4.1. Annual and seasonal rainfall variability
Sidama region is characterized by a diverse climate, which ranges from dry and hot lowlands to wet and cold highlands. The area-averaged annual maximum temperature of the region is approximately 24.5°C, and it varies from 13°C to 34°C. The average annual precipitation in the region is about 1243 mm (Fig. 2). In the Central Rift Valley Region (CRVR) part of the Sidama region, Matewos and Tefera (2020b) found that annual rainfall is 1101.27 mm. The region receives rainfall for eight (7) months from April to October although there was a modest reduction in June. The region receives a high average monthly rainfall between July and October with the second season between April and June.
Figure 3 showed the spatial distribution of seasonal and monthly rainfall from 1991 to 2020 over the Sidama region. The rainfall starts in March in the central part of the region though the amount is less than 100 mm/month (see Annex 1). Most parts of the region receive rain during April and May with an increasing tendency toward the southern and southeastern parts of the region. These areas include the kebeles such as Teticha, Hula, Chirone, Dara, and Dara Otilicho in the southern tips of the region while Aroressa and Hokko (southeastern part) also receive the relatively optimum amount of rainfall during these months. The monthly rainfall amount reaches up to 200 mm in parts of the region during this period. In June, rainfall showed a decreasing pattern throughout the region and was limited only to central and southeastern highlands. Thus, the northwestern and southwest parts of the region such as Hawassa Zuria, Boricha, Darara, and Loka Abaya remain moisture deficient. Meanwhile, the results revealed that the rainfall increased and its distribution covered back the entire region starting from Mid of July to October. In Sidama, this season stays from July to October and it is locally called Hawado. Thus, during Hawado, the majority of the districts in the Sidama region receive modest rainfall except for Cabe Ganbeltu and Dara. Similarly, Matewos and Tefera (2020b) reported that the Kiremt season provided > 100mm/month rainfall consecutively for four months for the northwestern part of the region. The region received minimal (< 50 mm/month) rainfall between November and February, which is locally known as Arro are the regions located in the southeastern and northwestern. During this season, these areas receive limited precipitation. The region received rainfall in two seasons, Badheessa (March, April, and May) and Hawado (June, July, August, September, and October) (Fig. 3). The dry (Arro) season extends from November to February. Following conventional seasonal classification does not reflect the region’s reality because rainfall extends from March to October almost without any jump between. Therefore, we classified only into two: from March to May as Badheessa and Hawado between June to October.
Regarding zonal (rainfall values in latitude wise) precipitation distribution in the region, high precipitation (200 mm/month) was received between August and September in areas located between 6.4 and 6.8 North latitudes (e.g., Daarra, Hula, Teticha, Shafamo, and Arbegona). It also revealed a slight weakening of the amount of rainfall during November. This result revealed that the northern part of the region receives rainfall from only June to September (Fig. 4). Meridionally, the central highlands (e.g., Arbegona, Bursa, Hula) of the region received rainfall for a longer period than the eastern and western parts of the region. For example, between 32.4 east and 38.8 east, the region received rainfall from April to October (Fig. 4). However, the western and eastern edges of the region received maximal rainfall for a shorter period (July to September) compared with central longitudes. Eventually, rainfall stays longer in the southern part of the region (lower latitude) compared to its northern part. An area located between 38.6° East and 6.6° North was identified as the wettest in the region. These are the areas that receive the most rainfall over the course of the year. According to these results Bursa, Arbegona and Hula receive higher rainfall for a longer period compared with other kebeles in the region, yet verification using ground stations was not possible because stations with long-time climate data series are missing in these areas. Moreover, the region lacks similar studies to compare our results with.
The mean annual rainfall of the region is 1244 mm/year for 1991–2020. The mean for the dry northeastern part of the region, for example, Lokka Abaya, Boricha, and Hawassa Zuria reported having 1101.27 mm/year for 1983–2014 (Matewos and Tefera, 2020b). It is found that the central and eastern highlands receive up to 1700 mm/year from 1991–2020. The region receives an annual rainfall ranging from 800 to 1800 mm (Figure. 5). The distribution of annual rainfall over the region has shown local characteristics. Annual rainfall gradients corresponding to latitude/longitude values respective to each pixel were estimated for the region. The maximum and strong rainfall gradients are oriented along with the central and southern tips of the region which receives total annual rainfall exceeding 1700 mm (Fig. 5). Western and south-and north-eastern margins of the region receive the lowest total annual rainfall from 1991 to 2020.
4.2. Precipitation trends and anomalies
The annual rainfall anomalies presented in Fig. 6 revealed that there were variations in the amount of rainfall spatiotemporally. Many areas in the region have received lower than long-term average annual rainfall in 1991, 1994, 1995, 1999, 2001, 2002, 2003, 2012, and 2019. However, in 1992, 1993, 1997, 1998, 2005, 2006, 2008, 2011, 2014, 2016, and 2020 many places in the region received higher than long-term annual average rainfall. However, the years 1991, 1999, and 2002 were identified as the driest years recorded in the region. In 2016 and 2020 the region received extremely high rainfall, particularly in the northern and northwestern parts. However, annual rainfall in the remaining years showed a slight deviation from the long-term mean, which indicates the region had received nearly normal annual rainfall distribution during these years. These findings are consistent with other studies in the northeastern part of the region conducted by Matewos and Tefera (2020b), who identified 1999 and 2012 as severe drought years. High inter-annual rainfall variability, combined with precipitation deficit, resulted in long-term drought in several areas. Many studies on climate change noted that Ethiopia has experienced eight major droughts since 1980: in 1984, 1987, 1991, 1994, 1999 2002, 2012, and 2015 (Seleshi and Zanke 2004; Suryabhagavan 2017; Matewos and Tefera 2020a). Additionally, the year 2019 can be categorized as a drought year, particularly in our study area. On the contrary, few regions received rainfall having high regional variability exceeding the long-term mean which caused flash flooding in 2005, 2006, 2016, and 2020.
Figure 7 showed monotonic trends in annual and seasonal rainfall per pixel over the region and stipples on the plot indicate trend significances at P < 0.05. The mean annual rainfall over the region showed positive trends across the region with values ranging from 20 mm to 60 mm per decade. According to the findings, the annual pattern of rainfall revealed only a significant increasing trend (ranging from 20 to 60 mm decade–1) over the southeastern (Aroressa and Hokko) and southwestern (Darra) parts of the region. At the same time, rainfall data from the Hawassa weather station showed a tendency of increasing trend (18 mm decade–1) although the trend is not statistically significant during the study period (Ware et al. 2022). Similar to these results, Ayehu et al. (2021) reported increasing trends in annual (2.48 mm year− 1), summer (1.16 mm year− 1), spring (0.92 mm year− 1), and autumn (0.67 mm year− 1) rainfall and decreasing trend in the winter season (-0.15 mm year− 1) over the Upper Blue Nile area. Moreover, Alemayehu et al. (2020) reported statistically significant increasing trends in annual and Bega rainfall and a non-significant increasing trend in Belg in the Alwero watershed, western Ethiopia. On the contrary, Matewos and Tefera (2020b) found decreasing trends in annual, Kiremt, and Belg rainfall over the CRVR although it is statistically significant only in Belg. Besides, Belihu et al. (2018) reported significantly decreasing trends in annual rainfall in the Gidabo catchment in the Sidama region.
Seasonal trend analysis shows a non-significant (except for small areas) increasing trend in rainfall during both Kiremt (Hawado) and Belg (Badhesa) seasons in the region. Only the Hawado season revealed a significant increase in rainfall in the region's northwestern and south-eastern regions. However, we discovered a decreasing trend in the Arro (NDJF) season across the central and northern parts of the region, with only a few areas of the northern tip of Hawasa Zuria showing a statistically significant trend during this season. Unlike this result, a non-significant increasing trend is reported by Matewos and Tefera (2020b) during the season over the Central Rift Valley part of the region.
The spatial and temporal variability in annual and seasonal rainfall is assessed using a coefficient variation (CV). The CV for over 30 years (1991 to 2020) indicates that the variability in seasonal rainfall is much higher than that of annual rainfall variability (Fig. 8). The CV in annual rainfall ranges from 6 to 13% and in seasonal rainfall from 12 to 65%. The seasonal rainfall variability showed the CV ranging from 12 to 18%, 23 to 30%, and 50 to 63% for the Hawado, Badhessa, and Arro seasons, respectively (Fig. 8). The highest seasonal variability was observed in the Arro season and it is the most significant contributor to annual rainfall variability. As the finding of Matewos and Tefera (2020b) showed the CV in the Bega rainfall is 35%, which is higher compared to annual (13%), and Kiremt and Belg account for 23% of each in the CRVR.
4.3. Annual and seasonal temperature variations
The overall temperature of the region ranges between 8–28°C (Fig. 9). Northwestern parts of the region are hotter compared with the central and southeastern parts of the region. Regarding seasonalities in the temperature, January to May was hotter relative to June to August, which is colder in over the lowlands. The average temperature over the central Sidama highlands was less than 20°C throughout the year.
4.4. Trends in annual minimum and maximum temperature
Figure 10 shows the decadal monotonic trend in maximum and minimum temperatures (°C decade–1) significantly increased during the last three decades (1991–2020). Similarly, Matewos (2019) reported significantly increasing trends in temperature over the northeastern parts of the Sidama region. Our results revealed significantly increasing trends in both minimum and maximum temperatures across the region. However, the southeastern part of the region experiences higher temperature incidences much faster than the rest of the region, which is probably due to the alarming conversion of forest areas into agriculture, particularly in Aroressa, Chirre, Daeela, Gambeltu, and surrounding areas. Meanwhile, Hamesso (2018) conducted a survey study in three districts from different agroecological zones and concluded that the majority of respondents reported a reduction in rainfall and a rise in temperature. For the Gidabo catchment of the Sidama region, Belihu et al. (2018)found the Gidabo catchment is getting warmer from 0.03°C to 0.07°C year–1, which is similar to our finds. Additionally, the national climate change adaptation program of action (NAPA) states that the mean temperature has been increasing by about 0.37°C decad–1 (NMA 2007).
4.5. Anomalies in annual mean temperature
The mean temperature anomaly in the region indicated that the years are getting hotter from time to time. Most of the years in the first decade (1991–2000) were colder than the long-term mean for 1991–2020. The year 2019 was the hottest compared to the years considered for this study. The frequency of having hot years increases from time to time. These results go hand in hand with the rising global temperature (Hansen et al. 2006). As temperature anomaly refers to a departure from a reference value or long-term average, in this study the latter was applied. The positive anomaly indicated that the respective year is hotter than the long-term mean. On the other hand, the negative (blue colors) in Fig. 11 indicates years colder than the long-term mean. Therefore, the years 1992, 1993, 1994, and 1999 were considerably colder than the long-term mean during the study time. However, the years 2002, 2006, 2009, 2010, 2011, 2015, 2016, 2017, 2019, and 2020 were hotter than the long-term mean temperature of the region. This is probably due to the fact that changing climate leads to significant increases in temperature extremes (Esayas et al. 2018). Meanwhile, the number of cold days and nights is significantly decreased across all agroecologies in Southern Ethiopia (Esayas et al. 2018) eventually this leads to higher annual mean temperature compared to its long-term mean.