Modelling water infiltration and transport in an agricultural compacted soil
MetadataVis full innførsel
A number of different soil physical properties, including the size, form, and distribution of pores in the soil matrix, affects water movement in the unsaturated zone and, consequently, its ability to deal with various amounts of precipitation. Knowledge regarding the extent and structure of variability of the properties are important for predicting the outcome of the variations that might exist. Despite the high importance of the physical properties, their quantification is a rear prioritization due to their high cost. The solution might be to use pedotransfer functions (PTFs) as they provide a pragmatic alternative to direct measurements of physical properties. PTFs are implemented to predict physical properties from parameters collected during soil surveys. The main objective of this thesis is to characterize the spatial variability of the physical properties that might exist on cultivated land. Through the implementation of different methods, including electrical resistivity tomography (ERT), double-ring infiltration tests, and water retention characteristics and grain size distribution analyses, a field plot southeast in the Skuterud catchment has been studied with the intention of characterizing the spatial variability in the topsoil, examining spatiotemporal variability, and simulating changes in unsaturated flow with varying precipitation patterns and amounts. As compaction is a stressing concern in conventional agriculture, mainly due to vehicular activity, the effect that topsoil and subsoil compaction might have on the flow through the soil matrix has been studied in the graphical computer platform HYDRUS using the van Genuchten parameterization of the water retention curve. Two vertical cross-sections have been created in HYDRUS, where the designs are based on findings through soil exploratory methods. They were implemented to simulate the effect of topsoil and subsoil compaction in combination with different patterns and amounts of precipitation on the vertical movement of water in the unsaturated zone. Based on these findings, it is evident that compaction of the soil decreases the rate of flow through the soil matrix, especially during unsaturated conditions. Though verification of the two models is still required, the double-ring infiltration tests and the geostatistical analyses of the soil physical parameters support these findings as they imply how compacted areas have decreased hydraulic conductivities and, hence, rate of flow. To indirectly explore the vertical spatiotemporal variability of the soil, time-lapse ERT surveys have been conducted from December 2015 to April 2016. The ERT was implemented to investigate the temporal change of resistivity values in the soil matrix during a period of snowmelt and induced infiltration. Though the resistivity values were generally expected to decrease between the measurements, the resulting images portrayed increasing values. Measurements of soil temperatures reveal that there had been a slight decrease in the temperature between a few of the measurements, which could possibly lead to increasing resistivity values due to increasing fluid viscosity. A plausible theory as to why the resistivity values continued to increase during a period of induced snowmelt might include preferential flow ways through macro-pores that generated large amounts of water rapidly, which resulted in low water contents by the time of the ERT measurements. Further analyses are needed to support this theory or to propose an alternate advocate suggestion.