Scales and locations of time stability of soil water storage in a hummocky landscape

Different factors and processes operating in different intensities and at different space-time scales result in strong spatio-temporal variability in soil water storage. However, there is similarity between the overall spatial patterns of soil water storage measured at different times, which has been identified as time stability. The objective of this study was to examine the scales and locations of time stability of soil-water storage spatial patterns at different seasons and depths in a hummocky landscape. Soil water storage was measured up to 140 cm depth over a 4-year period using time domain reflectometry and a neutron probe along a transect in the St. Denis National Wildlife Area, Saskatchewan, Canada. The transect was 576 m long with 128 sampling points (4.5 m sampling interval) and traversed several knolls and depressions. There were high Spearman's rank correlation coefficients between any two-measurement series, indicating strong time stability of the spatial pattern of soil water storage. The spatial patterns of soil-water storage from the same season (intra-season) had stronger time stability than those from different seasons (inter-season). Strong time stability was also observed between the measurement series from a season of 1 year and a measurement series from the same season of another year (inter-annual). Wavelet coherency analysis indicated that the large-scale (>72 m) spatial pattern was time stable irrespective of seasons due to the alternating knolls and depressions in the study area. There were also near replica spatial patterns at small (<18 m) and medium (18-72 m) scales during summer and fall, possibly resulting from evapotranspiration of vegetation established in mid-summer. The inter-season time stability was only present within large depressions with long slopes and fewer landform elements. However, intra-season and inter-annual time stability was observed at all scales and locations. The time stability of surface soil water storage was different from that of the whole soil profile, indicating a depth dependence of time stability. The change in the scales and locations of spatial pattern of soil water storage indicates the change in the hydrological processes, which can be used to identify the change in the sampling domain. (C) 2011 Elsevier B.V. All rights reserved.