Percolation: Irrigated conditions

Exercise objective:

·    Effect of percolation rate and plow sole conductivity; irrigated conditions.

Suggested reading:

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

Exercise: 

Ex-V.2. Check in the experimental data file EXPLORE.DAT that we use ORYZA2000 in the water mode (PRODENV = 'WATER BALANCE'), that the start time is day 4 (STTIME=4.), and that we supply 75 mm of irrigation water each time the ponded water depth drops below 10 mm (SWITIR=2). 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.1 (at SWITVP=-1). Explain the differences. Q. Why is the yield slightly lower in the last two reruns? Compare graphs of the depth of ponded water (WL0) and of the leaf expansion stress factor (LESTRES) of run 4 with that of run 0 to find the answer.

View ANSWERS from the Tutorial_answer_sheet.pdf file.

Ex-V.3. In the soil data file, scroll toward the array with values for saturated conductivity KST. Note that the third soil layer, i.e. the plow sole, has the lowest value and effectively ‘blocks’ water from flowing out of the puddled topsoil. Create a rerun file with the following values for KST in the plow sole: 0.03, 0.1, 0.5 and 1.0. Since these values are part of an array, we need to give the whole array in the rerun file:

KST = 2*127.0, 0.03, 3*35.0, 2*103.0, 42.0

KST = 2*127.0, 0.1,  3*35.0, 2*103.0, 42.0

KST = 2*127.0, 0.3,  3*35.0, 2*103.0, 42.0

KST = 2*127.0, 0.5,  3*35.0, 2*103.0, 42.0

KST = 2*127.0, 1.0,  3*35.0, 2*103.0, 42.0

Run ORYZA2000 and study the output in OP.DAT. Q. Why are all values still the same? A. The parameter SWITVP is still set to –1, indicating that the fixed percolation rate of 3 mm d-1 is used to calculate water movement in the topsoil. Thus, setting the saturated conductivity for soil layer 3 has no effect. Change SWITVP from –1 into 1 to use the Van Genuchten equations (that use the KST values in their calculations) to calculate water flow in the topsoil, run ORYZA2000 and fill out the second part of Table V.1 (at SWITVP=1).

Ex-V.4. Restore the selection SWITVP=-1, and make a rerun for a non-puddled soil (SWITPD = 0). Run ORYZA2000. Note that the simulated values are exactly the same. It does not matter whether the topsoil is puddled or not: as long as its hydraulic properties are the same (especially the percolation rate!), its simulated water balance is the same. Restore SWITPD to 1 after the exercise.