CHAPTER 1 Overview of the hydrological modeling of small coastal watersheds on tropical islands A. Fares College of Tropical Agriculture and Human Resources, University of Hawaii-Manoa, Honolulu, HI, USA. Abstract Increased population growth especially in coastal areas has resulted in substan- tial land use and land covers changes that in turn have generated concerns about the effects of such activities on their natural resources and especially on the qual- ity and quantity of water resources. Watershed models based upon sound physical theory and well calibrated can provide useful tools for assisting hydrologists and natural-resources managers to choose the best management practices for these sites. This chapter presents an overview of coastal-watershed modeling. It depicts the basic hydrological components of coastal watersheds; it also discusses the different governing equations implemented in the different models to describe the surface and subsurface water fl ow processes simulated by these models. In addition, governing equations for erosion and contaminant transport mechanisms were also presented for physically based and empirical modeling approaches. The chapter discusses the two main approaches (numerical and analytical) of solving the water fl ow and sediment transport governing equations models. Salt water intrusion as a result of natural disasters (Tsunami and hurricanes, e.g. Katrina) was also discussed. This chapter provides an overview of a few coastal-watershed hydrology case studies using different watershed models. By addressing various issues of coastal watershed modeling, this work is intended to assist resource managers, researchers, consultant groups and government agencies to select, use and evaluate different watershed models to be able to adopt sustainable watershed-management practices. 1 Introduction Rapid growth of global population and changes in economic environment have triggered land-use change that can be linked to changes in climate, biodiversity, and WIT Transactions on State of the Art in Science and Engineering, Vol 33, © 2008 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) doi:10.2495/978-1-84564-091-0/01 2 Coastal Watershed Management water quantity and quality. The impacts of these changes have more pronounced effects on coastal watersheds, especially those of small islands, i.e. Caribbean Islands, Hawaiian Islands, and Pacifi c Islands. A watershed is defi ned as a geo- graphic area of land that drains water to a shared destination such as a river sys- tem or any other water body. The size of a watershed can be small, representing a single tributary within a larger system, or quite large and cover thousands of square kilometers. Small islands are characterized by a large number of small and steep watersheds with highly permeable volcanic rocks and soils. Rainfall is spatially and temporally variable resulting from a combination of both the location within the island and altitude. Tropical rainfall comprises more than two-thirds of the global rainfall [1]. Great variations of rainfall occur within small distances on tropical islands. For example, on the island of Kaua’i, Hawaii annual rainfall increases from 500 mm near Kekaha to over 11,000 mm at Mt. Wai’ale’ale, an average gradient of 0.42 mm/m [2]. This is caused mainly by orographic charac- teristics of rains, which are formed by humid air above oceans carried by trade winds from the sea over the steep and high terrain of the islands. These coastal watersheds contain some of the most productive and diverse natural systems. They comprise complex and highly specialized ecosystems, which extend from the mountains to the adjacent coastal areas that include estuaries, coral reefs, and stream delta, which are vital natural resources for different stakeholders. Intensive management practices in these relatively sensitive environments have generated concerns about the effects of land use/cover changes on the quality and quantity of surface water in adjacent coastal areas and groundwater of the whole system. Hydrologists are often requested to describe, interpret the behavior of these complex systems. Although some conclusions can be made using best physical and biological science judgments, in many instances human reasoning alone is inadequate to synthesize the collection of factors involved in analyzing complex hydrological problems. Intensive fi eld experiments can be conducted to answer many of these practical management questions; however, such investigations are commonly site specifi c, dependent upon climatological and edaphic conditions, and costly in time and resources. Hydrological watershed models based upon sound physical theory can provide practical management tools to assist natural-resources managers meet the chal- lenge of description and interpretation. Such management tools combine the sub- tlety of human judgment with the power of personal computers to allow more effective use of available data and account for more complexity. Watershed models have been successfully used to perform complex analyses and to make informed predictions concerning the consequences of proposed actions. They also increased the accuracy of estimates for alternative practices to a level beyond the best human judgment decisions. 1.1 Characteristics of small coastal watersheds on tropical islands Many unique characteristics of coastal islands result from their isolation, small size and exposure to the marine environment. Most of the tropical islands are the WIT Transactions on State of the Art in Science and Engineering, Vol 33, © 2008 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) Hydrological Modeling of Small Coastal Watersheds 3 results of volcanic activities, which make them mountainous in nature, e.g. Hawaii. These islands are continuously exposed to winds, waves, tides, salts, animals, and human activities making them vulnerable to natural and man-made stresses. Generally, the larger the island, the more diverse is its ecosystem, the more varied and numerous are its plants and animals life, and the more tolerant it is to distur- bance. The tropical island climate is strongly moderated by the ocean. Island soils are acidic, infertile, and shallow, with a thin organic layer. Larger islands often contain marshes and bogs. Vegetative cover varies, depending on local conditions, soil type, and past clearing practices. Most of the larger islands are forested and mature softwood stands predominant on their landscapes. Groundwater is the main source of freshwater on islands, but its depletion and contamination is limiting its use. In tropical islands, groundwater is generated entirely by rain on the island, which percolates into the aquifer. Most of the islands are highly rocky and have impervious soil layers that reduce water infi ltration, causing more surface runoff. Sometimes high groundwater demand under limited source causes saltwater intrusions into the groundwater supply [3]. A methodical understanding of hydrologic cycle components and characteristics of coastal watersheds on tropical islands is needed to select a hydrological model suitable for a particular scenario. This chapter covers the following aims: 1) to describe the main characteristics of hydrological models; 2) to give an overview of available hydrological models applicable to small island coastal watersheds; 3) to review major environmental problems in coastal watersheds; and 4) to present case studies on the application of hydrological models to coastal watersheds. 2 Classifi cation of models Models are simplifi ed representation of real systems and are often used to predict the response of the modeled system under the infl uence of different management scenarios. Models are classifi ed based on process description (deterministic vs. stochastic), timescale (single event vs. continuous), space scale (distribute vs. lumped), techniques of solution (analytical vs. numerical), and their use (watershed, groundwater) (Table 1). Physical models are based on the mathematical-physics equations of mass and energy transfer intended to avoid and/or minimize the need for calibration. The phys- ical models are physical representations of a smaller- or larger-scale real system. A physical model is used to simulate some phenomenon on a large-scale by using a small-scale experiment either in a fi eld or a laboratory. Geometric and dynamic scales of physical models are important characteristics. Models can be also classifi ed as linear or nonlinear, deterministic or stochastic, steady state or transient, and lumped or distributed. A linear model is the one in which objective functions are expressed by linear equations. A steady-state model does not account for the element of time, while a transient model is one with an explicit time dimension. A deterministic model is one in which its variables do not vary randomly. Stochastic models have some ran- domness and uncertainty that are described by statistical properties, such as trend, seasonality, mean, variance, skewness, covariance, correlation, and variance function. WIT Transactions on State of the Art in Science and Engineering, Vol 33, © 2008 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) 4 Coastal Watershed Management Coastal Watershed Table 1: Characteristics of some watershed models. W w w I T w T . Simulation Runoff Overland Channel Watershed w r a i t n p s Model type generation fl ow fl ow representation Use r a e c s t s i . o c n o HSPF Continuous Soil moisture Kinematic Kinematic Lumped Watershed hydrology s m o , n I accounting