148
J. D. OVINGTON
water supply) are not always operative and that water loss is affected t o
a limited extent by differences in the vegetation cover because: (i) the
amount of energy reflected depends on vegetation colour and the nature
of its upper surface (Monteith, 1959); (ii) different plant covers vary in
their resistance t o wind; and (iii) the root systems of plants vary enormously, some taking water from a much bigger soil volume than others.
Recently Rider (1957) has emphasized the need t o apply cautiously the
concept of equal potential transpiration irrespective of crop size.
4. Run-off
Water that is not evaporated or transpired may be retained within
the ecosystem but the bulk passes from the ecosystem either as surface
run-off over the mineral soil or as ground water through the soil. The
relative proportions of the two types of water loss vary greatly, where
surface run-off is excessive, flash floods and soil erosion occur commonly.
Normally, more water leaves woodland ecosystems as subsurface tliaii
surface run-off. The organic matter overlying the mineral soil of woodlands tends t o absorb water readily and Dunford (1954) found that
when litter was removed from Ponderosa pine woodlands surface run-off
increased almost seven-fold. I n addition, water percolation into the
mineral soil is facilitated by numerous old root channels and animal
tunnels. The effect of the fauna on the water cycle is largely ignored but
animal activity is extremely important in creating conditions favourable
t o rapid infiltration of water into the soil. The traditional role of forests
in flood control by delaying and reducing the magnitude of peak run-off is
generally recognized.
B. W A T E R B A L A N C E
Detailed studies of isolated factors of the hydrologic cycle are ditficult t o integrate into a composite picture. At present, the most coinplete data available have been obtained by lysimeter and water catchment investigations, but the interpretation and accuracy of results
obtained by the two techniques have been subjects of controversy.
Nevertheless, both types of installation have repeatedly given results
which are broadly similar and consistent. I n Table VIII, summarized
water budgets obtained by lysimeter and water catchment studies are
presented, these are based on the assumption that long-term changes in
the water content of the ecosystem are insignificant. I n all cases, comparative data for woodland and for neighbouring treeless areas, or for
the same wooded area after tree felling are given.
The removal of water attributable to evaporation and transpiration
in Table VIII varies from 42 t o 72% of the precipitation in the wooded
areas whilst in areas devoid of vegetation or with a predominantly grass
cover the range is 23 t o 69%. On the whole, in dense woodlands evapora-
J. D. OVINGTON
water supply) are not always operative and that water loss is affected t o
a limited extent by differences in the vegetation cover because: (i) the
amount of energy reflected depends on vegetation colour and the nature
of its upper surface (Monteith, 1959); (ii) different plant covers vary in
their resistance t o wind; and (iii) the root systems of plants vary enormously, some taking water from a much bigger soil volume than others.
Recently Rider (1957) has emphasized the need t o apply cautiously the
concept of equal potential transpiration irrespective of crop size.
4. Run-off
Water that is not evaporated or transpired may be retained within
the ecosystem but the bulk passes from the ecosystem either as surface
run-off over the mineral soil or as ground water through the soil. The
relative proportions of the two types of water loss vary greatly, where
surface run-off is excessive, flash floods and soil erosion occur commonly.
Normally, more water leaves woodland ecosystems as subsurface tliaii
surface run-off. The organic matter overlying the mineral soil of woodlands tends t o absorb water readily and Dunford (1954) found that
when litter was removed from Ponderosa pine woodlands surface run-off
increased almost seven-fold. I n addition, water percolation into the
mineral soil is facilitated by numerous old root channels and animal
tunnels. The effect of the fauna on the water cycle is largely ignored but
animal activity is extremely important in creating conditions favourable
t o rapid infiltration of water into the soil. The traditional role of forests
in flood control by delaying and reducing the magnitude of peak run-off is
generally recognized.
B. W A T E R B A L A N C E
Detailed studies of isolated factors of the hydrologic cycle are ditficult t o integrate into a composite picture. At present, the most coinplete data available have been obtained by lysimeter and water catchment investigations, but the interpretation and accuracy of results
obtained by the two techniques have been subjects of controversy.
Nevertheless, both types of installation have repeatedly given results
which are broadly similar and consistent. I n Table VIII, summarized
water budgets obtained by lysimeter and water catchment studies are
presented, these are based on the assumption that long-term changes in
the water content of the ecosystem are insignificant. I n all cases, comparative data for woodland and for neighbouring treeless areas, or for
the same wooded area after tree felling are given.
The removal of water attributable to evaporation and transpiration
in Table VIII varies from 42 t o 72% of the precipitation in the wooded
areas whilst in areas devoid of vegetation or with a predominantly grass
cover the range is 23 t o 69%. On the whole, in dense woodlands evapora-