[1] Topography is an important control on hydrological processes. One approach to quantify this control is the topographic ln(a/tanb) index. This index has become widely used in hydrology, but it utilizes a relatively small portion of the information contained in a digital elevation model (DEM). One potentially important feature not considered in the implementation of the ln(a/tanb) index is the enhancement or impedance of local drainage by downslope topography. This effect could be important in some terrain for controlling hydraulic gradients. We propose a new way of estimating the hydraulic gradient by calculating how far downhill (L d , [m]) a parcel of water must move in order to lose a certain amount of potential energy (d, [m]). Expressed as a gradient, tana d = d/L d , values tend to be lower on concave slope profiles and higher on convex slope profiles compared with the local gradient, tanb. We argue that the parameter d controls the deviation of hydraulic gradient from surface slope. While we determine this subjectively, landscape relief, DEM resolution, and soil transmissivity should be considered at the selection of d. We found the downslope index values to be less affected by changes in DEM resolution than local slope. Three applications are presented where the new index is shown to be useful for hydrological, geomorphological, and biogeochemical applications.

Hydrologic modeling of catchments is frequently hampered by lack of information on subsurface stratigraphy and zones of preferred flow. We evaluated the usefulness of soil penetration resistance, easily measured by a dynamic cone penetrometer, together with measurements of ground water level fluctuations, as a cost-effective means to infer subsurface flow patterns. At our field site at Sleepers River, Vermont, penetration resistance was lowest in the surficial 10 to 30 cm, then typically increased to a local maximum at 60 to 80 cm, which we interpreted as the soil/till interface. Below this depth usually lies a zone of decreased resistance in the till, giving way to either a gradual or abrupt increase in resistance toward the bedrock surface at 1 to 4.5 m depth. Penetration resistance had a weak but significant negative correlation with saturated hydraulic conductivity determined by bail tests (r 2 = 0.25, p < 0.05). At many wells, monthly ground water levels tended to cluster at or just above the resistant zone near the soil/till interface. Chemical and isotopic dynamics in nested wells finished above and below the resistant zone suggest that the zone may temporarily isolate the deeper ground water reservoir from meltwater inputs, which were clearly identified by low δ 18 O values. In ground water discharge zones, δ 18 O values tended to converge throughout the profile. In contrast, dissolved organic carbon (DOC) maintained a gradient of increasing concentration toward land surface, even in otherwise well-mixed waters, reflecting its rapid release from organic horizons. Understanding the effect of soil penetration resistance on ground water behavior may be useful in future catchment modeling efforts. 964Shallow Water Table Fluctuations conditions on these aquifer properties thus determines the position of the water table-and ultimately, streamflow. The position of the water table also plays a role in stream water chemistry. Infiltrating rain and snowmelt, modified by passage through the organic-rich surficial soil, recharge the ground water. The chemistry of this newly recharged shallow ground water may be markedly different than that of the deeper pre-existing ground water (Christopherson and Hooper 1992). The latter is generally dominated by solutes derived from weathering of minerals in the soil and bedrock. Freshly recharged ground water is generally dilute in weathering solutes but enriched in surficial soil leachates such as DOC, nitrate, and silica . It may take some time for these waters to mix, during which time the shallow ground water system remains chemically stratified. Younger ground water readily discharges to stream water because of higher hydraulic conductivities typically found in surficial soils (Bishop 1991;Kendall et al. 1999). The chemical and isotopic signature of this discharging shallow ground water will reflect its recent origin.In this study, conducted at Sleepers River, Vermont, we analyzed a 10-year record of monthly water table fluctuations and one snowmelt period of chemi...