The energy balance, evapo-transpiration and nocturnal dew deposition of an arid valley in the Andes

The arid Elqui valley, part of the Norte Chico of Chile between 27°S and 33°S latitude, is located south of the hyper-arid Atacama desert. The region is characterized by complex terrain and great different of the surface properties, the latter due to cultivated and irrigated areas along the valley floor and sparse vegetation in the arid surrounding. Here the energy balance of the valley has been investigated to compare the evapo-transpiration of the cultivated area with the one of the natural vegetation and to estimate the dew deposition, which proves to be an important water resource in arid areas. Analyzes are based on two Bowen ratio and two eddy covariance stations, operated in the Elqui valley from 2000 to 2002 and in November 2004, respectively.The Bowen ratio of the natural vegetation is about β≈11β≈11, but much lower in the cultivated and temporally irrigated area along the valley floor. A typical value of the Bowen ratio of a field covered by potatoes is β≈2.5β≈2.5 after harvesting or during dry periods and is β≈0.7β≈0.7 to 11 after irrigation events or precipitation.For an area with natural vegetation, the annual accumulated evapo-transpiration reaches 65 mm yr−1, which is 65% of the mean annual precipitation and 3.5% of the equivalent net radiation Rn/LRn/L. For the cultivated fields along the valley floor, the annual accumulated evapo-transpiration reaches 750 mm yr−1, which is 40% of the equivalent net radiation, Rn/LRn/L, and about 100% of the sum of estimated irrigation and mean annual precipitation (I+P). The annual nocturnal dew deposition is about 5–10 mm yr−1 and, thus, serves as an additional water source for natural vegetation, especially during dry years.