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Interesting question, The catchment is the entire area that may contibute water from rainfall (Queensland, Australia so no snow) to the stream system including the ground water. Obviously seasonal variations in rainfall amounts and patterns would cause differences in the amount of water reaching the aquifers and stream but these average out over time particularly with 10 year rolling averages. Firstly though, is this observation accurate? Data from Catchments in SE Queensland supports it as do observations in some other parts of the world particularly Africa. How much water is or was expected ie. the expected Catchment yield as a percentage of rainfall? Zhang et al created a model from world wide data that suggests that for a rainfall of 1000mm/year between 20% and 30% of rainfall should leave the catchment via the stream or aquifer, less as the rainfall reduces. The variation is to do with vegetation density and rainfall patterns. For a particular catchment the model can predicts catchment yield. The model also predicts the changes in yield as more Forest Plantation is added to the catchment.
This is the key. In a natural forested catchment we would expect the forest to use up to 80% of the rainfall. Plants evaporate water to keep cool, evapotranspiration. Measured yields however are now as low as 5% to 10% of rainfall. A catchment made up of Forest Plantation will rarely yield water at all. Even a small percentage of Plantaion can eliminate the yield if it is placed across the riparian zone and aquifer.
In South Africa there are restrictions on Plantation establishment, particularly near streams. Australia has a further problem, one shared with those who imported Eucalypts. Disturbed Eucalypt forest regrows much thicker than any plantation and would use even more water if it was available. Because these trees are drought tolerant they do not die but simply drop leaf while waiting for rain. The problem on the East Coast of Australia at least is dense regrowth and woodland thickening generally caused by fire patterns that are different from Aboriginal patterns. Double or treble the amount of water can be had by returning bushland to a natural density.

2006-12-13 23:02:57 · answer #1 · answered by Gary K 3 · 0 0

Water flows over the ground either from rain or from a stream into your catchment. When the water is in the catchment and not flowing it is susceptible to the other two types of movement: evapotranspiration and ground water discharge. Depending on how high the water table in your area is some water will leech from your catchment into the ground and travel underground to and area of lower water potential. Evapotranspiration is the water that evaporates into the air and the water that is absorbed by plants. Evapotraspiration occurs all year long but it occurs at a higher rate the hotter it is outside especially in low humidity areas.

2006-12-13 10:23:02 · answer #2 · answered by ncsudancelover 2 · 0 1

It depends on where you live and the precipitation patterns. Maybe one of these two things is occurring:

In much of the mountain West, snowmelt provides much of the stream flows in summer. Streams are starting to dry up in summer because winter snows are melting earlier and faster - causing faster runoff and lower baseflows in summer. This has been pretty-well documented.

In an area that has not typically had much winter snow, a shift toward more precipitation in winter, and less at other times of the year can cause streamflows to be low for much of the year, even though the amount of annual precipitation doesn't change. In this situation, the ground becomes saturated in winter, causing most of the precipitation to run off quickly, so the total amount of groundwater recharge decreases, and baseflows which rely on groundwater are diminished. This sort of thing has not been documented as a long-term pattern, but it happens regionally in some years.

2006-12-13 11:46:56 · answer #3 · answered by formerly_bob 7 · 1 0

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