ANNEX C: CROP WATER REQUIREMENT AND IRRIGATION SCHEDULE AGRICULTURE & IRRIGATION. December Paradis Someth Timo Räsänen

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1 ANNEX C: CROP WATER REQUIREMENT AND IRRIGATION SCHEDULE December 2012 MK3 Optimising cascades of hydropower AGRICULTURE & IRRIGATION Paradis Someth Timo Räsänen

2 Authors Produced by Suggested citation More information Image Project Team Copyright Paradis Someth (ITC), Timo A. Räsänen (Aalto) Mekong Challenge Program for Water & Food Project 3 Optimising cascades of hydropower for multiple use Lead by ICEM International Centre for Environmental Management Someth Paradis and Räsänen Timo Annex to Land Use Suitability for Agriculture in the Sesan Catchment: Crop Water Requirement and Irrigation. Project report: Challenge Program on Water & Food Mekong project MK3 Optimizing the management of a cascade of reservoirs at the catchment level. ICEM International Centre for Environmental Management, Hanoi Vietnam, Cover image: Ou Chum upper reservoir, Rattanakiri (Photo Peter-John Meynell). Inside cover: Wet season irrigation canal, Lower Sesan, Cambodia (Photo Peter-John Meynell) Peter-John Meynell (Team Leader), Jeremy Carew-Reid, Peter Ward, Tarek Ketelsen, Matti Kummu, Timo Räsänen, Marko Keskinen, Eric Baran, Olivier Joffre, Simon Tilleard, Vikas Godara, Luke Taylor, Truong Hong, Tranh Thi Minh Hue, Paradis Someth, Chantha Sochiva, Khamfeuane Sioudom, Mai Ky Vinh, Tran Thanh Cong 2013 ICEM - International Centre for Environmental Management 6A Lane 49, Tô Ngoc Vân Tay Ho, HA NOI Socialist Republic of Viet Nam

3 WLE MEKONG Optimising cascades of hydropower (MK3) TABLE OF CONTENTS 1 INTRODUCTION DATA AND METHODS Water balance in paddy fields Crop evapotranspiration Effective rainfall Rainfall and temperature data Rice growth stage and crop coefficient Soil type Irrigation scheduling RESULTS FINAL REMARKS... 6 REFERENCES... 7 iii

4 WLE MEKONG Optimising cascades of hydropower (MK3) 1 INTRODUCTION Soil water balance for the paddy fields was calculated using FAO CROPWAT 8.0 (Allen et al., 1998 ) for The main purpose of this soil water balance analysis is to calculate rice crop water requirements and irrigation schedules for the wet and dry seasons for two sites in the Sesan catchment, where are supposed to be high suitability for paddy rice cultivation. The data and methodology used for water balance calculations together with the main results are explained in this report. 2 DATA AND METHODS 2. 1 WATER BALANCE IN PADDY FIELDS The water balance in a paddy field was calculated by field storage and water volumes entering and leaving the field. The field storage consisted of ponded water and soil moisture. The inflow to the field consisted of precipitation, irrigation, surface inflow and seepage inflow, while the outflow was composed of crop evapotranspiration, infiltration, surface outflow and seepage outflow. Therefore, the water balance equation was is the change of field storage and all terms are expressed either in mm or in m 3. The water balance for the paddy fields was calculated for the whole irrigated area in two selected locations for both the rainy and dry season. In this study, surface drainage, runoff and seepage from one plot to another is assumed as negligible since these parameters are not available at the time of the study, and especially in Cambodia, paddy fields are characterised by high bunds without drainage and water is distributed and used very carefully. Therefore, the consumptive use principally consisted of crop evapotranspiration and infiltration. Watanabe (1999) also reported that, in a plot-to-plot irrigation system, water consumption consisted principally of total evapotranspiration and total infiltration. Hence, supply terms are only precipitation and irrigation CROP EVAPOTRANSPIRATION Rice crop evapotranspiration ( ) is one of the most important factors for evaluating water consumption in the paddy fields. In this study, was estimated by the single crop coefficient approach (Allen et al., 1998): where is crop evapotranspiration (mm/day); is crop coefficient; is reference evapotranspiration (mm/day) and daily calculated by the FAO Penman-Montheith method using minimum and maximum temperature. Other parameters are estimated using the temperature: where is net radiation at the crop surface (MJ/m 2 /day); is soil heat flux density (MJ/m 2 /day); is mean daily air temperature at 2 m height ( C); is wind speed at 2 m height (m/s); is mean saturation vapor pressure (kpa); is actual vapor pressure (kpa); is slope of vapor pressure curve (kpa/ C); and is psychrometric constant (kpa/ C). 2

5 2. 3 EFFECTIVE RAINFALL Effective rainfall was calculated by USDA Soil Conservation Service, for decade (daily rainfall is aggregated per decade) as follows: for for mm mm 2. 4 RAINFALL AND TEMPERATURE DATA The daily rainfall and temperature data required for paddy field water balance calculations were derived from the hydrological model calibrated for Sesan River catchment (for description of hydrological modelling see Annex 1). We used two locations in the Sesan catchment to extract the rainfall and temperature data in order to take into account the differences in hydroclimate within the basin. The location 1 was selected from the central part of Lower Sesan basin in Cambodia and the location 2 was selected from the central part of Upper Sesan Basin in Vietnam. The two locations were selected also so that they reflect the hydroclimate of existing or potential future agricultural areas. The two locations for hydroclimate data were following: Location 1 (Cambodia): Latitude: N Longitude: E Altitude: 108 m Location 2 (Viet Nam): Latitude: N Longitude: E Altitude: 600 m 2. 5 RICE GROWTH STAGE AND CROP COEFFICIENT Two types of rice varieties were selected for the crop water requirement and irrigation scheduling: medium variety of 125 days for wet season rice and early variety of 105 days for dry season rice. Growing date started from 15 May for wet season rice, while from 01 December for dry season rice for the two selected sites. The root depth for these types of rice ranged from 30 cm to 70 cm. Crop establishment is transplanting for Cambodia and direct sowing for Vietnam. Therefore, nursery stage is only applicable to irrigation in Cambodia. Growing length and crop coefficient (Kc) for both rice varieties of each development stage were adopted as follows: - Wet season rice (146 days): 21 days and Kc = for nursery stage, 60 days and Kc = for vegetative stage, 25 days and Kc = for reproductive stage, and 40 days and Kc = for late season stage. - Dry season rice (119 days): 14 days and Kc = for nursery stage, 55 days and Kc = for vegetative stage, 20 days and Kc = for reproductive stage, and 30 days and Kc = for late season stage SOIL TYPE Soil type was assumed to be Sandy Loam for Site 1 (Cambodia) and Loam for Site 2 (Viet Nam). Puddling depth was set to 0.30 m for both sites. Maximum percolation was set to 3.1 mm/day for Sandy Loam in Cambodia and 3.4 mm/day for Loam in Viet Nam. 3

6 2. 7 IRRIGATION SCHEDULING There were two applications of irrigation scheduling for land preparation. First application (prepuddling) brought the soil to saturation and second application (puddling) is for flooding the paddy fields. Soil was refilled to saturation and then to standing water depth standing of 100 mm for each application. It is assumed that there is no irrigation (rain-fed) for wet season rice and 7-day rotation irrigation of 100 mm each time for dry season rice. Irrigated water is distributed from plot-to-plot, which is typical irrigation in Cambodia. Irrigation efficiency for both sites was set to 70%, considering unlined conveyance canals and plot-to-plot irrigation. Yield reduction due to soil moisture stress (percentage of the maximum production achievable in the area under optimal conditions) is not considered in this study 4

7 3 RESULTS The rice crop water requirements and irrigation schedules were calculated for the wet and dry seasons for , for two sites in the Sesan catchment. The results of the calculation were presented in Table 1. Rainfall in Cambodia was found to be little higher than in Vietnam. Similarly, evapotranspiration in Cambodia was generally higher in Vietnam. Irrigation water for wet season rice for both locations were only for land preparation (pre-puddling and puddling) and not for crop growing purpose. High amount of water would be needed in the land preparation process and usually rainfall amount is not enough at the beginning of the growing season. Average water requirement for land preparation at field level was 190 mm for Cambodia and 153 mm for Vietnam. On the other hand, dry season cultivation main depended on irrigation water. Net irrigation for dry season cultivation at field in Cambodia and Vietnam was 930 mm and 874 mm, or 1328 mm and 1248 mm for gross irrigation, respectively. The requirement for water abstraction for irrigation in the upper and lower Sesan catchments were on average m 3 /ha and m 3 /ha annually of which 2200 m 3 /ha and 2700 m 3 /ha were for wet season irrigation, respectively. The slightly higher irrigation demand in lower Sesan catchment resulted mainly from the different hydrometeorological conditions within the basin. The lower Sesan catchment was on average warmer and drier that the upper Sesan catchment. The weekly irrigation patterns for dry and wet season rice are shown in Figure 1A and the annual irrigation volumes in Figure 1B. The annual irrigation demand in the upper and lower Sesan catchments were lowest in 2002 ( m 3 /ha and m 3 /ha) and highest in 2003 ( m 3 /ha and m 3 /ha) thus the inter-annual variation was relatively small. The assumed irrigation efficiency of 70% resulted in annual water losses 4800 m 3 /ha and 4440 m 3 /ha in upper and lower Sesan catchments, respectively. Cambodia (mm) Vietnam (mm) Wet rice Rain Eta Perc Net Irri Gros irri Rain Eta Perc Net Irri Gros irri Avg Dry rice Avg

8 Figure 1 Estimated irrigation abstraction water requirements for upper (Vietnam) and lower (Cambodia) Sesan catchment on A) weekly and B) annual scale for transplanted irrigated dry and wet season rice. 4 FINAL REMARKS This study described briefly method used to estimate soil water balance for the paddy fields. The calculation was performed without calibration. The results could be refined with observation when applicable, especially soil type. This parameter affects the most the water consumption in paddy fields. In addition, it is also important to note that meteorological data used in this study is derived from interpolation of other observed locations. And only the temperature was used to estimate rice crop evapotranspiration. It is recommended to have a full set of meteorological data for better estimation of the reference evapotranspiration. However, with a given limited data available in the study area, the results in this report could be used as a preliminary analysis. 6

9 REFERENCES Allen, R.G., Pereira, L.S., Raes, D. and Smith, M., 1998 Crop Evapotranspiration - Guidelines for Computing Crop Water Requirements. Irrigation and Drainage, Paper 56. Food and Agriculture Organization (FAO) of the United Nations, Rome. Watanabe, T., Irrigation water requirement. In: Mizutani, M., Hasegawa, S., Koga, K., Goto, A., Murty, V.V.N. (Eds.), Advanced paddy field engineering. Editorial Committee of Advanced Paddy Field Engineering. The Japanese Society of Irrigation, Drainage and Reclamation Engineering (JSIDRE). Shinza-Sha Sci. Tech., Tokyo, pp