Flux-Based Estimation of Field Capacity

Field capacity has been defined as the water content at which the drainage flux from a soil ceases, or becomes negligible. Field capacity is most commonly estimated as the water content at a pressure head of 1/3 bar, which ignores its flux-based nature. A number of authors have proposed estimating field capacity as the water content at a given (negligible) flux, assuming unit gradient conditions. This flux-based method of estimating field capacity is discussed and the differences between this method and the more common pressure-based method are illustrated. The drainage flux considered to be negligible will depend on the particular application; values between 10⁻⁶ and 10⁻⁸ cm/s were considered here. The observed differences in field capacity resulting from the negligible flux range considered produce significant differences in the available water capacity. These differences directly impact the calculated drainage. The effect of available water capacity on the water budget for waste disposal facility covers at an arid and humid site was illustrated using the HELP code. At the arid site, drainage decreased by a factor of 4 (from 25 to 6 mm/year) as the available water capacity increased from 0.101 to 0.199. At the humid site, drainage decreased from 190 to 173 mm/year over the same range of available water capacity. Finally, a simple procedure for the determination of an appropriate value for the negligible flux is discussed. Analysis of data from a lysimeter at Coshocton, Ohio, indicated that field capacity for the lysimeter soils was associated with a negligible drainage flux of about 3 × 10⁻⁷ cm/s.