Effects of large gullies on catchment-scale soil moisture spatial behaviors: A case study on the Loess Plateau of China

- Gullies clearly increase soil moisture spatial variability but not spatial means.
- Gullies increase the required number of samples to represent spatial means.
- Effects of sample counts on spatial variability differ from spatial means.

Large gullies which occur globally affect greatly ecohydrological processes in gullied landscape. However, the effects of large gullies on spatial behaviors of soil moisture, a critical ecohydrological variable, at catchment scale are poorly understood. To this end, we conducted spatially intensive soil moisture measurements in the 0-60 cm during the spring, summer and fall of 2010 in the Yuanzegou catchment on the Loess Plateau of China. Statistical and geostatistical analyses showed that the presence of gullies clearly increased the spatial variability in terms of standard deviation (by 26.4% on average), nugget (by several folds) and sill (by 41.3% on average), but had weak effects on spatial means. Particularly, the large gullies also markedly strengthened soil moisture spatial correlations. Slope aspect was shown to be a stronger topographic control than elevation and gradient on soil moisture spatial variability whether or not gully observations were included. The role of elevation in soil moisture variability depended on the presence of gullies. As gully observations were included, significant (P < 0.01) and stronger correlations were observed in summer; however, as gully observations were excluded, stronger correlations were found in spring and fall. The presence of gullies increased the required number of samples (N) by 33 to 100% to accurately represent spatial means. A random combination method showed that the effects of sample counts on soil moisture spatial variability were more dependent on gully observations and showed clear seasonality compared with spatial means. This means that both spatial means and spatial variability should be considered for optimizing sampling strategy for gullied topography.