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Chapter 3
Land resources

The land resources inventory brings together two layers of information on physical environmental resources (climate and soil) and allows the creation of unique ecological land units (agro-ecological cells) within which soil, landform and climatic conditions are quantified. This information, compiled at the national level by province and district, constitutes the inventory of the physical land resources.

To create a computerized inventory of land resources, the individual climate and soil inventories have been compiled in map form at 1:1 million scale, and digitized.

The climatic resources inventory consists of three seperate thematic layers: thermal zones, length of growing period zones, and pattern of number of length of growing period zones. The Exploratory Soil Map of Kenya (KSS 1982a), forms the soil base, providing information on soils, landform and geology/parent material.

Six additional layers (1:1 million scale) of information have also been digitized and overlaid on the land resources inventory. These layers provide information on cash crop zones, forest zones, parkland areas, irrigation schemes, tse-tse infestation areas, and province and district boundaries.

The individual map layers have been digitized using the Comprehensive Resource Inventory and Evaluation System (CRIES 1983) a GIS developed at Michigan State University. The digitized information derived from the individual map layers has been converted to a data base of 576,072 grid cells. Each cell (one millimeter square) corresponds to 100 ha.

Subsequent to digitizing the individual layers of the land resources inventory, the soil mapping unit composition of each mapping unit and the associated ecological conditions have been incorporated.

The make-up of the national land resources data base is schematically presented in Figure 3.1, and is described in Technical Annex 1. A map of provinces and districts is presented in Figure 3.2, and their extents are given in Table 3.1.

3.1 Climate Resources

Temperature and water are the major climatic factors that govern crop distribution (both in space and time). In combination with solar radiation, these climatic factors condition the net photosynthesis and allow plants to accumulate dry matter (and to accomplish the successive development stages), according to the rates and patterns which are specific to cultivated plants.

FIGURE 3.1
Make-up of tend resources data base

FIGURE 3.1

FIGURE 3.2
District map of Kenya

FIGURE 3.2

TABLE 3.1
Extents of Districts and Provinces of Kenya

CodeName'000s hapercent of total
101Kiambu256.70.45
102Kirinyaga138.60.24
103Muranga254.60.44
104Nyandarua333.40.58
105Nyeri339.00.59
 Central Province1 322.32.30
    
201Kilifi1 262.52.19
202Kwale814.41.41
203Lamu654.01.14
204Mombasa25.40.04
205Taita1 735.23.01
206Tana River3 939.26.84
 Coast Province8 430.914.63
    
301Embu271.20.45
302Isiolo2 542.14.41
303Kitui3 037.25.27
304Machakos1 463.12.54
305Marsabit7 087.012.31
306Meru976.71.70
 Eastern Province15 377.326.69
    
401Nairobi75.40.13
 Nairobi75.40.13
    
501Garissa4 408.77.65
502Mandera2 617.84.55
503Wajir5 722.99.93
 North Eastern Province12 749.422.13
    
601South Nyanza588.51.02
602Kisii214.20.37
603Kisumu217.40.38
604Siaya247.20.43
 Nyanza Province1 267.32.20
    
701Baringo1 066.01.81
702Elgeyo Mar.263.00.46
703Kaijado2 151.93.74
704Kericho478.10.83
705Laikipia927.51.61
706Nakuru742.71.29
707Nandi281.40.49
708Narok1 799.13.12
709Samburu2 051.13.57
710Trans Nzoia246.70.43
711Turkana6 586.811.43
712Uasin Gishu382.60.66
713West Pokot535.70.93
 Rift Valley Province17 512.630.40
    
801Bungoma319.30.56
802Busia191.40.33
803Kakamega361.50.63
 Western Province872.21.52
    
 KENYA57 607.2100.00

The growing period has been used as a framework for the assessment of climatic resources (FAO 1978–81). It is defined as the period in which temperature and moisture permit crop growth. Prevailing temperature regimes have been inventoried by identification of thermal zones in order to take into account temperature requirements of crops (including pasture and fuelwood species).

The inventory of climatic resources allows:

  1. A differentiation of the country into reference thermal zones, reflecting the geographical and ‘seasonal’ distribution of the prevailing temperature regimes.

  2. A differentiation of the country into reference length and pattern of growing period zones, reflecting the prevailing moisture regimes including the year-to-year variations.

  3. A quantification of potential yields (of crops, livestock and fuelwood) that can be attained under constraint-free conditions.

  4. An assessment of various agro-climatic constraints to take into account yield losses likely to occur.

The climatic data bank, growing period analysis and the creation of the climatic resources inventory are described in Technical Annex 1.

3.1.1 Climatic Data Bank

The climatic data1 bank compiled for the assessment consists of three data sets. Data set 1 (Historical Data, from Jaetzhold and Kutsch 1980) consists of the following information for 437 stations:

-   Average decadal (10-day total) potential evapotranspiration (mm)

-   Historical decadal rainfall (mm) for individual years.

1 The primary source of the climatic data is the Kenya Meteorological Department.

Data set 2 (Average climatic data, from FAO/AGPC data bank, FAO 1977) consists of 45 stations with mean monthly values for the following 11 climatic parameters:

-   Precipitation (mm)

-   Mean daily temperature (°C)

-   Maximum temperature (°C)

-   Minimum temperature (°C)

-   Day-time temperature (°C)

-   Night-time temperature (°C)

-   Mean water vapour pressure (mbar)

-   Mean wind velocity (m sec-1)

-   Hours of bright sunshine as a percentage of maximum possible sunshine hours (%)

-   Solar radiation (cal cm-2 day-1)

-   Potential evapotranspiration (mm).

Data set 3 (Average climatic data, from Kenya Soil Survey) provides average data on the following for 1489 stations:

-   Annual daily temperature (°C)

-   Annual potential evaporation (mm)

-   Annual potential evapotranspiration (mm)

-   Annual Rainfall (mm)

-   Monthly rainfall (mm)

-   Type of rainfall pattern; monomodal (M), bimodal (B) or trimodal (T).

Extracts of the three data sets are presented in Tables 3.2, 3.3 and 3.4 respectively. The complete climatic data bank is available on diskettes (ASCII).

3.1.2 Growing Period Model

The definitions and model used to quantify the reference length of growing period have been described in Technical Annex 1. The growing period is the time period when moisture supply exceeds half potential evapotranspiration; it includes the time required to evapotranspire up to 100 mm of soil moisture storage2. The calculation of the reference growing period is based on a water balance model, comparing rainfall with potential evapotranspiration. The length of growing period (and the number of growing periods and dry periods per year) from a climatic viewpoint alone, and independent of crop, soil and landform, is therefore quantified in a reference manner (Kowal and Kassam 1978; Doorenbos and Kassam 1979; Kassam, van Velthuizen, Higgins, Christoforides, Voortman and Spiers 1982; Brammer, Antoine, Kassam and van Velthuizen 1988).

2 The computer program is able to handle a storage term in the range 0–250 mm.

Two types of growing periods are schematically shown in Figure 3.3. The distinction between ‘normal’ and ‘intermediate’ is useful because in the latter it is unlikely that full water requirements can be met during the rainy season without moisture conservation or a supply from groundwater or irrigation.

Two more growing period types have been identified (Figure 3.3). These are: (a) all year-round humid with rainfall exceeding full potential evapotranspiration throughout the year, and (b) all year-round dry with rainfall not exceeding half potential evapotranspiration throughout the year.

3.1.2.1 Length of Growing Period (LGP)

Mean length of growing period and frequency distribution for each individual group of years have been computed. Where there are more than one length of growing period per year, the total mean length as well as the individual mean lengths (e.g. two, three) and their frequency distribution are calculated (Figure 3.4).

TABLE 3.2
Extract agroclimatic data bank - Data set 1 - Historical data

STATION: EMBUNUMBER: 9037008 LAT: 0.32° SLONG: 32.27° EALT: 1410 FT40 YEARS' RECORDS
  JanFebMarAprMayJunJulAugSepOctNovDecYear
AVERAGE PETDECAD 140.843.244.840.037.634.432.832.837.640.836.836.8 
DECAD 242.443.245.637.637.633.632.032.839.241.635.236.8 
DECAD 342.444.044.037.636.833.632.034.440.040.036.038.4 
 MONTH125.4130.4134.4115.2112.0101.696.8100.0116.8122.4108.0112.01375.0
RAINFALL (1927)DECAD 11.80.029.020.047.30.38.12.96.60.019.314.5 
DECAD 20.02.0167.864.519.152.22.17.00.058.368.116.8 
DECAD 30.01.552.375.33.10.30.82.84.182.88.90.0 
 MONTH1.83.5249.1159.869.552.811.012.710.7141.196.331.3839.6
RAINFALL (1928)DECAD 115.60.040.910.2101.116.20.02.60.06.440.721.9 
DECAD 239.10.00.0140.2103.01.16.70.00.00.0108.20.0 
DECAD 30.00.07.180.462.13.80.05.65.117.349.20.0 
 MONTH54.70.048.0230.8266.221.16.78.25.123.7198.121.9884.5
RAINFALL (1929)DECAD 151.60.00.012.466.70.810.90.032.81.0108.7102.9 
DECAD 26.60.066.512.47.97.914.30.027.234.0104.538.0 
DECAD 36.40.05.1153.514.32.012.55.80.080.84.612.3 
 MONTH64.60.071.6178.388.910.737.75.860.0115.8217.8153.21004.4
RAINFALL (1930)DECAD 10.028.776.557.866.912.50.010.72.626.7122.842.9 
DECAD 20.00.020.6109.728.30.07.66.15.628.494.728.0 
DECAD 343.00.080.7107.35.46.60.833.02.566.153.91.0 
 MONTH43.028.7177.8274.8100.619.18.449.810.7121.2271.471.91177.4

TABLE 3.3
Extract agroclimatic data bank - Data set 2 - Average climatic data

COUNTRY: KENYA STATION: LOKITAUNGNUMBER: 63610 LAT: 4.15°LONG: 35.45E ELEVATION: 730 m
 JanFebMarAprMayJunJulAugSepOctNovDecYear
PRECIPITATION (mm)121753119472232137133228395
TEMPERATURE (°C; AVERAGE)28.028.528.226.026.626.625.725.726.826.827.126.826.9
TEMPERATURE (°C; MEAN MAX.)33.233.833.231.031.031.030.530.531.631.632.131.631.8
TEMPERATURE (°C; MEAN MIN.)22.723.223.221.022.122.121.021.022.122.122.122.122.1
TEMPERATURE (°C; MEAN DAY)29.830.430.027.828.128.127.527.528.528.528.928.528.6
TEMPERATURE (°C; MEAN NIGHT)26.126.626.424.224.924.924.024.025.125.125.225.125.1
VAPOUR PRESSURE21.122.723.023.022.520.820.520.120.121.623.022.021.7
WIND SPEED (at 2m ELEV.)3.74.34.02.93.33.73.83.74.14.14.34.03.8
SUNSHINE (%)84817578838376838884768181
TOTAL RADIATION530548545552550535515557587563510510541
EVAPOTRANSPIRATION1881841981571711711691781941911741732148
COUNTRY: KENYA STATION: LODWAR NUMBER: 63612 LAT: 3.07° LONG: 35.37°E ELEVATION: 515 m
 JanFebMarAprMayJunJulAugSepOctNovDecYear
PRECIPITATION (mm)15827562762310292116220
TEMPERATURE <°C; AVERAGE)28.829.830.229.629.729.028.228.529.329.829.028.629.2
TEMPERATURE (°C; MEAN MAX.)35.536.536.035.034.834.133.033.334.835.234.634.534.8
TEMPERATURE (°C; MEAN MIN.)22.223.124.324.324.624.023.523.523.824.523.522.623.7
TEMPERATURE (°C; MEAN DAY)31.232.232.331.631.530.930.030.231.331.831.030.731.2
TEMPERATURE (°C; MEAN NIGHT)26.527.428.127.727.827.226.526.627.327.927.026.327.2
VAPOUR PRESSURE17.017.019.322.022.020.319.519.518.718.719.019.019.3
WIND SPEED (at 2m ELEV.)2.62.83.02.82.82.62.42.83.03.22.82.42.8
SUNSHINE (%)84817578838376838884768181
TOTAL RADIATION537553546550545529509553587567516517542
EVAPOTRANSPIRATION1971952051861881711671861982081781702249

TABLE 3.4
Extract agroclimatic data bank - data set 3 - Average data

Station codeStation name°Lat (N/S)°Long (E)Alt (ft)Average Annual DataAverage Monthly Rainfall DataYrsRP type
TempEoPETPJanFebMarAprMayJunJulAugSepOctNovDec
9439000Kilindini4.0339.39642621681734105928135617126210668646688835450B
9439001Kwale Agr. Dept.4.1139.2812942420751662108934196015922798816467991027960B
9439002Mombasa Old.4.0439.41532621691735119326166219931311386676887995781B
9439003Ramisi Ass. Sug.4.3139.2550262175174014262318782713591551218261841037139B
9439004Gazi Kenya Sug.4.2539.3015026216117291350232470256347150100866879895850B
9439005Waa4.1039.376826217517401289281071240340118287712290966910B
9439008Mrere Works4.1239.2465026216117291049262052109255102717069104987323B
9439009Changamwe4.0239.3820026215317221093815471823089460717096717123B
9439010Msumbweni Hosp.4.3039.306226217115731376201662293356142111796693865230B
9439013Vanga Mudir's4.4039.1340262171173711342619861982748879675186986236B
9439014Gazi Mudir's4.2839.2920262175174013882118692753771331088166861045035B
9439015Kwango Mudir's4.0839.19650262193175481420335211915055584939841055027B
9439016Tiwi Disp.4.1439.3530262179174312902517582593279895828681936925B
9439019Mombasa Met. St.4.0339.39522621691735120226176319632012089646487966054B
9439020Ros Serani4.0539.4130262172173712411013422133589284928099976111B
9439021Mombasa Air.4.0239.37185262155172410543419631672317065677894976926B
9439023Bamburi4.0039.4315262173173811460715228253891505881118461013B
9439024Mkomani4.0339.4150262170173696921014317883117716191911312B
9439025Kinango Pump.4.0939.25400262193173893034215515117167645248811018524B
9439026Kisauni4.0239.405026217817411096208402362758898626365806111B
9439027Mwangulu4.2539.07400272243179483924277411213548545947701226712B
9439028Ndavaja4.1539.10500272239179181032216110913447676442761035422B
9439029Makuja4.1039.3440026215117219472913471542515774514775985111B
9439030Muhaka4.2039.3115026217317381129231950204260104937959931054019B
9439031Mrore I4.1339.251335242081166598734194815019576637352871177322B
9439032Mrore II4.1539.231160252108168610403626471612198383735299738812B
9439033Mrore III4.1739.267202521391711109413265315423793968350130758410B
9439034Mrore IV4.1739.2198025213217061077302645162219849679501017710810B
9439038Waa Disp.4.1039.35100262172173810702415421912661077962608099451BB
9439040Mombasa Rest.4.0339.397026216717341246541271267241110866785811056710B
9439041Mombasa Fields4.0339.40552621691735114547165823923311485617573875711B
9439043Simba Hills4.2239.258002521201696129044137122327010610481661271226319B
9439044Kikoneni4.2839.17500262172173812422514922092341171078970841247716B
9439045Kimansi W.4.0339.395026217017361257281662192369114876872681016078B
9439046Vangalunga4.3339.07200272205176491131217714617661514438721237120B
9439050Changamwe4.0139.37100262164173186031474687211658004283109592B
9439051Timbwani4.0739.4050262175174081410631542281306941382524412B
9439054Mwena School4.2939.0825027224717987522111756299907244389983583B
9439057Puma Camp4.0639.1460027222617814420012641276464223022552B

Stations 8535201 to 8940003 latitude North and stations 9034001 to 9439057 latitude South.

Temp: Mean daily temperature (celcius)

Eo: Potential evaporation (mm)

PET: potential evapotranspiration (mm)

P: Rainfall (mm)

RP type: Rainfall pattern type

M = Monomodal,

B = Bimodal,

T = Trimodal

FIGURE 3.3
Schematic presentation of types of growing periods

FIGURE 3.3

FIGURE 3.4
Number of growing periods and dry periods per year

FIGURE 3.4

TABLE 3.5
Patterns of growing periods (LGP-patterns) - Historical profiles of occurrence of number of length of growing periods per year

CodeLGP-patternProportion
(%)
11100
2H - 160:40
31 - H70: 30
41 - H - 265: 20: 15
51 - 2 - H65 : 20: 15
61 - 265:35
71 - 2 - 350: 35 : 15
81 - 3 - 240 : 35 : 20
91 - 2 - D40 : 35 : 25
101 - D - 240 : 35 : 25
111 - D60:40
122100
132 - 170: 30
142 - 1 - H55 : 30 : 15
152 - 1 - 355 : 25 : 20
162 - 375: 25
172 - 3 - 160:25: 15
182 - 3 - 450: 30: 10
192 - 1 - D70: 15: 15
203 - 260:40
213 - 2 - 150: 35: 15
22D100

H = 365+ days (i.e. year-round humid)
D = zero days (i.e. year-round dry)

TABLE 3.6
Relationships between mean total dominant and mean total associated lengths of growing period

LGP-PatternRelationship
1 - 2L2 = 80.40 + 0.75 L1
1 - 2 - H 
1 - H - 2 
1 - 2 - 3L2 = 71.56 + 0.66 L1
1 - 3 - 2L3 = 77.14 + 0.66 L1
1 - 2 - D 
1 - 2 - D 
2 - 1L1 = -86.09 + 1.28 L2
2 - 1 - HL3 = 25.29 + 0.82 L2
2 - 1 - 3 
2 - 1 - D 
2 - 3L3 = 30.11 + 0.83 L2
2 - 3 - 1L1 = -98.72 + 1.35 L2
2 - 3 - 4L4 = 114.54 + 0.58 L2
3 - 2L2 = 45.05 + 0.80 L3
3 - 2 - 1L1 = -9.86 + 0.88 L3

L1 = Total length of one growing period per year
L2 = Total length of two growing periods per year
L3 = Total length of three growing periods per year
L4 = Total length of four growing periods per year

For a group of years with one length of growing period, the length is designated the code L1, and the dry period is coded Dl (Figure 3.4a). For a group of years with two lengths of growing periods per year, the lengths are coded L21 and L22, and the first length(L21) is followed by the first dry period (D22) and the second length (L22) by the second dry period (D22) (Figure 3.4b) The sum of lengths L2 and L22 is coded L2. For a group of years with three lengths of growing periods per year, the lengths are coded L31, L32 and L33, and there are dry periods in between (D31, D32, D33) (Figure 3.4c). The sum of lengths L31, L32, L33 is coded L3.

3.1.2.2 Pattern of Length of Growing Period (LGP-Pattern)

To inventory the year-to-year variation in the number of lengths of growing periods per year, a historical profile is compiled showing groups of years each with a different number of growing periods per year. The proportional representation of each group in the total historical series is computed.

This information represents the pattern of growing period. Twenty two patterns are recognized in the climatic resources inventory. The patterns of number of length of growing period and their composition are presented in Table 3.5.

The pattern of growing period code represents the number of growing periods per year in order of frequency of occurrence, e.g. in the pattern coded as 2-1-3, the numeral 2 represents the number of lengths of growing periods per year (i.e. two)that occur in the majority of the years (i.e. 55 percent) - the dominant length number; the numeral 1 represents number of lengths of growing periods per year (i.e. one) that has the next most commonly occurring frequency (i.e. 25 percent) - the first associated length number; and the numeral 3 represents number of lengths of growing periods per year (i.e. three)that has the smallest occurrence (i.e. 20 percent) - the second associated length number.

For each pattern of growing periods, the mean total length of the dominant number is correlated with the mean total length of the associated numbers. Also, when the mean total length is a summation of more than one mean length, the latter are again correlated with the former. These relationships are presented in Tables 3.6 and 3.7.

TABLE 3.7
Relationship between individual component mean length and mean total length of growing period

LGP-PatternRelationship
2L21 = -1.11 + 0.55 L2
1 - 2L21 = 4.94 + 0.62 L2
1 - 2 - H 
1 - H - 2 
1 - 2 - 3L21 = 5.87 + 0.64 L2
1 - 3 - 2L31 = 22.12 + 0.39 L3
1 - 2 - DL32 = 1.58 + 0.32 L3
1 - D - 2 
2 - 1L21 = -5.48 + 0.64 L2
2 - 1 -HL31 = 0.14 + 0.46 L3
2 - 1 - 3L32 = -0.98 + 0.33 L3
2 - 1 - D 
2 - 3L21 = -3.05 + 0.61 L2
2 - 3 - 1L31 = 1.68 + 0.43 L3
2 - 3 - 4L32 = -3.00 + 0.34 L3
 L41 = 26.35 + 0.34 L4
 L42 = -20.88 + 0.38 L4
 L43 = -17.66 + 0.27 L4
3 - 2L21 = -2.33 + 0.63 L2
3 - 2 - 1L31 = 5.62 + 0.45 L3
 L32 = 1.25 + 0.31 L3

L21 = First length of two growing periods per year
L31 = First length of three growing periods per year
L32 = Second length of three growing periods per year
L41 = First length of four growing periods per year
L42 = Second length of four growing periods per year
L43 = Third length of four growing periods per year

In the climatic inventory map of Kenya, only the mean total dominant length has been inventoried on the map. The relationships in Table 3.6 are further presented in terms of length of growing period zones in Technical Annex 1; giving the mean total dominant (mapped) and the corresponding mean total associated (unmapped) lengths of growing periods. Similarly the relationships in Table 3.7 are further presented in terms of length of growing period zones in Technical Annex 1; giving the mean total length of growing period zones and the corresponding individual component lengths of growing periods.

3.1.2.3 Variability of Length of Growing Period

In addition to the frequency distribution mentioned in Section 3.1.2.1, coefficient of variation was calculated to allow a comparison of the variability in the mean length of growing period, and to take into account the likely losses in production. An aggregate relationship is given as follows.

Mean length of growing period (days)Coefficient of variation {%)
< 30> 50
30 – 5950
60 – 8945
90 – 11940
120 – 14935
150 – 17930
180 – 20925
210 – 23920
240 – 26915
270 – 29910
> 299< 10

3.1.2.4 Intermediate Lengths of Growing Periods

From the frequency distribution information (Section 3.1.2.1) occurrence of intermediate lengths of growing periods was quantified by relating P/ET ratio and moisture excess values with length of growing period.

The P/ET ratio for the intermediate lengths of growing periods of less than 150 days corresponds to values in the range 0.70 – 0.75.

The relationship which exists between the individual length of growing period and occurrence of intermediate periods is shown here.

Mean length of
growing period
(days)
Occurrence of intermediate
periods {%)
< 30100
30 – 5965
60 – 8925
90 – 11910
120 – 1495
> 149< 1

3.1.3 Thermal Zones

To identify thermal zones, temperature criteria corresponding to the requirements of crops (including pasture and fuelwood), were taken into account (Technical Annexes 3, 4, 5, and 6)1.

1 Field crops, pasture and fodder grasses and legumes, and fuelwood tree species have been classified into temperature-photosynthesis adaptability groups. Four temperature adaptability groups are recognized for field crops, four for pasture and fodder grasses and legumes and two for fuelwood species (each with three productivity classes).

To cater for differences in temperature requirements of crops in the compilation of the country inventory, commensurate with the scale of the assessment (1:1 million), thermal regimes have been defined based on 2.5 °C intervals. A thermal difference of 2.5 °C corresponds to an altitudinal change of some 385 m, thus allowing a sufficiently fine matching of crop thermal requirements to prevailing thermal conditions as inventoried2.

2 Data from meteorological stations above 150 m altitude conform closely to the following relationship between average daily temperature in degree Celcius (T) and altitude in metres (A): T = 30.2 – 6.496 ( A /1000 ). Temperature seasonality effects of latitude are minor due to the equatorial position of Kenya.

For Kenya nine reference thermal zones have been recognized as shown here.

3.1.4 Area Inventory of Climatic Resources

The area inventory of thermal zones, pattern of growing period zones and length of growing period zones, by district was prepared at 1 : 1 million scale.

Thermal
zone
code
Mean daily
temperature
range (°C)
Altitude (m)
1> 25.0< 800
222.5 – 25.0800 – 1200
320.0 – 22.51200 – 1550
417.5 – 20.01550 – 1950
515.0 – 17.51950 – 2350
612.5 – 15.02350 – 2700
710.0 – 12.52700 – 3100
85.0 – 10.03100 – 3900
9< 5.0> 3900

The mapped inventory was compiled by:

  1. plotting the individual station data of temperature, pattern of length of growing period and mean total dominant length of growing period1; and

  2. constructing boundaries of thermal zones, pattern of number of length of growing period zones, growing period zones and isolines of mean total dominant length of growing period with the values 0, 30, 60, 90, 120, 150, 180, 210, 240, 270, 300, 330, 365 and 365+ days respectively, delineating the mean total dominant length of growing period zones of 0, 1–29, 30–59, 60–89, 90–119 120–149, 150–179, 180–209, 210–239, 240–269, 270–299, 300–329, 330–364, 365- and 365+ days.

1 The individual component lengths of both the dominant (mapped) and associated length of growing period zones are presented in Technical Annex 1.

In addition to normal extrapolation techniques, extensive use was made of landsat images, climatic maps, vegetation maps, land use maps, topographic maps and soil maps to guide the delineation of boundaries and isolines.

The three climatic inventories in map form were digitized, and the digitized information from the maps was converted to a grid cell data base.

A generalized map thermal zones is presented in Figure 3.5, and their extents are presented in Table 3.8. A generalized map of mean total dominant length of growing period zones is presented in Figure 3.6, and their extents are presented in Table 3.9. A generalized map of pattern of length of growing period zones is presented in Figure 3.7, and their extents are presented in Table 3.10.

Extents of mean total dominant length of growing period zones by pattern of length of growing period zones for each of the thermal zones, and for all thermal zones combined, are presented in Technical Annex 1.

3.2 Soil Resources

3.2.1 Exploratory Soil Map of Kenya

The Exploratory Soil Map of Kenya (Siderius and van der Pouw 1980; Sombroek, Braun and van der Pouw 1982) at 1:1 million scale was used to compile the soil resources inventory for this assessment.

FIGURE 3.5
Generalized map of thermal zones

FIGURE 3.5

FIGURE 3.6
Generalized map of mean total dominant length of growing period zones

FIGURE 3.6

FIGURE 3.7
Generalized map of pattern of length of growing period zones

FIGURE 3.7

TABLE 3.8
Extents of thermal zones

Thermal zone
code
Mean daily temperature range
(°C)
Extent
(ha)
Percentage of total area
1> 25.038 120 60066.17
222.5 – 25.05 783 60010.04
320.0 – 22.54 070 1007.07
417.5 – 20.04 484 4007.78
515.0 – 17.53 448 3005.99
612.5 – 15.01 268 2002.20
710.0 – 12.5307 1000.53
85.0 – 10.0107 4000.19
9< 5.017 5000.03
1 – 9 57 607 200100.00

TABLE 3.9
Extents of mean total dominant length of growing period zones

LGP zone codeLGP zone (days)Extent (ha)Percentage of total area
106 837 70011.87
21 – 2914 941 30025.94
330 – 599 544 30016.57
460 – 896 027 10010.46
590 – 1195 019 1008.71
6120 – 1494 437 1007.70
7150 – 1792 377 9004.13
8180 – 2091 380 5002.40
9210 – 2391 204 8002.09
10240 – 2691 192 8002.07
11270 – 2991 711 6002.97
12300 – 3291 627 2002.82
13330 – 3641 223 3002.12
14365-57 3000.10
15365+25 2000.04
1 – 15 57 607 200100.00

This soil map, published by Kenya Soil Survey in 1980, provides the latest country-wide soil data base and includes information on distribution and characteristics of soils, landform and geology/parent material.

3.2.2 Soil Mapping Units

The soil mapping units are soil associations or soil complexes composed of dominant soils, associated soils and inclusions. The soil mapping units have been registered on the map by a symbol reflecting the landform in which they occur. In Table 3.11 the occurrences of soil mapping units are presented by landform.

The Exploratory Soil Map consists of 390 different soil mapping units. Extents of the individual soil mapping units are presented in Technical Annex 1.

TABLE 3.10
Extents of pattern of length of growing period zones

Pattern zone codePattern zone symbolExtent (ha)Percentage of total
area
1130 8000.05
2H - 125 2000.04
31 - H330 4000.57
41 - H - 2611 2001.07
51 - 2 - H52 3000.09
61 - 23 687 3006.40
71 - 2 - 3615 2001.07
81 - 3 - 217 7000.03
91 - 2 - D3 488 0006.05
101 - D - 26 080 70010.56
111 - D6 218 90010.80
122533 8000.93
132 - 120 332 50035.30
142 - 1 - H42 9000.07
152 - 1 - 33 326 8005.77
162 - 31 633 8002.84
172 - 3 - 12 696 1004.68
182 - 3 - 453 9000.09
192 - 1 - D698 7001.21
203 - 276 5000.13
213 - 2 - 1219 1000.38
22D6 835 40011.87
1 - 22 57 607 200100.00

For each soil mapping unit the following semi-quantified information in terms of description, classes and extents has been transferred to the soil resources data base of this assessment:

-   Landform

-   Geology/Parent material

-   Soil units (with implied characteristics)

-   Slope-gradient classes

-   Soil texture classes

-   Soil phases

3.2.3 Landform

Landform is the first entry in the legend of the Exploratory Soil Map. It provides information on physiography, altitudinal position and slope patterns. A generalized map of landforms in Kenya is presented in Figure 3.8, and a description of landforms and their extents is presented in Table 3.11.

For the Exploratory Soil Map of Kenya six slope classes have been employed in 12 combinations. The slope classes are: A, 0–2%; B, 2–5%; C, 5–8%; D, 8–16%; E 16–30%; and F > 30%.

TABLE 3.11
Occurrence of soil mapping units by landform

Landform symbolLandform descriptionSoil Mapping unitsExtent
(000 ha)
Percentage of total area
AFloodplain1 - A183 1795.52
A8 + A12  
BBottomlandB1 - B169731.69
DDunes or dune landD1 - D3900.16
D1 + P13  
FFootslopesF1 - F192 1113.66
FYFootslopes and piedmont plain(undifferentiated)FY1 - FY36141.07
HHills and minor scarpsHI - H223 2045.56
HsStep-faulted scarp of the Rift ValleyHs15150.89
LPlateau and high level structural plainL1 - L314 0527.03
LaLava flowLava9601.67
LcCoastal plateauLc1 - Lc312130.37
LsStep-faulted floor of the Rift ValleyLs1 - Ls38111.41
LuPlateau/upper-level upland transitionLu1 - Lu2980.17
MMountains and major scarpsM1 - M122 3764.12
PchHigher-level coastal plainPc1 - Pc34300.76
PclLower-level coastal plainPc4 - Pc75991.04
PerReef coastal plainPc8 - Pc10830.14
PdDissected erosional plainPd1 - Pd61 8953.29
Pf1Sedimentary plain of large alluvial plains(older fans)Pf1 -Pf33120.54
Pf2Sedimentary plain of large alluvial plains(younger fans)Pf4 - Pf53770.65
PILacustrine plainPI1 - PI138631.50
PnNon-dissected erosional plainPn1 - Pn356 00710.43
PshHigher-level sedimentary plainPs1 - Ps63 8856.74
Ps3 + Ps15  
PslLower-level sedimentary plainPs21 - Ps273 1386.64
PsmMiddle sedimentary plain (‘enclosed’)plain and sealing loam plain)Ps7 - Ps205 6189.75
Ps11 + D1  
PsxSedimentary plain of undifferentiated levelPs28 - Ps292470.43
Ps28 + D1  
PtSedimentary plain of upper river terracePt1 - Pt43230.66
PvVolcanic plainPv1 - Pv129981.73
RVolcanic footridgesR1 - R143 1215.42
SSwampS1 - S3950.16
TMangrove swampT1340.23
UcCoastal uplandUc1 - Uc115330.93
UhUpper middle-level uplandUh1 - Uh197861.36
UlLower-level uplandUI1 - UI211 4002.43
UmLower middle-level uplandUm1 - Um29 Up12 0903.63
UpUpland/high-level plain transitional land- Up83860.67
UuUpper-level uplandUu1 - Uu31320.23
UxUpland, undifferentiated landUx1 - Ux101 7443.03
VMinor valleyV1 - V21120.20
WBadlandW1 - W27221.26
YPiedmont plainY1 - Y132 1343.70
Z1Older coastal beach ridgeZ1790.14
Z2Younger coastal beach ridgeZ2390.07
Z3Lakeside beach ridgeZ350.01
Lakes  1180.20
Towns  110.02
Total extent  57 607100.00

A generalized map of slope-gradient classes in Kenya is presented in Figure 3.9. The combinations of slope classes employed and their extents are presented in Table 3.12.

To each of the 12 slope classes, inventoried in the Exploratory Soil Map, associated slope classes have been assigned. These associated slope classes, covering up to 10% of the land area of the 12 slope classes, are used for evaluation purposes and included in the land resources inventory. The inventoried slope classes and associated slope classes are presented in Table 3.13. For the same purposes assumed mean slopes of quartiles of the land area of each of the slope classes have been assigned. These values are presented in Table 3.14.

FIGURE 3.8
Generalized map of landforms

FIGURE 3.8

FIGURE 3.9
Generalized map of slope-gradient classes

FIGURE 3.9

TABLE 3.12
Extents of slope classes

Slope class symbolSlope class (%)Extent (ha)Percentage of total area
A0 – 219 868 85034.49
AB0 – 514 540 60025.24
B2 – 53 098 1505.38
BC2 – 87 351 20012.76
C5 – 8759 2001.32
BCD2 – 161 707 3002.96
CD5 – 161 557 4502.70
D8 – 161 616 2002.81
DE8 – 30885 8501.54
E, EF, F> 166 093 40010.58
Lakes 117 6000.20
Towns 10 7000.02
Total extent 57 607 200100.00

TABLE 3.13
Associated slope classes

Slope class symbol% Associated slope classes
A0 – 2100%A   
AB0 – 5100%AB   
B2 – 5100%B   
BC2 – 890%BC5%A5% D
C5 – 890%C5%AB5% D
BCD2–1690%BCD5%A5% E
CD5 – 1690%CD5%AB5% E
D8 – 1690%D5%BC5% E
DE8 – 3090%DE5%BC5% F
E16 – 3090%E5%BCD5% F
EF16 – 5695%EF5%BCD 
F30 – 5695%F5%DE 

TABLE 3.14
Quartiles of slope classes

Slope class symbol%Gentlest
Q1
Lower
Q2
Upper
Q3
Steepest
Q4
A0 – 20112
AB0 – 50245
B2 – 52345
BC2 – 82468
C5 – 85678
BCD2 – 16261116
CD5 – 16591216
D8 – 168111316
DE8 – 308162230
E16 – 3016212530
EF16 – 5616304256
F30 – 5630394756

3.2.4 Geology

Geology is the second entry in the legend of the Exploratory Soil Map. The geological subdivisions reflect mainly resistance to weathering and richness of parent material in order to provide linkage with soil formation. The first level subdivision comprises three types of rocks: igneous rocks, metamorphic rocks and sedimentary rocks.

Igneous rocks and metamorphic rocks are futher subdivided from basic to acid. The sedimentary rocks are futher subdivided from fine to coarse textured.

Each soil mapping unit is accordingly characterized for its geological setting/parent material. A generalized map of geology/parent material is presented in Figure 3.10 and the descriptions and extents of geological units are presented in Table 3.15.

3.2.5 Soil Units

The individual soil units of the soil associations or soil complexes (soil mapping units of the Exploratory Soil Map) have been defined in accordance with the FAO-Unesco legend of the Soil Map of the World (FAO 1974). The soil units adopted were selected on the basis of present knowledge on the formation, characteristics and distribution of the soils, their importance as resources for agricultural production and their significance as a factor of the environment.

In the legend of the Exploratory Soil Map some adaptations of the FAO-Unesco legend have been introduced. At the first level (great group) the terminology for Lithosols and Nitosols has been modified. At the second level (unit level) new subgroups have been introduced (cambic and orthic Rendzinas) and others modified (vertic Gleysols, mollic Nitisols, chromic Acrisols, chromic Luvisols and chromic Cambisols). In order to reflect the greater amount of detail of the Exploratory Soil Map of Kenya, a third level of terminology (subunit level) has been introduced for subdivision of soil units into subunits. The prefixes used to distinguish subunits are ando, calcaro, chromo, ferralo, luvo, nito, ortho and verto.

In the Exploratory Soil Map, 123 different soil units and five miscellaneous units occur. Table 3.16 presents the soil units and miscellaneous units and their extents.

The soil units have been defined in terms of measurable and observable properties of the soil itself, and specific clusters of such properties are combined into ‘diagnostic horizons’ and ‘diagnostic properties’.

The diagnostic horizons have been used as defined in the FAO-Unesco legend. Diagnostic horizons and properties of the soil units are given in Technical Annex 1.

Complete definitions of soil units are given in Volume 1 (Legend) of FAO-Unesco Soil Map of the World (FAO 1974) and Exploratory Soil Survey Report (KSS 1982a).

FIGURE 3.10
Generalized map of geology/parent material

FIGURE 3.10

TABLE 3.15
Geology/parent material

Geology symbolGeology descriptionExtent (ha)Percentage of total area
A(Alluvial) Sediments from various sources12 241 6003.89
BBasic and ultra-basic igneous rocks6 786 80011.78
B+As in B, but with volcanic ash admixture233 3000.39
BPAs in B, but volcanic ash predominant52 2000.09
DMudstones, claystones102 9000.18
EAeolian sediments (cover sands)235 4000.41
FGneisses rich in ferromagnesian minerals, hornblende gneisses1 680 6002.92
GGranites, granodiorites477 9000.83
G+As in G, but with volcanic ash admixture14 4000.02
GFBiotite-hornblende granites38 5000.07
GF+As in GF, but with volcanic ash admixture70 7000.12
GPAs in G, but volcanic ash predominant19 3000.03
GRComplex of G and R50 7000.09
IIntermediate igneous rocks (syenites etc.)499 9000.87
I+As in I, but with volcanic ash admixture90 9000.16
JLagoonal deposits1 154 6002.00
KSiltstones1 466 6002.55
KTComplex of K and T94 1000.16
LLimestones, calcitic mudstones2 056 4003.57
NBiotite gneisses400 6000.70
N+As in N, but with volcanic ash admixture160 3000.28
0Plio-pleistocene bay sediments9 606 60016.28
PPyroclastic rocks1 897 4003.29
QQuartzites405 9000.70
RQuartz-feldspar gneisses58 9000.10
SFeldspar, grits, arkoses647 4001.12
TShales164 0000.28
UUndifferentiated basement system rocks14 007 00024.31
U+As in U, but with volcanic ash admixture1 172 4002.04
UPAs in U, but volcanic ash predominant36 7000.06
VUndifferentiated or various igneous rocks8 536 30014.28
WMarls168 5000.29
XUndifferentiated or various rocks1 350 2002.34
X+As in X, but with volcanic ash admixture11 7000.02
YAcid igneous rocks (rhyolite, aplite)155 8000.27
Y+As in Y, but with volcanic ash admixture79 0000.14
-Not defined1 263 4002.19
Lakes 117 6000.20
Towns 10 7000.02
Total extent 56 607 200100.00

1 If the source of alluvial sediments and bottomland infills is known (e.g., basalts), then the code for this rock is used, otherwise the code A applies.


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