Complex sentence: Their main source of income is Government, Military, Air Force and Navy Service as their agricultural lands are dependant on rain for irrigation and no other irrigation system is available, therefore they left their lands in search of better income resources.
Split into simpler sentences: Their main source of income is Government, Military, Air Force and Navy Service as their agricultural lands have no irrigation system. Their lands are dependant on natural rain for irrigation, therefore they left their homelands in search of better income resources.