Old and New Water Contributions to Storm Runoff

There have been many studies in the last 30 years that have made use of the natural geochemical characteristics of runoff to give at least an approximate separation of the storm hydrograph into a contribution from the rainstorm itself (the "new" water) and water stored in the catchment prior to the event ("old" water) (see, e.g., Sklash, 1990). Most such studies have used a two-component separation (into old and new water), which requires the assumptions that the geochemical characteristics of the two components are distinctly different and are constant in both space and time. The old water concentration is usually taken as that of the stream water baseflow component, measured prior to the start of a storm. The results, under these assumptions, have often suggested that a large proportion of the storm hydrograph may be made up of old water.

The assumptions, however, have often been questioned, particularly that of the constancy of the old water component. A number of studies have therefore introduced a third component, or end member, with the chemical characteristics of the "soil water," as determined from direct sampling. A separation or end member mixing analysis (EMMA) into three components requires measurements on at least two different tracers and may also be subject to some uncertainty (see, Bazemore et al., 1994; Fig. 3). The results, however, still usually suggest that a large proportion of the hydrograph is made up of water stored in the catchment prior to the event. This proportion would be reduced if it were shown that the new water rapidly takes on the tracer concentration characteristics of soil water, especially for reactive natural tracers such as silica, since it is known that equilibration times for dissolution or desorption can be short relative to storm duration in some circumstances.

Although these results have led to a réévaluation of the Hortonian concept that storm runoff is predominantly provided by rain water running off over the surface of the soil, there is no mystery about the hydrograph showing a large proportion of old water. This requires a displacement of water stored in the catchment prior to an event by the incoming rain water during the event. The simplified kinematic analysis of the different subsurface velocities presented above suggests that the wave speed will be greater than the mean pore water velocity and therefore displacement of old water into the stream would be expected. In addition, at least for moderate storms in humid catchments, the volume of water stored in the profile prior to an event may be much greater than the volume of event water (a 1 -m soil profile, with an average of 25% moisture content in the profile, contains the equivalent of 250 mm of water per unit area). Thus, there will often be more than enough water available to be displaced. However, the proportion of old water might be expected to decrease as storm magnitude increases, but very few measurements have been reported for large-magnitude events.

feature of the melt process is that it will have a characteristic spatial pattern since, in general, south facing slopes will melt before north facing slopes (in the Northern Hemisphere) and a low elevation snowpack before a high elevation pack. There may also be spatial variations in melt associated with differences in the storage of snow as a result of drifting during the winter period.

The response of a catchment during the snowmelt period may depend very much on the state of the soil. If the soil is frozen, then it is likely that infiltration rates may be limited and there is a greater chance of the melt generating a downslope surface runoff through the base of the pack. If the soil is unfrozen, then the low intensity of the melt will usually mean that the bulk of the melt will infiltrate into the soil profile. Depending on the weather conditions prior to a pack being established, it is quite possible that in some years the soil surface remains frozen all winter, while in other years the surface is unfrozen at the start of the melt season. The responses expected during melt might then be very different in different years.

Melt rates can be greatly accelerated, if warm rain falls on a ripe snowpack, and in some parts of the world rain on snow events can be a significant cause of flooding. The rain adds both water to the event volume and heat resulting in increased rates of melt. This type of event was involved in the northern California floods of early 1996.

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