Improved understanding of soil water content at field capacity and estimates from pedotransfer functions

The soil water content at field capacity, θ_FC, is a fundamental variable for irrigation and agriculture. This study determines the optimal tension (−33 or −10 kPa) of deriving θ_FC which best matches the in situ field measurements following the Veihmeyer procedure. The Veihmeyer method refers to a profile water status for which there is a negligible drainage rate of bare soil without evaporation. In addition, we derive a set of linear and nonlinear pedotransfer functions (PTFs) which estimate θ_FC. θ_FC was measured in 69 plots in north-western Iran, which has a cold semi-arid climate. The soil properties used for developing PTFs include texture, bulk density, and organic matter. The results show that θ_FC cannot be derived at a fixed tension. We therefore developed PTFs with satisfactory performance in reproducing in situ measured θ_FC. The findings show that PTFs developed at a fixed tension consistently underestimate θ_FC derived in situ. It was also speculated that tension less than −10 kPa could yield improved predictions of in situ θ_FC.