Sudy on he quaniaive feasibiliy of rainwaer harvesing in small islands Jaehong Ki 1, Mooyoung Han 2 1 Seoul Naional Universiy, Korea, marchsixh@naver.com 2 Seoul Naional Universiy, Korea, myhan@snu.ac.kr Absrac Many islands in Korea have problems relaed o waer source securiy and supply. Especially, waer supply condiion is worse in small islands remoe from inland. Couples of alernaives are developed and suggesed o supply waer o islands including waer hauling, groundwaer exracion, and desalinaion. However, hese alernaives require high energy, cos and consideraion in insallaion and operaion processes. Rainwaer harvesing is susainable opion ha supplies waer wih low energy and cos. Lack of quaniaive sudy in rainwaer harvesing sysem hinders he promoion of he sysem. herefore, his research sudied quaniaive feasibiliy of rainwaer harvesing as a waer supply opion in islands. Operaion of rainwaer harvesing sysem was esimaed wih compuer simulaion and he mehod o deermine sysem design is suggesed based on he simulaion resul. he mehod applied o condiion of exising island and required capaciy of he sysem was found o be in accepable exen. ha rainwaer harvesing is esimaed o be a feasible waer supply opion under Korean climae, which is unfavorable condiion for rainwaer harvesing, implies high poenial applicabiliy of rainwaer harvesing echnology for oher regions over he world suffering from waer shorage. 1. Inroducion Korea is he counry locaed in peninsula and surrounded by sea along hree sides of he erriory. Many islands are disribued along he counry s coasal lines, especially on wesern and souhern pars. he number of officially regisered islands is abou 3,200 and abou 500 islands among hem are inhabied island. oal populaion in hese islands reaches up o approximaely 800,000[1]. Mos islands suffer from he problem relaed o waer source securiy and waer supply. Only 38.2% of residens are served by waerworks ou of oal islands populaion, while he percenage of residens served by waerworks ou of oal Korean populaion is as high as 91.3%[2]. Mos islands where waerworks sysem is disribued are relaively huge islands carrying large number of populaion and conneced wih inland. For hese islands, waer source o be supplied is acquired from heir own reservoir or by ransporing surface waer from inland. However, for small islands which carry less han 100 inhabians, he number of which is more han 50% of oal number of inhabied islands, waer supply condiion is worse. Narrow area and precipious opography combined wih rocky geology induce runoff o
direcly flow ino sea surrounding island. his propery makes i impossible for small islands o sore surface waer and cause waer source securiy problem. Groundwaer is common source of waer in small islands where waerworks sysem is no disribued. However, his way of waer supply is resriced islands where aquifer is formed while here are many islands which do no have subsurface waer body. Furhermore, overexracion of groundwaer causes groundwaer level o drop and seawaer peneraion ino aquifer happens. When his occurs, saliniy of groundwaer ges high and such phenomenon is observed frequenly in islands[3]. he oher waer supply alernaives include freshwaer hauling from inland and roof runoff uilizaion, which is called rainwaer harvesing in oher word. Recenly, desalinaion gains ineres as one promising waer supply alernaive for easy seawaer source acquisiion. Alhough many waer supply alernaives for islands exis and are developed, high cos and energy required for he consrucion and operaion hinders he applicaion of such echnologies. Small populaion in islands decreases he economic efficiency of waer supply faciliies and residens can no afford o pay for produced waer. herefore, i is highly recommended o promoe waer supply sysem ha requires low cos and energy. Rainwaer is radiionally he mos imporan waer source in small islands where access o oher sources is limied. o dae, many small islands sill relies much on i o supply domesic waer. his waer source is cheap and simple o uilize so ha inhabians insalled and are operaing rainwaer harvesing sysems by hemselves. he sysem collecs roof runoff and supplies i aferward. hese days, rainwaer harvesing echnology is highly ineresed, as a low energy waer supply alernaive, in regions no only where access o waer is limied bu also where waer supply sysem is esablished. And he echnology is suiable for small islands waer supply sysem because i has advanages in small-scale applicaion, waer source acquisiion, and low requiremen of energy and cos. However, lack of inensive researches on he promoion of rainwaer harvesing sysems make he sysem remains in domesic-level applicaion. Especially, waer deficiency in dry season caused by no enough sysem capaciy is suggesed o be he mos imporan problem in rainwaer harvesing. herefore, his sudy focuses on he quaniaive feasibiliy of rainwaer harvesing as a waer supply alernaive in small islands. o deermine under wha condiion rainwaer can saisfy waer demand of island is he firs issue of his sudy. And esimae if he condiion found is feasible o be realized in small islands is he second issue. o achieve goals design crieria of rainwaer harvesing faciliy o mee he waer demand will be firsly esimaed. Laer, he pracical applicaion of rainwaer harvesing sysem will be simulaed under environmens of exising island in Korea, Guja-do. 2. Maerial and Mehod
2.1. Quaniaive analysis of rainwaer harvesing sysem using compuaional simulaion Compuaional simulaion is adoped for quaniaive analysis of rainwaer harvesing sysem. Figure 1 describes componens of rainwaer harvesing sysem used for he simulaion. Jenkins e al. (1987) suggesed wo operaing algorihms for rainwaer harvesing sysem performance esimaion. One algorihm is Yield Afer Spillage(YAS) and he oher is Yield Before Spillage(YBS). I is found ha adoping YAS operaing algorihm give a conservaive esimae of sysem performance[4]. And esimaion in his sudy was conduced using YAS algorihm. By he use of his algorihm, rainwaer harvesing sysem operaion is simulaed according o equaions shown below. Y V = min( D, V 1 ) = min( V 1 + Q Y, S Y ) Where, Y : Yield from sorage ank (m 3 ) a during h ime inerval D : Rainwaer demand (m 3 ) during h ime inerval ( D = Y + M ) V : Rainwaer volume (m 3 ) in sorage during h ime inerval 1 V : Rainwaer volume (m 3 ) in sorage during -1 h ime inerval Q : Rainwaer runoff (m 3 ) during h ime inerval ( Q S : Sorage ank volume (m 3 ) during h ime inerval = R A ) Figure 1 Rainwaer harvesing sysem configuraion(feweks, 1999). A: Cachmen area(m 2 ), R : Rainfall(mm) during h ime inerval, O : Overflow(m 3 ) during h ime inerval, M : Municipal waer supply(m 3 ) during h ime inerval(occur when yield is smaller han demand) Simulaion calculaes rainwaer yield(supply) and remainder in ank for each consecuive ime inervals regarding inflow ino he sysem and domesic waer demand. In his sudy, ime
inerval is se o one day and daily precipiaion daa of Seoul in 2008 is seleced as he represenaive precipiaion daa of Korea. his daa shows ha annual rainfall deph is 1356mm and variance of daily precipiaion is 201. 9mm 2. Domesic daily waer demand was calculaed from muliplying he number of populaion o Lier Per Capia Day (LPCD) and LPCD in his simulaion was se as 271L, which is average LPCD in Korea[2]. 2.2. Applicaion o he condiion of exising island, Gujado Island ha will be he sie for he virual applicaion of rainwaer harvesing sysem is Guja-do, which is locaed in souh-wes par of Korea. his island has area abou 0.7ha and around 80 inhabians reside in 11 households. Mos residens are working on fishing and aquaculure for living. Every household in his island relies mainly on rainwaer harvesing sysem for waer supply and freshwaer is hauled when supply is insufficien. Average LPCD in his island was esimaed o be 200L. In simulaion, daily precipiaion daa of 2008 in his region is adoped: annual rainfall is 1302.5mm and variance of daily precipiaion is 164.11 mm 2. 3. Resul and Discussion 3.1. Quaniaive analysis of rainwaer harvesing 3.1.1. Analysis of variables influencing rainwaer harvesing I is known ha here are several variables influencing operaion of rainwaer harvesing including rainfall inensiy and paern, cachmen area, runoff efficiency, loss raio, waer demand, capaciy of rainwaer sorage[4,6]. Among hese variables, rainfall daa and waer demand are se as described in secion 2.1. Runoff efficiency was assumed as 0.81 combining runoff efficiency of 0.9 and sysem efficiency of 0.9. Researches sudied he operaion of rainwaer harvesing sysem used generalized values of waer demand and sorage capaciy be dividing wih cachmen area[4,6]. his sudy also use generalized values in esimaion: waer demand(m 3 ) is divided by cachmen area(m 2 ) and parameerized as D/A(m 3 /m 2 ); sorage capaciy(m 3 ) is divided by cachmen area(m 2 ) and parameerized as S/A(m 3 /m 2 ). Generalized parameers have advanage in ha hey can represen rainwaer harvesing sysem performance regardless of scale. 3.1.2. Seing rainwaer supply esimaion parameer Many parameers are suggesed o esimae rainwaer harvesing performances including rainwaer uilizaion raio, cycle number, supply days, waer saving efficiency[6,7]. Among
hem, Waer Saving Efficiency(WSE) suggesed by Dixon e al. (1999) is he parameer ha indicaes for how much porion of domesic demand rainwaer can be supplied and is expressed as equaion below. E = 1 = = 1 Y D Where, : he las ime inerval Knowing from is mahemaical expression, his parameer is he value sum of rainwaer supply for cerain period divided by sum of waer demand for he same period and clearly describes if rainwaer can fully saisfy demand or no. he value of WSE lies beween 0 and 1 and he WSE value of 1 is derived when rainwaer can fully saisfy demand. However, in simulaion using daily precipiaion daa, no rainfall even of firs day case make i impossible for WSE o have he value of 1 even if rainfall sufficien o cover waer demand is followed. o avoid his deficiency, modified WSE is suggesed and used in his sudy. Modified WSE is defined by adding remained sorage volume of he las ime inerval a he numeraor of upper equaion and described by following equaion. E, mod = 1 = Y + V = 1 D 3.1.3. Esimaion of quaniaive feasibiliy of rainwaer supply Figure 2 describes modified WSE calculaed by simulaion for ranges of D/A and S/A values. Verical axis is se in logarihmic scale for clearer expression. he value of modified WSE should exceed 1 o achieve full saisfacion of waer demand by rainwaer supply. he curve crosses he plane marked 1 deermines design crieria of rainwaer harvesing sysem. Rainwaer harvesing sysem designed according o D/A and S/A values lying under he curve can supply rainwaer o fulfill demand. Figure 2 shows ha he value of modified WSE increases wih increasing value of S/A and decreasing value of D/A. In some cases wih large value of S/A and small value of D/A, he value of modified WSE is even larger han 10, which indicae rainwaer harvesing sysem can supply or sore rainwaer o he amoun ha can supply 10-year demand by jus collecing 1-year rainfall. However, his is he case of very inefficien insallaion and operaion of
rainwaer harvesing sysem and i is recommended o selec design parameers around he 1- marked curve. From figure 2, various design of rainwaer harvesing sysem ha saisfy waer supply can be made. Le us assume ha values of S/A and D/A are seleced as 0.05 and 0.0007(coordinae marked wih he circle on lef side), respecively. Regarding LPCD of 0.271m 3 (271L), i is calculaed ha rainwaer harvesing sysem wih cachmen area of 387m 2 ( 0.271(m 3 ) / 0.0007(m 3 /m 2 ) ) and sorage ank volume of 19.35m 2 ( 387(m 2 ) 0.05(m 3 /m 2 ) ) saisfies waer demand for one person. Oherwise, when S/A and D/A are seleced o have values of 0.45 and 0.002, respecively, 136m 2 of cachmen area 61.2m 2 of sorage ank can comprise rainwaer harvesing sysem o mee one person s demand. he prior case is sui under he condiion where insallaion of large cachmen area is possible and space for sorage ank is no sufficien. If he acquisiion of large cachmen is difficul and large space for sorage ank insallaion is available, he laer case is preferable. In his manner, one can decide which design crieria o choose under he consideraion of environmenal condiion for he sie ha rainwaer harvesing sysem is supposed o be insalled. Figure 2 (Modified) Waer saving efficiency for daily rainfall daa of Seoul. 3.2. Feasibiliy esimae simulaion hrough he applicaion o he exising island 3.2.1. Descripion of curren waer supply condiion of island
7his chaper aims o sugges how waer supply sysem can be esablished by supplying rainwaer. Guja-do, which is he demonsraion sie, currenly own rainwaer harvesing sysem in household level. Survey revealed ha he number of residens for each household averagely is 7 and exising domesic rainwaer harvesing sysem consiss of 200m 2 of cachmen area and 50m 3 of sorage ank in average. Rainwaer supply simulaion wih hese condiion wih LPCD of 0.2m 3 (200L) revealed ha exising sysem can supply abou 211m 3 of rainwaer annually, which is abou 40% of annual household waer demand. 3.2.1. Sraegy for rainwaer harvesing sysem esablishmen in island Addiional rainwaer harvesing sysem is required o make up anoher 60% of waer demand ha exising supply sysem can no cover. Annual waer deficiency of his island is calculaed o 3300m 3. Possible design scenarios of rainwaer harvesing sysem o saisfy island s demand, derived by he mehod inroduced in chaper 3.1, are shown in able 1. Designs of rainwaer harvesing sysem shown in able 1 require large area or space and searching for he suiable place o insall he sysem is needed. able. 1 Required cachmen area and rainwaer sorage ank volume A (m 2 ) 18,082 9,041 4,521 3,616 S (m 3 ) 362 452 1,130 1,447 Every small islands, including Guja-do, have wharf srucure for ship s docking. his srucure normally is huge and has wide surface area wih empy space inside i. Such condiion implies ha his srucure can be used for he sie o insall rainwaer harvesing sysem; using surface area as cachmen and fill colleced rainwaer inside he srucure. Wharf srucure in Guja-do has abou 4200m 2 of surface area wih heigh of abou 3m. Rainwaer harvesing sysem designed in accordance wih he las condiion (shaded) in able 1 is possibly insalled on he wharf srucure and will successfully supply rainwaer o solve he waer shorage problem in he island. Furhermore, insallaion of elevaed arificial cachmen and sorage ank design o increase hydraulic reenion ime can helpful in qualiaive managemen of he sysem. 4. Conclusion
Many islands in Korea are suffering from waer shorage. Islands small and remoe far from inland, especially, waer source securiy and waer supply condiion is worse. Freshwaer hauling or desalinaion is possible waer supply alernaives for hese regions, bu high requiremen for energy and cos of hese alernaives does maer. Rainwaer harvesing, which requires low energy and cos wih high suiabiliy in small-scale applicaion, is needed o be promoed for he primary waer supply alernaive. Rainwaer harvesing originally was one of he radiional waer supply alernaive in mos islands. However, lack of inensive research on quaniaive aspec of rainwaer harvesing le curren sysems remain sill in experimenally designed household level, which is no only inefficienly designed bu also quaniaively insufficien. his sudy suggess how o deermine opimized design crieria of rainwaer harvesing sysem for he coverage of demand. And applicaion of he mehod o exising island s condiions show ha addiional insallaion of rainwaer harvesing sysem in accepable exen can saisfy island s demand. Relaively large sysem capaciy can be carried by island s wharf srucure. Rainfall paern of Korea is unfavorable condiion for effecive rainwaer harvesing. In fac, rainfall variance of Korea scores one of he world s highes value. Uneven disribuion of rainfall paern - concenraed in summer season require larger capaciy of rainwaer harvesing sysem o supply waer for dry winer and spring. ha rainwaer harvesing sysem is feasible o be applied as waer supply alernaive under climae condiion of Korea implies ha he sysem can show more effecive performance wih less capaciy in oher regions where rainfall is more evenly disribued hrough he year. herefore, rainwaer harvesing can be a feasible opion of waer supply for many islands in he world suffering from waer shorage. Also he mehod suggesed in his sudy is expeced o offer a framework for rainwaer harvesing sysem design where he need for rainwaer supply exiss. Acknowledgemen his research was suppored by a gran from Gyeonggi Sea Gran Program funded by he Minisry of Land, ranspor and Mariime Affairs of Korean governmen. References
[1] Korea Research Insiue for Local Adminisraion (2008) Building daa base(db) for islands. [2] Minisry of Environmen (2007) 2006 Saisics of waerworks. [3] Y. Kim, K. Lee, D. Koh, D. Lee, S. Lee, W. Park, G. Koh, N. Woo (2003) Hydrogeochemical and isoopic evidence of groundwaer salinizaion in a coasal aquifer: a case sudy in Jeju volcanic island, Korea, Journal of Hydrology, 270, pp. 282-294. [4] A. Feweks (1999) Modelling he performance of rainwaer collecion sysems: owards a generalised approach, Urban waer, 1, pp. 323-333. [5] D. Jenkins, F. Pearson, E. Moore, J. K. Sun, R. Valenine (1987) Feasibiliy of rainwaer collecion sysems in California, Conribuion No. 173, California Waer Resources Cenre, Universiy of California. [6] M. Han, M. Han, S. Kim (2004) A consideraion in deermining he ank size of rainwaer harvesing sysem in buildings, Journal of Korean Sociey of Waer and Wasewaer, 28(2), pp. 99-109. [7] A. Dixon, D. Buler, A. Feweks (1999) Compuer simulaion of domesic waer reuse sysems: invesigaing greywaer and rainwaer in combinaion. Waer Science and echnology, 38(4), pp. 25-32.