=Paper=
{{Paper
|id=Vol-2761/HAICTA_2020_paper40
|storemode=property
|title=A Spatial Interpolation Approach for Environmental Flow Assessment in Bulgarian-Greek Rhodope Mountain Range
|pdfUrl=https://ceur-ws.org/Vol-2761/HAICTA_2020_paper40.pdf
|volume=Vol-2761
|authors=Ekaterina Ivanova,Dimitrios Myronidis
|dblpUrl=https://dblp.org/rec/conf/haicta/IvanovaM20
}}
==A Spatial Interpolation Approach for Environmental Flow Assessment in Bulgarian-Greek Rhodope Mountain Range==
https://ceur-ws.org/Vol-2761/HAICTA_2020_paper40.pdf
A Spatial Interpolation Approach for Environmental
Flow Assessment in Bulgarian-Greek Rhodope Mountain
Range
Ekaterina Ivanova1, Dimitrios Myronidis2
1
Space Research and Technology Institute—Bulgarian Academy of Sciences, Bulgaria; e-
mail: ivanovae@spase.bas.bg
2
School of Forestry and Natural Environment, Aristotle University of Thessaloniki,
54124 Thessaloniki, Greece; e-mail: myronid@for.auth.gr
Abstract. Nowadays, the environmental flow (e-flow) is globally recognized as
an essential component of the sustainable water resources management.
Therefore, defining flow requirements is an important step forward, especially
in transboundary regions where different water management practices exist.
This study aims to test a spatial interpolation approach for estimating the e-flow
in the Bulgarian-Greek Rhodope Mountain Range, incorporating hydrological
methods, GIS techniques and expert judgment. It was found that the minimum
flow required to maintain rivers and riverine ecosystems in the region with a
probability of exceeding 90% of the time ranges from 0.027 to 6.11 m3/s, which
represents from 1.92 to 32.98 percentages of the mean annual flow. The base
flow variability index varies from 2 (low) to 20 (extremely high). Based on the
Tennant method and low-flow duration indices, the rivers were regionalized into
5 ecological management classes that identify the quality of ecosystems and
their conservation status.
Keywords: Environmental flow; Rhodope; GIS; river ecosystem.
1 Introduction
The concept of environmental flow (e-flow) is nowadays recognized as an essential
step towards sustainable management of the natural resources, the need of which is
constantly increasing for the demand to ensure the human livelihoods in the context of
global climate change and growing exigency. This concept, widespread in the last 3
decades, defines e-flow as the flow regime (i.e. quantity, quality, and timing of water
flow) required to sustain freshwater and riverine ecosystems (Clausen and Biggs,
2000; EC-Guidance No 31, 2015; Acreman, 2016; Karakoyun et al., 2018; Palmer and
Ruhi, 2019).
One of the main reasons for increasing the use of water is to generate electricity via
hydroelectric power plants (HPPs) (Karakoyun et al., 2018). As a consequence of the
abundant water reserves of the Rhodope Mountain, a significant part of Bulgaria's
hydropower plants is located here, whereat many of the largest country’s dams have
been constructed during the 50s and 60s of the 20th century. The construction of small
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hydroelectric plants (SHP) is also a strategic goal for Greece, which can reduce the
electricity imports and contribute to balance of payments (Myronidis et al., 2008).
Even supporting the economic development of the countries, the HPPs cause damage
to biological diversity in rivers and their ecosystems, changing the basic components
of the river flow (i.e. volume and timing), as well as the natural interrelation between
the river and its flood areas (Karakoyun et al., 2018). Degradation of watershed
ecosystems can also affect soil erosion, since the healthy vegetation is one of the best
protections against erosion. Myronidis et al., (2010) address the issue of land
degradation caused by excessive erosion in the Mediterranean region, pointing out the
need of sustainable plan to mitigate the negative impact on natural ecosystems.
In the past, many of these environmental problems have not been taken into account
in the watershed management and the construction of HPPs, which in turn requires a
scientific approach to ensure the future sustainable development of the Bulgarian-
Greek transboundary region. Recently, the rapid development of information
technology has significantly increased the technical capacity to assess the variability
of flow regimes in both temporal and spatial perspective, and hence the variability in
ecosystem processes, using a broad range of spatial scales, resolutions and data
availability (Tharme, 2003; Smakhtin et al., 2006).
This study aims to test a spatial interpolation approach to reveal the spatiotemporal
variability in the flow regime in the Bulgarian-Greek Rhodope Mountain Range,
integrating Geographic Information Systems (GIS) and the most commonly used
hydrological indices that might predict the magnitude, frequency and timing of flow
events to further define environmental flow requirements.
2 Methods
2.1 Study area and data
The entire territory of The Rhodope Mountain Range was considered in this study,
extending on an approximately 23 500 km2 between longitudes 23o40’E and 26o40’E
and between latitudes 40o50’N and 42o15’N in Bulgarian-Greek transboundary region
(Figure 1). Both the humid continental climate of the North and the Mediterranean
climate of the South influence the local climate of the region. The average annual
temperature varies from 5 to 10-13 °C whilst average annual precipitation ranges
between 600-1100 mm (Yordanova et al., 2002). The relief differs from low-
mountainous in the south-east to high-mountainous in the west (0–2191 m) with mean
elevation 630 m.
The Rhodope Mountain Range is famous for the highest species diversity in the
Balkans and rivers play a significant role in their conservation (Tsiftsis and Tsiripidis,
2012). According to the Bulgarian Ministry of Environment and Water and the Greek
Ministry of Reconstruction, Environment and Energy the region includes 36 Natura
2000 protected sites (17 in Bulgaria and 19 in Greece) with a total area of 11 000 km2.
The flow inventory data, employed in a previous study (Myronidis and Ivanova, 2020),
contain monthly records for maximum, minimum and mean discharges in m3/s for
275
time period of at least 10 years of measurements (between 1936 and 1995) for 22
pristine watersheds with mean annual discharge from 0.19 to 27.7 m3/s.
Fig. 1. Location map of the Bulgarian-Greek Rhodope Mountain Range.
2.2 Environmental flow assessment methodology
A large number of methods have been used for environmental flow assessment,
varying from simple statistics to complex models. Generally, they have been
categorized in Hydrological, Hydraulic Rating, Habitat Simulation and Holistic
methods (Gopal, 2013; Karakoyun et al., 2018). In this study simple methodology was
compiled in order to assess environmental flow and classify flow regime for water
management and habitat maintenance. The methodology includes following steps:
1. Selecting hydrological indices that adequately characterize flow regime;
2. Statistical analysis of the hydrological data to arrive at index values;
3. Spatial interpolation of the index values to characterize the spatial variability
of flow regime;
4. Compiling a classification of the flow regime to predetermine the state of
riverine ecosystems, based on sustainable flow requirements, in terms of
Ecological Management Classes (EMC) proposed by the South African DWAF
(1997).
The hydrological methods, selected and applied in a historical record, are the most
widely used method of Tennant (1976), based on percentages of mean annual
discharge, and Flow Duration Curve (FDC) method, which is a cumulative frequency
curve representing the percentage of time during which the flow rate is equal or
exceeds particular value (Gopal, 2013). Several low-flow indices were obtained from
FDCs in a monthly step in order to determine e-flow, including Q50, Q90 and Q95
(daily flows exceeding 50%, 90% and 95% of the time) expressed in both m3/s and %
of mean annual flow (MAF). In addition, Low Exceedance Flow Index – LEFI
(Q90/Q50) (Pyrce, 2004; Clausen and Biggs, 2000) and Baseflow Variability Index –
BVI (Q50/Q90) (Nelms et al., 1997; Pyrce, 2004) were calculated.
276
Spatial interpolation was applied to all indices to determine flow regime
requirements in their spatial and temporal variability, enforcing “Topo to Raster” tools
in the ArcGIS software, which represents an interpolation technique based on the
ANUDEM program, specially designed to create a surface closer to natural drainage
surface (Hutchinson et al., 2011). The procedure uses interpolation methods, such as
inverse distance weighted (IDW) interpolation, without losing the surface continuity.
Finally, a holistic approach based on calculated indices was applied to classify rivers
according to certain requirements for maintaining the whole riverine ecosystem in its
ecological integrity.
3 Results
3.1 Instream flow regime based on the Tennant method
Since the development of Tennant's hydrological methodologies (Tennant, 1976)
involves the collection of field habitat, hydraulic and biological data, this method
differs from many others and is considered one of the most suitable for e-flow
assessment (Tharme, 2003; Pyrce, 2004). The methodology consists of linking certain
percentages of mean annual flow (MAF) to eight categories of river condition on a
seasonal basis to sustain fish, wildlife, recreation, and related environmental resources.
To apply this method, the mean monthly flow was obtained for all gauging stations in
the Rhodope Mountain Range, averaging the mean monthly discharge data for all
observed years. Then, the percentage of the MAF was calculated month by month
(Table 1).
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Table 1. Mean monthly flow, calculated as a percentage of MAF.
Gauge N: % of MAD (October-March) % of MAD (April-September)
X XI XII I II III IV V VI VII VIII IX
1 52 62 64 56 48 65 159 252 197 117 71 54
2 43 75 97 81 73 88 173 238 148 89 49 42
3 42 66 76 110 87 164 199 162 137 79 44 37
4 63 64 80 100 143 208 203 116 101 66 37 40
5 50 63 83 105 115 169 189 154 119 66 45 43
6 41 67 75 83 72 101 218 221 141 90 49 39
7 51 82 119 94 120 139 183 171 131 50 33 30
8 33 69 108 117 108 135 185 175 120 65 47 42
9 36 87 100 62 105 148 235 192 97 72 35 31
10 46 93 120 89 205 183 243 198 162 66 28 27
11 39 80 139 139 122 146 168 153 104 56 28 30
12 30 78 97 139 123 158 185 159 112 65 30 25
13 42 59 98 164 114 143 174 188 129 68 39 32
14 33 89 112 144 133 158 165 120 75 36 73 33
15 46 63 71 74 67 99 244 243 125 81 44 38
16 24 81 138 144 121 150 187 139 107 70 22 19
17 25 55 106 150 141 135 234 172 98 49 18 16
18 33 99 183 194 166 146 140 107 74 31 13 17
19 23 80 104 125 95 152 226 169 116 75 18 16
20 30 79 170 165 206 145 122 101 79 39 18 21
21 30 85 185 263 239 142 82 66 45 37 8 9
22 25 32 71 78 111 176 270 189 111 68 28 40
Once obtained, these percentages were interpolated via ArcGIS software to reveal
the spatial and temporal variability of in-stream flow regimens with respect to different
aquatic and riverine habitat conditions. Following the Tennant’s environmental flow
recommendations, we assumed the threshold of 10% of the MAF as the lowest limit
corresponding to “severe degradation” of a riverine ecosystem. The category up to
10% (max to 30%) of MAF (Apr.–Sept.) was assumed to be “poor”. The other
categories are “fair”, “good”, ”excellent”, “outstanding”, “optimum” and “flushing”,
joined to ranges of 10%–20% (Oct.-Mar.) – 30%–40% (Apr.-Sept.); 20%–30% (Oct.-
Mar.) – 40%–50% (Apr.-Sept.); 30%–40% (Oct.-Mar.) – 50%–60% (Apr.-Sept.);
40%–100% (Oct.-Mar.) – 60%–100% (Apr.-Sept.); 100%–200% and over 200%,
respectively.
As can be seen from Figure 2 and 3, the river flow maintains an ecological optimum
over the year, ranging from good to flushing, except for the period August–September,
during which the ecological status of rivers dramatically degrades to poor, leading to
damage of the river and riverside habitats.
278
Fig. 2. Instream flow regime in the Bulgarian-Greek Rhodope Mountain Range for the period
October–March based on the Tennant (1976) method.
Fig. 3. Instream flow regime in the Bulgarian-Greek Rhodope Mountain Range for the period
April–September based on the Tennant (1976) method.
3.2 Classification of natural flow regimes for e-flow estimation
The FDC combined with other methods, as Vogel and Fennessey (1995) indicated,
has been used in many hydrologic studies including flood control, water quality
279
management and aquatic habitats maintenance, due to its easy application and
expression of wealth hydrologic information (Dakova et al., 2000; Smakhtin and
Anputhas, 2006; Smakhtin et al., 2006; Shaeri Karimi et al., 2012; Efstratiadis et al.,
2014; Ridolfi et al., 2020).
In an attempt to find a more comprehensive approach to determine environmental
flow requirements for the Rhodope Mountain transboundary area, FDCs were prepared
for all gauging stations based on long-term data (from 10 to 28 years), which is
sufficient to assess the availability of water in the study area. Focusing on duration of
low flow events, several indices, obtained from FDCs, were used for the purpose of
this study, which are most often employed in the government and academic literature
regarding environmental flow assessment (Table 2).
Table 2. Low-flow duration indices calculated from FDCs.
Gauge MAF Q50 Q90 Q95 LEFI BVI EMC BV class
N:
3 3
m /s % m /s % (Q90/Q50) (Q50/Q90)
1 0,94 0,65 0,31 32,98 0,26 27,66 0,48 2,10 A Low
2 0,37 0,27 0,12 32,43 0,11 29,73 0,44 2,25 A Low
3 0,69 0,52 0,19 27,54 0,17 24,64 0,37 2,74 A Low
4 0,70 0,45 0,18 25,71 0,14 20,00 0,40 2,50 A Low
5 2,63 1,79 0,65 24,71 0,5 19,01 0,36 2,75 A Low
6 0,35 0,24 0,086 24,57 0,068 19,43 0,36 2,79 A Low
7 25,85 20,1 6,11 23,64 3,61 13,97 0,30 3,29 B Moderate
8 17,48 13,4 4,06 23,23 3,45 19,74 0,30 3,30 B Moderate
9 21,82 15,1 4,73 21,68 3,83 17,55 0,31 3,19 B Moderate
10 21,68 19,14 4,58 21,13 2,70 12,45 0,24 4,18 B High
11 4,74 3,55 1 21,10 0,76 16,03 0,28 3,55 B Moderate
12 3,52 2,4 0,64 18,18 0,3 8,52 0,27 3,75 B Moderate
13 5,88 4,46 1,03 17,52 0,85 14,46 0,23 4,33 C High
14 0,19 0,077 0,027 14,21 0,024 12,63 0,35 2,85 C Low
15 0,39 0,23 0,05 12,82 0,038 9,74 0,22 4,60 C High
16 2,90 2,09 0,32 11,03 0,25 8,62 0,15 6,53 D Very high
17 0,89 0,54 0,081 9,10 0,059 6,63 0,15 6,67 D Very high
18 2,55 1,73 0,23 9,02 0,16 6,27 0,13 7,52 D Very high
19 1,71 1,14 0,15 8,77 0,071 4,15 0,13 7,60 D Very high
20 27,68 19,8 1,91 6,90 1,47 5,31 0,10 10,37 D Extremely
high
21 0,42 0,17 0,027 6,43 0,02 4,76 0,16 6,30 D Very high
22 5,74 2,22 0,11 1,92 0,06 1,05 0,05 20,18 E Extremely
high
Average flow magnitude (Q50), Q90 and Q95 exceedance flows overall years,
expressed as well in percentages of the MAF, were obtained directly from the FDCs.
Low exceedance flows index (LEFI) was calculated dividing mean magnitude of flows
exceeded 90% of the time (Q90) by Q50 (Clausen and Biggs, 1997, 2000), while Base
flow variability index (BVI) was obtained dividing Q50 by Q90 (Nelms et al., 1997;
Pyrce, 2004). Those indices combined with expert opinion were utilized to define
ecological management classes (EMC), which express the state of the riverine
ecosystems, based on the e-flow regimes, following the procedure proposed by
Smakhtin and Anputhas (2006) and some of the steps proposed by South African
Water Research Commission (King et al. 2008). The relationship between the low-
flow indices and the biological conditions of the benthic biota (e.g. elements such as
biomass, total number of species, etc.) was also taken into account (Clausen and Biggs,
280
1997). The five EMCs were predetermined (see Table 3) assuming that higher EMC
requires more water as a percentage of MAF with low baseflow variability for
ecosystem maintenance and conservation.
Table 3. Ecological management classes (EMC) assigned to the Bulgarian-Greek Rhodope
Mountain Range.
EMC Description of water, habitat and ecosystem quality
A Negligible modifications from natural conditions: Rivers with minor
changes in in-stream and riparian habitats. Negligible risk to intolerant
biota.
B Slight modifications from natural conditions: Ecologically important
rivers with largely intact biodiversity and habitats. Slight risk to intolerant
biota.
C Moderate modifications from natural conditions: The habitats and
dynamics of the biota have been disturbed, but basic ecosystem functions
are still intact. Moderate risk to intolerant biota.
D High degree of modifications from natural conditions: Large changes in
natural habitats, biota and basic ecosystem functions have occurred. Habitat
diversity and availability have declined. High risk of loss of intolerant
biota.
E Critical degree of modifications from natural conditions: Modifications
have reached a critical level and ecosystems have been completely
modified with almost total loss of natural habitats and biota.
Q90 exceedance flow, expressed in percentages of MAF, was selected in order to
pre-define the spatial extend of the EMCs. The values for Q90, which vary from 32.8%
to 1.9% of MAF, were interpolated in the ArcGIS software and were classified into
five classes, corresponding to A, B, C, D and E of the EMC, respectively (Figure 4a).
The same procedure was applied to BVI, identifying five baseflow variability classes
(Figure 4b).
Fig. 4. Environmental flow classification in the Bulgarian-Greek Rhodope Mountain Range: a)
ecological management classes (EMC); b) baseflow variability classes.
Looking at the continuity of the FDCs distribution (Figure 5: a, b), the five classes
can be clustered into two groups of rivers: (1) rivers of high quality habitats (classes A
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and B) with negligible to slight risk of degradation (15% of the total area) and (2) rivers
of low quality habitats (classes C, D and E) with moderate to high risk of degradation.
The second group differs significantly in the stability of the flow regime, expressed
through greater flow variability over time.
Fig. 5. Flow duration curves (FDCs) of the rivers in the Bulgarian-Greek Rhodope Mountain
Range: a) rivers of EMC “A” and “B”; b) rivers of EMC “C”, “D” and “E”.
4 Conclusions
This study is the first attempt for a comprehensive environmental flow assessment
in Bulgaria, where other ecological information (e.g. biological parameters) is still
scarce. The Bulgarian-Greek Rhodope Mountain Range was chosen for this purpose,
which is the most important transboundary area for both countries. A simple procedure
combining hydrological methods, a holistic approach and geoinformation techniques
was applied, emphasizing the spatiotemporal variability of the flow regime.
The results indicate that to maintain the rudimentary functions of the rivers in the
Rhodope Mountain Range requires an average daily flow in the range of 0.027 to 6.11
m3/s with a probability to exceed 90% of the time, which varies from 1.92 to 32.98%
of the MAF. On the other hand, the base flow variability index changes between 2
(low) and 20 (extremely high). High resolution gridded surfaces were generated by
282
spatial interpolation of the obtained data. Based on the calculated indices and applying
expert judgment of the flow regime, the rivers were regionalized into 5 EMCs
according to their potential to maintain the whole riverine ecosystem.
Finally, a disadvantage of this study is that it relies solely on hydrological data. We
therefore recommend building а more holistic methodology, involving biological
surveys and socio-economic information, which can better define the environmental
flow requirements in the transboundary region.
Acknowledgments. Part of the present study was conducted during D. Myronidis
Erasmus+ Staff Mobility for training mission in the Space Research and Technology
Institute (SRTI) research unit of the Bulgarian Academy of Sciences (BAS) between
28/10–01 and 11/2019.
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