An objective analysis of the dynamic nature of field capacity

Abstract

Field capacity is one of the most commonly used, and yet poorly defined, soil hydraulic properties. Traditionally, field capacity has been defined as the amount of soil moisture after excess water has drained away and the rate of downward movement has materially decreased. Unfortunately, this qualitative definition does not lend itself to an unambiguous quantitative approach for estimation. Because of the vagueness in defining what constitutes “drainage of excess water” from a soil, the estimation of field capacity has often been based upon empirical guidelines. These empirical guidelines are either time, pressure, or flux based. In this paper, we developed a numerical approach to estimate field capacity using a flux‐based definition. The resulting approach was implemented on the soil parameter data set used by Schaap et al. (2001), and the estimated field capacity was compared to traditional definitions of field capacity. The developed modeling approach was implemented using the HYDRUS‐1D software with the capability of simultaneously estimating field capacity for multiple soils with soil hydraulic parameter data. The Richards equation was used in conjunction with the van Genuchten‐Mualem model to simulate variably saturated flow in a soil. Using the modeling approach to estimate field capacity also resulted in additional information such as (1) the pressure head, at which field capacity is attained, and (2) the drainage time needed to reach field capacity from saturated conditions under nonevaporative conditions. We analyzed the applicability of the modeling‐based approach to estimate field capacity on real‐world soils data. We also used the developed method to create contour diagrams showing the variation of field capacity with texture. It was found that using benchmark pressure heads to estimate field capacity from the retention curve leads to inaccurate results. Finally, a simple analytical equation was developed to predict field capacity from soil hydraulic parameter information. The analytical equation was found to be effective in its ability to predict field capacities.

Parameterization of a bucket model for soil-vegetation-atmosphere modeling under seasonal climatic regimes

“…Therefore, Romano and Santini (2002) argued that a field drainage experiment is the test to be preferred for determining the water content value at field capacity, namely the condition when drainage fluxes become virtually nil during a redistribution process started from an initially saturated soil profile and with no flux from the soil surface. Using the HYDRUS-1D software to simulate field drainage experiments for real soils retrieved from different soil databases, Twarakavi et al (2009) confirmed the inaccuracy of determining field capacity from pre-fixed points of the water retention curve. They also suggested that the value of 0.01 cm day −1 can be considered as a negligible drainage flux being imposed at the lower limit of the rooting zone to meet the condition of field capacity for a wide range of soils.…”

“…The actual transpiration rate is then calculated on the basis of the actual soil water extraction rate over the whole rooting depth. Further details on this module of the model can be found in Kroes and van Dam (2003) and van Dam et al (2008). Of specific interest for this study is the semi-empirical way with which the uptake reduction function, α(h), is modeled by SWAP (see Fig.…”

“…2 Description of the models 2.1 The SWAP model SWAP (Soil, Water, Atmosphere and Plant) is a widespread used model in the soil hydrology community to simulate soil water flow in the vadose zone at field scale and during entire growing seasons (van Dam et al, 2008). The vertical soilwater movement is described by the Richards equation with a sink term accounting for the root water uptake, as follows:…”

Mapping soil water holding capacity over large areas to predict potential production of forest stands

“…However, this classical static approach based on benchmark pressure-can be criticized, field capacity being attached to water retention values rather than hydraulic conductivity characteristics of soils. The evaluation of SWHC using field capacity values defined as the soil water content when the drainage flux become negligeable could be a solution to overcome these limits (Twarakavi et al, 2009). Improvements could also be made by using the integral energy principle, a method based on the entire soil water retention curve to evaluate the energy required by plants to remove soil water, avoiding to consider water between the two potentials is equally available (Minasny and McBratney, 2003).…”

Revisiting Field Capacity (FC): variation of definition of FC and its estimation from pedotransfer functions

“…Based on this dependence and on the operational difficulties of the field test, FC is also commonly evaluated in a laboratory setting as the water content of undisturbed soil samples (or disturbed samples, which is less acceptable) at a specific soil water matric potential. Cassel & Nielsen (1986), Romano & Santini (2002) and Nemes et al (2011) reported that a wide range of matric potentials (from -2.5 to -50 kPa) have been used for this purpose, although suctions of 5, 6, 10, and 33 kPa are more common choices; however, there is no satisfactory general criterion for the selection of the suction values for this kind of FC determination (Hillel, 1998;Twarakavi et al, 2009;Nemes et al, 2011). Taking into account the dynamic nature of drainage in field FC evaluations, some authors (Nachabe et al, 2003) argue that the definition of FC must be based on a particular choice for the "negligible" downward flux, instead of the usual drainage time of two or three days, or the suction at FC.…”