3.1.1. Annual and seasonal rainfall patterns
Mean annual rainfall of Alwero watershed is 1665.5 mm (Table 1), and its inter-annual variability is low with a CV of 8.7%. The Kiremt and Belg rainfall also show low inter-annual variability (CV, 9.2% and 19.7%, respectively). Bega rainfall shows moderate variablity (CV, 24%). As indicated by the PCI values, rainfall shows moderate concentration in few months of the year (11.8). It can be observed that above average mean monthly rainfall is recorded from May to October. These months have recorded above 130 mm rainfall (Fig. 2). About 80% of annual rainfall is concentrated in these six months which ranges from 11% (October) to 14.4% (August). August supplies the largest amount of rainfall in the watershed. July and September also represent high contributions to the annual totals.
As elsewhere in Ethiopia, Kiremt rainfall contributes the largest to the annual rainfall (56%), followed by Belg rainfall (25%). Unlike other parts of the country, the contribution of Bega rainfall is high (19%) as well. The highest monthly rainfall is 395.9 mm which contributed 22% of annual and 89% of Belg rainfall totals, respectively (Table 1). Unlike other areas of the country where high concentration of rainfall is recorded in the month of August followed by July, the highest monthly rainfall is recorded in May which is the dry month in most parts of the country.
Annual and Bega rainfalls show statistically significant increasing trends at p = 0.01 level. Kiremt and Belg rainfalls show no significant trends. These results contradict the findings of Shanko and Camberlin (1998), NMA (2001; 2007), Osman and Sauerborn (2002), Seleshi and Zanke (2004), Verdin et al. (2005), Cheung et al. (2008), Jury and Funk (2013), Urgessa (2013), Viste et al. (2012), Wagesho et al. (2013), Addisu et al. (2015), Alemayehu and Bewket (2017a), Asaminew et al. (2017) and Haile et al. (2019) who reported declining trends of annual and seasonal rainfall totals in their respective study areas in different parts of the country.
Regarding monthly rainfall trends, November and May showed statistically significant increasing trends at p = 0.05 level. October shows significant increasing trend at p = 0.1 level, and March shows significant decreasing trend at p = 0.1 level. June to September show non-significant increasing trends. December, January and February showed statistically non-significant increasing trends. March contributes the highest to the overall declining trend of annual rainfall while November makes considerable contribution to the overall increasing trend of annual rainfall.
Table 1
Trends of annual and seasonal rainfall in Alwero watershed.
Parameter
|
Amount (mm)
|
Contribution (%)
|
CV
|
LT
|
Wettest
year
|
Driest year
|
HMR
|
PCI
|
Annual
|
1665.5
|
|
0.087
|
66*
|
2011
|
1986
|
365.9
|
10.8
|
Kiremt
|
932.9
|
56
|
0.092
|
0.63
|
2003
|
1995
|
|
|
Belg
|
410.8
|
25
|
0.197
|
20.27
|
2014
|
1987
|
|
|
Bega
|
321.9
|
19
|
0.241
|
45.09**
|
2008
|
1984
|
|
|
* = Significant at 0.05 level; ** = Significant at 0.01 level; LT = linear trend (mm/10 year) |
In Ethiopia the driest year is 1984, which is the well-known drought year. The wettest year is 2006, which is the major flood year. The country has witnessed large scale losses of life and property during those periods. However, the driest and wettest years for Alwero watershed are 1986 and 2011, respectively. The wettest year for Kiremt is 2003, which is a recovery period from the second major drought period affecting large parts of the country. The years 2008 and 2014 are wettest periods for Bega and Belg seasons, respectively, while the dry years for Bega and Belg respectively are 1984 and 1987 (Table 1).
Figure 3 shows standardized annual and seasonal rainfall anomalies. Since 1983, annual rainfall showed declining trend up to 1990 except for the year 1988. Conversely, standardized rainfall anomalies are positive from 1990–1999 except the years 1994 and 1995. Annual rainfall shows considerable inter-annual variations between 2000 and 2009. From 2010 to 2016 standardized rainfall anomalies are positive except the year 2013. In the watershed, positive and negative anomalies account for 53% and 47% of the total observations, respectively. About 29% of the total number of observations is under the different drought categories. The 1990s is wet compared with the 1980s and 2000s. The results of this study are consistent with Bewket and Conway (2007) and McSweeney et al. (2008) who concluded the 1980s was drier than its preceding decade and the decades following.
Negative anomalies in Bega rainfall were observed from 1983–1995 except the years 1988 and 1992. Similarly, negative anomalies are observed from 2002–2007 except the year 2006, which is the major flood year in the country. Positive anomalies are observed from 1996–2001 and 2008–2016 except the year 2013. The proportion of positive and negative anomalies is equal and represents 50% of the total observations. The 2000s is wetter than the preceding decades for Bega.
Kiremt rainfall shows considerable inter-annual variations throughout the period of observation. Large proportion of negative anomalies is observed in Kiremt rainfall (56% of the total observations). Relatively, the 1990s is wet compared with the 1980s and 2000s. Drier conditions for Belg rainfall were experienced for the period 1986–1992 except the years 1989 and 1991. Wetter conditions for Belg were experienced for the period 1993–2000 except the year 1995 and 1998. Since 2007, positive anomalies are observed except the years 2010 and 2012. Positive and negative anomalies account for 56% and 44% of the total observations, respectively. The 1980s is the driest decade for Belg.
Standardized monthly rainfall anomalies were also computed for the wettest and driest periods to assess the abnormality of wetness and dryness in the watershed. As shown in Table 1, the driest and wettest years over the period of observation are 1986 and 2011, respectively. In the driest period the highest and the lowest SRA values are 1.45 and − 1.25, respectively, while the highest and the lowest SRA values during the wettest period are 1.26 and − 1.19, respectively. The highest and the lowest SRA during driest years are observed during June and January, respectively. May and February observed the highest and the lowest SRA values during the wettest years. About 67% of observations showed below average rainfall during the driest period, but only 25% of the observations fall under the different drought categories. About 50% of the observations showed below average rainfall during the wettest period. From these, nearly 33% fall under the different drought categories. This shows that rainfall concentration is not uniform and few months remain dry even in years of good rain conditions.
Figure 4 and Table 2 show the spatial distribution of annual rainfall over the period of analysis. East and southeast part of the watershed receive high amount of annual rainfall. More than half of the watershed receives annual rainfall of 1550–1700 mm. Another large proportion of the watershed (26%) receives annual rainfall of between 1411 and 1550 mm. More than 20% of the watershed receives annual rainfall of between 1700 and 1850 mm. About 1% of the watershed receives annual rainfall of > 2000 mm. The spatial distribution of Belg and Bega seasons revealed similar pattern in which rainfall increases from west to east. Almost all parts of the lower sub watershed receives Kiremt rainfall > 950 mm over the period of observation. The northern part of the upper sub watershed receives Kiremt rainfall of 768–850 mm.
Table 2
Rainfall distribution in Alwero watershed.
Rainfall Class
|
Upper (eastern) sub-watershed
|
Lower (western) subwatershed
|
Alwero watershed
|
Area (ha)
|
%
|
Area (ha)
|
%
|
Area (ha)
|
%
|
1411–1550 mm
|
77962.5
|
28.6
|
54650.0
|
22.7
|
132612.5
|
25.8
|
1550–1700 mm
|
87243.8
|
32.0
|
180487.5
|
75.0
|
267731.3
|
52.2
|
1700–1850 mm
|
68800.0
|
25.3
|
5431.3
|
2.3
|
74231.3
|
14.5
|
1850–2000 mm
|
32131.3
|
11.8
|
0.0
|
0.0
|
32131.3
|
6.3
|
2000–2104 mm
|
6318.8
|
2.3
|
0.0
|
0.0
|
6318.8
|
1.2
|
Total
|
272456.3
|
100.0
|
240568.8
|
100.0
|
513025.0
|
100.0
|
3.1.2 Trends in annual and seasonal rainfall in the upper (eastern) sub-watershed
Mean annual rainfall of upper sub-watershed is 1686.2 mm with coefficient of variation of 8.9% which shows low inter-annual variability. Kiremt and Belg rainfalls show low inter-annual variability of 9% and 19%, respectively. Only Bega rainfall shows moderate inter-annual variability with coefficient of variation of 24%. Kiremt rainfall contributes the largest to the annual rainfall (55%), which are followed by Belg (26%) and Bega (19%), respectively. The driest and wettest periods over the period of observation are 1986 (1419.3 mm) and 2016 (1987 mm), respectively. Annual rainfall shows statistically significant increasing trend at p = 0.05 level, while Bega rainfall also shows statistically significant increasing trend at p = 0.01 level. Belg and Kiremt rainfall show statistically non-significant increasing and decreasing tendencies, respectively.
More than 85% of the upper sub-watershed receives annual rainfall of between 1411 and 1850 mm. The other 12% of the upper sub-watershed receives annual rainfall of between 1850 and 2000 mm. The remaining 2% of the upper sub-watershed receives annual rainfall of > 2000 mm (Fig. 4 and Table 2).
3.1.3 Trends in annual and seasonal rainfall in the lower (western) sub-watershed
Mean annual rainfall of the lower sub-watershed is 1600.6 mm. Annual rainfall shows low inter-annual variability (10.9%). Kiremt rainfall also shows low inter-annual variability (12.5%). Belg and Bega rainfalls show moderate inter-annual variability with coefficient of variation of 29% and 32%, respectively. Kiremt rainfall contributes the largest to the annual rainfall (67%). Belg and Bega contribute the remaining 19% and 14% of annual rainfall, respectively. The driest and wettest periods over the period of observation are 1995 (1179 mm) and 1991 (1857 mm), respectively. Annual and Kiremt rainfalls show statistically non-significant increasing trend, while Bega rainfall shows statistically significant increasing trend at p = 0.01 level.
Large part of the lower sub-watershed (75%) receives annual rainfall of between 1411 and 1550 mm, and about 23% receives annual rainfall of between 1550 and 1700 mm. A small portion of the lower sub-watershed receives annual rainfall of between 1700 and 1850 mm (Fig. 4 and Table 2).
Annual rainfall shows considerable inter-annual variations from 1983–2009 Positive annual rainfall anomalies accounts for 59% of observations. The 2000s is the wettest decade. Positive anomalies are observed since 2007 except the year 2009. The 2000s is the driest decade for Belg and Bega rainfall. The proportion of positive and negative anomalies for Belg and Bega rainfalls are 44% and 56% of the observations, respectively. Positive anomalies for Kiremt rainfall account for 53% of observations. The 1990s is the wettest decade.