Relations between available and extractable soil water and evapotranspiration from a bean crop

• Published in: Agricultural water management 1984-01, Vol.9 (3), p.193-209
• Main Authors: Reid, J.B., Hashim, O., Gallagher, J.N.
• Format: Article
• Language: English
• Subjects: Agricultural and forest climatology and meteorology. Irrigation. Drainage; Agricultural and forest meteorology; Agronomy. Soil science and plant productions; Biological and medical sciences; Fundamental and applied biological sciences. Psychology; General agronomy. Plant production; Water balance and requirements. Evapotranspiration
• ISSN: 0378-3774; 1873-2283
• DOI: 10.1016/0378-3774(84)90002-7

The accuracy of ‘available’ and ‘extractable’ soil water estimates was investigated using irrigated and unirrigated beans ( Vicia faba) grown in an alluvial silt loam in Canterbury, New Zealand. Available water capacity was defined as the difference between soil water contents in the root zone at the drained upper limit (DUL) and at the lower limit (LL) as estimated by laboratory procedures. Extractable water capacity was specified as the difference between field estimates of DUL and LL for the whole profile affected by roots. DUL was estimated in the laboratory by equilibrating soil cores at matric potentials at −10, −20 or −30 kPa, and in the field by neutron moderation. Laboratory estimates of LL were made from soil samples equilibrated at −1.5 MPa matric potential. In the field LL was measured by neutron moderation on plots where evaporation had apparently ceased due to drought stress. When compared at intervals down the profile laboratory estimates of DUL and LL showed poor agreement with field observations. However, the final estimates of available and extractable water capacities were similar because of compensatory inaccuracies in the laboratory estimates. Furthermore, field measurements of evapotranspiration, using neutron moderation and tensiometry, indicated that the accuracy of the available water estimates was much reduced by upward fluxes of water into the rooting zone. These fluxes resulted in water extraction to at least 1.0 m although the apparent maximum rooting depth (measured by counting roots washed from soil cores) was only 0.7 m. Particular attention was paid to the influence of subsoil textural variability, which is pronounced in such soils. Laboratory and field estimates of the LL had to be carefully matched texturally before relevant comparisons could be made. Problems associated with subsoil textural variability affected laboratory methods of DUL estimation more than field methods.