Modeling water resources allocation in a run-of-the-river rice irrigation scheme

Seventy-five percent of the available water resources in Malaysia are used for rice irrigation. Proper water management must be given due emphasis to effectively manage the water resources. This study analyzed field level practices, which could save irrigation water and thus increase area to be irrigated. The analyses were conducted for both the pre-saturation and normal supply periods using field data collected at the Besut irrigation scheme, Malaysa. Based on field water requirements and available flows at the intake structures, canal simulation was performed using the CanalMan model (Utah State University) together with water balance. The results have shown that pre-saturation should not be done continuously unless flow rates are at least 9.00 and 3.00 m³s⁻¹ for the Besut and Angga barrages, respectively. If the flow rate falls below these values, then pre-saturation should be done in two phases. However, when the flow is between 5.00 and 5.50 m³s⁻¹ at Besut barrage, pre-saturation should be done in three phases. The simulated schedules were compared with present pre-saturation schedules and it was observed that irrigable area could be increased by 10% than that at present. During the period of normal irrigation supply to the fields, there must be flow rates of at least 5.00 m³ s⁻¹ and 1.50 m³ s⁻¹ at the Besut and Angga barrages, respectively; in order to maintain irrigation supply to the whole irrigation scheme. Otherwise, selective irrigation or irrigation on a rotational basis will have to be enforced.