Hydrology as a science

7. TABIATDAGI SUV SIKILI
Katta suv aylanishi materiklar va okeanlardagi barcha turdagi suv aylanishini o'z ichiga oladi. Suvning quruqlikdan okeanlarga yoki qo'shni dengizlarga daryo oqimlari shaklida qaytishi katta suv aylanishi jarayonini yakunlaydi. Shunday qilib, Yer okeanlari, atmosfera namligi va suvlari bir tizim sifatida o'zaro bog'langan. Quruqlik yuzasida hosil boʻlgan daryo suvlarining bir qismi okean va dengizlarga quyiladi, bir qismi esa materiklar ichida qoladi. Quruqlik yuzasining katta qismi (78 foizi) Yer okeanlari tomon qiyalik bo'lib, u erda hosil bo'lgan daryolar okeanlarga quyiladi. Quruqlikning bu qismi okeanik yoki chekka oqim zonalari deb ataladi. Daryolar okeanga quyilmaydigan hududlar ichki oqim zonalari yoki yopiq zonalar deb ataladi. Yer sharida chekka oqim maydonlari 117 mln km2, ichki oqimlari esa 32 mln km2 ni tashkil qiladi. Eng yirik ichki oqim hududlariga Orol-Kaspiy havzasi, Afrikadagi Chad ko'li, Sahroi Kabir, Arabiston va Markaziy Avstraliya cho'llari misol bo'la oladi.

8.THE ARAL SEA PROBLEM
The first President Islam Karimov, in his book "Uzbekistan on the threshold of the XXI century: pays special attention to the tragedy of the Aral Sea, a threat to security, conditions of stability and guarantees of development", he states: "the drying up of the sea is a dangerous, extremely acute problem, arguably a national catastrophe. The Aral Sea crisis is one of the greatest environmental and humanitarian tragedies in human history. About 35 million people living in the sea basin have been affected by it. We are witnessing the disappearance of one of the largest indoor water bodies in the world in 20-25 years. But in the eyes of a generation, the death of an entire sea had never happened before. " Indeed, the natural hydrological regime of most lakes on Earth is undergoing drastic changes under the influence of human economic activity, i.e. anthropogenic factors.
The Aral Sea is located in the lowest part of Central Asia at the confluence of the Karakum, Kyzylkum, Ustyurt, Big Borsik, Little Borsik deserts. Until the 1960s, it was the largest surface area in Central Asia and the fourth largest in the world after the Caspian Sea, Upper Lake in North America, and Lake Victoria in Africa. Continuous monitoring of the Aral Sea water regime has been carried out since 1911. During the 50-year period from that year to 1961, the absolute altitude of the average perennial sea level was 53.04 meters.