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Ecological Indicators

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Ecological drought monitoring through fish habitat-based flow assessmentin the Gam river basin of KoreaSeo-Yeon Parka, Chanyang Surb, Joo-Heon Leea,b, Jong-Suk Kimc,⁎

a Dept. of Civil Engineering, Joongbu University, Goyang-si, Gyeunggi-do 10279, Republic of KoreabDrought Research Center, Joongbu University, Goyang-si, Gyeunggi-do 10279, Republic of Koreac State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, PR China

A R T I C L E I N F O

Keywords:Ecological drought indexEcological flowFish habitatIndicators of hydrologic alterationRiver2D

A B S T R A C T

Global warming has reinforced irregular hydrologic cycle patterns causing serious changes and threats to theecosystem. This study targeted the Gam River in South Korea and assessed ecological droughts that could occurin the river ecosystem due to long-term meteorological droughts. To assess ecological droughts, this study (1)focused on a representative fish species in the river, (2) analyzed the required ecological flow in the river toensure a healthy fish habitat, (3) developed a fish habitat-based ecological drought index (EcoDI_FH) calculatedby analyzing the river’s ecological flows and flow regimes with double thresholds to monitor how ecologicaldroughts proceed in extreme low flow, and (4) evaluated the applicability of the EcoDI_FH. The following are thesalient points of this study: (1) The dominant species, Zacco platypus, and endangered species, Squalidus gracilismajimae were chosen as the target fish species in the study area. To determine the ecological flow considering itshabitat the optimal ecological flow was estimated using the River2D hydrodynamic model. (2) Due to theconstruction of Buhang Dam in the Gam river basin, the pattern of river flow has changed significantly. (3) Byapplying the developed ecological drought index, the ecological flow was used as the first threshold for de-termining drought occurrence. Droughts were divided into four stages, and the extreme low flow calculatedusing indicators of hydrologic alteration was set as a second threshold or a serious stage. (4) For the past droughtperiods that took place in the Gam river basin, the drought state was analyzed before and after the constructionof the dam. In the drought period that occurred before Buhang Dam was built, an ecological drought occurred inthe serious stage during the drought period. During the droughts that occurred after the dam was built, theserious drought stage was alleviated quite substantially. It is expected that the results of this study will beutilized as one of the main tools for establishing countermeasures and adaptation strategies against ecologicaldroughts in the river ecosystem.

1. Introduction

A drought caused by a lack of precipitation develops into a me-teorological, agricultural, and hydrological drought over time (Wilhiteand Glantz, 1985). If a meteorological drought, which occurs due tolack of rainfall, continues, soil moisture content will fall, affecting plantgrowth and leading to an agricultural drought. If such drought condi-tions continue, river runoff will decrease, leading to water shortages incities and hydrological droughts (Wilhite and Glantz, 1985). Decreasein runoff due to a drought expectedly places restraints on water use, andsimultaneously causes significant damages to the aquatic ecosystem andenvironment as the amount of water flowing into reservoirs and wet-lands decreases. The impacts of long-term drought on the surrounding

environment may assume various forms such as destruction of wildlifehabitats, water quality degradation, forest fires, soil erosion, and ve-getation stress. Monitoring all these aspects is difficult.

While many studies have been conducted to monitor meteor-ological, agricultural, and hydrological droughts (Kogan, 1997;Vicente-Serrano et al., 2010; Lee et al., 2018; Park et al., 2018), only afew have focused on ecological droughts compared to agricultural andhydrological droughts. This is because the ecological drought coversmany aspects that are difficult to monitor and quantify. Recently, Kimet al. (2019) quantitatively assessed risks to water quality by applyingnonparametric kernel density estimation and assuming the probabilitythat the water quality of a river after an extreme drought exceeds thewater quality target. McEvoy et al. (2018) analyzed five watershed-

https://doi.org/10.1016/j.ecolind.2019.105830Received 4 June 2019; Received in revised form 6 September 2019; Accepted 12 October 2019

⁎ Corresponding author at: Department of Hydrology and Water Resources, School of Water Resources and Hydropower Engineering, Wuhan University, Wuhan430072, PR China.

E-mail address: jongsuk@whu.edu.cn (J.-S. Kim).

Ecological Indicators 109 (2020) 105830

1470-160X/ © 2019 Elsevier Ltd. All rights reserved.

T

scale drought plans in southwestern Montana, USA, by utilizing a newframework for ecological drought to understand if and how the ecolo-gical impacts of drought are currently being assessed. Crausbay et al.(2017) defined that the term “ecological drought” as an episodic deficitin water availability that drives ecosystems beyond thresholds of vul-nerability, impacts ecosystem services, and triggers feedback loops innatural and/or human systems. However, as yet, no comprehensivemethod exists to assess the impact of an ecological drought. Vegetationstress monitoring, which uses satellite imagery, has made some at-tempts in this direction (Kogan, 1997; Zhang and Jia, 2013).

Meteorological, agricultural, and hydrological droughts are mon-itored from the perspective of humans, while an ecological droughtshould be monitored from the perspective of the ecosystem and as-sessed by separating the terrestrial ecosystem from its aquatic coun-terpart (Fig. 1). However, it is difficult to set the threshold for droughtseverity for an aquatic ecosystem. Typically, assessing the drought statefor both hydrological and ecological droughts involves analyzing theextent of precipitation shortfall compared to rainfall in previous years.Ultimately, the quantity of water available in rivers and reservoirs isassessed to determine the drought severity. Just as the severity of ahydrological drought is assessed in terms of the amount of water storedin reservoirs, it should be possible to assess the severity of an ecologicaldrought by examining the minimum flow required for living organismsin the river ecosystem to survive. Therefore, to monitor an ecologicaldrought in a natural ecosystem, this study focuses on the fish ecosystemin a river. By doing so, the study assesses the ecological flow in the riverrequired to sustain fish habitats. We developed an ecological droughtindex to monitor how an ecological drought progresses with the river’secological flow and extreme low flow as the double threshold, andevaluated its applicability in the river ecosystem.

In this study, the Gam river basin in South Korea was selected tomonitor ecological drought, considering the fish habitat of the river.The dominant species, Zacco platypus, and endangered species,Squalidus gracilis majimae were selected as the target fish species in thestudy area. To calculate the ecological flow considering its habitat, theoptimal ecological flow was estimated using the River2D. TheEcological Drought Index for Fish Habitats (EcoDI_FH) was developedto examine the ecological flow reduction in the river and to evaluate the

severity of the ecological drought.

2. Materials and methods

2.1. Target river and hydrometeorological data

To evaluate the ecological impact of drought, data from SunsanGauging Station were analyzed. This station is located at a pointdownstream of the Gam River, which belongs to the Nakdong riverbasin of South Korea. The river bed structure is simple because it ismostly composed of sand. Table 1 shows the physical characteristics ofthe selected channel reach of Gam River.

As shown in Fig. 2, Buhang Dam was completed in the upstreampart of Gam River in 2013 to facilitate flood control and efficient watersupply in the basin. To evaluate the impacts of drought on the riverecosystem, the standardized precipitation index (SPI), a representativemeteorological drought index (Mckee et al., 1993, 1995), was utilized.SPI can be calculated using precipitation data alone. As meteorologicaldata of at least 30 years are required for the calculation of SPI, theprecipitation data of three automated surface observation system points(Chupungnyeong, Gumi, and Geochang) from 1975 to 2016 providedby the Korea Meteorological Administration were utilized. To convertpoint precipitation into areal average precipitation, the Thiessenpolygon method, a spatial interpolation technique, was applied withdaily precipitation. The calculation of the SPI index is based on his-torical records of precipitation accumulated over a specific period oftime. These time-series precipitation data are applied to the gammadistribution, which is converted to a standard normal distributionthrough equal probability transformation (Guttman, 1999). In thisstudy, past drought events were derived through SPI (180 days), whichappears to be effective in monitoring mid- to long-term droughts.

The runoff data of Gam River measured by Sunsan Gauging Stationwere obtained for 2007 to 2018 from the Nakdong River Flood Control

Fig. 1. Concept of ecological drought monitoring for terrestrial and aquaticecosystems.

Table 1Characteristics of the target river for the fish habitat study.

Water level gauging station (Sunsan) River width (m) Channel width (m) Bottom structure (%) Roughness coefficient

Rock Gravel Sand Silt

100–150 45–80 0 5 95 0 0.035

Fig. 2. Location of the Gam river basin in South Korea.

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Office (http://www.nakdongriver.go.kr). Fig. 3 shows the flow dura-tion curve (FDC) of Sunsan Gauging Station (2007–2018). It was foundthat the drought flow (Q355), indicating an exceedance probability of97% of the total duration (i.e., 355 days of the year), increased dra-matically from 0.25m3/s to 1.52m3/s after Buhang Dam was com-pleted in the upstream area of Gam River in December 2013. The re-lease of instream river flow from the dam was confirmed to be thereason for this increase in drought flow after the construction of BuhangDam.

2.2. Ecological data

To select representative fish species among various species in GamRiver for the fish habitat simulation, we used the results of the riverecosystem survey in the “Master Plan for the Gam River Management”(MOLIT, 2016). Fish surveys in Gam River were conducted thrice oversix selected sites before the construction of Buhang Dam. The surveyedsites contained 7 families, 21 species, and 1,303 fishes. No legallyprotected species were found to exist therein. Zacco platypus accountedfor 65.1% of the total population, followed by Pseudogobio esocinus(11.4%), Rhinogobius brunneus (4.5%), Opsariichthys uncirostris, andSqualidus gracilis majimae (3.5%), Cobitis hankugensis (3.3%), and others(4.5%).

To evaluate an ecological drought in Gam River with regard to itsfish habitat, Zacco platypus, the dominant species, and endangeredspecies, Squalidus gracilis majimae were selected as the target fish spe-cies in this study (Fig. 4). The habitat suitability index (HSI) representsthe quantitative correlation between the characteristics of physicalhabitats in a river and the suitability of the target fish species. Kang(2010) proposed a habitat suitability index (HIS) for 41 species usingmonitoring data in South Korea. In this study, we used the HSI relevantto species, as proposed by Kang (2010), to simulate the physical habitatof the target species (Fig. 5). When the HIS value is 1, the target fishspecies represents the most preferred flow velocity and water depth.After suitability was analyzed according to the fish habitat conditionsthrough a hydrodynamic simulation of streamflow, habitat simulationwas performed using HSI.

2.3. Ecological flow and its application assessment for ecological drought

Various criteria exist to assess river maintenance flows dependingon different flow rates, such as river instream flow, ecological flow,

mean annual low flow, and environmental flow. The application of eachcriterion (terminology) depends on the varying environmental situationof the river and consensus among decision-makers with regard to theextent of allowable social, economic, and ecological impacts on theriver.

Vélez et al. (2015) accurately described the differences betweenecological flow and environmental flow. Although some differencesexist across countries with regard to definitions, the ecological flowrefers to the concept of minimum flow, whereas the environmental flowis defined as the hydrological regime required to maintain the biota ofthe river alongside its social goods and ecosystem services (Richteret al., 1997). Various studies have been conducted to set concretequantitative criteria for ecological flow. Ormazábal (2004) definedecological flow as the minimum flow required to preserve the riverecosystem, and King and Tharme (1999) suggested that the minimumecological flow should be maintained during a drought period for thesurvival of the river ecosystem.

Natural rivers, which are not altered by human activities, canmaintain its minimum flow without any human intervention, and theriver ecosystem establishes an environment that can adapt to theminimum flow and survive. However, the natural flow of the riverchanges greatly due to changes in the natural environment arising fromman-made development activities for various purposes such as a basinor river development and dam construction (Poff et al., 1997; Nilssonet al., 2005). Hence, the ecological flow should be defined legally as theminimum flow to protect and maintain the river ecosystem, and it mustbe maintained and managed properly for different sections of the river.Minimum flow is usually a single threshold beyond which water cannotbe withdrawn for consumptive use, and it frequently provides sub-op-timal habitat conditions for aquatic species (Stalnaker et al., 1995).

Hydrological, hydraulic, and habitat simulations, as well as manyother models, have been developed to calculate the ecological flow.This study selected a representative fish species in the river as the basisto monitor ecological drought in the river basin and calculated theecological flow required for its habitat so as to use it as the threshold foran ecological drought assessment. Hence, this study applied the hy-drodynamic fish habitat simulation method to simulate the river’s fishhabitat and assess its ecological flow. In addition, by using the river’slong-term continuous hydrological observation data to analyze the flowregime, we estimated changes in the ecological flow caused by a hy-draulic structure (dam) built in the basin and reviewed its effect on anecological drought.

Fig. 3. Flow duration curves (FDC) of Sunsan Gauging Station before (“pre”: green color) and after (“post”: red color) the construction of Buhang Dam (2007–2018).Q95, Q185, Q275, and Q355 of the FDC represent the criteria for averaged-wet flow, normal flow, low flow, and drought flow, respectively.

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2.4. Hydrodynamic modeling for fish habitat assessment

Methods for simulating fish habitats using one-dimensional hydro-dynamic models, such as the Physical Habitat Simulation (PHABSIM),have been frequently used in previous studies to calculate the ecologicalflow and environmental flow. In addition, two-dimensional hydro-dynamic models have been applied for more detailed simulations ofinhabitable areas for fish due to the constructions of instream hydraulicstructures and stream inflows (Steffler and Blackburn, 2002; Wu andMao, 2007; Lee et al., 2006, 2010; Im and Kang, 2011).

In this study, the ecological flow was calculated by applying theRiver2D model, a two-dimensional hydrodynamic model, consideringthe habitat environment of the target fish species. The physical habitat

model involves ecological hydrodynamic analysis and is thus capable ofanalyzing the effects of the change in flow rate on the physical habitatof the river. The method is mainly based on the assumption that aquaticcreatures directly respond to changes in their hydrological environ-ment. The hydrodynamic elements of River2D are based on the St.Venant equations, which expresses the two-dimensional average waterdepth in a conservative form. River topographic information, flows,water depth, and HIS were used as input data for the River2D model inthis study. The model employs the mass conservation equation as wellas the momentum conservation equations in the x- and y-directions (Leeet al., 2010), as seen below.

Equation for conservation of mass:

Fig. 4. Relative abundance according to fish species in the Gam River.

Fig. 5. Habitat suitability index (HSI) for target fish species according to the water depth and flow velocity of Gam River, Korea. The figures are modified from Kang(2010).

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+ + =q qH

t0x

x

y

y (1)

Equation for conservation of x-direction momentum:

+ + +

= + +

qt x

Uqy

Vq gx

H

S Sx

Hy

H

( ) ( )2

gH( ) 1 ( ) 1 ( )

xx x

x fx xx xy

2

0(2)

Equation for conservation of y-direction momentum:

+ + +

= + +

qt x

Uqy

Vq gy

H

S Sx

Hy

H

( ) ( )2

gH( ) 1 ( ) 1 ( )

yy y

y fy yx yy

2

0(3)

where H is the average water depth of the flow; U and V are the averageflow velocities in the x- and y-directions, respectively; qx and qy are theflow rates in the x- and y-directions, respectively; g is the gravitationalacceleration, is the density of water; S andx0 S y0 are the bottom slopesin the x- and y-directions, respectively; Sfx andSfy are the friction slopesin the x- and y-directions, respectively; and xx , xy, yx , and yy are theshear stress values in the indicated directions.

The fish habitat modeling elements used in this study were based onthe weighted usable area (WUA) concept (Bovee, 1982) used in thePHABSIM fish habitat model, and following equation is as follows.

= ×=

WUA A Ci

ni i1 (4)

where Ai is the sectional area of each cell, Ci is the composite HSI ofeach cell according to the water depth, flow velocity, and river bedmaterial.

The initial and boundary conditions were determined by applyingthe data measured at Sunsan Gauging Station. Through this process,WUA, which is the total area of the physical habitat according to dif-ferent flow discharges, was calculated. The ecologically critical flow(QECF) is defined as the flow at which the sum of theWUA for each ratioof population is maximized, taking into account species diversity.

= ×Q Q WUAmax[ ( )]ECF jk

N

k k(5)

where K is the number of fish species, k is the number (n) of Kth fishspecies for the total number of fishes (N), and j is iteration for the flowchange. ECF can be estimated by targeting the entire species that live inor by selecting the representative species. In this study, to evaluate theecological impacts of drought on the Gam River, Zacco platypus, adominant species, and Squalidus gracilis majimae, which is small inquantity but is an endemic species in South Korea, were selected as thetarget species. The WUA was calculated corresponding to a flow forZacco platypus and Squalidus gracilis majimae (Figs. 6 and 7).

3. Results and discussion

3.1. Hydrologic alteration by Buhang dam and its influence on ecologicalflow

As mentioned in the previous section, the ecological flow estimatedby applying the habitat simulation method is the flow required toprovide the optimal habitat environment for a representative fish spe-cies. However, many studies define ecological flow as the minimumflow required to protect and maintain the river ecosystem (King andTharme, 1999; Ormazábal, 2004). Furthermore, natural rivers typicallyhave different magnitudes of flow throughout the year, such as extremelow flows, low flows, high flows, small floods, and large floods, and theriver ecosystem adapts to these different flows magnitudes to maintaina healthy ecosystem.

This study applies a double threshold to assess an ecologicaldrought. In other words, while the optimal ecological flow may be usedfor the assessment, the extreme low flow, that is, the minimum flowthat could occur under natural flow conditions could also be used forthe assessment. The first threshold determines the occurrence of anecological drought based on the habitat-based optimal ecological flow,and the second threshold determines the occurrence of an ecologicaldrought based on the minimum ecologically critical flow currently re-quired for the fish ecosystem.

In this section, we assessed the minimum threshold to determine theoccurrence of an ecological drought and used the indicators of hydro-logic alteration (IHA) approach suggested by Richter et al. (1997) toassess the hydrologic alteration of the Gam river basin. The IHA ap-proach can quantitatively assess the historical characteristics of thenatural flow and flow changes at a particular point in time, and cal-culate 34 environmental flow components (EFCs) that affect the eco-system (Conservancy, 2003; Nature Conservancy, 2007; Magilligan andNislow, 2005). IHA defines five major ecologically important compo-nents of flow: extreme low flows, low flows, high flow pulses, smallfloods, and large floods.

The Gam river basin underwent a huge change in the flow regimeafter a multipurpose dam was built upstream in 2013. Although dataare not available over a long enough period, the flow regimes for thehydrological periods before and after the construction of the dam wereanalyzed and are provided in Fig. 8 and Table 2.

According to Table 2, the extreme low flow increased from 0.25m3/s to 0.725m3/s, while its duration declined. Regarding the character-istics of monthly low flow changes, the monthly low flows increased inautumn and winter, which are traditionally characterized by low flow,and mostly decreased in the flood season (summer). As for high flowpulses, the peak flow declined by 13.7%, the frequency increased by57.1%, and the rise and fall rates dropped by 46.6% and 16.6%, re-spectively. No small and large floods were observed after the dam wasconstructed, and they were excluded from the analysis as they were notthe target flow magnitudes in this study.

As illustrated in Fig. 8, the monthly mean flow showed the largestvariation in July, August, and September due to flood control effects fortyphoons or other floods. In addition, the different magnitudes of flowsthat appeared before the dam’s construction disappeared subsequent to

Fig. 6. Result of habitat simulation using the River2D model and the dischargeweighted useable area (WUA) for the target fish species.

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it, and a simple unchanging flow pattern was observed after the dam’sconstruction was complete. In other words, the construction of the damhad a huge control effect on large floods during summer, and the ex-treme low flow during the low flow season increased as the dam con-trolled the maintenance flow discharge. While the dam-controlledfloods and ensured the minimum flow to guard against drought, it isexpected to have a significant effect on the Gam River ecosystem as theflow pattern in the overall natural flow regime changed radically. Thisstudy applied the EFC assessment results of Gam River as per the IHAapproach. It considered the extreme low flow to determine the occur-rence of an ecological drought in terms of an ecologically critical flow,namely, the concept of the minimum flow required for the survival andpreservation of the river ecosystem.

3.2. Ecological drought monitoring considering fish habitats in the river

To analyze the decrease in the ecological flow due to drought aswell as the impacts of drought on the river ecosystem, daily SPI, a

representative meteorological drought index, was calculated usingprecipitation data from the Gam river basin. SPI was less than −1 for2008, 2009, and 2015, indicating severe drought for those years, butrelatively wet conditions were observed in 2012 and 2013 (Fig. 9).According to the National Drought Information Center of South Korea,the Gam river basin had a limited water supply in 2009 due to a severedrought. Therefore, this study evaluated the ecological impact ofdroughts in 2008, 2009, 2015, and 2017, which were known to besevere drought years.

To assess ecological drought considering ecological flow (3m3/s)for Gam River’s fish habitat as the threshold, the number of days tosatisfy the ecological flow for Zacco platypus and Squalidus gracilis ma-jimae was examined by conducting a comparison with the observedflow at Sunsan Gauging Station (Table 3). In the severe drought years of2008 and 2009 before the construction of Buhang Dam, the number ofdays for which ecologically optimal flow was satisfied was 150 days and60 days, respectively. However, the number of days for which ecolo-gically optimal flow was satisfied in 2015, which had similar drought

Fig. 7. Simulation results of River2D according to the optimal ecological flow (3m3/s) for a selected channel reach of the Gam River.

Fig. 8. Changes in monthly mean flows after the construction of Buhang Dam on Gam River.

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severity as 2008 and 2009, was higher than 200 days, and threats to theecological flow caused by drought improved significantly after BuhangDam was built. In addition, the extreme low flow calculated by the IHAapproach was set as the threshold to examine the number of days inwhich the observed flow satisfied the extreme low flow (Table 4). Theanalysis of the number of days for which the extreme low flow(0.725m3/s) was satisfied was 328 days and 337 days in 2008 and2009, respectively. In the drought periods in 2015 and 2017 after theconstruction of the dam, the ecologically critical flow was satisfiedmore than 97% of the time.

3.3. Development of ecological drought index for fish habitats (EcoDI_FH)

To evaluate the ecological impact of drought on fish habitats,EcoDI_FH was developed to allow the monitoring of ecological drought.Ecological drought for fish habitats was divided into four stage-s—attention, caution, alert, and serious—and analyzed for the entireperiod from 2007 to 2018 using Eqs. (6) and (7).

= Q QQ

EcoDI_FH OBS ECF

ECF (6)

=Q serious stage Q extreme low flow( ) ( )ECF ELF (7)

As shown in Table 5, an index higher than 0 represents a normalstate, and an index lower than 0 represents an ecological drought state.The attention stage was classified as an index range between −0.25 and0, the caution stage ranged from −0.50 and −0.25, the alert stageranged from −0.75 and −0.50, and the serious stage was classified asan index lower than −0.75.

The ecological flow of 3m3/s calculated from fish habitat modelingserved as the threshold for an ecological drought, and it was found thatthe corresponding flow rate of the serious stage was similar to the ex-treme low flow of 0.725m3/s after the construction of Buhang Dam,calculated using the IHA approach. In other words, a flow lower thanthe fish habitat-based ecological flow corresponds to an ecologicaldrought, and when the flow is lower than the extreme low flow ex-amined in past flow regimes, the ecological drought is said to be ser-ious.

The percentage of each stage classified as ecological drought duringthe entire period (Table 6) shows that approximately 34.17% of theperiod corresponded to a drought stage for Zacco platypus and Squalidusgracilis majimae. In addition, 5.76% of the total analysis period wascategorized as serious drought.

Fig. 10 shows a schematic of the results of ecological droughtmonitoring in the four drought stages based on the fish habitat-basedecological flow for Zacco platypus and Squalidus gracilis majimae, thetarget fish species in the Gam River, along with the meteorologicaldrought index, SPI. Based on SPI, serious droughts (SPI≤−1.5) oc-curred in the Gam river basin in 2008, 2009, and 2015. In particular,even though the meteorological droughts were very serious in 2015 and2017, the ecological drought was not considered to be serious. Thisphenomenon could be attributed to the fact that Buhang Dam, whichwas built in 2013, released the river maintenance flow, confirming thatthe flow regime was controlled and changed by the dam thereby alle-viating an ecological drought.

While rivers and wetlands require the appropriate quantity of waterat any particular time to maintain healthy aquatic ecosystems, it isdifficult to ensure that the ecological and environmental flows aremaintained as required because of the various user demands on watersuch as those imposed by households, industries, and agriculture.

Drought is a natural phenomenon in the river basin, and fish haveoriginally evolved according to the occurrence of natural phenomenasuch as droughts and floods. So far, droughts have been monitored froma human perspective for planning water shortages. In other words,methods have been developed to assess drought severity, amount ofavailable water, and response to droughts. Nevertheless, droughts have

Table 2Analysis of environmental flow components (EFCs) using indicators of hydro-logic alteration (IHA).

EFC type Hydrologic parameters Pre Dam Post Dam Variation

Extreme lowflows

Minimum flow duringevent (m3/s)

0.25 0.725

Duration (d) 8 4.25Timing (Julian date ofpeak flow)

169 163.8 –

Frequency (number) 1 0

Monthly lowflows (m3/s)

January 2.3 3.8February 2.3 3.2March 4 3.9April 4.95 4.1May 3.7 2.95June 2.6 2.25July 3.9 4.4August 4.25 4.1September 4.55 5October 4.4 6.225November 5.2 5.5December 4.55 4.95

High flow pulses Peak flow (maximum flowduring event, m3/s)

15.3 13.2

Duration (days) 4 4 –Timing (Julian date ofpeak flow)

194 189 –

Frequency (number) 7 11Rise rates (m3/s·d) 8.8 4.7Fall rates (m3/s·d) −2.35 −1.96

Fig. 9. Historical drought events of Gam river basin using SPI (180Days) (2007–2018).

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not been assessed based on the flow demanded by the ecosystem. Whenthe river flow decreases due to drought, fish may migrate to search fornew habitat and find shelter in a puddle or pool where they can ensurethe minimum habitat. During the decline in flow due to a short-termdrought, the aquatic ecosystem may be protected for a limited amountof time, but if the drought continues over a longer-term, it can pose aserious threat to the survival of the aquatic ecosystem.

4. Summary and conclusions

This study developed the fish habitat-based ecological droughtindex called EcoDI_FH to assess ecological droughts that can occur in ariver ecosystem due to long-term meteorological drought. The fol-lowing conclusions were drawn after analyzing its applicability.

1) Zacco platypus and Squalidus gracilis majimae were selected as thetarget fish species. The optimal ecological flow considering thetarget fishes’ habitat in South Korea’s Gam River was determined tobe 3m3/s as per the River2D hydrodynamic model.

2) The flow regime changed significantly due to the construction ofBuhang Dam in the Gam River basin, and the calculation of EFCsusing the IHA showed that the extreme low flow, as well as the lowflow, increased during the dry seasons of autumn and winter.However, the diversity in the magnitudes of flows, including largefloods during summer, has disappeared, and the changing patternsof the ecological and environmental flows are expected to haveserious effects on the health of the river ecosystem.

3) The ecological drought index (EcoDI_FH) was developed to monitoran ecological drought considering Gam River’s fish habitat. Theecological flow calculated from habitat modeling was used as the

Table 3Number of days for which seasonal flow observed at Sunsan Gauging Station satisfies the ecologically optimal flow for fish habitats.

Year Drought year Zacco platypus & Squalidus gracilis majimae (Ecologically optimal flow: 3m3/s)

Entire period (Jan–Dec) Spring (Mar–May) Summer (Jun–Aug) Autumn (Sep–Nov) Winter (Dec–Feb)

2007 324 72 71 91 902008 ○ 150 30 64 7 492009 ○ 60 7 48 5 02010 220 81 63 61 152011 232 42 69 89 322012 274 81 68 91 342013 273 80 68 91 342014 275 65 57 91 622015 ○ 284 79 60 46 912016 203 75 58 – –2017 ○ 284 75 63 84 622018 317 92 63 91 71

Note: Flow data were missing from October to December 2016.

Table 4Number of days for which the seasonal flow observed at Sunsan Gauging Station satisfies the ecologically critical flow (extreme low flow) for fish habitats.

Year Drought year Zacco platypus & Squalidus gracilis majimae (Ecologically critical flow: 0.725m3/s)

Entire period (Jan–Dec) Spring (Mar–May) Summer (Jun–Aug) Autumn (Sep–Nov) Winter (Dec–Feb)

2007 360 90 92 87 912008 ○ 337 83 92 92 802009 ○ 328 91 83 64 902010 365 90 92 92 912011 365 92 92 91 902012 366 92 92 91 912013 365 92 92 91 902014 364 92 91 91 902015 ○ 365 92 92 91 902016 270 92 92 – –2017 ○ 355 92 82 91 902018 365 92 92 91 90

Note: Flow data were missing from October to December 2016.

Table 5Classification of ecological drought by EcoDI_FH.

Drought classes Normal Attention Caution Alert Serious

Index range > 0 0.25 0 0.5 0.25 0.75 0.50 <−0.75Corresponding flow rate (m3/s) > 3.00 2.25~ 3.00 1.50~ 2.25 0.75~ 1.50 <0.75

Table 6Percentage of ecological drought months classified into the four ecologicaldrought stages during the entire analysis period (2007–2018).

Drought Stage Percentage of ecological drought months based on theecological optimal flow (%)

Remark

Attention 9.57Caution 11.35Alert 7.49Serious 5.76Total 34.17

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first threshold to determine a drought. Droughts were categorizedunder four stages, and the extreme low flow calculated using IHAwas set as the second threshold or the serious stage. In other words,monitoring the level of stress that fish undergo from decreasinghabitats as a result of a reduction in flow caused by drought canprovide adequate and timely information to river managers, so thatproper measures can be established to maintain sustainable ecolo-gical flow.

4) For a river with a water storage facility such as a dam, it is expectedthat its ecological flow can be maintained or discharged dependingon the monitoring results of the ecological drought. During twodrought periods in the Gam river basin, the ecological drought wasclassified as serious before the construction of Buhang Dam(2008–2009), but this stage was alleviated to a great extent in thedrought period after the dam’s construction (2015 and 2017).

5) However, small and large floods that occurred during summer dis-appeared due to the effect of the dam’s flood control. In order tosecure the soundness of the river ecosystem, which has adapted tothe scale of various flows, it is necessary to establish dam operationplans that not only discharge the minimum ecological flow but alsoreleases artificially different environmental flows in consideration ofthe ecosystem.

This study developed a drought index to assess the ecologicaldrought that could occur in the Gam river ecosystem for representativefish species and suggested a method to monitor and provide warning ofdrought by using the developed ecological drought index. The mon-itoring method proposed in this study is expected to be used to preparefor future ecological droughts in a changing climate.

Declaration of Competing Interest

The authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influ-ence the work reported in this paper.

Acknowledgements

This subject is supported by Korea Ministry of Environment (MOE)as “Water Management Research Program” (RE201901059). This re-search was also supported by Basic Science Research Program throughthe National Research Foundation of Korea (NRF) funded by theMinistry of Education (NRF-2017R1D1A1A02018546).

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