I think sitting in england and browsing on net will never bring u real facts, Oman, Egypt, Iran Afghanistan,and others u mention, are dry countries, but rich in agriculture, they dot fall in drought,, due measure if u think every year 100000 ltr will fall, u r mistaken here, due measure means its equaly distributed to every part of year. whether u believe it or not, I told already, Quran is book of signs, guidance, u can't call it book of science, it contains topic or evvery subject, in short manner, it cannot give u formulas or more explanations, the verses are clear for sensible and educated ppl to understand.
But for u, u dont have answer to the questions, for u there is no sign in Quran
even ur science can;t give u correct answer