In the Basin and Range province of the western US the heat flow at the Earth's surface is considerably enhanced. In a 300 m deep borehole in Owens Lake in the eastern California portion of the Basin and Range, the temperature increased by 18°C from the surface to the base of the borehole. The conductivity of the materials is 4 W m^-1 °K^-1. 

• What is the heat flux (also sometimes called the heat flow) at the surface of the Earth? Please express this first in W/m², and then in heat flow units (HFU, where 1HFU = 41.84 m W/m²). Note that the worldwide average heat flow is 1.67 HFU.