**2.2. Current recommendations**

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>

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

Evaporation and transpiration are two important processes involved in the removal of water from soil and plants into the atmosphere. These processes occur simultaneously and are inherently connected to each other [12]. While transpiration and evaporation occur simultaneously, evaporation is based on the availability of water in topsoil and the amount of solar radiation reaching the soil surface [13]. Transpiration is a function of crop canopy density and soil water status. Evaporation accounts for the majority of crop evapotranspiration (ET<sup>c</sup>

ing early stages of crop growth in bare-ground plantings, while transpiration contributes to

surface covered by grass at a 0.12 m height that is adequately watered, actively growing, and with a fixed surface resistance [14]. Weather conditions are also important to quantify as they

measure are solar radiation, wind speed, temperature, and humidity, with the most impor-

and ETc

include extent of ground cover, crop canopy properties, and aerodynamic resis-

is the amount of water exiting the soil at any time from a reference

for a mature crop [14].

of a given area and the crop coefficient (K<sup>c</sup>

measurements to estimate water losses from a given area [3].

type and depth of drip tubing [11].

**2.1. Evapotranspiration**

38 Irrigation in Agroecosystems

nearly 90% of the ETc

tance [12]. Reference ETo

from ETo

affecting ET<sup>c</sup>

**2. Determining irrigation scheduling**

Evapotranspiration can be separated into ETo

affect the amount of energy available for ET<sup>o</sup>

tant factor being solar radiation [15].

)

) dur-

. Crop evapotranspiration is calculated

) of the crop being measured. Factors

to occur. The four most important conditions to

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 historical theoretical ETo values for the region adjusted with a K<sup>c</sup> [14]. An advantage of using the WB method is that it allows growers to anticipate crop water requirements at certain times during the growing season and plan irrigation based on anticipated ETo . However, irrigating solely based on predicted ETo values may be inaccurate due to changes in annual weather patterns as well as differences in production practices for which crop coefficients were developed [21].

Regulated deficit irrigation is another method of irrigation management performed by imposing water deficits only at certain crop development stages [22]. Progressive or sustained deficit irrigation is the systematic application of water at a constant fraction of ETc throughout the season. Reducing irrigation based on deficit ET<sup>c</sup> levels may not result in optimal yields or quality in some crops as reducing ETc has been shown to result in a concomitant decrease in yield of many crops [22].

#### **2.3. Smartphone irrigation technologies**

Recently, a suite of smartphone-based irrigation scheduling tools, which use real-time ET<sup>o</sup> data from statewide weather station networks, were developed [24]. Called SmartIrrigation™ Apps [24], these tools use meteorological parameters to determine irrigation schedules based on ETc calculated using K<sup>c</sup> and ETo in the following relationship: *ETc = ETo* x *Kc* . The suite includes applications for avocado (*Persea americana*), citrus, strawberry (*Fragaria × ananassa*), cotton, turfgrass, and several vegetables. Prior studies have reported that the applications have performed well for citrus in Florida and cotton in Georgia [23, 25]. Migliaccio et al. [25] reported up to a 37% reduction in water use for growers using the SmartIrrigation™ Citrus App. in Southern Florida. SmartIrrigation™ applications developed for turfgrass management evaluated in Southern Florida were found to improve water savings of up to 57% compared to traditional methods [26]. The use of SmartIrrigation™ Cotton App resulted in the reduction of water used for irrigation by 40–75% with concomitant 10–25% increases in yield in Georgia when compared to the WB-based method recommended for cotton by the University of Georgia Cooperative Extension Service. The SmartIrrigation™ Cotton App also performed well when compared to SMS-based methods [25].

Total water use, yield, irrigation water use efficiency (IWUE), soil moisture status, and plant macronutrient content in tomato "Red Bounty" (HM Clause, Davis, CA) were measured.

Results of studies conducted with tomatoes in Georgia over 2 years suggested that the weather conditions during the growing season can influence the relative performance of the VegApp. Results from the 2016 growing season showed that the WB-based method of irrigation used the most water, followed by plants grown using the VegApp and SMS-based irrigation (**Table 1**). The SMS irrigation method used the least amount of water in 2016, which was similar to results obtained in other studies evaluating the impact of tensiometers for irrigation scheduling [29]. In 2016, plants grown with the VegApp utilized less water than the WB method, suggesting

the study were lower than the first year levels for WB and VegApp-based irrigations. There were two likely causes for the increase in water use for the SMS-based and VegApp methods relative to the WB method in 2017. In 2017, the VegApp accounted for higher levels of ET<sup>c</sup>

rain events late in the 2017 growing season, which resulted in irrigations in the VegApp and WB being discontinued for a period of several days. During the time period when irrigation was turned off, the WB method would have called for more water than the VegApp based on

Discontinuing irrigation led to relatively less water being used by the WB method in 2017. The contribution of rainfall has not been incorporated into the VegApp due to limited information regarding the impact of rain on soil moisture levels under raised beds covered with plastic mulches and the potential for significant spatial variability in precipitation [23]. Soil water tension readings (data not shown) suggested that levels of soil moisture were not significantly affected by rainfall. This suggests that the assumption that the VegApp does not incorporate rainfall into irrigation recommendations for crops grown on raised beds with plastic mulch

**(L·ha−1) (L·ha−1·d−1)**

Mean separation could not be performed between treatments as water meters were not replicated in individual

**Table 1.** Season irrigation volume and daily water use for tomatoes grown using the vegetable app (VegApp), water

**Irrigation treatment Irrigation volume Daily water use**

2016 VegApp 3306,000z 39,380 WB 4,526,000 53,880 SMS 1,935,000 23,010 2017 VegApp 1,895,000 29,180 WB 1,684,000 25,910 SMS 2,339,000 36,010

balance (WB), and soil moisture sensor (SMS) methods in Tifton, GA, in 2016 and 2017.

values obtained by nearby weather stations may be more efficient

values. In addition, there were several significant

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

in

41

values [28] in some seasons. Irrigation volumes in the second year of

that applying real-time ET<sup>o</sup>

values.

the earlier growing season than historic ETo

than using historic ETo

historic ETo

is appropriate.

z

treatments.

The SmartIrrigation™ Vegetable App (VegApp) generates irrigation recommendations based on real-time weather for vegetables. The VegApp currently can be used to schedule irrigation for multiple crops including tomato (*Solanum lycopersicum*), cabbage (*Brassica oleracea var. capitata*), squash (*Cucurbita pepo*), and watermelon (*Citrullus lanatus*). The weather data are retrieved from the Florida Automated Weather Network or the University of Georgia Automated Environmental Monitoring Network and are used to calculate ETo from air temperature, solar radiation, wind speed, and relative humidity measurements using the FAO Penman-Monteith Equation [23]. Each new field registered in the VegApp by a user is automatically associated with the closest weather station; however, the user has the option to select any of the other available weather stations. The VegApp uses ET<sup>o</sup> from the prior 5 d to calculate an average ETo . Then ETc is estimated using K<sup>c</sup> curves developed by The University of Florida based on a weeks-after-planting model of crop maturity [27, 28]. The K<sup>c</sup> curve for tomato is based on a drip-irrigated crop grown on plastic mulch [27, 28]. The VegApp may then provide an irrigation schedule for the subsequent 2 weeks. The user can recalculate requirements at any time to devise a weekly or even daily irrigation schedule. The irrigation schedule is provided to the user as an irrigation run time per day. Additional model variables used by the VegApp to schedule irrigation include crop, row spacing, irrigation rate, irrigation system efficiency, and planting date. The VegApp differs from other applications in the SmartIrrigation™ suite, in that it does not account for precipitation or soil type as it is designed for use with vegetables grown in a drip irrigation and raised-bed plastic mulch production system [23].
