Nonparametric Monte Carlo Simulation for Flood Frequency Curve Derivation: An Application to a Korean Watershed1

Abstract: A mix of causative mechanisms may be responsible for flood at a site. Floods may be caused because of extreme rainfall or rain on other rainfall events. The statistical attributes of these events differ according to the watershed characteristics and the causes. Traditional methods of flood frequency analysis are only adequate for specific situations. Also, to address the uncertainty of flood frequency estimates for hydraulic structures, a series of probabilistic analyses of rainfall‐runoff and flow routing models, and their associated inputs, are used. This is a complex problem in that the probability distributions of multiple independent and derived random variables need to be estimated to evaluate the probability of floods. Therefore, the objectives of this study were to develop a flood frequency curve derivation method driven by multiple random variables and to develop a tool that can consider the uncertainties of design floods. This study focuses on developing a flood frequency curve based on nonparametric statistical methods for the estimation of probabilities of rare floods that are more appropriate in Korea. To derive the frequency curve, rainfall generation using the nonparametric kernel density estimation approach is proposed. Many flood events are simulated by nonparametric Monte Carlo simulations coupled with the center Latin hypercube sampling method to estimate the associated uncertainty. This study applies the methods described to a Korean watershed. The results provide higher physical appropriateness and reasonable estimates of design flood.     

Methane Ice Worms: Hesiocaeca methanicola Colonizing Fossil Fuel Reserves

 During a research cruise in July 1997 in the Gulf of Mexico we discovered a gas hydrate approximately 1 m thick and over 2 m in diameter which had recently breached the sea floor at a depth of 540 m. The hydrate surface visible from the submarine was considerably greater than that of any other reported hydrate. Two distinct color bands of hydrate were present in the same mound, and the entire exposed surface of the hydrate was infested (2500 individuals/m²) with 2 to 4 cm-long worms, since described as a new species, Hesiocaeca methanicola, in the polychaete family Hesionidae (Desbruyères and Toulmond 1998). H. methanicola tissue stable isotope values are consistent with a chemoautotrophic food source. No evidence of chemoautotrophic symbionts was detected, but geochemical data support the presence of abundant free living bacteria on the hydrate. The activities of the polychaetes, grazing on the hydrate bacteria and supplying oxygen to their habitats, appears to contribute to the dissolution of hydrates in surface sediments. Received: 16 August 1999 / Accepted in revised form: 19 January 2000     

WATER MANAGEMENT AND N,P LOSSES FROM PADDY FIELDS IN SOUTHERN KOREA1

ABSTRACT: Assessment and control of nutrient losses from paddy fields is important to protect water quality of lakes and streams in Korea. A four‐year field study was carried out to investigate water management practices and losses of nitrogen (N) and phosphorus (P) in rice paddy irrigation fields in southern Korea. The amount and water quality of rainfall, irrigation, surface drainage, and infiltration were measured and analyzed to estimate inputs and losses of N and P. The observed irrigation amount surpassed consumptive use, and approximately 52 to 69 percent of inflow (precipitation plus irrigation) was lost to surface drainage. Field data showed that significant amounts of irrigation water and rainfall were not effectively used for rice paddy culture. Water quality data indicated that drainage from paddy fields could degrade the recipient water environment. The nutrient balance indicated that significant amounts of nutrients (29.5 percent of total N and 8.6 percent of total P compared to input) were lost through surface drainage. Furthermore, up to half the nutrient losses occurred during nonstorm periods. The study results indicate that inadequate water management influences N and P losses during both storm and nonstorm periods. Proper water management is required to reduce nutrient losses through surface drainage from paddy fields; this includes such measures as minimum irrigation, effective use of rainfall, adoption of proper drainage outlet structures, and minimized forced surface drainage.     

Microhabitat-associated variability in survival and growth of subtidal solitary ascidians during the first 21 days after settlement

Newly settled juvenile ascidians were transplanted into shaded and unshaded sites at two subtidal depths to test the hypothesis that photonegative settlement behavior enhances juvenile survival. Silt, filamentous algae and grazing gastropods (Margarites pupillus) were identified as sources of mortality associated primarily with non-cryptic microhabitats. Silt inflicted heavy mortality on all six species tested. Algal overgrowth was important in shallow water, where it increased mortality and depressed growth rates over a 14-d period. Laboratory experiments demonstrated that gastropods can remove juvenile ascidians from the substratum. The sources of post-settlement mortality investigated may help determine distributional patterns of ascidians in the San Juan Islands, Washington, USA, and may also represent selective pressures that maintain negative phototaxis in the behavioral repertoires of ascidian tadpoles.     

Kinetics of nitrosation of the herbicide glyphosate.

The herbicide glyphosate was nitrosated by third-order kinetics to N-nitrosoglyphosate. The nitrosation at 25 degrees C was maximum at the reaction pH of 2.5 and had a pH-dependent rate constant of 2.43 M-2 sec-1. An activation energy of 9.5 kcal mole-1 also suggested that glyphosate is nitrosated very readily. Thiocyanate increased the rate 4.6 fold. The possibility of using these results to predict the formation of N-nitrosoglyphosate under normal agricultural practice is discussed.     

Asbestos exposure among Seoul metropolitan subway workers during renovation of subway air-conditioning systems

Air quality in underground spaces has seldom been evaluated in Korea. Accordingly, the current study recently evaluated asbestos exposure among Seoul metropolitan subway workers during the renovation of the subway's air-conditioning system. To identify possible routes of asbestos exposure, suspected sources, including gaskets, ceiling boards, ceiling materials, and dust settled inside ducts, were all sampled. Personal air samples were also taken to evaluate any asbestos exposure during the renovation. The asbestos fibers found in the samples were analyzed using a transmission electron microscope (TEM) equipped with an energy dispersive X-ray analyzer (EDX). Twelve out of eighteen bulk samples contained asbestos, the majority of which was chrysotile fibers. Asbestos was detected in 9 out of 72 personal air samples and the level ranged from 0.003 to 0.02 fibers/cm(3). While asbestos levels were below Korean occupational limit of 2 fibers/cm(3), they were still detectable and therefore further monitoring would be appropriate.