Statistical analyses and controls of root-zone soil moisture in a large gully of the Loess Plateau

Soil moisture variability and controls are little known in large gullies of the Loess Plateau which represent complex topography with steep slopes. This study analyzed spatial-temporal variability of soil moisture at the 0-20, 20-40, 40-60, and 60-80 cm depths in a large gully of the Loess Plateau based on root-zone soil moisture measurements for 3 years (2009-2011). The result showed that mean soil moisture, standard deviation (SD), and coefficient of variation, were highly dependent on depth; the highest mean value was observed at the 20-40 cm depth, while the lowest one was at the 0-20 cm depth. The SD increased with mean soil moisture for various depths as soil moisture was relatively wet; however, a transition that SD decreased with mean soil moisture occurred when soil moisture was relatively dry. Positive correlations exist between moisture contents over different depths, and that the relationships of the neighboring layers are relatively high with R (2) from 0.70 to 0.76. Correlation analysis, principle component analysis, and stepwise multiple regression analysis showed that soil particle size distribution and topography (slope and elevation) were the main environmental factors controlling soil moisture variability in the large gully.