Ninghu Su

Oxygation Unlocks Yield Potentials of Crops in Oxygen‐Limited Soil Environments

Subsurface drip irrigation (SDI) offers well‐documented potential for improving water use efficiency in irrigated agriculture. However, SDI in common with other forms of irrigation is liable to exclude soil air (and therefore oxygen) around the root zone during and following irrigation events, thus reducing root function and crop performance. When SDI is practiced with oxygation (i.e., aerating the rhizosphere by way of the irrigation stream) it could transform the irrigation industry, for it provides a source of oxygen in a root environment that suffers from temporal hypoxia, and occasionally from anoxia. The oxygen is introduced into the irrigation stream by way of the venturi principle, or with solutions of hydrogen peroxide. Oxygation assures optimal root function, microbial activity, and mineral transformations, and leads to enhanced yield and water use efficiency under hypoxic conditions. It also improves plant performance and yield under irrigated conditions previously considered to be satisfactory for crop growth, and offers scope to offset some of the negative impacts of compaction and salinity, related to poor soil aeration, on crop growth. Representing minimal capital investment and recurrent costs, economic returns appear very favorable, as do associated benefits to the environment, measured as reduced drainage, containment of rising water tables, better nutrient use efficiency, and reduced demand by agriculture for irrigation water. The aeration status of irrigated soils deserves more attention than it has received in the past if we wish to unlock yield potential constrained by soil oxygen limitations and effect the yield increases essential to keeping pace with future food (and fibre) demand.

Soil pH buffering capacity: a descriptive function and its application to some acidic tropical soils

Calculation of soil acidification rates requires knowledge of pH buffering capacity (pHBC), which is measured using titration methods. The pHBC is often quoted as a single value for a particular soil, implying a linear relationship between pH and the amount of acid or alkali added. However, over its whole range, the relationship is sigmoid rather than linear, and in many soils pH is low or high enough to be outside of the linear range. In this work we fitted a simple sigmoid function to pH buffer curves of 8 tropical Australian soils obtained using one titration method and 58 Papua New Guinean (PNG) soils obtained using another titration method. The function described the curves well for all soils (adjusted r2 > 0.93 for all samples and >0.99 for 90% of samples), irrespective of the titration method, allowing pHBC to be calculated as a function of pH across the range of pH values established. Using the function, the contribution of variable charge to pHBC was calculated for the PNG soils; on average it was 93% at the pH buffer curves’ inflection point, which corresponds with the soil’s minimum pHBC. Factors other than variable charge became important at pH (1 : 5, 0.002 M CaCl2) values 6.0. The relationship between pHBC and soil organic matter content was closest at pH 6.0–6.5. Application of the sigmoid function could facilitate more accurate assessments of acidification risks, acidification rates, and potential management interventions, particularly as soils become increasingly acidic.

Tides as phase-modulated waves inducing periodic groundwater flow in coastal aquifers overlaying a sloping impervious base

This paper presents procedures for analysing the interactions between tidal waves and coastal aquifers overlaying a sloping impervious bed, using Boussinesq equation and wave equation. Fourier series solutions of a linearised Boussinesq equation are presented subject to a periodic boundary condition (BC). The periodic BC is a phase-modulated periodic solution of the wave equation which has been shown to satisfactorily simulate uneven twin peaks of semi-diurnal tides as observed in a study area on the east coast of Queensland, Australia. Numerical analyses show that the Fourier series solutions of Boussinesq equation subject to periodic BCs reveal two important features of the tidal waves. First, the tidal waves damp towards landward, and second, the half amplitude of the tide above the mean sea level is greater than that below it. While the first feature is clearly expected, the second feature is physically more meaningful and important, and is confirmed by the field data.

Analysis of turbulent flow patterns of soil water under field conditions using Burgers equation and porous suction-cup samplers

This paper presents results using the Burgers equation in spherical coordinates for the investigation of soil water dynamics around porous cup samplers under field conditions. The analysis reveals that the yield (V) of the porous suction-cup sampler is a power function of inflow duration (T) and source moisture strength (A). It also shows that the shock front velocity and moving boundary of the front are both power functions of A and time. The analysis further demonstrates that the asymptotic shock front velocity derived from a similarity solution is the same for different versions of Burgers equation, indicating the validity of different versions of Burgers equation for the same flow patterns. The observation using time domain reflectometry shows that a moisture shock was induced around the sampler following application of a vacuum, which justifies the application of Burgers equation. The values of A were derived using field data to determine the location and the front velocity of the shock.

Generalisation of various hydrological and environmental transport models using the Fokker–Planck equation

Abstract The Fokker–Planck equation (FP) and its special derivatives, the generalised Feller equation (GF) and the advection equation (AE), have been shown to be universal in linking many hydrological and environmental transport models. A similarity solution of the GF is a hypergamma function, which is shown to generalise the various similarity solutions of different transport models. The hydrological and various transport models generalised using FP include the models for: (1) groundwater flow; (2) generalised diffusion wave with inertial effects for open-channel flows and flood waves; (3) flood waves, shock waves, kinetic overland and saturated subsurface flows; (4) solute transport in groundwater in fractal porous media; (5) isothermal gas flow through porous media; (6) moisture and solute transport in unsaturated soils; (7) transport and distribution of sediment in stream channels and movement of stream beds; (8) hillslope erosion; (9) dynamic rainfall and cloud fields; (10) transport in stream channels subject to estuarine tidal effect, and (11) transport in the atmospheric boundary layer.

A two-dimensional finite volume method for transient simulation of time- and scale-dependent transport in heterogeneous aquifer systems

In this paper, solute transport in heterogeneous aquifers using a modified Fokker-Plauck equation (MFPE) is investigated. This newly developed mathematical model is characterised with a time-, scale-dependent dispersivity. A two-dimensional finite volume quadrilateral mesh method (FVQMM) based on a quadrilateral background interpolation mesh is developed for analysing the model. The FVQMM transforms the coupled non-linear partial differential equations into a system of differential equations, which is solved using backward differentiation formulae of order one through five in order to advance the solution in time. Three examples are presented to demonstrate the model verification and utility. Henrys classic benchmark problem is used to show that the MFPE captures significant features of transport phenomena in heterogeneous porous media including enhanced transport of salt in the upper layer due to its parameters that represent the dependence of transport processes on scale and time. The time and scale effects are investigated. Numerical results are compared with published results on the same problems.

N-dimensional fractional Fokker-Planck equation and its solutions for anomalous radial two-phase flow in porous media

In this paper we present a time fractional Fokker-Planck equation (fFPE) for radial two-phase flow of liquid and gas in porous media. The fFPE of order @a is solved for both two- and three-dimensional flow patterns using the Laplace transform method. The general solutions of the fFPE for both two- and three- dimensional flows are given as a convolution integral of the input and a kernel in the Laplace domain. Special solutions for a large value and a periodic boundary condition are also given in the time domain when the inverse Laplace transform can be found analytically. The fFPE for two-phase flow in porous media presented in this paper is the first report of its kind.

Sustainable Irrigation to Balance Supply of Soil Water, Oxygen, Nutrients and Agro-Chemicals

The socio-economic pressure for improvements in irrigation efficiencies is increasing due to intense competition for water between agricultural, domestic and industrial users as well as demands for compliance with environmental regulations. Precision irrigation technology involving less irrigation water and uniform application across the field is therefore important. In the context of declining water allocation for irrigation and the variations in weather and drought patterns attributed to global climate change, efficient and precise applications are necessity. Traditional irrigation methods such as furrow, flood and sprinkler are neither efficient nor environmentally benign. Precision irrigation methods such as drip and subsurface drip irrigation are advocated because they are more water use efficient and because they offer a possible approach to meet projected food demand, a doubling by the 2050. In spite of greater water use efficiency afforded by minimal soil surface evaporation and deep drainage, and ease of automation, wide scale adoption of surface and subsurface drip irrigation technology is limited. This is due to their high investment cost for installation. They often lack a significant yield benefit when compared to conventional irrigation practice. Reasons are probably linked to a sustained wetting front around emitters. These emitters impose a condition of low oxygen content in the root-dense rhizosphere surrounding emitters that impede root respiration, and negatively impact on plant uptake of water and nutrients, leading to constrained yield performance.

Fractal tidal waves in coastal aquifers induced both anthropogenically and naturally

In this paper, we present a methodology developed for quantifying fractal properties of salt concentration profiles induced anthropogenically in coastal aquifers of Bundaberg, Queensland, Australia. Based on data observed in the field and multifractal models, it is found that salt profiles induced in the coastal aquifers are periodic fractals, and the fractal dimensions of the salt profiles and associated tidal waves are predominant in urban and industrial areas where frequent human activities impose severe impact on the waveforms. It is found that the fractal tidal waves are heterogeneous as indicated by variable dimensions of the waveforms at different orders. It is also shown that the results from multifractal analysis are more consistent and reliable than those from spectral analysis. The methods developed in this paper can also be applied to characterise time series data in other fields such as data on hydrographs, water levels, pollutant and air quality etc.

Modified Richards equation and its exact solutions for soil water dynamics on eroding hillslopes

The modified Richards equation (MRE) is presented by using a rotated coordinate system to accommodate the geometry of a hillslope and a moving boundary to represent an eroding surface on a hillslope. Exact analytical solutions of MRE are developed subject to Fujitas [1952] diffusivity and Sander et al.s [1988] unsaturated hydraulic conductivity. The mathematical analysis presented here for soil water dynamics and infiltration in particular on an eroding hillslope deviates from the traditional way in which infiltration has been investigated since Green and Ampts [1911] pioneering work. The MRE clearly improves mathematical representation of physical reality. Field data are used to derive parameters in a solution of MRE to illustrate the effect of erosion rates on soil moisture profiles in a moving boundary.