The following sections discuss the temporal, spatial patterns and trends of drought identified across the main river basins of Ethiopia using CHIRPSv2-based EDI and SPI drought indices.
EDI-based drought assessment
EDI-based temporal pattern of drought
Time series plots of basin-wise average monthly EDI for the six river basins for the period 1982 to 2016 indicate that CHIRPS-based EDI can capture the droughts efficiently. This finding holds true for both, the smaller as well as the main rainfall season, in each river basin. For example,1984, 1985, 1986, 1994, 1995, 2002, 2009, 2011,2012, and 2015 were some of the RHD years in the country (Table 2). A Temporal drought analysis shows that CHIRPS-based EDI is able to correctly detect the RHD with different severity levels either during the main rainfall season, smaller rainfall season, or in both seasons. Moreover, the study highlights the importance of doing river basin wise analysis as the severity of drought varies from one basin to other basins at a particular year. On the other hand, EDI correctly identifies the year 2007 as a non-drought year in all the study river basins (Figs. 3 and 4), which is in agreement with the RHD information. This finding also agree with the finding of Mohammed et al. (2018) and Mekonen et al. (2020) who found that North eastern parts of Ethiopia are drought prone areas of the country. To have more insight into the performance, a detailed analysis of two historically worst drought years such as 1984 and 2011 and one non-drought year of 2007 (highlighted in Figs. 3 and 4) across the six river basins is reported in the following sections.
In 1984, the EDI shows negative values (EDI < 0) during both smaller rainfall season (Fig. 3a–d) and main rainfall seasons (Fig. 4a–d), indicating there was drought in 1984. The 1984 drought has occurred due to the deficit of rainfall during both rainfall seasons. However, the severity level of drought varies temporally across each river basin. On the other hand, during 2011 the EDI show negative values (EDI < 0) during smaller rainfall season (Fig. 3a–d) and positive values (EDI > 0) during the main rainfall season (Fig. 4a–d). This implies that the 2011 drought has occurred only due to the deficit of rainfall during smaller rainfall season. Previous studies at different parts of the mentioned river basins also reported similar findings during 1984 and 2011 (Gidey et al. 2011; Suryabhagavan 2017; Bayissa et al. 2017). In both the RHD years of 1984 and 2011, specifically during the main rainfall season, the EDI values indicate moderate to severe drought at Awash, Danakil, and Tekeze river basins. This is understandable because these river basins are usually drought-prone areas (rainfall deficit areas) of the country and previous studies also categorized them as drought-prone areas of the country (Gidey et al. 2011; Bayissa et al. 2017). In addition, the 1984 drought has started in February 1984 and ended in April 1985. However, the 2011 drought has started in February 2011 and ended in May of the same year 2011.
In 2007, during the main rainfall season (Fig. 3a–d) except for Danakil (shows mild drought during June) as well as during the smaller rainfall season (Fig. 4a–d) except for Awash and Danakil (show mild drought from March to May), the EDI shows positive values (EDI > 0), indicating there was no drought.
EDI-based spatial pattern of drought
The temporal drought analysis represents only the average situations at each river basin; however, there is large variability within a basin. Therefore, representing the drought severity in grid-wise would be significant to study the spatial extents of drought severity. The obtained results (not shown here) for the early twenty-first century i.e. from 2000 to 2016 indicated that EDI detected correctly the spatial extent of the recorded historical droughts. However, for illustration purpose a detailed spatial assessment has been done only for the two historically worst drought years of 1984 and 2011 as well as one non-drought year of 2007 was reported hereunder.
Figure 5a and b show the maps of the RHDs in the six river basins during 1984 and 2011 respectively. All the selected river basins (except in the southwestern part of Baro, the northern part of Omo and southern tips of Awash and Blue Nile) were affected by the 1984 drought (Fig. 5a). Similarly, during 2011 (Fig. 5b) all the selected river basins (except Omo, Baro, and some portions of Blue Nile river basins) were affected by drought. The spatial maps of RHD for each study year across the selected river basins were also developed from the available RHD information (Table 2). This variability within the river basin also confirm that climate variability and associated extreme weather events are the possible cause for the occurrence of drought in different parts of each river basin.
Figures 6 and 7 show the EDI identified spatial pattern of drought in the selected river basins during smaller and main rainy months respectively. During smaller rainy months of 1984, Awash, Danakil, and Tekeze river basins were affected by moderate to severe droughts (Fig. 6a). During the same year, the Blue Nile river basin is partially affected by moderate drought severity. Likewise, during the smaller rainy months of 2011, mild to severe drought was observed in all the selected river basins except in Tekeze and Blue Nile river basins.
Similarly, during the main rainfall months of years, 1984 (Fig. 7a) and 2011 (Fig. 7b) EDI shows varied drought severity classes. In 1984, the major portion of the Blue Nile river basin, Baro and Omo basins show no drought (Fig. 7a). Likewise, in 2011, a major portion of the Blue Nile basin, and the Tekeze river basin show no drought (Fig. 7b). On the other hand, a major portion of the Awash and Danakil river basins show severe drought during the main rainfall months of 1984 and 2011. These results confirm that the 1984 drought has occurred due to the deficit of rainfall during both smaller and main rainfall seasons, whereas in 2011 drought has occurred only due to the deficit of rainfall during smaller rainfall season. This finding coincides with the findings of previous studies at different parts of the mentioned river basins during 1984 and 2011 (Gidey et al. 2011; Suryabhagavan 2017; Bayissa et al. 2017). Besides, the spatial pattern of EDI identified drought during both smaller and main rainfall months of 1984 matches with the RHD maps of the selected river basins (Fig. 5a). Whereas in 2011, the spatial pattern of EDI identified drought matches with its corresponding RHD maps (Fig. 5b) only during smaller rainfall months. It indicates there is a discrepancy between the yearly RHD maps (Fig. 5b) and the monthly EDI identified drought patterns during the main rainfall seasons of 2011 (Fig. 7b). It is observed that in these two drought years, the drought severity is higher at Awash, Danakil, and Tekeze river basins. This is plausible because these river basins are usually drought-prone areas of the country and previous studies also identified as a drought-prone area of the country (Gidey et al. 2011; Viste et al. 2013; Bayissa et al. 2017; Suryabhagavan 2017).
On the other hand, as represented in Figs. 6c and 7c, during smaller and main rainfall months, the 2007 drought-free year is well captured by EDI except in some pocket areas (which shows only a mild drought) of the selected river basins. Hence, it can be concluded that CHIRPSv2 based EDI clearly identified both drought and non-drought years in the selected river basins.
SPI-based drought assessment
Temporal pattern of SPI identified drought
Here the seasonal SPI at two and four monthly intervals have been used to capture the drought events. This is because, at shorter time intervals (monthly SPI), the SPI values tend to fluctuate frequently above and below the zero-line depending on the monthly rainfall fluctuation (Ionita et al. 2016; Trenberth et al. 2014; McKee et al. 1993). Hence, the main and smaller rainfall months of the selected river basins drought was calculated using four months’ time interval SPI (further referred to as SPI-4). Therefore, here only seasonal SPI (SPI-4) based temporal drought assessment results were reported. Also, the discussions are restricted to the two reported worst historical drought years of 1984 and 2011 as well as one non-drought year (2007) (highlighted in the figures) in the selected river basins. Figure 8a and b show the seasonal time series plots of SPI values in selected river basins during smaller and main rainfall seasons respectively. In 1984, during smaller rainfall season (Fig. 8a) and the main rainfall season (Fig. 8b), SPI was able to correctly detect the RHDs as reported in Table 2.
However, in 2011 during the smaller rainy season (Fig. 8a) except in Awash and Danakil river basins and during the main rainfall season (Fig. 8b) except in the Tekeze river basin drought was not captured by SPI. Whereas, in 2007 during both smaller and main rainfall season SPI showed no drought. Overall, the obtained results indicate that the temporal pattern of SPI identified droughts during smaller and main rainy seasons of 1984, 2007, and 2011 agrees with the temporal pattern of EDI identified droughts of the same years. In addition, this study agreed with the findings of Mekonen et al. (2020) that stated drought tends to be more frequent and more severe in main rainfall season in north east highland of Ethiopia.
Spatial pattern of SPI identified drought
Figure 9a–f show that SPI identified spatial pattern of drought in the selected river basins during smaller and main rainfall seasons of 1984 and 2011 as well as for 2007 drought-free year. Mild to extremely severe drought was observed by SPI at Awash, Danakil, and Tekeze during the smaller rainy season (Fig. 9a) and main rainfall seasons (Fig. 9b) of 1984. Whereas in 2011 mild to severe drought was observed in most of the regions of the selected river basins during the smaller rainy season (Fig. 9c). These results are clearly in line with RHD maps (Fig. 5a–b) and correlate well with the EDI identified drought patterns in the selected river basins (Figs. 6 and 7).
Likewise, as shown in Fig. 9e and f, the year 2007 was identified as drought-free year except in some pocket areas (such as Awash, Danakil, and Tekeze) which shows mild drought, particularly during the smaller rainfall season (Fig. 9e). Similar to CHIRPSv2 based EDI, CHIRPSv2 based SPI clearly identified both drought and non-drought years in the selected river basins.
Temporal trends of EDI and SPI identified drought
In this section the temporal trends of the detected droughts across each river basin using both EDI and SPI were described. Figure 10a–f illustrates the time series/trend of EDI and SPI-4 across the main river basins of Ethiopia for the period 1981 to 2016. As indicated on the trend plot the two drought indices (SPI and EDI) show a positive association and both of them detected most of the RHDs such as 82, 84, 88, 91, 92, 94, 2003, 2005, 2008, 2010, 2011, 2012, 2013, 2014 and 2015, however with different severity level, while in the other years positive EDI and SPI values were observed which indicate non-drought years. As seen in Fig. 10a–f, the frequency and severity of drought were higher during the 1980th and 2000th, while during the 1990th it seems less frequent and severe. Studies by Senamaw and Addisu (2021) also confirm that the drought will increase at an alarming rate due to climate change for the future in North eastern part of the country.
In almost all river basins both indices detected maximum severity during 1984/85, 1991/92, 2003, 2013 and 2014. Indeed, these years were among the worst drought years in the history of Ethiopia (Bayissa et al. 2015; Edossa et al. 2010; Yisehak et al. 2021). Many studies also show that severe drought have been occurred in these years and caused substantial damage in terms of life and economic losses (Edossa et al. 2010; Gebrehiwot et al. 2011; Bayissa et al. 2015; Yisehak et al. 2021). However, among the mentioned river basins Denakil, Awash and Tekeze were frequently strikes by meteorological drought. The current study coincides with Gidey et al. (2018) on the increasing frequencies and persistence of drought. Their findings stated that drought frequencies, durations, and severity are higher in the lowland area than in the mid and highlands during the last 15 years. Besides, this result also coincides with the work of Lemma et al. (2017) across rainfall regime 1 and 2 of Ethiopia.