Agriculture Reference
In-Depth Information
2.3.1
WATER BALANCE
The water balance at a site describes the partitioning and fates of the rain water that falls
onto a defined area of ground over a specified period. It depends markedly on the type
of vegetation present and the proportion of the soil surface that it covers. The factors
of importance in a simple water balance model (Figure I.27) for a closed forest with a
single canopy layer are the incident precipitation (P)‚ the amount lost from the canopy
by evaporation (Ec) of the intercepted precipitation stored by the canopy (Sc)‚ that lost
by transpiration (T) through the stomata of the leaves and the net amount of precipitation
that reaches the forest floor by flowing down the stems (stemflow‚ SF) and by penetrating
the canopy (throughfall‚ TF). In drier environments‚ substantial water may rise by
capillary action (C) from deeper in the soil and enter the root zone. Where the capillary
rise attains the surface‚ salts transported in this water may accumulate to salinate the
surface horizons to such an extent that plant growth is inhibited.
Once water reaches the forest floor‚ part will be involved in recharging the store of
water contained in the litter (Sl)‚ some will be lost by evaporation from the surface of
the litter (El) and the soil (Es). A further quantity (runoff‚ R) may move by surface flow
to sites lower in the landscape‚ and part will enter the soil. Some of the latter is retained
