Evaluation of Performance of an Irrigation Scheme: The case of Mwea Tebere Irrigation Scheme, Kenya

EXECUTIVE SUMMARY.

In the supply of water through canals, lined or earthen, water losses do occur. In irrigation schemes, these conveyance losses are very significant as they would in turn determine the production per unit area of land. It thus follows without saying that water losses and food production by an irrigation project are directly related. The main objective of this case study was to evaluate irrigation infrastructure of an irrigation scheme, specifically the water conveyance losses that tend to occur in the open canal irrigation network of Mwea Tebere Irrigation Scheme. The scope was geared towards contributing to sustainable production from irrigated rice agriculture in the area especially after the low performance decline that faced the scheme due to low water levels in the season commencing 2004. The research was carried out in the main, secondary, and tertiary canals. The presence of calibrated instruments in these canals like weirs largely aided in the computation of flows, while a current meter was used to establish the velocities at the various flows. Statistical relationships between canal types, canal shapes, canal length and type of lining was also investigated. The total percentage conveyance loss was found to be 48.56% in the Thiba main canal and the total percentage conveyance loss was found to be 22.88% in the Nyamindi main canal. The average loss in the main canals was 35.72%.  At the secondary canal level the average water conveyance loss for the trapezoidal canals on the Thiba main canal was 28.42% and for the Nyamindi main canal was 28.35%.The total percentage conveyance loss for all the tertiary canals, three on Nyamindi and four on Thiba was found to be 28.39%. The agricultural performance of the scheme shows that the production rate obtained varied between 3.687 tons/ha and 5.086 tons/ha, per unit of cropped area and 1.842 tons/ha to 4.486 tons/ha per unit of command. The output obtained out of a unit of water volume consumed varied from 1255.2 tons/m3/s to 3046.1 tons/m3/s. The results showed the scheme is not performing to its designed capacity as both the agricultural performance and the water delivery capacity of the scheme are way below optimum. The conveyance losses in the scheme are higher than the values determined in a similar project 40 years ago and this is a clear indication of the continued deterioration of the infrastructure in the scheme and the overall, maintenance and repair work on the conveyance canals were not sufficient.  

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