Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 19

Table 2.1 Aridity, drought and desertification (from Chapman, 1999)

Drought (temporary) Aridity (permanent) Desertification (human-induced)

Can occur in any climate Climatic state associated with global circulation

patterns

Resulting from:

Unpredictable occurrence Low annual precipitation normal Mining of groundwater

Uncertain frequency Rainfall highly variable in both time and space Over-grazing and unwise cultivation

Uncertain duration High evaporation normal Attempts to extract more from land than natural

productivity allows

Uncertain severity High solar energy input Unwise irrigation practises leading to salinisation,

Long periods of lower than average precipitation Large annual temperature variations normal Producing symptoms of:

Protracted periods of diminished water resources Low productivity of natural ecosystem normal Reduction of perennial vegetation cover

Diminished productivity of natural ecosystems Low productivity of farms and rangeland normal Aquifer depletion, land subsidence

Diminished productivity of farms and rangelands Sparse human settlement normal Damaged surface soil and subsoil, decreased

infiltration

Deterioration of farm land and range land Increase in soil temperature

Compaction and salinisation of soils
Oxidation of soil organic matter
Reduction of water-holding capacity of soil
Increased propensity for flash flooding and
further erosion
Loss of productivity of natural ecosystems and of
farms and rangeland
Raised surface albedo (tends to diminish rainfall)
Invasion of former farms and rangeland by woody
weeds