Scale and Time Dependent Hydrologic Response of Sites with Expansive Soils

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Clay swelling is a time-dependent process that, for some soils, requires up to a month to complete, even with abundant water supply. Our preliminary experiments on soils from the ?Drylands? of the coast range of central Chile demonstrate this behavior in laboratory and field infiltration tests. The importance of such time-dependent hydration has not been studied in the field nor included in hydrologic models, but is expected to be of profound application with respect to runoff generation and aquifer recharge. The objective of this study is to observe the influence of time-dependent swelling dynamics on infiltration and runoff processes and develop a scalable process-based model to predict hydrologic response of swelling soils at scales from 0.02 to 100,000 ha.The authors will examine the effects of the preferential flow paths in slow-swelling clay soils on water flow at the plot and catchment scales. At the plot scale, surface water runoff and water content in the upper 2 m of soil will be measured during a series of simulated rainstorms. Throughout two full years, stream flow will be recorded at a temporal resolution of 5 min or less from 9 catchments within a 1000 ha basin. The catchments will have the following sizes: 700 ha, 100 ha, 45 ha, 15 ha, 5 ha (x2), 1 ha (x3). Soil moisture profiles in above runoff plots as well as in other locations will be monitored and continuous rainfall and other atmospheric data will be recorded during this period. Infiltration tests adjacent to each of the 9 plots will be carried out during the summer, fall, winter and spring to observe the degree to which field saturated hydraulic conductivity changes by season. Soil samples be analyzed in the laboratory to measure:1) swelling/shrinkage characteristics, 2) time-dependent changes in saturated hydraulic conductivity, 3) water retention characteristic, and 4) particle-size distribution.A spatially-distributed hydrologic model that accounts for slow swelling and shrinking of soils on infiltration will simulate stream flow from each of the 9 catchments. In addition to the scientific activities, a hands-on three-week field hydrology camp in Chile will be funded for 12 university students, with up to 6 more students allowed to pay their own way. The field camp will include presentation of scientific principles of hydrologic measurement, and small-group practice of each of 6 basic measurements. These measurements will be part of the characterization work required to address the scientific hypothesis of the proposal, giving the students involvement in actual research. The scientific outcomes of the project apply to swelling soils found around the world with prominent examples in the USA (Ex.: Texas), Canada, New Zealand, and Chile.