Chapter 5
Climatic inventory

The climatic resources inventory of Kenya (Technical Annex 1) quantifies both heat and moisture conditions.

The quantification of heat attributes has been achieved by defining reference thermal zones, representing the prevailing temperature regimes¹. Temperature seasonality effects of latitude are minor due to the equatorial position of Kenya.

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

To cater for differences in temperature adaptability between species, nine thermal zones (based on 2.5°C intervals) are distinguished in the climatic inventory of Kenya (Table 5.1).

The temperature threshold values used in these definitions accord with those differentiating the four temperature adaptability groups of crop, pasture and fuelwood species described in Technical Annexes 1.3, 1.4 and 1.5, and in Section 6.1.

TABLE 5.1
Thermal zones

Quantification of moisture conditions was achieved through the concept of reference length of growing period (LGP), being 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 available soil moisture supply from rainfall exceeds half potential evapotranspiration. It includes the time required to evapotranspire upto 100 mm of stored available 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’.

TABLE 5.2
Patterns of length of growing periods (LGP-patterns) - historical profiles of occurrence of number of length of growing periods per year

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

TABLE 5.3
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.

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, summarized as a historical profile of pattern of number of length of growing periods per year (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 total lengths comprising the individual dominant and associated 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 5.2. 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 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 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 with the former total length. These relationships are presented in Tables 5.3 and 5.4.

TABLE 5.4
Relationship between individual component mean length and mean total length of growing period

L2¹ = First length of the 2 growing periods per year
L3¹ = First length of the 3 growing periods per year
L3² = Second length of the 3 growing periods per year
L4¹ = First length of the 4 growing periods per year
L4² = Second length of the 4 growing periods per year
L4³ = Third length of the 4 growing periods per year.

In the climatic inventory of Kenya, only the mean total dominant length has been inventoried on the map as 15 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, 240–269, 270–299, 300–329, 330–364, 365^- and 365⁺days¹.

¹ 365^- : 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 5.3 and 5.4 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 tablet 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.