Rainfall is varied along the lowland coastal region and it is highest
(1200 mm) in the south-west in ecological zone Coastal Lowland 3 (CL3,
coconut [Cocos nucifera L.]– cassava [Manihot esculenta Crantz] zone).
Further north, in the Kilifi-Malindi area (CL4, cashew nut [Anacardium
occidentale L.]– cassava region) annual rainfall decreases to 1000 m and
in Lamu (CL5, livestock-millet [Panicum miliaceum L.] zone) it further
drops to 900 mm (van Einnatten 1979; Jaetzold and Schmidt 1983).
Similarly, rainfall decreases from the coast to the hinterland.
Consequently, the number of wet months (≥ 100 mm m
-1
) decreases from
5 months on the southern coast to 3 months in Lamu. The El Niño rains of
1997-98 resulted in flooding of the low-lying areas and destruction of
infrastructure.
Rainfall is bimodal with a long rainy season from April to June and
the short rains from October to December. There are two distinct dry
seasons, which normally occur from January to March and from June to
September. This bimodal rainfall allows for two crop growing seasons: the
more reliable long rains and the short rains, which are seldom enough to
support a good crop. Access to irrigation in the lowland region is limited.
However, in the bordering district of Tana River, cotton (Gossypium
hirsutum L.) is grown under irrigation. Potential evapotranspiration is
high, 1900-2300 mm, and exceeds annual precipitation in most months,
resulting in water deficits. Mean annual temperatures are also high,
ranging from 22 to 35 °C. The warmest period is from January to April
when the daily temperatures average more than 30 °C. Relative humidity
is high and ranges from 70 to 90% for most of the year.