# An Optical and SAR Based Fusion Approach for Mapping Surface Water Dynamics over Mainland China
## 5 Discussion
### Operational Water Monitoring  
Over the past decades human activities and climate change have driven complex physical and ecological changes to China's inland water bodies. One of the most visible effects has been the steady annual increase in reservoirs since the 1970s [22]. Hydrological processes are significantly influenced by dam construction and regulation and there are valid concerns for the impact large dams may have on the flow regime. It is well known that spaceborne sensors can support monitoring of the state, change and release of large reservoirs [91], and our study verifies that by showing the consistency by which surface water area changes of large reservoirs can be accurately tracked. Furthermore, a recent study demonstrates how our algorithm can be used in conjunction with altimetry and gravity missions to obtain a more complete picture of the spatio-temporal variation of water resources over the North China Plain [90]. Whilst monitoring large dams is important, especially in China, where the greatest rate of increase in storage capacity comes from large reservoirs (>10 km\({}^{2}\)), the monitoring of smaller reservoirs and lakes (<0.1 km\({}^{2}\)), which by far outnumbers the larger ones [22], is also important. Science and government measures have traditionally focused on studying and monitoring larger water bodies [92]. However, there is growing awareness of the importance of small waterbodies for freshwater biodiversity and in delivering ecosystem services, as well as their increasing exposure and threats from human activities [3]. In China, small waters have provided hydrologic, biogeochemical, ecological, and socioeconomic benefits for thousands of years, but the hydrological regime of small waters is changing, and their management represents a challenge [93]. One of the primary requirements for improving our understanding of small water bodies, specifically with regards to recommended policy actions that protect and integrate them into water management, are reliable monitoring programs [3]. Our study shows that only higher resolution Sentinel imagery is capable of capturing and tracking these smaller water bodies. In the context of climate change, the information and mapping approach presented in this paper is also valuable. For example, the Tibetan Plateau has seen an increase in surface waters caused by higher temperatures and annual precipitation, which has accelerated stream runoff and glacial meltwater [94]. On the contrary, Inner Mongolian lakes have shrunk and even vanished in response to an observed warming trend beginning in the 1950s [22]. The ability to consistently map surface water extent over time and at scale and compare it to related climatology trends can improve our understanding of the mechanisms behind, and the extent to which climate change impacts on surface waters. Whether driven by human or climatic factors better monitoring of water resources in both space and time is needed to prevent a potential water crisis unfold unabated. It is therefore important to consider how this dynamic information is synthesized and operationalized. Monthly deviation measures could help prevent water stress, through actionable management of upstream reservoirs, and behavioral changes in downstream use. Longer-term changes could focus on the annual aggregations of permanent and seasonal water, which have been shown to be consistent through time. Since permanent water represents the minimum water supply available, it reflects potential water scarcity and hence it is an important indicator for the water available to ensure basic societal needs for, e.g., irrigation, hydropower, navigation, and domestic usage. However, seasonal waters represent the intermittent freshwater supply that is important for recharging groundwater and restocking local man-made water reservoirs and thereby helping to meet the year-round demand. They are also vital for sustaining ecosystem integrity, providing variable river flows to natural river systems and wetlands, which support 40% of all species.