Monitoring of soil moisture variability in relation to rice cropping systems in the Vietnamese Mekong Delta using MODIS data

Information on soil moisture is important for crop and water management. This study investigated soil moisture variability in relation to rice cropping systems in the Mekong Delta (MD), Vietnam, using MODIS data acquired from January to April, 2002 to 2007. The soil moisture was estimated using the temperature vegetation dryness index (TVDI). This index was empirically calculated by parameterizing the relationship between the MODIS land surface temperature (LST) and the normalized difference vegetation index (NDVI) data. The rice cropping systems were classified using time-series MODIS NDVI data for 2002 and 2006. The data were processed using empirical mode decomposition (EMD), principal component analysis (PCA) and maximum likelihood classification (MLC) methods. Various spatial and non-spatial data were also collected for accuracy validation of the TVDI and classification results. Comparisons reveal good agreement between the TVDI and daily rainfall data. The areas with low soil moisture were mainly distributed in coastal areas from 2002 to 2005, but expanded into the middle of the MD in 2006 and 2007. The largest area of low soil moisture was observed in 2006. Classification of rice cropping systems reveals an overall accuracy and Kappa coefficient for 2002 of 76.3% and 0.68, respectively while the values for 2006 were 76.9% and 0.69, respectively. To relate soil moisture with rice cropping systems, we aggregated the composite soil moisture maps (considering dry and very dry classes only) with the rice crop maps for 2002 and 2006. The results indicate a remarkable increase in the area of double and triple irrigated–rice cropping systems in areas of low soil moisture (i.e., dry and very dry conditions) during this period. This study demonstrated the merits of using MODIS data for studying soil variability in relation to rice crops which is important for crop and water management.