## 1 Introduction  
To facilitate environmental resource management of intensively populated countries like the Netherlands, integrated information systems which are capable of real-time monitoring of fundamental processes in the environment, as well as providing vital hazard warnings, are required. Traditionally, sensor networks covering various geographical and temporal scales are an important source of information for this task. They allow vast amounts of relevant information to be collected with a high temporal frequency for a network of point locations that are remote, inaccessible, or lack the necessary resources to acquire such information in a different manner [1]. For example, ground water levels in the Netherlands are monitored through a network of 4,000 semi-automated groundwater wells [2]. Recent developments in the miniaturization of electronics and wireless communication technology will enhance the opportunities of sensor networks for real-time monitoring of the natural environment [3]. Next to _in situ_ sensor networks satellite remote sensing systems are also a key source of information for many applications. Although space based sensors have a superb spatial coverage, they can frequently incur a significant data delivery latency, have a poor signal to noise ratio, and possess coarse resolutions. However, for a comprehensive monitoring system to provide timely information, a combination of _in situ_ and space based sensors offers a synergetic configuration [4]. In an integrated approach, the sensor observations provide data and information; scientific models use these data and produce predictive results which are provided to end-users to assist the decision making process [5]. Although space-based and _in situ_ sensor data have not been integrated in a fully self-consistent way, advanced technologies of today make it possible to pursue more integrated approaches to environmental resource assessment [4]. Although information technology is an important facilitator in this process, integrated information systems are often limited by interoperability problems due to individual components which cannot easily communicate with each other [6]. To overcome this problem, efforts at the sensor network level are required which deal with issues such as fusion of sensor data and interoperability among networks and their connections to information systems. This effort should not only pay attention to the technical facilitation but also should include organizational and standardization aspects. The concept of sensor webs as introduced by [PERSON] in 2002 [7] \"allows for the spatio-temporal understanding of the environment through coordinated efforts between multiple numbers and types of sensing platforms, including both orbital and terrestrial and both fixed and mobile.\" Compared to sensor networks, sensor webs are unique in their feature that sensors communicate with each other, share information with other sensors and are aware of their environment. Communication between the sensor web and user can be in two directions: the user receives information from the sensor web but can also send instructions to it [8]. In an initiative called sensor web enablement (SWE), the Open Geospatial Consortium (OGC) has been developing a framework of open standards for exploiting web-connected sensors and sensor systems of all types [9]. The available services include access to sensor measurements, retrieval of sensor metadata, controlling sensors, alerting based on sensor measurements and automatic processing of sensor measurements. Although the developed SWE concepts are being applied in a broad range of environmental domains (e.g., hydrology [9, 10], ecology [11, 12], risk management [6, 9, 13]), only a limited number of studies [4] describe the combined use of space-based and _in situ_ sensor sources in a sensor web based approach.