ABSTRACT
Soil water potential may be considered as the ‘‘energy status’’ of the water in the soil pores,
relative to some standard reference condition or datum. Soil water potential is defined along
with the units of measurement now in common practice (see Section 69.3). Soil water
potential is used primarily for determining the direction and rate of water flow between
locations with differing potentials (i.e., flow due to a potential gradient or hydraulic head
gradient). The potential of soil pore water varies over several orders of magnitude, ranging
from positive values in saturated soil to extremely negative values in dry soil. There are
numerous instruments and techniques for direct and indirect measurement of soil water
potential, but no single approach applies for the entire water potential range commonly found
in soils or other natural porous materials. Direct measurement of soil water potential involves
determining water pressure or water surface elevation relative to a datum (e.g., pressure
transducer, standpipe water level, etc.), while indirect measurement involves measuring
some surrogate property that correlates with water potential (e.g., electrical resistance or
conductivity, water vapor pressure, water content, plant xylem potential, etc.). For example,
a direct determination of positive pore water potential is obtained from the elevation of the
water surface in a piezometer pipe, while an indirect determination of negative water
potential can be obtained from electrical resistance or relative humidity.
