**1. Introduction**

Fruit and vegetable farmers in the USA rely on irrigation to produce high-value crops. Though drip irrigation is perceived to be efficient compared to other forms of irrigation, mismanagement can result in excessive water applications with water migrating through macropores (worm holes, cracks, root channels) to below the root zone. Previous experiments have demonstrated that water used for irrigation can be detected in a pan lysimeter within 20 min of drip irrigation

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

initiation on tomatoes [1]. When the water used for irrigation migrates below the root zone, there may be associated leaching of fertilizer and pesticides [2]. Efficient irrigation scheduling requires that farmers manage the timing and duration of irrigation in a manner that maintains yield and quality, while efficiently using water. Many irrigation scheduling methods exist including: the water balance (WB) method, soil moisture monitoring, hand feel and soil appearance, and crop phenology observations. Water balance-based irrigation scheduling relies on reference (ET<sup>o</sup> ) measurements to estimate water losses from a given area [3].

Crop coefficients are an adjustable constant that define the amount of transpiration occurring within a plant at a given stage of development. Crop coefficients are computed as the ratio

stage, climate, and soil type [14]. Plant developmental stage encompasses the relative activity of the plant. Plant size is also impacted by the crop development stage, thus affecting leaf area and canopy density, which in turn impacts transpiration. Accounting for environmental and management factors that influence the rate of canopy development is also important in

important to account for the variability in weather, irrigation, drainage, and runoff [16, 17].

Hargreaves method. The Priestly-Taylor equation is a modification of the Penman-Monteith equation that approximates parameters established by the Penman-Monteith, using solar

area during months with significant rainfall, corresponding to peak early summer vegetable

method. This equation is an empirical model that considers incoming solar energy, evaporation, monthly maximum and minimum temperature, and a temperature coefficient [19]. This method has a high correlation with the Penman-Monteith model for estimates of average

to use than the Penman-Monteith method; however, this can also result in reduced precision

Current recommendations for drip-irrigated tomatoes in Georgia and Florida are based on variations of the WB method [20]. The WB method estimates daily crop water use based on

the WB method is that it allows growers to anticipate crop water requirements at certain

weather patterns as well as differences in production practices for which crop coefficients

Regulated deficit irrigation is another method of irrigation management performed by imposing water deficits only at certain crop development stages [22]. Progressive or sustained defi-

values for the region adjusted with a K<sup>c</sup>

times during the growing season and plan irrigation based on anticipated ETo

cit irrigation is the systematic application of water at a constant fraction of ETc

in humid regions [19]. These methods of calculating evapotranspiration are easier

include crop type, crop growth

Using Smartphone Technologies to Manage Irrigation http://dx.doi.org/10.5772/intechopen.77304

are rainfall frequency, wind speed,

estimates from several seasons are

for the region [18]. Priestly-Taylor

for the Georgian Coastal Plain

has been the Hargreaves

[14]. An advantage of using

values may be inaccurate due to changes in annual

has been shown to result in a concomitant decrease in

levels may not result in optimal yields or

. However,

throughout

rate, such as the Priestley-Taylor method and

. However, calculations at a research site in the humid Southeastern

development are

39

. Environmental and physiological factors affecting K<sup>c</sup>

. Climatic factors that significantly affect K<sup>c</sup>

water table depth and soil porosity. Therefore, regional K<sup>c</sup>

USA found that Priestley-Taylor could overestimate ET<sup>o</sup>

has also been reported to overestimate the cumulative ETo

production [18]. Another method that has been used to estimate ET<sup>o</sup>

Several WB-based methods exist to calculate ET<sup>o</sup>

temperature, and photoperiod [14]. Soil profile characteristics that affect K<sup>c</sup>

ETo :ET<sup>c</sup>

calculating K<sup>c</sup>

weekly ETo

radiation to determine ETo

over the course of a season.

historical theoretical ETo

were developed [21].

yield of many crops [22].

**2.2. Current recommendations**

irrigating solely based on predicted ETo

quality in some crops as reducing ETc

the season. Reducing irrigation based on deficit ET<sup>c</sup>

A majority of vegetable growers use traditional methods of measuring soil moisture, by observing soil dryness and through feeling the soil itself. Recent surveys conducted in Georgia (US) found that this method accounts for over 40% of the irrigation scheduling occurring on farms. In addition, an estimated 88% of growers in Georgia may allow crops to be visibly stressed before watering [4]. Other methods of soil moisture-based irrigation may utilize tensiometers, granular matrix probes, or resistance-based sensors to determine thresholds for irrigation management [5, 6]. While soil moisture sensor (SMS)-based irrigation has been shown to be more efficient than a time-based system [7–9], proper placement of sensors to accurately reflect conditions experienced by the plant can be challenging [10]. Furthermore, placement of sensors within an irrigation zone can be problematic for growers with heterogeneous soils or topography within a field. Irrigation thresholds may also be impacted by factors such as soil type and depth of drip tubing [11].
