along the Nile valley from Cairo to the south the presence of rain is a rare sight, the fertility of the land being due to the existnece of the river itself.
帮忙分析下结构，谢谢

逗号前是主句。(along the Nile valley from Cairo to the south状语) (the presence of rain主语) (is谓语) (a rare sight表语),

逗号后是分词独立结构(或称独立主格)，作补充说明.（the fertility of the land主语) (being非谓语动词) (due表语) (to the existnece of the river itself状语).

翻译：沿尼罗河峡谷从开罗到南方，雨是很罕见的，土地肥沃是由于尼罗河本身的存在。