Estimation of root-zone soil moisture over gullies using an exponential filter

To obtain more effectively root-zone soil moisture over gullies of the Loess Plateau, this study used an exponential filter to estimate root-zone (20—80 cm) soil moisture at transect and gully scales based on surface observations (0—20 cm). The results showed that the approach reliably retrieved root-zone soil moisture whereas larger errors were observed at deeper layers. If the initial root-zone soil moisture was available, the estimation accuracy was improved to some extent. However, note that significant accuracy improvement was only observed at the first several points in soil moisture time series. The optimal characteristic time length T (T_opt) increased with depth, and it generally increased by one factor compared to that in the 20—40 cm with an increase of 20 cm depth. Furthermore, as the T_opt was exchanged to calculate root-zone soil moisture between transects and scales, the estimation accuracy only changed negligibly. This indicated the exponential filter method was non-sensitive to T_opt. However, it is worth noting that strong spatial heterogeneity of soil properties exists at region scale on the Loess Plateau, the sensitivity of the exponential filter to T_opt should be investigated at larger scales.