Naritaka Kubo

Performance diagnosis of Mae Lao Irrigation Scheme in Thailand (I) development of unsteady irrigation water distribution and consumption model.

The Mae Lao Irrigation Scheme is one of the largest irrigation projects in Northern Thailand. According to the field reconnaissance, water shortage usually occurs during the dry season. And it is very difficult to equally distribute available water to the paddy fields from the upstream to the downstream parts of the system. To understand and identify the causes of the problems, the measurement of water level and flow rate along all canals may be effective. However, it is not easy to achieve this in such a large-scale irrigation system. Thus, the numerical simulation becomes the second option. The objective of this study is to identify and quantify the real water shortage causes by developing an Unsteady Irrigation Water Distribution and Consumption model which can simulate the water movement and consumption in the whole irrigation system. The beneficial area of the right main canal is modeled based on the physical aspect of the system. The components of the model consist of canal networks, control structures, and paddy fields. A canal is divided into several portions called reach. The Saint-Venant equations are applied to describe the unsteady water movement in each reach. Flow movement at the control structure is expressed by the boundary condition. The paddy fields are modeled to make paddy block and connected to each reach. The water consumption in each paddy block is estimated by Paddy Tank model. The numerical model is successfully developed showing the ability to simulate the water movement and consumption properties in this irrigation system.

Performance diagnosis of Mae Lao irrigation scheme in Thailand (II). Application of the UIWDC model for water distribution system analysis

The Unsteady Irrigation Water Distribution and Consumption (UIWDC) model is applied to analyze causes of uneven water distribution between the upstream and downstream beneficial areas of the Mae Lao Irrigation Scheme (MLIS). The uneven water distribution may be caused by inadequate water distribution facilities or improper operation rule; therefore, its causes are examined systematically and quantitatively from the aspects of “water allocation” and “operation rule”. The water allocation is considered focusing on the dry season irrigation, where equity and efficiency should be especially balanced because of the scare water resources. The “EQTY index” (the equity index) is defined to widen the range of consideration between the equity and the efficiency, instead of alternative judgment of which has a priority. The operation rule for facilities in the MLIS is assumed considering their capacities, and two coordinate values of “ineffective spillage” and “water deficit” in the scheduled areas are incorporated into operation rule to quantitatively diagnose the system performance. As a result, the original causes of uneven water distribution will presumably be identified. The informative and quantitative results are utilized to set a new benchmark performance of the MLIS for the water distribution. It can be described by the “Expected Ratio of Irrigable Area” (ERIA) and “Present Ratio of Irrigable Area” (PRIA). Based on this standard, the general recommendations can be more concretely proposed to raise the water distribution performance of the MLIS such as by improving distribution facilities and/or by installing vertical pumps.

Reduction of Air Radiation Dose by Ponding Paddy Fields

Radioactive cesium (Cs) released by nuclear accidents is sorbed and fixed onto soil surfaces, which then radiate strong gamma rays (γ-rays). Decontamination around dwelling areas is now eagerly being implemented but more efforts are necessary to reduce the air radiation dose. Paddy field ponding, from the viewpoint of cost-effectiveness, is considered to be an effective practice for reducing the air radiation dose in the environment. In this study, field experiments were conducted at Sasu and Komiya regions in Iitate Village to verify the effectiveness of paddy field ponding, and numerical experiments were also conducted using the formula for uncollided γ-ray fluxes passing through the shield material.