Groundwater transit times are greatest in streams draining north-facing hillslopes with lower solar radiation, thicker soils and deeper flowpaths. These longer transit times may enhance mineral weathering rates and increase dissolved solute loads to surface waters. In addition, north-facing catchments may have greater net ecosystem production due to increased water availability. Cooler and wetter soils should reduce heterotrophic soil respiration, increasing both soil carbon pools and dissolved organic matter export relative to south facing catchments.

Transit time modeling

The Surface Water Hydrology Group developed a generalized modeling approach to track the time-variable hydrologic response of catchments and their associated water transit time distributions. The method is based on the realization that there is a distinct difference between the hydrologic response (i.e. unit hydrograph) and the particle response (i.e. water transit time distribution), although both are controlled by processes taking place in catchment storage. A 'particle' is a small parcel of water with an associated clearly defined tracer concentration. The observed streamflow hydrograph after storm rainfall is usually composed of pre-event and event water, and their ratio depends on initial storage conditions and precipitation intensity characteristics. If no water is in storage (a hypothetical case) then any measurable hydrologic response has to be event water and thus equals the water transit time distribution. In any other case, pre-event water will be mobilized together with event water en route to the outlet and the sampled mixture will represent, at any given time, the distribution of ages of the water. Only when the system is in steady state (inflow balances outflow and thus storage does not change) will the catchment exhibit a time-invariant transit time distribution. We therefore need to be able to track not only the time-varying nature of water transit times but also the variations in hydrologic response.

If sufficiently long time series data of precipitation and streamflow and their associated tracer concentrations are available for a given study site, applying our method will generate an ensemble of water transit time distributions (TTD) and associated hydrologic response functions (HRF). Each of these functions has a certain probability of occurrence and their combined distribution function represents a more realistic catchment descriptor to study similarities between catchments or landscape/climate controls on water transit time. We have introduced the concept of the master transit time distribution (MTTD) and associated master hydrologic response function (MHRF) as new catchment descriptors. Convergence of the master functions will have been reached once the addition of further TTDs or HRFs will not significantly alter their shape.