Variations in unit hydrographs on a very small agricultural catchment

Unit hydrographs (5-min 1 mm) were derived from six storms in the period November 1976 to March 1979 on a 1.2 ha catchment at Greenmount, about 25 km south of Toowomba in south-east Queensland. Peak rates of runoff in the six storms ranged from 1,12mm/h to 50.13 mm/h. All unit hydrographs were derived from multiperiod storms using an iterative procedure. Considerable variation occurred among the unit hydrographs with peak ordinate values ranging from 2.18 mm/h to 9.45 mm/h. It was found that each unit hydrograph could be separated into two components, surface runoff and interflow. The proportion of surface runoff in the unit hydrograph increased as the intensity of rainfall excess increased, and ranged from 5% in the storm with peak runoff of 1.12 mm/h to 86% in the storm with peak runoff of 50.13 mm/h. A large amount of runoff from the catchment occurred as interflow, consisting of water moving downslope through the porous cultivated surface layers of soil. The traditional separation of interflow from surface runoff prior to derivation of the unit hydrograph could not be done because of the complexity of the storm patterns. The study showed that it is possible to derive unit hydrographs which include an interflow component and to separate surface runoff and interflow in the unit hydrograph itself. A simple model was developed in which the proportions of surface runoff and interflow were allowed to vary from storm to storm. The proportions were adjusted by trial and error to match estimated runoff with actual runoff. The simple model reproduced observed runoff hydrographs with only minor errors of fit. The surface runoff component of the simple model behaved in accordance with traditional linear unit hydrograph theory. Variations in the unit hydrographs were accounted for by variations in the proportions of surface runoff and interflow from storm to storm.