Groundwater Management in an Uncommon and Artificial Aquifer Based on Kc Approach and MODIS ET Products for Irrigation Assessment in a Subtropical Island

Groundwater is a critical resource in remote and isolated islands where rainfall hardly provides a continuous and even water supply. In this paper, in a very rare and uncommonly found artificial aquifer on Miyako Island, far away from the main continent of Japan, with limited experimental results of evaluations of crop water requirement, MODIS ET together with crop ETc estimated from Kc coefficient from the nearest island were compared to determine the reliability of the MODIS ET and FAO-56-based ETc value. The testified Kc approach for sugarcane ET was used to assess the risk of irrigation water shortages using historical metrological data and to predict the future risk of irrigation agriculture under different scenarios of GCM models. It was shown that FAO-56-based ETc and MOD16A2 were both applicable for crop evapotranspiration on the island. Then, the response of groundwater storage to gross irrigation water requirement was analyzed to clarify the effect of irrigation on groundwater storage and the risk of groundwater depletion under current and future climatic conditions. Results showed that the construction of the dam efficiently secured the irrigation of sugarcane. Using historical climatic data (1951–2021), the influence of estimated irrigation water requirements on groundwater showed that in 296 out of 852 months, irrigation was heavily required. Over a 71 year period, there was absolutely no water for irrigation four times, or nearly once every 18 years. Under the future projected climate from four bias-corrected GCM models with two emission scenarios (2022–2100), the risk of groundwater depletion both in terms of frequency and duration will increase. Therefore, there is a need for either improvement of irrigation water management or additional construction of artificial aquifers on the island. The study proved the value of ET derived from remote sensing in areas lacking the support of experimental results. The methodology developed in the study can be potentially used to evaluate long-term irrigation demand and groundwater management over dry periods for engineering design or dam construction globally.