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Analysis of Agricultural Drought in a Rainfed Lowland Rice System: a Case Study in North-west Bangladesh

Abul Fazal M. Saleh
Professor
Institute of Flood Control and Drainage Research, Bangladesh University of Engineering and Technology Dhaka -1000, Bangladesh

Sadiqul I. Bhuiyan
Deputy Head
Soil and Water Science Division, International Rice Research lntitutc, P.O. Box-913, 1094 Manila, Philippines

Abstract/ Executive Summary:

The nature, extent and frequency of droughts in the monsoon season in a rainfed lowland rice system of northwest Bangladesh were analysed using the method of water balance from long term rainfall data. Probability analysis showed that in the study area drought are expected at the beginning of the crop season prior to transplantation and from the beginning of the ripening period. The seasonal relative water supply (RWS) from rainfall in an average year is 0.79 which means that 79% of the crop water requirement is available from rainfall. The RWS is expected to go down to 0.51 twice in ten years because of late transplantation due to early drought. Probability of 10 day and 15 day droughts during the ripening period are 73% and 53% respectively. Field-rainfall analysis showed that although seasonal rainfall is uncorrelated with crop yield there is a good correlation between the rainfall during the ripening period and crop yield. In order to stabilize the rice yield measures of drought alleviation have been suggested.

Keywords: Drought, Lowland, Rainfed, Rice

Location: Godagari Thana, Rajshahi district, Bangladesh

Start Date: 00-07-1994

End Date: 00-11-1994

Program Area: Crop-Soil-Water Management

Non-commodity: Drought

Objectives:

Studying the nature extent and frequency of droughts during Aman season in the rainfed lowland rice systems of the Barind Tract from long term rainfall data; and
Studying the impact of droughts on the yield of rice during the Aman season.

Methodology:

For drought quantification it is important to analyze the time intervals between rainfall of a particular magnitude, keeping the unit of time short enough so that. it can be related to crop's requirements (Hershfield, et al., 1973) The nature and extent of droughts in the area were studies by using the water balance method for finding out the numbers of 5-day periods and continuous 5-day periods when the rainfall was inadequate to meet the crop water requirement during the Aman season. The methodology followed is similar to Thornthwaites method of water balance (Paul and Tiwari, 1992). But in this analysis the seepage and percolation loss was also considered as this is an integral part of rice crop water requirement. The rainfall data of 1963-93 period was used for this purpose. From 5-day period water balance the incidence of either drought or adequacy of water supply was determined by the following procedure: a) if the rainfall is> = crop evapotranspiration (ET) but <= ET + Seepage and Percolation (S&P) then thc supply is adequate and the crop water requirement is satisfied: (b) if the rainfall is >=Et + S&P then there is storage which is available in the subsequent period; and (c) if the rainfall + previous storage (if any) is storage (if any) is < ET then there is drought. The water availability during the crop growth season was determined by using the concept of relative water supply (R WS), which is defined as the ratio of supply of water to the demand by the crop. Thus, RWS for a given period can be written as : RWSt= (R/(ET + S&P)t (I) , where R is the rainfall during time period and ET is the crop evapotranspiration. A RWS value greater than 1 indicates that the water supply is abundant, where as a value less than 1 indicates drought. For lowland rice, the pan evaporation data are good indicators of crop evapotranspiration (Tomar and O'Toole, 1980) The rainfall probability analysis was carried out using the Gamma distribution (Thom, 1968). The field study was carried out during 1994 Aman (July- November) season at Rajabari Union of Godagari Thana of Rajshahi District. For drought analysts, 31 years of daily rainfall data (1963-93) of Godagari station were collected from Bangladesh Water Development Board. The crop production practices of 50 randomly selected farmers in three toposequences - high, medium and low. were surveyed during the study period using a pre-designed questionnaire in order to characterize their land use and productivity. Field water status and water use of 30 farms in three toposcquences were closely monitored by install- ing 10.0 cm long PVC tubes, 20 cm above and the rest below the ground surface The water level readings in the PVC tubes (standing or perched water level) were taken every alternate day.<et then="" there="" is="" drought.="" the="" water="" availability="" during="" crop="" growth="" season="" was="" determined="" by="" using="" concept="" of="" relative="" supply="" (r="" ws),="" which="" defined="" as="" ratio="" to="" demand="" crop.="" thus,="" rws="" for="" a="" given="" period="" can="" be="" written="" :="" rwst="(R/(ET" +="" s&p)t="" (i)="" ,="" where="" r="" rainfall="" time="" t="" and="" et="" evapotranspi="" ration.="" value="" greater="" than="" 1="" indicates="" that="" abundant,="" less="" lowland="" rice,="" pan="" evaporation="" data="" are="" good="" indicators="" evapotranspiration="" (tomar="" o'toole,="" 1980)="" probability="" analysis="" carried="" out="" gamma="" distribution="" (thom,="" 1968).="" field="" study="" 1994="" aman="" (july-="" november)="" at="" rajabari="" union="" godagari="" thana="" rajshahi="" district="" .="" drought="" analysts,="" 31="" years="" daily="" (1963-93)="" station="" were="" collected="" from="" bangladesh="" development="" board.="" production="" practices="" 50="" randomly="" selected="" farmers="" in="" three="" toposequences="" -="" high,="" medium="" low.="" surveyed="" duri="" ng="" pre-designed="" questionnaire="" ordcr="" characterize="" their="" land="" use="" productivity.="" status="" 30="" farms="" toposcquences="" closely="" monitored="" install-="" ing="" 10.0="" cm="" long="" pvc="" tubes,="" 20="" above="" rest="" below="" ground="" surface="" level="" readings="" tubes="" (standi="" or="" perched="" level)="" taken="" every="" alternate="" day.

Source of Information: Journal of Indian Water Resources Society, Vol. 2, No. 1. January 1996

Article Link (Online Journal):

Funding Source:

1.

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Although the study area is considered a drought prone droughts area expected only at the beginning of the season prior to transplantation and from the beginning of the ripening stage. The total seasonal rainfall is not related to crop yield but there is a fair correlation between rainfall in the ripening stage and crop yield. The possible strategies for drought alleviation which need further research are crop establishment through occurring at the beginning of the season and on-farm reservoirs for supplementary irrigation for droughts at the ripening stage.

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