## Conclusions & Future Work Directions  
This paper provided a review of the state of the art in the use of EO data to derive operational estimates of ET and SSM. It also discussed emerging applications that can potentially benefit from the integration of ET/SSM operational products. The review suggested there is a significant progress made in recent years in the estimation of ET and SSM parameters from space but further work is required in order to be able to use those products in support of sustainable water management, particularly so at a spatio-temporal scale for the monitoring of hydrological processes and practical applications. Evidently, there are still a number of challenges to be addressed to optimize parameter accuracy and uncertainty and to improve the operational capability of services and practical applications developed using EO data. Despite the breadth of space technology and the promising accuracies reported by different techniques measuring SSM from EO sensors currently in orbit, still primary challenges in using existing operational ET and SSM data remain. Among the key directions on which future work should be directed to address those challenges in both ET and SSM products are included the following:  
* Thanks to advances in instrumentation, space technology and algorithm development, the new generation of satellite sensors are rapidly increasing their capabilities in terms of spatio-temporal resolution and accuracy of retrieved parameters. Also, new technologies such as cubesat and nano-satellites, if well designed, have the potential to provide high spatial-temporal resolution at low costs [71]. * Although new satellites are planned or already in orbit for estimation of SSM and ET, a better characterisation of the retrievals can only be obtained after development of more robust algorithms. As science is progressing at a good pace, more research devoted to the improvement of present SSM and ET retrieval algorithms is needed to enable a seamless integration of EO-derived information in practical applications. Present challenges to obtain reliable estimates of ET and SSM include the reconstruction of the environmental parameters from the measured signal by using a minimum of auxiliary data. Furthermore, development of new algorithms especially designed for different crop types are needed for an effective monitoring of ET and SSM over agro-ecosystems. * In some areas, agricultural fields might be too small for satellite observations for precision irrigation and measurements would have to be taken locally. This is probably especially the case in regions which do not have the financial means for these types of measurements. To address, this issue many researchers are working to produce fine resolution maps for local applications by using the spatial downscaling/disaggregation techniques [55]. Spatial downscaling/upscaling and synergistic approaches can be used to integrate data from different sensors and provide ET/SSM information at the required spatio-temporal resolution, covering the need of agro-hydrological applications. These spatially disaggregated maps are well validated in many regions and could be an alternative to poorly gauged basins/areas. Note, however, that there is generally a trade-off between high spatial resolution and high accuracy and therefore providing the final product with uncertainty bounds is paramount. * Data assimilation can be used to constrain models with in-situ and satellite ET/SSM data in decision support systems that enable improved natural resource management, disaster prevention and response and other benefits to society. Also, recent improvements on radar physical modelling and on new SAR and TIR sensing capabilities hold great promise for ET/SSM soil moisture measuring at very high resolution.