The Importance of Accounting for Precipitation When Using Soil Moisture Probe Based
Irrigation Scheduling In the Sub-Tropical Climate of the Southeastern United States
Parker Grace Adams
Abstract
This paper discusses the importance of irrigation scheduling and the various methods
available to determine when irrigation should occur. Irrigation scheduling is an extremely
beneficial technology introduced to the agricultural industry in the past several decades that has
allowed the cultivation of land not in close proximity to water as well as improved the crop
yields of that cultivated land. But unrestricted irrigation can lead to a soil excessively saturated
with water, ultimately reducing crop yields. Therefore, in the interest of producing maximum
crop yields as well as reducing the money, water, and energy inputs used in irrigation, it is
prudent to schedule irrigation times to receive maximum benefits. Soil moisture probes are often
used to determine how much water to apply based on the amount of moisture available in the
ground. When that moisture reading is lower than a specified point irrigation systems can be
turned on. In arid and semi-arid climate regions were rainfall is scarce a schedule of irrigation
can be created from soil moisture probes. However, in the sub-tropical climate of the
Southeastern United States putting together an irrigation schedule based solely on soil moisture
probes is a bit more complicated due to the noteworthy amount of rainfall received during the
growing season. This rainfall must be taken into account when irrigating so as not to over-water
the crops. A simple check-and- balance system can be used to determine the proper amount of
water application and derive an irrigation schedule. In this paper, the importance of using both
weather data and soil moisture probe data to derive an irrigation schedule for crops grown in the
sub-tropical climate of the Southeastern United States will be explored and expounded upon in
addition to the other factors which contribute to efficient irrigation scheduling.
Keywords: irrigation scheduling, soil moisture probes, rainfall, weather data, balancing method,
sub-tropical climate
Introduction
Irrigation has been an invaluable resource to farmers for many ages. Irrigation practices
have continually been improved upon enhancing benefits as well. Crop yields have increased
under the practice of irrigation and have done so further under precisely scheduled irrigation
events. Related factors of production such as input costs of water and energy use are reduced.
Nutrient, fertilizer, and pesticide losses are lowered as well. While irrigation and irrigation
scheduling have many benefits, there are a few concerns that must be taken into account.
Irrigation occurs in conjunction with precipitation and as such over-watering can easily transpire.
Over-watering results in decreased crop yields and increased input costs. Using soil-moisture
probes, irrigation can be precisely scheduled to avoid over- or under-watering crops. However, in
the subtropical climate of the southeastern United States, annual rainfall is high and soil moisture
probes cannot detect or predict coming storms. Therefore, it is important to weather data with a
soil moisture probe based irrigation scheduling system in the Southeast.
Irrigation
Water is arguably the most important and greatest limiting factor in plant growth and
crop yield. Without water the plant will not grow. Plants receive their needed supply of water
through precipitation events like rainfall or other natural sources such as ground water. In
production settings, irrigation is used to supplement precipitation. Sometimes this is because
precipitation events alone do not provide the plant with its needed amount of water. In most
cases irrigation supplements rainfall because it increases crop yields (Knappenberger 5).
Increased crop yield can result in an increase in profit or a decrease in inputs such as the amount
of land required to produce the same amount of crop. Therefore, irrigation is considered a
beneficial and often necessary farming practice.
Hazards of Over-Watering
Irrigation can easily result in an overwatering of crops. Overwatered crops have a lower
crop yield. This occurs due to the over-saturated moisture content of the soil. An over water-
saturated soil often leads to “plant disease, nutrient leaching, and reduced pesticide
effectiveness” (Evans 1996).
Furthermore, over-irrigating crops leads to other unfavorable effects. Over-irrigation
wastes water, a declining, valuable resource (Evans 1996). Water for irrigation is pulled from
ground water reserves. This can be a water body such as a lake, a reservoir, or an aquifer
(Kebede 2918). Many producers in the delta region of Mississippi use the Mississippi River
Alluvial Aquifer to irrigate their crops, but the Aquifer is not being replenished to meet the
demands of producers (Kebede 2918). With proper techniques, water can be applied in an
efficient method. One of the most important methods of efficient irrigation is the timing of water
(Evans 1996). The timing of water application to crops has been shown to not only increase crop
yields but decrease the water required as well (Harrison 2005). Prudent scheduling of irrigation
times can also minimize wasted water in the form of runoff and percolation losses (Evans 1996).
Minimizing these losses assists in sustaining irrigation water reserves. Scheduling irrigation
leads to an efficient use of water as a resource.
Another negative effect of over-irrigation is wasted energy use. Energy use is related to
irrigation and can be translated into a dollar amount. Irrigation systems require energy in the
form of electricity to run including pumping water from a reserve to the field. If over-irrigation
occurs, not only is water being wasted which decreases crop yields, but energy is being wasted in
running the system that is over-irrigating the crops. It is a double loss.
Energy can be saved by “no longer pumping water that is [being] wasted” (Evans 1996).
Mentioned previously, the amount of water needed can be reduced by applying it at opportune
times. By not using as much water, irrigation systems are not running as long and therefore, the
energy required to run the system is reduced. Decreasing both the energy and water required for
irrigation decreases input costs. Decreased input costs translate into an increase in profit (Evans
1996). That increase in profit is further maximized by the increase in crop yield stemming from
the efficient irrigation practices.
Using Soil Moisture Probes to Schedule Irrigation
The purpose of irrigation scheduling is to “determine the timing and the amount of water
to be applied based upon the crop’s water needs, soil water storage capacity, and climate
conditions” (Kebede 2922). Soil moisture probes are often used to schedule irrigation (Kebede
2923). They are based upon the moisture content in the soil. Plants experience stress when water
levels in the soil dip below a certain amount (Martin 2009). A stress level threshold is often set
for crops at a matric potential of 50kPa, below which “plants are considered to be under stress”
and which is why “[it] is considered as an irrigation trigger” (Knappenberger 5). Soil moisture
probes can be used to detect the matric potential of the soil in effect measuring the moisture
available in the soil to the plants. This information can be used to determine when to begin
irrigation and how much water to apply (Kebede 2922).
Soil water storage capacity plays a role in scheduling irrigation due to the differences in
the ability of different soils to hold water (Harrison 2005). For example, sandy soils hold less
water than clay soils. Therefore a crop grown in a sandy soil will need to be irrigated more
frequently than a crop grown in a clay soil (Harrison 2005). This volume ultimately dictates
irrigation, as it determines how much water can be made available to crops as stored in the soil
(Evans 1996). Once the soil water storage capacity is reached, excess water becomes runoff and
percolation (Evans 1996). Again, this harkens back to wasted energy and water that translates
into increased input costs. Furthermore, runoff and percolation reduce pesticide effectiveness and
are a cause of nutrient leaching (Harrison 2005). The water storage capacity of a soil plays a part
in overwatering as well. Excess soil moisture results in plant disease and decreased crop yields as
stated previously (Harrison 2005).
Associated to the amount of water available in the soil, moisture probes account for the
amount of water used by the plant (Kebede 2922). This crop water use is defined as “the amount
of water given up to the atmosphere by transpiration through plant leaves and evaporation from
the soil and plant surfaces” (Kebede2922). These two processes, evaporation and transpiration,
are combined in the term evapotranspiration (Kebede2922). Daily crop water use changes
throughout the growing season depending on different factors such as the life stage of the crop,
temperature, and wind-speed (Kebede 2922).
In arid and semi-arid regions such as the western United States, soil moisture probes
work very well in conjunction with irrigation systems. Once the probes detect that the soil
moisture is below a set threshold amount they trigger the irrigation system to begin irrigating
(Martin 2009). The threshold value is usually set above the stress-inducing matric potential value
so as to “insure that water stress will not be so severe as to cause any appreciable yield losses”
(Martin 2009). Thus, the whole system can be automated. This is in part due to the minimal
amount of annual rainfall that occurs in arid/ semi-arid regions and the even smaller amount
which occurs during the growing season (NCSU 2012). Because the annual rainfall is minimal,
overwatering is not a concern. In effect, with precipitation not a concern irrigation scheduling
operates in a controlled environment where efficiency is increased.
A soil moisture probe based irrigation system can work in sub-tropical areas such as the
southeastern United States. Jason Krudtz, a Mississippi State University irrigation specialist at
the Delta Research and Extension Center in Stoneville, Mississippi, is a huge proponent of soil
moisture sensors. In an article on the use of the sensors in Mississippi Krudtz explained their
efficiency. “‘It sounds absurd to say that you can apply half the water you normally apply and we
can maintain or improve your yield almost guaranteed and improve your profitability by cutting
way down on your water cost,’” Krutz said at the 2013 MSU Row Crops Short Course (Coblentz
2013).
Accounting for Precipitation
The main downfall of using soil moisture probes to schedule irrigation is they cannot
detect coming precipitation events. Annual rainfall must be considered when scheduling
irrigation. The southeast — including Alabama, Arkansas, Georgia, Louisiana, Mississippi,
North Carolina, South Carolina, and Tennessee — receives an annual rainfall amount of between
40 and 70 inches (NCSU 2012). While annual rainfall is high only 30% occurs during growing
season (Kebede 2918). Therefore, irrigation is used to supplement precipitation and maximize
crop yield. If irrigation were to occur without consideration for precipitation overwatering would
transpire leading to reduced crop yield and decreased efficiency. Therefore, combining a
moisture probe based system with a weather based system is wise.
Combining Scheduling Methods
In its simplest form, irrigation scheduling occurs in what is commonly referred to as the
checkbook method. The checkbook method is a simple accounting approach that seeks to
balance the crops’ water needs with incoming water sources based on water levels in the root
zone (Evans 1996). Water inputs such as irrigation and precipitation are compared against water
outputs such as crop water use, evapotranspiration, and percolation losses (Harrison 2005).
Building upon the checkbook method, other factors may be included. Annual rainfall that
occurs during the growing season in the southeast should be taken into account. The timing of
rainfall is most important as it has a major impact on irrigation decisions (Evans 1996). Irrigation
can be automatically scheduled to occur at certain times, however, these times do not account for
precipitation occurrences. Chances are that irrigation and precipitation will overlap causing over-
watering to occur reducing overall efficiency and crop yield (Evans 1996). In some instances, it
might be more wise to postpone irrigation in anticipation of a precipitation event than irrigate.
Final judgement is left to the producer.
More complex methods and models include factors such as wind speed, temperature,
solar radiation, past weather data, evaporation estimates, crop water use and more (Kebede
2924). One such model is the Mississippi Irrigation Scheduling Tool or MIST (Kebede 2924).
Many more programs exist, most are programmable and calibrated to local areas.
Conclusion
Irrigation scheduling requires knowledge of soil types, soil moisture capacity, crops, crop
stress, the potential yield of a crop if it remains stressed, and system operation. Scheduling is
often used by producers to maximize their crop yields while simultaneously reducing their input
costs. Other benefits occur such as minimized runoff that prevents nutrient, fertilizer, and
pesticide losses. Irrigation scheduling often utilizes soil moisture probes to determine when
irrigation should occur based on the amount of moisture in the soil. In the sub-tropical region of
the southeastern United States weather events play a role in scheduling irrigation. Annual rainfall
is high enough in the growing season to risk over-watering when irrigation also takes place.
Therefore, it is wise and advantageous to use soil moisture probe based irrigation and weather
data when scheduling irrigation events.
References
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