Environmental Engineering Reference
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Figure 24.2 The form of the logistic curves of vegetation growth with different maximum possible vegetation biomass values.
model (Tilman, 1982) using essentially a daily growth
period and making growth proportional to water effi-
ciency and then partitioning the growth to leaf, stem
and root according to partition coefficients. Respiration
needs are also budgeted in the model. An important
result emerged - that the rate of recovery was heavily
determined by the amount and pattern of rainfall (as
modelled of course), so that generation that started at
different times and therefore having different rainfall
histories during the regeneration period have different
rates of recovery. This process is shown in Figure 24.3,
where each curve of regeneration starts at a different year
and years with large extreme rainfalls usually show faster
regeneration.
The same logistic growth is assumed to hold for soil
erosion. That is, the more erosion there is, the more
there will be. Erosion starts slowly, accelerates and then
slows down as equilibrium is reached. The positive feed-
back occurs because shallower soils produce more runoff,
more erosion and therefore shallower soils. The rate of
erosion slows down eventually as more and more soil is
removed. This slowing is mainly because lower horizons
are denser and have a higher stone content (Thornes,
1990). Ultimately, when there is no soil left, there can
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Normal
Wet
Dry
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Days since abandonment
Figure 24.3 The modelled recovery of vegetation cover from bare ground, starting at different times and therefore having different
rainfall histories in the recovery period (after Obando, 1997).
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