Field Capacity and Available Water Capacity

Plant-extractable water capacity of soil is the amount of water that can be extracted from the soil to fulfill evapotranspiration demands. It is often assumed to be spatially invariant in large-scale computations of the soil-water balance. Empirical evidence, however, suggests that this assumption is incorrect. In this paper, we estimate the global distribution of the plant-extractable water capacity of soil.

A representative soil profile, characterized by horizon (layer) particle size data and thickness, was created for each soil unit mapped by FAO (Food and Agriculture Organization of the United Nations)/Unesco. Soil organic matter was estimated empirically from climate data. Plant rooting depths and ground coverages were obtained from a vegetation characteristic data set. At each 0.5°×0.5° grid cell where vegetation is present, unit available water capacity (cm water per cm soil) was estimated from the sand, clay, and organic content of each profile horizon, and integrated over horizon thickness. Summation of the integrated values over the lesser of profile depth and root depth produced an estimate of the plant-extractable water capacity of soil.

The global average of the estimated plant-extractable water capacities of soil is 8ċ6cm (Greenland, Antarctica and bare soil areas excluded). Estimates are less than 5, 10 and 15 cm—over approximately 30, 60, and 89 per cent of the area, respectively. Estimates reflect the combined effects of soil texture, soil organic content, and plant root depth or profile depth. The most influential and uncertain parameter is the depth over which the plant- extractable water capacity of soil is computed, which is usually limited by root depth. Soil texture exerts a lesser, but still substantial, influence. Organic content, except where concentrations are very high, has relatively little effect.

Global distribution of plant‐extractable water capacity of soil

Modeling of soil water retention curve using soil solid phase parameters

Publisher Summary This chapter provides an overview on the way soil texture can be characterized and described, the way those data are considered in different pedotransfer functions (PTFs), and the methods that can be used to fill in missing data required by some PTFs. It also presents a study that compares different representations of soil particle-size distribution (PSD) in estimating soil water retention. Textural classification of soils is based on particle-size analyses in most parts of the world. A number of standards exist internationally for the description of soil PSD and particle-size classes. Soil texture classes are defined based on sand, silt, and clay content limits and are usually displayed in soil texture diagrams. Even when the same particle-size class system is used, texture class definitions may differ. Soil-texture-class information is primarily used as input to class PTFs. A class PTF does not use detailed particle-size data as input but gives calculated, usually averaged, values of the output variable in a tabular format for each soil class/unit. Detailed PSD and/or parameters derived from it are used in almost all PTFs. The chapter compares the usefulness of some of the most common representations of PSD in estimating water retention at -−10, −33, and −1500 kPa and the available water content (AWC) of the soil, which is defined as the difference between water contents at −33 and −1500 kPa matric potentials. To relate the above soil hydraulic properties to soil texture, a number of PTFs are developed using three databases and the group method of data handling (GMDH). The GMD and its standard deviation are also used as predictors.

Soil texture and particle-size distribution as input to estimate soil hydraulic properties

Quantifying the interactive effect of water absorbing polymer (WAP)-soil texture on plant available water content and irrigation frequency

Sands and loamy sands within the Glacial Lake Souris basin in North Dakota vary considerably from one area to another in their particle size distribution. The objective was to relate water retention of these sandy soil textural classes to particle size percentage. Water retention percentages, on an oven-dry basis, were determined for samples at field capacity under field conditions and also at 1/10-, 1/15-, and 1/20-bar soil water section for air-dried, < 2-mm samples in conventional porous ceramic plate-pressure pot equipment. No single soil water suction produced water retention values adequately representing field capacity for all textures. In most instances the percentage of very fine sand alone and in combinations with the percentages of silt and clay were significantly correlated with soil water values.

Soil Water Retention as Related to Particle Size in Selected Sands and Loamy Sands

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Soil Water Retention as Related to Particle Size in Selected Sands and Loamy Sands