1. Introduction
Drought is an abnormal weather event resulting primarily from the shortfall of precipitation for a prolonged period [
1]. Shortage of soil moisture, depletion of surface and subsurface water, and crop failure are the various forms of drought manifestations that are exacerbated by the reduction in the readily available water for different use, mainly agriculture. Although agriculture is the backbone for the economy of many developing countries including Ethiopia, it is often referred to as the first and the most drought vulnerable and less resilient sector [
1]. According to the USAID report (2018), the overall Gross Domestic Product (GDP) of Ethiopia depends heavily on rainfed agriculture which is sensitive to the seasonal variability of rainfall. Unfortunately, drought is a frequently recurring phenomenon in Ethiopia followed by a rigorous impact on human lives and the socioeconomic sector. The country has experienced the most severe and prominent drought events that covered the majority of its parts. The 1983–1984 drought was one of the worst droughts in the history of the country that affected more than one million people and left many people into further destitution [
2]. Among the recent drought events, the 2015 drought was noted as the worst and widely spread in the eastern, central, and northwestern parts of the country. According to the United Nations International Children’s Emergency Fund [
3], many people needed emergency humanitarian food assistance during this drought event. The major crop-producing and livestock farming regions in Ethiopia (i.e., Oromia, Amhara, Afar, and Somali) were rigorously struck by this drought event. Thus, the severity of the drought disaster in Ethiopia reflects its multi-dimensional nature since it manifested in different forms as meteorological, agricultural, hydrological, and/or socio-economic droughts.
The proactive drought monitoring approach is the essential step towards developing a drought management, early warning, and prediction models and strategies to minimize drought-induced impacts in the socioeconomic sector. Drought indices are used to monitor drought and measure its severity across space and time. Based on application and data requirement, drought is broadly classified into four types that include meteorological, agricultural, hydrological, and socioeconomic droughts [
4,
5,
6]. Meteorological drought is mainly attributed to prolonged deficits of rainfall events from the long-term average. Often, meteorological drought is a precursor for the occurrence of other drought types. Some of the widely and commonly used meteorological drought indices include the Standardized Precipitation Index (SPI, [
7]), Palmer Drought Severity Index (PDSI, [
8]), Palmer Z-Index, Standardized Precipitation and Evapotranspiration Index (SPEI, [
9]), Effective Drought Index (EDI, [
10]), etc. Agricultural drought, on the other hand, is manifested and characterized by the deficit of the readily available soil moisture for plant water use. The shortage of soil moisture adversely affects crop yield and triggers vegetation loss and impairment [
6]. The amount of soil moisture is highly connected to several drivers’ mainly climatic variables and other biophysical parameters (e.g., land use/land cover change, soil type, topography). There are several agricultural drought indices, however, the Soil Moisture Deficit Index (SMDI, [
11]) and Evapotranspiration Deficit Index (ETDI, [
11]) are some of the widely used indices in recent times. Soil moisture deficit could trigger the reduction in surface and subsurface flows, which instigates the occurrence of hydrological drought. Hydrological drought indices include the Streamflow Drought Index (SDI, [
12]), Standardized Runoff Index (SRI, [
13]), Surface Water Supply Index (SWSI), etc. Drought indices utilize various observed data and models to quantify drought situations.
There are several challenges in drought monitoring; however, the inaccessibility of accurate and uniformly distributed meteorological observation data is the most deterrent factor in many developing countries [
13]. For large-scale drought monitoring, remote sensing technology has become an alternative source of information [
14,
15] that provides information relatively at high spatial and temporal resolutions in the near real-time. Therefore, blending ground-based climate data with satellite images is quite essential for a compressive drought assessment across space and time. Even some of the satellite-based rainfall products merge the ground observation data to augment the accuracy. As compared to station data, satellite data often provide continuous and public domain datasets that are widely applied to detect the onset, duration, and magnitude of drought [
16]. Remote sensing-based drought monitoring approaches often use to monitor the vegetation condition, greenness, and health in response to drought. Naturally, drought has a strong association with vegetation condition and cover, and therefore, vegetation indices are also widely considered for capturing the extent and severity of drought impacts [
17,
18].
Given the complex nature of drought and its different forms, drought-monitoring using a single drought index derived from a single variable sometimes lacks the ability to provide comprehensive information to successfully characterize the different drought types that occur concurrently [
19]. Although the decision-makers need concise and timely facts to trigger informative decisions, considering the multidimensional aspects of drought through employing several drought indices facilitates a pragmatic decision. In a transboundary river basin, integrated effort on drought monitoring and management is vital to mitigating its adverse impact on surface and subsurface flow components. However, hydrological drought indices are often derived from point-based single variables (e.g., streamflow) that are constrained to capture the detailed spatial distribution and drought disaster picture of the entire basin. Therefore, any drought-related negotiation and talks among the riparian countries for shared water resources should incorporate the multidimensional aspects of drought and its characteristics across time and space in the upstream reaches of the basin to reflect the shared responsibility of drought disasters among the parties.
The Nile river basin in Eastern Africa is home to millions of people and the main source of water supply. However, climate change/extreme variabilities coupled with rapid population growth led to pronounced pressure on the sustainability of water resources in the region. For instance, there are storage facilities (e.g., dams) being constructed downstream riparian countries (e.g., Egypt and Sudan) in the Nile river basin. These storage dams prompted the downstream countries to become more resilient to meteorological drought as compared to the upstream countries where there is a limited number of such storage dams to delink meteorological drought from other forms of drought. There are limited research efforts conducted thus far on drought monitoring using several drought indices in the basin. Therefore, drought-monitoring using several drought indices that represent the different components of the water cycle is vital and a crucial step forward for a better understanding of the different forms of drought and making informative drought-related decisions to secure water resource sustainability and management in the basin.
The objective of this paper is to demonstrate the different forms of droughts that occurred in the Abbay/Upper Blue Nile River basin and their national and regional scale implications. Multiple drought indices derived from in situ and earth observation-based hydro-climatic variables were used to characterize meteorological, agricultural, and hydrological droughts in the basin. The temporal pattern of each drought was assessed from 1982 to 2019 and the spatial patterns of meteorological and agricultural droughts were shown for the selected drought and wet years to explore the drought extents across the basin. The findings of this study argue for the efforts to develop a robust drought monitoring system for the basin and for drought impact assessment of the socioeconomic sector.
2. Study Area and Drought History
The Upper Blue Nile (UBN) Basin is located at the geographic coordinates of 7°40′ and 12°51′ N latitudes, and 34°06′ and 40°00′ E longitudes in the northwestern part of Ethiopia (
Figure 1). The basin covers a total area of ~176,000 km
2 upstream from the Ethiopia-Sudan border [
20]. The annual rainfall ranges from 780 mm to 2200 mm, with the highlands having the highest rainfall (ranging from 1500 to 2200 mm) and the lowlands receiving the lowest rainfall ranging from 1500 to 780 mm [
20,
21,
22]. The main rainfall season (locally called ‘
Kiremt’) spans from June to September, and 82% of the annual streamflow generates during this season [
23]. The long-term average annual volume of flow at the outlet of the basin is ~48 billion m
3 and represents about 40% of the total surface flow of the country [
20]. More than 85% of the annual crop is produced during
Kiremt season [
24]. The mean annual temperature ranges from 13°C in southeastern parts to 26°C in the southwestern part near the Ethiopia-Sudan border [
25].
Drought is becoming a critical issue in the UBN basin and previous studies indicated the frequent occurrences of historic drought events in the past few decades. Frequent droughts have affected the socioeconomic sector, which largely relies on rainfed agriculture and less resilient to drought. For example, Bayissa et al., [
26] demonstrated the drought vulnerable parts of the basin and the historic drought events in the basin. The 1984 and 2015 droughts were some of the severe drought years in the basin that affected the socioeconomic sectors and annual water budget. UBN is the major tributary for the Nile River and contributes more than 60–-69% of the total annual flow [
20,
21,
27]. In addition, population growth, poor land use and water management are becoming the recurring challenges that aggravated water resource insecurity in the basin. Thus, integrated drought management and planning at the regional scale is crucial to mitigating its adverse impact on the water resources.
5. Discussion
Further observation of the temporal drought analysis confirms that different forms of droughts can occur concurrently (multiple events) or as single events, complicating monitoring, and management of drought in the Upper Blue Nile. For instance, in 1984, 2002, 2008, and 2015 concurrent (multi-dimensional) drought episodes occurred, and the scale of the disaster was significant. Especially in 1984, where the drought coping capacity of the country was low, hundreds of thousands of lives were lost in Ethiopia, while the impact of stream level (hydrological drought) impact on downstream countries, for example, Egypt and Sudan, was minimal under all measures. In the rest of the multi-dimensional years of 2002, 2008, and 2015, Ethiopia managed to offset the impact of multi-dimensional drought on life, but the economic toll was still significant. A single event agricultural drought occurred in 1990 despite relatively higher rainfall and runoff (
Figure 6). This is likely related to the high-intensity rains occurring for short periods affecting rainfed agriculture but not continuously enough to supplement more runoff. In contrast, there were meteorological and hydrological droughts (
Figure 6) that did not affect agriculture (low or no agricultural drought). This is likely related to the low-intensity continuous availability of rainfall that supplemented soil moisture enough to support crop cultivation but was less available as runoff.
The spatial study indicates that the nexus between catchment level drought manifestation and stream level hydrological drought is complex and is not evident at different scales of the basin. While all dimensions of drought affect catchment-related socio-economic activities in the Upper Blue Nile (Ethiopia), only hydrological drought affects stream-related socio-economic activities in the downstream riparian countries (e.g., Sudan and Egypt). Thus, drought disasters affecting catchment-related socio-economic activities are multi-dimensional, regular, and more intense as observed in the recurring droughts in the Ethiopian Blue Nile catchment than those in Sudan and Egypt, which depend on the stream-related activities of the Blue Nile basin. This triggers countries to compete for the less drought-affected and storable water system in the hydrologic cycle, which is the surface runoff.
This complex and multi-dimensional drought manifestation in the UBN basin (especially in Ethiopia) needs to be seen from a basin perspective rather than a country perspective. This necessitates regional cooperation for a shared understanding of the multidimensional droughts focusing on entire basin communities at different spatial scales and responding jointly. In this way, the shared water resources like the Blue Nile River and the communities that depend on the rainfall part of the hydrological cycle can be addressed to avoid a repeat of the 1984 disaster in the communities living in the UBN subbasins. This study underlines the need for shared multi-scale drought monitoring and management practices to mitigate drought impacts across the basin.
Due to relatively short record length and inaccessibility and missing records of the streamflow data, we were unable to scale down the analysis at the subbasin/catchment level. The subbasin analysis would have captured the variability of the hydrological drought across the basin.
The study attempts to convey a scientific understanding of the linkages of different drought types at the basin scale and how the drought interplays affect different parts of the basin communities differently. In addition, there are several factors that largely affect the accuracy of the results of this study. The coarser spatial resolution of the input variables certainly lacks to represent and capture the high variability of the rainfall and soil moisture especially in a rugged topography like the UBN basin. The result may improve depending on the availability of the fine spatiotemporal resolution and good quality data of the input variables. In addition, the UBN basin is characterized by different land-use and soil types, which are certainly contributing to the accuracy of the results. The spatial correlation coefficient analysis between the SPI and stdSMDI showed a low correlation in areas where mixed, deciduous, and evergreen forests are dominant (e.g., central, southern, and southwest parts of the basin) [
47]. This indicates the need for a detailed drought analysis for each land-use type for future study. The water holding capacity of the soil that varies with the type of soil also plays a significant role in terms of retaining the readily available soil moisture for a long period of time to enhance drought resilience.
6. Conclusions and Recommendations
This study demonstrates the implication of the different forms of droughts at national and regional scales through drought monitoring approach using multiple drought indices in characterizing the different forms of droughts (i.e., meteorological, agricultural, and hydrological) in the Upper Blue Nile Basin. Remote sensing and station based observed input data were used to derive the time series of the drought indices that were eventually used to characterize the severity, extents, and frequency of the historic drought episodes across the basin from 1982 to 2019. SPI, stdSMDI, and SRI drought indices were used in this study to monitor meteorological, agricultural, and hydrological droughts, respectively. The UBN basin is the main source of water for millions of lives in the region and integrated drought management and planning at the regional scale is crucial to mitigate drought-related impacts to sustain the water resources in the basin.
The UBN basin is experiencing severe historic drought episodes that affect the livelihood of many populations in the basin. Based on the result obtained in this study, the amount and timing of the rainfall events significantly affected the agricultural sector in the basin and led to food insecurity and economic losses. The meteorological, agricultural, and hydrological droughts concurrently occurred in the basin, which implies the intensified and multi-dimensional aspects of drought. Multi-dimensional drought mitigation necessitates regional cooperation and watershed management to protect both the common water sources of the Abbay/Upper Blue Nile basin and the socio-economic activities of the people in the basin. This study also underlines the need for multi-scale drought monitoring and management practices to mitigate the multi-dimensional socio-economic crisis and other drought impacts across the basin.