# Smart Irrigation Systems from Cyber-Physical Perspective: State of Art and Future Directions
## 3 Problem Space
### Cost-Effective Dimension
#### 3.3.3 Water Utilization  
Water resources for irrigation mainly come from the following two sources: surface water and groundwater. After a rain, the actual amount of water that remains in the soil is affected by several factors. (i) _Water Runoff:_ Because of the slide of the ground and the type of soil texture, water might run off from the soil and move to nearby lakes. The type of irrigation method can also affect the water runoff rate. For instance, surface watering has the lowest utilization because of the runoff water and poisoned water by fertilization. On the contrary, the dripping method has the highest water utilization, because all the water is applied in the root zone, which can have up to a 95% utilization rate [89; 102]. (ii) _Water Gravity:_ Water gravity will let water go down to the deep underground, which reduces the surface water and increases the groundwater amount. (iii) _Evaporation:_ Heat can increase the speed of evaporation. Water changes from liquid in the ground to vapor in the air. Evaporation is part of the three steps of the water cycle: evaporation, condensation, and precipitation. In addition to heat, wind also increases the evaporation process. (iv) _Absorption:_ Plant roots, the type of soil texture, such as clay, organisms such as bacteria, fungi (such as mushrooms), earthworms, small animals, and organic matter will absorb the water in the soil as well. Thus, even for the same plant with different evaporation and absorption rates, the soil moisture value will differ when we utilize different depths. In vertical farms, water gravity is paramount to water collection and circulation. IoT devices such as soil moisture sensors, pH sensors, CO\({}_{2}\) sensors, acoustic sensors, and light sensors are employed to gather data related to plants and their environment [6; 14; 16; 18]. By analyzing these data and designing efficient input and output water flows, vertical farming systems can minimize water usage while also saving land area [103]. Integrating natural resources, like rainwater, further enhances water utilization in smart irrigation systems. Rain sensors provide real-time rain status information, enabling irrigation systems to halt operations during rainy periods [36; 42; 44]. However, these approaches often overlook the effects of runoff water from rainfall. [PERSON] and [PERSON] [40] used the weather forecast to estimate the soil moisture and then activate or stop the irrigation system based on the actual soil moisture data plus the weather forecast rainfall status. However, it still ignores the facts related to runoff water's impact on the soil and various soil textures with different absorption rates from rainfalls [104]. The traditional way to hold the runoff water is to use a water tank or build a dam to keep the runoff water. Thus, water level sensors send alerts to the user once the water reserved is under the minimum level or exceeds the maximum level. Tubes are used to guide water to the water tank or dam; leakage of the tube or tunnel will cause water loss during transportation. [PERSON] and [PERSON] [105] used water flow sensors to collect real-time water flow speeds at different interfaces with users, sending the data to the server to perform data mining to ensure the data processed are accurate, nothing is missed, and errors are removed before entering it into the machine learning models. Then, a neural network will be used to detect leakage and water consumption prediction. Water gravity will affect the underground water level. Unfortunately, urbanization development has already overdrawn underground water, which has caused the shrinkage of underground water levels. With increased wastewater, polluted rivers, and runoff water from contaminated soil merging into the groundwater, according to the WHO, 80% groundwater is contaminated, and 40% of the US stream river water is not suitable for livestock anymore [104]. IoT can be used to test the water quality before irrigation.