Instrumentation & Measurement Magazine 23-2 - 37

How Thirsty the Crops Are:
Emerging Instrumentation for
Plant-Based Field Measurement
of Water Stress
Dinko Oletic and Vedran Bilas

T

o grow crops in areas with arid climates, porous
karst soils, or increasingly unpredictable seasonal
droughts, farmers struggle for their yield and spend
precious water resources on irrigation. This ecological and economic problem of excessive water usage in agriculture may be
alleviated by application of precision irrigation techniques.
Precision irrigation implies precise spatial and temporal control of water supply based on the actual water demand. This is
also critical for maximizing the yield-quality of cultivars like
grapevine, which dynamically change their water requirements throughout different parts of their vegetative season [1].
However, to timely identify onset of drought, it is necessary
to be able to monitor the dynamics of the plant's physiological
processes related to water stress. This is a non-trivial task. In
the plant-physiological research community, there still remain
gaps in understanding the complex relations between the
physiological specifics of different plant species, their physiological drought adaptation mechanisms, and especially, how
they affect the measured plant-hydraulic quantities [2], [3]. To
fill these gaps, botanists strive for new types of autonomous instruments enabling them to conduct novel types of long-term
field-studies of the effects of drought.
For electronic engineers, this opens many opportunities
for research of novel electronic instrumentation for planthydraulic measurements. Here, we provide an overview of
state-of-the-art in measurements of common plant-hydraulic quantities, open challenges, and research opportunities.
We especially focus on novel, emerging methods showing
potential for field application, but which are still on their
way from botanists' laboratories into the vineyards and crop
fields.

Measuring the Water Demand in
Precision Irrigation
Measurement techniques used for irrigation scheduling are
usually grouped into three categories. The first category is

soil-based irrigation scheduling, relying exclusively on either measurement or estimation of soil-water status. The
second are plant-based irrigation techniques, determining the water-demand based on measured plant's water
status. Finally, the third category contains combined techniques, combining both soil-water and plant-water status
measurements.
Soil-based irrigation scheduling techniques aim to calculate the amount of water which needs to be supplied
in order to compensate for given soil-water deficit. The
least accurate is the model-based estimation of the soil
water balance. It estimates the total amount of available
soil-water from a mathematical model accounting for the
water drainage, evapotranspiration, and meteorological data (precipitations, air temperature, humidity, wind,
insolation).
More accurate soil-based irrigation scheduling is obtained
by direct measurement of physical quantities describing the
soil-water status: soil water potential or water content. The soil
water potential is usually measured using tensiometers and
psychrometers, and water content measurement are made by
using gravimetric sensors, capacitive time-domain reflectometry and neutron probes [4].
In the context of irrigation, a methodological drawback of using exclusively environmental and soil-based
measurements is that they provide only information on soilwater availability, and not on an actual crop's water deficit.
Therefore, they need to be combined with plant-based measurements to assess the actual plant's water demand [4].
This can be obtained from two types of plant-based measurements. The first are the plant's physiological responses to
drought, most usually estimated from stomatal gas exchange
measurements or change of the growth rate. The second type
of plant-based water stress measurements tries to quantify the
hydraulic quantities describing the water transport within the
plant's tissues.

This research has been supported by Croatian Science Foundation under the project IP-2016-06-8379,
SENSIRRIKA-Advanced Sensor Systems for Precision Irrigation in Karst Landscape.
April 2020	

IEEE Instrumentation & Measurement Magazine	37
1094-6969/20/$25.00©2020IEEE



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