One of the main crop production factors is soil water availability; therefore, determination of soil water inputs and outputs by the water balance method can be a valuable tool for improving crop management. The aim of this study was to assess soil water balance components in the field (soil water storage variations, drainage, capillary rise and evapotranspiration) and the root system profile of Brachiaria decumbens for nine months in 2011 (March to November). The ratio between evapotranspiration (ET) and reference evapotranspiration (ETo) was evaluated as well. The study was carried out in an experimental area at the "Riacho do Papagaio" farm, municipality of São João, state of Pernambuco, Brazil. TDR probes were set up at depths of 0.10, 0.20, 0.30 and 0.40 m to measure soil water content. Rainfall was monitored with a rain gauge installed on a tower in the center of the area. For the determination of drainage and capillary rise, infiltration tests were performed in the field to obtain the saturated hydraulic conductivity. The soil water retention curve was determined in the laboratory. ET was obtained as a residual term of the water balance equation and ETo was obtained by the Penman-Monteith method. The upper limit of the control volume used for water balance was the soil surface, and the lower limit was a plane parallel to the soil surface at a depth of 0.30 m. Each month, the root system profile was analyzed. It was observed that substantial soil water storage, drainage and ET values occurred in the high rainfall periods. Water flux through the plane of the lower limit (z = 0.30 m) occurred only downwards, with a loss of 103.14 mm by drainage, which is 24.12 % of the water supplied to the crop. The total value and average value for actual evapotranspiration of Brachiaria decumbens were 324.96 mm and 1.2 mm d-1 respectively. The grassland must have undergone water stress throughout the entire experimental period since the values of the ET/ETo ratio are much lower than one. The low value of the ET/ETo ratio indicates the need for irrigation to reduce water deficit. However, due to high drainage values, management practices for increasing soil water retention are recommended.
soil water content; hydraulic conductivity; retention curve; internal drainage
