Keith L. Bristow

An overview of APSIM, a model designed for farming systems simulation

The Agricultural Production Systems Simulator (APSIM) is a modular modelling framework that has been developed by the Agricultural Production Systems Research Unit in Australia. APSIM was developed to simulate biophysical process in farming systems, in particular where there is interest in the economic and ecological outcomes of management practice in the face of climatic risk. The paper outlines APSIMs structure and provides details of the concepts behind the different plant, soil and management modules. These modules include a diverse range of crops, pastures and trees, soil processes including water balance, N and P transformations, soil pH, erosion and a full range of management controls. Reports of APSIM testing in a diverse range of systems and environments are summarised. An example of model performance in a long-term cropping systems trial is provided. APSIM has been used in a broad range of applications, including support for on-farm decision making, farming systems design for production or resource management objectives, assessment of the value of seasonal climate forecasting, analysis of supply chain issues in agribusiness activities, development of waste management guidelines, risk assessment for government policy making and as a guide to research and education activity. An extensive citation list for these model testing and application studies is provided.

Comparison of different approaches to the development of pedotransfer functions for water-retention curves

Abstract Pedotransfer functions (PTFs) for estimating water-retention from particle-size and bulk density are presented for Australian soil. The water-retention data sets contain 733 samples for prediction and 109 samples for validation. We present both parametric and point estimation PTFs using different approaches: multiple linear regression (MLR), extended nonlinear regression (ENR) and artificial neural network (ANN). ENR was found to be the most adequate for parametric PTFs. Multiple linear regression cannot be used to predict van Genuchten parameters as no linear relationship was found between soil properties and the curve shape parameters. Using the prediction set, ANN performance was similar to the ENR performance for the prediction dataset, but ENR performed better on the validation set. Since ANN is still considered as a black-box approach, the ENR approach which has a more physical basis is preferred. Point estimation PTFs were estimated for water contents at −10, −33 and −1500 kPa. Multiple linear regression performed better for point estimation. An exponential increase trend was found between particles

Current Capabilities and Future Needs of Root Water and Nutrient Uptake Modeling

The importance of root function in water and nutrient transport is becoming increasingly clear, as constraints on agricultural resources are imposed due to water limitations and environmental concerns. Both are driven by the increasing need to expand global food production. However, the historical neglect of consideration of water and nutrient uptake processes below ground has created a knowledge gap concerning the plant responses of nutrient and water limitations to crop production. The review includes sections on (i) notation and definitions of water potential, (ii) the physical coupling of plant transpiration and plant assimilation by way of the principles of diffusion of water vapor and carbon dioxide, (iii) apoplastic and symplastic water and nutrient pathways in plants, (iv) active and passive nutrient uptake, and (v) a discussion of the current state-of-the-art in multidimensional soil water flow and chemical transport modeling. The subsequent review of water uptake, nutrient uptake, and simultaneous water and nutrient uptake addresses shortcomings of current theory and modeling concepts. The review concludes with an example illustrating a possible multidimensional approach for simultaneous water and nutrient uptake modeling. Specific recommendations identify the need for coupling water and nutrient transport and uptake, including salinity effects on root water uptake and the provision of simultaneous passive and active nutrient uptake. It considers the requirement for multidimensional dedicated root water and nutrient uptake experiments to validate and calibrate hypothesized coupled root uptake models.

Indirect estimation of soil thermal properties and water flux using heat pulse probe measurements: Geometry and dispersion effects

from 1.0 to >10 m d � 1 . We also demonstrate the general application of inverse modeling to estimate soil thermal properties and their functional dependence on volumetric water content in a separate numerical experiment. We suggest that inverse modeling of HPP temperature data may allow simultaneous estimation of soil water retention (when combined with matric potential measurements) and unsaturated hydraulic conductivity (through water flux estimation) from simple laboratory experiments. INDEX TERMS: 1866 Hydrology: Soil moisture; 1875 Hydrology: Unsaturated zone; 1894 Hydrology: Instruments and techniques; KEYWORDS: Soil water flow; soil heat flow; inverse modeling; dispersivity

Measurement of thermal properties and water content of unsaturated sandy soil using dual-probe heat-pulse probes

Abstract Soil thermal property data, especially as a function of water content, are currently not readily available. Demand for these data is, however, on the increase because of improvements in and wider applications of soil heat and water transport models. Small dual-probe sensors have been developed that will assist to overcome this shortage in soil thermal property data, and in this paper, we demonstrate their capability through discussion of measurements carried out on unsaturated sandy soil which was subjected to a wetting and drying cycle. The dual-probes employ heat-pulse methodology and yield the soil thermal diffusivity, heat capacity and conductivity from a single heat-pulse measurement. Thermal properties measured in this study are compared with independent estimates made using standard procedures from the literature. These standard procedures require knowledge of the soil mineralogy, and our dual-probe measurements highlight the fact that we cannot always rely on particle size data for accurate mineralogical information. The consequences of using inappropriate mineralogical data and hence, incorrect thermal properties in soil physical analyses, are illustrated. We also show how volumetric water content can be determined from dual-probe heat capacity measurements and other basic soil data (bulk density and specific heat). These data showed the presence of strong hysteresis in the water retention of the material used in this study, and highlight the fact that the dual-probes have an important role to play in monitoring soil water content as well as providing soil thermal property data.

Soil surface heat flux: some general questions and comments on measurements

Abstract Soil surface heat flux is often measured incorrectly owing to a lack of understanding of the processes occurring at the soil surface. To determine accurately soil surface heat flux from measurements of heat flux at some depth below the surface, both heat storage above the plate and latent heat loss from below the plate must be taken into account. Large errors can be introduced if heat storage is neglected, and even larger errors can be found if latent heat processes are ignored. In this paper we describe a general framework for the correct interpretation of field measurements of soil heat flux.

A small multi-needle probe for measuring soil thermal properties, water content and electrical conductivity

Abstract Ready access to reliable low cost instrumentation for use in laboratory and field experimentation remains a priority for many working in the soil and environmental sciences area. In this paper we provide an overview of the multi-needle heat-pulse probe which can now provide measurements of soil temperature, soil thermal diffusivity, volumetric heat capacity, thermal conductivity, volumetric water content, and bulk soil electrical conductivity, all at the same position and time. We show that the multi-needle probe can provide high quality measurements, in some cases as good as or better than those obtained using other current methodology. As an example, multi-needle probes have been shown to yield measurements of volumetric water content with a root mean square error of 0.01 m 3 m −3 . Because of its small size the multi-needle probe should prove particularly useful for measurements near the soil surface, near to plant roots, and in other situations requiring fine spatial resolution in measurements. These probes can also be easily automated for unattended use to facilitate data collection as a function of time and space. We also highlight issues that need further analysis and research in helping guide further development of more robust multi-needle probe designs, especially for field applications.

Crop residue effects on surface radiation and energy balance — review

SummaryCrop residues alter the surface properties of soils. Both shortwave albedo and longwave emissivity are affected. These are linked to an effect of residue on surface evaporation and water content. Water content influences soil physical properties and surface energy partitioning. In summary, crop residue acts to soil as clothing acts to skin. Compared to bare soil, crop residues can reduce extremes of heat and mass fluxes at the soil surface. Managing crop residues can result in more favorable agronomic soil conditions. This paper reviews research results of the quantity, quality, architecture, and surface distribution of crop residues on soil surface radiation and energy balances, soil water content, and soil temperature.

Soil-dependent wetting from trickle emitters: implications for system design and management

For trickle irrigation systems to deliver improved water- and nutrient-use efficiency, distance between emitters and emitter flow rates must be matched to the soils wetting characteristics and the amount and timing of water to be supplied to the crop. Broad soil texture ranges (e.g. sand, loam, clay) are usually the only information related to soil wetting used in trickle system designs. In this study, dimensions of wetted soil were calculated from hydraulic properties of 29 soils covering a wide range of textures and soil hydraulic properties to assess the impact of soil texture and/or type on soil wetting patterns. The soils came from two groups that differed in the extent to which hydraulic properties depended on soil texture. Vertical and radial distances to the wetting front from both surface and buried emitters were calculated for conditions commonly associated with daily irrigation applications in a widely spaced row crop (sugarcane) and horticultural crops. In the first group of soils, which had least expression of field structure, the wetted volume became more spherical (i.e. the wetted radius increased relative to the depth of wetting below the emitter) with increasing clay content, as is commonly accepted. However, in the second group of soils in which field structure was preserved, there was no such relationship between wetted dimensions and texture. For example, five soils with the same texture had as great a variation in wetting pattern, as did all 11 soils in the first group, indicating the considerable impact of field structure on wetting patterns. The implications of the results for system design and management were illustrated by comparing current recommendations for trickle irrigation systems in coastal northeastern Australia with the calculated wetted dimensions. The results suggest that (1) emitter spacings recommended for sugarcane are generally too large to allow complete wetting between emitters, and (2) the depth of wetting may be greater than the active root zone for both sugarcane and small crops in many soils, resulting in losses of water and chemicals below the root zone. We conclude that texture is an unreliable predictor of wetting and there is no basis for adopting different dripper spacing in soils of different textures in the absence of site-specific information on soil wetting. Such information is crucial for the design of efficient trickle irrigation systems.

How useful are small-scale soil hydraulic property measurements for large-scale vadose zone modeling?

A major challenge that recurs throughout the geophysical sciences is the downscaling (disaggregation) and upscaling (aggregation) of flow or transport processes and their measurement across a range of spatial or temporal scales. Such needs arise, for example, when field-scale behavior must be determined from soil hydraulic data collected from a limited number of in situ field measurements or analysis of small soil cores in the laboratory. The scaling problem cannot be solved by simple consideration of the differences in space or time scale, for several reasons. First, spatial and temporal variability in soil properties create uncertainties when changing between scales. Second, flow and transport processes in geophysics and vadose zone hydrology are highly nonlinear. We present a historical overview of the theory of scaling procedures, and demonstrate the application of various aggregation techniques, such as scaling and inverse modeling, to aggregate laboratory-scale soil hydraulic properties to larger scale effective soil hydraulic properties. Examples of application of these aggregation techniques from the pore scale to the watershed scale are demonstrated. We conclude that the development of new instrumentation to characterize soil properties and their variation across spatial scales is crucial. Moreover, the inherent complexity of flow in heterogeneous soils, or soil-like materials, and the need to integrate theory with experiment, requires innovative and multidisciplinary research efforts to overcome limitations imposed by current understanding of scale-dependent soil flow and transport processes.