HYDROLOGIC RESPONSE OF UNDISTURBED SOIL CORES TO SIMULATED RAINFALL

Experiments to characterize runoff and drainage from large undisturbed soil cores obtained from a site under volunteer pasture are presented . The objectives were to evaluate if a simple kinematic wave model cou ld be used to characterize drainage under conditions of by-passing flo w and to investigate the variability of runoff generation under wet an d dry antecedent conditions in this well structured soil. Cores were t aken to depths of 0.4 and 0.7 m, with six cores obtained from each dep th. Results show that variations in soil structure between cores have a greater influence on the hydrologic response than variations in ante cedent water content. It may be inferred that, in this soil, good stru ctural continuity substantially enhances soil drainage and, in consequ ence, decreases surface runoff. During wetting, kinematic celerities r anged from 337 to 3200 mm h-1 in the short cores and from 357 to 1400 mm h-1 in the long cores. The corresponding estimated ranges of mobile or 'macropore' fractions were from 0.015 to 0.154 m3 m-3 in the short cores and 0.033 to 0.132 m3m-3 in the long cores. Both parameters wer e approximately log-normally distributed. Measured drainage hydrograph s were reasonably well described by the kinematic model but the physic al significance of the model parameters was unclear.