# An Optical and SAR Based Fusion Approach for Mapping Surface Water Dynamics over Mainland China
## 1 Introduction  
The dedicated goal of water in the 2030 agenda for sustainable development has put the spotlight on water policy at the global level and in national planning. It also highlights the need to take action on water resource management in order to meet the challenge of a looming crisis of water scarcity, i.e., a situation where freshwater resources cannot meet standard demands [1]. A recent study from the World Resource Institute (WRI) documented that water scarcity is already real and far more commonplace than previously thought. WRI found that water withdrawals globally have more than doubled since the 1960s due to growing demand, and show no signs of slowing down. Population growth, socioeconomic development and urbanization are all contributing to increased water demand, while climate change induced impacts on precipitation patterns and temperature extremes further exacerbate water resource availability and predictability [2]. The Sustainable Development Goals (SDGs), especially the goal of 'clean water for all' (SDG 6) and the 'climate action goal' (SDG 13), therefore need particular attention to avoid an accelerating 'water crisis' towards 2030. A 'water crisis' is ultimately a management issue that can be mitigated through the application of sound policies and initiatives. The need for proper and timely information onwater (non) availability is probably the most important requirement for water management activities and to guide actions in response to the global water agenda. In large, remote, and inaccessible regions, it is costly and difficult to conduct effective field-based monitoring of inland waters. Therefore, in most countries, water resources monitoring is limited to major dam resources and river flow stations. This, however, represents only a small fraction of the overall water resources, as substantial portions of freshwater reside in smaller inland water bodies, such as streams and ponds, as well as man-made infrastructures such as micro-hydropower dams, weirs, irrigation ditches, canals, and fishponds. These small water bodies are very important because they are vital for the livelihoods of millions of people and for sustaining healthy ecosystems [3]. Better monitoring of inland water bodies is needed to support evidence-based planning and management of water resources required to protect and restore freshwater ecosystems and preserve freshwater ecosystem services. In the past decades, Earth Observation (EO) has established itself as an essential tool for large-scale monitoring of water resources by providing the information needed to address critical issues such as disaster response and mitigation [4], irrigation management [5], disease transmission [6; 7], climate change [8] and ecosystem restoration and protection [9]. More recently EO has also been recognized as an important source of information for reporting the state and progress of many of the SDGs, including indicator 6.6.1 on the change in water related ecosystems [10]. Global maps of long-term surface water dynamics derived from satellite observations are already readily accessible, as compared to the European Commission Joint Research Center's Global Surface Water Explorer (JRC-GSWE) [11] and the Global Land Analysis and Discovery group's Global Surface Water Dynamics (GLAD-GSWD) [12]. These global products are based solely on optical Landsat data, which has gained wide prominence in surface water mapping and monitoring, owing to its long history (operational since 1972), medium-spatial resolution (30 m), moderate spatial coverage, and a free and open data policy [13]. However, a monitoring system based only on Landsat data will inevitably have some shortcomings. In particular, Landsat acquisitions will have temporal and spatial gaps due to long revisit time (16-18 days), cloud cover and insufficient sunlight in higher latitudes [14]. Moreover, [PERSON] et al.