Drainage of lowland sugarcane fields:Mumias, Kenya
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The Mumias Sugar Company (MSC) has a sugarcane nucleus estate measuring 3,000 ha lying astride the Nzoia river. The nucleus estate comprises of well drained uplands and poorly drained lowlands lying below a spring line where the water table is within the top 50 cm for at least six months. The present study was done to find out the causes of the poor drainage in the lowlands, the long term effects of the poor drainage on sugarcane yields and then give remedial measures to improve the drainage. Drainage investigations which involved monitoring the groundwater behaviour in a section of the lowlands; water balance studies to ascertain the sources of excess water; soil and topographic studies to ascertain the causes of the drainage problem and the yield studies to compare yields from poorly drained and well drained fields were carried out. It was found that upslope seepage into the lowlands and the excessively high rainfall amounts in April - June were the main sources of excess water especially where the seepage interceptor drains were wrongly sited. Lack of Lind grading, poor drains maintenance, lack of adequate grade and reverse slopes of field and collector drains were found to be the causes of poor surface drainage whereas the virtually impermeable two metre thick clay (glei) layer underlying the a - 50 cm top soil was the cause of poor subsurface drainage. Hence, land grading is recommended as a prerequisite to any other drainage design after which the field and the collector drains should be regraded. Cambered beds with beds spacing in multiples of 1.5 m which is the sugarcane spacing are recommended. French drains laid within polyfelt cloth are given as an alternative to act as surface drains as well as subsurface drains for the top 50 cm soil. Without disrupting the present fields layout, a surface drainage system showing drains dimensions and grades is proposed based on long term rainfall data as well as rainfall storm data. Subsurface drains at 1 m depth and spaced at 12 m would control the groundwater table preventing waterlogging of the rootzone for more than 14 days. The yields versus field drainage analysis showed that on average, yields in the well drained fields are 1.84 tonne/ha/month higher than In the poorly drained fields.