# A Review of Irrigation Information Retrievals from Space and Their Utility for Users
## 5 The User Perspective: Observation Requirements and Current Obstacles
### Operational Observation Requirements  
In this section, we aim at understanding the current gap between the existing capabilities of satellite observations to observe soil moisture, evapotranspiration, and information on irrigation and the real needs of the users. The discussion is based on the reviewed literature from Section 3 and the questionnaire implemented.  
A general issue is the fact that satellites can provide only information on the few first centimeters of the soil, whereas information on the root zone soil moisture (0-100 cm) ismost important for the crops and hence most desired by users. While this information cannot be directly obtained from satellites, models can be integrated with observations to provide information of the water stored in the root zone. Based on the size of the agricultural fields in the Mediterranean, the spatial resolution at which remote sensing products should provide information on irrigation is below 1 km (10-100 m for most of the users). This of course represents a big challenge for the current state of the satellite missions. Currently, microwave data from S1, S2, and Landsat-8 could indeed meet these needs and models too can run at these spatial resolutions, assimilating satellite data with a spatial support larger than/equal to 1 km to correct states and fluxes at smaller spatial scales, providing information on irrigation. There is thus a realistic potential to cover the desired spatial scales identified by the users. Concerning the temporal resolution, 1-7 days is the desired window for farmers, while for basin authorities, monthly to seasonal scales seem to be sufficient. Even though current satellite missions cannot provide retrievals of irrigation information with daily temporal resolution at a spatial resolution of 10 m, weekly estimates at a resolution of 100 m-1 km are indeed realistic.  
Concerning the spatial resolution of evapotranspiration data, the requirements are similar to the one of soil moisture with values below 1 km for farmers and above for basin authorities interested in water management. Overall, the spatial/temporal resolution of 100-500 m daily can theoretically be provided from satellites, for example from MODIS observations, while lower spatial resolution could be obtained with Landsat [143] and the new Sentinel satellites [144]. In addition, models can run at this resolution and can benefit from the assimilation of satellite soil moisture/backscatter from S1 data or by LST information from thermal sensors [127].  
Regarding the desired accuracy (i.e., the difference between the estimated and true irrigation volumes) of the irrigation data, it seems that a value below 30% would be desired (based on the answers of the farmers). However, due to the lack of reliable ground reference information about irrigation, it currently remains difficult to provide exact accuracy estimates. Nevertheless, in a recent study, [PERSON] et al. [102] reported irrigation retrieval accuracies that span from 10% to 40%, which is not far from the defined targets.