The safe application of farm dairy effluent (FDE) to land has proven to be a challenge for dairy farmers and regulatory authorities throughout New Zealand. Poorly performing FDE systems can have deleterious effects on water quality because contaminants such as phosphorus, nitrogen and faecal microbes enter receiving waters with minimal attenuation by soil. We present a decision framework that supports good management of effluent, particularly during its application to land. The framework considers how FDE management can be tailored to account for soil and landscape features of a location that pose varying levels of contaminant transport risk. High risk soils and landscapes are vulnerable to direct losses via preferential and/or overland flow pathways and include sloping land (e.g. slopes greater than 7 degrees) and soils with mole drainage, coarse structure, poor natural drainage or low surface infiltration rates. Soil types that are well-drained with fine structure typically exhibit matrix flow characteristics and represent a relatively low risk of direct contaminant loss following FDE application. Our framework provides guidance on FDE application timings, rates and depths to different landform and soil types so that direct losses of contaminants to water are minimal and the opportunity for plant uptake of nutrients is enabled. Some potential limitations for using the framework include the potentially severe effects of animal treading damage during wet conditions that can reduce soil hydrological function and consequently increase the risk of overland flow of applied FDE. The spatial distribution of such treading damage should be considered in the framework's application. Another limitation is our limited understanding of the effects of soil hydrophobicity on FDE infiltration and application of the framework.

Reynolds number (Re), pore water pressure (P), and water flow shear force (tau) are primary indicators reflecting the characteristics of subsurface flow. Exploring the calculation of these parameters will facilitate the understanding of the hydrodynamic characteristics in different subsurface flows and quantify their differences. Hence, we conducted a study to monitor soil water content, matrix potential, and pore water pressure in two typical soil profiles (with and without fissures). The distribution of Re, P, and tau in both matrix flow (MF) and preferential flow (PF) were calculated with an improved calculation method, focusing on their energy changes. Results showed that these hydrologic parameters are quite different between MF and PF. Re values in MF remained below 0.1, indicating lower water flow velocities, while the Re values ranged from 0.8 to 2 in PF, indicating higher flow velocities. The P values in PF was tens to hundreds of times higher than that in MF, which is mainly due to the rapid accumulation and leakage of water within soil fissures. Additionally, the larger hydraulic radius and gradient in PF also resulted in higher tau values in PF (2 similar to 6 N m(-2)) than in MF (0 similar to 1.5 N m(-2)). In PF, the pressure potential was the significant factor for tau, while tau in MF was dominated by the matrix potential and varies with the magnitude of the matrix potential gradient. This study suggests that Re, P, and tau could be considered as the major indexes to reflect dynamic characteristics of subsurface flow.

This study examines the influence of preferential flow (PF) on seepage under different rainfall infiltration scenarios, addressing a critical gap in current modeling practices, which often overlook the interactive dynamics between matrix flow (MF) and PF domains within soil environments. In this study, an integrated saturated and unsaturated subsurface flow of dual-permeability (DP) model is developed to calculate seepage and slope stability using pore water pressure. This study aims to conduct numerical experiments of shallow landslides induced by rainfall to quantify the temporal and spatial impact of preferential flow on hydrological mechanisms and slope stability. For low-rainfall intensity, the variation in pore water pressure is greater in the MF domain than in the PF domain. 90 % of rainwater infiltrates downward through the MF domain. Water exchange predominantly occurs in the PF domain, as opposed to the MF domain. The factor of safety decreases from 1.61 to 1.55 when comparing before and after rainfall, which reduces by 3.73 %. For high-rainfall intensity, the pore water pressure variation in the PF domain is more pronounced than in the MF domain. The entirety of precipitation infiltration downwards through the PF domain. Water exchange mainly flows from the PF domain to the MF domain. The factor of safety decreases from 1.61 to 1.45 when comparing before and after rainfall, resulting in a reduction of 9.94 %.