Most of the world's river basins and aquifer systems are poorly gauged or completely ungauged. It is essential to strengthen the hydrological monitoring network, which is currently declining. A positive experience in this context is offered by various IHP research programmes (e.g. FRIEND, ISARM, among others). The international community should take advantage from existing large scale river flow archives which are crucial to advance hydrological science and operational issues. However, resources need to be increased to promote and implement open data sharing.
Remote sensing data includes data obtained through satellites and other airborne devices such as airplanes or balloons. These data are used more and more in hydrology. Globally and freely available space-borne data (e.g. SRTM and all current and historical Landsat data) provide the only information readily available, especially in developing countries that lack in situ hydrological monitoring networks, and can remove an obstacle to the application of hydrological models for global and regional predictions. In this context, the UNESCO-ESA TIGER initiative, which is focusing on the use of space technology for water resource management in Africa, is an encouraging example. The Global Observing System (GEO) provides an important set of data to the water community in general. However, the potential of remote sensing techniques to monitor hydrological extremes such as floods and droughts, to monitor water quality, and to support hydrological models is not yet entirely explored nor is it adequately used. In addition, there is a need for improved frameworks to assimilate or integrate remote sensing data into hydrological modeling systems. Having sufficient ground truth information against which remote sensing algorithms can be validated and improved is also critical to effective measurement of water resources. New data sources, such as satellite remote sensing, wireless sensors, acoustic Doppler profilers, and radar are triggering the need for continued training.