Page 220 - Biosystems Engineering

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6.3.2 Assessing Plant Water Needs
When the soil water content is maintained between the FC and the
permanent wilting point (PWP), the water is accessible to plant roots.
The water content within this range is known as available moisture
(AM) (ASAE 1990). However, as the water content of the soil decreases
from FC to PWP, the plant roots progressively access water from
smaller and smaller pores. As a result, the capillary pressure increases
and the plants have to work increasingly harder to take up water
from the soil. Ideally, if the soil water content is maintained near FC,
the plants will uptake water easily. This will require more-frequent
replenishment of the soil water by irrigation. Depending on the type
of water application method used to supply water to the plants, it
may not be practical to apply water more frequently. Generally, the
soil water content is allowed to deplete about half the available water
(ASAE 1990). The water content from FC to the midway point between
FC and PWP is known as readily available moisture (RAM). Biosys-
tems engineers design irrigation systems based on supplying the
RAM in a single irrigation sequentially to all the fields and plan to
come back to the same field just before the RAM is depleted.
Many methods are used to assess plant water needs. Biosystems
engineers have helped develop many of these methods that have
helped meet plant water needs accurately. The different methods can
be broadly classified as soil based, weather based, and plant based,
which are described as follows.

Soil-Based Methods
Soil moisture sensors are strategically embedded within the root zone
and the data are manually read by the farmer or transmitted tele-
metrically to a central station. The farmer will use the current soil
moisture status to determine the level of soil moisture depletion of a
particular field and plan the next depth and timing of irrigation.
Some of the methods measure the soil water content by measuring
the electrical resistance, which is proportional to the water content.
The dielectric constant of water, dry soil, and air are 80, 2-7, and 1,
respectively (Topp et al. 1980). This difference in dielectric constant
can be used in a dielectric mixing model based on volumetric ratios
to determine the volume of water present in a soil sample. Topp et al
(1980) presented an empirical equation to determine the volumetric
water content of soil based on apparent dielectric constant measured
by time-domain reflectometry (TDR). This is a nondestructive method
whereby a probe is inserted into the soil and a reading can be obtained
using a TDR. Sri Ranjan and Domytrak (1997) have presented the
design of TDR miniprobes that nondestructively measure the soil
water content in a soil volume the size of a finger. These miniprobes
are ideal for determining soil water depletion from different parts of
the plant root zone.