SIMULATION OF CANAL CONVEYANCE EFFICIENCY FOR AHERO IRRIGATION SCHEME USING HEC-RAS MODEL

Abstract

Irrigation plays a critical role in addressing food security as envisaged in one of the six key pillars (agriculture) of the Kenyan government's economic model. The government's strategy, among others, seeks to ensure that agricultural activities are anchored on technology to improve productivity and profitability while minimising the cost of production. However, Ahero Irrigation Scheme, established in 1969, experiences challenges with canal conveyance efficiencies, due to years of operation under imprecise maintenance practices. Moreover, the performance of any open channel irrigation system is a function of its canal conveyance efficiency among other factors, requiring determination of the same. To overcome challenges with irrigation water conveyance at Ahero Irrigation Scheme, the current study aimed to simulate conveyance efficiencies for the Ahero Irrigation Scheme canal network to inform effective maintenance practices. The Hydrologic Engineering Centre River Analysis System (HEC-RAS) model was used to simulate the canal conveyance characteristics at the tail-end section of the canal network, covering a total length of 2.6 km to inform on improved maintenance practices. Geometry, flow and velocity data gathered from the scheme network was used to simulate different flow scenarios to determine the optimal flow scenario for the canal network. Calibration was conducted before the application of the HEC-RAS software with the simulated against actual depth data, yielding a significant R2 value of 0.857. The study also estimated the crop water requirement for rice using the FAO-CROPWAT model to determine any flow deficit at the tail-end section of the scheme. The measured canal capacity in its unmaintained state revealed a discharge capacity of 0.228 m3/s, which was significantly lower than the minimum crop water requirement estimation of about 0.38 m3/s (a 40% water deficit within the canal network). A comparison with the original canal design flow capacity (0.45m3/s) also suggested a 49% drop in the carrying capacity due to dilapidation and degradation. Simulations of the network suggests that the concrete lined trapezoidal cross-section of the canal had a safe carrying capacity throughout the studied canal network. Consequently, this study recommends canal upgrade (levelling bed undulations, dredging, and smooth concrete lining) to attain the optimal flow capacity at the tail end of the network.