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Characterization of Rainfed Rice Growing Environment in Bangladesh Based on Relative Water Supply for the Year 2006-07

Debi Narayan Rudra Pual
CSO
Head,Agril. Statistics Division Bangladesh Rice Research Institute(BRRI) Gazipur-1701

Abstract/ Executive Summary:

The objective of the study is to determine and compare the effect of rainfall and relative water supply on the yield of rainfed Aman rice. Rgression analysis was performed and for mapping ARC View software were used.T. Aman rice yield is positively related to relative water supply (RWS) capacity of soils in the month of October. A map has been created characterizing the rainfed T.Aman environment based on RWS in October that may be used as a decision making tool for planning rainfed rice cultivation in Bangladesh. The resulting surface for rainfed rice growing environment based on relative water supply computed from nearly assured October rainfall (RWS_Oct) shows that, droughts commonly occur all over the country in October indicating the need of supplemental irrigation in order to increase crop yield. About 28% area of Bangladesh in the Northwest part including some other scattered areas has the least favourable environment for growing rainfed rice where water deficit is about 70%-90% of the requirement.

Keywords: T.Aman, Evapotranspiration, GIS

Location: All over the Bangladesh

Start Date: 00-00-2006

End Date: 00-00-2007

Program Area: Socio-economic and Policy

Non-commodity: GIS

Objectives:

Determine and compare the effect of rainfall and relative water supply on the yield of rainfed Aman rice
Construct map of rainfed rice growing environment of Bangladesh.

Methodology:

Water availability during the crop growth season may be determined by using the concept of relative water supply (RWS), which is defined as the ratio of water supply to the crop to water demand by the crop. RWS for a given period t is defined as: RWS_t = [R/(ET + S&P)]_t

RWS is used as an index of water availability. It has no unit. An RWS value greater than 1 indicates that the water supply is abundant, whereas an RWS value less than 1 indicates a drought.

In this study, daily ET (Evapotranspiration) values at different locations of Bangladesh were sum up to obtain monthly ET values of the respective locations. It was assumed that 50% of the applied water is lost through seepage (S) and percolation (P). Hence S&P values were assumed to be 50% of the monthly rainfall. Thus, considering rainfall (R) as the water supply and ET+S&P as the water requirement by the crop, the RWS values were calculated for the crop season (July-October) and those for grain filling period (October) for the rainfed T.Aman rice at different weather stations. In order to find out the effect of rainfall on T.Aman rice yield, multiple regressions were fitted for rice yield (Yld) on seasonal (July-October) rainfall (RN_Seas) and grain filling stage (October) rainfall (RN_Oct) including another two independent variables namely (i) percent of irrigated area in T.Aman season (PA) as it might be treated as a measure of supplemental irrigation applied when there is no rainfall and the crop demands water and (ii) Year (Yr), which might be taken as a variable for the measurement of other variables not accounted for the analyses. The regression analysis was performed only for the greater districts of Dhaka, Comilla, Mymensingh, Jessore, Sylhet, Rajshahi, Khulna and Bogra due to the fact that long-term data on yield and irrigated area were available only for these districts. Rainfalls of all interpolated points (created through krigging) falling within a district boundary were averaged to represent the rainfall for that district. This is due to the fact that the observed rainfall would not represent the whole district as it was recorded at a particular rainfall station located within the district.

Relative water supply being a determinant of water availability during the crop growth season, multiple regressions were also fitted for rice yield on relative water supply computed from seasonal (July-October) rainfall (RWS_Seas) and that computed from grain filling period (October) rainfall (RWS_Oct) in order to evaluate the effect of relative water supply based on monthly rainfall on T.Aman rice yield. Multi-collinearly among all the independent variables was examined. Results indicated that there was significant correlation between RWS_Seas and RWS_Oct and that between Yr and PA in most of the districts. Thus, RWS_Seas and PA had been dropped from the regression analysis, as they did not show significant effect on yield and the resultant multiple regressions.

Source of Information: BRRI Annual Report 2006-07

Article Link (Online Journal):

Funding Source:

1. Government

Budget:

Total Budget (Tk.) :

Achievement/Findings:

The resulting surface (Map 17) for rainfed rice growing environment based on relative water supply computed from nearly assured October rainfall (RWS_Oct) shows that, droughts commonly occur all over the country in October indicating the need of supplemental irrigation in order to increase crop yield. About 28% area of Bangladesh in the Northwest part including some other scattered areas has the least favourable environment for growing rainfed rice where water deficit is about 70%-90% of the requirement. To increase the rainfed rice yield, the location-wise water requirement from supplemental irrigation in October may be computed by using surface map generated for nearly assured October rainfall along with For example, in Rajshahi station the nearly assured October rainfall was found to be 26 mm from surface map generated for nearly assured October rainfall and here RWS_Oct in Rajshahi station is found to be 0.28, so (26/0.28)*(1.00-0.28) mm = 66.86 mm of water is needed from supplemental irrigation in October in order to get the best performance of rainfed rice yield from Rajshahi area.

Knowledge Management: Report/Proceedings