Title: Simulation of local controls on the temporal stability of soil water content Authors
Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 17, 2012
Publication Date: January 2, 2013
Repository URL: http://handle.nal.usda.gov/10113/56580
Citation: Martinez, G., Pachepsky, Y.A., Vereecken, H., Hardelauf, H., Herbst, M., Vanderlilnden, K. 2013. Simulation of local controls on the temporal stability of soil water content. Journal of Hydrology. 481:106-118. Interpretive Summary: Surveys of various landscapes have shown that some locations have soil water contents consistently higher than the average across the entire area, while other locations have lower than average water contents. This phenomenon is termed “temporal stability of soil water contents”. In spite of its wide spread occurence, no research has been done to understand the mechanisms that control this temporal stability. We hypothesized that spatial variability in soil hydraulic conductivity may be the primary control. Simulation studies were conducted with vertical soil water flow in soil columns with and without root activity of grass These simulations reproduced several essential features of soil water content temporal stability that were previously reported by various authors. We showed that the temporal stability data can provide information about the spatial variability of soil hydraulic conductivity, which is essential for modeling water flow and storage in soils and landscapes. Results of this work will be useful to agronomists, agricultural and environmental engineers, and hydrologists in that they explain the core mechanism controlling temporal multilocational dynamics of soil water for characterization of soil and provide a method of relating the temporal stability data to the spatial variations in the key soil hydraulic property, i.e. soil hydraulic conductivity.
Technical Abstract: Occurrence of temporal stability of soil water content (TS SWC) has been observed for a range of soil and landscape conditions and is generally explained as a consequence of local and non local controls. However, the underlying factors for this phenomenon are not completely understood and have not been quantified. This work attempts to elucidate and quantify the effects of several local controls, i.e. those affected by the soil properties at a certain location, like soil texture, depth, vegetation and spatial variability in the saturated hydraulic conductivity (Ksat), through water flow simulations. One-dimensional water flow was simulated with the HYDRUS code for bare and grassed sandy loam, loam and clay soils at different levels of the standard deviation (0.1 to 1.0) of the scaling factor of lnKsat. Soil water content at 0.05 and 0.60 m and the average water content of the top 1 m were analyzed. Temporal stability was analyzed by calculating the mean relative differences (MRD) of soil water content in 100 soil columns used for each combination of soil and weather conditions. Log-normal distributions of Ksat resulted in MRD distributions that were commonly observed in experimental studies of soil water content variability. Ksat was the lead factor of the soil column rank by its MRD. Nonlinear relationships were observed between scaling factor of ln Ksat and spread of the MRD distributions. For the same scaling factor and soil texture, shapes of the MRD distributions were found to depend on the length and weather of the simulation period. Variation in MRD was higher in coarser textures than in finer ones and more variability was seen in the topsoil than in the subsoil. Established vegetation decreased variability of MRD in the root zone and increased variability below it because it moved deeper the effects of transpiration. Further simulations would be necessary to also consider the effect of non-local controls on TS.