Kyoochul Ha

A method to improve the stability and accuracy of ANN- and SVM-based time series models for long-term groundwater level predictions

The prediction of long-term groundwater level fluctuations is necessary to effectively manage groundwater resources and to assess the effects of changes in rainfall patterns on groundwater resources. In the present study, a weighted error function approach was utilised to improve the performance of artificial neural network (ANN)- and support vector machine (SVM)-based recursive prediction models for the long-term prediction of groundwater levels in response to rainfall. The developed time series models were applied to groundwater level data from 5 groundwater-monitoring stations in South Korea. The results demonstrated that the weighted error function approach can improve the stability and accuracy of recursive prediction models, especially for ANN models. The comparison of the model performance showed that the recursive prediction performance of the SVM was superior to the performance of the ANN in this case study. ANN- and SVM-based long-term groundwater level prediction models were developed.We proposed a weighting factor approach for recursive prediction models.The method can enhance the performance of the long-term groundwater level prediction model.

A distributed water balance approach to groundwater recharge estimation for Jeju volcanic island, Korea

Groundwater is a major source of water use in Jeju volcanic island of Korea. Due to highly permeable surface conditions, the island does not have any perennial streams or rivers. Therefore, accurate estimation of groundwater recharge and understanding groundwater system are critical to make a reliable water budget in the island. A distributed quasi steady state water balance model, WetSpass (Water and Energy Transfer between Soil, Plants and Atmosphere under quasi Steady State) was adopted to estimate spatially-varying groundwater recharge considering soil type, land use, topography, and groundwater depth. The results from the study using weather data from October 2006 to September 2007 indicated that the eastern and northern parts of the island showed relatively higher rate of recharge as compared with the western region. A total of 69% of recharge occurred in the middle of the Island where topographic elevation is greater than 200 m from the mean sea level. The total amount of recharge was 1,935.6 million tons, 41.8% of the total precipitation. The higher rate of recharge was observed in the watersheds where forest and bare soil as land uses, and sandy loam and clay loam as soil types are dominant. The total runoff was 1,142.7 million tons (24.7%) and the total evapotranspiration was 1,555.7 million tons (33.5%). The zonal hydraulic conductivities estimated with groundwater flow modeling and parameter estimation process showed a wide range of values from 1.2 m/d to 5,380 m/d. The results of groundwater flow modeling indicated that the crest of groundwater table is off-centered from the peak of Mt. Halla toward south.

Assessing sustainability of groundwater resources on Jeju Island, South Korea, under climate change, drought, and increased usage

Numerical groundwater models were used to assess groundwater sustainability on Jeju Island, South Korea, for various climate and groundwater withdrawal scenarios. Sustainability criteria included groundwater-level elevation, spring flows, and salinity. The latter was studied for the eastern sector of the island where saltwater intrusion is significant. Model results suggest that there is a need to revise the current estimate of sustainable yield of 1.77u2009×u2009106 m3/day. At the maximum extraction of 84xa0 % of the sustainable yield, a 10-year drought scenario would decrease spring flows by 28 %, dry up 27 % of springs, and decrease hydraulic head by an island-wide average of 7 m. Head values are particularly sensitive to changes in recharge in the western parts of the island, due to the relatively low hydraulic conductivity of fractured volcanic aquifers and increased groundwater extraction for irrigation. Increases in salinity are highest under drought conditions around the current 2-m head contour line, with an estimated increase of up to 9 g/L under 100 % sustainable-yield use. The study lists recommendations towards improving the island’s management of potable groundwater resources. However, results should be treated with caution given the available data limitations and the simplifying assumptions of the numerical modeling approaches.ZusammenfassungNumerische Grundwassermodelle wurden verwendet, um Grundwassernachhaltigkeit für verschiedene Klima- und Grundwasserentnahmeszenarien auf der Insel Jeju, Südkorea, zu beurteilen. Nachhaltigkeitskriterien umfassen Grundwasser-Pegel-Messung, Quellfluss und Salzgehalt. Letzterer wurde für den Ostteil der Insel untersucht, wo das Eindringen von Salzwasser bedeutend ist. Modellergebnisse deuten darauf hin, dass es notwendig ist, die aktuelle Schätzung des nachhaltigen Ertrags von 1.77u2009×u2009106 m3/day zu überarbeiten. Bei der maximalen Entnahme von 84xa0% des nachhaltigen Ertrags würde sich beim zehnjährigen Dürreszenario der Quellfluss um 28xa0% verringern, 27xa0% der Quellen vertrocknen und die hydraulische Höhe der Insel im Durchschnitt um 7xa0m verringert werden. Hydraulische Höhewerte sind besonders anfällig auf Veränderungen der Grundwasserneubildung im westlichen Teil der Insel, aufgrund der relativ geringen hydraulischen Leitfähigkeit des gebrochenen vulkanischen Grundwasserleiters und der erhöhten Grundwasserentnahme zur Bewässerung. Erhöhter Salzgehalt ist am höchsten unter Dürre um die gegenwärtige 2-m hydraulische Höhe-Konturlinie, mit einem geschätzten Anstieg von bis zu 9xa0g/l bei 100xa0% Gebrauch des nachhaltigen Ertrags. Die Studie nennt Empfehlungen zur Verbesserung der Verwaltung des Trinkwasservorkommens der Insel. Allerdings sollten die Ergebnisse mit Vorsicht behandelt werden, angesichts der eingeschränkten verfügbaren Daten und der vereinfachenden Annahmen der numerischen Modellierungsansätze.ResuméDes modèles numériques ont été utilisés pour évaluer la durabilité des eaux souterraines sur l’île de Jeju, Corée du Sud, pour différents scénarios climatiques et de prélèvements des eaux souterraines. Les critères de durabilité comprennent l’altitude du niveau piézométrique, les débits des sources et la salinité. Cette dernière a été étudiée dans le secteur est de l’île où l’intrusion saline est forte. Les résultats des modèles suggèrent qu’il est nécessaire de réviser l’estimation actuelle du débit durable de 1.77u2009×u2009106 m3/j. Au maximum de prélèvement de 84xa0% du débit durable, un scénario de sécheresse de période de retour 10 ans réduirait le débit des sources de 28xa0%, assècherait 27xa0% des sources et diminuerait les niveaux piézométriques de 7xa0m en moyenne à l’échelle de l’île. Les niveaux piézométriques sont particulièrement sensibles aux variations de la recharge dans les parties ouest de l’île, en raison de la perméabilité relativement faible des aquifères volcaniques fracturés et de l’augmentation des prélèvements d’eau souterraine pour l’irrigation. Sous conditions de sécheresse, les augmentations de la salinité sont maximales aux alentours de la ligne isopièze 2xa0m, avec une hausse estimée atteignant 9xa0g/L pour l’usage de 100xa0% du débit durable. L’étude dresse la liste des recommandations destinées à améliorer la gestion des ressources en eau potable souterraines. Cependant, les résultats doivent être considérés avec prudence compte tenu des limites des données disponibles et des hypothèses simplificatrices des approches de modélisation numérique.ResumenSe usaron modelos numéricos de agua subterránea para evaluar la sustentabilidad del agua subterránea en la isla Jeju, Corea del Sur, para varios escenarios de clima y de extracción de agua subterránea. Los criterios de sustentabilidad incluyeron la elevación del nivel de agua subterránea, los flujos de manantiales, y la salinidad. Esta última se estudió para el sector este de la isla donde es significativa una intrusión de agua salada. Los resultados sugieren que hay una necesidad de revisar las estimaciones actuales de rendimiento sustentable de 1.77u2009×u2009106 m3/d. Con una extracción máxima del 84xa0% del rendimiento sustentable, un escenario de sequía de diez años reduciría los flujos hacia los manantiales en un 28xa0%, secándose hasta un 27xa0% de los manantiales, y una disminución de la carga hidráulica en toda la isla en un promedio de 7xa0m. Los valores de carga hidráulica son particularmente sensibles a los cambios en la recarga en las partes oeste de la isla, debido a la relativamente baja conductividad hidráulica de los acuíferos volcánicos fracturados y a un incremento de la extracción de agua para irrigación. Los incrementos en la salinidad son más altos bajo condiciones de sequía alrededor de la línea de carga hidráulica actual de 2xa0m, con un incremento estimado de hasta 9xa0g/L bajo un rendimiento sustentable de 100xa0%. El estudio lista las recomendaciones para una mejora del manejo de los recursos de agua subterránea potable de la isla. Sin embargo, los resultados deben ser tratados con precaución dadas las limitaciones en la disponibilidad de datos y los supuestos simplificados de las aproximaciones del modelado numérico.الملخصاستخدمت النماذج العددية لمحاكاة حركة الجريان الجوفي لتقييم استدامة المياه الجوفية في جزيرة جي جو بكوريا الجنوبية في ظل سيناريوهات مختلفة للمناخ ومعدلات سحب المياه الجوفية . شملت معايير الاستدامة على منسوب مستوى المياه الجوفية، وجريان الينابيع ودرجة الملوحة . وتم دراسة درجه الملوحة في القطاع الشرقي من الجزيرة حيث يتعاظم تداخل مياه البحر المالحة . أوضحت نتائج النمذجه الحاجة إلى تعديل القيمة الحالية لمعامل الانتاج المستمر (1.77u2009×u2009(106 × 1.77 m3/day). عند اقصى سحب بقيمة %84 من معامل الانتاج المستمر، سيناريو عشر سنوات جفاف سيقلل جريان الينابيع بنسبة %28 ويجفف 27xa0% من الينابيع ويقلل الضاغط المائي بقيمة متوسطة 7 أمتار بالجزيرة . بالإضافة الى ان قيم الضاغط المائي حساسة، على وجه الخصوص، للتغيرات في شحن الخزان الجوفي في الأجزاء الغربية من الجزيرة وذلك نتيجة للانخفاض النسبي لقيم معامل التوصيل الهيدروليكي للخزانات الجوفية الفولكانيه ذات التشققات وأيضا زياده سحب المياه الجوفية لغرض الري . وأوضحت النتائج أيضا أن زيادة الملوحة للمياه تصل لأعلى قيمة في ظل ظروف الجفاف عند خط الكنتور الحالي ذو القيمة 2 متر للضاغط المائي مصحوبا بزياده متوقعه إلى 9xa0g/Lعند استخدام معامل الانتاج المستمر بقيمة 100xa0% . ويجب الإشارة إلي توخى الحذر عند تطبيق ما انتهت اليه الدراسة من توصيات لتحسين إدارة الجزيرة لمصادر المياه الجوفية المستخدمة للشرب وذلك لمحدودية البيانات المتوفرة إلي جانب الافتراضات المبسطة التي اسس عليها النموذج العددي المستخدم في الدراسة .摘要数值地下水模型用于评价韩国Jeju岛各种气候和地下水抽取情况下的地下水的可持续性。可持续性标准包括地下水位海拔、泉流和盐度。对岛的东部进行了盐度的研究,因为那里的海水入侵很严重。模型结果显示需要修订目前可持续出水量为1.77u2009×u2009106 m3/day的估算值。在最大抽取量为可持续出水量的84%时,10年干旱方案泉流将降低28%,27%的泉将干涸,全岛范围内的水头将降低7米。水头值对岛西部补给变化特别敏感,因为断裂火山岩含水层导水系数相对较低及农业用地下水抽水量增加。在干旱条件下盐度在目前2 米水头等高线附近增加最高,在100%可持续出水量利用情况下,估计增加到每升9克。本研究列出了改进岛上饮用水管理的建议。然而,研究结果应该谨慎对待,因为现有的资料有限,数值模拟方法假定简化。초록대한민국 제주도에서 지하수 유동 모델링 기법을 이용하여, 다양한 기후변화 및 지하수 이용 시나리오에 따른 지하수 지속가능성을 평가하였다. 지속가능성을 평가하는 기준으로는 지하수위, 용천수량, 염도 등을 포함시켰다. 염도 기준은 지하수 내 염수침입이 빈번한 제주도 동부지역에 적용되었다. 모델링 결과, 현재의 지속가능 개발량인 1.77u2009×u2009106 m3/day가 수정될 필요가 있음을 알 수 있었다. 지속가능 개발량의 최대 84%의 지하수를 사용하고 10년 빈도 가뭄 시나리오를 적용했을 경우, 용천수량은 28%정도 줄어 들어, 전체의 27%가 말라버리고, 지하수위는 평균 7m정도 감소하는 것으로 평가되었다. 지하수위는 제주도 서부지역에서 지하수 함양량에 특히 민감하였는데, 이는 이 지역의 대수층이 상대적으로 수리전도도가 낮고 지하수가 농업용 관개용수로 많이 사용되기 때문이다. 염도는 가뭄시 지속가능 개발량의 100%의 지하수를 사용하였을 때 가장 높았으며, 현재 지하수위 2xa0m 지점에서는 9xa0g/L까지 증가하였다. 또한, 본 연구에서는 모델링 결과를 토대로 섬지역에서 가용한 지하수 자원을 효과적으로 관리하기 위한 대안들을 제시하였으나, 모델링 결과는 입력자료가 불충분하고 모델링에 필요한 간략화된 가정들 때문에 한계를 가지고 있어, 주의깊게 다루어 져야 한다.مقدمهجهت بررسی پایداری آبهای زیرزمینی جزیره جیجو، کره جنوبی، با توجه به سناریوهای برداشت آب زیرزمینی و اقلیم گوناگون، مدلهای عددی آب زیرزمینی به کار گرفته شدهاند. معیارهای پایداری، تراز سطح آب زیرزمینی، دبی چشمهها و شوری در نظر گرفته شده است. معیار شوری برای بخش شرقی جزیره که نفوذ آبهای شور قابل توجه است مورد مطالعه قرار گرفته است. نتایج مدل بیانگر نیاز به اصلاح برآورد حاضر از جریان آورد پایدار به میزان m3/day 107xa0×xa077.1 میباشد. با تخلیه بیشینه 84xa0% از جریان آورد پایدار، و در نظر گرفتن یک سناریوی خشکسالی ده ساله، 28xa0% از دبی جریان چشمهها کاهش خواهد یافت، 27xa0% چشمهها خشک شده، و هد هیدرولیکی به طور میانگین 7 متر در سطح جزیره کاهش مییابد. مقادیر هد به علت هدایت هیدرولیکی نسبتاً پایین آبخوانهای آتشفشانی گسسته و افزایش استخراج آب زیرزمینی برای کشاورزی، خصوصاً به تغییرات سطح تغذیه در بخشهای غربی جزیره، حساس میباشد. میزان سطح شوری تحت شرایط خشکسالی، پیرامون خط تراز هد 2 متر، با یک سطح افزایش تخمینی تا حدود g/L9 با 100xa0% استعمال از آورد پایدار، به بالاترین میزان خود میرسد. این تحقیق پیشنهادهایی را جهت بهبود مدیریت منابع آب زیرزمینی آشامیدنی جزیره ارائه میکند. هر چند که این نتایج باید با توجه به محدودیت دادههای موجود و فرضیات سادهسازی روشهای مدلسازی عددی، با احتیاط لازم مورد بحث قرار گیرند.ResumoForam utilizados modelos hidrogeológicos numéricos para avaliar a sustentabilidade das águas subterrâneas na Ilha de Jeju, Coreia do Sul, para vários cenários climáticos e de extração de água subterrânea. Os critérios de sustentabilidade incluíram os níveis de água subterrânea, a produção de nascentes e a salinidade. Es

Temporal and spatial variability of rainfall and climate trend on Jeju Island

The temporal and spatial analyses of rainfall and temperature were conducted for understanding spatial variability and climate trend on Jeju Island. Rainfall data from 1992 to 2009 were used for the spatial analyses, and missing data were adjusted on Mt. Halla and along the northeast coast to reduce uncertainty of spatial variability. In addition, rainfall time series data of both Jeju City and Seogwipo City from 1961 to 2009 were analyzed for a long-term trend and identification of droughts. Mean annual rainfall for the period 1992–2009 shows an area of maximum rainfall centered around Mt. Halla where maximum annual rainfall reaches more than 4,300 mm and mean island rainfall is 2,082 mm. Upward trends in rainfall intensity, magnitude, and dryness conditions at the Jeju City and Seogwipo City from 1961 to 2009 suggest that rainfall has intensified with greater quantities of rainfall occurring over shorter durations, with longer dry periods between storm events. The annual cycle shows a distinct monsoon signature with peak rainfall typically occurring in August. Rainfall seasonality shows a shift in peak rainfall from June, July, and August, to July, August, and September. The most severe droughts in the period 1961–2009 were identified using a 12-month composite SPI. Three severe droughts each lasting around two years were identified over a relatively short nine-year period from 1964–1972.

Simple equations for temperature simulations on mid-latitude volcanic islands: a case study from Jeju (Republic of Korea)

Volcanic islands can be characterized by remarkable surface air temperature variability. The distribution of weather stations in these settings, however, is typically too sparse to reliably describe temperature patterns which can complicate regional-scale hydrologic analyses. Here, a simple method is presented to estimate near surface air temperatures for such a setting (Jeju Island, Republic of Korea). The method utilizes temperature lapse rates (TLR; the change in temperature with elevation) which are computed for Tmin, Tave, and Tmax for two distinct hemispheres: the more arid northern flanks and the more humid southern flanks of the central orographic divide. Computed TLRs vary greatly with season and location relative to the orographic divide and, unlike in continental mountainous settings, are generally highest for Tmin in winter. Cross-validation results indicate a good match between modeled and measured values particularly for high altitude stations that are characterized by highest precipitation rates. Because temporally changing TLRs perform better than the often applied theoretical constant environmental lapse rate of 6.5 °C/km, monthly mean TLRs compiled from temperature data from Jeju Island are presented here as proxies for surface air temperature simulation efforts in similar settings for which only limited climatic data are available.

Groundwater response analysis to multiple earthquakes on Jeju volcanic island

Earthquakes have been known to induce a groundwater response. This study statistically analyzed the relationship between the changes in groundwater level (oscillation) and seismic waves, as well as the relationship between the changes in groundwater level and earthquakes of various magnitudes and epicenters more than 1,000 km distance from Jeju Island. The analysis showed that the groundwater level in Jeju Island is changed mostly by earthquakes larger than M = 7.0 within a 3,000 km from the epicenter. A positive relationship appeared between the earthquake magnitude and groundwater level change. A similar positive trend was observed between the maximum groundwater level and the seismic amplitude of the earthquakes. Geologically and spatially, the northern area of the island showed the highest response to earthquakes, the southern area showed an intermediate response, and the western and eastern areas showed the lowest response. The highest groundwater response in the northern area might be linked to its permeable structures and geological layers.