Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 3, 2000
Publication Date: April 1, 2001
Interpretive Summary: Soil water retention is an important property to estimate water available to crops, partitioning precipitation, partitioning into runoff and infiltration, and evaporation from soil. The overwhelming part of information on soil water retention has been obtained in laboratories from measurement on small samples. There have been indications that field water retention values may be significantly less than the laboratory ones. Our objective was to use a large database to compare field and laboratory data for the same soils. We worked with data on 135 samples of soils of different texture. The water retention was measured in tensiometric range, i.e., soil moisture ranged from high to moderate. We found that the differences between laboratory and field data can be large and have a random and a deterministic component. The random component was attributed to the spatial variability of soil properties. The deterministic component was found in soils having sand content less than 80%. This component resulted in field water retention 10 to 30% smaller than the laboratory one. We put forth an explanation based on differences in scale of measurements. Field water contents are measured with a neutron probe in volumes of soil that are much larger than the laboratory samples. Fractal scaling theory predicted a decrease in water contents with the size of the sample close to the one we found from our data. A scale-based correction needs to be made when soil water retention is estimated from other soil properties.
In laboratory water retention studies, soil water content and matric potential are measured in the same sample. In field measurements, water content has commonly been measured usually with neutron probes, and capillary pressure has been measured with tensiometers. Water content and capillary pressure are measured in different soil volumes and at different spatial scales in this case. The differences in scale and location of the two measurements in the field can potentially create differences in water retention data obtained in the field and in the laboratory. The objective of this work was to use a large database of 135 datasets to compare field and laboratory water retention. Coarse-textured soils (mostly sands, loamy sands) have the average difference between field and laboratory water contents close to zero. On the contrary, fine textured soils with sand content less than 50% have field water contents substantially smaller than the laboratory water contents in the range of water contents from 0.45 to 0.60 cc/cc. A polynomial regression explained 70% of variability in field water contents as computed from the laboratory data. A fractal scaling of the bulk density could be used to explain the observed "field - lab" differences in volumetric water contents in the range of high water contents. Pedotransfer functions built from the laboratory water retention data can overestimate available water content, and underestimate saturated hydraulic conductivity and sorptivity values. There is a need to include scale effects in pedotransfer functions.