Plant uptake
Simon DE OLIVEIRA
1. Primary Issue Location
Component: SWAT+ Editor & SWAT+ Executable
Area of impact: Model functionality / Crop Growth & Nutrient Balance Output
2. Software Versions
GIS Interface: QSWAT+ (latest compatible)
SWAT+ Editor: v3.1.4 (2026-07-06)
SWAT+ Revision: rev.61.0.2 (MODULAR Rev 2024.61.0.2)
3. Project Characteristics
Watershed Details: Marateca River Basin, Portugal (Mediterranean climate, warm/dry summers). Standard HRU discretization.
Modeling Aim: Water quality assessment, specifically tracking agricultural nitrogen dynamics, fertilizer management, and crop yield.
4. Description of Issue
The Problem: The model experiences a total failure in plant growth and nutrient uptake. In lsunit_nb_mon.txt, the nuptake and puptake columns remain strictly at 0.000 for all spatial units throughout the entire simulation, despite scheduled fertilizer applications (75.89 kg/ha).
Checking pl_grow_sum.txt, the generic agricultural land community (agrl) shows a catastrophic 182.26 days of Nitrogen Stress (astress), 0 days of water/temperature stress, and a final maximum LAI (lai_mx) of only 0.15, leading to a total crop failure (yield = 0).
Looking closely at plant.ini generated by the GUI, agrl_comm is written with an initial LAI of 5.00000 but an initial biomass (bm_init) of 0.00000 on January 1st. This mathematical inconsistency (LAI = 5 but Biomass = 0) seems to paralyze the transpiration/growth engine right from Day 1, locking the plant into an infinite nutrient starvation loop and preventing it from detecting any spring fertilizer inputs.
Steps to Reproduce:
Setup a standard agricultural management schedule using the Editor GUI.
Let the GUI write the default plant.ini file where agrl_comm gets lai_init = 5.0 and bm_init = 0.0 with lc_status = y.
Run the SWAT+ executable.
Check lsunit_nb_mon.txt (nuptake is 0) and pl_grow_sum.txt (astress is ~182 days).