# Sentinel-1 and -2 Based near Real Time Inland Excess Water Mapping for Optimized Water Management
## 1 Introduction  
Due to its physical geography, the Carpathian Basin in Central Europe is under the influence of extreme differences in weather patterns during the year. On one hand, periods of high precipitation can cause floods at the end of the winter; on the other hand, during the summer, long dry periods can cause drought at the same locations [1]. The floods cover large parts of flat areas with a shallow layer of water for a period of several weeks to months. Contrary to riverine and coastal floods, the inundations occur when, due to limited runoff, infiltration and evaporation, the superfluous water remains on the surface or at places where groundwater, flowing towards lower areas, appears on the surface by leakage through porous soils [2]. The phenomenon exists in other low-lying countries, though especially in Hungary a long tradition of research related to the floods exists. Sometimes, the floods are referred to as inland water or excess water, but the authors prefer to use the term inland excess water (IEW) to indicate that they develop due to surplus water and that their geographic location is inhomre. This prevents confusion because inland water can refer to any inshorse water body, while excess water can be riverine or coastal floods as well. In terms of daily precipitation intensity, many climate change models predict that in the summer an increasing number of days with high rainfall (more than 20 mm daily precipitation) will happen, but that due to a decrease in available water resources (e.g., lower groundwater levels), no significantchange in IEW hazard is expected at regional scale. Local environmental factors, like agricultural practices, relative relief differences and soil characteristics, play an important role in the development of IEW; thus, increased inundations in the study area are expected in summer and autumn resulting from heavy rainfall, which may even be enhanced by wet microbursts [3; 4; 5]. These extremes cause severe water management problems. A common technical-engineering solution for the problem is to try to pump away the water as fast as possible. Although sometimes feasible, when the water level in rivers is also not high, this solution is not sustainable. One of the potential integrated and sustainable solutions to the inland excess water problem is to store the surplus water in agricultural areas for later periods of drought [6; 7]. Also, IEW can be allowed to remain on areas designated as (temporary) wetlands, supporting ecosystem restoration. For such complex water management, it is important to understand where IEW develops. The methodology presented in this article aims to support water management for the sustainable use of the surplus water. When the needs of the population for agricultural land, urban settlements or others meet the inundations, it can result in serious financial, environmental and social problems. The length of IEW periods fluctuates between days, weeks and several months [1; 8]. In Hungary more than 24% of the arable land is in areas moderately or highly endangered by inland excess water [9]. The IEW problem is not new; already in the 19 th century IEW was mentioned as a natural hazard [7]. Many engineers and scientists have worked to find solutions to reduce the damage from the extreme amount of water. Before it is possible to take action against the problem, it is necessary to understand the phenomenon and identify the factors and processes that cause the formation of inland excess water. Also, it is important to determine the location and size of the inundations to be able to plan storage possibilities or take operative measures to mitigate and prevent further damage. When it is known precisely where and when IEW occurs, it may be possible to forecast the location, size and duration of future floods and to develop preventive policies [10]. Four major approaches to map and monitor IEW can be identified [10]. The oldest approach is visual observation of inland excess water patches.