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4. FURTHER HYDROLOGIC STUDIES Although hydrologic data in Hawaii have greatly increased since 1959, they are still insufficient for advanced hydrologic analysis to develop better criteria for the design of drainage facilities. For example, only scanty coordinated rainfall and runoff data are available that are useful for deriving unit hydrographs. In most cases, rainfall and runoff data are collected separately by different agencies. Thus, the rain gauge and stream flow gauge networks are designed and maintained often for uncoordi- nated purposes. The data so obtained are usually unsuitable for use in the analysis of rainfall runoff relationships because they do not adequately match geographically and are unsynchronized on many occasions. The hydrologic condition on the Island of Hawaii is extremely hetero- geneous because of irregular orographic characteristics and soil types. Each drainage basin often has its own unique hydrologic condition, even within one basin, the hydrologic condition varies radically. On the wind- ward side of the island, for example, there is a radical change in rainfall amounts from sea level to the peak of Mauna Kea. The upper slopes have little or no runoff because of light rainfalls and extremely permeable soils and the lower slopes are subject to high intensity storms and runoff. Thus, a unit hydrograph developed from runoff data measured at a downstream gauging station cannot be reliably applied to the upstream areas. In view of the lack of hydrological homogeneity, a regional analysis of rainfall and runoff can only be considered approximate. Since the hydrologic condition is heterogeneous, dense hydrologic net- works should be developed and more hydrologic data must be collected. As the drainage basins show individual characteristics, further analyses of the accumulated data must be made for all individual basins and at various places inside the basins. The results of such analyses would be most use- ful and reliable for the design of drainage facilities at a given place in a given drainage basin. Extensive hydrologic analysis of individual drainage basins would be too costly and time consuming, therefore, an approximate regional analysis such as the type of envelope curve shown in Plate 6, must be further im- proved and refined. When more peak discharges of record are measured, a frequency analysis will be incorporated in it to determine envelope curves for various recurrence intervals. Also, synthetic peak discharges may be computed from available unit hydrograph information and may be used to develop peak discharge frequency envelope curves separately for a number of regions on the Island of Hawaii. 4