BY DIRRIBA TESHOME
Ethiopia is located between latitudes of 3.8°N to 14.5°N and longitudes of 33°E to 48°E with an area of about 1.12 million km2. The varied topography of the country shows extreme changes in altitude with its lowest point at about 120 meters below sea level (Kobat Sink Afar Depression) and its highest point about 4,620 meters above sea level (Ras Dashen). These physiographic variations create a large difference in meteorological and hydrological condition both by time and space.
From meteorological point of view, there are three seasons in Ethiopia; Belg, Kiremt and Bega. Belg refers to February to May. It is the second rainy season for North East, East, Central and Southern Highland of the country.
On a recently held National Climate Outlook Forum (NCOF), Chali Debele, National Metrology Agency (NMA) Weather Forecasting Director said that much of the northeastern, central, southern, southwestern, eastern and southeastern parts of the country receive considerable amount of rainfall during this season.
The rainfall is highly variable and less predictable, due to its erratic rainfall nature. As well, it is the time of the year where daily maximum temperature to be pick.
Kiremt is the period from June to September which is the main rainfall season for most parts of the country except for the lowlands of southern and southeastern Ethiopia.
And Bega is from October to January which is mostly a dry season for most parts of the country except for southwestern as well as the lowlands of south and southeast Ethiopia.
As a whole, the mean annual rainfall amount ranges from 2400mm over south western to 500 and below over the northeastern and southeastern lowlands. Hydro meteorologically a rainy day is considered as the one with 2.5 mm of rain or more but in this publication a rainy day is one regardless of the amount, according to Chali.
Likewise, according to the report from NMA, in Ethiopia, water resources availability in terms of space shows a marked discrepancy when one goes from east to west.
The eastern part of the country compromise 7 catchments with only 11 percent of the water resource and while the west compromise 5 catchments with 89 percent of water resources.
The Bega 2020/21 seasonal outlook Hydro meteorological impact outlook over different river basins more or less is going on as forecasted by the agency.
It has been mostly a dry season for most parts of the river basin except Ogaden, lower and middle Wabishebele, middle and lower Genale Dawa as well as lower Rift Valley. Sunny and windy condition dominates across many river basins.
Thus, it has been increasing the loss of water by evaporation. Surface runoff has not occurred during Bega season. Hence, the flow of river water has become low.
Springs and ponds start to dry beginning from mid Bega season. Availability of water mainly decrease across upstream of main river basins. In some years unseasonal rain slightly fevers for water availability.
The selected analogue years
For the Bega 2020/21 season, the selected analogue years were 1995/6, 2005/6 and 2016/17 which were compared based on probabilistic seasonal forecast and viewed out on catchments-based map using geostatistical kriging method.
The Thornthwaite precipitation effectiveness index was used to calculate the Aridity index. It is computed from the monthly values of rainfall and evaporation.
The evaporation is computed empirically from mean monthly air temperature. In assessing the effectiveness of rainfall, in terms of water availability, relationships between the rainfall and air temperature have been worked out in terms of moisture indices.
The aridity indexes for the selected years were described in monthly basis for October, November, December and January. Based on the analogue years, during the month of October, most main river basins over Sothern and south western Ethiopia, Baro Akobo, Omo Gibe, Abay, Genale Dawa and upper Wabishebele have experienced humid to wet condition. This condition has positive impact for water resource developmental activities as shown below in figure.
Similarly, the wet moisture condition decreased from western and northern to southern catchments towards Bega benefiting catchments. During this month parts of Baro Akobo, lower Omo Gibe, and pocket areas of Rift valley and Genale Dawa experienced Humid to wet condition.
Differently, during December 2020, most of the catchments experienced dry condition index except parts of middle and lower Omogibe, Baro Akobo and pocket areas of lower awash, Abay and upper Waishebele catchments for analogue year 1995.
These areas obtained humid to sub humid conditions. This situation has negative impact on water availability especially for water resource developmental activities. Therefore, proper water resource management was required.
The computed Aridity Index for January 2021 for the selected analogue years except 1996, was completely dry. Humid to wet condition obtained during Jan 1996 over eastern Baro Akobo, western Awash and central lakes and rift valley catchments.
According to the Tercile probability, The Bega 2020/21 season is anticipated to be dry over Bega Benefiting catchments, we recommended to harvest and properly gained rain water. This dry Bega is forecasted to have a negative impact on water availability especially for the Bega Benefiting southern and southeastern catchments.
Due to a good rain of Kiremt all reservoirs and dams are at full capacity, therefore, “we recommend the responsible bodies to perform water resources managements for wise use of different developmental activities,” the agency endorsed in its report.
Based on the rainfall Tercile probability, as forecasted, most parts of Bega benefiting river basins have been under wet to very wet moisture condition. The North-western, western and South-western parts of catchments such as lower Tekeze, most part of Abay, Omogibe, BaroAkobo and upper and middle rift valley have got Normal Rainfall and unseasonal rainfall as anticipated.
This has positive impact over the North-western, western and South-western catchments especially for irrigation schemes and drinking water availability.
The Bega 2020/21 season is anticipated to be arid to sub humid over Bega Benefiting catchments. Therefore, we recommend the responsible bodies to carry out catchment management activities to control negative impacts such as flood and flash floods especially over Bega benefiting catchment, harvesting of gained water resources for different economic activities is highly recommended.
Equally, the agency has made forecasts for Belg that refers to the period from February to May. Western half of the country is anticipated to have normal to above normal rainfall. Eastern and Central including Southern high ground of Ethiopia will be under the domination of normal rainfall whereas, northeastern parts of the country are expected to receive normal to tending to below normal rainfall. South and southeastern Ethiopia are expected to get normal to below normal rainfall for the coming Belg season.
Equally, day time maximum temperature is expected to enhance over some low land areas. The delay onset while, the cessation is likely to late across Belg rain benefiting regions, Chali noted.
Generally, in view the current and projected La-Nina phenomena and variability nature of the Belg season, the rainfall is likely to experience erratic rain in the coming Belg season. In view of the projected climate scenarios, neutral ENSO Episode is likely to have the contribution of near normal rainfall of the Kiremt 2021. Therefore, it is important to pay attention to daily, weekly and ten days forecasts available in any means.
The Ethiopian Herald January 29/2021
