The most important decision is your first job after college usually sets the tone for the rest of your life career. If your first job is outside of your specialist, then it is unlikely that you’ll ever get back into it. For example, if you are keen on a career in research on-farm irrigation engineering, make sure that you get a suitable first job without compromising too much.
Regarding the five years out of school, I would suggest not thinking too much about money and instead focus on gaining and developing in-demand skills and on getting exciting research projects in which you learn loads of things that are important to your specific research field of irrigation engineering. This will help you learn the ropes of professional research. You should also make it a priority to get a job that will give you opportunities to be mentored by senior irrigation engineers.
If you are sure you want to get into R&D and/or advanced irrigation engineering, then plan to get a graduate degree right after your undergrad. A graduate degree is a minimum requirement for a serious researcher job, so save yourself time and get that before you start. You can instead do internships during some summers.
As a junior irrigation engineer, prioritize professional growth and learning over money. The money will come later on if you do this part right.
Could you tell us your experience about even it is a different field?