13
Measuring Water Availability and Uptake
in Ecosystem Studies
Robert B. Jackson, Laurel J. Anderson, and William T. Pockman
Introduction
Terrestrial productivity depends strongly on the
availability of water in the environment (Lieth
1972). Measuring the availability and movement of
water among soil, plants, and the atmosphere requires methods from ecosystem studies, plant physiology, soil science, and biogeochemistry (Casper
and Jackson 1997). Techniques for estimating the
water status of soils have been reviewed recently
(Rundel and Jarrell 1989; Boyer 1995) and those
for measuring plant water status at cellular and
whole-plant levels are thoroughly described in
physiological ecology and plant physiology texts
(e.g., Koide et al. 1989; Kramer and Boyer 1995).
Rather than reviewing all methods for estimating
plant and soil water, we emphasize techniques that
are increasing in importance or changing rapidly
for ecosystem studies. We include methods for determining the water content of soil, the availability
of that water for plant uptake, and the transport of
water through the plant (Fig. 13.1). Since many of
these methods are also used in physiological ecology, our chapter is designed to bridge the gap between the scales of physiological and ecosystem
ecology, as a foundation for other contributions in
this book.
We begin by defining terms that describe water
in the environment, including a brief discussion of
water potential, the currency that allows the water
status of soil, plants, and the atmosphere to be compared and the direction of flow to be predicted
(Slatyer and Taylor 1960). We describe various
techniques for measuring soil water in the field, emphasizing such recent innovations as time domain
reflectometry and remotely sensed data. We also
discuss methods for estimating the vegetative component of ecosystem water. fluxes, including sap
flow measurement and whole root/shoot hydraulic
conductivity. Such techniques for estimating
whole-plant water use are important for interpreting
canopy and ecosystem water fluxes in eddy covariance and other net ecosystem approaches (see
Chapter 11). We summarize the advantages and disadvantages of various techniques and recommend
some future directions for research.
Theory and Currencies
for Measuring Water
in the Environment
To understand differences among methods it is
helpful first to describe common ways of estimating
water availability. The three most common terms
for expressing soil water attributes are mass water
content, volumetric water content, and soil water
potential. Soil water content calculated on a mass
basis, em, is defined as
e =
m
soil water mass
soil dry mass
(wet soil mass - oven-dry soil mass)
oven-dry soil mass
(13.1)
em is a proportion; the mass water percentage (Pm)
is calculated by multiplying em by 100%. The volumetric water content, e v , is defined as
199
Measuring Water Availability and Uptake
in Ecosystem Studies
Robert B. Jackson, Laurel J. Anderson, and William T. Pockman
Introduction
Terrestrial productivity depends strongly on the
availability of water in the environment (Lieth
1972). Measuring the availability and movement of
water among soil, plants, and the atmosphere requires methods from ecosystem studies, plant physiology, soil science, and biogeochemistry (Casper
and Jackson 1997). Techniques for estimating the
water status of soils have been reviewed recently
(Rundel and Jarrell 1989; Boyer 1995) and those
for measuring plant water status at cellular and
whole-plant levels are thoroughly described in
physiological ecology and plant physiology texts
(e.g., Koide et al. 1989; Kramer and Boyer 1995).
Rather than reviewing all methods for estimating
plant and soil water, we emphasize techniques that
are increasing in importance or changing rapidly
for ecosystem studies. We include methods for determining the water content of soil, the availability
of that water for plant uptake, and the transport of
water through the plant (Fig. 13.1). Since many of
these methods are also used in physiological ecology, our chapter is designed to bridge the gap between the scales of physiological and ecosystem
ecology, as a foundation for other contributions in
this book.
We begin by defining terms that describe water
in the environment, including a brief discussion of
water potential, the currency that allows the water
status of soil, plants, and the atmosphere to be compared and the direction of flow to be predicted
(Slatyer and Taylor 1960). We describe various
techniques for measuring soil water in the field, emphasizing such recent innovations as time domain
reflectometry and remotely sensed data. We also
discuss methods for estimating the vegetative component of ecosystem water. fluxes, including sap
flow measurement and whole root/shoot hydraulic
conductivity. Such techniques for estimating
whole-plant water use are important for interpreting
canopy and ecosystem water fluxes in eddy covariance and other net ecosystem approaches (see
Chapter 11). We summarize the advantages and disadvantages of various techniques and recommend
some future directions for research.
Theory and Currencies
for Measuring Water
in the Environment
To understand differences among methods it is
helpful first to describe common ways of estimating
water availability. The three most common terms
for expressing soil water attributes are mass water
content, volumetric water content, and soil water
potential. Soil water content calculated on a mass
basis, em, is defined as
e =
m
soil water mass
soil dry mass
(wet soil mass - oven-dry soil mass)
oven-dry soil mass
(13.1)
em is a proportion; the mass water percentage (Pm)
is calculated by multiplying em by 100%. The volumetric water content, e v , is defined as
199