Estimation of field capacity for aggregated soils using changes of the water retention curve under the effects of compaction

The objective of this study is to present a new method for field capacity estimations using the CPV function (changes in pore volume). The method requires water retention data of an aggregated soil before and after a compaction event. The CPV function estimates the threshold of equivalent pore diameter (D-threshold) between structural and textural porosity using changes in the water retention curve. The D-threshold corresponds to a water potential associated with field capacity according to the TFC concept (textural field capacity), which assumes that (i) the textural pores define the field capacity conditions and (ii) the D-threshold and consequently the water potential at field capacity are constant parameters for each soil regardless of its compaction state. Published datasets for different aggregated soils subjected to different compaction processes and data from a lysimeter drainage experiment were used to evaluate the method. The analysis of the drainage experiment was supported by the BUDGETv.6.2 model, which was used to estimate the optimum value of the water content at field capacity (parameter of the model) using observed soil water contents and drainage rates during the drainage experiment (concept of inverse problem). The results for different soils subjected to compaction showed that the estimated water potentials by the CPV method follow the commonly accepted assumptions that govern field capacity conditions, justifying also the assumptions of the TFC concept. The field capacity conditions estimated by the model demonstrated an adequate correspondence to the results of the CPV method, indicating its ability to assess the field capacity conditions of the lysimeter soils.