WRRCTR No.99 Low-Flow Frequency and Stochastic Analysis of Irrigation Ditch Flows for Central Maui, Hawaii Final Report

Abstract

The central, portion of the island of Maui, state of Hawaii, suffered from droughts during the summers of 1971 to 1975, which resulted in heavy crop losses. In order to learn more about the frequency of droughts, a study of the irrigation water low-flow frequency was made during the earlier stage of this research, and a set of low-flow frequency curves for the central Maui area was prepared and presented in this report. Low-flow frequency curves can only provide the probability of occurrence of a drought in terms of a given magnitude of flow, its duration and its recurrence interval; therefore, their applicability is limited to an event basis, and the sequential, characteristics of the droughts are not provided. In order to study the drought occurrence on an event-sequence basis, with the aim to explore the water manageability during a drought, a stochastic analysis of the low flows in the summer periods of the central Maui region was conducted. The methods and results of the stochastic analysis are presented in this report. Since most of the irrigation water is collected and delivered by ditches to the central Maui sugarcane fields, and among these ditches, the Wailoa ditch delivers about 90% of the summer months' irrigation water; therefore, the Wailoa ditch monthly flow recorded by U.S. Geological Survey for 1923 to 1972 has been used for the stochastic analysis of the summer low flows. The results obtained show very little improvement from using Pearson Type III distribution against the normal distribution of the random error term in the stochastic simulation of the summer ditch flows. Four sets of summer ditch flows for the period 1974-2020 have been generated and presented for water management references.