Water-Storage-Monitoring of Seasonal Wetlands in a Semi-Arid Environment by the Integrated Use of Long-Term Satellite Images and UAV Topography Measurement

Abstract

Semi-arid North-central Namibia is susceptible to poor crop harvests due to flood and drought. To mitigate the risk on the food security caused by flood and drought, rice cultivation could be introduced into the seasonal ponds (oondombe), which is one of topographical features in this region. The successful introduction of rice cultivation will depend on a comprehensive understanding of the hydrology of water storage in such oondombe; however, few data exist to support this study. We therefore undertook spatiotemporal monitoring of oondombe water-storage volume by integrating satellite remote sensing with measurements using structure-from-motion multiview stereo (SfM-MVS) analysis with an unmanned aerial vehicle (UAV). SfM-MVS is a recently developed technique that enables precise, simple, and inexpensive measurement of topography. First, we conducted UAV surveys at 16 oondombe to generate a regression relationship between oondombe water expanse and oondombe water-storage volume. Then we observed daily oondombe water expanse using several different sources of long-term satellite data (AMSR-E, AMSR2, MODIS, Landsat ETM＋) interpreted with the assistance of recent data-fusion technique (database unmixing). Finally, we applied a regression relationship to the satellite data measurements of water expanse and obtained estimates of oondombe water-storage volume for the period 2002-2015 at three test sites in Namibia. The estimated oondombe water-storage volume closely reflected seasonal change and year-to-year variation in flood and drought status. The accuracy of UAV measurements was up to error margins of several centimeters in the vertical direction and 10cm in the horizontal direction. Comparison among oondombe water-storage volume estimates with three different spatial resolutions revealed that measurement with an insufficient spatial resolution may lead to the overestimation of water-storage capacity. This study not only revealed valuable data about oondombe in this region but also proposed a new approach for spatiotemporal hydrological monitoring over wide areas that merits additional research.