The Definition of Desert - Lecture Slides | ATOC 4750, Study notes of Meteorology

Download The Definition of Desert - Lecture Slides | ATOC 4750 and more Study notes Meteorology in PDF only on Docsity! The Definition of “Desert” A Place May Be Defined As Desert Based on -  Climate – amount of precipitation only; or combination of meteorological variables that affect the water balance  Vegetation type or amount  Type of surface/soil/substrate  Water drainage (similar areas regardless of how defined) Simple Precipitation Thresholds for Defining a Place Desert  25 cm (10 inches) annually– common geographer’s definition  5 cm annually  25 cm annually in cold climates and 50 cm in warm climates (why the distinction?)  Have experienced 12 consecutive months without precipitation  Number of precipitation days (> 2.5 mm) per month, regardless of amount But only considering precipitation is an oversimplification Consider Two Locations With About the Same Annual Precipitation (15-20”) Sonoran Desert Taiga Apparently annual-total precipitation alone is not a very satisfactory basis for defining a place as desert To develop a better definition, answer the following. Why does it seem to be a lot less desert-like at the high latitude location? Consider the amount of water loss at a location as well as water gain bia Ae Stee ee s ae er |_| (es ner oe a al aN tliat IF || laa Mid-Latitude Storms Inter-Tropical Convergence Zone  60 N 30 N EQ 308 60S 60E 120E 180 120 W 60 W =< 0.5 mm day! =~ 18cm year! 0.5-1.5mm day"! =~18-55cm year |  BLMex.23: 08.29.02 Thus, dry climates (surfaces) are related to:  Low annual precipitation, dew and fog deposition  High wind speed  High temperature  High fraction of possible sunshine  Low humidity  One more thing - Precipitation concentrated during the warm season A simple definition of desert that relies on the surface water balance concept Deserts - the “land of the empty bucket” What does this mean?  If precipitation (gain) exceeds evaporation (loss), the bucket will generally be full.  If evaporation (loss) exceeds precipitation (gain), the bucket will generally be empty.  Thus, an empty bucket means water loss > water gain, and the place is called a desert.  Loophole – what if the bucket is full during some seasons and not others?  Try this yourself with a bucket, and we’ll see what happens. Using Soils/Substrate to Decide Whether a Place is a Desert  Salt content  Extremely desiccated through a deep layer Using Vegetation to Decide Whether a Place is a Desert  Vegetation is either water conserving, or has deep roots to reach water table o Se PEE as Se ie re e\se i S WES Se O be 2 | [© SSC 1 in wo > lll ° 7 Se = = SSE! | Neo = OE N it = & Some Quantitative Indices of the Degree of Aridity “P over PET” P = Water supply (precipitation) ET = Evapotranspiration, the sum of evaporation and transpiration PET = Potential evapotraspiration, the ET when the water supply is unlimited (water loss) P/PET is the ratio of the amount of water gain to the maximum loss under the existing meteorological conditions (temperature, wind speed, etc.) So low P/PET result from low precipitation amounts and high rates of loss Based on UN Definition  Hyper-arid: P/PET < 0.03  Arid: 0.03 < P/PET < 0.20  Semi-arid: 0.20 < P/PET < 0.50  Subhumid: 0.50 < P/PET < 0.75 PET is calculated from observed solar radiation, wind speed, humidity, etc. Does not apply to “cold deserts”