**6. Irrigation scheduling**

Irrigation scheduling simply means application of water to crops at the required time and in the required quantity. Irrigation scheduling is one of the most effective way to increase water productivity in fertile crop production. Marketable yields for most shallow rooted vegetables can be easily damaged by short-term moisture stress of two to three days. Deficit irrigation generally results yield loss and inadequate quality in vegetables, while excess irrigation increases susceptibility to diseases, irrigation energy cost and environmental pollution risk from the nutrient leaching [31].

Different techniques of irrigation scheduling in irrigation of vegetable crops are used. They can be classified as monitoring of soil water status, water balance approach from crop water demand and observing of plant traits. Continuously monitoring soil moisture throughout crop growing period is very crucial and also requires accurate measurement with precision-based devices [32]. Soil moisture sensors can be used to regulate the interval of irrigation and, possibly, the water quantity by continuously monitoring water content or tension of the soil [31]. Technological advances in automatic soil water sensor-based irrigation systems are aimed to save an optimum soil water range in the root zone for high-quality plant growth. These algorithms are? used in automated irrigation management and scheduling agricultural activities. Furthermore, these algorithms are developed to observe water content and ensure irrigation with automated activation when necessary [33]. Smart irrigation automation systems can be less used in open field agriculture, while they are used increasingly in greenhouse production with soil or without soil to save considerable amounts of water and nutrients that are heavily applied.

Irrigation scheduling in crop water demand method consists of supplying the crop evapotranspiration (ETc) (daily, five or ten days' averages) for each crop growing stage. Thus, this method is known as crop evapotranspiration method. The ETc value of a fully-irrigated vegetable crop can be calculated either empirically by multiplying daily reference of evapotranspiration (ETo) by crop coefficient (Kc) or experimentally (2). However empirical way is commonly preferred to save time, cost and labor.

$$\mathbf{ETc = ETo \times Kc} \tag{2}$$

ETo is calculated by well-known Penman-Monteith (FAO) equation using daily or daily average (of five or ten days) air temperature, relative humidity, wind speed and solar radiation data [22, 34]. Kc values are selected from the tables prepared for different growth stages of vegetable crops [34]. Irrigation scheduling programs such as CROPWAT can be used as the model-based scheduling. This program uses Penman-Monteith (FAO) method with collected soil, crop and climatic data in the region [22, 35]. The precision of ET-based scheduling method depends strongly on the accuracy of the ETo estimation value, a correct Kc value determined with site-specific calibration approach, correct determination of the soil's available water holding capacity, and measuring site-specific precipitation [35, 36].

Irrigation interval can. be calculated by the ratio of readily available water (RAW) to the crop daily net irrigation water requirement (Inet) which are calculated as:

$$\text{Int}\left(\text{mm}\,\text{day}^{-1}\right) = \text{ETc} - \text{Effective Precipitation},\tag{3}$$

$$\text{Irrigation interval} \left( \text{day} \right) \text{=RAW} / \text{Inet.}\tag{4}$$

RAW is calculated by, below equation [35].

$$\mathbf{RAW} = \left(\theta \mathbf{fc} - \theta \mathbf{w} \mathbf{p}\right) \times \mathbf{D} \times \mathbf{MAD} \tag{5}$$

where RAW is the readily available water content (mm), θfc is the volumetric water content at field capacity (m3 m−3), θwp is the volumetric water content at the permanent wilting point (m3 m−3), D is the effective rooting zone depth or the soil layer depth considered (mm) and MAD is the fraction of the total available water that is allowed to be depleted. MAD value should be kept low in vegetable irrigations to protect plants from water stress. RAW value is also equal to the net irrigation quantity applied to the soil.

Plant-based scheduling techniques have been improved from the relationship between crop water stress and soil moisture deficit to define an optimal moisture content level for crop growth. Measuring crop water stress for irrigation scheduling has been also recommended considering the variations in plant species, tissues, and phenological stages [35, 37]. The approaches have been categorized as the measurements of tissue water potential and the measurements with plant physiology-based (sap flow, stomatal conductance, thermal sensing with infrared thermometers).

Stomatal conductance is a good indicator in determination of irrigation need in many plants sensitive to water insufficiency, thus improvement of this technique among plant-based irrigation scheduling approaches has drawn increaasing attention [37]. In recent years, the use of irrigation scheduling based on the crop water stress index (CWSI), which is calculated based on the canopy temperature measured with an infrared thermometer, has gained importance. Many researchers have reported that the CWSI value can be used for preparation of irrigation scheduling [38]. This approach argues that significant increases in canopy temperature exceeding air temperature has been a good indicative for stomatal closure and water deficit stress [37].

Furthermore, a systematic method can be applied as a practical scheduling approach. In this method, water applications are managed on a time or volume basis applying every day for the same duration or in the same quantity. Moreover, it is quite practical to base irrigation scheduling on evaporation from a Class A pan as a result of the combined effect of climatic factors. This approach requires a correction coefficient (kp) (mostly changed between 0.6–0.8) to convert potential evaporation value measured in pan to the ETo value. The Kp coefficient is also expressed as Kcp when it includes the crop coefficient. Water use from fully developed vegetation can be about 75–80% of the amount of water evaporated from the pan, in other words, Kcp = 0.75–0.80. When plants do not completely cover the soil surface, actual water consumption will be less than 75% of pan evaporation. Water use for vegetable crops can be considered as 10–15% of pan evaporation during the first 1/3 of the season, 40–50% of the pan evaporation during the mid-season, and 60–80% of the pan evaporation during the last 1/3 of the growing season [39].

#### **7. Irrigation methods**

In irrigated agriculture, when operating an area for irrigation, firstly the most suitable irrigation method under the conditions should be selected, then the system required by this method must be planned, installed and operated. In general, the irrigation method to be selected must meet some conditions such as to provide uniform water distribution, to minimize deep percolation and run-off losses, not cause soil erosion, not prevent agricultural mechanization, help leaching the salt from soil.

Due to the shallow rooting depth of most vegetables and their high response to lack of water, irrigation is frequently required in small amounts. This situation is more important in greenhouses with intensive production. Vegetable growers consider drip irrigation method as an effective way to save water and that plant needs, as well as to reduce weeds, fungi and diseases. Drip irrigation minimizes water loss from run-off and deep percolation, decreases evaporation losses. It has been determined that water savings of 50–80% are achieved when compared to conventional surface irrigation methods [27]. Drip irrigation method also provides more efficient water and fertilizer usage than the sprinkler method. It also reduces disease problems because leaves are not wetted. Drip irrigation lowers energy need because of the low pump pressures required and provides more applicable opportunity of automation. However, compared to sprinkler nozzle sizes, drip system emitters have very small openings, emitters gets clogged. Therefore, it requires water quality control and some preventive solutions such as filtration and dilute acid applications. Many researchers have declared considerable benefits of drip irrigation method over other conventional irrigation methods to improve yield and water productivity (WP) of fruits and vegetables [40]. The water application efficiency is about 80–90% in drip irrigation systems [39]. Maximizing the water productivity with decreased water loss and increased yield in drip irrigation is a practical way to manage finite water supplies. Plants use large amount of the water applied from increased water efficiency. This also minimizes leaching of agrochemicals out of the field or vegetable growing containers into the environment. In the last few decades, water productivity of vegetable crop values has been improved with the use of efficient micro irrigation techniques such as micro sprinkler and drip irrigation [41]. Jha et al. [40] determined that drip irrigation method resulted higher water productivity with more than five-folds increase in potato and cauliflower compared to the furrow method. It was also observed that drip irrigation method conserves approximately 70–80% water compared to conventional flood irrigation method.

In drip irrigation systems, to avoid possible plant stress, irrigations are usually scheduled to start when allowable percentage of usable water in the soil has been consumed. This level ranges from 30% in drought-sensitive plants to 70% in droughtresistant plants. In drip irrigation, this value is usually taken as 30% (MAD value) [39].

Irrigation volume in drip irrigation system considering soil available water depletion approach can be calculated with equation below.

$$\text{Irrigation volume} \left(\text{L}\right) = \left(\theta \text{fc} - \theta \text{wp}\right) \times \text{D} \times \text{MAD} \times \text{P} \times \text{A} \tag{6}$$

where P is the wetting factor, A is the irrigated area (m<sup>2</sup> ), and other terms are as mentioned before. Wetting ratio are considered minimum %30 in semi-arid regions, and it is 35% and 25% in arid and humid regions, respectively [42]. Wetting factor is less than 1 because especially during irrigation of plants with wide row spacing, a dry area remains between the laterals that is not wetted. In some cases, the plant covering ratio is also considered instead of this value in order to apply water according to the plant growth rate.

Irrigation volume can be also determined using the Class A pan evaporation with following equation [43].

$$\text{Irrigation volume} \left(\text{L}\right) = \text{Ep} \times \text{Kcp} \times \text{P} \times \text{A} \tag{7}$$

*Principles of Irrigation Management for Vegetables DOI: http://dx.doi.org/10.5772/intechopen.101066*

where Ep is the cumulative pan evaporation measured using a standard Class A pan at considered duration (mm), Kcp is the coefficient of crop-pan evaporation, P is the wetting factor and A is the irrigated area (m<sup>2</sup> ).

Due to (Ep x Kcp) is equal to the ETc (ETc = Ep × Kcp), bunun yerine (7) de can be also used to calculate irrigation volume from the ETc values determined using other approaches empirical (e.g. Penman-Monteith) or experimental.
