Chapter 5
Agro-climatic suitability classification

The agro-climatic suitability classification is based on:

1. matching the attributes of temperature regimes in each thermal zone to crop temperature requirements for photosynthesis and phenology, and determining whether the crop qualifies for further consideration in the matching exercise;

2. computation of constraint-free yield of the qualifying crop for each length of growing period zone;

3. computation of agronomically attainable yields for each crop in each length of growing period, taking into account yield losses that occur due to agro-climatic constraints of temperature and moisture stress, pests and diseases, and workability.

To enable crops to be matched to climatic conditions, the climatic inventory of Kenya was compiled to permit the interpretation of the climatic resources in terms of their suitability for production of crops. The appropriate climatic adaptability attributes of the crop dictate what parameters need to be taken into account in the compilation of the climatic inventory. The climatic adaptability attributes of crops form the basis of defining the crop climatic requirements, and are outlined in the next section. The climatic resources inventory of Kenya is decribed in Technical Annex 1, and is briefly outlined in Section 5.2.

5.1 Crop Climatic Adaptability

Crops have climatic requirements for photosynthesis and phenology both of which bear a relationship to yield. The rate of crop photosynthesis and growth are related to the assimilation pathway and its response to temperature and radiation. However, the phenological climatic requirements, which must be met, are not specific to a photosynthetic pathway.

In the FAO Agro-ecological Zones methodology (Kassam, Kowal and Sarraf, 1977), crops are classified into climatic adaptability groups according to their fairly distinct photosynthesis characteristics. Each group comprises crops of ‘similar ability’ in relation to potential photosynthesis, and the differences between and within groups in the response of photosynthesis to temperature and radiation determine crop-specific biomass productivity when climatic phenological requirements are met.

TABLE 5.1
Average photosynthetic response of Individual leaves of four groups of crops to radiation and temperature

¹For further information on crop adaptability groups see Tables 3.1 to 3.5 in FAO (1978).

Crop adaptability groups and their characteristic average photosynthesis response to temperature and radiation are presented in Table 5.1. Barley and oat have a C3 photosynthesis pathway. They belong to group I and are adapted to operate under cool conditions (<20°C mean daily temperature). Cowpea, green gram and pigeonpea have a C3 photosynthesis pathway. They belong to group II and are adapted to operate under warm conditions (>20°C) with a potential rate of photosynthesis that is greater than in group I crops. Crops in group III (e.g. pearl millet, sugarcane) have a C₄ photosynthesis pathway. They are adapted to operate under warm conditions (>20°C) but with a potential rate of photosynthesis that is greater than in group II crops. Crops in group IV (e.g. highland maize) have a C₄ photosynthesis pathway. They are adapted to operate under cool conditions (<20°C) with a potential rate of photosynthesis that is similar to that in group III crops.

The time required to form yield depends on the phenological constraints on the use of time available in the growing period, and the location of yield in the plant (e.g. seed, leaf, stem, root) has an important influence. Temperature has a rate controlling/limiting effect on growth, and it may influence the growth of a specific part and the accumulation of yield if located therein. For example, in barley and oat, cool night temperatures are required for tillering but the optimum temperatures at the time of flowering and subsequent yield formation are higher. Similarly, optimum temperatures for growth in sugarcane are greater than 20°C but during the ripening period, and because the yield is located in the stem, a lower temperature in the range 10–20°C is required for concentration in the cane of sugar of the right kind. On the other hand, optimum temperatures for growth, development and yield formation in cowpea, green gram and pigeonpea are greater than 20°C and most of the specific temperature requirements are also met when temperatures are optimum for photosynthesis and growth.

TABLE 5.2
Climatic adaptability attributes of crops

C - Cereal

L - Legume

TI - Terminal inflorescence

LI - Lateral inflorescence

LT - Last one third of growth cycle

ME - Middle to end period of growth cycle

Thermal zones: 1 - <25.0 °C

2 – 22.5–25.0

3 – 20.0–22.5

4 – 17.5–20.0

5 – 15.0–17.5

6 – 12.5–15.0

7 – 10.0–12.5

1 thermal zones 3 & 4

2 thermal zone 5

3 thermal zones 6 & 7

4 thermal zones 1,2 & 3

The attributes that are helpful in assessing the climatic adaptability of the crops in the matching exercise are given in Table 5.2.

Barley and oat (C₃-species, group I) are annuals with a botanically determinate growth habit. Their yield is located in terminal inflorescences in seeds, and the crop yield formation period is the last one-third of their growth cycle. Their climatic adaptability attributes qualify them to be considered for matching in areas with mean daily temperatures less than 22.5 °C and more than 10°C (i.e. thermal zones 3, 4, 5, 6 and 7).

Cowpea (C₃-species, group II) is an annual with botanically indeterminate growth habit, offering cultivars that may be morphologically determinate (bunch types) or indeterminate (spreading types). Its yield is located in the lateral inflorescences in seeds, and the crop yield formation period is from the middle to the end of its growth cycle. Its climatic adaptability attributes qualify it to be considered for matching in areas with mean daily temperatures greater than 20°C (i.e. thermal zones 1, 2 and 3).

TABLE 5.3
Thermal zones

Green gram (C3-species, group II) is an annual with botanically indeterminate growth habit, offering cultivars that may be morphologically determinate in growth and stature. Its yield is located in the lateral inflorescences in seeds, and the crop yield formation period is from the middle to the end of its growth cycle. Its climatic adaptability attributes qualify it to be considered for matching in areas with mean daily temperatures greater than 20°C (i.e. thermal zones 1, 2 and 3).

Pigeonpea (C3-species, group II) is a short-term perennial with botanically indeterminate but morphologically determinate growth habit. Its yield is located in the lateral inflorescences in seeds, and the crop yield formation period is from the middle to the end of its annual cultivated life-span. Its climatic adaptability attributes qualify it to be considered for matching in areas with mean daily temperatures greater than 20°C (i.e. thermal zones 1, 2 and 3).

5.2 Climatic Inventory

The climatic resources inventory of Kenya (Technical Annex 1.1) quantifies both heat and moisture conditions.

Quantification of heat attributes has been achieved by defining reference thermal zones representing the prevailing temperature regimes ¹. Temperature seasonality effects of latitude are minor due to the equatorial position of Kenya.

¹ The following equation closely represents the relationship between average daily temperature in degree Celcius (T) and altitude in metres (A): T = 30.2 - 6.496(A/1000).

To cater for differences in temperature adaptability between nine thermal zones (based on 2.5 °C intervals) are distinguished in the climatic inventory of Kenya (Table 5.3). The temperature threshold used in these definitions accord with those differentiating the four temperature adaptability groups of crops as described above.

Quantification of moisture conditions was achieved through the concept of reference length of growing period (LGP) being defined as the duration (in days) when moisture supply can permit crop growth. A moisture supply from rainfall of half, or more than half, potential evapotranspiration has been considered to permit crop growth. The following main concepts, definitions and methods form the basis of the quantification of moisture conditions in the climatic inventory.

The growing period is the time when moisture supply from rainfall exceeds half potential evapotranspiration. It includes the time required to evapotranspire up to 100 mm of stored moisture from the soil profile. A ‘normal’ growing period has a humid phase, i.e. a period when moisture supply is greater than full potential evapotranspiration. When there is no humid period, the growing period is defined as ‘intermediate’.

The quantification of moisture regime is based on the analysis of the length of growing period for each year separately and the computation of:

1. number of separate lengths of growing periods per year, summerized as a historical profile of pattern of number of growing periods per year (referred to as LGP-Pattern);

2. length of each growing period and its various moisture periods,summarized as mean total dominant length, first associated length and second associated length, and the mean individual dominant and associated lengths making up the total lengths;

3. the quality of moisture conditions during the growing period and its various moisture periods;

4. year-to-year-variability (frequency distribution) of each length of growing period and the associated moisture condition.

Twenty two LGP-Patterns are recognized, and these with their composition are presented in Table 5.4. The LGP-Pattern code represents the number of growing periods per year in order of frequency of occurrence, e.g. in the pattern coded 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 number 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.

TABLE 5.4
Patterns of growing periods (LGP-patterns) - historical profiles of occurrence of number of growing periods per year

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

D = zero days (i.e. year-round dry).

For each LGP-Pattern type, 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 lengths are again correlated to the former total length. These relationships are presented in Tables 5.5 and 5.6.

In the climatic inventory of Kenya, only the mean total dominant length has been inventoried on the map as 14 LGP zones. The boundary or isoline values used are 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, 240269, 270–299, 300–329, 330–364, 365^- and 365⁺ days ².

² 365^- - year-round growing period but not humid year-round; 365⁺ - year-round humid growing period.

Additionally, the LGP-Pattern zones have been inventoried. Consequently, the relationships in Tables 5.5 and 5.6 together with the map of dominant LGP zones and the LGP-Pattern zones provide the historical profile of any mean total dominant length of growing period in any of the 22 LGP-Pattern zones.

Reference tables relating the mean total dominant LGPs (mapped) and the corresponding mean total associated LGPs (unmapped) are presented in Technical Annex 1, together with generalized coefficients of variation of mean LGPs, and frequency of occurrence of intermediate LGPs.

TABLE 5.5
Relationships between mean total dominant and mean total associated lengths of growing period

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.

5.3 Matching of Crops to Thermal Zones

The initial step in the matching process is comparison of the temperature requirements of individual crops with the identified thermal zones. This step indicates the crops which should be considered from a temperature/growth and phenology viewpoint, in each thermal zone.

Crop/thermal zone suitability ratings for each crop and zone are presented in Table 5.7. Five suitability classes are employed (i.e. S1, S2, S3, S4, and N), and the ratings apply to both levels of inputs: where requirements are fully met, the zone is adjusted S1: where requirements are sub-optimal, the zone is adjudged S2, S3 or S4; where requirements are not met, the zone is adjudged as N (not suitable).

A rating of S1 indicates that the temperature conditions for growth and yield physiology, and phenology development are optimal and that it is possible to achieve the maximum attainable agro-genetic yield potential,if there are no additional climatic or edaphic limitations. Ratings of S2, S3 and S4 indicate that temperature conditions for growth and development are sub-optimal and that there would be a suppression of yield potential in the order of 25, 50 and 75 percent respectively. A rating of N indicates that the thermal requirements are not met and the zone is not suitable for father consideration.

TABLE 5.6
Relationship between individual component mean length and mean total length of growing period

L2₁ = First length of the two growing periods per year

L3₁ = First length of the three growing periods per year

L3₂ = Second length of the three growing periods per year

L4₁ = First length of the four growing periods per year

L4₂ = Second length of the four growing periods per year

L4₃ = Third length of the four growing periods per year.

5.4 Matching of Crops of Growing Period Zones

Matching of crops of growing period zones is according to the following procedure:

1. assessment of net biomass and constraint-free crop yields by individual lengths of growing period zones, assuming optimum temperature conditions for production (i.e. S1 crop/thermal zone rating);

2. inventory of agro-climatic constraints for each length of growing period zone by crop and by input level;

3. application of the thermal zone suitability ratings (Table 5.7) and agro-climatic constraints (ii) to the constraint-free yields (i) to determine (agro-climatically attainable) crop yields by individual lengths of growing period zones in each thermal zone.

TABLE 5.7
Thermal zones suitability ratings

The above matching exercise results in a basic agro-climatic suitability classification of each length of growing period zone by thermal zone only. From this, the agro-climatic suitability classification of each mean total dominant growing period zone (inventoried) can be derived for each crop according to agro-climatically attainable yields by thermal zone and by pattern zone. This is achieved by computing agro-climatically attainable yields as affected by year-to-year variability (i.e. LGP-Pattern) from the basic agro-climatic suitability classification of each length of growing period. Agro-climatic suitability by LGP-Pattern zone is not attempted in this annex but is considered in a separate annex (Technical Annex 1.4) dealing with crop productivity potential of each length of growing period by thermal zone and by LGP-Pattern zone.

5.4.1 Potential net biomass and yield

The methodology for the calculation of net biomass and constraint-free yields by suitable thermal zone is according to Kassam (1977), and is presented in this section.

Net total biomass (Bn) is calculated from the equation:

Ct = C30 (0.0044 + 0.0019 T + 0.0010 T^2,)

At 30 °C, C = 0.0283 for a legume crop and 0.0108 for a non-legume crop.

Constraint-free yield (By) is calculated from net biomass (Bn) from the equation:

The maximum rate of gross biomass production (bgm) is dependent on the maximum rate photosynthesis (Pm) which is dependent on temperature and photosynthesis pathway of the crop. Maximum rates of photosynthesis (Pm) for the five crops by temperature is presented in Table 5.8.

For Pm = 20 kg CH₂0 ha^-1 hr^-1and LAI of 5, bgm is calculated from the equation:

When Pm is greater than 20 kg CH₂₀ ha^-1 hr ^-1, bgm is given by the equation:

When Pm is less than 20 kg CH₂₀ ha^-1 hr^-1, bgm is given by the equation:

Table 5.8
Maximum rate of photosynthesis. (Pm in kg CH₂₀ ha^-1 hr^-1)

An example calculation of net total biomass and yield of cowpea for Lamu, is presented below.

TABLE 5.9
The photosynthetically active radiation on very clear days (Ac) in cal cm^-2 day^-1 and the daily gross photosynthesis rate of crop canopies on very clear (bc) and overcast (bo) days in kg CH₂₀ ha^-1 day^-1 for Pm = 20 kg CH₂₀ kg ha^-1 hr^-1 (from de Wit 1965)

Climate

Station : Lamu, Kenya
Location : 2° 16' S and 40° 54' E
Altitude : 30 m
Growing period : 140 days
Start growing period : 5 April
End growing period : 25 August
Average radiation (Rg) : 471 cal cm^-2 day^-1
Average day-time temperature : 26.5 °C
Average 24hr mean temperature : 25.3 °C

Crop

Crop : Cowpea
Growth cycle : 100 days
Leaf area index at maximum growth rate : 4
Harvest index : 0.30
Crop adaptability : Photosynthesis pathway C3, group II

Calculation of daily rate of gross biomass production (bgm)

Photosynthesis rate Pm at 26.5 °C : 35 kg CH₂ ha^-1 hr^-1.

Difference in Pm relative to Pm = 20 kg CH₂₀ ha^-1 hr^-1: 75%.

Average photosynthetically active radiation on clear days (Ac) : 351 cal cm^-2 day^-1 (Table 5.9).

Fraction of the day-time when the sky is overcast (F): 0.41 (from equation F = (Ac -0.5Rg)/0.8Ac).

Average rate of gross biomass production for perfectly clear days at Pm = 20 kg CH₂₀ ha^-1 hr^-1 (be) : 418 kg CH₂₀ ha^-1 day^-1 (Table 5.9).

Average rate of gross biomass production for totally overcast days at Pm = 20 kg CH₂₀ ha^-1 hr^-1 (bo) : 222 kg CH₂₀ ha^-1 day^-1 (Table 5.9).

TABLE 5.10
Crop characteristics considered in the potential biomass and yield computations

¹ Days to maturity of barley and oat assumed to increase by 6 days for each 0.5 °C decrease in mean temperature from 20 °C.

Rate of gross biomass production at Pm = 20 kg CH₂₀ ha^-1 hr^-1 at LAI of 5 : 338 kg CH₂₀ ha-1? day^-1 (from equation 5.3).

Rate of gross biomass production at Pm = 35 kg CH₂₀ ha^-1 hr^-1 at LAI of 5 (bgm) : 444 kg CH₂₀ ha-^-1 day^-1 (from equations 5.3 and 5.4).

Calculation of total net biomass production (Bn) and Yield (By)

Maintenance respiration coefficient at 30 °C : 0.0283 (for legume crop).

Maintenance respiration coefficient at 24.4 °C (Ct): 0.017 (from equation C_t = C₃₀ (0.0044 + 0.0019 T + 0.0010 T²).

Net total biomass (Bn) : 10.3 t/ha (equation 5.1).

Yield (By) : 3.1 t/ha (equation 5.2).

Crop characteristics considered in the biomass and yield calculations are presented in Table 5.10. The constraint-free net biomass and yield values are presented in Table 5.11. In Tables 5.10 and 5.11, values apply to the respective component length of growing period under which they appear in the tables. For a growing period range, the values continue to apply as stated as long as the largest growing period value in the range is equal to or shorter than the longest growth cycle or days of active growth considered. For any growing period range, the lower yield figure refers to the shortest crop growth period considered in the range.

The yield formation period has been assumed to be the end one-third of the growth cycle in barley and oat, and the second half of the growth cycle in cowpea, green gram and pigeonpea. Within the growth cycle range considered, leaf area index (LAI) of the shorter duration crop has been reduced proportionately relative to the LAI of the longer duration crop. Where the growth cycle is curtailed due to the growing period being shorter (e.g. 100-day cowpea at 75 days growing period), LAI has been reduced proportionately relative to the LAI for the normal growth cycle considered.

TABLE 5.11
Constraint-free potential net biomass (Bn) and yield (By) In t/ha dry weight

When the yield formation period is curtailed due to growing period being shorter, harvest index (Hi) has been reduced proportionately in relation to the Hi for the normal yield formation period considered.

Values for barley and oat (group I) apply to areas where mean temperatures during the growing period are less than 22.5 °C but greater than 10°C (thermal zones 3,4,5,6,7). Values for the remaining crops (groups II) apply to areas where the mean temperature during the growing period is greater than 20°C (thermal zones 1,2,3).

5.4.2 Agro-climatic constraints

The methodology for matching the climatic inventory with the crop adaptability groups, and for the calculation of net biomass and yield of crops provides quantification of yields that can be anticipated under conditions that are essentially free from soil constraints but also from agro-climatic constraints to production within the growing period.

To complete an agro-climatic suitability classification, yield losses likely to occur due to the agro-climatic constraints must be deducted from the constraint-free yields in Table 5.11.

Agro-climatic constraints have their origin primarily due to the prevailing climate, and cause direct or indirect losses in yield and quality of produce as follows:

1. yield losses due to water stress constraints on crop growth;

2. yield losses due to the effects of pest, disease and weed constraints on crop growth;

3. yield losses due to water stress, and pest and disease constraints on yield components, yield formation and quality of produce; and

4. yield losses due to workability constraints.

The assessment of the severity of the four sets of constraints by crops, growing period zones and levels of inputs is presented in Table 5.12. The severity of constraints are rated as follows:

In general, the major agro-climatic constraint in the shorter growing period zones for all crops is water stress. The major agro-climatic constraints in the longer growing period zones for annual crops are those of pests, diseases and weeds, and workability.

5.4.3 Anticipated yields and agro-climatic suitability classification

Application of the appropriate reduction factors, according to the occurrence and severity of the temperature constraints (Table 5.7) and agro-climatic constraints of water stress, pests and diseases and workability (Table 5.12), to the constraint-free yields (Table 5.11), allows quantification of attainable yields with the agro-climatic constraints for the agro-climatic suitability assessment and classification. The reductions were made consecutively according to the presence or absence of the constraints and the severity of their occurrence in each growing period zone and at each level of inputs. The computation was made in the order of presentation of the four sets of constraints, i.e. group ‘a’ constraints applied first and group ‘d’ constraints applied last, after applying reductions due to temperature constraints.

TABLE 5.12
Agro-climatic constraints by crop

Crop yields for representative growth cycles of each crop for the three input levels are presented in Tables 5.13, 5.14 and 5.15. Because of the yield reducing effects of low soil fertility and management limitations inherent in the low input level conditions, the constraint-free yields for this level have been assumed as 25 percent of the constraint-free yields under high input conditions in Table 5.11. Anticipated yields at the intermediate level are taken to be half-way between the high and low level yields.

Anticipated yields at each level of inputs are also presented in Tables 5.16, 5.17, 5.18, 5,19 and 5.20 for barley, oat, cowpea, green gram and pigeonpea respectively. Further, in Tables 5.16–5.20 the anticipated yield for each growing period zone is shown as a percentage of the maximum yield attainable, i.e. in the case of barley with 90–120 days growth cycle at high input level, the maximum attainable yield is 4.55 t/ha grain dry weight in the 150–179 days zone. The yield of 3.10 t/ha from the 90–119 days zone is therefore as shown, 68 percent of the maximum.

The agro-climatic suitability classification is also presented in Tables 5.16–5.20 in terms of suitability bar charts. The basis of these charts is comparison of attainable crop yields in the different lengths of growing period zones, in terms of percentages of maximum attainable. If the yield of a crop from a particular zone is 80 percent or more of the maximum attainable, that zone is assessed as agro-climatically ‘very suitable’ (VS) for that crop. Zones with yields of 60 to less than 80 percent are classified as ‘suitable’ (S); 40 to less than 60 percent as 'moderately suitable' (MS); 20 to less than 40 percent as ‘marginally suitable’ (mS); and less than 20 percent as 'not suitable' (NS). On the bar charts, the dividing line between the agro-climatic suitability classes is located at the nearest 15-day position in relation to the growing period zones.

Suitabilities and yields in Tables 5.16–5.20 are matched to individual component length of growing periods, i.e. L1, L2₁, L2₂, L3₁, L3₂, L3₃, L4₁, L4₂, L4₃ and L4₄ in the land suitability assessment. The LGP-Pattern evaluation for each crop is achieved by taking into account all the constituent component lengths in each LGP-Pattern, thus providing a profile of variability in potential yields over time (e.g. average yield, maximum yield, minimum yield). From such information it is then possible to assess yield stability of crops. This stability assessment has been incorporated in the crop productivity model (Technical Annex 1.4).

Yields in Tables 5.13–5.20 apply to normal lengths of growing periods. For intermediate growing periods, yield reductions are of the order of 50% on all soils except Fluvisols and Gleysols. The percentage of occurrence of intermediate lengths of growing periods, in all LGP-Pattern zones combined, is 100% in zone 1–29 days; 65% in zone 30–59 days; 25% in zone 60–89 days; 10% in zone 90–119 days and 5% in zone 120–149 days.

An exception to the general methodology for agro-climatic suitability classification applies to Fluvisols because the length of growing period does not fully reflect their particular circumstances with regards to moisture regime.

TABLE 5.13
Crop yields (t/ha dry weight) for Individual lengths of growing periods at low level of Inputs

TABLE 5.14
Crop yields (t/ha dry weight) for Individual lengths of growing periods at intermediate level of Inputs

TABLE 5.15
Crop yields (t/ha dry weight) for Individual lengths of growing periods at high level of Inputs

TABLE 5.16
Agro-climatic suitability classification and yields (with agro-climatic constraints) in t/ha dry weight for barley

LOW INPUTS

Growth cycle 90–120 days

INTERMEDIATE INPUTS

Growth cycle 90–120 days

HIGH INPUTS

Growth cycle .90–120 days

LOW INPUTS

Growth cycle 120–150 days

INTERMEDIATE INPUTS

Growth cycle 120–150 days

HIGH INPUTS

Growth cycle 120–150 days

LOW INPUTS

Growth cycle 150–180 days

INTERMEDIATE INPUTS

Growth cycle 150–180 days