Contents & Foreword, Characterizing And ... - IRRI books

Analytical procedureThe nature and extent of droughts in the area were studied by using the water balancemethod to find out the numbers of 5-d water-deficient periods and their continuity(consecutive 5-d periods) during the aman season. In these periods, the rainfall wasinadequate to meet the crop water requirement. Since rainfall during one day may beadequate for the entire period considered, the chances of a drought within the periodare higher if the period is longer than 5 d. As soil water may be available to the riceroots 2 to 3 d after standing water has disappeared, a shorter duration was not considered.The methodology followed is similar to Thornthwaite’s method of water balance(Steenhuis and Van der Molle 1986, Paul and Tiwari 1992). In this analysis,however, the seepage and percolation loss (S&P) was also taken into account alongwith evapotranspiration (ET), as S&P is an integral part of the water requirement forlowland rice. The 5-d water balance is written as follows:H t = R t + H t–1 – ET t – (S&P) t (1)where H is bund storage, R is rainfall, and subscripts t and t–1 denote time in 5-d timesteps (present and previous 5 d, respectively). R, ET, and S&P are all expressed inmm d –1 (in each 5-d time step). Thus, the incidence of either drought or adequacy ofwater supply was determined by the following criteria:If (R t + H t–1 – ET t ) < 0, then (S&P) t = 0 and H t = 0; there is drought (2)If ET t < (R t + H t–1 ) < ET t – (S&P) t and then H t = 0; there is no drought (3)If R t + H t–1 – ET t – (S&P) t > 0 and H t > 0; there is no drought (4)If R t + H t–1 – ET t – (S&P) t > H max and H t = H max = 20 cm; there is no drought (5)Water availability during the crop growth season was determined by using the conceptof relative water supply (RWS), which is defined as the ratio of water supply todemand by the crop. Thus, RWS for a given period t can be written asRWSt = [(R/(ET + S&P)]t (6)An RWS value greater than 1 indicates that the water supply is abundant, whereas avalue less than 1 indicates drought. For lowland rice, pan evaporation data are goodindicators of crop evapotranspiration (Tomar and O’Toole 1980).Rainfall probability and drought occurrences were analyzed by using gammadistribution (Thom 1968).For drought analysis, 31 years of daily rainfall data (1963-93) of Godagari station,situated about 15 km northwest of the study area, were collected from theBangladesh Water Development Board as long-term rainfall data were not availablefor the study site. Daily rainfall and U.S. Class A pan evaporation data for the studyseasons were collected from a nearby temporary weather station. Past crop yield datawere collected from the Bangladesh Bureau of Statistics (1979-83) and the Depart-Agrohydrologic and drought risk analyses of rainfed cultivation . . . 235