Hydrology is becoming increasingly diversified, embracing different disciplines and people from different backgrounds. While this is potentially an advantage, as it promotes a stimulating cultural environment and opportunities for exchange, there is also the risk of an increasing fragmentation of this discipline, with separated compartments that tend to develop in their own direction, with own languages and habits.

In fact, it is a common attitude of researchers to build their own 'niche' of expertise, an attitude that can be justified considering that it is virtually impossible for one single person to master all the disciplines that hydrology embraces. As a result, while it is common to see specialized studies that focus on single aspects of the hydrological system, it is seldom that these different works are brought together, in the effort to provide an overall picture of reality. The problem is that catchment systems seem to care little about the career of researchers, and the apparent progress of research has not brought us to a better understanding of how catchments operate. Catchments are hydrosystems characterized by a huge complexity. In trying to interpret their behavior we need to consider that they are whole environmental and social systems. The different 'niches' of specialization provide, if not black and white, at best monochrome pictures of reality, but they don't allow us to see the 'catchment in colors'. One could blame the fact that measurements are lacking, particularly at places and scales where they are most needed. Certainly this is one of the main reasons that hampers the progress of hydrology. However, several new measurement techniques have been emerging during the past years. Have we done our best to make full use of them? Similarly, different disciplines have looked at similar problems with different eyes. Did we do enough to combine these different perspectives?

The EGU Leonardo conference 2010 has brought together researchers of different groups that have addressed these issues. This provided an extensive overview of new measurement approaches and new theories that are emerging in hydrology. The measurement techniques that have been presented embraced several approaches that went beyond the traditional instruments. These included video monitoring and photography, microwave links for rainfall estimation, remote sensing of soil moisture, new online monitoring techniques for long term continuous and high frequency data on water quality variables, distributed temperature sensing along the stream, distributed water level sensors, and thermographic systems to map thermal signatures and connectivity of flow pathways.

All these measurements are a source of potentially useful information for hydrology. Clearly there are problems of representativeness, commensurability and scale. This large amount of data is in fact often perceived as being redundant, excessive or unusable by the hydrological modeler. All these issues need to be taken into account in the non-trivial task of making sense and extracting useful information from these new data sources.

On the theoretical side, current developments have demonstrated an increasing interest towards 'holistic' approaches, which propose alternative ways of representing information. The modeling community showed a progressive shift towards an attitude where matching observed and simulated hydrological 'behavior' becomes more important than simply matching 'data'. The interpretation of 'hydrological behavior' requires the definition of appropriate 'signatures' that characterize catchment response and functionality, an issue that is far from being resolved. These research efforts are also useful in the context of catchment classification, and its application to Prediction in Ungauged Basins (PUBs).

This special issue wants to build a constructive debate along these lines, inviting researchers to combine different views where a single color dominates, and to show progress towards a the perspective of looking at the 'catchments in colors'. More specifically, we invite contributions that deal with the following topics:

• New observations to generate information Hydrological modeling and the interpretation of catchment behavior are dominantly based on traditional sources of information such as rainfall and discharge. Several new measuring techniques are emerging (e.g. thermographic systems such as temperature fiber optic cables and thermal IR cameras, video monitoring and full spectrum photography), and little is known, even qualitatively, on how they can contribute to the understanding of how catchments operate. Many of these techniques become progressively more routinely available (e.g. chemical, remote sensing data), but they are seldom used in hydrological modeling. Hence, there is a strong need to understand their potential contribution in generating useful information for hydrology at the catchment scale.


• New kinds of analysis/synthesis of existing and new information Many types of data, even if available, are not used in hydrological modeling. Although these data are often considered essential by the experimentalist for a qualitative understanding of catchment functioning, modelers often consider them as redundant or unusable, due to problems of representativeness or scale. Hence, it is important to develop new tools to filter this information and synthesize it, to assess their complementarity and their added value.


• New kinds of theories and modeling approaches derived from information New information can inspire new approaches to represent catchment systems. In particular, measurement approaches able to capture catchment scale processes, such as soil moisture patterns or thermal signatures may contribute to new perceptions and concepts. Similarly, new models can be developed to test emerging hypotheses and help the interpretation of different data sources.