Evaluation of water use and management on smallholder irrigation schemes : a case study of Mubuku irrigation scheme in Uganda

This thesis aims at evaluating water use and management practices on smallholderirrigation schemes by examining performance indicators such as relative water supply,relative irrigation supply, irrigation ratio and water delivery capacity. Water balanceratios such as application and storage efficiencies are examined to classify the irrigationsystem in respect to water supply. The research questions answered in are: How doesrelative water supply and water delivery capacity affect performance of the scheme?Does water logging at Mubuku irrigation scheme affect crop production? How does thecurrent institutional set up affect water management at the scheme? What measuresshould be put in place for proper water table management?The study area is Mubuku irrigation scheme in western Uganda. Mubuku irrigation scheme is currently in a transition to afarmer managed institution and is currently used as a regional seed multiplication centre.This qualifies it to be classified as a food basket of western Uganda. Production trends have been dwindling for the last 10 years and this has been attributedto the rising water table caused by inadequate drainage systems. The current irrigationand drainage network design does not incorporate subsurface drainage leaving asaturated root zone unconducive for crop growth. The current water managementpractice involves re-use of overland flow for the downstream farmers who in turnreceive excessive flow as they already have enough allocated to them. The problem of water logging is spreading steady fast starting with the downstreamtertiary blocks. There is evidence of partial water logging in the farms midstream especially. During the study, collection of primary and secondary data was carried in collaborationwith different stakeholders involved in the management of smallholder irrigationschemes in Uganda. Primary data included, field observations, measurements ofprimary canal discharge, soil moisture, depth of water applied on the fields,groundwater heads, yield trends and interviews with key stakeholders. The secondary data consisted of, cropping pattern, metrological data, River Mubuku and River Sebwe discharge measurements and topographical maps. It was established from discharge measurements at the head works that the total volumediverted each season is 6,630,000 m3. From the analysis of the crop water requirementusing the CROPWAT mode 531 mm depth of water is required to support crop growthfor the 514 ha of land currently cultivated. The analysis of the performance indicatorsreveal that, the relative water supply and relative irrigation supply is 2.4 and 4.6respectively. Water delivery capacity is 3.6 for phase I and 6.9 for phase II respectively. These results indicate an over supply of irrigation water compared to the required crop water demand. Subsequently a 3-dimentional groundwater flow model using the PMWIN-MODFLOWcode was set up to simulate groundwater movement in the study area. A steady statecondition was simulated for the peak demand during the first season.The model was calibrated with the PEST-ASP module with a correlation coefficientbetween the calculated and the observed being 0.92. Still there are strong localdeviations which need further validation when more geohyrological information isavailable from field measurements especially in the locations outside the irrigated area.However results from this study were accepted and simulations of the three scenarioswith the Modflow model reveal that application of 2.14 mm/d which represents areduction by a half yielded better results for an optimum water table management. It hasbeen recommended that a steady state flow groundwater model will be extended by aunsteady state model analysis to include the two crop growing seasons. The remainingamount of water can be used to open up more land for irrigation and hence increasingon the agricultural output.A recommendation has also been put across to have a sustainable water managementpractice the scheme should be institutionally managed in an integrated manner togetherwith the local government organs that has capacity to support part of the budget insteadof relying on farmers' contributions which are not forth coming.As an outcome of this research a schematisation of the irrigation and drainage networkthat can cope with the improved water management systems proposed has been provided and recommended for implementation. This research has illustrated the impact of water logging on crop yields, means ofimproving water table management by use of the groundwater flow model thatgenerated an adequate irrigation scheduling plan. This research answers in general howperformance evaluation can improve water management practices on smallholder irrigation schemes