Drought Analysis in T. Aman Season: 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). In this study drought was calculated by the water balance and relative water supply methods and drought assessment completed by farmer survey using predesign questionnaire. Although the drought analysis was carried out from (2003- 2007) T. Aman season. The last 10 days of the crop season was not considered in the analysis. It is customary to drain the rice fields at that stage to enhance ripening and for easier harvesting. (Tabbal et al., 1992).
Drought analysis by Water Balance Method: Quantitative representation of the hydrologic cycle for any basin or area is called the water balance. The quantitative changes may be expressed as a water balance equations in which inflow, outflow and bund storage in a period are represented. In its general form it may be given as;
Inflow – Outflow = Bund storage (1)
The methodology followed drought analysis in the study is similar to Thornthwaite’s method of water balance (Paul and Tiwari, 1992). But in this analysis the seepage and percolation loss (S&P) was also taken into account along with evapotranspiration (ET), as S&P is an integral part of rice field water requirement (Saleh et al., 1996).
All the components of the hydrologic cycle are interrelated, and changing of one component influences the other. If the inflow is greater than the outflow, the excess inflow results in an increase in the bund storage, and vice versa.
For a bounded rice field, the above equation (1) may be written as:
Ht = Rt + Ht-1 – ETt – (S&P)t (2)
Where, H = bund storage
R = rainfall
ET = evapotranspiration
S&P = seepage and percolation
The subscripts t and t-1 denote time in 1- day time steps (present and previous 1-day respectively). R, ET, H and S&P are all expressed in mmd-1(in each 1-day time step).
In this study, for accurate assessment of drought, the 1 – day time step was followed.
If water was balanced on the basis of 5- days or 7-days or 10-days time step then drought assessment cannot be made accurately. Many times it was found that during 5-days, 7-days or 10-days time step, no rainfall occurred on the first few days but abundant rainfall occurred on the last few days. In that case drought status was not identified on the basis of water balance but actually crop was affected by drought due to lack of rainfall at the beginning of the time step. Moreover as in this study the water balance was carried out for five seasons and four sight, the drought assessment did not involve lot of computations. So for the accuracy of drought analysis one day time step was considered. As drought occur after three days of disappearing standing water, so drought considered from fourth day.