IMPROVEMENTS IN PLANNING OF AN IRRIGATION PROJECT – A CASE STUDY OF TANZANIA

Abstract

Quality of performance of an irrigation project depends on the quality of planning carried out during planning process of the given project. Well performing project assures high food production and food security to the people who are depending on the project. A case study of Mahiga irrigation scheme in Tanzania has been used to study the possibilities of improving the planning process of an irrigation project to optimize utilization of the present scarce water resource to improve food productivity and food security with regards to crop water and irrigation water requirement, water availability and water distribution system. Sufficient data availability for the estimation of the above parameters is a challenging problem, especially in developing countries. Thus, methods which require minimum data may be useful for planning purpose. Due to this data scarcity, conventional methods have been used in planning the project giving results which are uncertain. The Mahiga irrigation scheme located in Tanzania is used to suggest some methods which can improve the planning of irrigation projects while using the limited available data. Accurate estimation of project irrigation water requirement is one of the most important steps in the planning of an irrigation project. The study demonstrated application of ETo Calculator software and empirical formulas for estimation of irrigation water requirement. The ETO Calculator software and empirical methods are rarely used, therefore the CROPWAT software which is recommended by FAO and widely used method has been employed to validate and ascertain the usefulness. The estimated crop water requirement of the ETo calculator software considering empirical formulas and CROPWAT software were 2.72 MCM and 2.20 MCM respectively. Minor differences were observed in the values of crop water requirement obtained from ETo calculator software considering empirical formulas and CROPWAT software. Hence, in case of the limited data availability with small command areas and absence of CROPWAT software the ETo calculator software and suggested empirical formulas can be employed to estimate irrigation water requirements. Water availability for the scheme was estimated using the HEC-HMS software developed by the US Army Corp of Engineers. HEC-HMS was used to estimate runoff volume and peak discharge which can be found from the watershed collecting water to the Mahiga irrigation iv project. The software has several models which uses different input data in such a way that it allows the use to choose the model according to the available data. The estimated available wet season and yearly runoff volume from the watershed at 75% dependability was found to be 18.51MCM and 24.35MCM respectively. The amount was found to be enough to fulfill the estimated demand of 2.70MCM and allowing increasing the project irrigation intensity. Utilized potential irrigation land was increased from 50% to 86% by increasing the command area to be irrigated from the previously design command area of 200ha to 344ha. But also the intensity was increased from to 200% by increasing the cropping intensity for an agricultural year, whereby the crop has to be grown in two season (wet and dry season) by cultivating 400ha in each season. This will only be possible if a reservoir will be constructed. Storm rainfall and flood discharge were also estimated and found to be 68.1m3/s and 1.858MCM respectively, which are to be used to decide the spillway capacity of the reservoir. Moreover, the performance of the project was improved by applying the block/ group (also referred as Golongan) system in water distribution, whereby the 400ha was divided into four groups of 100ha each and the crop growing was scattered by having an interval of 1 month consecutively. This resulted in decrease of the monthly peak irrigation demand and hence reduced the canal discharge at head from 0.61m3/s to 0.55m3/s and is reduced by about 9.8%. This show that the water distribution method is important in reducing the canal discharge at canal head, and reducing the requirement of labor and improve equity in water distribution during operation and management of the project.