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Modeling of Seasonal Water Balance for Crop Production in Bangladesh with Implications for Future Projection

Mohammed R. Karim
Graduate School of Environmental Science, Hokkaido University, Japan

Mamoru Ishikawa
Graduate School of Environmental Science, Hokkaido University, Japan

Motoyoshi Ikeda
Graduate School of Environmental Science, Hokkaido University, Japan

Abstract/ Executive Summary:

Water deficiency is considered to be one of the major climatic factors limiting crop production in Bangladesh, especially in the dry season. To better understand the response of crops to moisture variation, a quantitative analysis of major water balance components, namely, potential evapotranspiration (PET), actual evapotranspiration (AET), soil moisture storage (ST), water deficiency (WD), and water surplus (WS), was carried out using the Thornthwaite monthly water balance program. Analyzes were carried out for three different seasons, incorporating interannual variability, in 12 major rice-growing districts of Bangladesh, which represented the northern, central, southern, and coastal zones. Hind cast monthly average surface air temperature and precipitation data were collected from the Bangladesh Meteorological Department (BMD) for the period 1986 to 2006. The analysis results suggested that the PET trend was the same at every station and that generally higher values were observed in the months of July and August. Khulna, a coastal station, had the highest annual average PET of 1369 mm. The lowest annual AET of 1108 mm was estimated for Teknaf, while the second lowest value of AET was recorded in Dinajpur. The ST was found to be almost at field capacity from July to September, and the southern station of Chittagong had the highest average monthly ST. Future projection showed the northern part of Bangladesh would be less vulnerable regarding ST. The maximum WD was found in Bogra, and the second highest value was found in Dinajpur. Estimation of the average WD of 178 mm yr^–1 in northern Bangladesh indicated that this region was subject to the greatest degree of WD and that winter is the most crucial season in determining water scarcity. The smallest value of WS was noted for the coastal station of Khulna. A significant positive relationship (P<0.05) between soil moisture and current rice yields proved the importance of surplus water conservation in the drought-prone zone of Bangladesh. To boost rice production and help cope with the consequences of climate change, integrated adaptation and mitigation measures should be adopted for agriculture.

Keywords: Deficit, Evapotranspiration, Moisture, Season, Surplus, Water

Location: Dinajpur, Rangpur, and Bogra represented the northern part; Faridpur, Tangail, and Dhaka represented the central part; Chittagong, Cox’s Bazar, and Teknaf represented the southern part; Khulna, Patuakhali, and Bhola represented the coastal zones.

Start Date: 00-00-1986

End Date: 00-00-2006

Program Area: Socio-economic and Policy

Non-commodity: Performance

Objectives:

To take proper crop growth initiatives, intensive study of the water balance is essential. Owing to less rainfall, the delayed onset of monsoon in some years in Bangladesh, and the limited, expensive facilities for irrigation, there is an urgent need for soil moisture measurements. To satisfy the rising demand for rice in Bangladesh, production must be increased at a faster rate than that in the past. Therefore, the present study was under taken to satisfy the present and future demand of water for crop production.

Methodology:

Four regions of Bangladesh, represented by 12 meteorological stations, were chosen for the study. Stations named Dinajpur, Rangpur, and Bogra represented the northern part; Faridpur, Tangail, and Dhaka represented the central part; Chittagong, Cox’s Bazar, and Teknaf represented the southern part; Khulna, Patuakhali, and Bhola represented the coastal part. Bangladesh has a humid, warm, tropical climate, and of the four prominent climatic seasons, winter (December to February) is relatively cooler and drier, with average temperatures ranging from a minimum of 7.2 to 12.8°C to a maximum of 23.9 to 31.1°C. Pre-monsoon (March to May) is hot with an average maximum temperature of 36.7°C, but in some places, the temperature occasionally rises up to 40.6°C or more. Monsoon (June to early October) is both hot and humid and brings heavy torrential rainfall throughout the season, and the mean monsoon temperatures are higher in the western districts than in the eastern districts. Post-monsoon (late October to November) is a short-lived season characterized by withdrawal of rainfall and gradual lowering of nighttime minimum temperature. The mean annual rainfall is about 2300 mm, but there exists a wide spatial and temporal variation. Annual rainfall ranges from 1200 mm in the extreme west to over 5000 mm in the east and northeast (MPO, 1991). Generally, the eastern parts of the country experience higher rainfall than do the western parts.

Meteorological data were processed for use as input in the Thornthwaite water balance model. Monthly mean values of maximum temperature (°C), minimum temperature (°C), and total rainfall (mm) were collected from the Bangladesh Meteorological Department (BMD) for the selected stations for the period January 1986 to December 2006. Projected data on future climate were collected from the committed climate change experiment output for IPCC assessment report 4 in 2007. The output of several GCMs was used in the current study, depending on the resolution and availability of projection data. Finally, the outputs of the climate model CGCM3.1 from the Canadian Centre for Climate Modelling and Analysis were used for the Tangail station.

Soil moisture storage (ST) refers to the amount of water held in the soil at any particular time. The amount of water in the soil depends on soil properties such as soil texture and organic matter content. Mean monthly ST values were calculated for the study seasons to investigate the impact of ST on the rice yield. The ST can be determined by calculating the input, output, and storage changes in water in a particular place for a particular time. The major input of water is from precipitation, and the major output is due to evapotranspiration. When the precipitation amount (that remaining after direct runoff) becomes higher than evapotranspiration, excess water enters the soil. This excess water is added to the previous month’s reserve and is considered to be the ST. However, when precipitation becomes lower than evapotranspiration, part of the previously stored water is withdrawn (STW) to meet the demand for actual evapotranspiration. Thus, the current ST becomes lower as a result of the withdrawn moisture. The relation between the ST and the rice yield is determined by regression analysis, by calculating the correlation coefficient (r).

Source of Information: Italian Journal of Agronomy, Vol 10, No 4 (2015)

Article Link (Online Journal): http://www.agronomy.it/index.php/agro/article/view/ija.2012.e21/679

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Currently, the northern part of Bangladesh is the area worst affected by water scarcity, and in all the studied stations, winter is the most critical season influencing water scarcity. PET was the highest in the coastal parts and decreased toward the northern belt. In the stations of Khulna and Patuakhali, the average monthly PET was 113 and 111 mm, respectively; Rangpur had the lowest value of 107 mm month^–1. The average monthly AET was higher in the central stations and decreased toward the northern parts. WD was the lowest for Dhaka (128 mm yr^–1) and the highest for Bogra (202 mm yr^–1), indicating that the northern part of Bangladesh required more irrigation than did the central and southern parts. During the monsoon, WS was noted in all the studied stations, indicating that this season is the most suitable for the cultivation of a wide variety of crops in drought-prone areas. Projection results showed that almost all the studied stations in Bangladesh would suffer from low soil moisture, which might result in a high percentage of crop loss. However, a 13% increase in projected soil moisture for the northern region of Bangladesh, showed the less demand of irrigation water there in future. The soil moisture is expected to decrease by 21% on average in all regions in Bangladesh.

Most parts of Bangladesh suffered from WD in the months November to April, indicating that irrigation must be started in the later months of the year and continued till the early months of the next year. Since a homogeneous climatic pattern exists in all parts of Bangladesh, major crops such as rice, wheat, maize, and potato can be grown easily in all seasons, but only on the condition that irrigation is provided whenever necessary. A significant positive relationship (P<0.05) between the soil moisture and the current rice yields in the northern stations showed that conservation of surplus water is essential for the future, especially in the drought-prone zone of Bangladesh.

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