Sampai tahun 60-an para ahli masih memperdebatkan apakah air tawar berasal dari
dalam bumi (mata air, air hasil dari pembentukan batuan dsb) ataukah dari hujan.
Pada dasawarsa 70-an hampir semua ahli sumberdaya air sepakat bahwa 90% air
tawar berasal dari siklus hujan (siklus hidrologi). Ini yang menerangkan kenapa
hampir semua model ketersediaan air dihitung dengan konsep berdasarkan
ketersediaan air hujan di wilayah tersebut.
10 % yang bukan berasal dari hujan dikalsifikasikan sebagai air juvenil
(berasal dari inti bumi), air metamorfik (berasal dari proses pembentukan
batuan metamorf), air magmatik, air fosil dll. Mengingat komposisi kimianya
yang kaya dengan mineral, maka sulit untuk dikatakan sebagai sumber air tawar
yang layak dikonsumsi secara langsung.
Sepakat dengan Pak Awang ... Memang perkembangan dalam ilmu penentuan umur
secara umum membawa para ahli kepada kesimpulan bahwa air berasal dari `extra
terestial` yang dibawa oleh meteor dan komet 3,8 milyard tahun yang lalu.
Pertanyaan yang menarik adalah fenomena tabrakan meteor/komet/planetisimal
ini tentu saja akan berlangsung di planet-planet yang lain. Lalu mengapa hanya
di Bumi fenomena ini dapat berkondensasi menjadi lautan yang akhirnya memicu
terjadinya siklus hidrologi?
Salahsatu jawaban yang ada saat ini adalah akibat proses tektonik bumi yang
memicu letusan gunungapi yang pada akhirnya membentuk lapisan atmosfer. Tentu
saja masih banyak jawaban lain yang menunggu pembuktian dari para ahli kebumian.
Apapun jawaban yang benar ... Tampaknya alam Bumi ini adalah satu kesatuan
yang saling kait mengkait....
Keep on search .... Selamat Iedul Fithri 1428 H Mohon Maaf Lahir Batin,
Fajar (1141)
==============================================
Hystory of Water Cycle (from many sources) :
Water was delivered to primitive Earth by planetesimals, meteors and comets
during its accretionary phase, which was largely complete by 3.8 billion years
ago (bya).
Water was released from Earths crust in volcanic eruptions (degassing), but
remained in the atmosphere as long as Earths surface temperature was >100°C.
Once Earth cooled below 100°C, most water condensed to form the oceans.
Enough water vapor and CO2 remained in the atmpsohere to keep the temperature
of Earths surface above freezing; without this Greenhouse effect the Earth
might have remained frozen, like Mars.
There is good evidence of liquid water on Earth at 3.8 bya, and the volume of
water has not changed appreciably since then.
Throughout Earths history, changes in relative sea level have occurred
during times of tectonic activity that increases (or decreases) the volume of
submarine mountains, and/or global temperature changes that lead to
glaciation/deglaciation.
The geologic record shows large changes in ocean volume, as evidenced by
del-18O of marine carbonates, that accompanied repeated glaciation during the
Pleistocene Epoch (2 mya).
During the last peak glaciation (18,000 ybp) 42,000,000 km3 of seawater was
trapped in polar ice caps, or 3% of oceans volume, and this lowered sealevel by
120 m relative to the present day.
Continental glaciations thus represent a major disruption in the Earths
hydrologic cycle, or a loss of steady state conditions.
Some examples of perturbations in the hydrologic cycle due to glacial /
interglacials and the accompanying global cooling include:
- lower rates of evaporation
- reduced circulation of moisture through the atmosphere
- reduced precipitation
One model suggests that 18,000 ybp total ppt was 14% lower than at present,
deserts expanded, NPP and terrestrial biomass were lower, greater erosion
increased dust flux to the deep sea as recorded in deep sea sediments and ice
caps, and loess deposits on land. In contrast, the SW US climate ameliorated
during this time period.
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