Estimation and mapping of field capacity in Brazilian soils

Abstract

Modeling soil water dynamics is important to support decision in planning and management policies. Field capacity (FC) is an important parameter in this context and its proper determination allows the use of simple models to assess soil water balance components. In this study we aimed estimation and mapping of FC in Brazilian soils based on available soil global data and pedotransfer functions (PTF). Soil basic data were obtained from the SoilGrids system in a 0.05 × 0.05° spatial grid covering the Brazilian territory, with a resolution of approximately 5 km. Soil saturated hydraulic conductivity (Ks) and water retention curve (SWRC) were estimated by PTFs and used as input in the agro-hydrological model SWAP to simulate an internal drainage experiment without evapotranspiration, until the drainage at the bottom profile reached negligible flux density values (qfc) of: 0.25, 1.0 and 4.0 mm d−1, considered to correspond to FC. Simulations were performed considering three soil depths (zfc) of 30, 60 and 100 cm. FC was classified according to major soil textural and pedological classes. Uncertainty was evaluated by the sensitivity of the model to the soil hydraulic properties and the grid cell size. The highest soil matric potential at FC (hfc) was observed for loam texture while the lowest hfc was observed for sandy loam texture. Between soil pedological classes, Gleysols presented the lowest hfc. The results showed that FC estimation is sensitive to zfc and qfc. Spatial agreement between patterns of Ks, FC and soil class were observed according to the evaluated database. The qfc value of 4.0 mm d−1 presented reasonable results for estimating FC in Brazilian soils, in agreement with expected drainage time following saturation, with average values below 4 days for the evaluated depths and hfc around −30 cm. Estimation of FC with PTF showed underestimation of hfc varying from double to almost fourfolds times. The usually adopted hfc of −330 cm does not represent the FC for Brazilian soils. Furthermore, a qfc lower than 1.0 mm d−1 did not yield usable values for FC.