Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/14/2004
Publication Date: 9/14/2004
Citation: Grant, L., Seyfried, M., and McNamara J. 2004. Spatial variation and temporal stability of soil water in a snow-dominated, mountain catchment. Hydrological Processes. 18. 3493-3511.
Interpretive Summary: Traditionally, water supply forecasts from regions dominated by snowmelt are based the measured amount of snow on the ground at the end of winter. We know that a secondary control on the water supply is the amount of soil water storage at that time. Ideally, this would also be accounted for in water supply forecasts, but the high degree of spatial variability of soil water storage makes it difficult to characterize so that it is not clear when or where to measure it. We found that, although highly variable, the soil water storage at all points within an experimental watershed followed the same trends in time and that each site maintained the same relative position, in terms of the amount of soil water stored, over time. This means that, given background information, relatively few measurements made in late fall could describe the watershed soil water storage. This is important because it provides a basis for extending the limited number of measurements that are practical in important watersheds to the large basins that are critical for water supply. This information is of use primarily for scientists working on improving methods of estimating water supply based on snow measurements.
Technical Abstract: Soil is a critical intermediary of water flux between precipitation and stream flow. Characterization of soil water content (', m3m-3) may be especially difficult in mountainous, snow dominated catchments due to highly variable water inputs, topography, soils and vegetation. However, individual sites exhibit similar seasonal dynamics, suggesting that it may be possible to describe spatial variability in terms of temporally stable relationships. Working in a 0.36 km2 headwater catchment, we: (i) described and the spatial variability of ' over a two year period, (ii), characterized that variability in terms of temporal stability analysis and (iii) related changes in temporally stable soil water patterns to stream flow generation. Soil water data were collected for two years at representative sites and quantified in terms of '' and water storage to a depth of 75 cm (S75, cm). Both S75 and '''were normally distributed in space on all measurement dates. Spatial variability was high relative to other studies, reflecting catchment heterogeneity. However, the ranking of S75 values displayed temporal stability for all site locations, seasonally and annually. This stability was attributed to soil texture. Further temporal analysis indicated that estimates of catchment mean and standard deviation of S75 may be characterized with relatively few measurements. Finally, we used temporal linear regression to define catchment soil water conditions related to stream flow generation. Static, high S75 conditions in late winter and early spring indicate that stream flow response is highly sensitive to inputs whereas static, low S75 conditions in late summer and early fall indicate minimum stream flow sensitivity to water inputs. The fall transition was marked by uniform Sd across the catchment. The late spring transition was marked by nonuniform S75 decreases, with the highest S75 sites decreasing most. Threshold S75 values identifying catchment sensitivity to water input were identified.