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Can
use published rainfall and river discharge data.
The
Effective Precipitation (the amount of precipitation
that eventually becomes runoff) is more or less equivalent
to the runoff co-efficient. It has long been used
as a climate index by Jenny who related it to the
different ecosystems in the USA and to the type of
soil. For example calcium carbonate begins to accumulate
in the soil when the EPI is below about 500 mm and
water soluble salts at say 300 mm. It has been related
by Musgrave (1954) to the sediment collected in lakes
and by Anderson (various papers between 1948 and 1960)
to the sediment yield of American rivers.
The
EPI index can be used at a regional and continental
scale to compare sites but within any region allowance
must be made for the geology. The EPI is at a maximum
for areas with about 350 to 500 mm and this is the
region in which erosion is at a maximum (Schumm and
Lichty). Above these values, rainfall causes more
vegetation to grow. This evaporates more water so
that there is relatively less runoff and also less
erosion. Desertification in these areas will cause
more runoff.
In the areas with less than 350 to 500 mm, the opposite
might happen, as mentioned above. During wet years,
there will be more vegetation and the effective precipitation
in the summer will drop. During dry years, on the
other hand there will be less vegetation so that there
will be more runoff.
To
address this problem, the EPI should be interpreted
in combination with the precipitation data , i.e.
related to change in the amount of rainfall. It can
also be used for monitoring temporal changes.
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