Understanding the watershed hydrology of tropical islands

Abstract:

Streams systems in Hawaii is unique comparing to systems in other places in the worlds. Besides its volcanic geology, its steep topography contributes to the flashiness of stream flow. The prevalent trade wind results in wet (windward) and dry (leeward) sides of island, which receive unequal amount of moisture for cloud formation. To better understand and describe the mechanism of forming stream flow in Hawaii, a series of hydrologic model, from simple to complex, were used to simulate stream flow on two pilot watershed. We first used a lumped-model, Thornthwaite Monthly Water Balance Model, to describe water budget of each watershed. Second, we used a non-spatial explicit model, topographic index model (TOPMODEL), to describe the surface and subsurface components that contribute to stream flow. Finally, we used a semi-distributed model, Soil and Water Assessment Tool (SWAT), to understand the different land use effect on stream flow. In particular, rainfall coverage was obtained from dynamic downscaling model and validated with observation. The spatial distribution of rainfall data were served as input for all models. Through these processes, we strategically dissected the components in contributing to stream flow within a watershed, and revealed the dominate mechanism and drivers of stream flow in the watersheds. We then compared the processes in forming stream flow in watersheds of windward and leeward side. The difference in the two watershed were presented and described. Ultimately, the successful building of flow models allows us to further describe the nutrient and sediment transport that affects both instream habitats and downstream estuarine and coastal system.