Crop yields, associated to weather and soil conditions, depend on the availability of water and nutrients in the soil at the appropriate time and amounts. Lack and excess of water in the soil are limiting factors for plant growth and can reduce the productivity. Therefore, studies leading to a better understanding of how water behaves within the root zone of a field crop are of unquestionable importance for an adequate agricultural management. Objectives of this study were the evaluation of the methodology of soil water balance applied to the soil volume used by the root system of a citrus plant, considering five soil depths and five horizontal distances from the stem in two directions from the stem (one along and the other perpendicular to the plant row); and second, to verify the contribution of each evaluated soil layer to the actual plant evapotranspiration. The experiment was carried out on an Oxisol with a citrus orchard in Piracicaba, state of São Paulo, Brazil, during 40 days (dry period) and 37 days (rainy period). To attain the goal set, two sets of 25 tensiometers (5 depths x 5 distances from the stem) were installed: one along the plant row and the other perpendicular to it. Water storage was calculated from the readings of the two tensiometer sets whose matric potentials were converted into volumetric soil water contents by the respective soil water retention curve. The internal drainage and the capillary rise were determined by the Darcy-Buckingham equation through daily readings of the installed tensiometers and by the function K(phim) (hydraulic conductivity in function of the matric potential). The soil hydraulic conductivity for each depth was determined by the instantaneous profile method in an adjacent area. A rain gauge with automatic data acquisition installed in the area measured the rain. The obtained results allowed the conclusion that the proposed soil water balance methodology adequately evaluated how each soil layer of the plant root zone contributed to the total plant water use (actual plant evapotranspiration) in the two studied periods. In this sense, the 0.00 to 0.60 m soil layer should be the one in which citrus should have the greatest volume of active roots since the contribution of this layer to the plant water use was the highest.
Oxisol; hydraulic conductivity; matric potential
