Contributions of riparian and hillslope waters to storm runoff across multiple catchments and storm events in a glaciated forested watershed

Journal of Hydrology (2007) 341, 116– 130

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Contributions of riparian and hillslope waters to storm runoff across multiple catchments and storm events in a glaciated forested watershed Shreeram P. Inamdar a b

a,*

, Myron J. Mitchell

b

Bioresources Engineering, 260 Townsend Hall, University of Delaware, Newark, DE 19716, USA Environment and Forest Biology, SUNY-ESF, Syracuse, NY 13210, USA

Received 18 April 2006; received in revised form 17 February 2007; accepted 5 May 2007

KEYWORDS Riparian zones; Wetlands; EMMA; Runoff generation; Landscape organization; Storm events

The contributions of hillslope and riparian sources of runoff to streamflow were determined for four catchments (1.6–696 ha) in the Point Peter Brook watershed, a glaciated, forested, watershed in Western New York, USA. Investigations were performed for 10 storm events of varying size, intensity, and antecedent moisture conditions. Hydrometric, geochemical, and landscape analysis procedures were used to characterize the sources of runoff and the influence of topography on hydrologic response. Using end member mixing analysis (EMMA), throughfall, groundwater discharged at hillslope seeps, and valley-bottom riparian water were identified as the controlling end-members for storm-event runoff. Contribution from seep groundwater was highest during baseflow, contributions from throughfall increased through the rising limb of the hydrograph, while riparian water amounts were highest at or after the peak in discharge. The delayed response of riparian water was attributed to displacement by hillslope interflow. The relative contributions of the end-members varied with catchment size and storm event conditions. Riparian water contributions were greater at the large catchment size (696 ha) while seep groundwater was important for the small headwater catchments. Steep hillslope gradients and moist valley-bottoms allowed for a greater expression of hillslope seep water in runoff during baseflow conditions. Percent contributions of riparian water to streamflow were higher for larger storm events while small events and wet antecedent conditions increased the expression of seep groundwater. This study underscored the need for three-dimensional or volume-based assessments to characterize the contributions of valley-bottom riparian and wetland areas to streamflow. ª 2007 Elsevier B.V. All rights reserved.

Summary

* Corresponding author. Tel.: +1 302 831 8877; fax: +1 302 831 2469. E-mail address: [email protected] (S.P. Inamdar). 0022-1694/$ - see front matter ª 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2007.05.007

Contributions of riparian and hillslope waters to storm runoff across multiple catchments

Introduction Recently there has been considerable interest in identifying the spatial sources of runoff in watersheds through the use of tools such as end member mixing analysis (EMMA), tracerbased hydrograph separations, and hydrometric analysis (Bernal et al., 2006; Burns et al., 2001; Hangen et al., 2001; McGlynn and McDonnell, 2003; Subagyono et al., 2005; Wenninger et al., 2004). These studies have been successful in quantifying runoff amounts from various sources and have also shown that the contributions may vary with varying moisture conditions in the watershed (Burns et al., 2001; McGlynn and McDonnell, 2003). Identification of runoff sources or contributions from watershed units is important since such information can: (a) assist in developing more realistic models for watershed management; (b) assist in identifying the key sources of nonpoint pollutants; and (c) help better evaluate the impacts of land use change on water quality. Burns et al. (2001) identified the riparian zone, hillslope, and a rock outcrop as the three main sources of runoff for the Panola watershed, a small catchment in Georgia, USA. Riparian groundwater runoff was high during early parts of the event and during stream recession while runoff from the rock outcrop provided a significant portion of discharge at peak flow, especially during large storm events. McGlynn and McDonnell (2003) identified riparian and hillslope units as the primary contributors to catchment streamflow for the Maimai watershed in New Zealand. Riparian contributions were high for small events and for the early portion of large events while hillslope contributions were greatest during peak discharge for large storm events. They concluded that the riparian reservoir was an important modulator of catchment runoff during baseflow, small events, and early portion of large events. However, these results were based on only two storm events. For a small catchment in the Black Forest region of Germany, the valley-bottom riparian reservoir and hillslope were identified as the regulators of streamflow (Hangen et al., 2001). Hangen et al. (2001) presented a 3-stage model of runoff generation where saturation overland flow, soil and groundwater from the riparian reservoir, and hillslope interflow were the key contributors to catchment streamflow. In Japan, Subagyono et al. (2005) found that the near surface riparian reservoir and hillslope aquifer were more important during rapid storm flows while the deep riparian groundwater reservoir maintained slow flow and served as a recharge system for the watershed. Similarly, Bernal et al. (2006) identified hillslope and riparian groundwater and event water as the three end members for streamflow, but also found that the stream chemistry could not be explained by these end members during dry periods of the year. These studies clearly show that the proportions of nearsurface and deep groundwater flows and their spatial expression at key landscape positions such as riparian zones or hillslopes could be important determinants of catchment runoff. However, very little work has been done in determining how the source amounts vary with catchment scale and multiple storm events with varying antecedent moisture conditions. Topography and the areal extent of hillslope and

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riparian units vary with catchment size and therefore are very likely to influence the runoff contributions from these sources. Evaluation of storm events of varying sizes and antecedent conditions are more likely to provide insights into nonlinear behavior or threshold-mediated response of the landscape units (e.g., McDonnell, 2003; Sidle et al., 2000). We determined the sources of storm event runoff for the Point Peter Brook watershed, a forested, glaciated watershed located in Western New York, USA. Investigations were performed across four catchments ranging in size from 1.6–696 ha and for multiple storm events over a two-year period. The Point Peter Brook watershed has wide ridgetops, steep side slopes, and narrow valley-bottom areas which are moist year-round. Perennial seeps located twothirds of the way up the hillslope discharge groundwater as surface runoff. We hypothesized that the occurrence of the hillslope seeps, steep sideslopes, and saturated areas in the valley-bottom were important determinants of the hydrologic response for Point Peter Brook. Topographic variation across the catchments was characterized using the downslope wetness index (DWI) of Hjerdt et al. (2004). An end member mixing analysis (Inamdar and Mitchell, 2006) identified throughfall, riparian groundwater, and groundwater discharged at the hillslope seeps as the three controlling end members for streamflow. The overall goal here was to determine how the contributions of runoff from end members varied with catchment scale, event size, and antecedent moisture conditions and if topographic differences among the catchments could explain the differences in contributions. Specific questions that were explored included: (a) How do the temporal patterns and amounts of end member contributions (runoff sources) vary across the catchments? (b) How do storm event size and antecedent moisture conditions impact end member runoff contributions? (c) What is the role of topography in influencing the runoff sources and amounts across the catchments?

Site description and methods Site description This study was conducted in the Point Peter Brook watershed (Fig. 1), located in Cattaraugus County and 55 km southeast of Buffalo in New York State (42 26 0 3000 N; 78 55 0 3000 W). Mean annual winter temperature is 3C and the mean summer temperature is 21C. Annual precipitation averages 1006 mm of which 200–250 mm occurs as snow (20 y average based on the National Atmospheric Deposition Program Weather Station at Chautauqua, NY; 35 km southwest of Point Peter Brook; NADP, 2004). The parent material was derived from glacial till (Kent Drift of Woodfordian formed 19 000 y B.P.) (Phillips, 1988). Soils in the watershed belong to the Volusia-Mardin-Erie association (Phillips, 1988) and can be classified as typic dystochrepts, coarse-loamy, mixed, mesic. Vegetation on ridgetops and hillslopes was dominated by deciduous trees including sugar maple (Acer saccharum), black maple (Acer nigrum), American beech (Fagus grandiflora), yellow birch (Betula alleghaniensis) with larger proportions of conifers including hemlock (Tsuga canadensis) and white pine (Pinus strobus)

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Figure 1 Location of Point Peter Brook watershed (PPBW) in New York State (a); study subcatchments S1, S2, S3, and S5 (696, 3.4, 1.6, and 1.9 ha, respectively) (b); and instrumentation and sampling locations in the catchments (c). Contour intervals in (c) are in meters.

in valley-bottoms. Topography of the entire watershed is fairly distinct with wide ridgetops, steep hillslopes, and narrow valley bottoms. Slope gradients in the watershed range from 0 to 69%, with a mean gradient of 14%. Elevation ranges from 252 to 430 m above mean sea level. A low-permeability clay layer that generates perched water tables and forces water to move as shallow subsurface flow on the steep hillslopes underlies the soils. The depth to the clay/till measured using soil cores varies from 1.2–1.7 m in the valley-bottom locations, 0.3–0.5 m along the side slopes and 0.6 m at the ridgetops. The catchments that were studied (S1, S2, S3, and S5) are shown in Fig. 1. Outlet for S1 (696 ha) was located on the main drainage of watershed with S2 (3.4 ha) and S3 (1.6 ha) nested within S1. Catchment S3 drained a hillslope hollow with streamflow originating from two perennial seeps S3a and S3b that discharged at the channel head (Fig. 1). Surface saturation in S3 was limited to the channel head at the seeps, with limited surface saturation along the

stream channel. Hillslopes in S3 extended to the stream channel. Outlet S2 was located in a valley-bottom riparian area downstream of S3. The riparian area in S2 was variably saturated and the organic matter content of surficial (0– 20 cm) soils was between 3% and 11%. Soil thickness in the riparian area ranged from 1.5 to 2 m above loose gravel or unconsolidated material. The riparian area at S2 was located in a topographic convergence such that in addition to S3, two other intermittent seeps contributed to runoff at S2. Runoff from the seeps traversed rapidly as streamlets over the variably saturated riparian area before entering the stream S2. In many locations in the valley-bottom, seepage areas with reddish-brown iron-oxide deposits were visible. Roulet (1990) indicated that when deep groundwater low in dissolved oxygen (Hill, 1990) emerges to the surface, reduced iron is oxidized, thus producing the reddish brown coloration. Surface-saturation in S2 was also observed in multiple isolated pockets on the hillslope bench along the eastern hillslope flank (Fig. 1). Organic matter concentra-

Contributions of riparian and hillslope waters to storm runoff across multiple catchments tions of surficial soils in these isolated saturated areas ranged from 3% to 70%. Catchment S5 (1.9 ha) located downstream of S1 enclosed a valley-bottom riparian wetland. This saturated area in S5 was identified as a wetland since it was continuously saturated year-round and the organic matter concentration of the soil was 70%. Soil thickness in the S5 wetland was 1 m or less (above gravel/loose unconsolidated material) and lower than that observed for the riparian area at S2. Runoff to S5 also originated from a seep (S8) located more than two-thirds of the distance along the contributing hillslopes along the northeastern edge (Fig. 1).

Watershed monitoring, sampling, and analysis Hydrologic and surface water chemistry monitoring at the Point Peter Brook watershed was initiated in November 2002 and intensive storm-event sampling started in May 2003 and continued through May 2004. Precipitation was recorded using a tipping-bucket rain gage located 400 m downstream from S1. Streamflow discharge measurements at S1 were initiated in November 2002, at S2 and S3 in May 2003 and at S5 in April 2004. Stream flow stage was recorded every 15 min using a pressure transducer with a recorder (Global Water Inc.). At S1, a stage-discharge relationship was developed for the 3 m wide stream channel. Parshall flumes were installed on streams at S2 and S3 and a V-notch weir was installed at the stream channel at S5. Groundwater elevations were recorded using pressure transducers (Global Water Inc.) nested within logging wells that were constructed of 5 cm (ID) PVC tubing. The logging wells were constructed by coring to the depth at which an impeding clay or loose/unconsolidated gravel or till was intersected. Two logging wells R1 and R5 were located in the valley-bottom riparian and wetland areas of S2 and S5, respectively (Fig. 1). One hillslope well (H2) was positioned in a saturated area on the hillslope bench while the other was located at the base of the hillslope (H7) just above the S5 wetland. Grab sampling was performed on a bimonthly basis for: valley-bottom and hillslope groundwater wells, surface seeps, and lysimeters located in valley-bottom and hillslope-bench saturated areas. Groundwater sampling wells were constructed of 5 cm (ID) PVC tubing and were screened from 30 cm below the soil surface to the bottom. Three groundwater-sampling wells (RS1, RS2, and RS5) were established in riparian and wetland valley-bottom locations (Fig. 1). Seep samples were collected from surface seeps at S3a and S3b (Fig. 1) in the catchment S3. Starting in spring 2005, samples were also collected downstream of the seep S8 located in catchment S5. Zero-tension lysimeters were constructed of 5 cm (ID) PVC tubing and were inserted at a 45 angle to a depth of 30 cm from the soil surface. The lysimeters were installed to collect soil water from the A horizon. Lysimeters were installed in valley-bottom riparian and wetland areas (L1, L2, and L6) and hillslope-bench saturated areas (L3 and L4) (Fig. 1). Sample water was obtained from the groundwater wells and lysimeters using a hand-operated suction pump. Storm event sampling for the four catchments was conducted using three ISCO samplers. For S1, storm-event sam-

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pling was performed from May 2003 through April 19, 2004. Catchment S2 was sampled from May 2003 onwards. S3 was sampled from September 2003 onwards and sampling at S5 was initiated after April 19, 2004. The automated ISCO sampler was triggered for event sampling when the rainfall rate exceeded a threshold of 2.8 mm within a 2 h period. The sampler was programmed on the ‘‘variable time’’ mode so as to sample more frequently on the hydrograph rising limb than on the recession limb. Composite precipitation samples were collected in a collector placed in the open; throughfall samples were collected from two collectors, one placed under a coniferous canopy (Tc) and one placed under a deciduous canopy (Td) (Fig. 1). Precipitation and throughfall collectors were 3.8 L plastic containers connected to funnels, which had a plastic mesh on the mouth to prevent entry of debris. All samples were collected within 24 h of an event in 250 mL Nalgene bottles. Analyses performed on the samples included: DOC on a TekmarDohrmann Phoenix 8000 TOC analyzer, silica (Si) and cations on a Perkin-Elmer ICP-AEC Div 3300 instrument, and anions on a Dionex IC. The laboratory is a participant in the United States Geological Survey (USGS) performance evaluation program, that helps ensure data quality (Mitchell et al., 2001).

Digital Elevation Model (DEM) and landscape analysis A 2 m DEM was developed for the Point Peter Brook watershed via aerial photogrammetry (LaFave, White, and McGivern Inc., NY). The drainage network and catchment boundaries were delineated using the PrePro program (Olivera and Maidment, 2000). The DEM-computed catchment boundaries for S2, S3, and S5 were verified by comparing against field-surveyed boundaries using a Trimble GPS (Trimble, Inc.). Both the ln(a/tanb) index (Quinn et al., 1995) and the downslope index (DWI) (Hjerdt et al., 2004) were computed using the 2 m DEM. One important feature of the DWI is that it accounts for the enhancement or impedance of local drainage by downslope topography, something that is not considered for in the ln(a/tanb) index (Hjerdt et al., 2004). The DWI was computed using the GEASY program (Seibert, 2005) and procedures explained by Hjerdt et al. (2004) for four downslope elevation ‘‘d’’ values – 2, 3, 4, and 10 m. Field surveys of surface-saturated areas were performed in May 2004 for the S2, S3, and S5 catchments through visual identification and mapping of surface-saturation and soil coring. Since the DWI values better matched the field-observed areal and total extent of surface-saturation (Inamdar, Unpublished data), only the DWI results are presented here.

Concentrations of geochemical tracers and identification of runoff sources using end-member mixing analysis (EMMA) Spatial sources of streamflow in the Point Peter Brook watershed were identified by Inamdar and Mitchell (2006) using silica (Si), magnesium (Mg), and dissolved organic carbon (DOC) as tracers and the procedures and results are briefly described here. Silica and magnesium have been the tracers

120

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of choice in many studies and high concentrations of these solutes have typically been associated with deep groundwaters and/or waters with high residence time in the watershed (Hooper and Shoemaker, 1986; McGlynn and McDonnell, 2003; Shanley et al., 2002). Although we recognized the non-conservative nature of DOC, we chose DOC because we had consistently useful data on DOC and it represented the solute that typically originates from surficial soil layers and is exported with near-surface runoff. DOC has successfully been adopted in numerous studies to identify flow paths and geographic sources of runoff (Bernal et al., 2006; Brown et al., 1999; McGlynn and McDonnell, 2003). For the Point Peter Brook watershed, Inamdar and Mitchell (2006) found highest Si concentrations in riparian groundwaters followed by groundwater discharging at hillslope seeps (hereby referred to as seep groundwater) (Table 1). Concentrations of Mg2+ followed a trend similar to Si, except that Mg2+ concentrations for the wetland well RS5 were considerably lower than for other locations. The high Si concentrations sampled at the valley-bottom riparian wells suggest that the valley-bottom areas were likely discharge locations for upwelling deeper groundwaters. A similar deduction was also made by Hill (1993) for a glacial till catchment in Ontario, Canada, located about 200 km north of our watershed. While Si and Mg2+ were elevated in groundwaters, highest DOC concentrations were recorded for throughfall and riparian water (Inamdar and Mitchell, 2006). DOC concentrations in conifer throughfall were significantly greater than those recorded under the deciduous canopy. For the EMMA analysis (described below) an average of the deciduous and conifer concentrations was used to represent the throughfall value. EMMA procedures described by Burns et al. (2001) and Christopherson and Hooper (1992) were implemented to identify the controlling end members and the proportions of end member contributions to streamflow (Inamdar and Mitchell, 2006). Bivariate and U-space plots for S1 and S2 showed that stream chemistry could be constrained by three end members for most storm events – throughfall, hillslope-seep and riparian groundwaters. For example, for the event of June 8, 2003 (Fig. 2) stream concentrations for S2 evolved from seep groundwater to throughfall and then towards riparian groundwater. In contrast to S2, concentrations for S1 were displaced further away from seep groundwaters indicating a lesser influence of the seeps at this larger catchment scale (Fig. 2). Although the evolution of event runoff for the small headwater catchment S3 is not presented, these values were even more tightly clustered

Table 1

around seep groundwater since streamflow at S3 originated from the seeps. Clearly, this suggested that the influence of seep groundwater on stream chemistry increased as one moved from the valley-bottom to the ridge top. The proportions of streamwater derived from these three end-members were computed by solving the water and solute mass-balance equations (Burns et al., 2001, equations 4–6). The EMMA model was evaluated by comparing the model-predicted concentrations for Mg2+, Si, DOC, NO 3, Ca2+, and SO24 against observed stream concentrations assuming conservative mixing (Inamdar and Mitchell, 2006). The R2 values for fits between EMMA-predicted and observed concentrations ranged between 0.79 and 0.99, suggesting that the selected three-component EMMA model was a strong predictor of stream solute concentrations. The EMMA model also indicated that maximum riparian groundwater contribution occurred after peak discharge and during streamflow recession. This was supported by riparian groundwater depths which peaked after the discharge peak and remained elevated through recession (Inamdar and Mitchell, 2006).

Selection of storm events A total of 33, 21, 20, and 10 events were monitored for catchments S1, S2, S3, and S5, respectively over the study period (May 2003–June 2004). Of these, 10 events (June 8, July 27, August 2 and 9, September 22 from 2003; and May 9, 20, 22a, 22b, 27 from 2004) are evaluated here. These events were selected because they represented a wide range of storm amounts, intensities, and antecedent moisture conditions. Data from S1, S2, and S3 was available for the 2003 events while data from S2, S3, and S5 was available for the 2004 events. The start of the event was defined when a perceptible rise in discharge was observed after precipitation or the occurrence of first ISCO sample, whichever occurred earlier. The end of the event was defined by the first occurrence of when discharge returned to pre-event values or when a subsequent event began. Discharge per unit area or specific discharge (mm) was the total volumetric flow for the event divided by the catchment area. Antecedent moisture conditions for each storm were computed by: (a) summation of the precipitation amounts for seven days prior to the event (antecedent precipitation index – API7); and (b) average of ground water elevations (antecedent groundwater index – AGI7) for seven days prior to the event. AGI7 values were computed for both the riparian well

Mean concentrations of Si (lmol L1), Mg2+ (lmolc L1), and DOC (lmol L1) in various watershed components

Watershed component

Si (lmol L1)

Mg2+ (lmolc L1)

DOC (lmol L1)

Rainfall Conifer throughfall (Tc) Deciduous throughfall (Td) Topsoil water: average of lysimeters L1, L2, L6, L3, and L4 (TSW) Riparian water: average of RS1, RS2 (RW) Riparian water: wetland well RS5 Seep groundwater – (SGW)

0.4 (0.4) 4 (2) 2 (1) 160 (34) 234 (55) 187 (12) 168 (12)

8 (2) 31 (16) 14 (4) 1123 (113) 1217 (132) 896 (43) 1039 (89)

150 (43) 847 (266) 255 (80) 254 (207) 309 (64) 576 (115) 45 (11)

Standard deviations are provided within parentheses. Refer to Fig. 1 for spatial locations of the components in the watershed.

Contributions of riparian and hillslope waters to storm runoff across multiple catchments 1200

8

1000

6

2

RW

400

0 -20

-10

-2 0

200

10

SGW

-4

SGW

0 0

THF

100

200

-6

300

Si

S1

-8

U1

S1

3

1200

2

THF

1000 800

SGW

1 0

THF -20

600

-1 0

-10

U2

DOC

RW

4

THF

600

U2

DOC

800

121

400

10

-2 -3

RW

-4

200

-5

SGW

0 0

100

S2

-6 200

300

400

Si 1

RW

-7

S2

U1

1

Figure 2 Bivariate plots for Si (lmol L ) and DOC (lmol L ) (left) and U-space mixing diagrams (right) for the event of June 8, 2003 for catchments S1 and S2. The selected end-members throughfall (THF), seep groundwater (SGW), and riparian water (RW) enclose the stream concentrations (open circles). Tracers used in EMMA were – Mg2+, Si, and DOC.

R1 and the hillslope-bench well H2 to provide a more complete picture of the wetness in the catchments.

Results Topographic attributes of the catchment Based on mean catchment gradient, S3 was the steepest followed by S5, S2, and S1 (Table 2). The dominant slope aspect for all three small catchments was northwest, whereas 17% of the hillslopes for S1 were oriented to the west. Field-surveyed surface-saturated areas were highest for S5 at 5.9% of the catchment area followed by S2 (2.0%) and S3 (0.8%). The valley-bottom wetland in S5 constituted 4.7% with 1.2% of

Table 2

the saturated areas on hillslopes. The valley-bottom riparian area in S2 accounted for only 0.7% of the saturated area with the remaining saturation (1.3%) in discrete pockets on hillslope benches. Saturated areas in S3 were limited to the channel head (0.8%). The slope contrast between contributing hillslopes and valley-bottom saturated area was the greatest for S5 with contributing slope gradients averaging 22.8% above the flat valley-bottom wetland. For S2, the slope gradients above the riparian area were slightly less at 18.6%, whereas the mean of the slope gradients contributing to the stream channel in S3 was 20.3%. Visual comparisons of the DWI with field-surveyed valleybottom saturated areas indicated that the DWI corresponding to ‘‘d’’ = 3 m provided the best replication of the areal extent of valley-bottom saturated areas. The mean of the

Watershed characteristics and topographic attributes across the four catchments

Attribute

Catchments S1

S2

S3

S5

696 254–430 14.1 W (17)

3.4 255–307 14.3 NW (57)

1.6 260–307 15.0 NW (39)

1.9 255–304 14.9 NW (49)

Field surveyed saturated area in % and m2 in ( ) Total – Valley-bottom – Hillslope –

2.0 (675) 0.7 (231) 1.3 (444)

0.8 (129) 0 0.8 (129)

5.9 (1122) 4.7 (896) 1.2 (226)

Downslope wetness index (DWI) moments Mean Variance Skew % catchment area with DWI > 10

5.39 1.34 0.90 0.9

5.28 1.36 0.91 0.7

5.67 2.60 1.44 4.3

Area (ha) Relief (m) Mean catchment gradient Dominant aspect (% catchment area)

5.12 1.98 1.61 2.1

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S.P. Inamdar, M.J. Mitchell large event of September 22 (16.5 mm) occurred under dry antecedent conditions with only 2.8 mm of API7 and AGI7 for H2 at 0.68 m. In contrast, the event of May 27, 2004, a moderate intensity event, occurred during extremely wet conditions with ponded water (4 cm) in the hillslope-bench saturated area at H2. The remaining events were classified as moderate (June 8, August 9, 2003, and May 20, 2004) and small (August 2, 2003 and two sequential events on May 22, 2004) with relatively wet but not excessive antecedent moisture conditions. Based on peak 10 min rainfall intensity, the events of July 27, September 22, and May 9 were the most intense. Runoff ratios (event discharge divided by precipitation) and peak runoff values (mm h1) for S1 (696 ha) were always lower than those for the nested catchments S2 (3.4 ha) and S3 (1.6 ha) (Table 3). Among the smaller catchments (S2, S3, and S5), the hollow catchment S3 had the highest runoff ratios across all events except for the large event of May 9. However, peak runoff values for S2 exceeded those of S3 across all spring events except the small event of May 22a. Peak values for S5 were greater than S3 only for the large event of May 9. It is important to note here that although runoff ratios for S2 were lower than S3 this does not indicate that runoff from S3 was lost to infiltration in the riparian area at S2 (Fig. 1). On the contrary, when volumetric rates (m3 h1) are compared, the runoff at S2 was slightly greater than that for S3. The average volumetric discharges for S2 and S3 over the months of May–June 2004 were 16.2 and 14.7 m3 h1, respectively, indicating a 15% increase in runoff volume through the riparian area at S2. We attribute this slight increase in discharge to contributions from the other two small seeps and the slow groundwater discharge in the riparian area at S2 (Fig. 1). With regard to influence of large events on discharge, the greatest increase in runoff ratios occurred for S1. Runoff ratios for the S2, S3 and S5 did not show as large a change as that

DWI was highest for the S5 catchment followed by S2, S3 and S1 (Table 2). The DWI distribution failed to replicate the discrete pockets of saturation because the DEM despite its high resolution (2 m) did not accurately capture the abrupt break in slope at the hillslope benches. Surface-saturated areas for S1 (696 ha) could not be surveyed because of its large area. We compared the areas corresponding to various DWI index values against fieldsurveyed saturated area extents for catchments S5 and S2 to ascertain the extent of saturation in S1. A DWI value of 10 produced the best fits between field-surveyed saturated area extent and that generated from the DWI map (Table 2). A threshold value of 10 indicated a DWI area of 4.3% for S5 that was similar to the valley-bottom wetland area of 4.7%. For S2 the DWI value was 0.9%, which was again similar to the field measured value of 0.7%. Using a threshold of 10, the extent of surface-saturation for S1 was computed to be 2.1%. Although the value of 2.1% for S1 likely represents the valley-bottom saturated areas and does not include the more dynamic hillslope-bench saturated areas, this value provides a useful estimate for comparison against the other catchments.

Hydrologic attributes of the selected events The ten events (Table 3) were grouped into five categories based on precipitation amounts and antecedent moisture. Events of July 27, 2003 (24 mm) and May 9, 2004 (16 mm) were classified as large events that occurred under relatively wet antecedent conditions. Antecedent moisture conditions (Table 3) characterized by AGI7 values for H2 varied over a wide range, while corresponding values for R1 did not show as much variation. This suggests that moisture conditions in the valley-bottom riparian areas were more uniform over the year than for other parts of the watershed. In comparison to the large events of July 27 and May 9, another

Table 3 Events

Hydrologic attributes of the ten selected storm events Total rain (mm)

Rain duration (h)

Peak 10 min rain intensity (mm)

API7 (mm)

AGI7R1 (m)

AGI7H2 (m)

Peak runoff (mm h1)

Event runoff ratio Subcatchments

Subcatchments

S1

S2

S3

S5

S1

S2

S3

S5

Large events, wet antecedent conditions Jul 27, 2003 24.1 4.7 8.4 May 9, 2004 16.0 4.0 5.6

47.5 25.4

0.23 0.24

0.12 0.10

0.27 0.35

0.29 0.40

– 0.36

– 0.23

1.5 0.70

4.2 4.3

– 3.2

– 4.0

Large event, dry antecedent conditions Sep 22, 2003 16.5 5.3 6.1

2.8

0.33

0.68

0.10

–

0.13

–

0.33

–

1.9

–

Moderate event, very wet conditions May 27, 2004 10.7 1.5 3.8

83.0

0.12

–0.04

0.26

0.32

0.33

0.30

0.26

1.7

1.5

1.0

Moderate events, wet antecedent conditions Jun 8, 2003 12.1 2.7 4.3 Aug 9, 2003 11.2 1.5 4.1 May 20, 2004 11.2 0.5 6.9

12.7 31.5 23.1

0.26 0.27 0.19

0.03 0.02 0.12

0.23 0.16 –

0.25 0.27 0.18

– – 0.28

– – 0.17

0.24 0.41 –

1.0 1.2 3.4

– – 1.6

– – 2.6

Small events wet antecedent conditions Aug 2, 2003 5.6 0.3 4.0 May 22a, 2004 3.8 1 1.0 May 22b, 2004 5.6 0.33 2.0

28.2 38.9 33.8

0.25 0.18 0.17

0.07 0.09 0.08

0.07 – –

– 0.36 0.27

– 0.57 0.39

– 0.28 0.22

– – –

– 1.2 2.2

– 1.3 2.1

– 0.5 1.6

Contributions of riparian and hillslope waters to storm runoff across multiple catchments

throughout the event (Fig. 3). For May 9, discharges for S2 and S5 were lower than S3 prior to the event, but exceeded the values for S3 at peak flow, with a return to lower values at the end of this event (Fig. 4). For May 22b, the pre-event discharge for S3 was again greater than S2 and S5, but S2 discharge increased and approached that for S3 at peak flow (Fig. 5). Towards the end of the event, S2 and S5 discharges dropped below the S3 values.

observed for S1. This suggests that there was a greater mobilization of runoff at the S1 catchment scale during large events. Temporal patterns of specific discharge (mm/h) for three (July 27, May 9, and May 22b) of the ten events (Figs. 3–5) displayed distinct trends with catchment scale and event size. Across all three events, pre-event discharge was lowest for the largest catchment S1 and increased with decreasing catchment size for the nested catchments (S3 > S2 > S1). However, catchment S5 (1.9 ha) which was slightly larger in area than S3 (1.6 ha), did not fit this sequence and had pre-event discharges lower than S3. For the event of July 27, discharge for S2 was much higher than S1 at the start of the event and remained greater than S1

Percent contributions of end members to total event discharge for the 10 storms are provided in Table 4 while the temporal patterns of these contributions across the three 5

10 2 15

S1

3

0.0 0.20 R1 (m)

2 1

R1

0 7/27 12

20

0 7/27 10 7/27 02 7/27 06 7/27 10 7/28 02 date

-0.2

7/27 01

7/27 02 date

S1

7/27 03

0.30 7/27 04

0.4

S2

RW 0.8

0.8 RW

SGW fraction

fraction

0.2

0.25

S2

1.0

1.0

0.6

0.15

H2

H2 (m)

5

3

1

0.10

4 discharge(mm/hr)

precip

4

precip (mm)

discharge(mm/hr)

Runoff contributions from end members

0

5

123

SGW

0.4

0.6 0.4

THF

THF 0.2

0.2 0.0 7/27 10

7/27 2

7/27 6

7/27 10

0.0 7/27 12

7/28 2

7/27 01

7/27 02

7/27 03

7/27 04

date

date

Figure 3 Streamflow discharge (mm/h), groundwater elevations (meters below surface), precipitation (mm) [top panel], and fractional end member contributions [bottom panel] for the event of July 27, 2003 for catchments S1 and S2. End members were – throughfall (THF), seep groundwater (SGW), and riparian water (RW).

S2

0.20

1

5/9 06

5/9 07 date

0.22 5/9 09

5/9 08

0.1

S3 15

1

5/9 06

5/9 07 date

5/9 08

S2

H7

5/9 06

5/9 07 date

5/9 08

S3

0.05 5/9 09

0.7

S5 0.8

SGW

0.6

fraction

fraction

fraction

0.6

SGW SGW

0.4

0.6

RW

THF

0.4

THF

THF

5/9 06

0.4

0.04

S5

1 0 5/9 05

20 5/9 09

0.8

0.4

0.0 5/9 05

0.02

0.03 0.5 R5 (m)

R5 2

RW

0.2

0.3

3

RW

0.8 0.6

0.01

1.0

1.0

1.0

4

H7 (m)

10 2

0 5/9 05

0.2

5

3

discharge (mm/hr)

0.0 R1 (m)

4

precip (mm)

0.18 R1

2

discharge(mm/hr)

-0.1

H2 (m)

discharge(mm/hr)

H2

3

5

0 precip

0.16

4

0 5/9 05

5

-0.2

5

0.2

0.2

5/9 07 date

5/9 08

5/9 09

0.0 5/9 05

5/9 06

5/9 07 date

5/9 08

5/9 09

0.0 5/9 05

5/9 06

5/9 07 date

5/9 08

5/9 09

Figure 4 Streamflow discharge (mm/h), groundwater elevations (meters below surface), precipitation (mm) [top panel], and fractional end member contributions [bottom panel] for the event of May 9, 2004 for catchments S2, S3, and S5. End members were – throughfall (THF), seep groundwater (SGW), and riparian water (RW).

S.P. Inamdar, M.J. Mitchell

-0.10

2.0 0.12 R1 (m) -0.05 0.13

R1 1.5 H2

1.0

0.14

0.5 0.0 5/22 11

5/23 12

0.15 5/23 02

5/23 01

3.0

2

2.5

2.0

4

0.00

S3

1.5

8

0.5

10

0.0 5/22 11

0.05

6

1.0

5/23 12

0.035

0.6

0.040 R5 (m)

0.7

5/23 1

2.0 H7 1.5 1.0

0.045 0.5

0.8

S5

0.0 5/22 11

12 5/23 2

5/23 12

0.050 5/23 2

5/23 1 date

1.0

1.0

1.0

S3

S2

S5 0.8

0.8

0.8 SGW 0.6 0.4

fraction

SGW

fraction

fraction

0.5

R5

time

date

0.6 0.4

RW

RW THF 5/23 12

0.6 0.4 SGW 0.2

0.2

0.2 0.0 5/22 11

0.030

precip

H7 (m)

S2

0

2.5

3.0

-0.15

discharge (mm/hr)

0.11

precip (mm)

0.10

2.5

discharge (mm/hr)

3.0

H2 (m)

discharge (mm/hr)

124

THF RW

THF

5/23 01

0.0 5/22 11

5/23 02

5/23 12

5/23 01

0.0 5/22 11

5/23 02

5/23 12

5/23 01

5/23 02

date

date

date

Figure 5 Streamflow discharge (mm/h), groundwater elevations (meters below surface), precipitation (mm) [top panel], and fractional end member contributions [bottom panel] for the event of May 22b, 2004 for catchments S2, S3, and S5. End members were – throughfall (THF), seep groundwater (SGW), and riparian water (RW).

Table 4 Percent contributions of throughfall (THF), seep groundwater (SGW), and riparian water (RW) to storm event runoff for the 10 selected events Events

Catchments S1 (696 ha) THF%

SGW%

S2 (3.4 ha) RW%

S3 (1.6 ha)

S5 (1.9 ha)

THF%

SGW%

RW%

THF%

SGW%

RW%

THF%

SGW%

RW%

Large events, wet antecedent conditions Jul 27, 2003 31 4 65 May 09, 2004 – – –

27 14

7 33

66 53

– 15

– 37

– 48

– 43

– 38

– 19

Large event, dry antecedent conditions Sep 22, 2003 32 21 47

–

–

–

37

19

44

–

–

Moderate event, very wet conditions May 27, 2004 – – –

16

65

19

14

60

20

19

46

35

Moderate events, Jun 8, 2003 Aug 9, 2003 May 20, 2004

wet antecedent conditions 33 23 45 18 29 7 64 38 – – – 30

60 37 51

21 25 19

– – 13

– – 63

– – 24

– – 42

– – 47

– – 12

Small events wet Aug 2, 2003 May 22a, 2004 May 22b, 2004

antecedent conditions 17 52 31 – – – – – –

– 55 52

– 24 33

– 11 12

– 67 63

– 22 24

– 19 31

– 47 39

– 35 30

– 17 25

selected events (July 27, May 9, and May 22b) are presented in Figs. 3–5. Again, specific patterns in end member contributions were apparent with catchment size. Generally, riparian groundwater contributions were highest for the largest catchment S1 and lowest for the headwater hollow catchment S3. However, there were exceptions to this pattern, e.g., riparian groundwater amounts for S2 were similar to S1 for the large event of July 27 and those for S3 exceeded that for S5 for the event of May 9. In comparison, seep groundwater contributions were highest for the hollow

catchment S3 and lowest for S1. The elevated contribution of seep groundwater at S3 was not surprising since flow at S3 originated from the seeps (Fig. 1). Regarding throughfall, highest contributions occurred from the wetland catchment S5 while lowest amounts were generated from S3. Throughfall contributions from S2 and S1 were intermediate of those of S5 and S3. Large/intense events compared to the small events resulted in greater proportion of riparian groundwater amounts and lower contributions of seep groundwater (Ta-

Contributions of riparian and hillslope waters to storm runoff across multiple catchments ble 4). For S1, the contributions of seep groundwater were low for the large and moderate events but highest for the small event of August 2, 2003. The largest amount of riparian groundwater contribution from S1 was during the event of July 27, 2003. The influence of event size was especially obvious for S2 and S3 where the largest riparian groundwater contributions occurred with the large events while seep groundwater amounts were high for moderate and small events. A consistent trend with respect to water sources and event size was not apparent for catchment S5 (Table 4). The temporal patterns of end member contributions during events were very distinct and consistent across all catchments (Figs. 3–5). Streamflow at the start of the event was a mixture of riparian and seep groundwaters with greater proportions of seep water as catchment size decreased (S1 > S2 > S3). Contributions of seep groundwaters (expressed as fractions of streamflow in Figs. 3–5) were highest at the start of the event, declined sharply on the hydrograph rising limb, reached a minimum near the discharge peak, and then recovered to pre-event values through the recession limb. The decline in seep groundwaters was especially dramatic for the large events of July 27 and May 9 compared to the event of May 22b. Contributions from throughfall increased sharply on the rising limb, reached a peak near the discharge peak, followed by a decrease through the recession limb. Again, the rise in the contribution of throughfall amounts was especially striking for the larger events. Among the catchments, S5 and S1 showed much larger increases in throughfall amounts followed by S2 and S1. Riparian groundwater contributions increased through the rising limb and reached a maximum after the throughfall peak and on the hydrograph recession limb. Again similar to throughfall, riparian water amounts were higher for the larger events. Seep groundwater clearly had the greatest influence on discharge at the small catchment scale (S2, S3 and S5) for the event of May 22b where the proportions of seep water were highest across all three catchments.

Groundwater elevations during the events Groundwater elevations for valley-bottom (R1, R5, and H7) and hillslope-bench (H2) positions displayed a distinct response through the events (Figs. 3–5). Groundwater elevations increased through the rising limb, reached peak values at or after the discharge peak, and remained high through hydrograph recession. Valley-bottom wetland well R5 for which the water table was closest to the surface responded the earliest, in contrast to well H2 (located on a hillslopebench) which had the most delayed response. The lag in groundwater response and the elevated groundwater levels during hydrograph recession suggest that groundwater discharge continued to occur even after the streamflows had peaked and were receding. The delayed response of well H7 (located at the base of the hillslope) indicates the possibility of event water infiltrating on the hillslopes and moving downslope as interflow. The increase in groundwater elevations across all wells was largest for the large event of July 27 and lowest for the small event of May 22b. To explore the relationship between event size, groundwater response, and the amount of

125

riparian groundwater contributions, we determined the change in groundwater levels (peak groundwater elevation minus the elevation at the start of the event) for all events. Groundwater elevations for R1 and H2 were used since these results were available for all the 10 events. The correlation between change in groundwater elevations and riparian groundwater amounts was evaluated using Pearson r values. Correlations (r) between increase in R1 and corresponding % riparian water amounts for S1, S2, S3, and S5 were: 0.70, 0.68, 0.04, 0.09, respectively. These values indicate a strong correlation between valley-bottom groundwater and riparian water amounts generated from S1 and S2, but not for S3 and S5. Correlations between change in H2 elevations and riparian groundwater amounts were weaker than the R1 values. Correlation between total event precipitation and riparian groundwater amounts for the same order of catchments (S1, S2, S3, and S5) were: 0.70, 0.77, 0.87, and 0.62, respectively. Again, riparian water amounts were correlated with precipitation for the nested catchments (S1, S2, S3), but not for S5. Correlations between throughfall amounts and precipitation amounts for the four catchments were: 0.66, 0.10, 0.66, and 0.66, respectively.

Discussion Conceptual model for runoff generation Topography has a marked influence on the hydrologic response of this watershed, with the broad ridgetops, steep sideslopes, narrow valley-bottom areas, and the glacial till being the key topographic attributes. The difference in chemistry of the groundwaters discharged at the hillslope seeps and those measured in valley-bottom riparian areas (Table 1) suggests two separate groundwater systems. The local groundwater is discharged at hillslope seeps (where the till and clay layer intersects the surface) and the surface flow is rapidly transmitted downslope due to the steep slope gradients. The valley-bottom saturated areas are maintained by deep groundwaters that seep up at these landscape positions. The importance of deep groundwaters in maintaining wetness in valley-bottom areas in glaciated watersheds in eastern North America has previously been highlighted by Hill (1990) and Roulet (1990). The presence of wet valley-bottom conditions further expedites the movement of seep groundwater that traverses the riparian areas via streamlets. The high velocity of the seep groundwater at the foot of the hillslopes and its channelization along streamlets precludes much mixing with riparian groundwater (Roulet, 1990). The valley-bottom moist riparian/wetland areas also act as loci for interception of throughfall and its delivery to the stream as saturation overland flow (e.g., Hill, 1990). Interception of throughfall also occurs at the isolated hillslope-bench saturated areas from where it is delivered to the drainage network during peak moisture conditions when these distal saturated areas are hydrologically connected to the drainage network. The temporal patterns (Figs. 3–5) and total amounts (Table 4) of end member contributions provide two important observations – (a) riparian groundwater contributions to streamflow (at the start of the event and through the event) were much greater for the larger (696 ha) S1 catchment

126

S.P. Inamdar, M.J. Mitchell

than the smaller headwater catchments (S2, S3, and S5); and that (b) riparian groundwater contributions were at their maximum at or after the peak in streamflow discharge. The delayed expression of riparian water contributions is supported by the delayed response of groundwater elevations in wells H7 and R1 (Figs. 3–5). We attribute this delayed expression of riparian groundwater to its displacement by hillslope interflow. We hypothesize that event water (precipitation and throughfall) infiltrating on the hillslopes moved rapidly as interflow (over the restricting clay layer) and displaced a portion of the riparian groundwater. Prior to the event, the hydraulic gradient at the hillslope-riparian interface was not sufficient to displace riparian water to the stream, and only the largest catchment S1 with a much larger riparian reservoir was able to contribute substantially to streamflow. For the smaller catchments, the contributions of seep waters and their rapid downslope movement overwhelmed contributions from the riparian areas. Elevated hydraulic gradients associated with hillslope flux have already been recognized as an important mechanism for displacement of riparian waters to the stream (Hangen et al., 2001; McGlynn et al., 1999; Pionke et al., 1988; Subagyono et al., 2005; Wenninger et al., 2004). Hangen et al. (2001) and Wenninger et al. (2004) highlighted the role of hillslope interflow in displacing riparian water in forested hillslope catchments in Germany. Subagyono et al. (2005) observed elevated hillslope soilwater potentials associated with subsurface storm flow that was responsible for displacing the near-surface riparian groundwater to the stream. Pionke et al. (1988) working in a small watershed in Pennsylvania, found that riparian saturated areas acted as loci for interception of event water which contributed to streamflow on the rising limb of the hydrograph. Hydraulic gradients in the riparian areas were directed downwards during the rising limb of the hydrograph when event water contributions were at their maximum. However, at and after peak discharge the hydraulic gradients in the riparian

SGW

seep

RW THF Stage 1

2 2 3 3

Stage 2

1 1 Stream hydrograph And stages

RW Stage 3

Figure 6 Conceptual three-stage runoff generation model for the Point Peter Brook watershed. The three stages highlight the relative contributions of throughfall (THF), seep groundwater (SGW), and riparian water (RW) to streamflow. The thickness of the lines/arrows indicates the fractional contributions.

area were directed upwards. Pionke et al. (1988) attributed this change in hydraulic gradients to the arrival of the hillslope interflow at the riparian area and the displacement of riparian waters by this flux. We believe the hydrologic response at Point Peter Brook watershed is similar to that highlighted by Pionke et al. (1988) and others. We present a conceptual model to explain the temporal patterns of end member contributions in three stages over the duration of the storm (Fig. 6): Stage 1: Baseflow or conditions just prior to the storm event Valley-bottom riparian saturated areas are recharged by deep groundwater seepage (and therefore the high Si and Mg2+ concentrations in riparian waters, Table 1) while local groundwater discharged at seeps moved rapidly downslope as surficial runoff along the steep slope gradients. The hydraulic gradient for seep groundwaters is much greater than the seepage gradient in the riparian/wetland areas, especially for the headwater catchments. Although some seep groundwater likely recharges the valley-bottom riparian areas, a large fraction of the seep runoff is delivered to the stream along streamlets. This is the period when the potential for expression of seep runoff in streamflow is at its maximum. Riparian water contributions to streamflow are high at the large catchment scale (S1) which has a larger riparian reservoir. Stage 2: Rising limb of the hydrograph This stage comprises the sharp rise of the hydrograph with increasing contributions of throughfall. Throughfall is intercepted at surface-saturated areas and is delivered to the drainage network as saturation excess runoff. The relative fractions (Figs. 3–5) of seep groundwater drop rapidly as large amounts of throughfall are mobilized. Contributions of riparian groundwater also increase due to: (a) displacement of riparian water by incident throughfall and precipitation (e.g., Waddington et al., 1993); (b) mixing and transport with throughfall water in saturation overland flow; and (c) displacement of riparian groundwater by initial (and increasing) inputs of hillslope interflow. Stage 3: Just after peak flow and recession limb Riparian groundwater contributions to streamflow reach their peak due to elevated hydraulic gradients associated with hillslope event water flux. This is displayed by the elevated groundwater elevations in the valley bottom locations. Throughfall contributions continue to occur from distal saturated areas which are now hydrologically connected to the drainage network under peak catchment wetness. Towards the end of the recession limb, riparian and throughfall contributions recede and the relative proportions of seep groundwater recover to pre-event values.

Runoff contributions with catchment size Riparian water amounts were highest for S1, followed by S5, S2 and S3. Low contributions of riparian water for the headwater-hollow S3 catchment were not surprising, while the high riparian water amounts for S1 were likely associated with the larger riparian storage at this scale. Despite having

Contributions of riparian and hillslope waters to storm runoff across multiple catchments the largest % areal extent of saturated area, riparian water contributions from catchment S5 were much lower than S1. We attribute this difference to the total volume of riparian/ wetland storage in S5. Soil depths in the wetland area at S5 ranged from a maximum of 1 m to values as low as 0.3 m across most of the wetland area. Hence, despite having a large surface area (896 m2), the volume of riparian store in S5 was small and could have only been slightly more than the corresponding valley-bottom riparian store for S2 (S2 riparian area = 231 m2; soil depths = 1.5–2 m). The limited valley-bottom volume at S5 resulted in lower riparian water storage and the smaller expression of riparian groundwater during events. This suggests that use of two-dimensional indices like % saturated area or DWI may not provide a complete picture of the potential for riparian water contributions in catchments. One approach to resolve this problem would be to use a three-dimensional approach such as that proposed by McGlynn and Seibert (2003) for computing the riparian volume in watersheds. Another topographic attribute that was important to the moisture distribution in the Point Peter Brook watershed was slope aspect. Our visual surveys indicated that southfacing hillslopes and riparian areas were much drier compared to north-facing landscape positions. This was also true when we compared north- and south-facing positions corresponding to a specific DWI value (Unpublished data). These observations suggest that while McGlynn and Seibert’s (2003) volumetric index may provide a static measure of potential maximum riparian storage the dynamic nature of riparian (or hillslope) storage and contributions cannot be estimated unless topographic attribute such as aspect that influences both microclimate and evapotranspiration losses is taken into account. In comparison to riparian water contributions, the areal extent of saturated areas had a greater influence on the throughfall amounts. Throughfall contributions were highest for S5 followed by S1, S2, and S3 and in the same order as the % saturated area in the catchments (Table 2). This was not surprising since following our conceptual model, the saturated areas acted as loci for interception and transport of throughfall to the stream. The large surface area, the relatively limited storage volume, and the consistently high groundwater elevations in the wetland in S5 clearly provided a greater opportunity for transport of throughfall as saturation overland flow into stream discharge. The agreement between the patterns of throughfall amounts and the surface saturated area extent and DWI indices confirms that these indices can be useful for estimating throughfall contributions in watersheds like Point Peter Brook. The % throughfall amounts presented in Table 4 however, should be interpreted with caution considering the limits of geochemical tracers in characterizing event water contributions (Buttle, 1994; Kendall et al., 2001; Monteith et al., 2006; Pionke et al., 1993; Rice and Hornberger, 1998; Turner et al., 1987). Runoff at the headwater catchments was clearly influenced by seep groundwater contributions. These groundwater seeps occurred at hillslope breaks and at positions where the glacial till and the clay layer intersected the surface. Since subsurface geology affects the occurrence of these seeps, it is unclear whether the DWI index (which is based purely on surface topography) will be a useful predictive tool for esti-

127

mating seep runoff and therefore the runoff production in the headwater catchments where seeps are prevalent. McGlynn et al. (2004) observed high runoff ratios for headwater catchments and hypothesized that the headwater catchments were the primary regulators of runoff in the Maimai watershed. We also found high runoff ratios for the headwater hollow catchment S3. Although the S5 catchment was also small, it extended to the valley-bottom and contained a valley-bottom wetland and thus was not similar in topography to S3. We also recognize that there were many other headwater catchments in the Point Peter Brook watershed, especially on the south-facing slopes, where runoff (or seep discharge) was very low or negligible that we did not evaluate. Hence, it is difficult to make a conclusive statement on runoff production across catchment scales due to heterogeneities in topography, geology, etc. We believe a larger assemblage of headwater catchments need to be monitored to make such an assessment. We can, however, clearly state that the relative dominance of various watershed units for runoff production varied during non-storm and storm-event periods and for storms with varying size and antecedent moisture conditions as discussed further below.

Influence of storm event size and antecedent moisture conditions on runoff contributions While riparian water contributions to baseflow were substantial at the large catchment scale (696 ha, S1), seep groundwater runoff was the primary contributor to baseflow for the small headwater catchments. However, riparian water contributions increased dramatically across all scales in response to storm events suggesting that the ‘‘passive’’ riparian storage was hydrologically ‘‘activated’’ in response to elevated hydraulic gradients associated with hillslope flux. These observations suggest that the relative importance of runoff sources may shift dramatically between non-storm and storm event periods. Riparian water contributions varied with event size with the largest and most intense events (July 27, May 09) producing the largest contributions. In contrast, riparian water contributions were low for the small events (August 2 and May 22, Table 4). Not surprisingly, this suggests that hillslope interflow was more pronounced for large, intense, precipitation events that resulted in elevated hydraulic gradients for the displacement of riparian waters to the stream. The corresponding increase in riparian groundwater elevations (Figs. 3–5) associated with the large events supports this hypothesis. In addition to event size, antecedent moisture conditions also influenced the proportions of end member contributions. Events that occurred during wetter parts of the year (May, June events) resulted in greater seep groundwater contributions compared to events in August and September. This was especially true for moderate and small events (e.g., May 27, May 22) (Table 4). Wet antecedent conditions resulted in greater groundwater discharges at the seeps (Fig. 5), which for moderate and small events made up a significant portion of the streamflow discharge. The influence of catchment wetness and storm event size on source contributions has been reported in a number of

128 previous studies. McGlynn and McDonnell (2003) working in headwater forested catchments in Maimai, New Zealand, found that riparian water was the dominant runoff source between events, during small events, and on the early portion of large events. However, contributions from hillslope water increased substantially for large events, with hillslope runoff occurring during hydrograph recession. They indicated that while hillslope runoff likely displaced some riparian storage, a portion of the hillslope water also likely bypassed the riparian store and contributed to streamflow during peak runoff periods. This study was however conducted in a much wetter climate compared to Point Peter Brook with an annual rainfall of 2600 mm and with soils remaining within 10% of saturation for much of the year (McGlynn and McDonnell, 2003). This level of wetness suggests a greater amount of available moisture in both the hillslope and the riparian reservoirs compared to our study site. Working in a 10 ha unglaciated, forested catchment in Georgia, USA, Burns et al. (2001) identified riparian water, runoff from a rock outcrop, and hillslope water as the three end members for storm event discharge. While riparian runoff comprised the ascending and descending limbs and was the largest contributor to total event runoff, contributions from the rock outcrop which comprised 30% of the catchment area were significant during peak streamflow discharge. Burns et al. (2001) also suggested that runoff from hillslope and rock outcrop sources may mix and move through riparian zone during periods of high discharge. For a 14 ha mountainous forested catchment in Japan, Subagyono et al. (2005) reported hillslope soil water, near-surface riparian water and deep-riparian water as the key contributors to streamflow. However, while the near-surface groundwater contributed most of the streamflow across all events, the contributions from the deep riparian reservoir were found to be small. Contributions from hillslope soil water were high for a large summer storm event (Subagyono et al., 2005) but much less for smaller winter storm. In contrast to these studies, Bernal et al. (2006) working in a 1050 ha Mediterranean watershed found no consistent increases in hillslope water contributions with increase in catchment wetness or storm event size. Furthermore, although Bernal et al. (2006) also identified riparian groundwater, hillslope groundwater and event water as the end members, they also noted that the end members could only explain the stream chemistry for the wetter period of the year and failed to do so during the drier seasons. These studies clearly show that catchment wetness and storm event size may have important implications for the temporal sequencing and contributions from runoff sources. While we hypothesized that hillslope interflow for instrumental in displacing riparian water, unlike McGlynn and McDonnell (2003) and Burns et al. (2001) we did not see expression of hillslope interflow or soil water in streamflow at peak discharge. It is likely that the storm events that we monitored were not large enough for the hillslope flux to exceed the riparian threshold or it is also possible that the continuous release of hillslope groundwater at seeps reduced the magnitude of hillslope flux associated with the storm events. The

S.P. Inamdar, M.J. Mitchell occurrence of hillslope seeps was a direct consequence of the glaciated topography and geology of Point Peter Brook. Thus, catchment topography and volumes of riparian and hillslope aquifers in addition to catchment wetness and storm event size are critical factors that need to be accounted for when developing conceptual models runoff sources in catchments. The influence of upland and riparian aquifer volumes and catchment topography for riparian hydrology was recently characterized by Vidon and Hill (2004) who presented six possible conceptual scenarios of upland-riparian hydrogeologic settings for glacial till and outwash landscapes. While Vidon and Hill (2004) did not determine runoff source contributions, these conceptual scenarios can be used as initial templates to further explore the controls of topography and riparian-upland volumes on runoff contributions from riparian and hillslope reservoirs.

Conclusions The contributions of hillslope and riparian sources of runoff to streamflow were determined across four catchments (1.6–696 ha) and ten storm event with varying antecedent moisture conditions. A conceptual model explaining the sequencing of throughfall, seep groundwater, and riparian water to stormflow was developed. Important insights and deductions from this study included: • The relative amounts of runoff sources for streamflow shifted with storm and non-storm periods. Seep groundwater contributions were elevated during non-storm periods while riparian groundwater contributions were highest during peak discharge and recession. Steep slope gradients and narrow, moist, valley-bottom riparian zones enhanced the expression of seep groundwaters in streamflow. These results suggest that catchments with such topographic settings may provide limited opportunity for the riparian areas to buffer the contributions of hillslope runoff. • Runoff contributions from valley-bottom riparian areas were at their maximum at or after the streamflow discharge peak and were attributed to the displacement of the riparian water by hillslope interflow. • Riparian water contributions were much greater during large storm events suggesting a shift in contributions from watershed units/positions with event size and antecedent moisture conditions. The increased role of valleybottom riparian areas in runoff production was also apparent with an increase in catchment size. • Two-dimensional topographic indices (such as DWI and % saturated area) may explain throughfall or saturation overland flow contributions; however, three-dimensional or volume-based assessments are needed to better characterize runoff displacement from valley-bottom riparian and wetland areas. • A combination of hydrometric, geochemical, and landscape analysis procedures provided important insights into how watershed units respond during storm events. These observations are critical for furthering our understanding of how hydrologic processes ‘‘scale-up’’ from small to large scale catchments.

Contributions of riparian and hillslope waters to storm runoff across multiple catchments

Acknowledgements We would like to thank the USDA-NRI for funding this project via a New Investigator award to S. Inamdar (#2002–00847). We are extremely grateful to the Gowanda Water Department for providing access to the watershed. Joanna Tuk and Julia Graham are thanked for sample collection and data management. Pat McHale and David Lyons provided the water chemistry results expeditiously. We would also like to thank the Great Lakes Center at Buffalo State College for providing tuition support for the graduate students.

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