In situ Field Capacity in Brazilian Soils and a Derived Irrigation Management Practice Based on Water Suction

Field capacity (FC) is a fundamental parameter in soil and water engineering and hydrologic modeling. Despite its relevance, the in situ determination of this parameter is not standardized and its determination by indirect methods is dubious. This study presents a method of calculation of in situ FC and its corresponding water suction (hFC), using the van Genuchten equation for water retention and the pedotransfer function by Ottoni Filho et al. (2016) for standardized in situ determination of FC. The methodology was applied to HYBRAS, a database of hydrophysical data for Brazilian soils with 1,075 soil samples from 15 Brazilian states. FC and hFC were confirmed to depend on textural class and pedogenetic origin (weathered and unweathered soils). Our analysis justified why FC must not be determined based only on a single predetermined water suction value. A simplified method is proposed for the management of irrigated soils through the determination of water suction in the root zone and the mode and confidence interval values of hFC corresponding to soil groups formed from textural classes and pedological nature. Various statistical calculations of FC and hFC are presented for these groups.