Estimation of soil water storage using temporal stability in four land uses over 10 years on the Loess Plateau, China

An understanding of the temporal stability of soil water storage (SWS) in deep soil profiles is critical to optimize monitoring strategies and to predict the status of soil water on the Loess Plateau. This study tested and validated the feasibility of estimating mean SWS over multiple years by the SWSs at selected locations. The SWSs in 0-1, 1-2, 2-3, and 3-4 m layers were collected using neutron probes at 11 sites in each of four land-use types: cropland (CL), grassland (GL), fallow land (FL), and shrubland (SL). The most time-stable locations (MTSLs) for the various layers and the location at mid-slope for each land use were selected on 20 sampling occasions during a calibration period from July 2004 to December 2005. A validation data sets from January 2006 to October 2013 was used to test the length of time the estimates of mean SWS remained valid. The SWSs in SL and GL decreased with plant growth, and the temporal variations were larger in SL and GL than in FL and CL. The temporal stability of the SWSs was high for all soil layers in four land uses, with the rank correlations over the threshold of significance (α = 0.05) over 10 years. The degree of temporal stability of SWSs was ranked as CL > FL > GL > SL, and the temporal stability of SWSs in SL and GL decreased with increasing lengths of observation period, as indicated by lower mean Spearman's correlations for all soil layers. The MTSLs selected from the calibration period could accurately estimate mean SWSs for diverse layers under four land uses with estimation errors less than 10% over eight years. The study verified that a single location at mid-slope of each land use could be sampled in order to reduce the required number of samples and save time and labor while maintaining a high accuracy of prediction over multiple years.