Complex sentence: At one time, it was believed that they began food production here because of the rich soil, but although there was this rich soil present, it was still necessary for them to invent irrigation in order to successfully farm this alluvial desert.
Split into simpler sentences: At one time, it was believed that they began food production here because of the rich soil. Even so, it was necessary for them to invent irrigation in order to successfully farm this alluvial desert.