IntroductionThe time water spends travelling subsurface through a catchment to the stream network (i.e. the catchment water transit time) fundamentally describes the storage, flow pathway heterogeneity and sources of water in a catchment. The distribution of transit times reflects how catchments retain and release water and solutes that in turn set biogeochemical conditions and affect contamination release or persistence. Thus, quantifying the transit time distribution provides an important constraint on biogeochemical processes and catchment sensitivity to anthropogenic inputs, contamination and land-use change. Although the assumptions and limitations of past and present transit time modelling approaches have been recently reviewed (McGuire and McDonnell, 2006), there remain many fundamental research challenges for understanding how transit time can be used to quantify catchment flow processes and aid in the development and testing of rainfall-runoff models. In this Commentary study, we summarize what we think are the open research questions in transit time research. These thoughts come from a 3-day workshop in January 2009 at the International Atomic Energy Agency in Vienna. We attempt to lay out a roadmap for this work for the hydrological community over the next 10 years. We do this by first defining what we mean (qualitatively and quantitatively) by transit time and then organize our vision around needs in transit time theory, needs in field studies of transit time and needs in rainfall-runoff modelling. Our goal in presenting this material is to encourage widespread use of transit time information in process studies to provide new insights to catchment function and to inform the structural development and testing of hydrologic models. What is transit time?The terminology on time concepts associated with water movement through catchments can be confusing and a barrier to its use. Water transit time through the system can be defined as:where t w is the elapsed time from the input of water through a system input boundary at time t in to the output of that water through a system output boundary at time t out . In a catchment, the land surface and the catchment outlet may be considered as the main input and output boundaries for most of the water flow through the catchment (Figure 1). However, the land surface constitutes both a water input boundary and an output boundary for water that experiences evapotranspiration (ET). Considering also the subsurface depth dimension of a catchment, groundwater flow into and out of the catchment system is determined by prevailing groundwater divides and hydraulic gradients, which may vary in time and space and differ from the topographically determined catchment boundaries. For general transient flow conditions, water may thus flow into and out from the catchment system through different boundaries that are not all fixed in time and space. By analogy to the water transit time definition and quantification in Equation (1), one can similarly define and quantify the mean age o...

A 6,300-m 2 catchment in SW Sweden was covered by a roof and irrigated by deacidified lake water. The •80 of lake water differed from that of soil water and groundwater in the catchment, making it possible to follow the replacement of old (preirrigation) water by new (irrigation) water. After 7.5 months of irrigation the old water in the catchment had been replaced by new water. The breakthrough curve for new water gave a nearly exponential distribution of flow corrected transit times, with a mean transit time corresponding to a runoff of 54 mm. The estimated outflow of old water by runoff and evapotranspiration, 186 mm, compared well with independent estimates of the water storage at the onset of irrigation, 205 mm. The spatial pattern of the •80 of soil water and groundwater within the catchment demonstrated the strong influence of the topography on the water flow. RODHE ET AL.: TRANSIT TIMES FOR WATER IN A SMALL TILL CATCHMENT 140 rn 135 m 130m 125 in 3 5 7 9 11 13 i i ß ß ß ßßß ßß ß ß ß ß ß ß ß ß -o lus, 46B, 378-389, 1994. Rodhe, A., The origin of stream water traced by oxygen-18, doctoral RODHE

[1] Appropriate conceptual simplifications and assumptions are a central issue for hydrological modeling, especially when those models serve as the foundation for more complex hydrochemical or ecological models. A common and often unexamined assumption in conceptual modeling is that the relation between groundwater levels and runoff can be described as a succession of steady state conditions. This results in a singlevalued, monotonic function between the groundwater levels and runoff. Consequently, the simulated rise and fall in groundwater levels always follow the dynamics of runoff. We tested this assumption with an analysis of detailed groundwater level data along two opposing hillslopes along a stream reach in a Swedish till catchment at Svartberget. Groundwater levels in areas close to the stream followed the dynamics of the runoff. The correlation between groundwater level and runoff decreased markedly for wells farther than approximately 40 m from the stream. The levels were often independent of streamflow: Upslope area groundwater could be rising when riparian groundwater and runoff were falling, and vice versa. There was a high degree of correlation between groundwater levels at similar distances from the stream. The median Spearman rank correlation between wells within 35 m from the stream was 0.86 and for wells located more than 60 m from the stream was 0.96. This indicated that there is a common hydrological pattern even in the upslope area that can be identified and modeled. Despite the widespread acceptance of the steady state assumption previously in this and other study catchments, our study shows that it is not valid for the investigated hillslope site. If the divergence from steady state, with potential ramifications for other processes such as runoff chemistry, is common, then it will be worthwhile to reconsider the appropriate range of applicability for the steady state hypothesis, and the alternatives to that hypothesis.