Page 221 - Biosystems Engineering

Design of Sustainable W ater Management Systems 199

The soil water content is inversely proportional to the capillary
pressure of the soil, and this is depicted by soil moisture characteris-
tic curves. Tensiometers have been used to measure the capillary
pressure of the soil and the soil moisture characteristic curve of that
particular soil is used to determine the water content. Generally, ten-
siometers have been used to trigger irrigation when the soil reaches a
predetermined capillary pressure.
Weather-Based Methods
As described previously, weather plays a crucial role in controlling
the rate of ET. Weather stations that collect maximum and minimum
temperature, wind speed, solar radiation, rainfall, and relative
humidity can also transmit this data telemetrically to a central sta-
tion. Empirical relationships have been developed relating the ET of
well-watered grass to the weather variables (FAO 1998). The weather
data are entered into these empirical equations to predict the daily
reference ET rates for grass as the reference crop. For other crops,
crop coefficients have been developed to adjust the reference ET to
actual ET. The sum of this daily depletion (actual ET) since the last
irrigation is used as a measure of the soil water depletion level, and
the next depth of irrigation is planned based on this information. One
major drawback of this method is the lack of disconnect between the
soil water content and the ET rate. As described previously, plant ET
will not proceed at the maximum rate if the soil water content is
below FC; this phase is known as the soil-limiting phase (FAO 1998).
The weather-based methods do not account for this lowered ET. The
empirical equations were developed using grass growing in lysime-
ters, which were well-watered. A lysimeter is a large container in
which a crop is grown in the middle of a field; load cells monitor the
entire weight of the soil, plant, and water. The stage of growth of the
plant also determines the ET rate, and this is accounted for by incor-
porating another coefficient.

Plant-Based Methods
Leaves show different signs of water stress. The leaf angel may change
in response to water stress. Leaves will also lose turgor and appear to
be wilted. The color of the leaf will also change, which may not be
easily discernible by the naked eye. More details are presented as
follows.
Leaf Water Potential Method As the soil begins to dry, the plant leaves
have to generate a higher negative pressure to help uptake the water.
By measuring the leaf water potential, one can assess the water stress
experienced by the plant. Turner (1988) reported a method for deter-
mining the potential of the leaf water. In this method, a leaf is cut and
placed in to a chamber with the cut petiole exposed to the atmo-
sphere. When a leaf under water stress is cut, sap will retreat into the