water Article
Estimation of River Management Flow Considering Stream Water Deficit Characteristics Jang Hyun Sung 1 and Seung Beom Seo 2, * 1 2
*
Han River Flood Control Office, Ministry of Environment, Seoul 06501, Korea;
[email protected] Institute of Engineering Research, Seoul National University, Seoul 08826, Korea Correspondence:
[email protected]; Tel.: +82-2-880-8270
Received: 5 September 2018; Accepted: 25 October 2018; Published: 26 October 2018
Abstract: South Korea endured extreme drought through 2015 and 2016. This hydrological drought led to a socio-economic drought which is a restriction on stream water use. Previous studies have explored streamflow drought using a threshold level based on flow duration curves, but streamflow drought does not necessarily lead to stream water deficit, which is related to water demand. Therefore, this study introduced a threshold for stream water deficit in South Korea, which is termed as river management flow, and was applied to Geum River Basin where a severe drought recently occurred. The stream water coordination council has restricted the use of stream water to cope with the stream water deficit. The deficit characteristics for the upstream and downstream river management flow should be similar in order to ensure the feasibility of stream water restrictions. Thus, upstream and downstream river management flows, which reproduced similar deficit characteristics to those of the reference site, were estimated. The deficit characteristics of Bugang and Gyuam were estimated from their river management flows for the 2015 drought and were comparable to those of Gongju. We expect this study to minimize the conflict between upstream and downstream water users in future. Keywords: hydrological drought; stream water use; stream water deficit; river management flow
1. Introduction Hydrological drought is defined as a significant decrease in the availability of water in all its forms appearing in the land phase of the hydrological cycle [1]. To cope with the risk of droughts, a variety of hydrological drought indices are used to identify the start and end times of a drought and to evaluate drought phenomena such as severity and frequency [1–8]. Similarly, streamflow drought is generally defined by a threshold level based on the run theory by Yevjevich [9], which identifies the characteristics (duration, deficit, and magnitude) of droughts [10–13]. The seasonal, monthly, and daily threshold levels obtained from flow duration curves (FDCs) were associated with an anomalous deficit [7]. Because drought is defined as the condition when streamflow falls below the threshold, a cautious approach for the appropriate threshold level is required [12,14,15]. Streamflow and socio-economic droughts are closely related to the deficit in stream water necessary for socio-economic activities. In particular, stream water deficit (SWD) has an influence on socio-economic activities such as hydroelectricity production and recreation [16]. Thus, assessment of SWD is necessary not only for estimation of the statistical threshold using FDCs, but also for that of the threshold considering stream water use (SWU). Sung and Chung [13] proposed a severity-duration-frequency curve for SWD using threshold levels based on water demand. In their study, Sung and Chung [13] calculated SWD assuming that the desired yield of the dam was based on the demand of stream water [17]. However, the use of stream water in South Korea depends on contracts with dams and the permission of users of the stream water. Because the permission for Water 2018, 10, 1521; doi:10.3390/w10111521
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2 of 15 with dams and the permission of users of the stream water. Because the permission for stream water comprises more than 70% of the total SWU, SWU must be considered in the calculation of SWD. The permission formore SWUthan is given on SWU, a standard watershed. River management flow stream water comprises 70% ofbased the total SWU must be considered in the calculation (RMF), which is the threshold level for SWDs, is also calculated for each standard watershed. of SWD. However, SWD is related to streamflow andonRMF, such that the RMFRiver at a management site where streamflow is The permission for SWU is given based a standard watershed. flow (RMF), observed is necessary. Therefore, at a site where the outlet of the standard watershed and that of the which is the threshold level for SWDs, is also calculated for each standard watershed. However, observation siteto dostreamflow not coincide, the RMFsuch should appropriately at a nearby SWD is related and RMF, thatbe the RMF at a siteprocessed where streamflow is observation observed is site. A stream is continuous from upstream to the downstream, and a deficit or surplus in the necessary. Therefore, at a site where the outlet of the standard watershed and that of the observation upstream is propagated downstream, thereby creating the need for a balanced operation of upstream site do not coincide, the RMF should be appropriately processed at a nearby observation site. A stream and downstream is continuous fromflows. upstream to the downstream, and a deficit or surplus in the upstream is propagated A stream is public property, has the right to use it equitably. South Korea, the downstream, thereby creating the and needeveryone for a balanced operation of upstream andIn downstream flows. stream water coordination council (SWCC) has the authority to adjust SWU upstream and A stream is public property, and everyone has the right to use it equitably. In South Korea, downstream to cope with SWD.council Therefore, to secure validity of adjusting the stream water coordination (SWCC) has the the reliability authority and to adjust SWU upstreamSWU, and the SWD should be managed equitably via RMF to reproduce similar deficit upstream and downstream to cope with SWD. Therefore, to secure the reliability and validity of adjusting SWU, downstream. Forbethis purpose, it is necessary utilize RMFs at other sites because of downstream. their spatial the SWD should managed equitably via RMF to to reproduce similar deficit upstream and limit. To study adjustments in stream water flow, it is necessary to revisit the point-to-point For this purpose, it is necessary to utilize RMFs at other sites because of their spatial limit.transition To study of RMFs. adjustments in stream water flow, it is necessary to revisit the point-to-point transition of RMFs. In this this study, RMF was In study, RMF was estimated, estimated, which which isissimilar similarwith withthe theSWD SWDcharacteristics characteristicsatatsites sitesalong alonga stream that are at high risk of drought. The RMFs were calculated based on permission for the use of a stream that are at high risk of drought. The RMFs were calculated based on permission for the stream water in a standard watershed, and the upstream and downstream flows, which have use of stream water in a standard watershed, and the upstream and downstream flows, which have characteristics similar to to expanding thethe RMF at the observation site characteristics similar to tothose thoseofofSWDs, SWDs,were wereapplied applied expanding RMF at the observation corresponding to the outlet of the standard watershed. Section 2 of this paper summarizes site corresponding to the outlet of the standard watershed. Section 2 of this paper summarizes background theories theories on on drought drought and and deficit deficit and and introduces introduces SWU SWU and and deficit deficit management management in in South South background Korea. In Section 3 of this paper, calculation of RMF, considering stream water demand and transfer Korea. In Section 3 of this paper, calculation of RMF, considering stream water demand and transfer of of the deficit characteristics, is presented. the deficit characteristics, is presented.
2. Materials and Methods Methods 2. Materials and 2.1. Procedure The methodology adopted in this study for the estimation of RMF by considering considering the characteristics of an SWD SWD is represented represented by by the the flow flow chart chart in in Figure Figure1. 1.
Calculate RMF, which is the threshold level of SWD
↓ Calculate deficit characteristics for SWD, for streamflow below RMF; choose reference site as a site with high drought risk
↓ Estimation of the upstream and downstream RMFs with characteristics similar to the deficit characteristics at the reference site Figure 1. Flow chart of the proposed methodology. methodology.
The procedure is outlined below: 1. 2.
The RMFs were calculated based on the permission for SWU of a standard watershed in Geum River Basin. The RMFs of other sites were estimated by transferring the deficit characteristics of the site at high risk. First, the Drought Severity Index (DSI), representing the deficit characteristics,
The procedure is outlined below: 1.
The RMFs were calculated based on the permission for SWU of a standard watershed in Geum River Basin. 2. The 10, RMFs at Water 2018, 1521 of other sites were estimated by transferring the deficit characteristics of the site 3 of 15 high risk. First, the Drought Severity Index (DSI), representing the deficit characteristics, was calculated for a standard watershed. Second, a site with high drought risk was chosen as the was calculated reference site. for a standard watershed. Second, a site with high drought risk was chosen as the reference site. and downstream RMFs of the reference site were estimated such that the 3. The upstream 3. The upstream of and reference site wereThe estimated sucheither that the characteristics thedownstream reference siteRMFs could of be the similarly reproduced. RMF during the characteristics of the reference site could be similarly reproduced. The RMF during either irrigation or non-irrigation period was fixed, and the RMF during the other non-irrigation or the the irrigation or non-irrigation fixed, and RMF during non-irrigation or irrigation period was changed,period which was minimized thethe difference in thethe DSIother of the reference site. the irrigation period was changed, which minimized the difference in the DSI of the reference site. 2.2. Study Area 2.2. Study Area In South Korea, 50–60% of the annual precipitation is concentrated in the summer, which results In South Korea, the droughts annual precipitation is concentrated the summer, which results in in floods during the 50–60% summerofand during other seasons. The in Geum River Basin is in central floods during the summer and droughts during other seasons. The Geum River Basin is in central South 2 South Korea (Figure 2a). The watershed area of the basin is 9911.8 km (Figure 2b). Although Korea (Figure 2a). The watershed ofthere the basin is 9911.8 km2 (Figure 2b).drought Although streamflow streamflow is typically high in thearea river, is growing concern regarding because of the is typically high in the river, there is growing concern regarding drought because of the recent recent decrease in precipitation (Figure 3a). The average annual precipitation in the Geum River Basin decrease precipitation (Figureduring 3a). The annual in the Geum Basin is is 1271.5 in mm, and precipitation theaverage last 2 years hasprecipitation been 830.5 and 1163.1 mm,River respectively. 1271.5 mm, and precipitation during the last 2isyears has been and 1163.1 respectively. Particularly, precipitation during the summer decreasing. The830.5 precipitations inmm, August 2015 and Particularly, precipitation during the summer is decreasing. The precipitations in August 2015 and August 2016 were 68.8 and 68.3 mm, respectively, which was about 28% of the normal level (246.7 August 2016 were 68.8 and 68.3 mm, respectively, which was about 28% of the normal level (246.7 mm), mm), and the precipitations in July 2015 and July 2016 were 54% and 45% of the normal level, and the precipitations in July 2015 and July 2016 were 54% 45% River of the Basin normal level,considerably, respectively. respectively. The monthly streamflow at Gongju along theand Geum varies The monthly streamflow at Gongju along the Geum River Basin varies considerably, and the streamflow and the streamflow during August 2015 and August 2016 was approximately 21% of the normal during August 20153b). andThis August wasprecipitation approximately 21% the of the normal streamflow 3b). streamflow (Figure was2016 because during rainy season was very (Figure low during This was because precipitation during the rainy season was very low during August 2015 and 2016, August 2015 and 2016, and typhoons did not make landfall. In South Korea, for the purpose of and typhoons did notgovernment make landfall. In South Korea, for the purpose of common useinto amongst common use amongst agencies and individuals, the whole country is divided large, government agencies and individuals,Figure the whole is divided into large, medium, and standard medium, and standard watersheds. 4a–c country shows the large, medium, and standard watersheds watersheds. 4a–c shows the large, medium, standard of the Geum Basin. of the GeumFigure River Basin. The Geum River Basin isand composed of watersheds 1 large, 14 medium, and River 78 standard The Geum River Basin is composed of 1 large, 14 medium, and 78 standard watersheds. watersheds.
(a)
(b)
Figure Figure 2. 2. Location Location of of the the Geum Geum River River Basin. Basin. (a) (a) Korean Korean Peninsula; Peninsula; (b) (b) Geum Geum River River Basin. Basin.
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ave. 2015 ave. 2016 2015 2016
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ave. 2015 ave. 2016 2015 2016
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250
Streamflow3[m /s] Streamflow [m /s]
Precipitation [mm/month] Precipitation [mm/month]
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0 0
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Jun Jul Month Month (b) May
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Nov Dec Jun Jul Aug Sep Oct Nov Dec Month (a) Month (a) (b) Figure 3. Changes in annual precipitation (Geum River Basin) and monthly streamflow (Gongju). 3. Changes in annual (Geum monthly streamflow (Gongju). Figure 3. (Geum River (Gongju). (a) Monthly precipitation [mm].precipitation (b) Monthly streamflow at Basin) Gongjuand (m3/s). (a) Monthly /s). Monthly precipitation precipitation [mm]. [mm]. (b) (b) Monthly Monthly streamflow streamflow at at Gongju Gongju (m (m33/s).
Jan
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(a) (b) (c) (a) (b) (c) Figure4.4.Large, Large,medium, medium,and andstandard standardwatersheds watershedsofofthe theGeum GeumRiver RiverBasin. Basin.(a)(a) Large; (b)Medium; Medium; Figure Large; (b) (c) Standard. 4. Large, medium, and standard watersheds of the Geum River Basin. (a) Large; (b) Medium; (c)Figure Standard. (c) Standard.
2.3. Threshold Level Method 2.3. Threshold Level Method 2.3. Threshold Level level Method The threshold method can easily obtain the start and the end times of a drought or streamflow The threshold level method can easily obtain the start and the end times of a drought or deficit period, and has been used to define streamflow droughts or deficits (Figure 5). A drought starts The threshold level and method can used easilytoobtain start and the endortimes of (Figure a drought streamflow deficit period, has been define the streamflow droughts deficits 5). Aor when the streamflow falls below a threshold level, and the drought recovers when the streamflow streamflow deficit period, and has been used to define streamflow droughts or deficits (Figure drought starts when the streamflow falls below a threshold level, and the drought recovers when 5). theA returns above the threshold level. The duration (run-length, di ), total deficit (run-sum, vi ) which is drought starts when the streamflow falls below threshold(run-length, level, and the recovers whenvthe streamflow returns above the threshold level. Thea duration di),drought total deficit (run-sum, i) the sum of the monthly deficits, and magnitude (vi /di ) of each drought event can be readily obtained streamflow returns above the threshold The duration di), total deficit which is the sum of the monthly deficits, level. and magnitude (vi/d(run-length, i) of each drought event can(run-sum, be readilyvi) (Figure 5). Because South Korea has a large monthly variation in streamflow, the temporal resolution which is(Figure the sum the monthly anda magnitude (vi/dvariation i) of each in drought event the can temporal be readily obtained 5).ofBecause Southdeficits, Korea has large monthly streamflow, for drought analysis should be shorter than 1 month, but measuring the daily streamflow makes obtained for (Figure 5). Because Korea has a large in streamflow, temporal resolution drought analysisSouth should be shorter than 1monthly month, variation but measuring the daily the streamflow it difficult to ensure independence among droughts. Thus, for independence, pooling methods, resolution for drought analysis should be shorter than 1 month, Thus, but measuring the daily streamflow makes it difficult to ensure independence among droughts. for independence, pooling such as moving average (MA), sequential peakamong algorithm, and inter-eventfor time and volume criterion, makes itsuch difficult to ensure independence independence, pooling methods, as moving average (MA), sequential peakdroughts. algorithm,Thus, and inter-event time and volume were necessary [10]. methods, suchnecessary as moving average (MA), sequential peak algorithm, and inter-event time and volume criterion, were [10]. criterion, were necessary [10].
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Figure 5. 5. Definition of streamflow drought and deficit based on the Figure the threshold threshold level level method. method.
To To assess assess the the severity severity of of drought drought events, events, Pandey Pandey et et al. al. [12] [12] proposed proposed the the drought drought severity severity index index (DSI) (Equation (1)). Generally, the main characteristics of drought are magnitude and duration. (DSI) (Equation (1)). Generally, the main characteristics of drought are magnitude and duration. The The is categorized using severity a drought estimatedbased basedon onits itsmagnitude magnitudeand and duration. duration. DSI DSI is categorized using thethe severity of of a drought estimated Equation (1) is as follows: Equation (1) is as follows: V de (1) DSIe = d V d (1) DSI = TL m where V d is the deficit streamflow volume for the duration of the drought event, V TL is the expected streamflow at the threshold level for the duration of the deficit streamflow volume forthe theduration durationofofthe thedrought droughtevent, event,deVis TL is the expected where Vd is volume the independent drought event, and d is the maximum duration of an independent drought event. m for the duration of the drought event, de is the streamflow volume at the threshold level duration of In this study, DSI was used to define droughts as mild, moderate, severe, or extreme (Table 1). the independent drought event, and dm is the maximum duration of an independent drought event.
In this study, DSI was used to define droughts as mild, moderate, severe, or extreme (Table 1). Table 1. Drought classification based on Drought severity index (DSI).
Table 1. Drought classification based on Drought severity index (DSI). DSI Drought Strength
DSI0.5 2.4. Permission for SWU and RMF in South Korea
Weak Drought Strength Mild Weak Moderate Severe Mild ExtremeModerate
Severe Extreme
In this section, the river management 2.4. Permission for SWU and RMF in South system Korea and the system of permission for SWU in South Korea 5 in Figure 6) is the minimum streamflow required is reviewed. The instream requirement flow (IRF; In this normal section, functioning the river management and water the system for streamflow SWU in South to maintain of the river. system The stream intakeofispermission defined as the for Korea is reviewed. The instream requirement flow (IRF; ⑤ in Figure 6) is the minimum streamflow 4 vested water rights and is the sum of the customary water rights ( ) and SWU permitted water rights to maintain normal functioning the river. The stream water intake is defined as the 3 1 is the 10-year frequency of the streamflow maintained (required
). The standard drought streamflow (SDS;of ) streamflow for vested water rights and is the sum of the customary water rights (④) and SWU 3 + 4 + ) 5 is the minimum streamflow required to meet for more than 355 days per year. The RMF ( permitted water rights The standard drought streamflow is the 10-year frequency of the demand of SWU and(③). maintain the normal functioning of the (SDS; river, ①) which is the threshold level for the streamflow maintained for more than 355 days per year. The RMF (③ + ④ + ⑤) is the minimum 2 = 1 − ( 3 + 4 + )) 5 SWD. Water availability ( represents the new water rights, and is calculated by streamflow the required to meet demand of SWU and maintain subtracting RMF from thethe SDS, as shown in Equations (2)–(6).the normal functioning of the river, which is the threshold level for SWD. Water availability (② = ① − (③ + ④ + ⑤)) represents the new water is calculated by subtracting thetoRMF fromthe thenormal SDS, asfunctioning shown in Equations (2)–(6). IRFrights, (m3 /s)and = minimum streamflow required maintain of the river (2)
IRF (m3/s) = minimum streamflow required to maintain the normal functioning of the river Streamflow for vested water rights (m3 /s) = 3/s) = Streamflow for vested water rights (m customary water rights + SWU permitted water rights customary water rights + SWU permitted water rights
(2) (3) (3)
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33 RMF RMF(m (m/s) /s) == IRF + streamflow for vested water rights 3 3/s) SDS /s) = 10-year frequency streamflow maintained for more 355 days per year SDS (m(m = 10-year frequency streamflow maintained for more thanthan 355 days per year 3 3 Water availability /s)==SDS SDS− − RMF Water availability (m(m /s)
(4) (4) (5) (5)
(6) (6)
In South South Korea, Korea, because because of of climatic climatic variations, variations, if if stream stream water water use use is is permitted permitted based based on on Q95 Q95 In (the 95-day exceedance flow) or Q185 (the 185-day exceedance flow), then an SWD is likely to occur (the 95-day exceedance flow) or Q185 (the 185-day exceedance flow), then an SWD is likely to occur for more more than than 99 months months during during the the year. year. Thus, Thus, the the upper upper limit limit of of permission permission for for SWU SWU must must be be equal equal for to the the SDS SDS for for water water use use to to ensure ensure aa stable stable supply. supply. Japan Japan is is very very similar similar to to Korea Korea in in terms terms of of water water to rights,and andnormal normal streamflow is sum the sum the and RMF anduse. water use.availability Water availability is the rights, streamflow is the of theofRMF water Water is the difference difference SDS and normal streamflow. However, thefor data for SDS calculations between thebetween SDS andthe normal streamflow. However, the data used SDSused calculations are differentare in different in the two countries: Korea uses simulated flow due to length short record the two countries: South KoreaSouth uses simulated natural flow natural due to short record while length Japan while Japan considers observed data. considers observed data.
Figure 6. Concept of water rights in South Korea.
The RMF is estimated Figure based 6. onConcept the water budget ofinsupply and demand from downstream of water rights South Korea. to upstream. The supply is the main (Imain ) and tributary inflow (Iside ), return flow (Ireturn ), and wastewater standard watershed. The demand is the (Ointakefrom ) anddownstream SWU (Ointaketo ). release ) for a The RMF is(Iestimated based on the water budget of supply andIRF demand Here, the main and tributary inflow for a standard watershed is the SDS (Equation (7)). However, upstream. The supply is the main (Imain) and tributary inflow (Iside), return flow (Ireturn), and wastewater when a water resource facility as a (O dam, the dam supply contracted downstream (Ireleasethere ) for aisstandard watershed. Theupstream, demand issuch the IRF intake ) and SWU (Ointake ). Here, the main and becomes the SDS. To estimate the RMF, the end of the river should satisfy the IRF at all times, such that tributary inflow for a standard watershed is the SDS (Equation (7)). However, when there is a water the RMF at the end of the river tothe thedam IRF. supply Therefore, we estimate RMF from the initial IRF resource facility upstream, suchisasequal a dam, contracted downstream becomes the SDS. at end ofthe theRMF, river the to the upstream. Because thethe RMF should greater thanatthat Tothe estimate endRMF of the river should satisfy IRFupstream at all times, suchbe that the RMF the downstream, to estimate the RMF upstream, the RMF downstream was first compared to the IRF of end of the river is equal to the IRF. Therefore, we estimate RMF from the initial IRF at the end of the the corresponding basin. A larger value chosen, thereby adding the demand to the chosen value river to the RMF upstream. Because thewas RMF upstream should be greater than that downstream, to and subtracting the supply, which became the RMF of the corresponding basin. This is necessary estimate the RMF upstream, the RMF downstream was first compared to the IRF of the corresponding to guarantee thevalue RMF was downstream in the upstream (Figure 7). An example the abovementioned basin. A larger chosen, thereby adding the demand to the chosenofvalue and subtracting calculation shownbecame in Figure and Equations (8)–(10). This method proposedto byguarantee the Ministry the supply,iswhich the8RMF of the corresponding basin. Thiswas is necessary the of Land, Infrastructure and Transport in Japan, and South Korea employed the same method for the RMF downstream in the upstream (Figure 7). An example of the abovementioned calculation is estimation of RMF.8 and Equations (8)–(10). This method was proposed by the Ministry of Land, shown in Figure
Infrastructure and Transport in Japan, and South Korea employed the same method for the (7) estimation of RMF. RMF2,3 = max(IRF2,3 , RMF1,2 ) + Ointake − ( Imain + Iside ) where Iside = IRFside or standard drought streamflow
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RMFA∼B = IRFA∼B + Ointake − ( Irelease + Ireturn + Iside + Imade ) Water 2018, 10, x FOR PEER REVIEW = 15.0 + 10.0 − (1.0 + 0.5 + 1.0 + 2.5) = 20.0 m3 /s
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RMFB∼C = RMFA∼B + Ointake − ( Ireturn + Imade ) = 20.0 + 2.5 − (1.5 + 2.0) = 19.0 m3 /s
(9)
RMFA∼B > IRFB∼C
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(8)
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Figure 7. Schematic showing calculation of the RMF for the main stream.
RMF
= max IRF
,
, RMF
,
(7)
,
where Iside = IRFsideFigure or standard drought streamflow 7. Schematic showing calculation of the RMF for the main stream.
Figure 7. Schematic showing calculation of the RMF for the main stream.
RMF
,
= max IRF
,
, RMF
(7)
,
where Iside = IRFside or standard drought streamflow
RMF. Figure 8. Example of the calculation of RMF.
3. Results
RMFA~B = IRFA~B
3.1. Estimation of RMF
= 15.0
10.0
intake
1.0
release
0.5
1.0
return
side
2.5 = 20.0
made
/
(8)
In this section, the RMF estimation based on use permission Figure 8. Example ofwater the calculation of RMF. from standard watersheds is described. By establishing the RMF at the upstream and downstream sites that had similar risk based RMFB~C = RMFA~B return intake made on a site at which the deficit risk was highest, the restriction of stream water users at the upstream and (9) RMFA~B = IRFA~B return intake release side made downstream sites was equally use accounted = distributed. 20.0 2.5Agricultural 1.5 2.0 = 19.0 for/ 89.3% of the streamflow use (8) in the Geum River Basin, and the remaining was for = 15.0 10.0 1.0 use0.5 1.0residences, 2.5 = industries, 20.0 / and environmental improvement. The stream water used in theA~B standard of the Geum River is shown RMF IRFwatershed (10)in B~C Figure 9; the permission of SWU was concentrated downstream. In addition, because agricultural RMFB~C = RMFA~B return intake made water is primarily needed between May and October (i.e., the irrigation period), it was confirmed that (9) 3. Results / ensure stable water supply. = into 20.0irrigation 2.5 and 1.5 2.0 = 19.0 the RMF needed to be divided non-irrigation values to
3.1. Estimation of RMF
RMFA~B IRFB~C (10) In this section, the RMF estimation based on water use permission from standard watersheds is described. By establishing the RMF at the upstream and downstream sites that had similar risk based 3. Results on a site at which the deficit risk was highest, the restriction of stream water users at the upstream and downstream sites was equally distributed. Agricultural use accounted for 89.3% of the 3.1. Estimation of RMF streamflow use in the Geum River Basin, and the remaining use was for residences, industries, and In this section, the RMF estimation water permission from standard environmental improvement. The streambased wateronused in use the standard watershed of the watersheds Geum Riveris establishing the RMF at of theSWU upstream and downstream sites that had similar risk based isdescribed. shown inBy Figure 9; the permission was concentrated downstream. In addition, because on a site at water which isthe deficit risk was highest, restriction of stream water users at the upstream agricultural primarily needed between the May and October (i.e., the irrigation period), it was
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confirmed Water 2018, 10, that 1521
the RMF needed to be divided into irrigation and non-irrigation values to ensure 8 of 15 stable water supply.
Figure Figure9.9.Status Statusof ofthe theGeum GeumRiver Riverfor forRMF RMFcalculation. calculation.
The RMF for the main stream of the Geum River Basin was estimated considering the tributaries The RMF for the main stream of the Geum River Basin was estimated considering the tributaries flowing into the main stream (Geum River). In the Geum River, the RMFs at five sites, as enacted flowing into the main stream (Geum River). In the Geum3 River, the RMFs at five sites, as enacted by by the River Law, are 8.5, 10.5, 15.1, 17.1, and 19.93 m /s for Bugang, Maepo, Gongju, Gyuam, the River Law, are 8.5, 10.5, 15.1, 17.1, and 19.9 m /s for Bugang, Maepo, Gongju, Gyuam, and and Gangyeong, respectively. The Daecheong Dam (3008), Geum River Estuary (3014), Gapcheon Gangyeong, respectively. The Daecheong Dam (3008), Geum River Estuary (3014), Gapcheon (3009), (3009), Mihocheon (3011), Nonsancheon (3013) (a national river), Yongsucheon (301202), Daegyocheon Mihocheon (3011), Nonsancheon (3013) (a national river), Yongsucheon (301202), Daegyocheon (301203), Yugucheon (301206), Jicheon (301209), Seokseongcheon (301213), Geumcheon (301212), (301203), Yugucheon (301206), Jicheon (301209), Seokseongcheon (301213), Geumcheon (301212), and and Gilsancheon (301402) are the local tributaries that flow into the Geum River. Here, national rivers Gilsancheon (301402) are the local tributaries that flow into the Geum River. Here, national rivers are are “rivers managed by the central government for important national conservation or the national “rivers managed by the central government for important national conservation or the national economy.” Local rivers are “rivers managed by metropolitan or local governments that are closely economy.” Local rivers are “rivers managed by metropolitan or local governments that are closely related to the public wealth of the province.” related to the public wealth of the province.” IIside (2.400, 2.600, and 1.710 m3 /s, respectively) values were used for the IRF of Gapcheon, side (2.400, 2.600, and 1.710 m3/s, respectively) values were used for the IRF of Gapcheon, Mihocheon, Mihocheon,and andNonsancheon, Nonsancheon,which whichare aretributaries tributariesthat thatflow flowinto intothe themain mainstream. stream.The Theremaining remaining tributary inflow, which was not identified in the IRF, used SDS (Figure 9). The main inflow tributary inflow, which was not identified in the IRF, used SDS (Figure 9). The main inflow of of 3 Daecheong Dam 1 (300805) was 16.600 m /s, which was the value specified in the dam water supply 3 Daecheong Dam 1 (300805) was 16.600 m /s, which was the value specified in the dam water supply contract. supply waswas highhigh in the and 301201 standard watersheds because because of sewage contract.The Theinflow inflow supply in 301002 the 301002 and 301201 standard watersheds of treatment plants. The RMF in the main stream of the Geum River was the highest. The RMF was sewage treatment plants. The RMF in the main stream of the Geum River was the highest. The RMF 41.516 m3 /smin3/sthe 301208 watershed, andand thethe RMF downstream was high was 41.516 in the 301208 watershed, RMF downstream was highoverall overallbecause becauseofofthe the high water demand downstream. However, because tributary inflow was high in 301401 high water demand downstream. However, because tributary inflow was high in 301401and and301214, 301214, the theRMFs RMFswere weresmall small(Table (Table2). 2).The TheRMF RMFduring duringthe thenon-irrigation non-irrigationperiod periodwas wasestimated estimatedexcluding excluding agricultural agriculturalwater water(Table (Table2). 2).
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Table 2. Calculation of RMF (m3 /s). Standard Watershed 301403 301401 301214 301211 301210 301208 301207 301205 301204 301201 301002 301001 300805
1
2 Use
Downstream (Ointake ) (RMF[i–i–1] )
19.900 29.364 37.071 40.208 40.363 40.187 41.516 40.068 39.711 39.205 29.303 22.523 20.047
10.447 10.229 4.483 0.530 0.677 1.920 0.000 0.319 0.380 0.000 0.909 0.197 0.197
3 Side
Inflow (Insider )
4 Main
Inflow (Imain )
5 IRF
(IRF[i–i+1] )
0.350 1.710 1.078 0.000 0.578 0.000 0.960 0.000 0.547 9.620 7.500 2.400 0.000
0.633 0.812 0.268 0.375 0.275 0.591 0.488 0.676 0.339 0.282 0.189 0.273 16.600
19.900 19.900 17.100 17.100 15.100 15.100 15.100 15.100 15.100 10.500 10.500 8.500 8.500
6 RMF = max( , 1 ) 5 + 2 − 3 − ) 4
(RMF[i–i+1] )
Irrigation
Non-Irrigation
29.364 37.071 40.208 40.363 40.187 41.516 40.068 39.711 39.205 29.303 22.523 20.047 3.644
26.678 24.456 26.350 26.378 26.888 27.898 25.940 23.723 23.927 16.582 13.456 11.039 8.920
The RMFs of other sites were estimated by transferring the deficit characteristics of the site at high risk. First, the flow observation site must coincide with the standard watershed outlet, such that the streamflow and the RMF can be compared. Second, the flow observation data should be sufficient for a SWD. Third, to conservatively cope with the SWD, a site with high water demand should be chosen. The RMF during the irrigation period in the 301208 standard watershed was the highest. The flow observation site and outlet of the standard watershed were coincident, but the flow data were affected by the operation of a multi-purpose weir. In addition, the outlet of the 301211 standard watershed coincided with the Gyuam observation site, and the RMF during the irrigation period was also high. Owing to tidal influences, the flow data contained considerable uncertainty. On the other hand, it was confirmed that Gongju coincided with the outlet of the 301204 standard watershed (Figure 9), and the RMF of the standard watershed was comparable to that of Gyuam. Therefore, Gongju was chosen as the representative site for the RMF transition. The RMFs at Gongju were 40 m3 /s and 22.927 m3 /s (Table 2), which were lower than the statistical monthly threshold Q70 recommended by Sung and Chung [13]. Their threshold level was derived as the 70th percentile of the FDC for each month. They recommended that the Q70 should consider the monthly or seasonal variation in streamflow in their study. Thus, the statistical threshold levels of Gongju, which are defined as the 70–99th percentile value of each month’s FDCs, were compared with the RMF. In general, the threshold levels defined the SWD conservatively as compared to the RMF. The RMF was larger than Q99 and smaller than Q95 . Furthermore, the threshold levels in January, February, and August were similar to Q90 (Figure 10). 3.2. Estimation of RMF Using the Drought Severity Index The RMF upstream and downstream of the reference site were estimated such that the characteristics of the reference site could be reproduced in a similar manner. To do this, the RMF during either the irrigation or non-irrigation period was fixed, and that during the other non-irrigation or the irrigation period was changed, which minimized the difference in the DSI of the reference site. The independence of the SWDs should be ensured to calculate the deficit characteristics. Tallaksen et al. [10] recommended a 10-day MA. The SWDs at Gongju from 2007–2009 were determined by taking the 10-day MA for SWDs from 2012–2016. For the last 2 years, the streamflow at Gongju was less than the RMF once and twice during the irrigation and non-irrigation periods, respectively. A review of the results of the DSI shows that the SWD from the end of December 2015 to the beginning of February 2016 was the most severe event (Table 3).
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100
Threshold level [m3/s]
monthly threshold (0.70~0.99, by=0.01) RMF 80
60
40
20
0 Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Month Figure 10. RMF and monthly threshold calculated by the FDC. Figure 10. RMF and monthly threshold calculated by the FDC.
Table Streamflow deficit events and severity indexes Gongju (2007–2009, 2012–2016). Table 3. 3. Streamflow deficit events and severity indexes at at Gongju (2007–2009, 2012–2016). Start Start
09-17-2015
EndEnd
09-26-2015
09-17-2015 09-26-2015 12-26-201502-10-2016 02-10-2016 12-26-2015 02-20-201602-24-2016 02-24-2016 02-20-2016 03-24-2016 03-24-201604-04-2016 04-04-2016
3 Volume (86,400 Duration (day) (day) Volume (86,400 m3 )m ) Duration
25.5
25.5 234.0 234.0 4.24.2 28.8 28.8
10
10 4747 55 1212
DSI DSI 0.00186 0.00186 0.08034 0.08034 0.00015 0.00015 0.00253 0.00253
Gongjuexperienced experiencedan anSWD SWD from from 17 17 to anan irrigation period. The Gongju to 26 26September September2015, 2015,which whichwas was irrigation period. 3/s to 50 m 3/s, and DSIs were 3 /s RMF of of Gyuam, changed from from40.0 40.0mm The RMF Gyuam,downstream downstreamofofGongju, Gongju, was was changed to 50 m3 /s, and DSIs were 3/s, there was no beginning or ending calculatedforfor these RMFs. We found that a rate 40–41 calculated these RMFs. We found that at at a rate of of 40–41 m3m /s, there was no beginning or ending 3 3 3 3 wasthe themost mostsimilar similartotothe theDSI DSIfor forGongju Gongju the SWD Gongju. The RMF from forfor the SWD at at Gongju. The RMF from 42 42 mm /s/stoto 4343 mm/s/swas (Table 4). It lasted 9 days, which was not very different from the 10 days for Gongju, and the deficit (Table 4). It lasted 9 days, which was not very different from the 10 days for Gongju, and the deficit 3/s or 43 3m3/s. 3 was adequate. We determined that the RMF during the irrigation period was 42 m was adequate. We determined that the RMF during the irrigation period was 42 m /s or 43 m /s. Table Streamflow deficit events and severity indexes Gyuam (2012–2016, irrigation period). Table 4. 4. Streamflow deficit events and severity indexes forfor Gyuam (2012–2016, irrigation period).
RMF Start Start RMF 09-21-2015 42 42 09-21-2015 43 43 09-21-2015 09-21-2015 44 44 09-21-2015 09-21-2015 45 09-21-2015 45 09-21-2015 46 09-21-2015 09-21-2015 47 46 09-20-2015 48 47 09-20-2015 09-20-2015 49 48 09-20-2015 09-20-2015 50 09-16-2015
3 End Volume (86,400 Duration (day) End (day) Volume (86,400 m3 )m ) Duration 09-29-2015 43.2 09-29-2015 43.2 9 9 09-29-2015 43.2 9 9 09-29-2015 43.2 09-29-2015 52.2 9 09-29-2015 52.2 9 09-29-2015 61.2 9 09-29-2015 61.2 09-29-2015 70.2 9 9 09-29-2015 70.2 09-29-2015 79.4 10 9 09-29-2015 89.4 10 10 09-29-2015 79.4 09-29-2015 99.4 10 10 09-29-2015 89.4 09-29-2015 131.6 14
DSI DSI 0.00234 0.00234 0.00234 0.00234 0.00282 0.00282 0.00331 0.00331 0.00380 0.00380 0.00477 0.00537 0.00477 0.00597 0.00537 0.01107
49 09-20-2015 09-29-2015 99.4 10 0.00597 50 09-16-2015 09-29-2015 131.6 14 0.01107 During the non-irrigation period, SWDs occurred from 26 December 2015 to 10 February 2016 (event During 1); fromthe 20–24 February 2012 (event 2); and from 24from March 4 April 2014 3) at Gongju. non-irrigation period, SWDs occurred 26 to December 2015(event to 10 February 2016 The RMF1);atfrom Gyuam during the 2012 non-irrigation period was determined based on (event the three at (event 20–24 February (event 2); and from 24 March to 4 April 2014 3) atDSIs Gongju. Gongju. The three Euclidean distances were assumed to be the same, and the Euclidean distances for The RMF at Gyuam during the non-irrigation period was determined based on the three DSIs at Gongju. The three Euclidean distances were assumed to be the same, and the Euclidean distances for the three DSIs were calculated for each RMF (Table 5). For calculation of the DSI, the irrigation RMF
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the three DSIs were calculated for each RMF (Table 5). For calculation of the DSI, the irrigation RMF was fixed at 43 m3 /s; the Euclidean distance of the DSI corresponding to 33 m3 /s was the shortest. The DSI, according to the change in the RMF at Gyuam, was compared to RMF at Gongju (Table 6). The RMFs at Gyuam were generally in the order of event 1, event 3, and event 2, but 34 m3 /s was slightly different. Comparing each event, it was found that the deficit of event 1, which was the highest at 33 m3 /s, was the most similar to that at Gongju, such that it would be appropriate as the non-irrigation RMF. The RMF at Bugang, upstream of Gongju, was estimated in the same manner as that at Gyuam. Compared to the SWD at Gongju during the irrigation period, the SWD because of RMFs of less than 20 m3 /s occurred 1 month later than that at Gongju. Therefore, it increased by 1 m3 /s from 20 m3 /s compared to the deficit at Gongju. As the RMF increased, the SWD increased in severity as well. At an RMF of 20 m3 /s or greater, there were deficit events twice during September 2015. On the other hand, at 24 m3 /s, SWDs occurred once, but the RMF was too high to evaluate the deficit. Therefore, it was concluded that the RMF during the irrigation period was 20 m3 /s. The RMF at Bugang was determined based on the deficit events 1–3 at Gongju. The Euclidean distances of the three DSIs were calculated by fixing the irrigation RMF at 20 m3 /s and varying the non-irrigation RMF from 9 m3 /s to 1 m3 /s. During the same period as events 1–3 at Gongju, the RMF, which resulted in a lack of stream water, was 10–13 m3 /s, and the Euclidean distance was relatively short at 12 m3 /s (Table 7). Streamflow deficit events and their severity indexes for Gyuam (2012–2016, Table 5. non-irrigation period). RMF
Start
End
Volume (86,400 m3 )
Duration (day)
DSI
Euclidean Distance
02-11-2016 02-25-2016 04-03-2016
135.7 22.8 34.1
42 5 5
0.03423 0.00068 0.00102
0.04614
29
01-01-2016 02-21-2016 03-30-2016 01-01-2016 02-21-2016 03-25-2016
02-11-2016 02-25-2016 04-06-2016
177.7 27.8 48.2
42 5 13
0.04483 0.00083 0.00376
0.03554
30
01-01-2016 02-21-2016 03-23-2016
02-11-2016 02-25-2016 04-06-2016
219.7 32.8 65.5
42 5 15
0.05543 0.00099 0.00590
0.02515
31
01-01-2016 02-21-2016 03-23-2016 01-01-2016 02-21-2016 03-23-2016 02-21-2016 03-23-2016 12-10-2016
02-11-2016 02-25-2016 04-06-2016 02-11-2016 02-25-2016 04-06-2016 02-25-2016 04-12-2016 12-14-2016
261.7 37.8 80.5 303.7 42.8 95.5 47.8 68.6 17.2
42 5 15 42 5 15 5 21 5
0.06603 0.00114 0.00725 0.07662 0.00129 0.00860 0.00144 0.00866 0.00052
0.01510
32
33
34
0.00721
0.07938
Table 6. Streamflow deficit events and their severity indexes for Gongju (2012–2016, irrigation period). Start 09-15-2015 09-04-2015 09-14-2015
End 09-28-2015 09-11-2015 09-30-2015
Volume (86,400 m3 ) 13.7 9.8 30.2
Duration (day) 14 8 17
DSI 0.00256 0.00102 0.00669
22
09-04-2015 09-14-2015
09-11-2015 09-30-2015
25.8 47.2
8 17
0.00263 0.01022
23
09-04-2015 09-14-2015
09-11-2015 09-30-2015
25.8 64.2
8 17
0.00257 0.01358
24
09-04-2015
09-30-2015
107.6
27
0.03537
RMF 20 21
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Table 7. Streamflow deficit and their severity indexes for Bugang (2012–2016, Table 7. Streamflow deficit events andevents their severity indexes for Bugang (2012–2016, non-irrigation period). non-irrigation period). DSI Euclidean Distance RMF Start End Volume (86,400 m3) Duration (day) RMF Start End Duration DSI Euclidean Volume 01-12-2016 01-16-2016 3.2(86,400 m3 ) 5 (day) 0.00026 0.08011Distance 10 02-06-2016 02-11-2016 4.2 6 0.00040 01-12-2016 01-16-2016 3.2 5 0.00026 0.08011 02-06-2016 03-30-2016 02-11-2016 0.00040 10 03-25-2016 2.44.2 66 0.00023 03-25-2016 10-26-2015 03-30-2016 0.00023 10-16-2015 7.12.4 116 0.00121 0.07899 10-16-2015 01-17-2016 10-26-2015 0.00121 0.07899 01-09-2016 9.97.1 911 0.00138 11 01-09-2016 02-11-2016 01-17-2016 9.9 9 0.00138 01-31-2016 13.8 12 0.00255 11 01-31-2016 02-11-2016 13.8 12 0.00255 03-23-2016 04-05-2016 13.7 14 0.00296 03-23-2016 04-05-2016 13.7 14 0.00296 10-16-2015 18.1 1111 0.00296 0.051829 10-16-2015 10-26-2015 10-26-2015 18.1 0.00296 0.051829 01-06-2016 53.6 3737 0.02952 01-06-2016 02-11-2016 02-11-2016 53.6 0.02952 12 12 02-21-2016 02-26-2016 02-26-2016 12.3 0.00110 02-21-2016 12.3 66 0.00110 03-15-2016 04-06-2016 04-06-2016 37.1 0.01269 03-15-2016 37.1 2323 0.01269 10-15-2015 10-26-2015 29.8 12 0.00513 0.18401 10-15-2015 10-26-2015 29.8 12 0.00513 0.18401 12-26-2015 03-04-2016 104.1 70 0.10449 13 13 12-26-2015 104.1 7024 0.10449 03-14-2016 03-04-2016 04-06-2016 60.7 0.02089 03-14-2016 04-06-2016 60.7 24 0.02089 10-13-2015 11-06-2015 45.5 25 0.01575 14 10-13-2015 45.5 25 0.01575 12-26-2015 11-06-2015 04-11-2016 192.9 108 0.28819 14 12-26-2015 04-11-2016 192.9 108 0.28819
The based on on the the RMF RMF at atGongju, Gongju,isisshown shown TheSWD, SWD,compared comparedto tothe theRMF, RMF,at at Gyuam Gyuam and and Bugang, Bugang, based ininFigure 11. The red and orange boxes in Figure 11 indicate the occurrence of SWDs. It was confirmed Figure 11. The red and orange boxes in Figure 11 indicate the occurrence of SWDs. It was confirmed that at Gongju. Gongju. As Asaaresult, result,the thestart start thatSWDs SWDsoccurred occurredat atGyuam Gyuam and and Bugang Bugang when when an an SWD SWD occurred occurred at and thethe deficits were similar at Gyuam and and Bugang. Thus, the estimation of an of RMF andend endofof deficits were similar at Gyuam Bugang. Thus, the estimation an that RMFresults that inresults similar SWD upstream and downstream in a river may lead to the implementation of fairofwater in similar SWD upstream and downstream in a river may lead to the implementation fair restrictions by the SWCC. water restrictions by the SWCC. 2012 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
2013 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
2014 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
2015 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
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Feb
Mar
Apr
May
Jun
Jul
(a)
Figure 11. Cont.
Aug
Sep
Oct
Nov
Dec
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2013 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
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(b) 2012 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
2013 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
2014 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
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(c) Figure11. 11.Deficits DeficitsatatGongju, Gongju,Gyuam, Gyuam,and andBugang. Bugang. (a) (a) Gongju; Gongju; (b) (b) Gyuam; (c) Bugang. Figure
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4. Conclusions South Korea has been responding to SWDs using a RMF based on the River Law. When a SWD is expected, the SWCC restricts the SWU regardless of location (e.g., upstream or downstream). Through this study, we revisited South Korea’s response to SWDs. The SWCC should manage the RMF so that the upstream and downstream users face similar risks. First, because SWU in South Korea has been managed by standard watershed units, the RMF has also been estimated based on standard watershed units. Second, it is necessary to ensure consensus and reliability regarding the restriction of SWU because of SWDs through balanced management of rivers. To estimate the RMF based on standard watershed units, considering the SWD as a result of a survey of SWU, we found that the demand for agricultural water was dominant in the Geum River Basin and agricultural water was mostly extracted from the downstream section of the river. Results of the estimation of the RMF considering SWU and the IRF showed that the RMF during the irrigation period was the highest in the 301208 standard watershed, which had a higher RMF than the 301204 standard watershed even during the non-irrigation period. Sung and Chung [13] suggested that Q70 –Q95 was the appropriate threshold level for streamflow drought, but we found that the threshold for the SWD at Gongju was less than Q90 . Therefore, we could conclude that even if streamflow is less than normal, it does not necessarily lead to an SWD; SWDs occur only after a streamflow drought becomes extreme. Therefore, Q70 , as proposed by Sung and Chung [13], can be utilized for the purpose of monitoring movement from stream water drought to deficit. RMFs were estimated such that the characteristics of the SWDs upstream and downstream were similar. Applying the RMF at Gongju to the upper and lower sites, we found the RMF of Bugang and Gyuam, which minimized the difference in Gongju’s DSI. The estimated RMF simulated the main characteristics of the SWD, such as the deficit and duration upstream and downstream, which were similar to those at Gongju. In South Korea, a serious drought occurred from 2015 to 2016 and stream water users suffered from restrictions on the use of stream water based on the SWD. According to the results of the application of the proposed methodology, the characteristics of the SWD were similar upstream and downstream. It was determined that balanced river management is possible. This study, thus, (i) suggested a threshold level considering water demand, (ii) compared the threshold level to a threshold related to anomalies, and (iii) suggested the utilization of the described method. However, for calculating RMF, we used SDS (10-year frequency of the streamflow), in which the streamflow is a fixed flow that is maintained for more than 355 days per year. This method has a limitation in that the seasonal and monthly variations cannot be considered. Tallaksen et al. [10] and Sung and Chung [13] suggested that FDC should be calculated monthly or seasonally to consider the monthly variation. With the use of monthly or seasonal FDCs, the SDS during summer increased to a value greater than the fixed SDS, such that the available stream water for use also increased. This led to reasonable permissions for SWU. The SDS, considering the variation in streamflow, leads to changes in the main river and tributary inflow and allows a reasonable RMF to be calculated. Author Contributions: J.H.S. conducted the study and made the first draft of the article. S.B.S. supervised the study and helped in preparation of the first draft of the article. Funding: This research was funded by the National Research Foundation of Korea (grant number is NRF-2017R1A6A3A11031800). Conflicts of Interest: The authors declare no conflicts of interest.
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Abbreviations DSI FDC IRF MA RMF SDS SWCC SWD SWU
Drought Severity Index Flow Duration Curve Instream Requirement Flow Moving Average River Management Flow Standard Drought Streamflow Stream Water Coordination Council Stream Water Deficit Stream Water Use
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