Dryland systems are
ecosystems characterised by a lack of
water. They include cultivated lands, scrublands, shrublands, grasslands,
savannas, semi-deserts and true deserts.
The lack of water constrains the production of crops, forage, wood, and other
ecosystem services.
Four dryland subtypes are widely recognized: dry sub-humid,
semiarid, arid, and
hyperarid, showing an increasing level of aridity or moisture
deficit.
Drylands refer to land areas where the mean annual precipitation (P) is less
than two thirds of potential evapotranspiration (PET = potential evaporation
from soil plus transpiration by plants), excluding polar regions and some high
mountain areas which meet this criterion but have completely different
ecological characteristics.
Hyperarid areas, also referred to as true deserts, have a P/PET ratio of less
than 0.05.
Arid areas have a P/PET ratio of 0.05 to 0.20.
Semiarid areas have a P/PET ratio of 0.20 to 0.50.
Dry sub-humid areas have a P/PET ratio of 0.50 to 0.65.
