=Paper=
{{Paper
|id=Vol-1498/HAICTA_2015_paper37
|storemode=property
|title=Optimizing Soil Moisture Uniformity and Irrigation Management
|pdfUrl=https://ceur-ws.org/Vol-1498/HAICTA_2015_paper37.pdf
|volume=Vol-1498
|dblpUrl=https://dblp.org/rec/conf/haicta/GravalosKGGA15
}}
==Optimizing Soil Moisture Uniformity and Irrigation Management==
https://ceur-ws.org/Vol-1498/HAICTA_2015_paper37.pdf
Optimizing Soil Moisture Uniformity and Irrigation
Management
Ioannis Gravalos1, Dimitrios Kateris2, Anastasios Georgiadis2,
Theodoros Gialamas2, Avgoustinos Avgoustis2
1
Department of Biosystems Engineering, Technological Educational Institute of Thessaly,
41110 Larissa, Greece, e-mail: gravalos@teilar
2
Department of Biosystems Engineering, Technological Educational Institute of Thessaly,
41110 Larissa, Greece
Abstract. This research is a study on the relationship of irrigation water
treatments and soil moisture distribution uniformity (DU). Soil moisture
distribution was based on long-term data sets that were collected during wet
and dry soil conditions (from permanent wilting point to field capacity) using a
novel electromagnetic sensor-based platform moving inside subsurface
horizontal access-tubes. The irrigation treatments regarding two case studies
under dry and wet soil conditions were conducted for a period of 115 days and
110 days respectively. In dry soil conditions, the irrigation water treatments
strongly affect the DU of soil moisture that can be achieved constantly using
variable rate irrigation treatments. On the contrary, the DU of soil moisture in
wet soil conditions was maintained at a high percentage and was slightly
affected by irrigation treatments. However, obtaining accurate soil moisture
information at a large scale over a long period can be used to improve water
use efficiency.
Keywords: precision irrigation scheduling, sensor-based platform, uniformity.
1 Introduction
The main methods used to describe soil moisture content are gravimetric, volumetric
and depth of soil moisture per depth of soil. Many instruments exist for measuring
and monitoring soil moisture content and they are summarized as follows: neutron
moisture meter, tensiometers, electrical resistance blocs, and dielectric sensors and
probes. Dielectric sensors and probes have gained wide acceptance over the last
years. This group of sensors and probes determines soil moisture content by
measuring the dielectric constant of soil (Muñoz-Carpena, 2004).
Soil moisture content is highly variable in time and space. Soil moisture variations
are affected by different factors such as soil texture, topography, crop cover, climate
parameters and irrigation practices. Soil moisture variability is very important to
understand soil moisture redistribution after rain or irrigation event, infiltration,
evapotranspiration and pollutant transport. Various sensing approaches have been
developed for measuring spatial and temporal soil moisture variability, including soil
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moisture sensor networks, geophysical methods (Hu et al., 2011) and remote sensing
techniques (Moran et al., 2004). A novel horizontal access tubes sensing system for
monitoring soil moisture variability using an electromagnetic sensor-based platform
was first proposed by Gravalos et al. (2012).
The objective of the study reported here was to investigate the effects of uniform
rate irrigation and variable rate irrigation on soil moisture distribution and give
recommendations for improved irrigation scheduling and the design of automatic
irrigation systems. Soil moisture distribution is based on long-term data sets that
were collected during wet and dry conditions (from permanent wilting point to field
capacity) using a novel electromagnetic sensor-based platform moving inside
subsurface horizontal access-tubes with the task of monitoring the soil moisture
distribution.
2 Materials and Methods
Experiments were carried out under laboratory conditions in the Department of
Biosystems Engineering at the Technological Educational Institute of Thessaly
(Greece) during a period of eight months. In the laboratory room, the ambient
temperature was kept nearly constant (≈22 ˚C). For the soil moisture monitoring
experiments, an artificial soil tank was used and rigid polyvinylchloride (PVC)
plastic access tubes. The soil tank was made of water-resistant wood, having
dimensions 1.44 m long, 1.10 m wide, and 0.25 m deep (total volume 0.4 m3). The
three PVC access tubes were placed horizontally, along the soil tank, at a depth of
0.15 m under the soil surface, and at uniform distances. The type of soil used for all
series of experiments was clay loam. The total surface of the soil tank was divided
using a wood frame into 36 (3 columns x 12 rows) equal cell-rectangles. The wood
frame was used for the trapping of applied water in the individual cell-rectangles and
thereby ensuring uniform distribution of irrigation water.
A schematic illustration of the prototype electromagnetic sensor-based platform
that travels through subsurface access tubes and monitors the soil moisture content is
shown in Fig. 1. It was composed of a modified commercial soil moisture sensor
(Diviner 2000), which was placed on two articulated wheeled bases. The sensor-
based platform is presented in detail elsewhere, see Gravalos et al. (2012). According
to Sentek Pty Ltd. (2007), the Diviner 2000 sensor recorded moisture from a soil
volume outside the access pipe, which had a sphere of influence of: (a) 100 mm
horizontal length, and (b) 50–100 mm radial distance from the outer wall of the
access pipe.
The electromagnetic sensor-based platform recorded the soil moisture content at
fixed positions of the PVC access tubes spaced out initially at 6 cm and then every
12 cm of length increment (move-stop-measure case). Each position corresponded to
the center of each rectangle. Thus, for each access tube a number of 12
measurements have been conducted where every single value is the average value of
three readings. By use of a data display unit and a personal computer, the soil
moisture content was determined at each position one time per day.
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Fig. 1. Schematic illustration of the electromagnetic sensor-based platform.
(1) towing hook,
(2) driving wheel, (3) DC motor, (4 & 6) wheeled bases, (5) shaft, (7)
sliding wheel, (8)
universal joint, (9) soil moisture sensor.
In this study, the irrigation water treatments regarding two case studies under dry
and wet soil conditions (from permanent wilting point to field capacity) were
conducted for a period of 115 days and 110 days respectively. The irrigation water
was precisely measured by a volumetric flask and applied directly onto the surface of
the 36 equal cell-rectangles of the soil tank allowing high irrigation uniformity. On
the first days (2/12/2013 to 19/12/2013) under dry soil conditions, the irrigation
water was applied uniformly on the entire tank surface (0.25 l per each cell-
rectangle) with an irrigation frequency of 9 liters every 4 days. This way 45 liters of
water were initially irrigated. During the next days (20/12/2013 to 30/01/2014), non-
uniform irrigation was applied in the soil tank according to the observations provided
by the prototype electromagnetic sensor-based platform. In this case, only those cell-
rectangles in which the observed soil moisture was lower than 10m3m-3 were
irrigated. The purpose of the variable rate irrigation was to achieve distribution
uniformity of soil moisture content in the soil tank around the desired limit of 10
m3m-3. This period of variable rate irrigation a total of 15.25 liters of water was
consumed. The soil moisture content in tank was further increased after a period of
one month (01/02/2014 to 28/02/2014) with distribution uniformity of irrigation
water in all cell-rectangles (0.125 l per each rectangle). In this treatment the
frequency of irrigation was 4.5 liters every 4 days, and was applied in a total 22.5
liters of water. The last time interval (01/03/2014 to 27/03/2014) during the dry soil
conditions repeated the variable rate irrigation in the soil tank according to the
indications provided by the electromagnetic sensor-based platform. In this case only
those cell-rectangles in which the observed soil moisture content was lower than 16
m3 m-3 were irrigated. During this period of variable rate irrigation there was
consumed a total of 27.25 liters of water.
The observations regarding wet soil conditions were conducted for a period of 16
weeks (28/3/2014 to 15/7/2014). 36 l of water were applied on the first 4 days (9 l for
every irrigation session). The irrigation water was applied uniformly on the surface
of each of the 36 equal cell-rectangles of the soil tank. In the next two months
(01/04/2014 to 31/05/2014), the irrigation water was applied non-uniformly on the
surface of the soil tank according to the observations provided by the prototype
electromagnetic sensor platform. The aim of these irrigation treatments was to
achieve distribution uniformity of soil moisture content in the soil tank around the
field capacity. In this case study only those cell-rectangles in which the observed soil
moisture content was lower than 27m3m-3 were irrigated. This period of variable rate
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irrigation a total of 32.5 liters of water for the first month (five sessions) and 35.5
liters of water for the second month (thirteen sessions) was consumed. On the other
days (01/06/2014 to 15/07/2014), the change of the soil moisture content was only
recorded without any irrigation treatment. The soil starts to lose moisture
progressively while drying up from field capacity moisture content.
3 Results and Discussion
The distribution uniformity of the majority of irrigation systems is influenced by
different factors (such as sprinkler operating pressure, sprinkler spacing, etc.). During
this study the irrigation water treatments were conducted in the experimental soil
tank, without crop cover, under controlled laboratory conditions, and with high
application effort. The applied water cannot move laterally as surface flow due the
elimination action of the wood frame. The applied water can move only vertically
and then laterally due to capillary action of the soil in each cell-rectangle of the
experimental tank.
The results of the uniformity coefficients (CU) of the applied water in different
irrigation days in soil tank surface are shown in Fig. 2. The Christiansen’s coefficient
of uniformity (Christiansen, 1942) was used for calculating irrigation water
uniformity. The 100 % of application rate tests of CU represent the treatments where
the irrigation water was applied to all cell-rectangles evenly without application
losses in order to achieve rapid and uniform distribution of soil moisture at the
desired values. The low application rate tests (8.3 % to 68.3 %) of CU represent
these treatments, in which the irrigation water was applied only in selected cell-
rectangles according to the readings of the sensor-based platform in order to improve
the general DU of soil moisture in the soil tank. Therefore, the lack of uniformity in
the water application affects soil moisture distribution between cell-rectangles of the
soil tank.
Fig. 2. Applied water uniformity coefficients during different irrigation days.
Fig. 3 shows the evolution of soil moisture distribution uniformity (DU) (Merriam
and Keller, 1978) based on long-term data sets (2/12/2013 to 15/7/2014) that were
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collected during dry and wet soil conditions (from permanent wilting point to field
capacity) and were calculated based on the average of the lowest quarter of soil
moisture measurements (9 points) dividing by average of total soil moisture
measurements (36 points) in the soil tank.
In permanent wilting point (before any irrigation water treatment) the resulting
soil moisture DU was 80 %. On the twenty first days during which 100% of the
application rate tests were conducted (with irrigation frequency 9 liters every 4 days),
DU of soil moisture was found to be declining gradually from 80 % to 40.3 %. The
low DU of soil moisture, after uniform rate irrigation treatments, was due to different
water infiltration rate into 36 cell-rectangles of the soil tank that affects the moisture
movement in soil. Infiltration rate is unmanageable and varies both in time and
space. The investigation period from 20/12/2013 to 30/01/2014, under variable rate
irrigation, indicated that the soil moisture DU increased rapidly on the first fifteen
days and then followed a less upward trend. In this case, it only the cell-rectangles in
which the observed soil moisture was lower than 10 m3 m-3 were irrigated, and
15.25 liters of water were consumed in total. In the last period, from 01/03/2014 to
27/03/2014 of the dry soil conditions, variable rate irrigation on the soil tank was
repeated, but in this time it only the cell-rectangles in which the observed soil
moisture was lower than 16 m3 m-3 were irrigated. However, in this period of variable
rate irrigation 27.25 liters of water consumed in total and soil moisture DU remained
nearly constant at 67 %. In general, the irrigation water treatments strongly affected
DU of soil moisture during dry soil conditions. Constant DU can be achieved by
using variable rate irrigation that is based on rigorous soil mapping techniques and
schedule irrigation to specific points under the irrigator on a daily basis.
Fig. 3. Evolution of soil moisture distribution uniformity based on long-term data sets
(2/12/2013 to 15/7/2014) that were collected during wet and dry conditions.
Then the DU of soil moisture in wet soil conditions near the field capacity was
studied. On the first five days (28/03/2014 to 01/04/2014), the water was applied
uniformly on the soil tank surface with irrigation frequency 9 liters per day. The
resulting soil moisture DU was 87.7 % (significant increase). In the rest period of the
wet soil conditions (01/04/2014 to 31/05/2014), only these cell-rectangles in which
the observed soil moisture content was lower than 27 m3m-3 were irrigated. In this
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case the DU of soil moisture remains constant at 85.24 %, and 68 liters of water were
consumed in total. During the last period of the wet soil conditions (01/06/2014 to
15/07/2014) without any irrigation water treatment, the resulting soil moisture DU
was gradually reduced. According to the above results, DU of soil moisture was
maintained at a high percentage and slightly affected by the irrigation treatments. In
addition, DU exhibited lower sensitivity compared with DU in the case of dry soil
conditions.
4 Conclusions
The results analysis indicates that, the irrigation water treatments strongly affect DU
of soil moisture, which can be achieved by constantly using variable rate irrigation
during dry soil conditions. On the other hand, DU of soil moisture maintains at high
percentage and is slightly affected by the irrigation water treatments during wet soil
conditions. In wet soil conditions the irrigation water is transformed and smoothed
into less variable soil moisture values. Some regions of the experimental irrigated
area, which received higher amounts of applied water, indicate higher soil water
holding capacity than the others which received lower amounts of applied water. It is
obvious that the spatial distribution of the moisture values depend more on intrinsic
factors of the soil than on irrigation water distribution.
References
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Experiment Station Bulletin, No. 670.
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Tsiropoulos, Z.T., Xyradakis, P.I., 2012. Design of a pipeline sensor-based
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