Understanding Plant Competition and Resource Allocation in SWAT+ Plant Communities

[Ilef Abid]

Dear SWAT + Community, 

I am currently conducting research on how SWAT+ considers multiple plants within the same plant community. My objective is to understand and justify, from a mechanistic point of view, how SWAT+ represents multiple plants in a community, particularly how competition for resources is modeled, how resources are distributed among the plants, and how stress affects each plant.

I would specifically like to understand how SWAT+ internally allocates each resource among the different plants within a plant community. In particular, I would like to know the mechanism used to determine how water, radiation, nutrients, and other resources are allocated between competing plants, rather than assuming a fixed percentage-based or spatial distribution. I am interested in the actual internal logic and equations used by the model to determine the share of each resource received by each plant.

I have found the relevant Fortran source code, but I am having difficulty understanding the entire mechanism and how the different parts of the code are connected.

Is there any documentation, technical report, paper, or other reference that could help me better understand the underlying mechanism of plant competition and resource allocation between multiple plants in a SWAT+ plant community?

Thank you so much 

[Gafar Agunbiade]

Hello llef,

To set up your manual multiple-plant system in SWAT+, you need to construct a logical chain among your initialization, scheduling, and land-use files. Here is the step-by-step process for editing these inputs.

1. Initializing the Plant Community (`plant.ini`)

You must manually group your three plants under a single plant community identifier (spst_comm).

Each of the three plants you add must correspond to a valid species code inside the SWAT+ plant database (plant.sdb).

If your specific plant is not in the database, you must find a plant with similar physiological properties and use its code.

You will then manually edit the initial values for each plant in the community, setting parameters like initial biomass (bm_init), growing status, and the plant's initial age or years to maturity (yrs_init).

2. Building the Schedule Logic (lum.dtl)

Instead of fixed dates in a basic schedule, you create your dynamic schedule using decision tables within the lum.dtl file.

This file relies on a highly logical structure of conditions and actions.

You will define specific condition variables (such as the simulation year and Julian day) that, when met, trigger target actions (like `harvest` for tree thinning or `plant` for pasture grasses).

3. Linking Land Management (landuse.lum)

Finally, you must tie the plant community and the schedule to the physical landscape.

Create a new land management entry in the landuse.lum file.

This entry acts as a bridge, linking your specific Hydrologic Response Units (HRUs) to your newly created plant community and your overarching management schedule (management.sch).

The management. sch file will then continuously evaluate the rules you set in lum.dtl to execute your multi-plant system.

Getting this file sequence to read correctly is the foundational modeling step of SWAT+. Make sure you have a good, logical flow

Are you planning to have all three plants actively growing from the very first day of the simulation, or will some remain dormant until a specific planting rule and management?

​

Ilef Abid 9/8/2026 14:42

Dear SWAT + Community,  I am currently conducting research on how SWAT+ considers multiple plants within the same plant community. My objective is to understand and justify, from a mechanistic point of view, how SWAT+ represents multiple plants in a community, particularly how competition for resources is modeled, how resources are distributed among the plants, and how stress affects each plant. I would specifically like to understand how SWAT+ internally allocates each resource among the different plants within a plant community. In particular, I would like to know the mechanism used to determine how water, radiation, nutrients, and other resources are allocated between competing plants, rather than assuming a fixed percentage-based or spatial distribution. I am interested in the actual internal logic and equations used by the model to determine the share of each resource received by each plant. I have found the relevant Fortran source code, but I am having difficulty understanding the entire mechanism and how the different parts of the code are connected. Is there any documentation, technical report, paper, or other reference that could help me better understand the underlying mechanism of plant competition and resource allocation between multiple plants in a SWAT+ plant community? Thank you so much

​

Gafar Agunbiade

Ph.D. Candidate in Agricultural Systems Management,

Department Of Agricultural Sciences.

Clemson University.

Office/Lab: South Carolina Water Resources Center,

509 Westinghouse Road, Pendleton, SC, 29670.

Contact: 8647658794, gag...@g.clemson.edu

---

From: qswa...@googlegroups.com <qswa...@googlegroups.com> on behalf of Ilef Abid <abidi...@gmail.com>
Sent: Tuesday, September 8, 2026 09:28
To: QSWAT+ <qswa...@googlegroups.com>
Subject: [QSWAT+ User Group] Understanding Plant Competition and Resource Allocation in SWAT+ Plant Communities

 

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