The role of event water, a rapid shallow flow component, and catchment size in summer stormflow

Abstract. Compound-specific stable isotope analysis (CSIA) has, in combination with model-assisted interpretation, proven to be a valuable approach to quantify the extent of organic contaminant degradation in groundwater systems. CSIA data may also provide insights into the origin and transformation of diffuse pollutants, such as pesticides and nitrate, at the catchment scale. While CSIA methods for pesticides have increasingly become available, they have not yet been deployed to interpret isotope data of pesticides in surface water. We applied a coupled subsurface-surface reactive transport model (HydroGeoSphere) at the hillslope scale to investigate the usefulness of CSIA in the assessment of pesticide degradation. We simulated the transport and transformation of a pesticide in a hypothetical but realistic twodimensional hillslope transect. The steady-state model results illustrate a strong increase of isotope ratios at the hillslope outlet, which resulted from degradation and long travel times through the hillslope during average hydrological conditions. In contrast, following an extreme rainfall event that induced overland flow, the simulated isotope ratios dropped to the values of soil water in the pesticide application area. These results suggest that CSIA can help to identify rainfallrunoff events that entail significant pesticide transport to the stream via surface runoff. Simulations with daily rainfall and evapotranspiration data and one pesticide application per year resulted in small seasonal variations of concentrations and isotope ratios at the hillslope outlet, which fell within the uncertainty range of current CSIA methods. This implies a good reliability of in-stream isotope data in the absence of transport via surface runoff or other fast transport routes, since the time of measurement appears to be of minor importance for the assessment of pesticide degradation. The analysis of simulated isotope ratios also allowed quantification of the contribution of two different reaction pathways (aerobic and anaerobic) to overall degradation, which gave further insight into the transport routes in the modelled system. The simulations supported the use of the commonly applied Rayleigh equation for the interpretation of CSIA data, since this led to an underestimation of the real extent of degradation of less than 12 % at the hillslope outlet. Overall, this study emphasizes the applicability and usefulness of CSIA in the assessment of diffuse river pollution, and represents a first step towards a theoretical framework for the interpretation of CSIA data in agricultural catchments.

Section: Carbon and Hydrogen Isotope Ratiossupporting

confidence: 92%

A model-based assessment of the potential use of compound-specific stable isotope analysis in river monitoring of diffuse pesticide pollution

Lutz

Meerveld

Waterloo

et al. 2013

“…In more studies large groundwater contributions were found during the peak discharge (Frederickson and Criss, 1999;Pinder and Jones, 1969;Sklash and Farvolden, 1979;Wenninger et al, 2008). Although many others also found that with increasing intensity of the storms the event water contribution (surface runoff) also increases (Caissie et al, 1996;Hooper and Shoemaker, 1986;Brown et al, 1999).…”

Section: Hydrograph Separationmentioning

confidence: 98%

Spatial rainfall variability and runoff response during an extreme event in a semi-arid catchment in the South Pare Mountains, Tanzania

Mul

Savenije

Uhlenbrook

2009

Abstract. This paper describes an extreme flood event that occurred in the South Pare Mountains in northern Tanzania. A high spatial and temporal resolution data set has been gathered in a previously ungauged catchment. This data was analysed using a multi-method approach, to gather information about the processes that generated the flood event. On 1 March 2006, extreme rainfall occurred in the Makanya catchment, (300 km 2 ), where up to 100 mm were recorded in Bangalala village in only 3 h. The flood was devastating, inundating large parts of the flood plain. The spatial variability of the rainfall during the event was very large, even in areas with the same altitude. The Vudee sub-catchment (25.8 km 2 ) was in the centre of the rainfall event, receiving about 75 mm in 3 h divided over the two upstream tributaries: the UpperVudee and Ndolwa. The peak flow at the weir site has been determined using the slope-area method and gradually varied flow calculations, indicating a peak discharge of 32 m 3 s −1 . Rise and fall of the flood was very sharp, with the peak flow occurring just one hour after the peak of the rainfall. The flow receded to 1% of the maximum flow within 24 h. Hydrograph separation using hydrochemical parameters indicates that at the floodpeak 50% of the flow was generated by direct surface runoff (also indicated by the large amount of sediments in the samples), whereas the recession originated from displaced groundwater (>90%). The subsequent base flow in the river remained at 75 l s −1 for the rest of the season, which is substantially higher than the normal base flow observed during the previous rainy seasons (15 l s −1 ) indicating significant groundwater recharge during this extreme event.

“…These outlets were adjustable in height and had a protecting screen to avoid the collection of incident rainfall. This runoff would correspond to rapid subsurface flow production through the leaf layer, sometimes called the "thatched roof effect" (Weiler and McDonnell, 2004) or pseudo-Hortonian overland flow, whereby differences in saturated hydraulic conductivity at the organicmineral soil boundary create lateral flow in the near-surface horizon (Helvey and Patric, 1965;Brown et al, 1999).…”

Section: Test 2: Effect Of the Litter Layer-soil Matrix Interface On mentioning

confidence: 99%

Experimental analysis of drainage and water storage of litter layers

Guevara-Escobar

González-Sosa

Ramos-Salinas

et al. 2007

Abstract. Many hydrological studies of forested ecosystems focus on the study of the forest canopy and have partitioned gross precipitation into throughfall and stemflow. However, the presence of forest litter can alter the quantities of water available for soil infiltration and runoff. Little information exists regarding the value of storage and drainage parameters for litter layers. Vegetation parameters of this kind are required in physically-based and lumped conceptual models to quatify the availabilty and distribution of water. Using a rainfall simulator and laboratory conditions two main objectives were investigated using layers of recently seneced poplar leaves, fresh grass or woodchips: 1) Effect of rain intensity on storage. With this respect we found that: maximum storage (C max ), defined as the detention of water immediately before rainfall cessation, increased with rainfall intensity. The magnitude of the increment was up to 0.5 mm kg −1 m −2 between the lowest (9.8 mm h −1 ) and highest (70.9 mm h −1 ) rainfall intensities for poplar leaves. Minimum storage (C min ), defined as the detention of water after drainage ceased, was not influenced by rainfall intensity. Repeated wetting-draining cycles or layer thickness have no effect on C max or C min .2) The evaluation of drainage coefficient for the Rutter model. This model was found accurate to predict storage and drainage in the case of poplar leaves, was less accurate for fresh grass and resulted in overestimations for woodchips.Additionally, the effect of an underlaying soil matrix on lateral movement of water and storage of poplar leaves was studied. Results indicated that the soil matrix have no effect on C max or C min of the litter layer. Lateral movement of water in the poplar layer was observed at intermediate rainfall intensities (30.2 and 40.4 mm h −1 ), but not a the lowest or highest rates.