Hydrodynamic characterisation of soil subsurface flows for water conservation purposes.

Osame, Peter Uloho

Abstract

This study focuses on the hydrodynamic characterisation of soil subsurface flows using developed new soil column experimental setup to assess the hydraulic parameters of unsaturated soil in the laboratory. The soils in the inland valley of Nigeria's Niger Delta, where agriculture uses more than one-third of the clean water available, is the subject of the study. The Nigerian agriculture industry needs more effective water management as, when it comes to irrigation, flood irrigation being the most widely used technique, farmers primarily rely on experience rather than empirical data. The study seeks to provide a better understanding of the complex hydrodynamics of water flow through soil subsurface. A purpose-built cylindrical vertical soil column rig is constructed, and the volumetric soil water content θ (%) and soil matric potential Ψ (kPa) are measured using precision sensors. The soil water characteristics curve is obtained as a relationship between the volumetric soil water content and the soil matric potential. Gravity-driven flow experiments are conducted on two distinct monoliths of undisturbed soil samples from Ivrogbo and Oleh in the Niger Delta inland valley of Nigeria, and a homogeneous packed sample of the soil from Aberdeen in the United Kingdom for comparative purposes. Experiments are carried out on the monoliths of undisturbed soil samples once in each case, whereas experiment is conducted on the packed sample before it is further agitated to simulate ploughing and subsequent infiltration experiments are carried out on it, making four samples on the whole. The Van Genuchten model of the soil water characteristics curve has been used for the verification of the experimental data. The volumetric soil water contents and soil matric potentials of the four samples are compared at different depths, which reveal a marked difference in their behaviour. Nonetheless, the range of values is smaller than the predicted curve. At 200 mm depth, the value of n is noticed to be 15 with 𝜃𝑟 of 0.046 and 𝜃𝑠 of 0.23 for the packed soil sample, giving a percentage difference of 86.7 % compared to n equal to 2 in the Van Genuchten curve. Also, for the ploughed sample, n equals 10 giving a percentage difference of 80 % but 𝜃𝑟 = 0.03 while 𝜃𝑠 = 0.23. For the Ivrogbo sample and Oleh samples, the range of the matric potential is relatively too small for the comparison. The pre-experiment moisture content of the soil samples is part of the cause of this, in addition to differences in the soil type. Furthermore, the findings demonstrate a strong agreement between the measured behaviour and the predicted technique of the soil water characteristics curve. The results also show that ploughing improves soil homogeneity and uniform flow, which makes it more suitable for irrigation in agricultural settings. Additionally, the Oleh region soil sample needs less water to reach field capacity (i.e. the EU standard for crop comfort zone) and has the lowest negative matric potential (good water retention capabilities). Based on these results, farmers in the Oleh region can readily implement drip micro-irrigation.

Citation

OSAME, P.U. 2024. Hydrodynamic characterisation of soil subsurface flows for water conservation purposes. Robert Gordon University, PhD thesis. Hosted on OpenAIR [online]. Available from: https://doi.org/10.48526/rgu-wt-2795739

• Thesis Type: Thesis
• Deposit Date: Apr 18, 2025
• Publicly Available Date: Apr 18, 2025
• DOI: https://doi.org/10.48526/rgu-wt-2795739
• Keywords: Hydrodynamics; Hydraulics; Soil water content; Soil water retention; Irrigation
• Public URL: https://rgu-repository.worktribe.com/output/2795739
• Award Date: Oct 31, 2024