# HydroLab: A Module for the Investigation of Fertigation Strategies in Hydroponics
## 4 Case Study
### Compared Fertigation Strategies  
In this section, the compared fertigation strategies are detailed.  
_Treatment 1: Time-based strategy._ To prevent water stress and ensure proper hydration of the crop, a daily volume of 350 cc of nutrient solution was applied [17]. Four irrigation events were scheduled throughout the day, with each event delivering 1/4 th of the daily volume (i.e., 87.5 cc), i.e., (i) at 7:00 a.m., to hydrate the plants before the temperature rise; (ii) at 10:00 a.m., to provide a refreshing effect on the crop; (iii) at 2:00 p.m., to minimize excessive evaporation losses; (iv) at 5:00 p.m., to support crop recovery before the end of the day.  
_Treatment 2: Radiation-based strategy._ Evapotranspiration, which encompasses both plant transpiration and surface evaporation, is a fundamental process in plant physiology that involves the loss of water from plants. Accurate estimation of water losses is essential for effective fertigation management. In our study, we utilized the Hargreaves and Samani equation as the fundamental framework for estimating evapotranspiration rates and, consequently, the amount of nutrient solution provided during each fertigation [29]. This equation allows for the estimation of water lost by the crop during a given time period. Next, the Hargreaves and Samani equation is presented:  
\[ET_{0}=0.0135(T_{avg}+17.38)R_{s} \tag{1}\]  
where \(ET_{0}\) (cc) is the estimated evapotranspiration (per plant) in the considered time period, \(T_{avg}\) (\({}^{\circ}\)C) is the mean temperature, and \(R_{s}\) refers to the incident solar radiation converted to cc/\({}^{\circ}\)C.  
When converting the incident radiation to evapotranspiration in cc, it is important to consider the complex processes occurring within crops. Simply estimating it based on the energy values required for water evaporation is insufficient. To address this, the FAO guidelines were followed [30]. The guidelines offer a comprehensive framework for adjusting incident solar radiation values and obtaining accurate estimations of water loss through evapotranspiration. As recommended by the FAO, the following conversion factor was applied to \(R_{s}\):  
\[2.45[J/\text{cm}^{2}]=1[\text{cc}/^{\circ}\text{C}] \tag{2}\]  
Next, we show how this mathematical model was utilized to calculate the nutrient solution amount provided during each fertigation. Following the recommendations of  
Figure 5: Uniandes Controlled Environment Agriculture Laboratory.  
Senz Fety, a solar radiation threshold of 160 J/cm\({}^{2}\) was set. This means that when the accumulated solar radiation overcame this value, fertigation was triggered. By applying Equation (2), \(R_{s}\) was computed in 65.3 cc/\({}^{\circ}\)C. Then, \(ET_{0}\) was estimated through Equation (1), considering the average temperature between the previous and the current fertigation.  
It is recommended that the amount of nutrient solution supplied during fertigation matches or slightly exceeds the amount of water lost through evapotranspiration (\(ET_{0}\)). This approach ensures that plants have adequate water supply for their physiological processes, including nutrient absorption, photosynthesis, and growth. By preventing water deficiency, healthy plant development can be promoted [31]. In our experiment, the calculated \(ET_{0}\) value was used as the nutrient solution volume delivered during the fertigation process, ensuring that the plants received the necessary water and nutrients for optimal growth.