Abstract:
Eight stations with 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 area wide
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
season have a southward trend. The start of the long rains season is
widespread but the end of the season shews a northward trend. Durations 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 10-year periodicity is observed in the very wet and
widespread seasons in the short rains season. Periodicity is not
observed in the long rains reason or for the very dry seasons.
