Department of Geography, University of Otago, PO Box 56, Dunedin, New Zealand. Abstract The relative importance of headwater bogs and adjacent hillslopes.
Hydrology, Water Resources and Ecology in Headwaters (Proceedings of the HeadWater'98 Conference held at Meran/Merano, Italy, April 1998). IAHS Publ. no. 248, 1998.
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Hillslope-wetland hydrological linkages in the headwaters of a tussock grassland catchment at Glendhu, South Island, New Zealand B. FAHEY, W. B. BOWDEN Landcare Research New Zealand Ltd, PO Box 69, Lincoln, New Zealand
J. SMITH & D. L. MURRAY Department of Geography, University of Otago, PO Box 56, Dunedin, New Zealand
Abstract The relative importance of headwater bogs and adjacent hillslopes in the production of storm runoff and baseflow was investigated in a 3.64-ha first order basin with a central wetland (0.39 ha) at the Glendhu experimental site. Hydrometric data were collected from three throughfiow pits on the headwall slope, and three on the true right bank. Capacitance probes were used to record water-table movements in the central wetland. A weir was installed at the bog outlet later in the study, enabling the water balance for selected periods to be calculated. In the storms examined here the hillslope-wetland-stream system responds as a unit, making it unlikely that flood peaks are attenuated by the presence of the bog. During poststorm recessions, unsaturated flow from the surrounding hillslopes may contribute as much water to baseflow as the bog itself.
INTRODUCTION Headwater wetlands or bogs are common features of streams draining the uplands of east Otago on New Zealand's South Island. Opinions differ over their hydrological importance. Davoren (1978) and Bonell et al. (1990) suggest they may regulate stormflow and sustain baseflow between storms by serving as temporary storage areas as water travels from the surrounding hillslopes to the stream channels. Miller (1994) however, concluded that these features may not be an important source of baseflow. Rather, they may simply link the surrounding hillslopes and the streams that drain them. This paper uses hydrometric data and a water-balance approach to evaluate the relative importance of headwater bogs and adjacent hillslopes in the production of runoff and the maintenance of baseflow.
SITE DESCRIPTION The study was conducted on a 3.64-ha first order basin (GH5) containing a 0.39-ha wetland in the headwaters of the 218 ha tussock grassland catchment (GH1) at the Glendhu experimental site 60 km west of the city of Dunedin (Fahey & Jackson, 1997) (Fig. 1). Measurements began in February 1994 and continued through to May 1996, except for the winter months (June-September). The prevailing vegetation cover on the hillslopes of GH1 and GH5 is narrow-leafed snow tussock (Chionochloa rigida) with an understorey of sphagnum moss. The central bog at GH5 is dominated
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Fig. 1 Map showing the GH5 catchment with central wetland bog, and location of field sites.
by sphagnum, with a mixture of red tussock (Chionochloa rubra) and wire grass (Empodisma minus). Bedrock is weakly-to-strongly weathered quartzo-feldspafhic schist with a mantle of loess 0.5-3 m deep. Soils in the subcatchment are imperfectly drained with a distinct, perched gley horizon. The A- and B- horizons have low bulk density (wlOOO kg m"3), high macro-porosity, and high saturated conductivity (Ksal « 50 mm h"1). The C-horizon (a silty clay loam) has little or no macro-porosity and low hydraulic conductivity (Ksal « 0.5 mm h"1). The bog has a 5-10 cm layer of living sphagnum and wire grass underlain by peat. The profile grades downwards from weakly decomposed (fibric) through moderately decomposed (mesic), to strongly decomposed (humic) peat, and finally to a gleyed layer of silt and sand. The bog is about 3.5 m in depth at the centre. The mean annual rainfall at GH5 (elevation 625 m a.s.l.) is 1240 mm. The average annual temperature is 7.6°C.
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METHODS Six throughflow pits were established in GH5 (Fig. 1). Three were located on a transect down the head of the catchment (headwall, HW). The upper site (HWU) was on a 10° slope, the middle site (HWM) was on an 15° slope, and the lower slope (HWL) was just above the toe slope grading into the bog on a 20° slope. A similar sequence was established on the convex slope forming the true right (TR) bank (TRU on a 7° slope, TRM on a 13° slope, and TRL on a 16° slope). The throughflow pits were 1 m wide and 1-1.5 m deep. Water moving down the hillslope as subsurface flow through the moss (M) layer was trapped by troughs with edges inserted into the base of the layer. Water moving down-slope through the Aand B-horizons was collected by 1-m long sections of slotted 10-cm wide PVC pipe installed at the base of these horizons and separated by heavy plastic sheeting. The water from each layer was directed downslope via alkathene tubing to large tipping buckets connected to data loggers. For the purposes of this paper, the cumulative response from all three levels in each pit will be discussed. Contributing areas for each throughflow pit were assumed to represent a 1-m wide strip extending upslope to the catchment boundary. Water-table fluctuations were recorded with capacitance probes containing dedicated data loggers, installed in 6-cm diameter PVC wells at three locations approximately 50 m apart down the centre of the bog: at Well-A (near the headwall), at Well-B (mid-bog), and Well-C (the downslope end of the bog) (Fig. 1). The depth to water table was measured during each visit to serve as a check for the logged record. A probe was also installed at the outlet to the bog to measure stream stage heights. All probes were calibrated before installation and re-calibrated at regular intervals during the study. In late 1995 a 45° V-notch weir was constructed across the outlet to the bog. Records are available from mid December 1995 (Smith, 1996). Water levels were monitored with a battery-operated Belfort recorder installed in a stilling well.
HILLSLOPE AND WETLAND RESPONSE TO STORMS Two storms were selected to illustrate the hillslope-bog response to rainfall events. Storm 1 (22-24 December 1994) This was a large summer storm with wet antecedent soil moisture conditions (Fig. 2). Total rainfall was 38.3 mm with a maximum intensity of 9.2 mm h"1. It is clear from Figs 2(a) and (b) that hillslope runoff is very responsive to rainfall. Runoff began at the headwall at 0500 h, only 1 h after the storm began and peaked at 55.3 1 s"1 ha"1 at the lowest hillslope location (HWL) at 0700 h, only 2 h after the peak rainfall intensity. The temporal patterns on the true right slope were similar but area specific runoff was lower. In addition, peak runoff (25.7 1 s"1 ha"1) occurred at the upper location (TRU). The integrated runoff volumes from the headwall slope for the entire storm were 1498, 2484, and 9037 litres at site HWU, HWM and HWL, respectively. For the true right slope, the integrated runoff volumes were 997, 265 and 414 litres
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Fig. 2 Plots for the storm of 20-24 December 1994 of (a) cumulative flow through the moss layer and the A- and B-horizons at the GH5 headwall (HW) at the upper (U), mid (M) and lower (L) sites, (b) flow through the moss layer and the A- and Bhorizons on the GH5 true right (TR) bank at the upper (U), mid (M) and lower (L) sites, (c) water levels in the bog at GH5 for the upper (Well-A), mid (Well-B), and lower (Well-C) locations, (d) stage height at the GH5 bog outlet, and (e) flow and rainfall at the GH1 weir.
respectively for sites TRU, TRM and TRL. The water levels in all three wells in the bog began to rise at the same time in response to runoff from the hillslope. The water level at the upper bog site (Well-A) was at a depth of 214 mm at the start of the storm and rose to within 60 mm of the surface. The mid-bog water level (Well-B) started at a depth of 130 mm, and rose
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174 mm in 3 h during the storm. The error in our measurements of water level might be as much as 20 mm, but these data suggest that water may have been flowing over the surface of the bog at the height of the storm. At Well-C, the water level, which was 115 mm below the surface at the start of the storm, rose 120 mm in 3 h, bringing it at least to the surface. Water levels at the three bog locations and at the bog outlet all peaked simultaneously, and only 1-2 h after peak runoff on the hillslopes. Increased outflow from the bog began at 0500 h (the same time as flow began on the hillslopes), and peaked at a stage height of 237 mm at 0900 h. At the GH1 weir, stormflow was observed at 0500 h, but it peaked (9.5 1 s"1 ha"1) at 0700 h, 2 h earlier thanGH5. Storm 2 (20-24 March 1995) This was a moderately-sized mid-autumn storm with wet antecedent conditions (Fig. 3). Total rainfall was 21.8 mm with a maximum intensity of 3.8 mm h"1 between 2000 h and 2100 h. Although the temporal patterns of storms 1 and 2 differed substantially, the hillslope runoff patterns observed during storm 2 were similar in form to those observed during storm 1. Runoff began at all headwall sites at 2000 h and peaked at 0100 h (Fig. 2(a)). As in storm 1, peak runoff was highest at the HWL site (31.7 1 s"1 ha"1). The temporal patterns of runoff from the true right slope were similar to those on the headwall slope, although the magnitudes were lower. As before runoff was highest at the upper location (TRU), peaking at 11.7 1 s"1 ha"1. On the headwall slope, integrated runoff volumes increased downslope 5-10 times from sites HWU (453 1) and HWM (635 1) to HWL (3737 1). Runoff volumes from the true right slope decreased from 308 1 at TRU to 123 1 at TRM and 131 latTRL. The water level began to rise in the three wells in the bog, nearly simultaneously at 2000 h, approximately 1 h after the start of the storm. Water levels peaked first at Well-A near the headwall (0100 h) followed by Well-B (0300 h), and Well-C (0500 h) 4 h later. The maximum rise (153 mm in 7 h) was recorded at the mid-bog site. Flow at the bog outlet began simultaneously with the rise in the bog wells at 2000 h, but peaked 6 h later at 0200 h at a stage height of 146 mm. Flow at the GH1 weir began to rise about 1 h after the storm began. The peak discharge of 3.7 1 s"1 ha"1 occurred at 0100 h on the 21 March, roughly 4 h after the peak storm intensity. BOG BASEFLOW DYNAMICS Two distinct recession stages can be identified from water-table drainage curves recorded at Well-B (mid-bog position) during eight storms (Table 1). In the first stage, drainage is rapid over the first 24-48 h (mean drainage rate of 80 mm day"1), after which drainage continues at a reduced but relatively constant rate. Similar calculations were not done for other wells in the bog. However, the same two-stage drainage pattern was apparent in the data. It is the second stage that has the potential
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