# A Review of Irrigation Information Retrievals from Space and Their Utility for Users
## 6 Synthesis and Future Perspectives
However, VNIR observations--besides their inherent limitations due to cloud cover--can theoretically only provide the consumptive water use (i.e., the amount of water that is transpired by the crop and evaporated from the soil), and thus neglect the amount of water infiltrating to the subsurface, or MW observations have been demonstrated to be sensitive to noise and vegetation as well as to the satellite revisit time. Indeed, the temporal frequency is a crucial factor to reproduce the spatio-temporal dynamics of irrigation. In fact, the irrigation frequency depends on many factors (e.g., climatic conditions, crop type, water availability) and low-frequency data are often not able to detect irrigation events occurring at a not-negligible time distance from the acquisition. 7. Hardly affected by surface conditions, gravimetric measurements derived from GRACE and its successor GRACE-FO could provide important information on irrigation, but the spatial and temporal resolution achievable with these instruments have so far limited their application to only very large areas. 8. Most studies addressing the quantification of irrigation have employed modeling components, which potentially exhibit large uncertainties due to the need of a suitable parameterization and high-quality input observations (especially land cover and soil maps in land surface and hydrological models). For instance, the means of irrigation (i.e., sprinkler, drip, or flood) strongly affects the daily timing and quantity of irrigation water applied, while input data providing information on irrigated areas and starting of the growing season are required but rarely available. Additionally, modeled irrigation schemes generally ignore the source of applied water (i.e., surface water or groundwater), thus not allowing an integrated water resource analysis. 9. Considering that there has been increasing interest in understanding both the role of the irrigation on land-atmosphere interactions [148] and the impact of irrigation on water resources [149], coupling remote sensing information with land surface models for an improved representation of anthropogenic activities seems to be a key challenge to be addressed in the near future (e.g., [119; 150]). Notwithstanding the importance of the studies in irrigation retrieval research and its representation in Land Surface Models and Hydrological models to advance the knowledge of its impact on the climate and land systems, quantification, timing, and mapping of irrigation have a paramount importance also for agricultural and operational water resource management, where a plethora of actors are involved with very different needs. The scales of action are therefore very diverse, as are the capabilities of the different available satellite missions (see Table 2). Improving the efficiency of water use by agriculture and predicting the impacts on the different reservoirs requires the knowledge of irrigation water inputs and timing at the decision-making scales: (1) the plot/day for the farmer interested in irrigation scheduling on a day-to-day basis, (2) the irrigated perimeter/month for the irrigation district to help in the water distribution to farmers, and (3) the watershed/season for the manager for land use planning. For these scales, the different requirements have to face the diverse geographic/historical/cultural/economic conditions of the regions, which change over time, and the maturity of the technological advancement of satellite observations. There are many missions planned for the future which will help to fill these gaps. A potential benefit for irrigation retrieval could be derived from the Global Navigation Satellite System Reflectometry (GNSS-R) mission, such as the NASA Cyclone Global Navigation Satellite System (CYGNSS) [151; 152], and from the US-Indian NASA-ISRO SAR (NISAR) and Radar Observing System for Europe--L-band (ROSE-L) SAR missions [153],