Mechanistic modeling of root water uptake in tropical agriculture: a sensitivity analysis of drought stress dynamics

Background and aims

Drought stress is a major driver of crop yield reductions in Brazil and other tropical regions. This study explores the mechanistic underpinnings of drought stress using a process-based root water uptake (RWU) model. We aimed to perform a comprehensive sensitivity analysis of the SWAP/MFlux model to simulate drought stress in long-term scenarios of soybean and wheat cultivation under tropical winter-dry conditions.

Methods

The agro-hydrological model SWAP, incorporating the RWU function MFlux, was used to simulate 32 years of rainfed soybean and wheat cultivation across five soils with varying hydraulic properties in a tropical winter-dry climate. Sensitivity analysis of the MFlux function was conducted using three methods — local, global Morris, and global Sobol' — by varying seven RWU parameters within literature-supported ranges.

Results

Wheat, grown in the dry winter, experienced higher drought stress than soybean, grown in the wetter summer, across the years. Root length density was the most influential RWU parameter, contributing 35% to 50% of drought stress variation. Soil hydraulic properties were also influential, with Ferralsols linked to a 50% reduction in above-ground dry matter productivity and an Acrisol and a Nitisol to up to 30% in the standard scenario. The Sobol' method provided the most comprehensive parameter sensitivities.

Conclusions

Root length density is the most influential parameter in modeling drought stress, with soil hydraulic properties modulating crop responses. This study offers insights for informing management and breeding strategies to mitigate soil- and climate-induced limitations on soybean and wheat production in tropical environments.