Bund height: Rainfed conditions

Exercise objective:

·    Effect of bund height; rainfed conditions.

Suggested reading:

Chapter 7.4 of the book, ORYZA2000: modelling lowland rice.

Exercise:  

Ex-V.5. In the experimental data file, change the irrigation mode to purely rainfed (SWITIR = 0) and the start time to day 150 (STTIME=150.). Create a rerun file with the following values for the percolation rate FIXPERC: 1., 5., 10., 20. Run ORYZA2000 and fill out the top part of Table V.2 (at WL0MX=250). Use graphs of rainfall and ponded water versus time to explain the differences.

Table V.2

View ANSWERS from the Tutorial_answer_sheet.pdf file.

Figure V.1a. Amount and timing of rainfall (RAIN; mm) versus time; cv. IR72 grown under rainfed conditions, IRRI, Los Baños, wet season 1992.

Figure V.1b. Depth of ponded water (WL0; mm) versus time; cv. IR72 grown under rainfed conditions with three percolation rates, IRRI, Los Baños, wet season 1992.

With the lowest percolation rate of 1 mm d-1 (Run 1), large rainfall events raise the depth of ponded water to above the height of the bund (WL0MX=250 mm; check in the soil data file!), which results in relatively large runoff losses (RUNOFC in Table V.2). At a higher percolation rate of 3 mm d-1 (Run 0!), bund overflow is less. At the highest percolation rate of 20 mm d-1, there is no bund overflow anymore, but a large quantity of water is lost by percolation and yield is substantially reduced.

Ex-V.6. In the soil data file, reduce bund height WL0MX to 50 mm, run ORYZA2000 with the same rerun set as in Ex-V.5 on percolation rate. Fill out the bottom part of Table V.2 and explain the differences, using graphs of depth of ponded water versus time.