Dependence of Vertisol Surface Cracking On Long- and Short-Term Dynamics of Water: a Wavelet Analysis.

Observations of temporal cracking processes of a Vertisol on a gilgai landscape showed dynamic crack changes that have proved difficult to represent using current soil water content. Based on 10-year trends in a dataset of soil cracks measured at study site in a Texas Vertisol, we hypothesized that in addition to current soil moisture, antecedent soil moisture prior to cracking is another factor explaining long-term variation of surface cracking in field conditions. In this study, our objectives were to (i) analyze wavelets of a water available index (WAI) to model in situ observations of surface crack area density and (ii) evaluate the effect of short- and long-term variability of the water balance on in situ surface cracking using the wavelet results. A daily WAI was calculated from precipitation and reference evapotranspiration where surface cracking was measured over 100 m² of a Laewest clay (fine, smectitic, hyperthermic Typic Hapluderts) from 1989 to 1998. For wavelet analysis, the daily WAI values selected for about 3 years (1024 days) prior to 106 measurement dates with and without cracks. Discrete wavelet transformations of WAI values using the Haar wavelet were combined with multiple stepwise linear regressions to model crack area density taking microtopography into account. Full cross validation results showed crack area density predictions with adjusted R² values of 0.80 for microhighs and microslopes, and 0.73 for microlows. The R² values were a 10% improvement on regression results using current and antecedent soil moisture. Wavelet analysis results support the idea that seasonal oscillations of daily soil moisture triggers relatively short drying-wetting cycles, while multi-year cycle of antecedent soil moisture corresponds to longer periods of soil water recharging conditions.