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• Inverse modelling is a valuable tool for the determination of the soil hydraulic conductivity. • Hydraulic conductivities of slowly permeable horizons were determined with APSIM. • Inverse modelling highlighted the importance of the conductivity at field capacity. • Conductivity values were related to morphology characteristic and soil drainage classes. • Water retention parameters predicted by S-map are less important for slowly permeable soils. Soil hydraulic properties play a primary role in water infiltration and movement, and transport of nutrients and contaminants through the soil profile. However, the determination of soil hydraulic properties, especially the hydraulic conductivity at lower depths within the soil profile is very challenging. We investigated the use of inverse modelling based on APSIM (Agricultural Production Simulation Model) to determine likely ranges of the hydraulic conductivity at saturation (K sat) and field capacity (K DUL) for a slowly permeable subsurface horizon. Simulations were run for the poorly drained Otokia silt loam, in the Otago region of New Zealand. APSIM simulations were set up either according to the site specific soil profile description or based on the national soils database (S-map) pedotransfer functions. Simulated temporal soil moisture data were compared to measurements from a field experiment. Within a sensitivity analysis K sat values of the subsurface soil were varied from 2.4 to 192 mm/d, and values for K DUL from 0.05 to 1 mm/d. Finally simulations were run over 45 years using three climatic conditions, with annual rainfall amounts of 582, 628 and 730 mm. The effect of varying the hydraulic conductivity of the subsurface fragipan horizon (C x) on the wetness status of the overlying B 2g horizon was determined by calculating the average annual duration of soil moisture above field capacity. These were then used to estimate ranges of the hydraulic conductivity that contributes to a soil being imperfectly or poorly drained. The results highlight the significant influence that K DUL has on the reliability of modelling the hydraulic behaviour of soils with slowly permeable subsoils. [ABSTRACT FROM AUTHOR]

• Hydraulic conductivity at field capacity (FC) is important for bimodal pore system. • Hydraulic conductivity at FC should be adjusted for soil drainage classes/texture. • Saturated hydraulic conductivity is less important for the temporal soil saturation. • Hydraulic conductivity at FC affects pasture yield. Accurate simulation of soil water dynamics is a key factor when using agricultural models for guiding management decisions. However, the determination of soil hydraulic properties, especially unsaturated hydraulic conductivity, is challenging and measured data are scarce. We investigated the use of APSIM (Agricultural Production Simulation Model) with SWIM3 as the water module, based on Richards equation and a bimodal pore system, to determine likely ranges of the hydraulic conductivity at field capacity (K -10 ; assumed at a matric potential of −10 kPa) for soils representing different drainage characteristics. Hydraulic conductivity measurements of soils with contrasting soil drainage characteristics and values for K -10 were extracted from New Zealand's national soil database. The K -10 values were then varied in a sensitivity analysis from 0.02 to 5 mm d−1 for well-drained soils, from 0.02 to 1 mm d−1 for moderately well-drained soils, and from 0.008 to 0.25 mm d−1 for poorly drained soils. The value of K -10 had a large effect on the time it took for the soil to drain from saturation to field capacity. In contrast, the saturated hydraulic conductivity value had little effect. Simulations were then run over 20 years using two climatic conditions, either a general climate station for all seven different soils, or site-specific climate stations. Two values for K -10 were used, either the APSIM default value, or the soil-specific measured K -10. The monthly average soil saturation level simulated with the latter has a better correspondence with the morphology of the seven soils. Finally, the effect of K -10 on drainage and pasture yield was investigated. Total annual drainage was only slightly affected by the choice of K -10 , but pasture yield varied substantially. [ABSTRACT FROM AUTHOR]

• Groundwater discharge may hydrologically be as important as artificial drainage. • Nitrate concentrations in artificial drainage typically increased with flow. • The increase in nitrate concentrations varied between sites and seasons. • Nitrate, the main form of N exported, was not discharged through reduced groundwater. • Organic N and ammonium N exported via groundwater pathway can be important. When considering hydrological export pathways from drained pastoral fields, the shallow groundwater underlying the artificial drainage system may fulfil an as important role as the artificial drainage system itself. However, the important split between artificial drainage and groundwater flows at a field site is usually unknown. Consequently, the effects that the groundwater system's redox status may have on the forms and total amounts of nitrogen (N) exported from the site cannot be confidently assessed. We addressed these deficiencies by investigating the export of various forms of N in the artificial drainage and the shallow groundwater beneath two dairy farming sites (Tatuanui, Waharoa) within the Piako River catchment in New Zealand. Due to the very low hydraulic conductivity of the degraded peat in the saturated zone perched on a clay aquiclude, no significant water or contaminants were exported at Tatuanui via the unconfined shallow groundwater. Accordingly, artificial drainage discharge into the receiving surface collector drain was the only export pathway for N at this site. The sealed nature of the groundwater zone resulted in the accumulation of organic-N and ammonium-N in the peaty shallow saturated zone underlying the mineral soil. In contrast, the Waharoa site featured a more mobile shallow groundwater system, which on average conveyed approximately equal volumes of water offsite as the artificial drainage system. Nitrate (NO 3 -N) was at both sites the predominant form of N (76%) leached from the aerobic soil profile and discharged through the artificial drainage. As the shallow groundwater was at both sites in a reduced redox state, any NO 3 -N leached from the rootzone into the saturated zone was consumed. Accordingly, even the mobile shallow groundwater at Waharoa did not convey significant NO 3 -N. The NO 3 -N consumption in the reduced groundwater presumably occurred due to dissimilatory NO 3 -N reduction (concomitant increases in ammonium-N concentrations, with no organic material available for mineralisation in the saturated zone) and microbial denitrification (producing gaseous forms of N). The NO 3 -N reductions in the groundwater resulted in the average total N exports via the shallow groundwater pathway at the Waharoa site being substantially lower (31% of the exported N) than in the artificial drainage (69%). This is despite both pathways exporting similar water volumes. Because of the NO 3 -N consumption in the reduced shallow groundwater at Waharoa, over 98% of the N exported through this pathway was either as ammonium-N or organic-N. [ABSTRACT FROM AUTHOR]