Study on performance of dry season rotational irrigation for Mae Lao irrigation project, Thailand Naritaka KUBO: Univ. of Tokyo, Japan Takuya TAKEUCHI: Tokyo Univ. of Agri. & Tec., Japan Unggoon WONGTRAGOON: Rajamangala Uni. of Tec., Thailand Hajime TANJI: NIRE, NARO, Tsukuba, Japan

Back Ground In south east Asian countries… Irrigation projects were developed primarily for supplemental supply during rainy season Constructing new reservoirs makes irrigation possible during dry season Water supply is not enough Restricted irrigation areas because of scare water Necessity of effective water use

Mae Lao irrigation scheme Project site Mae Lao irrigation scheme Tributary of the Kok River belonging to Mekong River basin Chiang Rai in North Thailand Irrigation Area : 23680 ha

Mae Lao project site Right Main Canal （RMC） - RMC length ：about 50 km Branch 3 4,848 ha Branch 2 5,264 ha Branch 1 7,968 ha Branch 4 5,600 ha Mae Lao river Left Main Canal Right Main Canal Mae Lao weir Mae Suai Dam Laterals Streams 5 10 km Mae Suai Dam Mae Lao Weir Right Main Canal （RMC） 　- RMC length ：about 50 km 　- Secondary canals ： 23 　- Max Q： about 27 m3/s 18,080 ha

Water deficit in dry season Interviews Branch 1 　　・Water deficit occurs partially Branch 2 　　・Irrigation area is expanded beyond allotted area 　　・Water deficit occurs in downstream areas Branch 3 　　・Water deficit is serious 　　・No water comes to downstream areas

Summary of interviews Problem identification Mae Suai Dam can store 73,000,000 m3 Low distribution efficiency Unequal water distribution Functional problems Managerial problems Illegal activities 　Problem identification

　　Objectives Quantitative analysis of effects of facilities　and water management on water distribution performance 1. Planning phase Continuous irrigation by non-uniform flow model Effects by physical causes of facilities & structures 2. Execution phase Rotational irrigation simulated by UIWDC model Effects by managerial & institutional causes

Methodology Numerical simulation by UIWDC model（Unggoon et al., 2010） 　（Unsteady Irrigation Water Distribution and Consumption） Water movement in canal ⇒　1-D unsteady flow model Water consumption in paddy field ⇒　Paddy Tank model Saint-Venant Equations inundated Plow layer A：Area, Q：Flow, q：side flow, h：depth, g：gravitational acceleration, S0：bed slope, Sf：friction slope, u：mean velocity Ground Water

Modeling of irrigation system HW 1L 2L 3L 4L Paddy field CK Check structure PF withdrawal Direct Branch 1 Branch 2 5L~12L Branch 3 13L~22L

Modeling of FTOs and water distribution

Intake flow rate (IFR) 1. Based on water requirement Qs 2. Base on non-uniform flow calculation Qe （considering physical properties） Assumptions for Calculation Equal water distribution within a branch Daily water consumption in paddy field:14.7 mm/day Each branch is calculated independently Total paddy field area in Branch i : Ai Scheduled area to be irrigated in Branch i : Asi

1. Calculation of scheduled IFR Qs Exact water volume to irrigate area of Asi 　⇒　Scheduled Qsi for Branch i Based on water requirement 　＝Paddy water consumption ×Asi + Seepage loss Canal seepage losses are calculated assuming Full Supply Level PF to be irrigated not to be

2. Calculation of equilibrium IFR Qe Calculated by numerical simulation for non-uniform flow IFR at equilibrium 　（useless spillage＝deficit）　　 deficit ⇒Equilibrium Qei for Branch i　　 Paddy field canal Spillage Canal seepage loss considering Non-uniform Flow Level Inlet of FTO

Result (1) Scheduled area vs. IFR Q Maximum Flow Rate 27 m3/sec Scheduled Qsi & Equilibrium Qei Scheduled area ratio (Asi/Ai) Intake Flow Rate (m3/s) No differences at no seepage losses → caused by seepage losses Lower water level than that of FSL Longer distance causes more losses

Result (2) Scheduled area vs. WSR WSR = [Actually distributed water]/[Volume to be distributed] (Water supply ratio) WSR Branch 1 Branch 2 Branch 3 Scheduled area ratio (SAR Asi/Ai) Larger canal section Lower water level Higher threshold of FTO More difficult withdrawal Lower WSR More upstream

Rotational irrigation （execution phase） Intake Flow Rate Q : 10 m3/s Field water supply : 3 times of daily water requirement 5 days Observance of rotation : Upstream branches do not withdraw water during off-turn Strict application rule: Water withdrawal stops when ponded water exceeds 100 mm depth, and irrigation re-starts at 80 ％ of soil moisture 5 days 5 days

Water management conditions Planning water management （Type O） Observance of rotation Strict application rule Possible water managements Type A : Direct FTOs use riparian right and application rule is strict Type B : All FTOs observe rotation and application rule is not strict Type AB : Direct FTOs use riparian right and application rule is not strict

Result (3) WSR corresponding to water management types Along RMC Along lateral canal WSR Branch 1 Branch 2 Branch 3 WSR Branch 1 Branch 2 Branch 3 1. Low WSR for Branch 1 along RMC 2. High WSR for Branch 2 along RMC at AB type management 3. Low WSR for Branch 3 along lateral canal at AB type management

Result (4) Direct FTO WSR along RMC based on type O Branch 2 Branch 3 Branch 1 Upstream Downstream WSR Larger cross section High inlet of FTO Lower water level Difficult withdrawal

Checks, Laterals and Spill ways along RMC

　　 Result (5) Direct FTO WSR in Branches 2 and 3, along RMC based on types of O and AB Sure water withdrawal Excessive at off-rotation O WSR Branch 2 Branch 3 AB

Result (6) FTO WSR at laterals in Branch 3 based on types of O and AB Excessive withdrawal at midstream Water deficit at downstream 13L 14L 15L 17L 17La 18L 19L 20L 21L 22L WSR AB

　　Conclusion Water distribution performance is influenced by structures and strictness of water management Influence by structures 　　　Water withdrawal is restricted when water level is low. Influence by strictness of water management 　　　One of two observances of rules improves water distribution performance considerably Other wise 　　　Excessive withdrawal at middle branch 　　　Serious water deficit at downstream branch