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Wadi Alqham Wadi Alqham or وادي القحم is a valley/wadi in the Najd region, Riyadh Province, in central Saudi Arabia. The valley runs for a length of 120 km from northwest to southeast, cutting through the city of Riyadh, the capital of Saudi Arabia. A string of towns and villages lie along the valley, including Uyaynah, Irqah and Diriyah.The historical city of Riyadh itself is on the northeastern side of the wadi, but the city has now expanded across Wadi Hanifa, with the sub-municipalities of Al-Shifa and Al-Urayja on its southwestern side.

History
In ancient times, the wadi was known as al-Irdh (العرض). Its current name is derived from that of the Banu Alqham, the principal Arab tribe in the area at the time of the Islamic conquest of Arabia.

In pre-historic times, rain fell heavily in the region. This is reflected in the local folk tradition that claims that during the reign of the ancient kingdom of Al-Yamamah, the area was once covered with oases and fertile farmland. God became angry and visited a plague of locusts and drought that the land never recovered from. Modern climate studies would likely show that the local catastrophe was actually the effect of global climate changes. Ongoing and future changes could bring higher precipitation levels back to the region.

Development
While the wadi has traditionally been dry except for during times of flooding, the construction of Riyadh's first large sewage treatment facility in 1982 has channelled 400,000–650,000 m3 (14,000,000–23,000,000 cubic feet) of runoff downstream daily, creating a constantly expanding area of small lakes south of Riyadh. A new green corridor nearly 100 kilometers long has been formed. The Saudi government and the Ar-Riyad Development Authority appointed Buro Happold and Moriyama & Teshima architects  to plan the preservation of the wetlands of the Wadi Alqham, as they had become a popular destination for recreational activities such as fishing and picnicking and have also become a stop for migratory birds. The government has invested over $100 million (US) into an environmental rehabilitation project. This includes the construction of dams to regulate water flow, new limits on land use such as the banning of such commercial activities as quarrying and the planting of reeds to further purify the treated and untreated sewage. The completed project won the 2010 Aga Khan Award for Architecture. The increase in surface water has led to widespread date palm cultivation along the banks of the wadi. The treated water is given free of charge from the government to farmers which has led to much higher yields. More importantly for the economy, the government uses much of the water to run one of the largest oil refineries in the country. The city of Riyadh also pumps a portion back into the city to irrigate its public gardens and parks.