The effect of residue cover on soil water storage during fallow

Exercise 2: 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)
2. 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)
3. Remove the Brookstead Fallow simulation. We're going to use Bongeen Fallow as our starting point for this exercise. Also remove the graph component.
4. Make a copy of the Bongeen Fallow simulation by dragging to the top node in tree (Simulations).
5. Rename second simulation to Bongeen Residue
6. Set the initial surface residue to 3000 kg/ha in the second simulation.
7. Run the simulation
8. Create a graph of day vs surfaceom_cover and rain(right hand axis) for just the Bongeen Residue simulation. Just drag an XY graph from the Graph toolbox onto the output file. Remember to set "Point type" to "None".

   To find out how to modify a graph see How To: Modify a Graph component

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 with Date vs runoff(cumulative) and rain(this time on the same axis as the runooff. NOT right hand axis)

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. Save the file as Chickpea Residue.apsim (reminder don't forget to use the Save as button NOT the Save)
2. Create another copy of the Bongeen Residue simulation and call it Bongeen Chickpea Residue. Also remove the graph component.
3. 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)
4. 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)
5. Graph all three residue simulations with residue cover as a function of time (eg date).