Climate change impacts on rainwater harvesting potential and irrigation water requirement in Bangladesh

Abstract

In the present study a total of 58 years (1950 to 2007) of daily rainfall data from 34 stations of Bangladesh Meteorological Department has been analyzed for the entire country to identify the changes in rainfall distribution considering temporal as well as spatial distribution. From the rainfall analysis, it is observed that average annual rainfall of total 58 years is 2371 mm; while that of the recent 12 years (1996-2007) is 2505 mm.

From the decadal average rainfall analysis for each of the months considering 58 year rainfall data through GIS, it has been observed that for all of the decades during the months of January, February, November and December average rainfall is below 50 mm, while during June and July more than 400 mm of average rainfall is observed. From the comparison of decadal average monthly rainfall at different rinfall stations within eight hydrological regions, it has been observed that an increasing trend of regional average decadal rainfall is evident in NW region for the months of April and September; in NE region during April and in SW region during September. Regional average rainfall during the recent three decades (80s, 90s and 2000-2007) are higher than that of previous three decades as well as 58 year average in NW region during February and May, in NC region during September; in SW region during May, in SC region during July and in SE region during May.

Contribution of rainwater in meeting crop water requirement for three varieties of Rice and Wheat have been compared both temporally and spatially for the recent 12 years and total 58 years for eight hydrological regions of the country through normalization technique. It is noticed that for each month change of rainfall resembles to the change of rainwater contribution in meeting crop water requirement during different months of cropping period for each crop. It has been observed that during the cropping period of Boro (January to May), rainwater contribution in meeting crop water requirement has increased significantly in SC and RE regions during January, SW region during March, NW and NC regions during April, EH and RE regions during May. Again, for the same crop, rainwater contribution decreased significantly in NW and NE regions during January, NC and SE regions during February. For Aman (cropping period of July to November), rainwater contribution in meeting crop water requirement has increased significantly in SW region during September, NW, EH and RE regions during October. Again, for the same crop, rainwater contribution has decreased significantly in NW, SC and EH regions during November. So, in general, for Boro, during the initial two months (January and February) rainwater contribution appears to be decreasing. For Aman, rainwater contribution is increasing during October but decreasing in November. This suggests that rainwater can be stored in October for irrigation in later months. From spatial variation of yield for three varieties of rice and wheat, it has been observed that percentage of area with yield at least 2000 kg/Ha for Boro is relatively high in all of the regions and large area of NW region show higher yield than the other regions.

From groundwater level assessment, it has been found that among three cropping periods, Kharif I (March to July) and Rabi (November to April) show worse condition specially in NW and NC regions. As significant increase of rainfall is observed during April in NW region, ground water recharge schemes can be implemented in this region. SE is the most vulnerable region due to arsenic and iron contamination with ground water. From rainfall analysis for SE region, it has been noticed that during May recent three decades show higher rainfall relative to that for previous three decades. Salinity problem is experienced in the coastal areas of SC and SW regions most. As there is significant increase of rainfall during January in SC region and during September in SW region, rainwater may be helpful to overcome the soil salinity problem in these regions.

To find out the potential of rainwater harvesting in an urban area, Dhaka city has been considered as a typical urban case. From long term rainfall analysis for Dhaka city, it has been found that average annual rainfall of 55 years (1953 to 2007) is 2073 mm and that for recent 12 years (1996 to 2007) is 2166 mm, and the annual rainfall follows an increasing trend. Variation of 5 year (moving) average annual rainfall as well as average monthly rainfall during March, May and September has been analyzed for Dhaka city considering 55 year duration. Comparatively higher rainfall has been observed during the recent three decades relative to that for previous decades. From the mass curve analysis of Dhaka city, it is found that the maximum excess water is available during October and maximum deficit occurs during April. Thus excess water obtained during rainy season can be stored in a proper way for the use in dry season. This analysis suggests that in Dhaka, RWH can be used in conjunction with conventional water supply system to improve the present water scarcity. An analysis of roof size distribution of different wards of Dhaka city has been made through GIS to find out suitable catchment area that can be used for RWH.