Can soil water measurements at a certain depth be used to estimate mean soil water content of a soil profile at a point or at a hillslope scale?

Profile soil water contents (0) are important for scheduling irrigation, determining root water uptake and energy partition between sensible and latent heat. Soil water content measurements for deep soil profiles are time consuming and costly. The objective of this study was to test whether theta at a certain depth at a point can be used to estimate profile theta at a point or at a hillslope scale by time stability analysis. A total of 37 datasets of theta from 0.1 m to 3.8 m depths were collected by a neutron probe at 28 locations over four years on a representative hillslope in the Chinese Loess Plateau. Time stability of vertical patterns of theta was assessed by Spearman's rank correlation analysis. Soil water contents of the first two years were used to identify the time stable depth using mean absolute bias error, and theta of the second two years were used to validate if the time stable depth identified and associated mean relative difference can be used to predict mean theta of a soil profile at a point or at a hillslope scale. Results showed that vertical patterns of theta were time stable. The prediction error for mean theta varied with sampling locations and soil profile depths. At the most time stable location (location 4) in terms of vertical patterns, mean theta of soil profiles (i.e., 0-1.0 m, 0-2.0 m, 0-3.0 m, and 0-3.8 m) was predicted well by the most time stable depth, with absolute bias relative to mean <0.05 at a point scale and