Identification Of Crop-Growing Seasons Of Semi-Arid Kenya By Analyses Of The Soil Tloisture Patterns

ABSTRACT

Identification of Crop-Growing Seasons of Semi-Arid Kenya by

Analvses of the Soil Moisture Patterns. (May 1984)

David Kasina }1usembi, B.S., University of Nairobi

Chairman of Advisory Cormnit tee i- Prof. John F. Griffiths

Eight stations wit~ over 30 years of rainfall record were selected

in the dry region of Kenya, east of 37°E longitude, to study the

nature of the crop growing seasons during the short and the long rains

(October through June).

The rainfall data were reduced to soil moisture based upon areawide

assumptions on the soil characteristics. The start, end and

duration of the crop growing seasons were determined and tested for

normality using a 10 mm soil moisture threshold value at each

station. Mean seasonal conditions were computed and comparisons

were made between the two growing seasons within each station and

among all the stations. Stations were grouped according to the magnitude

of the coefficient of determination between start and duration

and regression equations were developed to predict season duration,

from the starting dates. Equations are also derived to estimate the

probable length of the growing season for various levels of crop

moisture requirements. The exceptionally wet and dry years are examined.

The distributions of the start, end and duration of the seasons

are not significantly skewed or significantly different from the

normal distribution. The start and end times of the short rains

iv

season have a southward trend. The start of the long rains season is

widespread but the end of the season shews a northward trend. Dur3-

tions in both seasons have no general patterns but reflect the influence

of local factors. The short rains growing season is found to

last longer and to have a higher soil moisture content than the long

rains season for the stations located south of the equator. The

opposite is true for the stations north of the equator.

The very wet and very dry seasons are not usually widespread.

The occurrence of a very wet season appears to weaken the other

season and a lO-year periodicity is observed in the very wet and

widespread seasons in the short rains season. Periodicity is not

observed in