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Rainfall in Ethiopia is characterized by high spatial and temporal variability as a result of the topographic variation and geographical location [1-13]. Topographic highs play a major role in releasing the conditional thermodynamic instabilities of the moist incoming air into the strengthening convective developments, whereas precipitation patterns over topographic lows greatly depend on the strength of large scale rainfall producing systems, such as the Intertropical convergence Zone (ITCZ), the Tropical Easterly Jet, the creation of convergence zones and local convective systems [14]. When the windward side of the mountains is characterized with a rising moist air, the leeward side of the mountains is characterized with a descending warm dry air. Thus, rainfall activity on locations with the same altitude can differ depending on whether they are found over the windward or on the leeward side. As a result, one can observe great difference in the rainfall between the different parts of Bale Highlands which are on the same homogeneous types of rainfall [15-17].

In Ethiopia, an onset and cessation of seasonal rainfall vary considerably within few kilometers distance due to altitudinal variations, orientation of mountain chains and their physical influence on atmospheric flow [17].

Therefore, the objective of the current study was to examine the onset, cessation and LGP of seasons over Bale Highlands. There are limited attempt that made to employ appropriate scientific explorations on the seasonal onset, cessation and length of growing over Bale Highlands in particular. The present study is therefore; supposed to show the start, end date and length of growing periods of the seasons over Bale Highlands.

Ethiopia is characterized by three distinct seasons. These are locally known as Bega (October to January), Belg (February to May) and Kiremt (June to September). The rainfall pattern is also named according to their rainfall distribution [18-22]. These classifications don’t encompass the southern and southeastern lowlands of the country, which have a bimodal rainfall with rainfall periods from March to May and from September to November. Accordingly, there are two cropping seasons in Bale Highlands: Ganna (March to June) and Bona (July to December) seasons.

Bega

Bega is the dry season and covers the period from October to January. Bega season is mostly associated with hot dry days and cool nights. It is frosty in early mornings accompanied by occasional frost over most of the highland areas.

Belg

Belg is the small rainy period for most parts of Ethiopia except southern and southeastern low lands. It covers the period from February to May. Rainfall during the season is highly variable in time and space and high maximum temperature values are common [22]. From Belg season March, April and May months are the warmest months [18].

Kiremt

Kiremt is the main rainy season in which about 85% to 95% of the food crops of the country are produced [8]. It covers the period from June to September following the Belg rains and is associated with frequent rains and homogeneous temperatures mainly in July and August. The magnitude of rainfall is higher as compared to the other seasons for many parts of the country [22].

Background of the study area

Highlands of Bale is located between 6.45°N to 7.45°N and 39.47°E to 40.77°E in the southern parts of Oromia National Regional State of Ethiopia. The elevation of the area ranges from 1500 to 3500 meter above sea level [9]. The study area is bordered in the south by Mena, in the west by West Arsi Zone, in the north by Arsi Zone, in the northeast by Legehida district, in the southeast by Guradamole district of Bale zone and in the east by Sewena district. The highest point in Bale Highland is Mount Tuludimtu (4377 m and Batu Mount (4307 m), Based on the topography Bale is divided into Dega (Highland), Waine Dega (mid lands), and Kola (low lands). Most of these districts are characterized by bimodal rainfall types. The average rainfall is about 590 mm in the Ganna and 560 mm in Bona season. In the study area, barley and wheat are highly produced [23-31]. Various studies indicate that the study area is dense or populous because the region is highly fertile and suitable for agricultural activities [20]. The current study is mainly focused on the Highlands (Figure 1) parts of Bale Zone.

Figure 1: Map of the study area (Bale highlands).

Observed meteorological data (Data sources)

All historical meteorological data were collected from National Meteorological Agency (NMA) of Ethiopia Bale Robe Meteorological Branch Directorate. The dataset was daily rainfall and the rainfall data was collected from 12 meteorological stations that govern by National Meteorological Agency in Bale Highlands.

Data analysis

Twelve selected stations were categorized as Central, Eastern, Western, Southern and Northern parts of Bale Highlands. The variability of seasonal rainfall was analyzed for its onset, end date, LGP. Statistical packages like the mean, standard deviation and coefficient of variation were determined and interpreted based on Australian Bureau of Meteorology rainfall index (Australian Bureau of Meteorology, 2010). For this study, the determining onset, end date and LGP was performed by adapting definition from the study by Stern et al. [29]. Accordingly, the day with accumulated rainfall of 20 mm over three consecutive days that were not followed by greater than 9 days of dry spell length within 30 days from planting day was said to be onset date. To determine the end of the growing season or rainy season, the stored soil water and its availability to the crop after the rain stops was a very useful criterion [29]. The end of the rainy season adopted in this case was defined as any day after the first of September/October/, when the soil water balance reaches zero [29]. In addition, a fixed average 3.5 to 5 mm of Evapo-transpiration per day, and 100 mm/meter of the maximum soil water holding capacity of the area [13] were considered. Given the above definitions, Instat Statistical programme Version 3.37 [29] was used for analysis using January to December calendar and in INSTAT climatic guide [29], threshold value 0.85 and 60 mm/day was used. The LGP was determined by subtracting onset date from end date.

Statistical methods

This method is based on statistical principles that describe random variation of a set of observations of process and they focus on the observations themselves rather than on the physical process which produced them. Statistics is a type of description but not causality because it assumes that the data is from pure random process [7]. The sample estimate of population parameters are given in the following equations [7].

(1)

Where, = is sample mean n=is number of observation in individual observation For i = 1, 2, 3…n

The variance (S2) and standard deviation (√S2) respectively are given as;

(2)

(3)

Similarly, the coefficient of variation CV is given as;

(4)

Results and Discussion

Onset Belg season over Bale Highlands

The first attempt of producing onset and cessation of the small season of Ethiopia were made by Tesfaye Haile in 1989. Onset of Belg season is difficult to define because of highly erratic characteristics. In Bale highlands Belg onset is differ from place to place. As Bale highland is partly Belg rain-benefiting areas, the Belg season is defined as the time when the first wet-spell of at least three days total rainfall amount of 20 mm or more occurs in March, provided there will not be continuous nine or more dry spell or dry days in the subsequent 30 days of the area [19].

The rains in April over eastern and southeastern regions are associated with the seasonal southward advance of the equatorial trough Inter-Tropical Convergence Zone (ITCZ) [19]. The Onset of Belg season over Bale Highlands was in the 3rd dekade of March but gradually advances eastward from western and southern parts of Highlands. The results presented in Table 1 revealed that, on average, the small rainy season (Belg) starts over Bale Highlands on March 28^th day of the year (DOY, 88). Starts of the season begins March 25th (DOY, 85) for Central Bale Highland, March 29^th (DOY, 89) for Eastern Bale highland, March 24^th (day DOY, 84), for Western Bale highland, March 26^th (DOY, 86) for Northern Bale Highland and March 31^st (DOY, 91) for Southern Bale Highlands.

Table 1: Mean onset, standard deviation and CV of Belg season from 1985 to 2014 over Bale Highlands.

In the previous studies by Messay [19] the regularity onset and cessation over the study area is not clearly cited due to less meteorological station but the region consider as a Belg-growing areas and the onset and cessation period was in average 1^st dekade of March and 1^stdekade of June, respectively. Table 1 shows that variability day of the mean onset days of the growing season. The mean standard deviation of onset day of Belg season over Bale highlands was 19 days. It is highly variable, with standard deviation across the highlands ranging from 14 to 35 days. South parts of the highlands have the highest onset variability with standard deviation of 35 days, whereas Eastern parts of the highlands have the lowest onset variability with standard deviations 14 days. Central, Western and Northern parts of the highlands have onset variability with standard deviation 17, 19 and 21 days, respectively.

The mean Coefficient of Variation (CV) of onset Belg season over Bale Highlands was 22% which is moderate variation. The Central, Eastern, Western, Northern and southern parts of Bale Highland experienced Coefficient of Variation (CV) of 20%, 16%, 23%, 24% and 38%, respectively (Table 1). Overall, start of the season ranges between March 24^th to 31^st days or 3^rd dekade of March as shown in Table 1, which indicate that start of the season does not vary significantly over Bale Highlands, this result agrees with the findings of Korecha and Barnston [17], which stated that Bale Highlands possess one Homogeneous Rainfall Regime.

Onset Kiremt season over Bale Highlands

Indicated that over Bale Highlands the onset of Kiremt start’s from June 25t^th [26]. The Onset of Kiremt season starts from the first month on the seasons that is June and defined the time when the first wet-spell of the first month of the seasons for Kiremt June month occurs at least three days total rainfall amount of 20 mm or more, provided there were no continuous nine or more dry spell or day days in the subsequent 30 days.

The mean onset of the main growing season (Kiremt) which is consistent with the previous work of Segele and Lamb [26] was July 1^stover Bale Highland which represent by Robe station only for their study. In this study 12 meteorological stations were used. On the average the main rainy season (Kiremt) starts on July 12 (DOY, 194) over Bale highlands which is similar to the Segele and Lamb [26] in Robe starts on 1^st dekade of July. But Kiremt season onset over Bale Highlands was gradually advances eastwards from Western and Northern parts of the highlands. As a result of Table 2 starts of kiremt season originates July 14^th (day of the year (DOY, 196), for Central parts of Bale Highlands, August 7th (DOY. 220), for Eastern parts of Bale Highlands, June 26^th (DOY, 178), for Western parts of Bale Highland, June 27^th (Day of the year (DOY, 179) for Northern parts of Bale Highland and July 18^th (Day of the year (DOY, 200) for Southern parts of Bale Highlands.

Table 2: Mean onset, standard deviation and coefficient of variation of Kiremt onset from 1985 to 2014 over Bale Highlands.

The mean onset in practicability and regularity within the region under kiremt season over Bale Highlands was July 1st (DOY, 183) over of south in eastern Ethiopia that represented by Robe station [26]. Table 2 showed that there was a slight agreement the onset of season over the Bale Highlands. Table 5 shows that the variability day of the onset of the main growing season (Kiremt) less than Belg season. The mean standard deviation of Kiremt season onset over Bale highlands was 16 days. Its variability’s, with standard deviation across the highlands ranging from 13 to 26 days. Southern and Eastern parts of the highlands have the highest onset variability with standard deviation of 26 days, whereas Western parts of the highlands have the lowest onset variability with standard deviations 13 days. Central and Northern parts of the highlands have onset variability with standard deviation 22 days.

The mean Coefficient of Variation (CV) onset of Kiremt season over Bale Highlands was 8% which is less variation. The Central, Eastern, Western, Northern and southern parts of Bale Highland experienced Coefficient of Variation (CV) of 11%, 12%, 7%, 12% and 13%, respectively (Table 2). Overall, start of the season ranges between June 26^th to August 7^th days or 3^rd dekade of June to 1^st dekade of August (Table 2) which indicate that start of the season does not vary significantly over Bale Highlands; this result coincides with the findings of Korecha and Barnstorn [17], which stated that Bale Highlands possess one Homogeneous Rainfall Regime.

Belg season Cessation over Bale Highlands

The end of rainy season is obtained when a dekadal rainfall amount is less than half of the corresponding reference evapo-transpiration (ETo) at the end of rainy season. In the previous studies there is no exact cessation of Belg season for Bale highlands due to less Meteorological stations data.

The mean end date of Belg (small rainy) season for Bale Highland was on June 17^th (DOY, 162). Table 3 has mention as Belg season cessation advances East-north-westward from south-central parts of Bale Highlands. South and Central Bale Highland has the earliest cessation on the first decade of June (June 3 and June 7, respectively), followed by eastern, northern and western parts of Bale Highlands, which have cessation in 2nd dekade of June (June 11, June 13 and June 15, respectively).

Table 3: Mean cessation, standard deviation and Coefficient of Belg cessation from 1985 to 2014 over Bale Highlands.

The mean cessation of the small growing season (Belg) is less variable (19 days) temporally than the mean onset, with standard deviation of 17 to 32 days (Table 3) over Bale Highlands. The highest variability of the mean cessation occurs over Northern, south and western parts of bale Highlands which have standard deviation of 30, 31 and 32 days, respectively, while the lowest variability is over eastern parts of Bale Highland, where the standard deviation is 18 days.

The mean Coefficient of Variation of cessation of the small rainy season (Belg) over Bale Highland is less variable which is 12 days shown in Table 3. The highest variability of the mean CV of cessation occurs over southern parts of bale Highlands which has 21 days and it is under moderate variation, while the lowest variability is over eastern parts of Bale Highland, where the coefficient of Variation is 11 days.

Kiremt season Cessation over Bale Highlands

According to Segele and Lamb [26] over Bale highland the Cessations of Kiremt season is from Oct-15 to Oct-26 from eastern to western of the highland, respectively. The mean value of end date of Kiremt (main rainy) season over Bale Highland is October 31^st (Day of year (DOY, 305). The main rainy season cessation advances eastwards from northern and southern parts of Bale Highlands in November 6 (DOY, 311), October 27^th (DOY, 301), October 28^th (DOY, 302), respectively.

The mean cessation in practicability and regularity within the region under kiremt season over Bale Highlands was October 28 (302) over of south in eastern Ethiopia that represented by Robe station. [26]. Table 4 showed that there was a slight agreement in the cessation of season over the Bale Highlands. The mean variability of cessation of the main rainy season (Kiremt) is less variability with standard deviation is 12 days over Bale Highlands. The highest variability of the mean cessation occurs over south and western parts of Bale Highlands which have standard deviation of 18 days, while the lowest variability is over northern parts of Bale Highland, where the standard deviation is 13 days.

Table 4: Mean cessation, standard deviation and coefficient of variation for Kiremtcessation from 1985 to 2014 over Bale Highlands.

The mean Coefficient of Variation of cessation of the main rainy season (Kiremt) over Bale Highland is less variable which is 4% shown in Table 4. The highest variability of the mean CV of cessation occurs over southern and western parts of Bale Highlands which has 6% and it is under less variation, while the lowest variability is over northern parts of Bale Highland, where the coefficient of Variation is 4% days.

Length of the growing period (LGP) in Belg season over Bale Highlands

The average value of length of growing period (LGP) for small rainy season over Bale Highlands was 73 days. It has less special variability with values ranging from 68 to 83 days. Western part of Bale Highland has the longest mean growing season 83 days, followed by north, east and central parts of Bale Highlands (78, 75 and 74, respectively). South part of Bale Highland has the shortest mean growing period in Belg season (i.e. 68 days) shown in Table 5.

Table 5: Mean length of growing period of Belg and Kiremt seasons over Bale Highlands.

Length of the growing period (LGP) in Kiremt season over Bale Highlands

The average value of length of growing period during main rainy season over Bale Highlands was 110 days cited in Table 5. The length of growing period has variability with values ranging from 91 to 126 days. Western part of Bale Highland has the longest mean growing season 83 days, followed by north, central and south parts of Bale Highlands (123, 110 and 102 days, respectively). East part of Bale Highland has the shortest mean growing period in Kiremt season (i.e. 91 days) shown in Table 5.

The mean variability of the mean length of growing period in both Belg and Kiremt season were shown in Table 5. Belg season mean LGP standard deviation ranges from 21 to 38 days in Bale Highlands. The mean LGP standard deviation of Belg season over Bale Highland is 26 days. Eastern and Southern portion of Bale Highlands have the lowest variability with standard deviation of 21 and 22 days, respectively, whereas Central, western and northern parts of Bale Highlands have the highest variability (standard deviations of 32, 33 and 38 days, respectively).

Similarly, Kiremt season mean LGP standard deviation ranging from 23 to 30 days in Bale Highlands. The mean LGP standard deviation of Kiremt season over Bale Highland is 19 days. Eastern and Southern portion of Bale Highlands have the highest variability with standard deviation of 30 days, whereas Central, western and northern parts of Bale Highlands have the lowest variability (standard deviations of 24, 23 and 25 days, respectively).

The mean coefficient of variation of mean LGP in both Belg and Kiremt season were shown in Table 5. In Belg season the mean LGP coefficient of variation ranging from 28 to 48% in Bale Highlands. The mean LGP coefficient of variation of Belg season over Bale Highland is 36%. Eastern and Southern portion of Bale Highlands have the lowest variability with coefficient of variation of 28 and 32%, respectively that is from moderate to high variability, whereas Central, western and northern parts of Bale Highlands have the highest variability (coefficient of variation of 43, 40 and 48%, respectively) that is shown in Table 5 as the highest variability.

Likewise, Kiremt season mean LGP coefficient of variation ranging from 18% to 33% in Bale Highlands. The mean LGP coefficient of variation of Kiremt season over Bale Highland is 17%. Eastern and Southern portion of Bale Highlands have the highest variability with coefficient of variation of 33% and 30% that is the highest variability, whereas Central and northern parts of Bale Highlands have the moderate variability (coefficient of variation of 22% and 20%, respectively).

This study presents analysis of latest characteristics of seasons and seasonal distribution for both Belg and Kiremt season over Bale highlands. After a thorough examination of the daily rainfall patterns for 12 stations in the study area, the current study has been able to objectively determine the onset and cessation and LGP of the Belg and Kiremt rainy seasons of Bale Highlands. It was found that there is more variability in the onset of the Belg rains than the cessation of Belg and onset of kiremt rain season.

Analysis of seasonal feature variability is very crucial for planning and management of agricultural practices in Bale Highlands. The mean standard deviation and CV values of onset seasons as 35 days and 38%, respectively in the southern parts of Bale Highlands during Belg season is higher than kiremt seasons mean standard deviation ad CV values 26 days and 13%, respectively over Bale highlands. The high variation was observed in areas with high number of mean standard deviation and CV values of Cessation of seasons in Belg season than Kiremt season. The mean CV value of LGP in Belg season is higher than Kiremt season. Seasons with CV value of >30% was persist in southern parts of Bale Highland for onset of Belg season. The CV value of LGP < 30% in eastern parts of Bale Highland for Belg season and >30% in eastern parts of Bale highlands for Kiremt season. The average onset dates of small rainy season in Bale Highland in DOY was found to be 88 (March 28), with CV of 22%; whereas for end dates of small rainy season was 162 (June 10) with CV of 12%. The mean LGP of Belg and Kiremt seasons were 73 days, 110 days with CV of 36%, 17% for Bale Highlands. The result is also in line with the works reported by Hoefsloot [13].

Belg is less likely to be operational for rain-fed agriculture discretely in Bale Highlands. Hence long-term strategy should be designed to select short cycle and/or drought resistant crops or support the season with additional water resources (irrigation/water harvesting). Mostly, the agricultural activities appropriate the length of the two rainy seasons. Hence, the output of the agricultural activities depends on the performance of these seasons. The small rainy season is used for sowing short and long cycle crops.

Information about the start of the rains is important for appropriate agricultural decision making so that reduction in yield of the crops is avoided by taking appropriate action. During those seasons, over Bale Highlands receives small and large rain lived. But, agriculturally small rainy and main rainy seasons are important over the study area. A delay or deficient Belg or Kiremt rainfall means absence of water and pasture, which may result in deaths of thousands of peoples and animals. Kiremt and Belg season rain is extremely important from agricultural economic point of view.

The short in LGP and the variation of onset and cessation of the seasons will be considered by the concerned body and this body would be assess the local community by aware creation them on the onset and cessation as well as Length of growing period for both seasons. It is more likely to deviate from the rain-fed agricultural practice (using Belg and Kiremt season) to use irrigation system.

The starts of the season, end of the seasons and length of growing periods of the seasons will investigate related with ENSO and IOD phenomenon over Bale Highlands.

First and foremost, thanks to almighty God for granting me His limitless care, love and blessings all along the way. I would like to thank NMA Bale robe branch Directorate and Cordex African regions for providing me Meteorological GCM data for my study area. I would like to acknowledge the invaluable input of all individuals and organizations, in particular those who provided materials and information and who helped me a lot during the data entry, quality control period: Mr. Fitsum Bekele, Mr. LaekmariamYohanes. I am highly indebted to all staff members of Bale robe Meteorology directorate for their ideal and technical supports. Thanks are also to all my friends who give me their comments, ideas, shared love and happiness.