Principles of terrestrial ecosystem ecology.pdf

94 4. Terrestrial Water and Energy Balance
transpiration (Eq. 4.8). Average runoff from an
ecosystem depends primarily on precipitation
and evapotranspiration because long-term
changes in storage are usually negligible.
Runoff responds to variation in precipitation
much more strongly than does evapotranspiration
(Fig. 4.17) because it constitutes the leftovers
after the water demands for evapotranspiration
and groundwater recharge have
been met. Runoff is therefore greater in wet
than in dry climates or seasons. Over hours to
weeks, runoff generally increases after rainfall
events and decreases during dry periods.
Changes in water storage buffer this linkage
between precipitation and runoff. The recharge
of soil moisture in grasslands, shrublands, and
dry forests, for example, may prevent large
increases in flow from occurring after rainfall
when soils are dry. In ecosystems with a small
capacity to store water such as deserts with
coarse-textured soils and a calcic layer or
ecosystems underlain by permafrost, runoff
responds almost immediately to precipitation,
and rainstorms can cause flash floods.
Conversely, slowly draining groundwater provides
a continued source of water to streams
(base flow) even at times when there is no
precipitation.
Glacial river mean daily discharge (m 3 sec -1 )
2000
1600
1200
800
400
0
Non-glacial
river
In ecosystems that develop a snowpack in
winter, precipitation inputs are stored in the
ecosystem during winter, causing winter stream
flows to decline, regardless of the seasonality of
precipitation. Much of the water stored in
the snowpack can move directly to streams,
when the snow melts, causing large spring
runoff events. Glacial rivers, for example,
have greatest runoff in midsummer, when
temperatures are highest, whereas nonglacial
rivers in the same climate zone have peak
flow in early spring following spring snow melt
(Fig. 4.18).
Flow in streams and rivers integrates the
precipitation, evapotranspiration, and changes
in storage throughout the watershed. In large
rivers, the seasonal variations in flow often
reflect patterns of precipitation and evapotranspiration
that occur upstream, hours to weeks
previously. These integrative effects of runoff
from large watersheds make this an effective
indicator of temporal changes in the hydrologic
cycle.
Seasonal variations in stream flow are a
major determinant of the structure and seasonality
of ecosystem processes in streams and
rivers. Periods of high flow in small streams,
for example, scour stream channels, removing
Glacial
river
J F M A M J J A S O N D
Month
Figure 4.18. Average daily discharge from a glacial
and a clearwater river. Runoff from the clearwater
river peaks at snow melt, whereas the glacial river
140
120
100
80
60
40
20
3 sec -1 )
0 Non-glacial river mean daily discharge (m
has peak discharge when temperatures are warmest
in midsummer. (Redrawn with permission from
Phyllis Adams; Adams 1999.)