Irrigation Water Source in SWAT+ for Large HRU Models

[Engr. Moazzam Mushtaq]

Dear Sir/Madam,

I am working on a basin-scale SWAT+ model that is running successfully. The model consists of more than 29,000 HRUs and represents kharif–rabi cropping systems, mainly rice–wheat and cotton–wheat.

My main question concerns irrigation water sourcing in SWAT+. When irrigation operations (e.g., surface or sprinkler irrigation) are applied at the HRU level, what water source does SWAT+ assume by default? Specifically, how does the model determine whether irrigation water is withdrawn from a channel, reservoir, or aquifer when water rights and allocations are not explicitly defined?

In large models with thousands of HRUs and multiple potential water sources (channels and aquifers), assigning irrigation sources individually to each HRU through the Water Rights module becomes impractical. I would appreciate clarification on how SWAT+ handles irrigation water abstraction under these conditions, and whether any default source prioritization or implicit assumption is applied.

Thank you for your guidance.

Kind regards,
Moazzam Mushtaq