How does catchment geology control thresholds and non-linearities in rainfall-runoff transformation?
Abstract:
IntroductionThe apparently inextricable natural diversity of streamflow generating mechanisms is a crucial challenge for a better understanding of the governing controls of rainfall-runoff transformation. The inter-comparison of catchments using paired catchment studies and various manipulation experiments was frequently employed, especially in forest hydrology, and allowed significant knowledge increase in runoff generation processes. The complexity of many catchments and the differences between catchments related to hydro-climatology, geology, soils, landuse, and topography often challenge the link of runoff response to the underlying controls. Runoff generation is largely determined by the feedbacks and interactions between these influences and how they distribute water in space and time, leading to non-linearities and emergent thresholds in the rainfall-runoff transformation. Thresholds are critical points in time or space that eventually express a rapid change in runoff behavior. They have gained increasing attention in recent years. Two types of thresholds exist: storage and intensity controlled thresholds. Most threshold studies focused on individual catchments, which lead to difficulties in identifying the combinations of hydro-climatic and physiographic factors that control catchment thresholds and non-linearities in rainfall-runoff transformations, as well as differences between catchments. The individual influence of the boundary conditions remains poorly understood. Here, we propose a catchment inter-comparison experiment capitalizing on the nested catchment setup of the Attert River basin (Luxembourg). We focus on three headwater sub-catchments with similar hydro-climatology but contrasting geologies and landuse. We use this as a field based experiment to investigate the landscape controls on non-linearities and thresholds with a focus on geological controls.
Study area
The catchment comparison was carried out in three sub-catchments of the Attert River basin in Luxembourg: Huewelerbach, Weierbach, and Wollefsbach catchments. The Attert basin lies on the contact zone between the schistose Ardennes massif (northern part) and the sedimentary Paris basin (southern part). The Huewelerbach catchment (2.7 km2) is dominated by Triassic and Liassic sandstone (70%) and marl (20%) and ranges from 280 to 400 m.a.s.l. The catchment is mainly covered by forest (92%) on the hillslopes and plateaus while grasslands (8%) are found along the river valley and the foot slopes. The Weierbach catchment (0.47 km2) is nearly exclusively underlain by devonian schist (99%) and completely covered by forest. The catchment consists of a plateau, deeply cut by V-shaped valleys and ranges from 422 to 512 m.a.s.l. The Wollefesbach catchment (4.7 km2) is dominated by Triassic marl (90%). The catchment is characterized by grassland (65%) and arable land (30%). Elevation range from 245 to 306 m.a.s.l and some influence anthropogenic influence exists in the catchment due to drainage.
The climatic conditions in the Attert River basin (~250 km2) are relatively homogeneous with slightly higher precipitation amounts in the schistose uplands compared the rest of the basin. Dominating westerly atmospheric fluxes are causing annual rainfall totals of about 900 mm/year in the western part of Luxembourg (1971-2000). December, January and February are the wettest months (about 100 mm/month), while April, August and September are the driest months (about 70 mm/month). The mean monthly temperature is characterized by significant seasonal fluctuations. Lowest monthly average temperatures occur in January (average temperature of 0°C) and the highest temperatures are observed in July (average temperature 16.9°C). Monthly potential evaporation varies significantly across the seasons, ranging from 13 mm/month in December to 82 mm/month in July. This strongly affects the runoff regime, which is characterized by high flows during winter (maximum runoff in February) and low flows during summer (minimum runoff in September).
Methods
A V-notch weir is installed at the Weierbach catchment outlet , while water levels are measured at stable nearly rectangular streambed sections at the outlet of the Huewelerbach and Wollefsbach catchments. Water levels of all three streams are transformed into discharge using rating curves obtained through frequent discharge measurements. Open range precipitation is measured for all three catchments using tipping bucket raingauges.
The soil storage was calculated as antecedent soil moisture index (ASI) in mm for the upper 30 cm of soil. Additional piezometer data were available in the Huewelerbach and Weierbach catchments.
We analyzed the hydro-meteorological time series of the three catchments to determine event runoff ratios. We chose characteristic times that separated precipitation events to allow differentiating their influence on catchment response. While we chose 48 h of dry conditions to define the end of a precipitation event in the Weierbach catchment, we retained 8 h of dry conditions for defining the end of a precipitation event in the Huewelerbach and Wollefsbach catchments. Within this study, we evaluated only precipitation events that reached a minimum depth of 5 mm. After identifying the onset of a precipitation event, we used the Straight Line (SL) method to separate direct runoff and baseflow. The SL extends from the discharge at the start of the event until it intersects the hydrograph (or a new precipitation event was initiated). We then calculate the amount of direct runoff for every event as the discharge above the SL until the end of the event. The runoff ratio is the event discharge divided by the event precipitation. Over the period 2009-2012, we analyzed a total of 138 storm events in the Huewelerbach, 158 events in the Weierbach, and 171 events in the Wollfesbach catchment. We linked runoff ratio to event precipitation (EP), ASI, and ASI+EP.
Results
Runoff ratios
Event runoff ratios vary clearly between the different catchments. In the sandstone catchment (Huewelerbach), event runoff ratios rarely exceeded 0.2 in the 4-year observation period. In the schistose catchment (Weierbach), event runoff ratios reached 0.35 and in the marl catchment (Wollefsbach) we observed event runoff ratios of up to 0.8.
Threshold response
Minor responses on a precipitation forcing with low values of ASI and ASI+EP were observed in the three catchments. Nevertheless, clear catchment responses were only observable after distinct thresholds of ASI and ASI+EP were exceeded (see Figure a-c). The three catchments showed distinct differences in these threshold values and in the slope between runoff ratio and ASI+EP. In the Wollefsbach (marl) the runoff ratios increase strongly to values of 0.8 after an ASI+EP value of 60 mm is exceeded. The Weierbach (schists) shows a sharp threshold at around 90 mm after that runoff ratios increase from <0.05 to values of 0.4. The Huewelerbach (sandstone) exhibits a threshold around 75 mm once the runoff ratios slightly increase at a low level; this threshold is the least pronounced among the three catchments. These thresholds are observable throughout the experimental period with no differences between the hydrological years.
Furthermore, we compared the runoff responses (in the form of runoff ratios) to dynamic storage calculations for the individual events (Figure d), i.e. the filling of the catchment storage was considered. We were able to find a clear threshold response for every catchment for every hydrological year. Nevertheless, the threshold values and the non-linear behavior were different from year to year and no underlying principle could be detected so far.
Conclusions and Outlook
Our catchment inter-comparison study in three catchments of contrasting geology showed the strong role of the underlying bedrock, especially its permeability, eventually leading to the occurrence of thresholds in rainfall runoff-response. We were able to document the characteristics of the non-linear relationship between rainfall and runoff. High bedrock permeability lead to a strong moderation and damping in the rainfall-runoff response, while low available storage capacities lead to lower thresholds (ASI+EP) in on-set of significant runoff and higher runoff ratios. Using catchment storage, in the form of dynamic storage calculations, showed to be a promising approach for catchment inter-comparison going beyond the use of soil moisture data. This is especially relevant, since we expect feedback between different catchment storage units to exert important controls on runoff generation, and the spatio-temporal dynamics of catchment storage controlling catchment response. In a next step, we will need to improve the inter-annual comparability of dynamic storage, e.g. develop a better accounting scheme for actual evapotranspiration and the point of reference for catchment storage. Such a tool can further help hydrologists on the way towards a better framework for runoff generation studies and for the detection of underlying controls on rainfall-runoff transformation. The nested catchment set-up within the Attert River basin, with its long term data set, proofed to be an excellent environment to investigate controls on runoff generation.