Drought is a silent and pervasive disaster that impacts a large area and propagates slowly. Unlike for other natural disasters such as floods, tornados etc., impacts of droughts do not manifest immediately. This makes it more difficult to monitor drought and mitigate adverse effects by early warning. Several drought indices exist to monitor drought. Individually, however, they are unable to provide an integral concise information to characterize and indicate the occurrence of meteorological, agricultural and hydrological droughts. A combined drought index (CDI) using several meteorological, agricultural and hydrological drought indices can indicate the occurrence of all drought types, and can provide information that facilitates the drought management decision-making process. Moreover, development of a CDI can be an impact-based, e.g. by optimizing for monitoring drought-related crop yield reduction. The economic growth in many developing countries relies on the agricultural products, hence developing crop yield monitoring and prediction methods is vital to enhance the economic growth.