Calculation of the field volumetric water content of cracking clay soils from measurements of gravimetric water content and bulk density

Uncertainty in estimating water use in shrinking soils from changes in gravimetric water content arises from the difficulty in ensuring that samples are taken from comparable depths at different sampling times, and of identifying the correct bulk density for the conversion to volumetric water content. Equations which express the amount of water held by the same mass of soil solid matter during soil drying and shrinking are derived for two models. In one the soil properties vary continuously with depth and in the other the values are averages for layers. The models are applied to field water content measurements made in the Sudan Gezira. Systematic errors (biases) are examined and their magnitudes calculated. The maximum overestimate of gravimetric water content arising from ignoring vertical shrinkage on drying was 2.3%. The maximum overestimate of volumetric water content from ignoring volumetric contraction was 24%; this was corrected by using for all sampling times bulk densities of layers before contraction.