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Accueil du site → Doctorat → États-Unis → 2006 → Development and application of laboratory and geophysical methods for the determination of hydraulic properties on desert pavements in the Mojave Desert


Development and application of laboratory and geophysical methods for the determination of hydraulic properties on desert pavements in the Mojave Desert

Meadows, Darren G.,

Titre : Development and application of laboratory and geophysical methods for the determination of hydraulic properties on desert pavements in the Mojave Desert

Auteur : Meadows, Darren G.,


Grade : Doctor of Philosophy (PhD) 2006

Interactions between pedologic development and hydrology have important implications for arid ecosystems and landscape evolution. The ability to rapidly evaluate hydraulic properties of a soil would greatly enhance our ability to study these relationships. The purpose of this work was 1) to develop a method for determining the hydraulic properties of structured soil peds ; 2) to use the method to investigate the impact of surface age on the hydraulic properties ; and 3) to investigate the efficacy of noninvasive geophysical methods for estimation of clay content and hydraulic properties of surface soils in arid environments. This work was conducted on multiple aged desert pavements in the Mojave National Preserve, California. To more fully investigate the mechanism of infiltration on desert pavements, we first ran a tension infiltrometer experiment on a well-developed pavement to determine the field hydraulic properties. The area directly underneath the disc was then excavated and brought to the laboratory. The peds that comprised the disc area were individually sampled using the newly developed method, which is a combination of the traditional evaporation experiment and parameter estimation. Hydraulic properties of the individual peds were then areally averaged and compared with the field-determined values. A comparison of the two methods provides an indication of the impact of the interped region on the hydraulic properties because the laboratory method sampled only the peds, whereas the field method sampled both peds and the interped cracks that separate them in the field. This method was then used on three different pavements of varying age (10 ka, 100 ka, and  600 ka), with triplicate measurements on each surface. Although the 600 ka is the oldest surface, the Av horizon and desert pavement mantling this surface is similar in age to the 10 ka because the original soil has been stripped and the horizon re-formed, making the Qf3 the oldest surface soil. Average field-determined hydraulic properties were compared to the average laboratory-determined values for each surface. Laboratory-determined values of Ks and α showed a progressive decrease with age. This is in agreement with the aggradation model of desert pavement formation, where an influx of fine-grained eolian material causes the surficial soil horizon to grow and structurally develop. Field-determined values showed a similar decrease in Ks and α from the Qf5 to the Qf3. However, these values both increased from the Qf3 to the Qf2, which supports the age data that suggests the Qf5 and Qf2 are similar in age at about 10 ka. Field-determined values of n were significantly higher than the laboratory determined values, which suggests the occurrence of interped flow. The results of a dye tracer experiment confirmed the occurrence of interped flow. Thus, it appears that the mechanism of infiltration evolves from a matrix-dominated system to a preferential flow dominated system as the surface pedologically develops. Perhaps most importantly, the changes in the hydraulic properties were minimal across the surface ages that we sampled. This suggests that the majority of the changes in hydraulic properties occur relatively early in the development of a desert pavement. After approximately 10 ka, the surface is relatively stable, and changes occur slowly. As a final experiment, we investigated the use of ground-penetrating radar (GPR) as a tool for estimating the clay content and hydraulic properties of the surficial horizon on a young and old desert pavement. As clay content increases with surface age, more electromagnetic energy should be dissipated in the form of conduction currents. Thus, the received signal amplitude should be decreased. Using a multiple linear regression, we show that combining the early-time signal amplitude with a nominal measure of pavement development can explain almost 90% of the variability in clay content and nearly 75% of the variability in Ks on the older surface. The results for the younger surface were poor. There appears to be a threshold value of clay content that is necessary to decrease the signal strength sufficiently for GPR to be a diagnostic tool. Although the predictive error was rather high, this method could be a useful reconnaissance tool to help guide more time-intensive point measurements. The technique also provides much higher spatial resolution than traditional electromagnetic methods.

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Page publiée le 16 septembre 2006, mise à jour le 31 octobre 2018