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Impacts of Air Temperature Variations on the Boro Rice Phenology in Bangladesh: Implications for Irrigation Requirements

Rezaul Mahmood
Department of Geography, College of Geosciences, University of Oklahoma, Norman, OK 73019, USA

Abstract/ Executive Summary:

Air temperature significantly affects crop phenology. Numerous experiments have shown that prevailing air temperature determines the length of crop growth stages. Irrigation field requirements depend on the length of the crop growth stages. In the present study, a physically based parametric model, YIELD, has been applied to estimate the impacts of fluctuating air temperature (due to inter-annual climatic variability and global warming) on evapotranspiration water requirements and the length of growth stages of the irrigated boro rice in Bangladesh. The YIELD model is crop-specific and crop-growth-stage specific which is a compromise between area-specific regression models and complex crop growth simulation models. The model was tuned to Bangladesh’s environment to represent appropriate agro-ecological conditions including soil type, depth of groundwater, field size, wind regime, and percolation losses. YIELD has been validated for the length of the growth stages, length of the growing season, final yield, and evapotranspiration. A baseline estimate for the boro rice phenology has been established by running the model for 12 meteorological stations located in the major rice growing regions. Based on the analysis of the past variations of air temperature and general circulation model (GCM) predictions, ten scenarios have been created to estimate the effects of these variations on the boro rice growth stages. These applications find that the planting date plays an important role in the boro rice phenology. This effect is most noticeable during initial growth stages. This study has found a non-linear relationship between decreasing air temperature and the length of the initial growth stage and a predominantly linear relationship with other growth stages. Model applications show that an increase in air temperature will provide longer and more stable thermal conditions for boro rice maturing stage. A 5% increase and a 4% decrease in seasonal total evapotranspiration will occur under each 1 ° cooler and warmer air temperature conditions, respectively. A rise in evapotranspiration will cause higher demands for irrigation water. Such conditions will put pressure on the current irrigation infra-structural facilities in Bangladesh and result in reduction of boro yields. Furthermore, variations in the percent of time required for the completion of different growth stages under various air temperature conditions will demand a reorganization of irrigation schedules.

Keywords: Air temperature variations; Boro rice phenology; Irrigation requirements

Location: In Bangladesh

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Water requirement

Objectives:

A set of 12 meteorological stations, well distributed over the major rice-growing regions in Bangladesh, have been selected and used in this study. Furthermore, these stations are consolidated into three groups based on their latitudinal locations.

Methodology:

The YIELD model evolved from a crop water balance model called WATER developed by Burt et al. (1980), Burt et al. (1981). It has been validated for several major agricultural crops including rice, grain com, wheat, barley, potato, and alfalfa. YIELD model has been successfully applied to several regional studies including Australia, the North American Great Plains, California (Hayes, 1986), and Bangladesh (Mahmood, 1993; Mahmood and Hayes, 1995).

The YIELD model’s evapotranspiration calculation is based upon methods developed by Doorenbos and Pruitt (1977) and Doorenbos and Kassam (1979). They have used the Penman (1948) equation to calculate evapotranspiration (ET). Penman’s equation combines energy balance and aerodynamic functions to calculate evapotranspiration. Doorenbos and Pruitt (1977) successfully modified this equation for reference crops (which is an extended surface of 8 to 15 cm tall, green grass cover of uniform height, actively growing, completely shading the ground and not short of water). The modified equation included the effects of crop type, crop-growth-stage, selected site factors, influence of unusual climatic conditions, ET adjusting crop coefficients ET for specific crops and specific crop growth stages, and soil moisture budget conditions. The submodels CROP1, 2, 3, 4, and 5 constitute the calculation procedure for ET of the reference crop and actual crop (Table 2). The model calculates each variable for a set time step. In this study time step is 5 days. ET is the sum of radiative and moisture terms, involving net radiation and atmospheric vapor pressure deficit, respectively, in CROP1 (Doorenbos and Pruitt, 1977; Burt et al., 1980). CROP2 adjusts CROP1 for wind regimes, radiation amounts, and mean daily maximum relative humidity. CROP3 is the adjustment for CROP2 by crop-specific, growth-stage-specific coefficients that consider varying effects of crop growth stages on plant water use. CROP4 adjusts CROP3 for ‘clothesline’ and ‘oasis’ effects. Optimal water conditions have been assumed up to CROP4. CROP5 deals with an array of possible water stress situations of a specific crop and specific growth stages. For each crop and season, a soil water budget is calculated. In the YIELD model, soil water budgets are a function of a number of variables that includes growth-stage-specific ET, soil water storage, effective precipitation, groundwater contributions, variable root depths, and percolation losses. As this study assumes optimum supply of water, CROP5 is equal to CROP4. Further description of the YIELD model can be found in Burt et al. (1980) and Hayes et al. (1982b).

Source of Information: Agricultural and Forest Meteorology 84:3-4 (April 1997), pp 233-247.

Article Link (Online Journal): DOI:10.1016/S0168-1923(96)02360-X

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

The YIELD model has been applied to 12 major rice-growing regions for ten air temperature variation scenarios. These scenarios represent cooler or warmer growing season air temperatures and possible global warming conditions. The present study estimates a non-linear relationship between decreasing air temperature and the length of the initial growth stage and a largely linear relationship with other growth stages. Under T_air – 5°C condition, the YIELD model pushes transplanting dates 2-7 weeks forward causing a relatively quick entrance of the boro rice plants into the summer season. This results in shorter initial growth stage under T_air – 5°C scenario compared with T_air– 4, – 3, and – 2°C scenarios. As air temperature increases from Tair – 5°C to Tair – 3°C conditions, the length of initial growth stages also increases. Increasing air temperatures push transplanting dates close to January 1 and force rice plants to grow under a cooler thermal climate. Such climate conditions require a longer time for boro rice to complete initial growth stage. This study finds that seasonal total evapotranspiration will increase up to 5% under each 1°C cooler than normal conditions. A rise in evapotranspiration will result in higher demands for irrigation water. These conditions will put tremendous pressure on the current irrigation facilities in Bangladesh and result in reduction of boro rice yields.

The model application shows that an increase in air temperature will provide longer and more stable thermal conditions for the boro rice maturing stage. A 4% decrease in seasonal total evapotranspiration will occur under each 1°C increased air temperature conditions. The warmer air temperature conditions will result in shorter growing season and reduce end of the season total evapotranspiration. Furthermore, the variations in the percent of time required for the completion of different growth stages under various air temperature conditions will require a reorganization of irrigation schedules.

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