Decision Tables

Decision tables are a precise yet compact way to model complex rule sets and their corresponding actions. Decision tables, like flowcharts and if-then-else and switch-case statements, associate conditions with actions to perform, but in many cases do so in a more elegant way (see Wikipedia article on decision tables).

Structure of decision tables:

1. Conditions

2. Condition alternatives

3. Actions

4. Action entries

Each decision corresponds to a variable, relation or predicate whose possible values are listed among the condition alternatives. Each action is a procedure or operation to perform, and the entries specify whether (or in what order) the action is to be performed for the set of condition alternatives the entry corresponds to. Many decision tables include in their condition alternatives the "don’t care" symbol, a hyphen. Using "don’t cares" can simplify decision tables, especially when a given condition has little influence on the actions to be performed. In some cases, entire conditions thought to be important initially are found to be irrelevant when none of the conditions influence which actions are performed.

Usage in SWAT+

There are four decision table sections in SWAT+: land use management, reservoir release, scenario land use, and flow conditions.

See the land use management documentation, under management schedules for how to choose a decision table for your HRUs.

Reservoir release decision tables are assigned from the reservoir properties section under connections in SWAT+ Editor.

Modifying Decision Tables in SWAT+ Editor

In the current version of SWAT+ editor, we do not have a GUI available for editing decision tables, however we have provided the ability to export the decision table file. You may then make changes to it in a text editor, and upload back into SWAT+ Editor.

Table Definitions

d_table_dtl

d_table_dtl_cond

d_table_dtl_cond_alt

d_table_dtl_act

d_table_dtl_act_out

Text File Example and Explanation

The following is an example of a decision table in the lum.dtl input file. It is a table for warm season annual crops, using continuous corn.

In the above table, there are 6 conditions, 4 alternatives and 3 actions.

Description of the conditions

1. soil_water – if soil water is too high (> 1.50*field capacity), it will be too wet to operate machinery

2. plant_gro – (“n”) Planting allowed if plant is not growing.

3. phu_base0 – (0.15) when the sum base zero heat units for the year (starting Jan 1) exceeds 0.15, indicating it’s warm enough to plant

Description of the alternatives

If all of the conditions for each alternative are met, outcomes are checked for ‘y’ to take action. Alternatives with dash (‘-‘) are not checked.

1. plant corn based on heat units: if soil water < 1.50*fc and if phubase0 > 0.15*phu_mat and if year_rot = 1 then check outcomes for ‘y’ and if ‘y’, take that action (plant)

2. Harvest corn based on crop accumulated heat units: if soil_water < 1.50*fc and if phu_plant > 1.15*phu_mat and if year_rot = 1 and then check outcomes for ‘y’ and if ‘y’, take that action (plant)

3. Harvest corn based on days since planting: if year_rot = 1 and if days_plant =200 then check outcomes for ‘y’ and if ‘y’, take that action (harvest)

Description of the actions

1. plant: corn – cross walked to plant name in plants.plt file

2. harvest_kill: corn – cross walked to plant name in plants.plt file grain – relates to harvest type in harv.ops file

3. rot_reset: rotation reset – for continuous corn (1 year rotation). The rotation year is reset to 1 at the end of every year.