Fallow Water Balance

Exercise 1: Fallow Water Balance

In this exercise you will explore the major elements of interest in soil water balance during a fallow - soil water storage, drainage, runoff, and evaporation. Changes will be examined over a one year period in the Dalby area.

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

1. Create a new simulation using Continuous Wheat Simulation as a starting point.
2. Rename the simulation to something more meaningful: Brookstead Fallow
3. Save the simulation file as Fallow.apsim
4. Choose Goondiwindi weather (C:\Program Files\Apsim73-r1387\Examples\MetFiles\Goond.met)
5. Starting date: 1/1/1989 Ending date: 31/12/1989
6. Open the "Soils" toolbox (it may take some time to load, so be patient). Drag a "Grey Vertosol-Cecilvale (Brookstead No004)" soil onto the simulation, removing the old soil (You can find it under Soils->Australia->Queensland->Darling Downs). Rename the soil to something short like "Soil". If you like you can also reorder the soil component so it comes straight under the paddock.
7. Set the starting water to 10% full - filled from the top. (expand the Soil to see InitWater)(by expand I mean click the "+" next to the Soil)
8. Set the starting NO₃ to 50.34 (kg/ha) and starting NH₄ to 3.23 (kg/ha), spread evenly throughout the profile. (You spread NO₃ and NH₄ evenly throughout the profile by only having one layer in the soil and making this layer span the entire soil depth) (ie. from 0cm to 200cm) (To only specify one layer you must first, using the mouse block all the layers you don't want and delete them [right mouse click, then delete]. Then in the remaining layer just click on the cells in the table and change them)



9. Check that the default initial residue type is wheat and the mass is 1000 kg/ha. (under 'surface organic matter')
10. Delete the Fertiliser, Wheat and Manager components out of the simulation. They are not needed for a fallow run.
11. Choose these variables to report (removing old ones):

    Component	Variable name
    	dd/mm/yyyy as Date
    Clock	Day
    	Year
    Met	Rain
    Soil	ESW - Extractable soil water (mm)
    	ES - Evaporation
    	Runoff
    	DRAIN - Drainage
    	NO3 - summed over profile (Do this by putting () next to the name in the "Variable name" column) eg. no3() (click "?" button next to variable list for more info)
    	DLT_N_MIN - N mineralised - summed over profile
    Surface organic matter	SURFACEOM_WT - Weight of all surface organic materials.
    	SURFACEOM_COVER - Fraction of ground covered by all surface organic materials.

12. Choose "end_day" reporting frequency for the output file. This can be found under Clock.
13. Run the simulation.
14. Create a graph of Date vs ESW and Rain(Right Hand Axis). To do this Click on the Graph toolbox to open it. Expand the "Graphs" folder. Then drag in an XY component onto the output file in your simulation. Click on the "+" symbol next to XY component to expand the node. Click on the Plot component. Now in the Plot window click on the X variables square to make sure the background of the square is pink. Now click on the "Date" column heading. It should appear in the list in the square. Now click on the Y variables square to make its background pink. Then click on the esw column heading, then the rain column heading. They should be added to the list in the square. Now to make the "rain" appear on the right hand axis, click rain in the square to highlight it, and then "right" mouse click on it again. In the popup menu click on "Right Hand Axis". Now we want a nice clean line to be plotted with no points so now under "Point type" choose "None". Now just click on the XY component to view the graph.

The graph should show the ESW (in mm) increasing with day of year. The sudden increases are due to rainfall events and the declines to evaporation and drainage loss. Daily rainfall will show this more clearly.



15. Create a graph of Date vs Runoff and Rain(right hand axis). This time in the Plot window under "Type" choose "Dot line" as well as "Point type" to "None".
    

The effect of runoff on the water balance.

Runoff is affected by weather and soil water storage capacity. This run will take an additional soil into account, and compare runoff from both soil types. The user interface still contains all the specifications provided for the previous simulation.

If you drag the Brookstead Fallow node in the Simulation Tree to the top node Simulations), a copy of it will be made and your file will then have 2 simulations in it. e.g.

This second simulation can then be modified to add the characteristics of a Macalister, Bongeen soil.

1. Rename the simulation to Bongeen Fallow. Click Save
2. Drag the "Black Vertosol-Bongeen (Macalister No026)" soil onto the paddock in the simulation tree (located under Soils->Australia->Queensland->Darling Downs) and then delete the old soil.
3. Rename your soil to something shorter. (eg. Soil)
4. Since now we have a new soil we will need to go and set the initial soil water (InitWater) to 10% filled from top and initial soil nitrogen (InitNitrogen) to NO3 to 50.340 kg/ha and NH4 to 3.230 kg/ha.
5. Save the simulations
6. Run APSIM.
7. Graph both the output files by dragging an XY graph onto the top node Simulations in the simulation tree.

Create a graph of day vs runoff(cumulative) and rain(right hand axis). To make the runoff be cumulative it is the same procedure as to make the rain appear on the right hand axis. Only select "Cumulative" from the popup menu instead of "Right Hand Axis". Set "Point Type" to "None".