HYDRAULIC VARIABILITY IN-SPACE AND TIME IN A DARK RED LATOSOL OF THE TROPICS

Detailed information of the variability of soil properties and process es in space and time is presented for a dark red latosol (Alfisol) of the county of Piracicaba, S.P., Brazil. Data were collected on 25 plot s along a 125 m transect during the years 1989-199 1, and consisted of the soil water content theta in the 0-150 cm soil layer and the water pressure heads h at 135 and 165 cm depths. These raw data were used t o characterize variabilities in space and time using classical statist ics and in a second step to analyse the difficulties in calculating so il water storage, soil hydraulic conductivities, hydraulic gradients, soil water fluxes and water balances. In general, there was a great va riability of hydraulic properties and processes, which is fairly const ant in time in the case of basic data like soil water content and pote ntial, but not in the case of calculated data like hydraulic conductiv ities and gradients, and soil water flux densities. A discussion is pr esented of the difficulties of using Darcy's equation to estimate soil water flux densities due to the exponential K(theta) and K(h) relatio nships of the hydraulic conductivity K, and of the influence of variab ility in space and time on the establishment of water balance componen ts.