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Made available in DSpace on 2022-04-28T19:46:33Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-01-01 The water retention curve (WRC) of arable soils from the southeastern United States at different levels of compaction (no compaction, and 10 and 20% increases in soil bulk density) was estimated using the van Genuchten–Mualem (VG) model. The VG water retention parameters of the noncompacted soils were obtained first by fitting measured soil hydraulic data. To construct the WRC of the compacted soils, gravimetric values of the permanent wilting point (θgw, 1,500 kPa) and the residual (θgr) water content were assumed to remain unchanged with compaction. The VG parameter α and exponent η after compaction were estimated using two approaches. In Approach 1, α and η were estimated from saturation, the permanent wilting point, and the residual water content. In Approach 2, the value of η was assumed to remain unchanged with compaction, which allowed α to be estimated immediately from the VG equation. Approach 2 was found to give slightly better agreement with measured data than Approach 1. The effect of compaction on the saturated hydraulic conductivity (Ks) was predicted using semitheoretical approaches and the VG-WRC function. HYDRUS-1D was further used to simulate vertical infiltration into a single-layered soil profile to determine the impact of compaction on the infiltration characteristics of the soils used in our analyses. Results showed that a 10–20% increase in soil bulk density, due to compaction, reduced cumulative infiltration (Ic) at time T = Tfinal (steady-state) by 55–82%, and the available water storage capacity by 3–49%, depending upon soil type. Univ. of Southern Queensland Centre for Agricultural Engineering CSIRO Agriculture and Food Black Mountain Science and Innovation Precinct Dep. of Earth Sciences Utrecht Univ. Center for Environmental Studies São Paulo State Univ. Dep. of Agricultural and Biosystems Engineering Iowa State Univ. Engineering Dep. Harper Adams Univ. Center for Environmental Studies São Paulo State Univ.

This study was designed to improve our understanding of, and mechanistically simulate, nitrate (NO3) dynamics in a steep 9.8 ha rural headwater catchment, including its production in soil and delivery to a stream via surface and subsurface processes. A two-dimensional modelling approach was evaluated for (1) integrating these processes at a hillslope scale annually and within storms, (2) estimating denitrification, and (3) running virtual experiments to generate insights and hypotheses about using trees in streamside management zones (SMZs) to mitigate NO3 delivery to streams. Total flow was mathematically separated into quick- and slow-flow components; the latter was routed through the HYDRUS software with a nitrogen module designed for constructed wetlands. Flow was monitored for two years. High surface-soil NO3 concentrations started to be delivered to the stream via preferential subsurface flow within two days of the storm commencing. Groundwater NO3-N concentrations decreased from 1.0 to less than 0.1 mg l−1 from up-slope to down-slope water tables, respectively, which was attributed to denitrification. Measurements were consistent with the flushing of NO3 mainly laterally from surface soil during and following each storm. The model accurately accounted for NO3 turnover, leading to the hypotheses that denitrification was a minor flux (