**1. Introduction**

Drought is a worldwide natural hazard and has a detrimental impact on society, the environment, and the economy [1]. Extreme hydrological events both high (flood) and low (drought) flow are of particular concern globally. Of these hydrological extremes, drought is the most complex and widespread [2]. It is one of the most common natural events that has devastating negative impacts on agriculture and water resources [3].

There is no universal definition for drought due to its complexity [4]. Therefore, meteorologists defined drought as a scarcity of precipitation [5–10]; hydrologists have defined hydrological drought as scarcity of surface and subsurface water [5, 6, 7–15]; agriculturalists and agronomists defined agricultural drought as related to soil moisture deficiency [3, 16, 17] and sociologists and economists defined the overall welfare crisis of the society caused by drought to be socioeconomical drought [4, 18–21]. These types of droughts have accumulating effects, thus meteorological drought results in losses, such as crop stress, predation by pests, and disease due to low moisture, to the agricultural systems while hydrological drought causes the shortage of water supply, decrease in reservoir water level and groundwater volume, lower irrigation and hydropower production [14]. The accumulation of meteorological and hydrological drought results in socioeconomical drought in which the overall ecosystem will be disturbed and human and animal lives will be negatively impacted and even lost [15].

Historically, Ethiopia has faced multiple seasonal drought events due to erratic rainfall and climate change [22]. The most drought-prone areas in Ethiopia are in Northeast Ethiopia and the Upper Blue Nile basin, including the Northern Tigray region, some parts of Amhara regions, such as South Wollo, North Wollo, South Gondar, and Afar Region, most parts of Somalia Region, and Eastern parts of Oromia Region [1, 3, 23–27]. Drought in Ethiopia occurs at a recurrence interval of 3–10 years [1], and even though this frequent recurrence is common, there still lacks any firmly established drought mitigation measure for these events. Only short-term response efforts are provided in the form of food aid when food supplies have decreased significantly due to extended drought.

Meteorological drought analysis has been studied frequently, yet hydrological and agricultural drought analysis and monitoring are not studied adequately. It is thought that Ethiopia is a water tower in East Africa but water resource management over the region is not well developed. This aggravates the natural hazard, such as drought impact on human life. Hydrological drought has a great influence on water supply irrigation and power production by reducing the availability of surface and subsurface water. There are few dams and reservoirs in the country and most of them are hydropower plants. But there is a lack of water conservation to reduce drought impact when it occurs. Generally, drought monitoring and forecasting studies are untouched and need a thorough investigation to alleviate socioeconomic problems related to drought.

The objective of this review chapter is to assess the status of hydrological drought studies in Ethiopia by reviewing different previously studied article papers related to drought. A total of 24 article papers was reviewed and the master plan of the eightriver basin was also reviewed. Of these, only two papers were related to hydrological drought and the remains were about meteorological and other drought-related topics. This implies that hydrological drought studies in Ethiopia require further analysis, monitoring, and forecasting investigation. Therefore, it is important to do this kind of review to show the gap of drought studies over the region for future researchers, stakeholders, and planners to develop a suitable early warning system.
