Southern Arabia (i.e. Oman and Yemen) has no permanent rivers, but it does have flowing water in the regions wadis for much of the year thanks in large part to the biannual Indian monsoon. As a result, Yemen, in particular, is far more lush than the Hejaz to the north. Techniques like terrace farming, the construction of cisterns, dams and aflaj irrigation systems allowed southern Arabia to be more suitable to permanent, settled agriculture than just about any other part of the peninsula.