Umidade do solo medida com TDR e estimada pelo método da FAO 56 em cultivo de cana-de-açúcar

Abstract

The study of water dynamics in cultivated soil is crucial for the planning of production, irrigation management and validation of computer simulation crops models. In order to show the calibration of a TDR equipment to measure moisture with it a profile of soil cultivated with sugarcane in the region of Rio Largo - AL and compare with the estimate obtained by the FAO 56 method, an experiment was conducted in a 240 m² area planted with ratoon cane, variety RB98710, planted in rows 12 m long and 1 m between rows and drip irrigated. The local soil was classified as yellow latosol distrocohesive argisolic, the local climate is megathermic and humid. The TDR probes were installed at depths of 0.1 m to a maximum depth of 0.9 m. The TDR equipment was calibrated locally following the methodology of SILVA (2013) , resulting in the third degree polynomial equations, relating the apparent dielectric constant of the soil (Ka) (directly verified by TDR) to soil moisture (θv). The FAO 56 method uses meteorological data to estimate the reference evapotranspiration and, from it , along with information of culture and soil , estimating the water balance and water storage in the soil. The calibration of the TDR resulted in individual equations of each layer covered by the probe (0.1 m) which had different settings. From the average of the values of Ka and θv were certain universal equations for estimating moisture in intervals of 0.3 m for a general profile up to 0.9 m . With the data from TDR locally calibrated, we determined curves of water distribution in the soil, which showed how the variation in density and particle size profile can influence the availability of soil water. The variations in the storage of each layer of 0.1 m thick during the period when the TDR has been installed differ, again showing that the uneven profile interfere with the storage of water. The presence of a cohesive horizon duripan type could also be seen in the results of each individual storage layer and the curves of water distribution. The estimated by FAO 56 method storage differed from verified by TDR, especially during the wet season. Calibration of TDR is recommended for each soil type for each layer and also in the case of the profile is heterogeneous. Universal empirical equations determined locally and are not recommended for estimating the moisture profile of soil from Rio Largo - AL. The presence of duripan interferes with the water distribution in the soil profile, significantly reducing drainage and retaining moisture, providing it to the culture by capillarity. The FAO 56 method to determine the storage does not apply to the region of Rio Largo - AL.