe’s largest
inland delta as recommended in the literature (Britannica 2011). It is 460.7 km long and historically known
in Norway for being a log-floating river. Equally, it is
maximized for hydropower production. Its hydroelectric
dams power the paper and cardboard mills at Rena town.
Hence, it has several run-of-river power plants situated
on it (Berge et al. 2008).
The main problem to be addressed in this study points
to the fact that drought has been a major issue across the
world and Norway is not exempted. Nevertheless, Norway
is a country that exploits its water by regulating it so it
can be used for multiple purposes, one of which is hydropower (Young et al. 2011). Hence, hydropower is the
country’s primary source of power supply (NVE 2020b).
This can however have a positive or a negative effect on
the biodiversity around the river. Accordingly, this study
aims to address the following research questions.
RQ1. What is the effect of reservoirs on low flow
periods in Glomma River?
RQ2. Is the water released during low flow period
sufficient to achieve sustainable low flow period?
Therefore, the objective of this current study intends to
investigate the effect of hydropower on the flow condition
in Glomma River in Norway especially with regards to
low flow. Glomma River is selected as a case study in this
research since it the longest and largest river in Norway.
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Literature review
Hydropower for energy production in Norway
Progressive environmental awareness over several years
has led to heightened usage of renewable sources for generating electricity (Zarfl et al. 2015; Olabiwonnu et al.
2021). In fact, renewable energy has become a generally
desired source of generating electricity for its economic
and environmental benefits (Fan et al. 2020; Igwe et al.
2020). Hence, hydropower which is one of the renewable energy sources currently making waves in the energy
sector has been improved over time as more research is
being carried out (Fasol 2002; Keller and Hartmann 2019).
Hydropower exploits water which can either be free-flowing or dammed water to generate electricity thus, it can be
classified as either run-of-river or reservoir projects (IPCC
2011). In addition, energy generation via the use of hydropower project produces a reliable energy which is also
affordable (Fasol et al. 2002; Fan et al. 2020). According
to International hydropower association, there are three
main types of schemes:
• Run-of-river hydropower: this entails the use of free-
flowing water to generate electricity with little or no
storage (IHA 2020).
• Storage hydropower: in this system a reservoir is used
to store water which can be used in periods of high
demand for electricity (IRENA 2020).
• Pumped hydropower: in this type of system water can
be recycled between a lower and upper reservoir for
use in providing peak load supply and recycled during
periods of low demand (IHA 2020).
Renewable energy is deeply promoted in Norway as
the country primarily generates electricity via hydropower
production by maximizing its mountainous and steep falls
environment. Furthermore, Norway presently has a total
number of 1667 of power plants and an approximate yearly
production of 136 TWh. However, according to NVE
(2020a) the estimated production by the end of 2020 will
be 153 TWh.
Background of low flow in rivers
It is generally believed that low flow is the flow that occurs
at dry periods of the year alone. However, it can also be
considered as a general reduction in the flow regime of
the river which can be attributed to changes in the natural
flow regime of a river (Smakhtin 2001). Yet, one of the
actions that can result in a higher or lower than anticipated
Sustainable Water Resources Management
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is regulation of flows in rivers. Though, according to
Stromberg et al. (2007) it is seen that river regulation in
fact increases flow rate during low flow periods and can
also be useful in dampening floods. Hence, researchers
such as Smakhtin (2001) argued that low flow can be sustained by additional release of water during these periods.
Ordinarily, the lowest flows occur due to snow storage
during the winter months while low flow events which
occur during the summer are a result of precipitation deficit
and high evaporation (Tallaksen 2000; Galvez et al. 2019).
Aside from this, there are transition regions which can experience low flow anytime, be it summer or winter (Hisdal
et al. 2001), hence, low flow can be experienced in any of
these two seasons (Hisdal et al. 2001; Tallaksen 2000). However, the occurrence of severe low flow considering future
increasing demand for water will have a critical impact on
the environment. An analysis of historical time series of
data therefore provides the necessary information needed
to model low flow and understand how to mitigate it (Tallaksen 2000; Shahraki 2019).
In cold countries such as Norway low flow is experienced
more during the winter due to the storage of precipitation as
snow. Therefore, river regulation can either help to sustain
low flow during these periods or worsen it depending on
how the reservoir is managed and operated (Smakhtin 2001;
Huokuna et al. 2020). Norway precipitation is experienced
as snow during the winter months, and this can go on for
three to five months. During this period, runoff is highly
reduced and demand for electricity is at its peak (Thaulow
et al. 2016). Specifically, depending on counties within Norway seasonal streamflow fluctuates. For example, it was discovered that the western part of Norway experiences higher
precipitation and has steep falls in comparison to the eastern
side which has low precipitation and wider valleys (L’AbéeLund and Villar 2017).
Nevertheless, periods of low flow can be very distressing
for aquatic life in rivers, as it can result in increased water
temperature and interrupt seasonal fish migration (Keefer
et al. 2009). According to Righter et al. (1996); Rolls et al.
(2012), hydrological attributes that affect the biodiversity
around a river are changes in the magnitude of the flow, the
time span of the low flow conditions, the frequency of low
flow in a river, and the timing of the low flow event in relation to season. The hydrological attributes of low flow in
rivers is illustrated as seen in Fig. 1.
Figure 1 depicts hydrological attributes of low flow in
rivers adapted based on findings from the literature (Rolls
et al. 2012). Figure 1 shows that hydrological attributes of
low flow in rivers mainly comprises of timing, magnitude,
duration, rate of change, and frequency. Similarly, the hydrological indices which act as Indicators of hydrological alterations were developed by The Nature Conservancy (TNC) and
described by Richter et al. (1996) to examine hydrological
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alterations in the environment. Hence, the indices can be
adopted and used as to assess streamflow regime as suggested by Olden and Poff (2003). Presently, Hydrological
indices or parameters are increasingly applied in research
for describing and assessing the different streamflow regimes
(Olden and Poff 2003) due to hydrologic alterations which
causes notable changes in hydrologic attributes (Kannan
et al. 2018; Tunji et al. 2020). Hydrological alteration can
affect the discharge of water which can in turn increase the
length of dry periods in a river (Dinpashoh et al. 2019). As
any variation to runoff inevitably affects the biodiversity in
the river, according to Richter et al. (1996) there are several important streamflow characteristics that can be used
in assessing riverine biotic and abiotic eco system integrity.
Some of the streamflow characteristics are the annual and
seasonal variability, timing of extremes, seasonal pattern of
flow, water temperature, dissolved oxygen level and many
more.
Related works
Prior studies have contributed to explore river regulation
in relation to low flow. A few of these studies are reviewed
in this section. Among these studies Huokuna et al. (2020)
researched on the effect of ice in reservoirs and regulated
rivers. Based on a case study the authors identified that the
hydrograph for regulated monthly mean discharge has modified low flow in comparison with hydrograph of unregulated flow. The authors focused on presenting some key
areas about ice in regulated river systems, mainly related
to reservoirs and dams. Their study was aligned to ice with
respect to dam failure, hydropower generation, and dam
removal. Another current study by Tunji et al. (2020) studied the development of a water surface area storage volume
relationship for Uganda’s Namodope Reservoir. The authors
provided a methods of measuring reservoir sedimentation
which is not time-consuming, laborious, less expensive,
and not weather dependent. Also, geospatial technology
was utilized to carry out reservoir capacity survey in afast,
frequently, and economically method to compute the reservoirs’ volume and area.
Additionally, Ždankus et al. (2019) studied the protection of river downstream of a hydropower plant. The authors
aimed to assess the potentials to prevent riverbed erosion
downstream of hydropower plant, towards protecting fish
population and enhance the navigation states and development of effective measures. Accordingly, the author proposed a new method for protecting the riverbed from erosion
and safeguarding the water fauna downstream of a hydropower plant. Also, Barton et al. (2016) employed a Bayesian belief network to analyze significant adverse impact of
the European Union (EU) Water Framework Directive on
hydropower production in Norway. The authors discussed
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Sustainable Water Resources Management
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Fig. 1 Hydrological attributes of low flow in rivers adapted from (Rolls et al. 2012)
the significance of further local and regional evaluation of
all available parameters to achieve better environmental
potential in Norwegian water courses.
Furthermore, Guo et al. (2012) investigated the effects of
the three gorges dam on Yangtze River flow and river interaction with Poyang Lake, China. Findings from their study
signifies that that due to impoundment by the three Gorges
dams. The results also suggest that there was reduced flow
hence the low flow period was not improved but after October release of water was observed due to hydropower generation. This helped to increase the outflow in the rivers during
the low flow seasons. The results also highlighted the needs
for employing strategies to balance the impacts of the dams
on flood control and water resources as well as their ecological and societal consequences within the Poyang Lake basin.
Rolls et al. (2012) explored mechanistic effects of low-flow
hydrology on riverine ecosystems ecological principles and
consequences of alteration. The authors argued that natural
periods of low flow can be sustained via flow regulation.
13
Zhang et al. (2012) examined the Three-Gorges Dam made
the Poyang Lake wetlands wetter and drier. The authors
stated that irrespective of the adverse effects of the dam construction the discharge released during the low flow periods
are higher. Stromberg et al. (2007) explored the importance
of low-flow and high-flow characteristics to restoration of
riparian vegetation along rivers in arid south-western United
States. Findings from their study revealed that reservoirs
help to increase flow in dry periods and also dampen floods.
McMahon and Finlayson (2003) investigated droughts
and anti‐droughts based on the low flow hydrology of Australian rivers. Findings from the authors indicated that in a
regulated river it was noticed that the periods of low flow
have higher discharges after regulation and the streamflow
during the summer is reduced. Thus, flow regulation reduces
the severity of low flow. Smakhtin (2001) researched on low
flow hydrology based on a review. Findings from their study
showed that river regulation can be used to modify low flow
conditions in river if the operation and maintenance of the
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river is properly carried out. Rørslett et al. (1989) investigated the effects of hydropower development on aquatic
macrophytes in Norwegian rivers present state based on
existing knowledge and evidence from case studies. The
study aimed to examine how river regulation can increase
discharge during the winter and reduce the summer discharge considerably in Norway.
Based on the reviewed 11 studies, it is evidence that reservoirs have helped in considerably increasing discharge
during low flow periods. For instance, Norway is experiencing high demand for electricity during winter months,
and this is also when runoff is extremely low (Thaulow
et al. 2016). But the ability to harness hydropower as an
energy source for generating electricity in Norway can also
be impactful on the low flow period. Hence, the hydropower
reservoirs can also be used to help reduce the effect of the
dry period on the biodiversity in the river by the release of
water to the downstream reaches to sustain periods of low
flow (Isaak et al. 2012). However, based on the reviewed
11 studies there are fewer studies that have explored how to
achieve sustainable low flow using hydropower reservoir for
ecological water management.
Therefore, this current study adds to the body of knowledge by examining how sustainable low flow using hydropower reservoir for ecological water management can be
achieved in Glomma River Norway. Also, this study aims to
examine how the effect of reservoirs on low flow periods in
Glomma River can be modelled and to verify the impact of
the reservoir on the seasonal flow periods in Glomma River.
Methods
This study employs WEAP for modeling Glomma River
to examine the impact of reservoirs on low flow periods.
Glomma River vassdraget which is a part of a river in Southeastern Norway, was selected to be the study catchment.
Glomma River was selected for this study due to its expansive use by hydropower operators.
Figure 2 depicts the location of Glomma River within
Norway. Glomma River catchment area is depicted in Fig. 2
and it is 20305 km2 with a specific discharge of 15.2 l/s*km2.
It is a river that is maximized for hydropower production as
there are several power plants that make use of the water
from this river. According to Berge et al. (2008), there are
26 hydropower reservoirs on Glomma River and 57 stations
linked to these reservoirs (Gooch et al. 2010). In addition,
Glomma River is a conducive river for fish population.
Hence, up to 24 fish species flourishes in this river and due
to the reservoirs on it a fish passage had to be constructed
to allow the free passage of the fishes around the river even
after river regulation (Linløkken 1993; Hesthagen and Sandlund 2004).
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Fig. 2 The location of Glomma River within Norway
Overview of WEAP
Water Evaluation and Planning System (WEAP) is an initiative of the Stockholm Environment Institute (WEAP.
org 2020). It is a modelling software that can be used to
simulate different water demand and supply amongst other
processes, and it can be used to assess water resource planning and management issues (Arranz and McCartney 2007).
According to Yates et al. (2009), WEAP21 can describe the
water-related infrastructure and institutional arrangements of
a region in a comprehensive, outcome-neutral, model-based
planning environment that can identify strategies and help
evaluate freshwater ecosystem services.
In addition, WEAP can be utilized for simulating and
analyzing different processes and scenarios involving water
planning and management of river basin (Arranz and McCartney 2007; Yates et al. 2005). Therefore, as described by
Yates et al. (2005) WEAP operates by the principle of water
supply versus water demand, and it understands precipitation that comes into the basin as the water supply while this
supply lessens over time depending on the pressing water
demand.
Moreover, WEAP is very user-friendly. Its interface
allows simulation time step to be set as desired by the
researcher (Arranz and McCartney 2007; Yates et al. 2005)
and it is able to simulate hydrologic processes which can
be made to permit assessment and management of water in
a river basin (Yates et al. 2005). Previous studies in Water
resources management study have made use of WEAP for
river simulation and scenario analysis (Mugatsia 2010).
Hence, WEAP can be used to study the water processes
before and after hydrologic alterations and is employed as
the modeling tool in this study to examine the impact of reservoirs on low flow periods in Glomma River and assess how
has the water subsequently sustained the fish population.
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Sustainable Water Resources Management
Modelling of the unregulated flow in Glomma River
NSE = 1 −
As this study involves modelling of the unregulated flow as
well as the present regulated river flow to properly assess
the low flow condition of the river. Hydrological data which
include precipitation, temperature, and runoff data were
retrieved from Norwegian Meteorological Institute database.
The period studied was chosen to be from 2009 till 2019 due
to availability of time series of data, and good data quality.
Hence, the data were retrieved by relying on information
gotten from the gauges in the river. In addition, shapefiles
for the river network with tributaries and precipitation fields
were retrieved from the Norwegian Water Resources and
Energy Directorate Map Catalog in WGS84 coordinate system for use in WEAP software.
Afterwards, two unregulated sub-basins were calibrated
and simulated with WEAP. This helped to calibrate the
Glomma catchment, to simulate the pre-regulation river
flow condition of Glomma River. Hence, the two sub-basins
were represented as sub-basin 1 and sub-basin 2, and they
were selected to represent the upstream and downstream
region of Glomma River as shown in Fig. 3. Sub-basin 1
has a catchment area of 110.6 km2 and specific discharge of
11.4 l/s*km2 while sub-basin 2 has an area of 1367.9km2 in
comparison and a specific discharge of 11.4 l/s*km2.
For the sub-basins, selected parameters in WEAP had to
be calibrated. Hence, with the use of Parameter Estimation
Tool (PEST) which the Stockholm Environment Institute
(SEI) has linked to WEAP software. An automatic calibration was carried out with emphasis on Land use and Climatic
data.
Afterwards, the performance of the two sub-basins were
assessed using Nash Sutcliffe Efficiency (NSE) and coefficient of determination (R2). The equations used are respectively represented below as Eq. (1) and (2).
Fig. 3 Locations of the sub-basins used in calibrating the unregulated
flow in Glomma River
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(2022) 8:53
∑T
(Qtm − Qto )2
t=1
∑T
(Qot − Qo )2
t=1
(1)
where Qo is the mean of observed discharge, Qm is the modelled discharge and Qto is the observed discharge at time t.
2
⎞
⎛
−
−
∑n
⎟
⎜
(O
−
O)
(S
−
S)
i
i
i=1
⎟
R2 = ⎜ �
�
⎟
⎜
2
2
−
−
∑n
⎜ ∑n (O − O)
(S − S) ⎟⎠
i
⎝
i=1
i=1 i
(2)
where Oi is the observed time series of data, Si is the modelled data, O is the mean of observed discharge, S is the
mean of modelled data.
Since the two models had a good fit between their
observed and simulated runoff and with the result gotten
from NSE and the R2, they were considered to be good
enough to use to model the entire Glomma catchment.
Hence, the two sub-basins were used in calibrating the
upstream and downstream part of the Glomma River. Therefore, the natural flow condition of the Glomma catchment
before its regulation was modelled and simulated. It should
be noted that this was carried out to properly assess how the
low flow has changed over the years and in WEAP software,
Rainfall Runoff (soil moisture) method was used chosen as
the catchment simulation method in this study.
Modelling of the regulated flow in Glomma River
To assess the effect that the reservoirs on Glomma River
has had on the flow especially the low flow condition. Three
gauges situated at strategic points in the Glomma River were
chosen from the Norwegian Water Resources and Energy
Directorate (NVE) measuring stations database. Thus, the
three gauges used are Noorfoss gauge, Atnasjoen gauge, and
Glomstadfoss gauge. Their discharge between year 2009 and
2019 were retrieved and used to generate a new simulation
result on WEAP software.
Specifically, to interpret the results simulated from
WEAP software, monthly averages were derived from the
pre regulation and post regulation discharge. Afterwards,
these results were processed using excel software to show
the low flow occurrence in the river for both winter and
summer. The winter months (December, January, and February) and the summer months (June, July, and August) were
separated and then the annual minimum discharge experienced during the summer and the annual minimum discharge
experienced during the winter months were extracted to
be graphically represented as the measure of low flow in
Glomma River. Also, from the data inputted into WEAP
software, the potential evapotranspiration and the actual
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Table 1 Result of the statistical analysis of the unregulated sub-basins
53
Result of the pre‑regulation runoff in Glomma River
Model performance evaluators
Sub-basin 1
Sub-basin 2
NSE
R2
Climatic factors
Elevation range (m.a.s.l)
Latitude (°)
0.90
0.85
0.76
0.78
The results of the simulation of the pre-regulation flow in
Glomma River using WEAP are presented as daily average
and annual total discharge in Figs. 4 and 5, respectively.
618–1850
62
150–550
60
Result of Glomma River post regulation
evapotranspiration rate of Glomma River was simulated,
and its result duly presented.
The result of the simulation was also computed for the three
gauges Noorfoss gauge, Atnasjoen gauge, and Glomstadfoss
gauge to show the post regulation effect of reservoirs stationed on Glomma River as illustrated in Fig. 6.
Result of the low flow in relation to critical periods
Results
The results showing the model performance are presented
based on Nash Sutcliffe efficiency (NSE) and coefficient of
determination (R2), where the optimum value is 1. Therefore,
for the calibration to be accepted the result must be as close
to 1 as possible. Hence, as illustrated in Table 1 results from
the statistical analysis of the two unregulated sub-basins suggest that the models are both acceptable as they are within
the required range.
Fig. 4 Result for the condition
of the daily average flow before
the regulation of Glomma River
Result showing the evapotranspiration rate
in Glomma River.
The average monthly evapotranspiration value as simulated
by WEAP software in Glomma River is represented in the
Table 2.
Cubic Meters per Second
The simulated natural daily average runoff in Glomma river
(Before regulaon)
1000.0
800.0
600.0
400.0
200.0
0.0
01-Jan
01-Feb 01-Mar 01-Apr 01-May 01-Jun
01-Jul
01-Aug 01-Sep 01-Oct 01-Nov 01-Dec
Days
Annual total runoff from Glomma river (Before regulaon)
Cubic Meters per Second
Fig. 5 Result showing the
annual total runoff in Glomma
River before its regulation
To properly show the effect of regulation on low flow periods, the annual minimum low flow simulated using WEAP
for all three gauges is presented in Fig. 7.
350
300
250
200
150
100
50
0
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
Year
Discharge
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Fig. 6 The hydrograph showing the pre and post regulation
result of Glomma River
(2022) 8:53
Hydrograph of simulated and observed flow at Glomma river
1200
1000
800
600
400
200
0
01-Jan
01-Feb 01-Mar
01-Apr
01-Jun
01-Jul
01-Aug
01-Sep
01-Oct
01-Nov
Noorfoss gauge / Before regulaon
Noorfoss gauge / Aer regulaon
Atnasjoen gauge / Before regulaon
Atnasjoen gauge / Aer regulaon
Glomstadfoss / Before regulaon
Glomstadfoss / Aer regulaon
Discussion and implications of study
Figure 4 shows the pre regulation flow simulated for
Glomma River. As Glomma River is extensively used for
hydropower, the daily average flow is simulated by WEAP
to be an average of 100 m3/s during the low flow periods and
an average discharge of 350 m3/s during the summer. This
may also be as a result of snow melt during the spring. Also,
from the model, Fig. 5 which presents the annual total runoff
shows a slight fluctuation and gradual decrease in discharge.
In addition, Fig. 6 depicts the results from the simulated
hydrograph which represents the natural runoff in Glomma
River as simulated using the three gauges Noorfoss, Atnasjoen, and Glomstadfoss gauges before regulation. This
hydrograph properly captured the low flow within the catchment. However, the simulated flow showed a reduced peak
before regulation as opposed to higher peak. This may be
due to the model simulating a much higher melting than
what occurs in reality or due to a deflection that may have
occurred as a result of the averaging of daily data in all the
years.
To properly show the effect of regulation in low flow
periods, the annual minimum flow during summer and
winter period for both pre and post regulation period were
presented for all the gauges in Fig. 7. For the Atnasjoen
Gauge, Fig. 7 showed that during the Winter, the minimum
flow has doubled by almost 50% as compared to how it was
13
01-May
01-Dec
before the regulation. However, during the Summer, the
result is more varied. In some of the years, the minimum
flow remained the same after river regulation while for
most of the years, the minimum flow has reduced.
Besides, findings from Fig. 7 established that in both
Glomstadfoss and Noorfoss Gauges the runoff during the
winter has tripled as compared to what it used to be before
regulation. This finding can be useful particularly to the
biotas around the Glomma River. Besides, this finding may
be specifically helpful in sustaining the fish population, as
it helps to reduce the stranding of fishes in isolated pool.
However, the result indicates that the flow in the summer
has reduced by 80% in majority of the years and has an
increase of around 20m3/s which is not so much. Hence,
after regulation the peaks have reduced during the summer. This is a positive impact that helps the downstream
reach as the reservoir has indeed helped to capture the
spring flood.
In addition to this, the evapotranspiration rate at Glomma
River was measured using WEAP for both the actual and
potential evapotranspiration as represented in Table 2. The
result however showed that for all the years simulated, the
potential evapotranspiration was higher than the actual evapotranspiration rate. This can be important in estimating crop
water need in the event of future water use for irrigation.
Findings from this study is similar to result from previous
study as detailed by Tena et al. (2019).
Sustainable Water Resources Management
(2022) 8:53
Page 9 of 12
53
Fig. 7 Annual minimum low flow gotten from the three gauges studied in relation to Glomma River
Table 2 shows the
Evapotranspiration rate in
Glomma River as simulated by
WEAP
Years
2009
236
Actual
Evapotranspiration
(mm)
752
Potential
Evapotranspiration
(mm)
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
198
232
210
191
207
205
195
191
182
195
512
552
516
538
559
532
545
527
569
543
In summary, the results (see “Overview of WEAP”,
“Modelling of the unregulated flow in Glomma River”,
“Modelling of the regulated flow in Glomma River”) show
that after the flow regulation, the water released downstream
during the low flow periods have increased significantly
in Gauges Noorfoss and Glomstadfoss but not as much in
Atnasjoen Gauge. This is notably important for the aquatic
life in Glomma and especially for the fish population, as
there is need for preservation of their species and population.
Since Brown trout and graylings are well known to occupy
Glomma River (Heggenes et al. 1996), the release of more
water (more runoff) during the periods of low flow will be
13
53
Page 10 of 12
useful to the brown trout and graylings. As periods of low
flow can affect the survival rate of graylings, particularly
young graylings. Also, the spring flood during the summer
has been successfully captured by the reservoir in comparison to how it was before the regulation. Hence, the river
regulation has helped to sustain the low flow period and
thereby the eco system around the river.
Although this research has explored and assessed the role
that hydropower reservoirs play in sustaining ecologically
water management specifically in Glomma River Norway it
does have some limitations. In this study, the role of ground
water to the precipitation and temperature data was not considered. Also, this study did not incorporate hydropower
plants and their influence due to system priority and energy
demand.
Conclusion, limitations, and future works
Over the decades the need for water has increased and the
availability of water is seen as an economic good as it can
be used for hydropower production. Hence, the need for efficient water management as well as for improvement of water
policies is needed for conservation of the eco-system. But
with the increasing need for water comes different challenges
arising from day to day of which one of them is the changing climate. Drought is seen as one of the most damaging
weather-related challenges as regards economic cost. Even
though drought occurs naturally due to climate change its
effect on hydrological processes has become more intense.
Usually, low flow in rivers occur as a result of an extended
period of dryness which is likely attributed to drought. The
advantage of this current study as compared to prior studies
is that this research study examined how to achieve sustainable low flow using hydropower reservoir for ecological
water management in Glomma River Norway which has not
been well investigated in the literature. The novelty of this
study relates to the fact that the effect of river regulation on
low flow condition was assessed using Glomma River in
Norway as case study.
Furthermore, the natural streamflow in Glomma River
was simulated by calibrating two sub-basins with historical
dataset, using WEAP software. Afterwards, streamflow data
were retrieved from three (3) gauges. Hence, the annual river
runoff was simulated. Results suggest that under present climate and with Glomma River being heavily regulated, the
period between January and March are critical periods even
though they are within dry season. In addition, the critical periods of low flow in Norway were taken into consideration, and the annual minimum flow during the summer
and the winter was calculated. The results were presented
in maps and graphs. Using this method, the pre regulation
flow could easily be compared to the post regulation flow
13
Sustainable Water Resources Management
(2022) 8:53
to effectively assess how hydropower has sustained periods
of low flow in the study area. Future works will investigate
the effect of the timing of the streamflow as studied by Dinpashoh et al. (2019) and the economic impact of climate
change on hydropower reservoirs. In addition, the reservoir
operating rules can be incorporated to better simulate the
effect of drought on low flow condition and the economic
consequences of the low flow condition and climate change
can be considered.
Author contributions FOO contributed to drafting the manuscript.
FOO also contributed to carrying out the literature review, conducting
the experiment and modelling. TAB contributed to providing links to
get the data required and checking the results and presentation of the
manuscript. BAJ contributed to carrying out the literature review and
ensuring the implications and significance of the study is presented.
Funding Open access funding provided by NTNU Norwegian University of Science and Technology (incl St. Olavs Hospital - Trondheim
University Hospital). This study did not receive any funding.
Availability of data and material All data used for this study are
included within the manuscript.
Declarations
Conflict of interest Not applicable.
Ethical approval Not applicable.
Consent to participate Not applicable.
Consent to publish Not applicable.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article's Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
the article's Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
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