**1. Introduction**

Water is an essential component of growing crops. Even in humid climates, precipitation is not enough to meet plant water requirements. Thus, additional water is applied through irrigation. Irrigation management can be complicated with unpredictable precipitation patterns. Not watering at the right time and correct amount can result in plant water stress and reduce the quality and yields of crops. On the other hand, over-watering can increase the risk of nutrients leaching below the root zone, waste resources (water, energy, and nutrients), and environmental impacts. Therefore, it is important to apply irrigation at the right timing and correct amount. Determining the appropriate amount of irrigation and the optimal timing of irrigation are challenging due to unpredictable weather conditions and climate changes.

Irrigation scheduling is a method of determining the appropriate amount of water to be applied to a crop at the correct time to achieve full crop production potential. Scheduling irrigation water has been based on the soil moisture measurement and/or weather data that are estimates of evapotranspiration.

This chapter reviews the various existing and recent advances in irrigation scheduling methods. The irrigation scheduling methods are:

• Feel and appearance.


#### **2. Irrigation scheduling method**

#### **2.1 Feel and appearance**

The most popular and quickest method is based on the feel and appearance of the soil. A soil probe is typically used to take soil samples. **Table 1** shows the soil moisture and appearance relationship. **Table 1** shows an approximate relationship between field capacity and wilting point. The top of each soil type corresponds to the condition of zero soil moisture deficiency, also known as field capacity. The bottom of each soil type corresponds to the condition of maximum soil moisture deficiency, also known as wilting point. The soil moisture deficiency also presents the available moisture range of the soil. The table provides general numbers for a specific group of soils and may not apply to all soil groups. This method is not quantitative and is judged by the individual, which lacks precision.

#### **2.2 Gravimetric method**

Soil moisture content is an important parameter for understanding the water movement in the soil. Taking soil samples is the direct method to measure the actual soil moisture level. This method requires weighing a sample of a known volume of soil and then reweighing it after drying in an oven at 105°C to calculate the mass of water lost by drying [2]. This method allows for calculating gravimetric water content (g/g) and soil bulk density (g/cm<sup>3</sup> ). Multiplying the gravimetric water content by the soil bulk density allows for calculating the volumetric water content (cm<sup>3</sup> /cm<sup>3</sup> ) [3]. The equation of volumetric water content is described in (Eq. (1)). Soil sample collection method is accurate, but it requires intense labor, time, and soil disturbance. Therefore, continuous soil moisture monitoring through soil sample collection on farmland can be difficult and limited.

$$\theta\_v = \frac{\theta\_\text{g} \* \rho\_{soil}}{\rho\_{water}} = \frac{\left(\left(\frac{\left(M\_{water} - M\_{dry}\right)}{M\_{dry}}\right) \* \left(\frac{M\_{dry}}{V\_{soil}}\right)\right)}{\rho\_{water}}\tag{1}$$

Where,

*θ<sup>v</sup>* = Volumetric water content (cm3 /cm3 ).

*θ<sup>g</sup>* = Gravimetric water content (g/g).

*Irrigation Scheduling Methods: Overview and Recent Advances DOI: http://dx.doi.org/10.5772/intechopen.107386*


#### **Table 1.**

*Relationship between soil moisture and appearance [1].*

*ρsoil* = Soil bulk density (g/cm<sup>3</sup> ). *ρwater* = Density of water = 1 (g/cm<sup>3</sup> ). *Mwater* = Weight of wet soil (g). *Mdry* = Weight of dry soil (g). *Vsoil* = Volume of soil sample (cm3 ).

#### **2.3 Weather-based irrigation scheduling method**

Weather-based irrigation scheduling method is based on the weather condition. Four major weather parameters determine evapotranspiration (ET), which drives the weather-based irrigation scheduling method. The weather parameters are solar radiation, air temperature, relative humidity, and wind speed. Higher the solar radiation, the greater ET. This is because sunlight is the main energy source for evaporating water. The warmer the air, the greater ET, because it can hold more water vapor. The drier the air, the greater the ET, because there is less water vapor it already holds. The greater wind, the greater the ET. In humid climate regions, solar radiation and air temperatures play a significant role in determining daily ET.

ET can be estimated in several ways. One method that is accepted as an international standard is the Penman–Monteith Equation, which is used to calculate the reference potential ET (rPET) using comprehensive weather data. The weather data includes net radiation, soil heat flux, average air temperature, wind speed, saturation vapor pressure, actual vapor pressure, the slope of vapor pressure curve, and psychrometric constant. rPET assumes a four grass-covered surface that is well-watered and unshaded. The actual ET of a crop at any given time depends on the amount of leaf area and the developmental stage, so to calculate the ET for a specific crop type, at a specific developmental stage, the rPET values must first be multiplied by a crop. Kc changes with crops as they grow, for example, the Kc of fruit trees increases rapidly in the spring as the trees leaf out to full canopy. **Figure 1** shows the change of Kc as a soybean grows. To estimate actual crop ET, the rPET is multiplied by the crop coefficient Kc to determine the actual water lost from the crop via ET (see (Eq. (2)).

$$ET\_C = K\_C \* rPET \tag{2}$$

Where,

*ETC* ¼Actual Crop Evapotranspiration (in/day).

*KC* ¼Crop Coefficient (unitless multiplier).

*rPET* ¼Reference Potential Evapotranspiration (in/day).

Based on each day of the actual crop evapotranspiration, the suggested irrigation amount can be calculated to ensure that the soil has adequate moisture for plant growth and improve irrigation water use efficiency. For example, if last week's cumulative actual crop evapotranspiration was 2.5 cm, the farmer should apply 2.5 cm of irrigation to maximize crop production and minimize environmental impacts.

#### **2.4 Soil moisture sensor-based irrigation scheduling method**

An alternative way to measure soil moisture content is using a soil moisture sensor. A typical soil moisture sensor estimates the volumetric water content (cm3/cm3) in soils. Soil moisture sensors allow monitoring the changes of soil moisture level over time without disturbing the soil. The sensors can be installed at multiple depths of soil to monitor the water flow in soil. In general, there are two types of soil moisture sensors. Soil tension sensors measure the required force for roots to pull water out of the soil. Volumetric water content sensor based on the electrical properties of the soil

**Figure 1.** *Crop coefficient (kc) changes as the soybean grows.*

to estimate the soil moisture level. Knowing some common terminologies used in soil moisture sensor-based irrigation scheduling would be helpful in interpreting these sensor data.

The descriptions of some useful terminologies follow:

**Saturation:** All soil pore spaces are filled with water.

**Field Capacity (FC):** Maximum amount of water that soil can hold after drainage. **Wilting point (WP):** Soil moisture level where there is no available water for the crop.

**Available Water Capacity (AWC):** The difference between FC and WP, is often expressed in inches of water per foot of soil. AWC of a soil is primarily related to the soil texture, organic matter content, and bulk density. The equation of AWC is shown in (Eq. (3)).

$$A\text{WC} = \frac{\rho\_{soil} \ast T \ast P\_W}{\rho\_{water} \ast 100} \tag{3}$$

Where,

AWC = Available water capacity in inches.

*ρsoil*= Soil bulk density.

T = thickness of soil horizon under consideration in inches.

*PW* = Moisture content between field capacity and wilting point in percentage by weight.

*ρwater* = Density of water = 1 (g/cm<sup>3</sup> ).

**Maximum Allowable Depletion (MAD):** The amount of available water that can be safely depleted without causing drought stress, which depends on the crop and the growth stage.

**Soil Matric Potential (SMP):** Physical force required for the plant to move water into its root system.
