Jayantha Obeysekera

Revisiting the Concepts of Return Period and Risk for Nonstationary Hydrologic Extreme Events

Current practice using probabilistic methods applied for designing hydraulic structures generally assume that extreme events are stationary. However, many studies in the past decades have shown that hydrological records exhibit some type of nonstationarity such as trends and shifts. Human intervention in river basins (e.g., urbanization), the effect of low-frequency climatic variability (e.g., Pacific Decadal Oscillation), and climate change due to increased greenhouse gasses in the atmosphere have been suggested to be the leading causes of changes in the hydrologic cycle of river basins in addition to changes in the magnitude and frequency of extreme floods and extreme sea levels. To tackle nonstationarity in hydrologic extremes, several approaches have been proposed in the literature such as frequency analysis, in which the parameters of a given model vary in accordance with time. The aim of this paper is to show that some basic concepts and methods used in designing flood-related hydraulic structures assuming a stationary world can be extended into a nonstationary frame- work. In particular, the concepts of return period and risk are formulated by extending the geometric distribution to allow for changing exceeding probabilities over time. Building on previous developments suggested in the statistical and climate change literature, the writers present a simple and unified framework to estimate the return period and risk for nonstationary hydrologic events along with examples and applications so that it can be accessible to a broad audience in the field. The applications demonstrate that the return period and risk estimates for nonstationary situations can be quite different than those corresponding to stationary conditions. They also suggest that the nonstationary analysis can be helpful in making an appropriate assessment of the risk of a hydraulic structure during the planned project-life. DOI: 10.1061/ (ASCE)HE.1943-5584.0000820.

Impacts of the 2004 tsunami on groundwater resources in Sri Lanka

The 26 December 2004 tsunami caused widespread destruction and contamination of coastal aquifers across southern Asia. Seawater filled domestic open dug wells and also entered the aquifers via direct infiltration during the first flooding waves and later as ponded seawater infiltrated through the permeable sands that are typical of coastal aquifers. In Sri Lanka alone, it is estimated that over 40,000 drinking water wells were either destroyed or contaminated. From February through September 2005, a team of United States, Sri Lankan, and Danish water resource scientists and engineers surveyed the coastal groundwater resources of Sri Lanka to develop an understanding of the impacts of the tsunami and to provide recommendations for the future of coastal water resources in south Asia. In the tsunami-affected areas, seawater was found to have infiltrated and mixed with fresh groundwater lenses as indicated by the elevated groundwater salinity levels. Seawater infiltrated through the shallow vadose zone as well as entered aquifers directly through flooded open wells. Our preliminary transport analysis demonstrates that the intruded seawater has vertically mixed in the aquifers because of both forced and free convection. Widespread pumping of wells to remove seawater was effective in some areas, but overpumping has led to upconing of the saltwater interface and rising salinity. We estimate that groundwater recharge from several monsoon seasons will reduce salinity of many sandy Sri Lankan coastal aquifers. However, the continued sustainability of these small and fragile aquifers for potable water will be difficult because of the rapid growth of human activities that results in more intensive groundwater pumping and increased pollution. Long-term sustainability of coastal aquifers is also impacted by the decrease in sand replenishment of the beaches due to sand mining and erosion.

Lysimeter study of evapotranspiration of cattails and comparison of three estimation methods

A lysimeter was designed and installed in a cattail marsh as part of the Everglades Nutrient Removal project in South Florida (26° 38’ N, 80° 25’ W) to measure evapotranspiration (ET) of cattails (Typha domingensis). The fully automated lysimeter with a surface area of 9.8 m2 was located inside the marsh to measure cattail ET in a marsh environment with sufficient fetch and minimum oasis effect on the lysimeter. The average measured ET rate was 3.9 mm per day for the period of 12 February to 19 December 1993. The Penman-Monteith equation was applied to estimate daily ET using physically based resistance parameter estimates and high resolution weather data. Also, average albedo (0.17) was computed for the vegetation for possible future computation of net radiation from solar radiation data. The Penman combination model was applied with new wind function coefficients developed for the study site and the Priestley-Taylor model was applied with an estimated average coefficient (a) value of 1.18. Seven-day mean of measured and estimated data were compared. The Penman-Monteith method had the least error of estimation of 0.39 mm day–1 with an r2 value of 0.89 while the Penman combination equation had an error of estimation of 0.57 mm day–1 with r2 value of 0.86 and a least intercept of 0.03 mm. The Priestley-Taylor method with an a value of 1.18 had standard error of estimate 0.53 mm day–1 with an r2 value of 0.79. The Priestley-Taylor method has the potential to estimate ET in south Florida when climatic data is limited. More data is needed to evaluate each equation to estimate ET of wetland features in humid areas such as south Florida.

The natural South Florida system I: Climate, geology, and hydrology

Developing hypotheses for sustainability requires an understanding of the natural forces that shaped the historical Everglades prior to extensive engineering of the landscape. The historical Everglades marsh covered 10,000 km2 in a 100-km-long basin that has an extremely low gradient (slope of only 3 cm · km-1). The region is characterized by a heterogeneous landscape that has developed over the past five millennia, functioning as an interconnected mosaic of wetland, upland, estuarine, and marine ecosystems. The boundaries of this system were defined as the historic drainage basin from the Kissimmee River system through Lake Okeechobee, the Everglades, Florida Bay, and out through the Florida Keys to the coral reef tract. This geographic area is interconnected through the regional hydrology, with its unifying surface and subsurface freshwater transport system. However, in the final analysis, the interaction of geologic and climatic processes determine the systems hydrology, a major determinant of community and landscape structure and the point of connectivity between natural and human systems. This review examines the role of climate, geology, soils and sediments, topography, and hydrology in shaping and modifying ecological systems through time. However, it is clear from the wetland nature of this system that the predrainage hydrologic features were critical to the sustainability of the Everglades. Important hydrologic features include sufficient water quantity, storage, and sheetflow, and the appropriate hydroperiod and timing of water releases over both annual and interannual variations in precipitation.

Selection of scale for Everglades landscape models

This article addresses the problem of determining the optimal “Model Grain” or spatial resolution (scale) for landscape modeling in the Everglades. Selecting an appropriate scale for landscape modeling is a critical task that is necessary before using spatial data for model development. How the landscape is viewed in a simulation model is dependent on the scale (cell size) in which it is created. Given that different processes usually have different rates of fluctuations (frequencies), the question of selection of an appropriate modeling scale is a difficult one and most relevant to developing spatial ecosystem models.The question of choosing the appropriate scale for modeling is addressed using the landscape indices (e.g., cover fraction, diversity index, fractal dimension, and transition probabilities) recently developed for quantifying overall characteristics of spatial patterns. A vegetation map of an Everglades impoundment area developed from SPOT satellite data was used in the analyses. The data from this original 20 × 20 m data set was spatially aggregated to a 40 × 40 m resolution and incremented by 40 meters on up to 1000 × 1000 m (i.e., 40, 80, 120, 160 … 1000) scale. The primary focus was on the loss of information and the variation of spatial indices as a function of broadening “Model Grain” or scale.Cover fraction and diversity indices with broadening scale indicate important features, such as tree islands and brush mixture communities in the landscape, nearly disappear at or beyond the 700 m scale. The fractal analyses indicate that the area perimeter relationship changes quite rapidly after about 100 m scale. These results and others reported in the paper should be useful for setting appropriate objectives and expectations for Everglades landscape models built to varying spatial scales.

Global and regional sea level rise scenarios for the United States

Environmental issues and disasters/Climatic and atmospheric; Environmental issues and disasters/Flood

A science-based strategy for ecological restoration in South Florida

The Everglades and associated coastal ecosystems of South Florida are unique and highly valued ecosystems. One of the worlds largest water management systems has been developed in South Florida over the past 50 years to provide flood control, urban and agricultural water supply, and drainage of land for development. However, this system has inadvertently caused extensive degradation of the South Florida environment, resulting in the loss of more than half the historical Everglades system and elimination of whole classes of ecosystems. The U.S. Man and the Biosphere Program (US MAB) instituted a project to develop ecosystem management principles and identify requirements for ecological sustainability of South Florida. A strategic process developed by the US MAB Project illustrates how ecosystem management and ecological risk assessment principles apply to South Florida, including the development of societal goals and objectives of desired sustainable ecological condition, translation of these goals/objectives into scientifically meaningful ecological endpoints, creation of a regional plan designed to meet the sustainability goals, and development of a framework for evaluating how well the plan will achieve ecological sustainability of South Florida. An extensive federal, state, and tribal interagency process is underway to develop a restoration plan for restructuring the regional management system, essentially following the elements in the US MAB project process. The Florida Governors Commission was established as an institution to reflect societal values and define regional sustainability goals. The U.S. Army Corps of Engineers is developing a science-based plan for Congressional approval to restructure the water management system to achieve the societal goals. Thus, South Florida may become the prototype example of successful regional-scale ecosystem management.

Climate Links and Variability of Extreme Sea-Level Events at Key West, Pensacola, and Mayport, Florida

Analysis of long-term coastal tide records at Key West, Pensacola, and Mayport, Fla. examines the variability of extreme high-water events. Linear trends of event water level at Key West and Pensacola support findings elsewhere that increases in extreme water level are consistent with increases in global mean sea level. A link between event variables and the Atlantic multidecadal oscillation (AMO) is found at the Key West and Pensacola stations. Mayport does not exhibit such a link, likely the data are confounded by estuarine flows and oceanographic interactions with the St. Johns River. Quantile regressions and extreme value distributions suggest dynamic trends in the event variables at Key West and Pensacola as a function of AMO, indicating an increased variability in relation to linear models that should be of interest to coastal planners and forecasters.

Scenario-Based Projection of Extreme Sea Levels

ABSTRACT Obeysekera, J. and Park, J., 2013. Scenario-based projection of extreme sea levels. Heavily populated urban centers and natural areas located in low-lying coastal regions are highly vulnerable to sea-level extremes. Historical data at many tide gages suggest that changes over time in extremes generally follow the rise in mean sea level. Assuming this observation to hold in the future, a relationship between mean sea-level rise and its associated extremes with a generalized extreme value distribution can provide future return levels of extreme sea levels. Current projections of future sea level, which include varying degrees of acceleration, may result in large increases in extremes that need to be accounted for in the evaluation of existing coastal projects or in the planning of new ones. Because precise quantitative estimates of the uncertainties in sea-level rise projections are not available, scenario-based approaches have been suggested for project evaluation and design. Here, we propose a general method based on the synthesis of extreme value statistics with sea-level rise scenarios that allows any combination of linear or nonlinear local and global sea-level rise components and can accommodate the nonstationary evolution of sea-level extremes. The temporal variation of the design level of protection for coastal projects, expressed as the return period of extreme events, and the future behavior of the risk are explored. The concepts are demonstrated through application to tide gage data at several locations in the United States.

Evapotranspiration Estimation for South Florida

A regional estimate of potential evapotranspiration (ETp) for central and south Florida is provided based on cumulative literature and lysimeter studies of evaporation and evapotranspiration measurements and estimations. General estimates of annual ETp for the south Florida Water Management District range from 122 cm in the north to 137 cm in the south. Open water and wetland systems evaporate at the potential rate. Otherwise, actual evapotranspiration will be lower due to limited availability of water or moisture. A network of weather stations and an ETp computation software developed in-house is currently being used at the South Florida Water Management District to develop a daily ETp database. A simple ETp estimation model that was previously calibrated with lysimeter data is being applied.