The influence of soil moisture on threshold runoff generation processes in an alpine headwater catchment

Abstract

Abstract. This study investigates the role of soil moisture on the threshold runoff response in a small headwater catchment in the Italian Alps that is characterised by steep hillslopes and a distinct riparian zone. This study focuses on: (i) the threshold soil moisture-runoff relationship and the influence of catchment topography on this relation; (ii) the temporal dynamics of soil moisture, streamflow and groundwater that characterize the catchment's response to rainfall during dry and wet periods; and (iii) the combined effect of antecedent wetness conditions and rainfall amount on hillslope and riparian runoff. Our results highlight the strong control exerted by soil moisture on runoff in this catchment: a sharp threshold exists in the relationship between soil water content and runoff coefficient, streamflow, and hillslope-averaged depth to water table. Low runoff ratios were likely related to the response of the riparian zone, which was almost always close to saturation. High runoff ratios occurred during wet antecedent conditions, when the soil moisture threshold was exceeded. In these cases, subsurface flow was activated on hillslopes, which became a major contributor to runoff. Antecedent wetness conditions also controlled the catchment's response time: during dry periods, streamflow reacted and peaked prior to hillslope soil moisture whereas during wet conditions the opposite occurred. This difference resulted in a hysteretic behaviour in the soil moisture-streamflow relationship. Finally, the influence of antecedent moisture conditions on runoff was also evident in the relation between cumulative rainfall and total stormflow. Small storms during dry conditions produced low stormflow amounts, likely mainly from overland flow from the near saturated riparian zone. Conversely, for rainfall events during wet conditions, higher stormflow values were observed and hillslopes must have contributed to streamflow.

Rainfall–runoff response and event-based runoff coefficients in a humid area (northwest Spain)

“…The absence of a straightforward relationship between monthly values of rainfall and water yield is not particular to the data shown here, and a similar effect has already been reported for humid temperate catchments (Endale et al 2006, Merz et al 2006, Penna et al 2011) and for Mediterranean catchments (García-Ruíz et al 2008), in particular. A possible factor explaining this nonlinearity is related to the number, the succession and the characteristics of the rainfall events, as well as to the seasonal dynamics of rainfall and evapotranspiration, which then affect the antecedent conditions for individual rainfall events.…”

“…5 it is clear that, as the catchment becomes wet, the hydrological response increases and, consequently, it represents a greater percentage of rainfall. Probably the antecedent soil moisture state plays an important role in the hydrological response of the catchment, as has been often suggested for temperate humid catchments (Dunne et al 1975, Pfister et al 2002, Norbiato et al 2009, Penna et al 2011. However, over the study period, the observed seasonality of the runoff coefficients was also possibly affected by rainfall characteristics, such as large and long rainfall events occurring in 2006.…”

“…2). Runoff showed a higher trend for variability than rainfall, indicating the nonlinear relationship between rainfall and runoff rates in the study area, as described by numerous authors (Jordan et al 1994, Merz and Blöschl 2009, Penna et al 2011. This means that water yield from this catchment depends not only on the rainfall amount but also on its temporal distribution, as other catchment characteristics, such as land use and management, have remained unchanged during the study period.…”

The significance of soil moisture in forecasting characteristics of flood events. A statistical analysis in two nested catchments

“…Our study underpins the expedience of using tensiometers in place of TDR probes, which are commonly used (Brocca et al, 2007;Penna et al, 2011;Zehe et al, 2010) to assimilate soil moisture into rainfall-runoff modeling. Our analysis shows that further studies on flood forecasting should concentrate on carefully selecting representative measurement sites to improve the quality of forecasts instead of labor-intensive and costly attempts to monitor the soil moisture of an entire catchment.…”

Convection-permitting regional climate simulations for representing floods in small and medium sized catchments in the Eastern Alps

“…Using a hydrological model for an evaluation of climate model results also enables the incorporation of other hydrological 25 quantities, which give indications about the performance of the climate model regarding the hydrological conditions. Soil moisture is an important variable to be analysed in terms of non-linearity and threshold processes in flood generation (e.g., Penna et al, 2011). It is continuously calculated by the hydrological model, and hence, can be used as a comparison between the different simulation runs.…”