North eastern Ethiopia is one of the sensitive regions to climate variation particularly to temperature and rainfall changes. Rainfall and temperature are one of the most determinant climate patterns for the study area because more than 80% of the area’s agriculture is reliant on rain-fed. The main aim of this study was to investigate trends in temperature and rainfall in the annual, seasonal and monthly time scale at Kombolcha and Dessie Meteorological stations. The daily rainfall and temperature data were collected from National Meteorological Agency of Ethiopia. The Mann–Kendall test and Sen’s slope estimator were used to assess rainfall and temperature trends. The Mann-kendall test revealed that anon- significant decreasing rainfall trend was observed during February to March and September at Kombolcha and during January to March and September at Dessie stations. The rainfall trends showed decreasing in belg while increasing in both kiremt and bega season at both stations. However, the detected trends are non-significant. Both maximum and minimum temperature revealed that statistically significant decreasing trends in annual and seasonal time scale at Dessie station while at kombolcha station a significant increasing trend of maximum temperature was observed at both annual and seasonal time scale. On the other hand, anon- significant warming trend of minimum temperature was observed during annual and kiremt time scale but a non-significant decreasing trend was experienced during bega and belg season at kombolcha station. Increase in temperature and decrease in amount of rainfall may have a negative impact on crop production and soil water balance. Therefore, this study could have an important role in identifying possible present and future production strategies.
Keywords:Rainfall; Temperature; Trend analysis; Mann-kendall test
Temperature and precipitation are two most important climate parameters that are most studiedin climate research because of their immediate impact in various socioeconomic sectors (e.g. agriculture and hydrology), including human comfort (sayemuzzaman 2014). The changes in temperature and rainfall patterns are widely observed in many semi-arid parts of the developing world that are likely to become even hotter and dryer with time (Collier et al. 2008). Inter-annual variability of precipitation and rising trends of temperature has observed (Meze 2004, Seleshi & Zanke 2004). Developing countries in East Africa like Ethiopia may experience greater variability of precipitation and evapotranspiration . Principally, climate is one of the most important factors among the leading factors for vulnerability, in most parts of Ethiopia . Hence, Ethiopia has been described as one of the most vulnerable countries to climate change in Africa .
Climate change is manifested in terms of increasing temperature and change in intensity and pattern of rainfall . Increasing or decreasing of average annual temperatures and rainfall are most likely due to climate change . IPCC  also confirmed that a change in precipitation was due to change in climate. According IPCC  report, the mean global surface temperature is become increasing. Many authors argue that the causes of increasing global surface temperature may be the alteration of the hydrological cycle at global and local scales [8,9]. However, the change in rainfall and temperature patterns is not globally uniform . Many parts of the world, particularly countries in sub-Saharan Africa, are affected by global warming owing to the changing in temperature and precipitation patterns [3,11]. The rising temperature and variability in rainfall pattern have direct impact on crop production and food security . Literature states that the coming decades will have experienced in higher temperature and change in precipitation intensity, and this
may cause crop yield reduction in many countries in the world
The northern part of Ethiopia is affected by climate change
and variability [14,15], coupled with high rainfall variability
. In Ethiopia, the average annual minimum and maximum
temperatures have increased by about 0.25oC and 0.1oC every ten
years respectively . The higher temperature accelerates the
evapotranspiration process that creating moisture stress, and this
becomes more severe regarding yield losses if it occurs during the
canopy formation . Therefore, the main objective of this study
is to detect the trend, magnitude and inter-annual variability of
rainfall and temperatures over North Eastern Ethiopia.
The study was conducted at Kombolcha and Dessie sites in
North Eastern parts of Ethiopia. The Kombolcha and Dessie sites
are situated in 11.08°N, 39.72°E and 11.13°N, 39.14°E respectively.
The altitudes are ranged from 1842-1915 and 2470-2553m above
sea level for Kombolcha and Dessie sites, respectively (Figure 1).
Based on current studies, kombolcha and Dessie sites receive a
mean annual rainfall ranging from 725.1 to 1361.6mm and 851.3
to 1612.6mm respectively. The mean annual temperature varies
from 18.7 to 20.90c and 14.8 to 19.3 0c at Kombolcha and Dessie,
Daily rainfall data (1983-2019), Maximum and minimum
temperature (1985-2019) were collected from the National
Meteorological Agency of Ethiopia (NMA). Missing values were
patched using Markov chain simulation model of INSTAT v.3.36
. Quality control check was also done for maximum and
minimum daily temperature values by running a macro automatic
The daily observed climate data were arranged in day of year
(DOY) format for processing. The data was analyzed using INSTAT,
XLSTAT, 2016 and Excel spread sheet tools. Both descriptive and
Mann-Kendall trend test were computed in order detect the trends,
directions and magnitudes as well the inter-annual variability
of rainfall and temperatures. Descriptive statistical, such as
minimum, maximum, mean, standard deviation, and coefficient
of variation were computed using simple Excel spreadsheet,
whereas the trend, directions, magnitudes and significances were
determined using Mann-Kendall trend test, Kendall’s tau and
Sen’s slope estimator (Mann 1945) .
Mann-Kendall’s test is a non-parametric method, which is less
sensitive to outliers and test for a trend in a time series without
specifying whether the trend is linear or nonlinear (Partal 2006;
Where xj and xi are the sequential precipitation values
in months j and i (j> i) and N is the length of the time series. A
positive S value indicates an increasing trend and a negative value
indicates a decreasing trend in the data series.
The variance of S, for the situation where there
may be ties (i.e., equal values) in the x values,
is given by as:
Where, m is the number of tied groups in the
data set and ti is the number of data points in the ith tied group for n larger than 10.
The standard normal ZMK test statistic is computed as follows
(Partaland Kahya 2006)
The presence of a statistically significant trend is evaluated using
the ZMK value. In a two-sided test for trend, the null hypothesis Ho
should be accepted if | ZMK | < Z1-α/2 at a given level
of significance. Z1-α/2 is the critical value of ZMK from the
standard normal table.
The magnitude of trend is predicted by the Sen’s estimator.
The slope (Ti) of all data pair computed as (Sen 1968). This test is
applied in cases where the trend is assumed to be linear, depicting
the Quantification of changes per unit time.
Where Xj and Xk are considered as data values of time j and
k (j>k) correspondingly, the median of these N values of Ti is
represented as Sen’s estimator of slope which is given a
Positive value of Qi indicates an upward or
increasing trend and a negative value indicates
downward or decreasing trend in the time series.
The temporal variations of mean monthly, seasonal and
annual rainfall at Kombolcha and Dessie meteorological stations
are presented in Figure 2 and Table 1. Kombolcha and Dessie
stations have received the maximum rainfall about 280.4mm
and 318.7mm respectively during August and July (Figure 2).
Except the four months (Dec, Jan, Feb and Mar), all other months
received a considerable amount of rainfall in the two stations
(Figure 2). The annual total rainfall was varied from 725.1mm to
1361.6mm with a mean of 1037.6mm at Kombolcha station, while
from 851.3mm to 1612.6mm with a mean of 1191.7mm at Dessie
station in the last 3 decades.
Refer Table 2 for the corresponding values for Belg and Bega
seasons. The coefficients of variation for rainfall at Kombolcha
were varied moderately with CV of 14.8% in annual and19.75%
in Kiremt rainy season, while it was extremely varied with 43 and
68.3% during Belg and Bega season respectively. This result agrees
with the finding of [20,21]. Both stations were experienced with
bimodal rainfall pattern, where much of the rainfall concentrated
in the main rainy season (June-September) and a small amount
of rainfall occurred the second rainy season (February-May). The
season from October to January is a relatively dry season (Figure
Minimum temperature is high during June and July at
Kombolcha and Dessie areas respectively while that of low is
during November and December in the previous order. The
highest maximum temperature was occurred in the month of June
and the low was in the month of December and January at both
stations (Figure 3). The annual maximum temperature was varied
from 25.5℃ to 28℃ and 22.9℃ to 26.2℃ at Kombolcha and Dessie
stations respectively. Similarly the annual minimum temperature
was varied from 11.8℃ to 13.7℃ and 6.6℃ to 12.4℃ at Kombolcha
and Dessie stations respectively (Table 2). As far as temporal
seasonal variation is concerned, high maximum and minimum
temperature is recorded during Kiremt (June-September) season
for both stations. The coefficient of variation showed that both
maximum and minimum temperature is more variable at Dessie
station than Kombolcha (Table 2).
A summary of the monthly, seasonal and annual rainfall
trend analysis at Kombolcha and Dessie stations are shown in
Table 3 & 4. The rainfall trend is decreasing and non-significant
(P>0.05) during the month of February to April and September
at Kombolcha station. Similarly, at Dessie station, the trends were
decreasing and non-significant in the month of January to March
and September. The rainfall trend was significantly increase
by 2.3mm per month at Kombolcha station, while increased by
0.17mm per month during August at Dessie station (Table 4).
The seasonal rainfall trend was increasing by 4.2mm/season
at Kombolcha and by 2mm per season at Dessie station during
Kiremt (Table 4). Whatever there were increasing or decreasing
trend, the detected trends were not statistically significant in
most of the months and seasons over both stations. Insignificant
annual rainfall trends may be due to high inter-annual variability.
In general, the rainfall trend was increased during the Kiremt and
Bega season and it was decreased during the Belg season at both
stations. However no significant trend was detected for seasonal
rainfall distribution (Table 4).
The maximum temperature trend was significantly increasing
and decreasing at Kombolcha and Dessie station in all months
Significant increasing trend of maximum temperature
(0.09ºC/month) was observed during March and April while the
minimum temperature trend was increasing significantly during
April to November at Kombolcha station. On the other hand,
the minimum temperature trend showed in decreasing trend
throughout the months at Dessie station (Table 5).
Anon- significant minimum temperature trend was increasing
by 0.002 and 0.01ºC/season at Kombolcha station during annual
and Kiremt time scale respectively (Table 6). Solomon et al. 
come across with an increasing trend in annual maximum and
minimum temperature at Lake Tana by a factor of 0.141oc and
0.423oc per year respectively. In case of Belg and Bega season,
decreasing trend of minimum temperature was detected at
Kombolcha (Table 6). The mean annual and seasonal temperature
have shown a significant increasing trend at kombolcha station
(Table 6). This is supported by Woldeamlake & Conway  annual
and kiremt rainfall showed significant increasing trend at Dessie
and Labella whereas significant decreasing trend was observed at
Debre Tabor. On the contrary a significant decreasing of annual
and seasonal mean temperature was observed at Dessie station.
The magnitude of the increasing trend of the annual and Kiremt
maximum temperature compared to the minimum temperature
at Kombolcha areas indicates higher daytime evaporative demand
and therefore a higher water requirement for crops [23,24].
The trend of climatic variables with their fluctuation and
variability of rainfall and temperature in the kombolcha and
dessie sites were analyzed. The study area is vulnerable and
susceptible to climatic fluctuation and variability; the climatic
trend is more likely to result in an increase in the number and
severity of natural disaster. Rainfall and temperature are one of
the most determinant climatic patterns for the study area because
more than 80% of the study area’s agriculture is reliant on rain
fed. Increased temperature and changes in rainfall amounts will
increase occurrence of drought and flood events. Hence, increase
in temperature and decrease in rainfall amount may have a
negative impact on crop production and soil water balance.
Anon–significant decreasing trends of rainfall was observed
during February, March, April and September at Kombolcha while
during January, February, March and September at Dessie station
in the entire study periods. The annual rainfall was increased with
the rate of 0.1mm per year at Kombolcha and decreased by 0.56mm
per year at Dessie station. Although the increasing and decreasing
trends were observed, the detected trends were not statistically
significant for both stations regarding to rainfall. On the other
hand, the maximum temperature was increased significantly
at Kombolcha and decreasing at Dessie station for all months.
Similarly, the minimum temperature was decreased significantly
in all months at Dessie station. In the annual and seasonal time
scales, the maximum temperature has shown an increasing
trend at Kombolcha and decreasing trend at Dessie station.
Moreover, a non-significant increasing of minimum temperature
was observed by 0.002 and 0.01ºC/year at Kombolcha station
during annual and kiremt time scale respectively. The analysis
results provide further knowledge to improve our understanding
on climate variability and change and would be useful for future
planning and management of water resources safely in the study
area. In general, the study reveals decreasing rainfall amounts
and warming trends across kombolcha than dessie sites because
kombolcha is the hometown of different industries and factories
which causes for air prolusion. Hence, information generated by
this study could be beneficial to agricultural and water recourses
planning and management especially in semi-arid environment were adaptability to climate variability and change is still
low. Therefore, the concerned stakeholders should take into
consideration the rainfall and temperature changes of the study
area into their climate change adaptation strategy.
IPCC (2007) Climate Change 2007: Impacts, Adaptation, and Vulnerability Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel.
IPCC (2014) Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. In: Pachauri RK, Meyer LA (Eds.), IPCC, Geneva, Switzerlan, p. 151.
Parry ML, Canziani OF, Palutiko JP, VLinden V, Hanson CE (2007). Technical Summary. Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change.IPCC Technical report. Proceedings of the National Academy of Sciences, UK, Cambridge University Press. pp. 23-77.
Warrick RA, Gifford R, Parry ML (1986) CO2, climatic change and agriculture. In the Greenhouse Effect, Climatic Change and Ecosystem. In: Bolin B, Doos BR, Janger J, Warrick RA (Eds.), Wiley: Chichester, pp. 393-473.
Stern R, Rijks D, Dale I, Knock J (2006) Instat climate guide. UK: Statistical Service Center, University of Reading.
Kendall MG (1975) Rank Correlation Measures; Charles Griffin: London, UK.
National Meteorological Services Agency (2001) Report Submitted to Initial National Communication of Ethiopia to the United Nations Framework Convention on Climate Change (UNFCCC), Addis Ababa, Ethiopia.
Solomon S (2007) Climate Change 2007 - the Physical Science Basis: Working Group I Contribution to the Fourth Assessment Report of the IPCC (Vol. 4). Cambridge University Press, Cambridge, UK.