AEZs: Kenya System

Introduction

Agro-ecological zones (AEZs) are land units defined on the basis of combinations of soil, landform and climatic characteristics. These zones consider not only the climatic factors but also other ecological factors such as soil properties, topography, and vegetation cover. Agro-climatic zones (ACZs) focus primarily on climatic factors as determinants of agricultural potential and suitability. These zones are primarily based on climatic variables such as temperature, rainfall, and growing periods.

 

 

Agro-ecological zones (AEZs)

Agro-ecological zoning (AEZ) refers to dividing an area of land into smaller units with similar characteristics related to land suitability, potential production, and environmental impact. An AEZ is a land resource mapping unit defined in terms of climate, landform, soils, and/or land cover, having specific potentials and constraints for land use (FAO, 1996). The key elements for defining an AEZ are the growing period, temperature regime, and soil mapping unit.

In Kenya, two systems are used to describe agro-ecological zones:

1. The FAO classification for tropics, which is a general classification system used worldwide.
2. An older Kenya-specific classification system applicable only in Kenya.

The Kenya system:

Kenya has a total area of about 582,646 square kilometers, with approximately 1.9% (11'230 km2) covered by water. The dry land mass is divided into seven agro-ecological zones, as shown in Table 1:

 

Table 1: Agro-ecological zones of Kenya

Zone	Approximate Area (km2)	% Total
I. Agro-Alpine	800	0.1
II. High Potential	53,000	9.3
III-IV. Medium Potential	53,000	9.3
V. Semi-Arid	48,200	8.5
VI. Arid	300,000	52.9
VII. Very arid	112,000	19.8
Rest (waters etc)	15,600	2.6

Source: Sombroek, et al., 1982.

Approximately 80% of the country lies in the semi-arid to very arid zones (ASALs), which are predominantly inhabited by pastoralists and agro-pastoralists. These areas, classified as rangelands, are unsuitable for rainfed cultivation due to physical limitations such as aridity and poor vegetation.

 

Agro-climatic zones of Kenya

Agro-climatic zones focus primarily on climatic factors as determinants of agricultural potential and suitability. These zones are primarily based on climatic variables such as temperature, rainfall, and growing periods. Agro-climatic zones provide insights into the specific climatic conditions required for different crops and agricultural practices. They are useful for understanding the climatic limitations and opportunities for agricultural activities within a particular region. Each zone is characterized by specific elevations, annual rainfall, and vegetation types that are adapted to the prevailing climate. Additionally, important grass and legume species found in the respective zone can also vary, as they have evolved to thrive under the specific climatic conditions of each zone. By considering these factors, agricultural planners and farmers can make informed decisions regarding suitable crop choices, agricultural practices, and the management of natural resources in different agro-climatic zones.

 

7 zones are classified based on a moisture index. Zones V, VI and VII have a moisture index of less than 50% where barely dry land crops can be produced. Zones I, II, III and IV have a moisture index of more than 50% as well as high rainfall and fertile soils, where there is a high potential for production of varied crops (Kogo et al., 2020).

Zone I Humid
This zone has no direct importance in agricultural production other than being the source of rain and some rivers/streams. It is confined to mountains and immediate surrounding such as Mt. Kenya and Mt Elgon. 

 

Zone II Sub-humid
This zone is generally restricted to the highlands of Kenya between 1980 and 2700 m and occurs as a forest or open grasslands. This zone is found in the surrounding of Mt Kenya (parts of Meru, Embu, Kirinyaga and Nyeri), isolated parts of the Rift Valley around Mau and Aberdares mountains (e.g around Kericho and Nyahururu respectively) and the surrounding of Mt Elgon (e.g around Kitale and Webuye). The minimum rainfall is 1000 mm. The main grasses are Pennisetum clandistenum (Kikuyu grass), Themeda triandra (Red oats), Andropogon Chrysostachyus, Andropogon pralonsia, Exotheca abysinica, Digiteria scalaram, Eragrostis lascantha, Seteria sphacelata, Pennisetum catabasis and Sporobolus filipes. The legumes include Trifolium johnstoni, Medicago sativa (Alfalfa or Lucerne), Sesbania sesban and Leuceana leucusephala.

Zone III Semi-humid
This zone occurs mainly at elevations between 900-1800 m with  annual rainfall between 950 and 1500 mm. Trees are numerous here and somewhat of shorter stature than in Zone II. This zone is the most significant for agricultural cultivation and several legume fodders are found here in crop-livestock systems. It is also the most resettled by human. It occurs in the vast parts of Nyanza, Western and Central provinces, good proportion of Central Rift-Valley (Nandi, Nakuru, Bomet, Eldoret, Kitale) and a small strip at the Coast province.  The major grasses are Hyperenia and Cymbopogon, Themeeda triandra, Panicum maximum, Seteria Sphacelata, Sporobolus pyramidalis, Bracharia brizantha (Congo signal), Bricharia siluta, Chloris gayana (Rhodes grass) and Cynodon dactylon (Star grass). 

Zone IV Semi-humid to semi-arid
This zone occupies more or less the same elevation (900-1800 m) as the previous or may be at times lower. However, it has lower rainfall of about 500-1000 mm. This is typically represented in surroundings of Naivasha, vast parts of Laikipia and Machakos districts vast parts of central and southern Coast Province. It is the home of most Acacia trees and shrubs including Acacia seyal,Acasia Senegal,Acacia brevispica, Acacia drepanolobiumand Acacia gerrardii. Euphobia trees occur in some drier parts of this zone. Combretum and Terchonanthus spp. are also common here. Grasses found include Themeda triandra, Pennisetum mezianum, Pennisetum straminium, Pennisetum massaiense, Eragrostis spp., Hyperenia spp. Seteria spp., Digiteria spp., Bothriochloa insculpta, Cenchrus ciliaris. Rare grasses include Chloris spp. and Cynodon spp. Besides acacia, other important legumes include Indigoferra and Crotolaria.

Zone V Semi-arid
This zone is much drier than Zone IV and occurs at lower elevations. Annual rainfall is 300-600. This Zone is prevalent in northern Baringo, Turkana, lower Makueni and vast parts of North Eastern Province. Low trees and shrubs found here include Acacia mellifera, Acacia tortilis,  Acacia horrida, Acacia reficiens, Acacia nubica, Acaia paslii, Acacia Zanzibarica, Adansonia digitata, Terminalia prunioides, Dobea spp., Dioppspyros spp. and Commiphora spp. Common grasses are Eragrostis superba, Cenchrus ciliaris, Cymbopogon spp., Bothriochloa spp. and  Heteropogon contortus. 

Zone VI Arid
This zone is considered as semi desert and is the driest part of Kenya. Annual rainfall is 200-400 mm and is quite unreliable. The zone is found in Marsabit, Turkana, Mandera and Wajir Districts.  Dominant in this zone are Acacia and Commiphora shrubs with scattered taller trees of Delonix elata, Acacia tortilis and Adansonia digitata. Balanites eagyptica, Boscia coriacea, Salvadora persica, Acacia mellifera andAcacia reficiens are important shrubs or low tree species. The very common and important dwarf shrubs are Indigofera spinosa and Sansevieria spp. Other important shrubs are Sericocomopsis, Barberia and Duosperma eromophylum. Being the most delicate zone both annual and perennial grasses are important here. Important grasses include Aristida adoensis, Stipagrostis hirtigluma are very characteristic and may occur as annuals or perennials. Other grasses also found here are Aristida mutabilis, Chrysopogon aucheri, Tetrapogon spp, Enneapogon cenchroides, Chloris roxburghiana. 

Zone VII Very arid
This is represented by Chalbi desert in Marsabit district. The Chalbi is a salt desert with very sparse salt bushes as the only vegetation found. It is vast and of beautiful scenery. Pastoralists use it as a source of mineral lick for livestock, particularly during the rainy season. 

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