| dc.description.abstract | Sugarcane production in Mumias Sugar Company Limited, Kenya, faced drainage problems in the
lowland fields. The existing cutoff-drain and other drainage structures had not provided adequate
solution to the problems. In view of the situation, this study was undertaken with the objectives of
identifying the causes of water logging, determining an appropriate means of controlling the water
logging and quantifying the potential flows for use in design of elaborate drainage and irrigation
systems.
Different parameters including land slope; soil texture, storage properties, hydraulic conductivity
and infiltration; ground water levels; depth to impermeable layer; rainfall; evapotranspiration under
sugarcane conditions; yields; and flows both subsurface and surface were studied over a period
covering dry and wet seasons. The results of the studies showed that the cause of drainage
problems is an interaction of rainfall, topography and presence of glei, i.e. relatively impermeabile
clay soils in the lowlands combined with the seepage of ground water coming from the uplands
coupled with consequences of wrongly placed interceptor drain and inadequate number of infield
drains previously installed in the lowland fields.
The mean annual rainfall in the area is 1980mm and therefore nearly balanced out the peak
evapotranspiration in the sugarcane plantation that was estimated at 2000mm per year. Ground
water levels were found to fluctuate with rainfall, a situation that in turn influenced the seepage
flows (springs), which compounded the waterlogging problem in the lowland cane fields. The
upland area comprises long sloping land of less than 3% slope and a plateau that are of highly
permeable coarse textured soil with infiltration rates of 6 to 14 cm/hr and hydraulic conductivity of
6m/day. From this upland area, estimated to occupy 230ha, water is taken in and gradually released
as seepage flows, below the existing cut off drain, at the slope-break into some 200ha lowland area.
The lowlands are on the other hand is dominated by the glei at shallow depth within 70cm below
surface, with low infiltration rate at 2cmlhr; hydraulic conductivity of O.1m/day and surface slope
that is less than 1%, all of which impede the internal drainage process as well as surface runoff,
with subsequent piling up of both local water from direct rainfall and water from the uplands. The
existing interceptor drain though excavated to depths beyond 2m was situated far above the
seepage or spring line and therefore could not intercept the seepage flows. Furthermore its bed
slope was not graded thereby limiting its function of discharging the surface flows; the limited
number of the existing infield drains not withstanding. The infield drains were placed without
reference to recommendation of any studies and failed to function as manifested in the persistence
of water logging problem in the lowlands. It was therefore established as to what causes the
waterlogging situation observed in the affected lowlands, even though some 20% of the area still
suffered water stress during the months of moisture deficits; owing to the glei presence.
Performance of a pilot drain dug on the route of the proposed interceptor drain around the slopebreaks
between the 91 m and 100 m contours on the study base map, indicated that such a drain of
only about 1m depths, would effectively protect the waterlogged lowlands from the excess waters
of the uplands. The intercepted flow could be estimated by use of a mathematical formula
developed for the site to relate the drain flow and groundwater levels below surface with recession
time on a known surface elevation. The total flow, estimated at 90mm/month, ~ver the lowland
area was found to be in excess of the established monthly water balance deficits of 55mm/month
over the same area. The seepage flows were of good water quality and could irrigate about 70 ha
of the lowland area with some considerable subsequent improvement of cropping schedule towards
a better management of the waterlogged fields; permissible by. both higher yield levels and lowered
seed rates through drainage and irrigation respectively. By installing regulating structures inside
the drain, manipulation of the water level within the drain to enable drainage or sub-surface
irrigation was considered possible since the permeable top layer of the lowland soil profile allows
for the desired movement of water, as would be the case of sub-surface irrigation in the lowlands. | en |
