Technical Report No. 74
The major tasks under Phase II of the project were to: (1) adapt and evaluate several existing watershed simulation models for their applicability under local conditions, (2) expand the data collection effort in the research watersheds, and (3) identify problem areas for subsequent study.
The instantaneous unit hydrograph (IUH) theory was explored in detail. The results indicate that the successive routing method developed by Nash (1957) has better applicability on urban watersheds near Honolulu. The parameter, N, representing the conceptual number of reservoirs, decreases as urbanization progresses.
In the adaptation of the Kentucky Watershed Model (KWM), it was found that the basic logic of continuous balancing of the water budget within a given watershed is applicable to Hawaiian conditions. The development of KWM to meet Hawaiian requirements would entail two to three more years as has been the experience in Kentucky. A preliminary computer program has been established and simulated daisy streamflow from Kalihi watershed obtained.
In the Road Research Laboratory Model (RRL) adaptation, there was poor correlation between the rainfall and runoff data collected from the St. Louis Heights research watershed. This may be attributable to deficiencies in instrumentation as well as inexperience in data colIection, as the data used for simulation were obtained during early stages of the study.
In the multiple regression method for determining peak flow, the statistical method appeared to establish a reasonable empirical relationship between peak flow and urbanized area of a given watershed, as well as between time to peak and urbanized area. However, additional data are required to fully establish such a relationship.
Of the mathematical models tested for rainfall-runoff relationship, the nonlinear time variant method gave the best peak discharge simulation. Also Nash’s IUH model (1957) gave good simulation.
All watershed simulation models tested indicated that more hydrological data are prerequisite to the development of a reliable urban hydrology simulation model for Oahu. The data collection program has been expanded to include evaporation, soil moisture, wind speed, solar radiation, and water quality in addition to rainfall and streamflow records gathered since initiation of the project.