The Thames RBD covers an area of over 16,000 sq km from the source of the River Thames in Gloucestershire through London to the North Sea. Dominated by Greater London, the eastern and northern parts of the river basin district are heavily urbanised, whereas the area to the west of London has considerable areas of rural land. The Thames RBD is one of the driest in the UK, with rainfall levels below the national average, however the River Thames is an important water source, providing around two-thirds of London's drinking water. Even though it is one of the most densely populated and urbanised parts of the UK, agriculture remains one of the most important industries in the landscape. Farming varies from intensive fruit and vegetable farming to mechanised arable farming on chalk to dairy and beef farming on grassland.