# Smart Irrigation Systems from Cyber-Physical Perspective: State of Art and Future Directions
## 3 Problem Space
### IoT Layer Dimension
#### 3.1.1 Perception Layer  
Sensors play a critical role in data collection and signal transmission, thus shaping the efficacy of IoT systems [29; 30]. Table 2 provides an overview of the various types of sensors in smart irrigation systems. These sensors enable comprehensive data gathering, foster informed decision-making processes, and facilitate real-time adjustments to optimize irrigation efficiency. The table offers a valuable snapshot of the Perception Layer in a smart irrigation system. Figure 3: IoT system architecture. Figure 2: Smart irrigation system overview. The workflow, as shown in Figure 4, illustrates the crucial facet of smart irrigation systems. This data-driven approach ensures that irrigation is initiated only when necessary, minimizing water wastage while maintaining optimal soil conditions for plant growth [32; 34; 38; 39; 40; 52; 60; 61; 62]. While soil moisture sensors are invaluable tools for gauging soil dryness, there are some concerns to address. * _Suitability of Soil Moisture Alone_: Relying solely on soil moisture data might not provide a comprehensive understanding of the irrigation needs. How much water to use is affected by soil composition, organic matter content, soil type, soil and air temperature, wind speed, and evaporation rates. Only using the soil moisture value without considering the potential impact from other resources might cause over-watering or under-watering. * _Reliability of Soil Moisture Data_: While soil moisture sensors provide valuable insights, their reliability relies on factors such as sensor accuracy and placement. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Purpose** & **Sensor Type** & **Paper IDs** & **Remarks** \\ \hline Soil Moisture & Soil Moisture Sensor & [31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58] & Two types: resistive and capacitive \\ \hline Soil Condition & Ultrasonic Sensor & [31; 45; 50; 58; 59] & Determines soil type \\ \hline Soil Temperature & Temperature Sensor & [31; 33; 47; 49; 50] & Such as DS18B20 to measure the soil temperature \\ \hline Soil pH Value & Soil pH Sensor & [31; 33; 42; 43; 49] & Measures soil pH \\ \hline Rain Detection & Rain Sensor & [37; 41; 42; 44; 46; 47; 49; 55] & Detects rain presence \\ \hline Freshwater Measurement &
\begin{tabular}{c} Water Meter, Water pH Sensor, \\ Water Flow Sensor \\ \end{tabular} & [31; 37; 48; 55; 56; 57] & Measures water flow, pH, and volume \\ \hline Surrounding Environment & Humidity and Temperature & [31; 33; 35; 37; 42; 43; 44; 46; 49; 51; 53; 54; 55; 57; 58; 60] & Such as DHT11 or DHT22 sensors to measure the air temperature and humidity \\ \hline Surrounding Environment & Light Sensor & [31; 57; 51; 58] & Measures light strength (e.g., in greenhouses) \\ \hline Surrounding Environment & Wind Speed Sensor & [31; 49] & Monitors wind speed \\ \hline Surrounding Environment & Solar Radiation Sensor & [31; 49; 58] & Measures solar radiation levels \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Sensors in IoT irrigation systems. Figure 4: Workflow of current smart irrigation systems. * _Comprehensive Irrigation Management:_ The distribution of plant roots can also lead to varied moisture levels even within the same soil at a given time. Moreover, a plant's water requirement varies with its growth stage. Using over-watering as an example, [PERSON] et al. (2018) explained that over-watering could be as harmful as under-watering. One of the common reasons for over-watering is ignoring weather forecasts and watering fields. Thus, combining soil moisture sensors with other sensors (temperature, humidity, etc. ), implementing predictive models, and adopting real-time adjustments can enhance system reliability. Temperature sensors are another widely adopted sensor used in irrigation systems. There are two types of temperature sensors: _waterproof temperature sensors_ (i.e., _DS18B20_) and _air temperature and humidity sensors_ (i.e., _DHT11_ or _DHT22_).