Understanding the Hydrology of a Rainforest Watershed in Hawaii


Dr. YinPhan Tsang, PI
Dr. Carl Evensen, Co-PI


National Institute for Water Resources, Water Resources Research Institute Program


3/1/2015 - 2/28/2017


Recent studies have shown an increase in rainfall intensity in Hawaii. However, total rainfall has been on the decline in the last two decades and stream flow is subsequently subsiding. Climate models project continuous decline in rainfall with uneven spatial distribution. The impact of decreased atmospheric circulation in the tropical Pacific on trade winds and cloud formation remains unclear. In addition to large scale climate circulation, local topography and geographical locations play an important role in determining characteristics of rainfall. All these factors add more challenges in assessing stream and groundwater flows and their influence on water availability and on the islands' ecosystems. Lyon Arboretum, located at upper Manoa Valley and being part of the Waikiki Ahupua'a, is the only accessible tropical rainforest on the island of Oahu. Lyon Arboretum, of a total of 78.3 hectares was used to conduct experiments for sugarcane and watershed reforestation. Presently, it is a rainforest with limited anthropogenic disturbance. Rainforests are recognized for their role in helping the water cycle by transpiring water to atmosphere and contributing to rain cloud formation. Forests also protect the soil layers that store water and prevent soil erosion. With its unique location and topography, the watershed containing Lyon Arboretum receives frequent and intense rainfall. Previous research at Lyon Arboretum has been focusing on assembling native and introduced plant species as well as preserving rare Hawaiian plants. Little has been done to understand how its rainforest responds to unique precipitation patterns at a watershed scale. This research proposes a proof-of-concept project to understand the hydrology of rainforest watershed in Hawaii. The research team will install two monitoring stations at the study site. A land cover survey and spatially distributed rainfall, along with monitored stream flow data, will be used to assess the stream response for the rainforest watershed. The data will aid in understanding hydrologic mechanisms and processes, including flow paths, groundwater recharge, evapotranspiration, and time of concentration. As it is naturally covered, the study watershed provides an excellent prototype for understanding stream flow of rainforest under an undisturbed condition. With this information, we should be able to assess nutrient and sediment transport and their contribution to downstream watersheds. The study will also develop a hydrologic model utilizing compiled monitoring data. This will set a baseline for future studies on exploring hydrologic mechanisms and nutrient transport under disturbed conditions (e.g., anthropogenic land use, stream diversions, and groundwater withdraw). The established knowledge of plant species in Lyon Arboretum provides a great source to understand rainforest evapotranspiration and its interaction with cloud formation. With the projected changes in rainfall under future climate, this project will help in assessing the impacts on rainforest and the respective freshwater availability for human and natural ecosystems.