8.3 Field Capacity
The concept of field capacity has often been used in irrigation planning, crop modeling, and hydrologic modeling. Field capacity is thought to be the volumetric water content at which drainage effectively ceases. This hypothetical water content value is often incorrectly considered a property of the soil. However, the whole field capacity concept is flawed because, as we have just seen, there are typically no clearly defined breakpoints at which drainage stops. For example, in a drainage experiment on a silt loam soil in Israel, the gravimetric water content in the 60- to 90-cm depth decreased from 0.29 g g-1 at the start of drainage, to 0.20 g g-1 after 1 day, 0.19 g g-1 after 2 days, 0.16 g g-1 after 30 days, and 0.15 g g-1 after 60 days [7]. At which of these water contents could we accurately say that drainage has ceased?
Procedures have been developed for estimating field capacity in the field and in the laboratory. The most common way to estimate field capacity is to assume that it is equal to the water content retained in the soil at a specific matric potential. Research has proven repeatedly that there is not any one matric potential value which universally represents field capacity, but the convenience of this approach to estimating field capacity leads to its continued use. In the United States, a matric potential of -33 kPa has most commonly been used for this purpose. In some other countries, a value of -10 kPa has been more widely-used. Recent research has shown that -33 kPa provides a poor approximation of field capacity in most cases, and less negative values of matric potential such as -10 or even -6 kPa appear to be more suitable [8, 9]. Alternative approaches for estimating field capacity based on the parameters of the soil water retention curve have also been proposed [10, 11].