Groundwater table depth: Rainfed conditions

Exercise objective:

·     Effect of groundwater; rainfed conditions.

Suggested reading:

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

Exercise: 

Ex-V.7. In the soil data file, restore bund height WL0MX to 250 mm. Create a rerun file with the following values for depth of groundwater:

ZWTB =   1.,100.,366.,100.

ZWTB =   1.,50.,366.,50.

ZWTB =   1.,20.,366.,20.

ZWTB =   1.,10.,366.,10.

Run ORYZA2000, and fill out the first column (under FIXPERC=3) of Table V.3. Next, change the fixed percolation rate (FIXPERC) in the soil data file to 30 mm d-1, run ORYZA2000 and fill out the second column in Table V.3. Explain the differences between the two columns, using graphs of the weight of the above-ground biomass, the soil water tension (e.g., at layer 1 and layer 4), and the leaf expansion reduction factor (LESTRES) versus time.

Table V.3.

View ANSWERS from the Tutorial_answer_sheet.pdf file.

Figure V.2a. Weight of above-ground biomass (WAGT; kg ha-1) versus time; cv. IR72 grown under rainfed conditions with 150 cm groundwater table depth (Run 0) and 10 cm groundwater table depth (Run 4), with 3 mm d-1 percolation rate (top graph) and 30 mm d-1 percolation rate (bottom graph); IRRI, Los Baños, wet season 1992.

Figure V.2b. Leaf expansion reduction factor (LESTRS; -) versus time; cv. IR72 grown under rainfed conditions with 150 cm groundwater table depth (Run 0) and 10 cm groundwater table depth (Run 4), with 3 mm d-1 percolation rate (top graph) and 30 mm d-1 percolation rate (bottom graph); IRRI, Los Baños, wet season 1992.

Figure V.2c. Soil water tension at 5 cm depth (MSPKA1; kPa) versus time; cv. IR72 grown under rainfed conditions with 150 cm groundwater table depth (Run 0) and 10 cm groundwater table depth (Run 4), with 3 mm d-1 percolation rate (top graph) and 30 mm d-1 percolation rate (bottom graph); IRRI, Los Baños, wet season 1992.

Figure V.2d. Soil water tension at 20 cm depth (MSPKA4; kPa) versus time; cv. IR72 grown under rainfed conditions with 150 cm groundwater table depth (Run 0) and 10 cm groundwater table depth (Run 4), with 3 mm d-1 percolation rate (top graph) and 30 mm d-1 percolation rate (bottom graph); IRRI, Los Baños, wet season 1992.

With a shallower groundwater table, capillary rise into the root zone during periods without ponded water increases. With a very shallow groundwater table, such as at 20 and 10 cm, part of the roots of rice grow even in the groundwater, while upper soil layers may be dry. In Figure V.2d, we see that the soil at 20 depth with 10 cm groundwater table depth is saturated (soil water tension is close to 0), whereas with 150 cm groundwater table depth, the soil at 20 cm depth is continuously non-saturated. With a high percolation rate (30 mm d-1), the soil dries out faster (higher tensions) than with a low percolation rate (3 mm d-1). In Figure V.2c., we see that the soil at 5 cm depth is also non-saturated for some periods of time with the 10-cm deep groundwater table, but that it is drier with the 150-cm deep groundwater table. In Figure V.2b, we see that the drier soil layers with the 150-cm deep groundwater table and the 30-mm d-1 percolation rate translate into a larger reduction in the leaf expansion rate. This, in turn, leads to reduced biomass accumulation (Figure V.2a) and, eventually, a lower yield (Table V.3). The effect of groundwater table depth is more pronounced in light-textured soils with a high percolation rate than in heavy (more clayey) soils with a low percolation rate. In lowland rice growing areas, groundwater tables are often very shallow and may be found within the rootzone of the rice crop.