Ethiopian Precipitation Cycles

Sorry readers! It’s been a while since my last post, but whilst I’ve been away I have been taking a look at the extraordinary situation of Ethiopia, on the east coast of Africa (see Figure 1). The pressures facing Ethiopia’s agricultural production are extreme and highlight well the precarious relationship between food and water across much of Africa. I will therefore be  focusing on Ethiopia throughout my blog. This week I will discuss some of the cyclical water-related issues facing Ethiopian agriculture before highlighting long-term and anthropogenic problems posing a threat to Ethiopia’s food security in my next post. Future posts will then explore the potentially devastating impact of these cycles and changes on food availability, before taking a look at some of the solutions that have and could be employed to combat Ethiopia’s chronic food insecurity. 

 Figure 1: Location of Ethiopia 

                                       

Across Africa, the relationship between water availability and demand is complex (Taylor, in press) and Ethiopia is no exception. Increasing water demands must be accommodated by a resource that is subject to extreme temporal and spatial variations (Taylor, in press). Achieving food security can therefore pose a significant challenge and although irrigation has driven enhanced agricultural production in many regions across Sub-Saharan Africa (SSA), only 5.4% (200,000 hectares) of Ethiopia’s 3.7 million irrigable hectares are currently irrigated, contributing just 3% of the nation's food production. Clearly then, green (soil) water, deriving largely from rainfall, is an important resource for Ethiopian agriculture, sustaining almost all its food production. Similarly, green water accounts for 84% of global water use during crop growing season and sustains almost all food production in Sub-Saharan Africa. Changes in precipitation levels can therefore impact food security, as illustrated by the 1980s famines in Ethiopia, which were largely due to the disruption of green water stability as precipitation levels dropped.

There are various reasons behind climatic variability throughout Ethiopia, however the primary cause of intra-annual temporal precipitation patterns is the Inter Tropical Convergence Zone (ITCZ; Taylor, in press). The ITCZ is a zone of convergence that generates rainfall as a result of rising air at the thermal equator, which, following latitudinal variations in solar radiation as a result of the earth’s orbital tilt (see Figure 2), moves north and south throughout the year (see Figure 3; Taylor, in press). The movement of the ITCZ thus dictates intra-annual temporal variations in rainfall in Ethiopia, and generates three distinct rainy seasons: June to September (Kiremt), October to January (Bega), and February to May (Belg). However, 80% of Ethiopia's total annual rainfall comes during Kiremt as the ITCZ migrates northwards. 

   

    Figure 2: Movement of the thermal equator                            Figure 3: Range of the ITCZ

Ethiopia is the most mountainous region in Africa, as the Rift Valley bisects the country, and topography plays a significant part in the spatial distribution of this ITCZ induced rainfall. In plateau regions over 2,500 metres above sea level, annual precipitation is 1400 -1800 mm per year, mid-altitude (600 - 2,500 metres) areas receive 1000 -1400mm per year and lowland regions get less than 200mm of rainfall per year. Topography, therefore, dictates the varying agricultural production and methods within Ethiopia.  In areas of high elevation crop cultivation dominates, in areas of low elevation livestock rearing is the primary livelihood strategy and in between these two topographic and precipitation extremes, agro-pastoralists utilise a mix of the two livelihood strategies (Taylor, in press). Figure 4 illustrates the relationship between topography, livelihood strategy and population density whilst also revealing areas suffering from food insecurity, which we will explore in more detail next time. 

Figure 4: Relationship between topography, population density, livelihood strategy and food insecurity. 

The amount of rainfall Ethiopia receives during summer months is not, however, down to chance. Rather, inter-annual variabilities are largely driven by changing sea surface temperatures in the Pacific and Indian Oceans. These are known as the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD).

ENSO, which many of you will be aware of, involves the irregular inter-annual change in wind patterns within the tropical Pacific Ocean, with subsequent changes in water temperatures and rainfall patterns in tropical regions across the world. For those unfamiliar with the phenomenon, the video below nicely summarises its causes and effects (Figure 5). Ethiopia’s ITCZ induced summer rainfall is strongly influenced by ENSO related shifts in global wind and precipitation patterns. The warming of ocean surface temperatures in the eastern equatorial Pacific, known as El Niño, presents risks for Ethiopia’s food security as El Niño years are associated with below average rainfall. In evidence of this, historical drought years of 1965, 1972, 1983, 1987 and 1997 have been linked to El Niño weather events.

Figure 5: Video illustrating causes and effects of ENSO

Inter-annual rainfall fluctuations from ENSO cycles are exacerbated in Ethiopia due to the lesser known IOD. Although there is continued debate over the causes of the IOD, with some linking it to ENSO cycles (see Figure 6 for ENSO and IOD cycles since records began in the 1960s), there is agreement that changes in sea surface temperatures in the western Indian Ocean have a significant impact upon precipitation totals in East Africa. The westward oceanic temperature gradient observed in the upper Indian Ocean is driven by surface winds known as the Wyrtki jet and influences rainfall across East Africa. The weakening of westerly winds over the Indian Ocean during a positive cycle is associated with increased rainfall over Eastern Africa and has been linked to flooding events in 1961, 1994 and 1997 (see Figure 7). Whereas the uncharacteristic strengthening of westerlies across the equator during the negative IOD results in an enhanced oceanic temperature gradient and reduced rainfall in East Africa (see Figure 8). IOD cycles are therefore further driving inter-annual precipitation variations within Ethiopia, causing floods and, as is the case at the moment,  droughts. 

Figure 6: ENSO and IOD cycles

Figure 7: Positive IOD cycle

Figure 8: Negative IOD Cycle

These cycles strongly influence agricultural production within Ethiopia. However, anthropogenic stresses have become increasingly important for food and water management in the country. Next week, I will therefore be taking a look at these additional factors which, together with natural cycles, are putting immense pressure on Ethiopian food production.