# A Review of Irrigation Information Retrievals from Space and Their Utility for Users
## 5 The User Perspective: Observation Requirements and Current Obstacles
### Current Practices Versus Current Satellite Capabilities
#### 5.2.3 Employed Technology  
The employed irrigation method determines the potential ability of the techniques described in Section 3 to detect and quantify irrigation. For instance, sprinkler and flooding/gravity types of irrigation systems cause significant (surface) soil wetting, which translates into a strong response of the remotely sensed signal. This facilitates irrigation detection and quantification with soil moisture-based techniques. In contrast, drip irrigation systems apply much smaller water amounts, whose detection can be very challenging, especially for coarse-resolution satellite sensors. In this case, evapotranspiration-based techniques (based on the use of thermal sensors) can provide better performance as the actual ET rate for crops located in a drip-irrigated field can be very close to their potential ET, which is generally not the case for non-irrigated crops. Moreover, thermal-based methods to retrieve evapotranspiration have the advantage of a higher spatial resolution suitable for the monitoring of small-scale fields, where drip irrigation is also more commonly used. Nevertheless, this is at the expense of the revisit time, reaching at best 8 days by combining Landsat-7 and -8 with the sensors currently in orbit. A significant way to reduce the revisit time could derive from the future satellite missions, such as Thermal InfraRed Imaging Satellite for High-resolution Natural resource Assessment (TRISHNA, [141]), a future high-resolution space-time mission in the thermal infrared (TIR) led jointly by the French (CNES) and the Indian (ISRO) space agencies for a launch planned in 2025, or by the high-priority candidate mission High Spatio-Temporal Resolution Land Surface Temperature Monitoring Mission (LSTM, [142]), which will provide enhanced measurements of land surface temperature with a focus on responding to user requirements related to agricultural monitoring.  
Other than the type of irrigation system, its relation to the farmer organizations can play a role. For instance, in traditional flood irrigation, irrigation is performed in turns, so there are a few events each month. With modern systems, such as sprinklers, irrigation can take place every day in order to compensate for water deficits caused by plant transpiration and soil evaporation. However, this also depends on the objectives of the farmers, as the different strategies lead to different quantities and qualities of the product and the farmers may bet on quality or quantity depending on the year. Therefore, the frequency is often tied to the irrigation method, but the relationship is not univocal. In summary, with traditional irrigation methods, the required temporal resolution to obtain information on irrigation can be larger than 1-2 days, whereas with modern systems (i.e., sprinkler), daily temporal resolution is needed. Furthermore, with modern systems that use daily irrigation, soil moisture will tend to be more constant than with classical irrigation systems, and techniques relying solely on soil moisture for the retrieval of irrigation amount might be suboptimal. With traditional systems, due to the turn-like method of irrigation, soil moisture will be more variable and will alternately increase and decrease depending upon whether irrigation will or will not take place, so techniques which exploit soil moisture for the retrieval of irrigation might provide better performance.  
Despite the limitations and difficulties in estimating irrigation from remote sensing observations, where the technical advancement is still low (e.g., Urgell area in Spain and developing countries) and the possibility to measure irrigation is very scarce as no counter nor modern irrigation systems exist to measure water in the soils, satellite data can be very important. This is also perceived by users, as there seems to be a growing interest in the use of remote sensing products to better plan and manage irrigation.