A study of the soil pattern, properties and hydrology of a mole and tile-drained, loess-mantled downland in Southland, New Zealand : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Dissected, loess-mantled terraces (downlands) are a dominant component of the soil-landscape in Southland, New Zealand and commonly contain soils with dense subsoils that impede drainage. Consequently mole and tile drainage systems, which removes excess soil moisture, are widespread. Alongside agronomic benefits, mole and tile drainage creates problematic secondary effects, including altered stream hydrographs, contaminant transfer into surface water and possible modification of soil properties. Today, water quality and quantity management rely heavily on environmental modelling. Models of soil – water interactions are informed by knowledge of soil hydraulic properties and processes, which is gained from a variety of disciplines, including pedology, soil physics and hydrology. Understanding the properties and processes relevant to mole and tile-drained, slowly permeable loess soils is, thus, criticical for reliable simulations of their water dynamics. Concerningly, this understanding remains elusive. There is a need to better understand the variability in soil properties that regulate water storage, movement and connectivity, as well as the intrinsic and extrinsic controls on water flow pathways. These research gaps were addressed by characterising and monitoring a small, mole and tile-drained basin on a typical Southland sheep farm. Surveying of the soil and loess mantle found vertical differentiation of soil properties was due to contemporary pedogenesis, and soil features inherited from buried loess sheets were important sources of heterogeneity in water-regulating soil properties. Catchment side slopes had greater soil variation relative to the interfluves. Buried loess sheets, which had been exhumed by hillslope procceses, were contributing to the subsoils on the basin slopes whereas the soils on the interfluves were formed solely into the uppermost loess sheet. The findings confirm that understanding the soil-geomorphology of loess-mantled downlands is important for improving soil – landscape models, characterising soil spatial variability and producing more accurate soil maps. Ground penetrating radar (GPR) was effective at identifying and mapping mole channels, thus allowing drainage density to be quantified and the mole network to be characterised. It demonstrated that the mole networks had non-systematic patterns and most likely were multi-generational. Ground truthing during the GPR survey showed mole channels were in good condition even after 30 + years without maintenance. From a survey of soil hydraulic properties it was apparent that the mole fracture network created when moles were installed had not persisted. The finding has implications for hydrological and water quality modelling, as preferential flow via the fracture network is commonly considered to be the major route for rapid water and contaminant transfer into moles. Mole channels were not found to influence soil properties directly; however, it was discovered that surface saturated hydraulic conductivity (Ksat) was controlled by antecedent soil moisture state in parts of the landscape where perched water tables (PWT) persisted. This was hypothesised to occur by priming of the macropore network when capillary rise from a PWT removed air pockets, signalling a potential indirect influence of drainage systems on Ksat. A water balance analysis refuted a common assumption of negligible deep drainage in slowly permeable loess, with or without artificial drainage. Deep drainage varied between events as well as seasonally and occurred via piston flow through the fragipan and underlying loess mantle. Runoff (tile and overland flow) responses to rainfall events could be discriminated by thresholds of precipitation volume and/or intensity, depending on antecedent soil moisture state (dry or wet). Infiltration-excess overland flow was the most common form of surface runoff, consistent with the low surface infiltration rates estimated from infiltration experiments. A corollary is that shifts in the distribution of precipitation intensity with climate change could change the overland flow component of the water balance of this landscape. Tile flow from small events was predominantly sourced from the interfluves and upper hollow. As events became larger, the source area expanded until most of the catchment was contributing to tile flow. When considering the mitigation of contaminant transfers to surface water, winter and spring should be be key time periods, and the upper hollow and interfluves key locations, on which to focus. Soil moisture spatial patterns appeared to be influenced by both PWTs and the artificial drainage network and were not well predicted by the topographic wetness index (TWI). In the dry soil moisture state, upslope areas were hydrologically disconnected. With increasing catchment wetness, hydrological connectivity expanded from the lower hollow to the PWTs at the mid- to upper hollow, and eventually up to the interfluve PWTs. Once the hydrological connection between the lower hollow and interfluve had been established, the catchment had completed its transition into the wet state, and the interfluve PWT and mole network became the primary regulator of tile flow. The transition from dry state to wet state soil moisture conditions took about two weeks and began at the lower catchment hollow. Only rainfall periods with exceptional precipitation volumes and intensities connected the slopes, and only when all other parts of the catchment were saturated. There was no evidence that natural interflow, either as matrix or preferential flow, led to hydrological connection. Surface-sourced preferential flow to the tile drain occurred when the soil was in the dry state, but the flux was hydrologically insignificant. The thesis highlights: The need for significant sources of soil variation associated with loess stratigraphy to be represented in soil classification and mapping; that, while there was no un-drained control in the experimental set-up, the effects of the artificial drainage network appear to be profound and must be considerd in hydrological-contaminant modelling; and finally, that several common assumptions about flow pathways (i.e., mole channel fracture networks, negligible deep drainage, significant natural interflow, insignificant overland flow) in artificially drained, low permeability loess may be inappropriate and, consequently, their use in modelling in support of management and mitigation needs careful evaluation.