ATS2545 Lecture Notes - Lecture 8: Water Cycle, Condensation, Advection

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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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