Diagnosis of Water Management Problems and Sediment Monitoring in EL Semeih Water Harvesting Project

Abstract

Spate irrigation is runoff farming where flash floods are diverted by intake structures to irrigate crops using basin system. The indigenous spate irrigation practices have been described to be not effective and of low crop productivity due to many problems (such as the destruction of headwork during large flood events and deposition of heavy sediment load in canals and irrigated fields). Spate irrigation system has been largely neglected in the literature and there are no available technical guidelines that determine its specificities. Yet, it is different from conventional irrigation in many ways and needs special skills and approaches of which practitioners are not always aware. The, standard design approaches cannot appropriately take into account the level of uncertainty related to floods, the hydraulic challenge of guiding flood flows, the heavy sediment loads, the exceptional nature of the water rights, or the management and maintenance models that are specific to spate irrigation. To improve productivity of spate irrigation it is recommended to select proper rehabilitation and modernization program. Basic step to reach this end is to quantitatively diagnose these problems and there by decide on program activities. Against this background, this study is directed to analyze the existing performance and mode of operation in El Semeih agricultural scheme, which is one of the old spate systems in Sudan. Therefore, the overall objective of this study is to assist planners and practitioners in designing and managing other spate irrigation projects. The study specific objectives are two folds: Firstly: to diagnose and evaluate performance of El Semeih Scheme with respect to water utilization and management, agricultural productivity, and allocation of seasonal budget. Secondly: to identify the type and XIII distribution of sediment load entering in El Semeih agricultural scheme. To achieve these objectives primary and secondary data is collected for Khor Abu Habil and El Semeih spate irrigation project during season2010/2011. The primary data covers measurement of discharge, sediment settling velocity, concentration, and Particle size distribution, Sediment removal rate, and land levels along the main canal. While secondary data covers soil, climate, areas, agricultural data, project budget and past studies. The obtained results are discussed with reference to the reviewed past studies, theoretical framework, and current practices. The study employed evaluation procedure for diagnosis of project performance covers water resource utilization and sediment monitoring. Water resource utilization is discussed in terms of: water use efficiency (WUE), hydraulic performance (adequacy, and equity indicators), agricultural performance (area utilization and crop productivity indices), and budget utilization. Sediment monitoring is discussed in relation to determination of sediment sizes, spatial and temporal distribution of sediment, and sediment removal rate. It is inferred from the results that indices of irrigation efficiency, crop productivity, area utilization efficiency, budget allocation, and water use efficiency are low due to poor system of water management. The equity indicator is low reflecting existence of top-tail ends problem in the project and adequacy of supply can be described as poor and function of available rain water. The study also reveals that low sediment removal rate (high sediment concentration) is found at reaches located at canal top and more sediment deposition occurred at these reaches (non uniform head- tail sediment distribution) due to the mode of water distribution. As conclusion the study recommend to improve the existing mode of water management and distribution, study alternative schemes to store water and select the most feasible method for sediment control. Key words: Spate Irrigation, Sediment monitoring, irrigation performance; agricultural production; water resources