Hiroki Mizuochi

Evaluation of the Surface Water Distribution in North-Central Namibia Based on MODIS and AMSR Series

Semi-arid North-central Namibia has high potential for rice cultivation because large seasonal wetlands (oshana) form during the rainy season. Evaluating the distribution of surface water would reveal the area potentially suitable for rice cultivation. In this study, we detected the distribution of surface water with high spatial and temporal resolution by using two types of complementary satellite data: MODIS (MODerate-resolution Imaging Spectroradiometer) and AMSR-E (Advanced Microwave Scanning Radiometer–Earth Observing System), using AMSR2 after AMSR-E became unavailable. We combined the modified normalized-difference water index (MNDWI) from the MODIS data with the normalized-difference polarization index (NDPI) from the AMSR-E and AMSR2 data to determine the area of surface water. We developed a simple gap-filling method (“database unmixing”) with the two indices, thereby providing daily 500-m-resolution MNDWI maps of north-central Namibia regardless of whether the sky was clear. Moreover, through receiver-operator characteristics (ROC) analysis, we determined the threshold MNDWI (−0.316) for wetlands. Using ROC analysis, MNDWI had moderate performance (the area under the ROC curve was 0.747), and the recognition error for seasonal wetlands and dry land was 21.2%. The threshold MNDWI let us calculate probability of water presence (PWP) maps for the rainy season and the whole year. The PWP maps revealed the total area potentially suitable for rice cultivation: 1255 km2 (1.6% of the study area).

Classification of Small Seasonal Ponds Based on Soil–Water Environments in the Cuvelai Seasonal Wetland System, North-Central Namibia

The objectives of this study were to classify the small seasonal ponds that develop in the Cuvelai Seasonal Wetland System in north-central Namibia based on soil physicochemical properties so that potential agricultural uses of the ponds could be assessed. Satellite imagery was used to examine the relationships between soil characteristics and the probability of water presence in the region. Soil samples were collected from 66 ponds at three sites and their physicochemical properties and levels of salt accumulation were investigated. Soil data were analyzed by using principal component analysis and cluster analysis. The ponds were classified into four types based on soil properties. Type A accounted for 20% of the ponds and was characterized by high levels of clay, silt, total N, and exchangeable Ca, Mg, and K. Type B accounted for 14% of the ponds and was characterized by high values of organic C, C/N ratios, and available P. Type C accounted for 20% of the ponds, and this type featured high levels of salinity and sodicity. The remaining ponds were classified as Type D. If these seasonal ponds are to be considered for crop production, Type C should be avoided and Type D would require soil fertility improvements.