Steve Thomas

Effects of irrigation intensity on preferential solute transport in a stony soil

ABSTRACT If irrigation intensity exceeds soil infiltration capacity, water may flow preferentially down cracks and large pores. In this situation, solute transport will involve only a fraction () of the soil’s water and leaching rate may be affected. To assess whether irrigation intensity affects preferential solute flow, an experiment was performed at Lincoln using 12 steel-encased lysimeters with a Lismore Stony Silt Loam soil under two irrigation intensities, 5 and 20 mm h–1. Burns’ equation was used to describe the measurements of non-reactive tracer concentration as a function of drainage. Under dry antecedent moisture conditions, bromide transport was not significantly different under the different irrigation rates, even though strong preferential leaching occurred, with of 0.23. For chloride, was 0.85 and 0.58, for 5 and 20 mm h–1 respectively, sufficient evidence to confirm the effect of irrigation intensity (P < 0.05). By assuming to be 1.00 for the median rainfall at Lincoln, an exponential function was fitted to the data, suggesting a lower limit of 0.35 for under moist conditions. Implications for nutrient leaching are discussed.

A nitrogen balance model for environmental accountability in cropping systems

Abstract In New Zealand and worldwide, growers are increasingly being required to demonstrate that their nutrient management does not have adverse effects on the environment. Since routine direct measurements of nitrogen (N) losses at an appropriate scale are currently unfeasible, a modelling tool that can be used to assess losses, both actual and potential, from farming systems is therefore needed. We present the upgraded N balance module for cropping systems (OVCrop) developed to be incorporated into the OVERSEER® Nutrient Budgets model. It presents an easy-to-use interface, requiring few but meaningful inputs, with the minimum flexibility needed to describe management of real farming systems. Following the developmental guidelines of OVERSEER® Nutrient Budget, OVCrop is designed to provide the long-term annual average of N leaching from any user-defined cropping rotation. OVCrop has been parameterized based on process-based model simulations for typical farm systems and environments of New Zealand. The intended use of the OVCrop is to help on-farm nutrient management and to demonstrate future compliance to N leaching regulations. OVCrop is a robust tool, simple enough to be used by people with a low to medium level of expertise, such as farmers and regional councils.

Nitrate leaching losses during pasture renewal – Effects of treading, urine, forages and tillage.

Pasture renewal is a key component of intensive temperate grassland farming. This practice is performed to improve pasture yields, but it may increase nitrate (NO3-) leaching losses, which can impact on water quality. Farmers face many choices when renewing pasture, however, there is limited information to guide decisions to reduce leaching losses. An experiment was established to study how different management practices and grazing affect biomass production and NO3- leaching during pasture renewal on a heavy soil. Long-term pasture was either re-sown into ryegrass (Lolium perenne) in autumn (GG) or into forage rape (Brassica napus) followed by ryegrass in spring (GCG). Rape was established following ploughing or direct-drilling. Grazing was simulated in winter, whereby split plots ±urine (600 kg N ha-1) and ±treading were established. Nitrate concentrations at 1 m depth were measured with suction cups and drainage predicted using a crop model. Estimated NO3- leaching losses at 1 m depth ranged from 16 to 38 kg N ha-1, with little difference between GCG and GG. However, the risk of future leaching was much greater below GCG plots. At the end of the study, soil NO3- between 0.6 and 1 m ranged from 28 to 130 kg N ha-1 in GCG plots and 1-28 kg N ha-1 in the GG plots. Timing of leaching differed between renewal systems, reflecting the differences in plant N uptake and fallow period. Overall, there was no difference in dry matter production between the two systems. Treading resulted in greater compaction, especially in tilled plots and reduced NO3- leaching by c. 40% - this can be attributed to increased denitrification. Our study demonstrates the complex nature of management and environmental factors and their interaction during pasture renewal. We show that management practices affect the risk and timing of N leaching. Practical implications for farmers are discussed.

Modelling soil-water dynamics in the rootzone of structured and water-repellent soils

Abstract In modelling the hydrology of Earths critical zone, there are two major challenges. The first is to understand and model the processes of infiltration, runoff, redistribution and root-water uptake in structured soils that exhibit preferential flows through macropore networks. The other challenge is to parametrise and model the impact of ephemeral hydrophobicity of water-repellent soils. Here we have developed a soil-water model, which is based on physical principles, yet possesses simple functionality to enable easier parameterisation, so as to predict soil-water dynamics in structured soils displaying time-varying degrees of hydrophobicity. Our model, WEIRDO (Water Evapotranspiration Infiltration Redistribution Drainage runOff), has been developed in the APSIM Next Generation platform (Agricultural Production Systems sIMulation). The model operates on an hourly time-step. The repository for this open-source code is https://github.com/APSIMInitiative/ApsimX. We have carried out sensitivity tests to show how WEIRDO predicts infiltration, drainage, redistribution, transpiration and soil-water evaporation for three distinctly different soil textures displaying differing hydraulic properties. These three soils were drawn from the UNSODA (Unsaturated SOil hydraulic Database) soils database of the United States Department of Agriculture (USDA). We show how preferential flow process and hydrophobicity determine the spatio-temporal pattern of soil-water dynamics. Finally, we have validated WEIRDO by comparing its predictions against three years of soil-water content measurements made under an irrigated alfalfa (Medicago sativa L.) trial. The results provide validation of the models ability to simulate soil-water dynamics in structured soils.