ATS2545 Lecture Notes - Lecture 8: Water Cycle, Condensation, Advection
Lecture 8 - Changing Global Hydrology: desertification and deforestation
Arguments:
• Global renewable fresh water resource is subject to perturbation by human
activity
• The state of the land surface as shaped by people influences regional and
global hydrology
• Causes of land surface change include overgrazing, logging, collection of fuel
wood for cooking
• Changes in vegetation can modify precipitation recycling, a key process in
large forested regions such as Amazonia
• Case study
o The WA wheat belt and the bunny fence that separates native
woodland from cropland
The Significance of Landsurface Properties (H and LE), roughness
• Energy balance at the surface: H and LE
o Surface properties, including moisture content, albedo and
aerodynamic roughness
▪ Affect the partitioning of surface radiation
o Moist soils, free water surfaces and actively transpiring plants convert
some of the received energy into latent energy (LE)
o Drier surfaces and surfaces of lower albedo, convert some of the
received energy into sensible heat (H)
o Evaporation rates and the mixing of H away from the surface area
affected by wind speed and eddy structure in the lowest layers of the
air - influenced by the surface roughness
• Changes in landsurface
o People have transformed > 33% of the earth’s landsurface
▪ Mainly for agriculture and settlements
o Been 20% reduction in forest cover since 1700
o 2.8 x 106 km2 is now irrigated land
o Deforestation has decreased global water vapour flows from land by
4% (3,000 km3 per year)
▪ But has largely been offset by increases from irrigation
o Effects may be large regionally
o Alteration of the hydrologic cycle due to forest clearing and its
consequences for rainforest succession
o Chile
▪ Forest interception loss ~30% but fell to 1% after clearing and
conversion to shrub
▪ Water table has risen and this may prevent forest re-growth
Precipitation Recycling - the principle
• Recycled rain is derived from evaporation and transpiration within a particular
region, and which later re-condenses and falls as rain within the same region
• Rain is composed of local and advected moisture - that is, water vapour
evaporated locally or carried inland from a source over the ocean (or another
moisture source upwind)
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