Water Balance Analysis for Irrigation Water Allocation in the Idano Eho Watershed, Nias Island, Indonesia

Abstract

This study aims to analyze the water balance for irrigation water allocation in the Idano Eho Watershed, Nias Island. The analysis was conducted by comparing water availability and demand in 27 irrigation areas within the watershed. Water availability was estimated through reliable discharge using the F.J. Mock method based on 15-day rainfall data for the 2015–2024 period, while evapotranspiration was calculated using the Modified Penman method using climatological data for the 2020–2024 period. Reliable discharge was determined using Weibull probability at reliability levels of 35%, 50%, and 80%, with Q80 used as the basis for irrigation planning. The results of the study indicate that water availability in the Idano Eho Watershed fluctuates throughout the year, with periods of relatively high discharge in April–May and October–November. Meanwhile, irrigation water demand increases from October, reaching a high value in November-I, and peaking again in March-I. In general, the water balance in the Idano Eho Watershed remains predominantly surplus, but seven irrigation areas experience deficits during certain periods: the Tuindrao, Faohe, Hilimbulawa, Guliho, Laza Gau, Sefa, and Olo-Olo areas. These results indicate that irrigation water allocation issues in the Idano Eho Watershed primarily occur during certain critical periods, necessitating adaptive water distribution management through adjustments to cropping patterns, distribution schedules, and irrigation service priorities.