| dc.description.abstract | Investigations involving mainly food crops
adapted to the Kabete area (the complete range
of crops investigated being maize, field beans,
Irish potatoes, sweet potatoes, wheat, sunflower,
soyabeans and linseed) sought to obtain data on
particular moisture aspects. These aspects
included:
(1) seasonal water use at different stages of
growth by the different crops in relation to
the natural rainfall, in both the short and
long rainsf
(2) the relationship between open free water
evaporation, as calculated by the modified
Penman method and Pan A data, and the actual
water use of the crops; and
(3) the yield of the crops studied during the
time of the experiment a8 affected by water
and other factors.
Two methods were employed, namely the neutron
probe and gravimetric methods for determining moisture
content of the soil to a depth of 180 centimetres.
Calibration of the probe machine showed that this
method may not be very accurate for predicting volumetric
moisture content in the Kabete soil, due to
lack of precision at the higher moisture range
(above 37%) and greater soil depths (90 + cm depth)
due to high clay content. However, it was shown
that volumetric moisture content and probe count
ratio are linearly related (r= 0.8675 for a 0 - 30
centimetre depth and r= 0.7432 for 30 - 180 cm
depth). Because of the difficulties encountered
in obtaining adequate calibration curves, water
use calculations for the crops were based on
gravimetric samplings.
The available moisture for the Kabete soil
in a profile of 180 centimetres was about 161.5
millimetres which was in close agreement with what
Pereira (1957) found for similar soils.
Periodic and seasonal total water use by
crops was found to be affected by the length
of the crop's growing season, amount and frequency
of the rainfall, soil dryness (mainly soil water
availability)-and crop development.
Irish potatoes used the least amount of water
(266.1 millimetres (mm) in the short rains and
477.5 mm in the long rains). These were followed
by field beans (299.5 mm (SR) and 478.9 mm (LR», ~-wheat
(324.8 mm) linseed (400.1 mm), soyabeans
(478.8 mm) sunflower (516.6 mm), maize (518.4 mm
(SR) and 619.2 mm (LR» and sweet potatoes
(629.3 mm (SR) and 641.4 mm (LR», respectively.
Irish potatoes and field beans had short
effective rooting depths of 50 - 70 cm range,
followed by linseed which had 90 - 120 cm range
of effective rooting depth. Wheat, maize, sweet
potatoes, soyabeans and sunflower had effective
rooting depth of 150 - 180 cm.
Open free water evaporation values obtained
from Pan A data were found to be consistently lower
than those calculated from the modified Penman
method (McCulloch, 1965). Thus the crop coefficients
(Et/EO Penman) were consistently lower during the
'twoseasons, for all crops, than the pan coefficients
(Et/E
o Pan A). These coefficients were found to be
influenced by the frequency of wetting and amount
of rainfall as found by Kowal and Andrews (1973)
and Wangati (1972), in addition to being influenced
by the crops development and phenology. They were
alao found to be'closely related to water consumption.
The most efficient crap in terms of water use
waa Irish potatoes, 36.42 kg/ha/mm of water use and
20.50 kg/ha/mm of water use were realised during
the short and long rains respectively. Maize was
the next most efficient crop, wlth\9.50 kg/ha/mm
and ?22 kg/ha/mm)of water use during the short
and long rains respectively. These were followed
by sweet potatoes which had 6.93kg/ha/mm and
5.13 kg/ha/mm, field beans (2.42 kg/ha/mm and
2.80 kg/ha/mm) during the short and long rains
respectively. Wheat, sunflower, soyabeans and
linseed had 2.10, 1.24, 1.03 and 0.23 kg/ha/mm
of water use during the short rains, respectively.
However, water use efficiency reflected total
yields and therefore moisture availability, in
addition to other factors, such as pests and
diseases, influencing yields. | en |
