Remote sensing - based water demand index mapping: Insights from Khanh Hoa Province, Viet Nam

Located on the South-Central Coast of Viet Nam, the former Ninh Thuan (now is Khanh Hoa province) is one of the driest areas in Viet Nam and frequently experiences severe water shortages during the dry season. This study aims to develop spatial maps of the Water Demand Index (WDI) for Ninh Thuan area during the dry months from March to August in the representative drought year of 2020. The methodology integrates big Earth data and GIS data to compute WDI using key influencing first-order variables, namely soil moisture (SM), vegetative growth (NDVI), and heat factor (growing degree day, GDD). WDI reflects the spatial imbalance between vegetation water demand and the available soil water in areas with condensed agricultural activity at each pixel level. The WDI maps developed for the agricultural areas of Ninh Thuan reveal substantial spatial and temporal variability. Agricultural zones located in former Ninh Son, Thuan Bac, and Ninh Phuoc districts generally exhibit high WDI values (above 250), indicating considerable water stress. In contrast, certain regions such as the former Thuan Nam and Ninh Hai districts show comparatively lower WDI values. The WDI values peaked in April and May, coinciding with the most intense drought period and the critical crop development stage. This study provides a practical and spatially explicit tool for monitoring drought conditions and support effective water allocation planning amid the growing challenges of climate change and increasing water scarcity in the South-Central Coast of Viet Nam.