Exploring Controls on Rainfall‐Runoff Events: 1. Time Series‐Based Event Separation and Temporal Dynamics of Event Runoff Response in Germany

Abstract

Analyzing a response of catchments to rainfall inputs allows for deeper insights on the mechanisms of runoff generation at catchment scale. In this study an automated time series‐based event separation procedure consisting of available base flow separation, runoff event identification, and rainfall attribution methods and of a novel iterative procedure for the adjustment of thresholds used to identify single‐peak components of multiple‐peak events is proposed. Event runoff coefficient, time scale, rise time, and peak discharge of more than 220,000 identified rainfall‐runoff events are then used to analyze dynamics of event runoff response in 185 catchments at multiple temporal scales. In mountainous catchments with poor storage event runoff response is strongly controlled by the characteristics of rainfall and is generated by event‐fed saturation or infiltration excess. A distinct switch between saturated and unsaturated states occurs in these catchments. A weak relation between rainfall and runoff event properties is instead observed in lowland and hilly catchments with substantial storage, where a gradual transformation between functioning states occurs and the response is driven by preevent saturation. The seasonality of their event characteristics is governed by the contribution of snowmelt and the seasonality of the aridity index rather than of rainfall properties. Long‐term changes of total precipitation amount alone do not explain season‐specific long‐term changes of event characteristics that are rather consistent with changes of seasonal indicators of the wetness state. The effects of land use changes are detectable only in a few cases and display themselves mostly in the characteristic response time of catchments.

Exploring Controls on Rainfall‐Runoff Events: 2. Regional Patterns and Spatial Controls of Event Characteristics in Germany

“…In this cluster runoff coefficients strongly depend on the volume of single rainfall events (Figure a, middle panel; Tarasova et al, ), implying possible event‐fed saturation (Berghuijs et al, ). According to the findings reported in the companion study of Tarasova et al (), this is the only region where a high correlation between event runoff coefficient and precipitation intensity exists, thus implying that infiltration excess is a possible runoff generation mechanism as well. Moreover, there is a distinct nonlinear relationship between runoff coefficient and antecedent soil moisture (Figure a, middle panel), which indicates two distinct functioning states for catchments of this cluster (Grayson et al, ).…”

“…However, snowmelt and fast components of runoff generation are pivotal especially in summer. In this cluster runoff coefficients are largely independent from the volume of single rainfall events (Figure b, middle panel), implying that event‐fed saturation is unlikely to occur (Tarasova et al, ). A near‐linear increase of runoff coefficients with increasing antecedent soil moisture indicates a gradual transition from dry to wet states.…”

Event‐based deep drainage and percolation dynamics in Vertosols and Chromosols

“…A lower bound on the site‐averaged deep drainage volume of 2 mm within a 3‐day period was used to define a deep drainage event. Consistent with other studies involving event delineation, the 2 mm bound is arbitrary, aiming at a balance between achieving a good sample of events and minimising spurious or insignificant events (Rodríguez‐Blanco, Taboada‐Castro, & Taboada‐Castro, ; Tarasova, Basso, Zink, & Merz, ). The event ceases at the beginning of the 15‐minute time‐step after which there is no more deep drainage for 3 days or more.…”

Parameter's Controls of Distributed Catchment Models—How Much Information is in Conventional Catchment Descriptors?

“…Streamflow of many German catchments is dominated by anthropogenic impacts such as reservoirs, dams, or control gates. Based on a regional analysis of the spatiotemporal patterns of runoff event characteristics (Tarasova, Basso, Zink, & Merz, 2018), a survey of metadata on dams and reservoirs (Lehner et al, 2011) and a manual screening of runoff time series we removed all catchments with reported or suspected flow disturbance.…”

“…Climate and catchment descriptors are highly correlated to each other (see, e.g., Tarasova, Basso, Zink, & Merz, 2018, Figure 6) as a result of the coevolution of climate and landscape. Kirkby (2005, p. 58), e.g., notes:…”