The effect of residue cover on soil water storage during fallow

Tracking the decline of cover as residues decompose

APSIM simulates the influence of crop residues on the efficiency with which water is captured and retained during fallows. But residue cover declines as residues decompose. Residue decomposition is simulated in APSIM in response to weather, as well as the chemical composition of the residues. By doing this simulation you will reinforce skills learned in previous exercises and learn to do some basic editing of default values to ‘customise’ your simulations.

The examples assume you have read and walked through the previous document: How to Build, Run and Graph a Simulation

This simulation will demonstrate how surface residue decomposes over time. You should use the previous simulation as a starting point for this simulation. You need to add an initial amount of surface residues.

1. Reopen the previous file (Fallow.apsim) and remove the Brookstead Fallow simulation. We're going to use Bongeen Fallow as our starting point for this exercise.
2. Make a copy of the Bongeen Fallow simulation by dragging to the top node in tree (Simulations).
3. Set the initial surface residue to 3000 kg/ha in the second simulation.
4. Rename second simulation to Bongeen residue
5. Save the file as Residue.apsim (reminder don't forget to use the Save as button NOT the Save, or you will save these changes to Fallow.aspim)
6. Run the simulation and plot day vs surfaceom_cover for just the Bongeen Residue simulation (Go to the outputfile component in Bongeen Residue and click on APSVis (Graph) button. As mentioned before clicking the graph button whilst the top node in the tree is selected will graph ALL output files. Use a ScatterChart and leave the default chart type for surfaceom_cover).
7. Add rain as a Y2 variable to this chart. (use the first bar chart as the chart type)

It can be seen that periods of high decomposition rate match with higher rainfall and low decomposition with dry periods.

The effect of cover decline on runoff and evaporation

In this activity, a comparison will be made of two simulations: ‘Bongeen Fallow’ and ‘Bongeen Residue’.

1. Graph both output files as a scatter chart of Date vs runoff with ‘runoff’ plotted as accumulated
2. Add rain to this chart. (NOT as Y2, use the first bar chart as the chart type). Return to the simulation when you have finished.
    

The effect of residue type on speed of decomposition

The APSIM residue model will decompose residues at differing rates according to the C:N ratio of the material. To demonstrate this we will reproduce the previous simulation but apply legume residues in the place of the wheat residues.
 

1. Create another copy of the Bongeen Residue simulation and call it Bongeen Chickpea Residue.
2. Change the residue parameters to 3000 kg/ha of Chickpea (type) residue. Set the C:N ratio to 25. (Remember you may want to change the Organic Matter pool name to something like chickpea as well)
3. Run this new simulation. (If you just select this simulation in the tree and click the run button it will only run this simulation instead of all of them)
4. Graph all three residue simulations with residue cover as a function of time (eg date).