Hydrology 2015; 3(6): 55-65 Published online October 14, 2015 (http://www.sciencepublishinggroup.com/j/hyd) doi: 10.11648/j.hyd.20150306.11 ISSN: 2330-7609 (Print); ISSN: 2330-7617 (Online) Calibration of Channel Roughness Coefficient for Thiba Main Canal Reach in Mwea Irrigation Scheme, Kenya Imbenzi J. Serede 1 , Benedict M. Mutua 2 , James M. Raude 3 1 Department of Planning and Design, National Irrigation Board, Nairobi, Kenya 2 Department of Agricultural Engineering, Egerton University, Nakuru, Kenya 3 Department of Biomechanical and Environmental Engineering, Jomo Kenyatta University of Agriculture and Technology, Juja, Kenya Email address: jaey03@gmail.com (I. J. Serede), bmmutua@gmail.com (B. M. Mutua), ramesso@yahoo.com (J. M. Raude) To cite this article: Imbenzi J. Serede, Benedict M. Mutua, James M. Raude. Calibration of Channel Roughness Coefficient for Thiba Main Canal Reach in Mwea Irrigation Scheme, Kenya. Hydrology. Vol. 3, No. 6, 2015, pp. 55-65. doi: 10.11648/j.hyd.20150306.11 Abstract: Canal roughness is one of the most sensitive parameter in simulation of irrigation canals. The present study attempted to calibrate the channel roughness coefficient (Manning’s “n” value) along the Thiba main canal reach, through simulation of canal discharges and water depths using HEC-RAS Model. After HEC-RAS model was calibrated and validated using two sets of observed discharges and water levels, it was used to simulate the hydraulic behaviour of Thiba main canal reach in Mwea Irrigation Scheme (MIS). The model was used to simulate different flows in the main canal as a result of varying the design discharges through the sluice gates and drop structures. Statistical and graphical techniques were used for model assessment to establish its performance. The results of the study showed that an increase in roughness coefficients caused a corresponding increase in the water levels for both Link Canal II (LCII) and Thiba Main Canal (TMC), while a decrease in roughness coefficients led to a decrease in water levels for both canals. The largest change in simulated water levels was 0.45 and 0.12 m in TMC and LCII respectively. It was concluded from the simulation study that Manning’s “n” value of 0.023 and 0.016 gave best result for LCII and TMC reaches respectively. Keywords: Calibration, Simulation, HEC-RAS Model, Reach 1. Introduction Water use and competition among different users has been growing at more than twice the rate of population increase over the last century. Water use for irrigation for instance, accounts for about 70-80% of the total freshwater available worldwide and irrigation has been ranked as one of the activities that utilize huge amounts of fresh water in many countries. Molden et al. (2007) affirms that in the near future, less water will be available for agricultural production due to competition with other sectors. At the same time, food production will have to be increased to feed the growing world population estimated at 81 million persons per year (UN, 2013) or about 9 billion people by 2050 (Munir and Qurreshi, 2010). Due to water shortage for irrigation in Mwea Irrigation Scheme (MIS), it is inevitable that the little available water needs to be utilized in an optimal way. This can be achieved through several strategies that include; proper design of canals, hydraulic structures and proper scheduling for water release to farmers. To achieve this, a total change in operation and maintenance of the systems is required (Maghsoud et al., 2013). In addition, further efforts have been developed to manage the limited available irrigation water. For instance, introduction of New Rice for Africa (NERICA) varieties which thrive in the uplands areas. Further, the use of System of Rice Intensification (SRI) which allows rice paddy to be grown in straight lines at a specified spacing leading to higher yields of rice is also another strategy being used. Maintenance of irrigation scheme infrastructure consisting of canals, roads and water management structures requires substantial amount of funds. Furthermore, preparation of a workable maintenance schedule may lead to disruption of the cropping programme leading to exorbitant losses to farmers. In order to address this challenge, proper understanding of the irrigation canal hydraulics and water management within the scheme can be assessed by use of irrigation simulation models which require calibration as was the case in this study. Among various canal hydraulic parameters, the channel roughness plays very important role in the study of open