Water Quality
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Hydrology

The Yarqon River drains an area extending from the national water divide in the Judean Hills and Samaria to the Mediterranean Sea, covering an area of approx. 1,800 km2.  Most of the River’s tributaries and drainage channels flow off the hills and then through the Shefela (inland plain) and the coastal plain.  The flat area accounts for about 25% of the overall catchment basin, the area is drained by a system of tributaries and is characterized by intensive human activity related to agriculture, urbanization as well as associated activities. 
The volume of water flowing in the Yarqon River is the second largest among Israel’s rivers with only the River Jordan exceeding it.  When it comes to the catchment basin the size of the Yarqon River’s basin is the third largest in the country as it is surpassed by the Besor and Jordan Rivers.     
 
The catchment basin is divided into two sub-basins: Yarqon and Ayalon.  Both of these basins drain areas of similar size, i.e., approximately 900 km2 each.  The main tributaries of the Yarqon sub-basin are Qana, Rabah and Shilo while those of the Ayalon sub-basin are Beit Arif and Natuf. 
 
The average annual rainfall in the basin ranges from 600 mm in the western sector to 500 mm in the eastern area.  The maximum rainfall on record was measured at the Beit Dagan weather station where 1,100 mm fell during the winter of 1991-2, and the probability of such a high seasonal rainfall occurring was calculated at 3%.