Abstract. The aim of this paper is to illustrate the effects of selected catchment storage thresholds upon runoff behaviour, and specifically their impact upon flood frequency. The analysis is carried out with the use of a stochastic rainfall model, incorporating rainfall variability at intra-event, inter-event and seasonal timescales, as well as infrequent summer tropical cyclones, coupled with deterministic rainfall-runoff models that incorporate runoff generation by both saturation excess and subsurface stormflow mechanisms. Changing runoff generation mechanisms (i.e. from subsurface flow to surface runoff) associated with a given threshold (i.e. saturation storage capacity) are shown to be manifested in the flood frequency curve as a break in slope. It is observed that the inclusion of infrequent summer storm events increases the temporal frequency occurrence and magnitude of surface runoff events, in this way contributing to steeper flood frequency curves, and an additional break in the slope of the flood frequency curve. The results of this study highlight the importance of thresholds on flood frequency, and provide insights into the complex interactions between rainfall variability and threshold nonlinearities in the rainfall-runoff process, which are shown to have a significant impact on the resulting flood frequency curves.

Abstract. The aim of this paper is to illustrate the effects of selected catchment storage thresholds upon runoff behaviour, and specifically their impact upon flood frequency. The analysis is carried out with the use of a stochastic rainfall model, incorporating rainfall variability at intra-event, interevent and seasonal timescales, as well as infrequent summer tropical cyclones, coupled with deterministic rainfall-runoff models that incorporate runoff generation by both saturation excess and subsurface stormflow mechanisms. Changing runoff generation mechanisms (i.e. from subsurface flow to surface runoff) associated with a given threshold (i.e. saturation storage capacity) is shown to be manifested in the flood frequency curve as a break in slope. It is observed that the inclusion of infrequent summer storm events increases the temporal frequency occurrence and magnitude of surface runoff events, in this way contributing to steeper flood frequency curves, and an additional break in the slope of the flood frequency curve. The results of this study highlight the importance of thresholds on flood frequency, and provide insights into the complex interactions between rainfall variability and threshold nonlinearities in the rainfall-runoff process, which are shown to have a significant impact on the resulting flood frequency curves.

[1] This model-based study examines the combined effects of catchment and lake thresholds upon the frequency and magnitude of lake-overflow events, and their impacts on flood frequency. A dominant control of lake-overflow events is antecedent storage, which is governed by the climate and by the properties of the contributing catchment. The next major control was shown to be the magnitude of storm depths, and their adequacy to replenish and exceed the lake storage deficit, which are governed by the ratio of catchment to lake areas, A C /A L . When A C /A L is large, lake-overflows can be triggered even at larger antecedent lake storage deficits. This points to the importance of A C /A L as a critical parameter governing the frequency and magnitude of lake-overflow events. As regards the catchment properties, model simulations indicate that fast draining catchments enhance the triggering of lake-overflow events due to the fact that the drainage is rapid and there is the opportunity for faster runoff contributions to combine with direct rainfall on the lakes to exceed antecedent lake storage deficit and overcome the reducing influence of evaporation during inter-storm periods. Slow draining catchments, on the other hand, will not release runoff fast enough to replenish the lake storage deficit before the evaporative effects take hold. The shape of the resulting flood frequency curve captures the dominant lake-overflow generating mechanisms. When the dominant lake overflow generating mechanism is catchment runoff, the shape of flood frequency curve of lake-overflows resembles the shape of the catchment flood frequency curve, including the effects of associated thresholds. On the other hand, when the dominant lake overflow generating mechanism is direct rainfall falling on the lake, the shape of the lake-overflow flood frequency curve exhibits a persistent truncation below a critical return period that is associated with the frequency of flow termination for the given climate in question. These results have provided valuable insights into the relative roles of climate, soil depth, the soil's drainage capacity as well as the ratio of catchment area to lake area on flood frequency of catchment-lake systems in general. The improved understanding of these process controls will be useful in assisting the management of such combined catchmentlake systems. In particular, they can provide valuable guidance towards the monitoring of catchment-lake systems in ways that are targeted towards those controls which are critical to the determination of the magnitude and frequency of lake-overflow events to assist in flood prevention and mitigation. (2008), Thresholds in the storm response of a catchment-lake system and the occurrence and magnitude of lake overflows: Implications for flood frequency,

No abstract

Unit hydrograph is a very practical tool in runoff prediction which has been used since decades ago and to date it remains useful. Unit hydrograph method is applied in Way Kuala Garuntang, an ungauged catchment in Lampung Province, Indonesia. To derive an observed unit hydrograph it requires rainfall and water level data with fine time scale which are obtained from automatic gauges. Observed unit hydrograph has an advantage that it is possible to derive it for various time steps including those with time step less than an hour. In order to get a more accurate unit hydrograph, it is necessary to derive a unit hydrograph with small time step for a small catchment such as those used in this study. The study area includes Way Kuala Garuntang and its tributaries, i.e. Way Simpur, Way Awi with areas are 60.52 km 2 , 3.691 km 2 , and 9.846 km 2 respectively. The results of this study highlight the importance of time step selection on unit hydrograph, which are shown to have a significant impact on the resulting unit hydrograph's variables such as peak discharge and time to peak.