RIPARIAN HYDRAULIC GRADIENT AND WATER TABLE  DYNAMICS IN TWO STEEP HEADWATER STREAMS

ABSTRACT

Patterns of riparian hydraulic gradients and flows in headwater catchments provide the hydrologic context for important ecological processes, but are not well understood.  Of particular importance is the relative dominance of down- and cross-valley hydraulic gradients, which are a primary control on stream-aquifer exchange dynamics and subsurface residence time of stream water.  We investigate hydraulic gradient and water table dynamics over different time scales in the riparian zones of two steep, forested, headwater catchments in the H.J. Andrews Experimental Forest, Oregon.  Groundwater level and stream stage data collected at high spatial and temporal resolutions over a period encompassing a 1.25-year storm and subsequent baseflow recession indicate that both riparian zones exhibit strong seasonal downvalley dominance in hydraulic gradients, and responses to rainfall input that do not adhere to simple conceptual models of riparian water table rise.  Four constant-rate tracer injections in each stream showed a seasonal increase in the intrusion of tracer-labeled stream water into riparian aquifers as stream discharge receded.  In one riparian zone this was linked to the seasonal changes observed in hydraulic gradients, which would tend to increase the potential for hyporheic exchange flows.  The similar hydraulic gradient response of the same riparian area to the storm implied the potential for increased stream-groundwater exchange due to the storm, contrary to studies finding increased hyporheic exchange with decreased flows (otherwise supported by seasonal tracer data here).  Despite similarity in size, location, and geology, only one watershed exhibited repeated diurnal fluctuations in stream flow and water table elevation during the dry summer, likely because the other lacked riparian vegetation.

 

Spatial and temporal patterns in water level fluctuations, showing increased magnitude with greater distance from the stream and increasing magnitudes through the season, support the idea that the stream has a buffering effect on water table dynamics that diminishes with distance.  Time lag between minimum vapor pressure deficit and minimum water level increased in all cases throughout the season, but at a much faster rate in the stream than in the riparian wells.  This points to the possible down-network accumulation of upstream ET signals that could distort the timing of minimum water level in the stream, as has been previously proposed.

 

 

TABLE OF CONTENTS

LIST OF FIGURES …………………………………………………………………………………………….. viii

LIST OF TABLES ………………………………………………………………………………………………. xvi

ACKNOWLEDGEMENTS …………………………………………………………………………………… xvii

Chapter 1  Introduction ………………………………………………………………………………….. 1

1.1   Prevalence of Headwater Streams ……………………………………………………… 1 1.2   Ecologic Importance of Headwaters ……………………………………………………. 1

1.3   Riparian Zones and the Terrestrial-Aquatic Interface ……………………………. 2

1.4   Movement of Water in Riparian Zones ……………………………………………….. 3

1.5   Riparian Water Table Fluctuations – Diurnal, Storm, and Seasonal …………. 5

1.6   Conclusion ……………………………………………………………………………………….. 7

1.7   Scope and Objectives ………………………………………………………………………… 8

Chapter 2  Site Description ……………………………………………………………………………… 11

2.1   Physical Description ………………………………………………………………………….. 11

2.2   Climate ……………………………………………………………………………………………. 11

2.3   Geology and Soils ……………………………………………………………………………… 13 2.4   Vegetation and History of Management ……………………………………………… 14

2.5   Study Reaches ………………………………………………………………………………….. 16

Chapter 3  Field Methods and Data Processing ………………………………………………….. 20

3.1   Monitoring Networks ………………………………………………………………………… 20

3.1.1   WS01 Monitoring Network ……………………………………………………….. 21

3.1.2   WS03 Monitoring Network ……………………………………………………….. 22

3.2   Constant-Rate Salt Tracer Injections …………………………………………………… 22

3.2.1   Injection of Salt Tracer ……………………………………………………………… 23

3.2.2   Manual Measurement of Well Electrical Conductivity ………………….. 23

3.3   Instrumentation ……………………………………………………………………………….. 24

3.3.1   Water Level Loggers ………………………………………………………………… 24

3.3.1.1   Pressure-based Water Level Loggers ………………………………… 24

3.3.1.2  Capacitance-based Water Level Loggers …………………………….. 26

3.3.1.3   Logger Deployment ………………………………………………………… 28

3.3.1.4  Reference to Datum andCorroboration with Depth Sounder.. 29

3.3.2   Electrical Conductivity (EC) Probes and Loggers ………………………….. 29

3.3.2.1   Handheld EC Meter ………………………………………………………… 29

3.3.2.2  In-stream EC Loggers ……………………………………………………….. 30

3.4   Other Data Sources …………………………………………………………………………… 30

3.4.1   Atmospheric Data ……………………………………………………………………. 30

3.4.2   Flow Data ……………………………………………………………………………….. 31

3.5   Water Level Data Processing ……………………………………………………………… 31

3.5.1   Disturbances to Water Level Records ………………………………………… 31

3.5.2   Referencing Water Levels to a Common Datum ………………………….. 32

3.5.3   Removing Small Errors from EC Measurement Disturbance………….. 33

3.5.4   Application of a Simple Smoothing Function ………………………………. 34

3.5.5  Lessons Learned ……………………………………………………………………….. 35

3.6   Interpolation of Water Elevations and Calculation of Hydraulic

Gradients …………………………………………………………………………………………. 35

3.6.1  Interpolation Scheme ……………………………………………………………….. 35 3.6.2  Calculation of Gradients ……………………………………………………………. 36

3.6.3  Resolution of Gradients into Down- and Cross-valley Components … 37

3.7   Water Table Elevation Anomalies ……………………………………………………….. 37

3.8   Timing of Daily Peak and Trough Water Elevation ………………………………… 38

Chapter 4  Hydraulic Gradient Dynamics …………………………………………………………… 39

4.1  Analysis over Three Time Scales ………………………………………………………….. 39

4.2  Spatial Definition of Gradients…………………………………………………………….. 40

4.3  Gradient Dynamics Over the Seasonal Time Scale …………………………………. 42

4.3.1  WS01 Gradient Time Series ……………………………………………………….. 42 4.3.2  WS01 Seasonal Salt Tracer Intrusion into Riparian Zone ……………….. 46

4.3.3  WS03 Gradient Time Series ……………………………………………………….. 49

4.3.4  WS03 Seasonal Salt Tracer Intrusion into Riparian Zone ……………….. 51

4.4  Gradient Dynamics Over the Storm Time Scale …………………………………….. 54

4.4.1  WS01 ………………………………………………………………………………………. 54

4.4.2  WS03 ………………………………………………………………………………………. 57

4.5  Gradient Dynamics over the Daily Time Scale ……………………………………….. 61

4.5.1  WS01 ………………………………………………………………………………………. 61

4.5.2  WS03 ………………………………………………………………………………………. 65

4.6  Summary and Discussion ……………………………………………………………………. 65

4.6.1  Seasonal Time Scale ………………………………………………………………….. 65

4.6.2  Storm Time Scale ……………………………………………………………………… 66

4.6.3  Daily Time Scale ……………………………………………………………………….. 67

4.6.4  Discussion of Related Literature …………………………………………………. 68

Chapter 5  Water Table Dynamics ……………………………………………………………………. 73

5.1  Water Level Anomalies ………………………………………………………………………. 73

5.1.1  WS01 Anomalies ………………………………………………………………………. 73

5.1.2  Investigating Patterns in WS01 Riparian Anomalies ……………………… 80

5.1.2.1  Relation of Connectivity to Distance from Stream in WS01 ….. 82

5.1.2.2  Consistency of Water Table Response in WS01 …………………… 84

5.1.2.3  Possible Role of Tree Proximity in Anomaly Magnitude in

WS01 ………………………………………………………………………………….. 86

5.1.3  WS03 Anomalies (or Lack Thereof) …………………………………………….. 87

5.1.4  Accounting for the Difference between WS01 and WS03 ……………… 90

5.1.5  Timing of Daily Peak and Trough Water Levels …………………………….. 92

5.2  Discussion of WS01 Conceptual Models ……………………………………………….. 97

5.3  Summary of Findings …………………………………………………………………………. 101

Chapter 6  Conclusions …………………………………………………………………………………… 103

References ……………………………………………………………………………………………………. 107

Chapter 1

 

Introduction

1.1   Prevalence of Headwater Streams

Many researchers in the fields of ecology, hydrology, biology, and chemistry have recognized the importance of first- and second-order headwater streams in the broader context of large stream networks and drainage basins.  Early work by Horton [1945] and later contributions by Strahler *1957+ established ‘stream order’ as a means of quantitatively measuring the prevalence of differently sized geomorphic features in drainage basins.  Their work revealed that low-order, headwater streams greatly dominated stream networks both in terms of stream channel length and upstream drainage area.  In the United States, headwater streams make up 70 to 80% of stream networks by channel length [Leopold et al., 1964; Meyer et al., 2007].  In an assessment of nitrogen pollution in European rivers Haycock et al. [1993] noted that it was commonly recognized that first- and second-order streams contribute more than 90% of flow to most rivers by the time they reach the ocean.

1.2   Ecologic Importance of Headwaters

Headwater streams’ prevalence in terms of channel length and drainage area leads logically to their disproportionately large influence on downstream water quality.  Regulating water quality in headwater streams is a priority first step in the larger scheme of remediating European rivers, strictly because of their dominant contribution to total river flow [Haycock et al., 1993].  Headwater streams perform a number of important functions, including maintaining natural discharge regimes, regulation and retention of sediment and nutrients, processing organic matter, and determining the character of water quality in that region [Lowe and Likens, 2005].  Many studies demonstrate the role headwater streams play in retention of sediment [Dieterich and Anderson, 1998], dissolved organic carbon [McDowell, 1985], particulate organic matter [Jones and Smock, 1991; Webster et al., 1994], and nitrogen, typically as nitrate [Haycock et al., 1993; Jacobs and Gilliam, 1985; Triska et al., 1989; Valett et al., 1996; Dieterich and Anderson, 1998; Peterson et al., 2001; Thomas et al., 2001, Alexander et al., 2007].  Nitrogen, generated in large quantities by human activity [Thomas et al., 2001], can be toxic to humans and animals in high concentrations and is therefore classified as a pollutant.  Alexander et al. [2007] find that headwater streams in the northeast U.S. contribute 55% of flow and 40% of nitrogen input to higher-order receiving rivers, highlighting the substantial downstream impacts of many headwater streams taken together.  Effective transformation and retention of inorganic nitrogen in headwater streams can help to prevent eutrophication in downstream waters [Peterson et al., 2001], such as that which affects waters in the Lower Susquehanna

Basin and Chesepeake Bay in the northeast U.S. [Millard et al., 2001].

1.3   Riparian Zones and the Terrestrial-Aquatic Interface

Riparian zones, commonly defined as the vegetated strips of land adjacent to stream channels in valley bottoms [Hill, 1996], play a special role in the transformation and retention of nutrients like nitrogen [Jacobs and Gilliam, 1985; Altman and Parizek, 1995; Hill, 1996; Burt et al., 1999; Vidon et al., 2010].  Riparian zones are also commonly thought of as the interface between the aquatic (stream) and terrestrial (hillslope and valley bottom) environments [Hill,

1996, Meyer et al., 2007], an interface identified as a critical point for nutrient flux and

chemical reactant availability and regulated by stream-groundwater exchange [Morrice et al., 1997; Dahm et al., 1998].  Presence of this interface leads to the development of so-called ‘hot spots’ and ‘hot moments’ in riparian or other areas of stream-groundwater exchange, where conditions are just right for substantial biogeochemical activity [Hill, 1996; McClain et al., 2003; Vidon et al., 2010].  Both time and space must favor this activity, however, as one without the other may fail to produce a beneficial outcome.  For instance, Burt et al. [1999] showed that great potential existed for denitrification in the organic-rich upper soil horizon of a riparian zone, but it was largely underexploited because the timing of nitrogen-bearing water flowing from the hillslope seldom brought the water into that soil horizon to cause a ‘hot moment’.  Similarly, a lack of sufficient organic matter or other necessary biogeochemical reactants (i.e. those required to generate a ‘hot spot’) obviates the potential for flow of nutrient-bearing water through that space, since one half of the equation is missing.  This underlines the importance of studying hydrologic flowpaths within riparian zones in order to establish the link between ecology and hydrology and to improve the prediction of the timing and location of hot spots and hot moments, with a view to undertaking management programs that use this knowledge to ultimately improve downstream water quality in affected regions [Haycock et al.,

1993].

1.4   Movement of Water in Riparian Zones

Because riparian zones (particularly those in headwater catchments) tend to be nearly saturated with water much of the time and intimately connected with the streams they interface with, flow in these systems under varying climatic conditions can be complicated and sometimes counter-intuitive.  The most notable aspect of flow in these systems that has been studied and discussed in recent years is the link between groundwater and surface water, which had previously tended to be discussed and managed as though they were separate entities [Jones and Holmes, 1996; Winter, 1998].  Recognizing that streams and their adjacent riparian aquifers were frequently well connected required that conceptual models of flow in those systems be expanded to account for this exchange and the more complex gradients and flow paths it implied [Larkin and Sharp, 1992; Woessner, 2000].  An important aspect of this conceptual expansion was the recognition that valley-bottom riparian flow does not occur only in a perpendicular (cross-valley) direction relative to the stream, but is instead threedimensional in nature, with groundwater often flowing oblique to the direction of streamflow

[Harbaugh and Getzen, 1977; Prince, 1980; Alley, 1993; Haycock et al., 1993; Altman and Parizek, 1995; Hill, 1996].  Many researchers have recognized this and analyzed threedimensional riparian flow systems in various headwater locations [Harvey and Bencala, 1993;

Morrice et al., 1997; Wroblicky et al., 1998; Kasahara and Wondzell, 2003; Vidon and Hill, 2004; Wondzell, 2006].  But comparatively fewer have conducted seasonal monitoring of riparian water table elevations at high temporal and spatial resolution in order to provide a detailed characterization of the relative dominance of down- and cross-valley hydraulic gradients, and combined this with solute transport data from tracer injections to assess the impact of gradient changes on riparian stream-groundwater exchange.

The three-dimensional nature of riparian flow, and in particular the relative magnitudes of down- and cross-valley gradients has implications for studying and engineering the ecological function of riparian buffer zones.  Alley [1993] suggests that in many cases it would be prudent to conduct several seasons of water level monitoring to establish patterns of gradients and flowpaths prior to collecting water quality samples, so that transformation or retention of nutrients or other groundwater constituents can be tracked accurately for a given parcel of water along its flow path.  From the perspective of riparian zones serving as buffers against nonpoint-source groundwater pollution, residence time is an important consideration that is directly determined by riparian hydraulic gradients [Haycock et al., 1993].  Riparian groundwater that tends to flow parallel to the streamflow direction (down-valley) will spend much more time in the soil before entering surface water (if it does at all), the impact being that even for narrow riparian corridors, actual retention times can be substantially greater than expected [Haycock et al., 1993].  Better understanding of the spatial and temporal distribution of down- and cross-valley gradient dominance in a riparian area can therefore greatly inform both scientific studies aimed at tracking biogeochemical cycling and management strategies for protecting headwater streams.

1.5   Riparian Water Table Fluctuations – Diurnal, Storm, and Seasonal

In addition to riparian-scale hydraulic gradient patterns that result from seasonal fluctuations in the water table, there are other aspects of riparian water table dynamics that have been studied and could teach us more about spatially characterizing riparian areas.  In particular, numerous studies have focused on fluctuations in groundwater levels and stream stage and flow that occur in a diurnal cycle, of which Gribovski et al. [2010] provide a good review.  Of these, the majority focus on diurnal fluctuations caused by regular variations in evapotranspiration (ET) demand from phreatophytic (water-loving) or other vegetation close enough to the saturated zone to produce effects on the groundwater level and stream stage.  These studies approach this phenomenon from different perspectives, with most focusing on estimation of streamflow volumes “lost” to vegetation through ET *White, 1932; Troxell, 1936; Dunford and Fletcher, 1947; Robinson, 1958; Tschinkel, 1963; Meyboom, 1964, 1967; Reigner,

1966; Weisman, 1977; Baird et al., 2005; Loheide et al., 2005; Lautz, 2007; Gribovski et al., 2008; Loheide, 2008], and others focusing more on what could be learned about physical watershed or riparian zone processes on the basis of the fluctuations or changes in them [Weisman, 1977; Burt, 1979; Bren, 1997; Butler et al., 2007].  A number of studies have also been conducted at the H.J. Andrews Experimental Forest (HJA) field sites used in this work, aimed at assessing the extent of vegetation influence on ET-induced fluctuations in streamflow and hillslope groundwater flow [Bond et al., 2002; Barnard et al., 2010] and how they fit into conceptual models about riparian zone hydrology [Gooseff et al., 2008; Wondzell et al., 2009] and the headwater stream hydrology on the whole [Wondzell et al., 2007].

The study of diurnal fluctuations in riparian water tables is useful in management of water resources and for improving hydrogeologic and eco-hydrologic characterization of watersheds [Gribovski et al., 2010].  Water table fluctuations that occur also in response to storm events, seasonal drying, and the accumulated effect of many consecutive diurnal fluctuations, found to speed flow recession [Federer, 1973; Weisman, 1977], are of similar interest and can be useful in learning more about the behavior of riparian zones and their role in mediating stream-terrestrial linkages.  Here, we seek to use high resolution water elevation data supplemented by thorough literature review to comment on the work performed at our field sites in HJA, in order to both test and refine, if necessary, conceptual models already proposed based on past (recent) data collected there.  Our objectives include assessing diurnal, storm, and seasonal water table fluctuations.

1.6   Conclusion

Since processes like denitrification tend to be focused in localized hot spots of low oxygen and high organic carbon [Parkin, 1987; Murray et al., 1995; Hill, 1996], for example, and hydrologic flowpaths are essential in creating and carrying nutrients and biochemical reactants and products to and from these sites [McCain et al., 2003; Vidon et al., 2010], understanding how water is likely to flow in morphologically and hydrologically distinct headwater catchments will enable us to predict how the efficacy of nutrient cycling and other ecological processes varies between regions.  Furthermore, due to the complex nature of groundwater flow in headwater riparian areas, there is a need for studies that assess patterns of hydraulic gradients and potential hydrologic flowpaths at the spatial scale of the riparian zone.  Although several researchers in the last few decades have shown experimentally and through the use of models that the down-valley gradient plays a prominent role in determining the complex combinations of hydraulic gradient magnitudes and directions that can occur in headwater riparian areas, none have explicitly examined the relative dominance of down- and cross-valley gradient components as they vary both in space across the riparian zone and in time throughout seasonally varying flow conditions.  Such variation in flow and water table elevation is found in the baseflow recession that occurs during summers in the western Cascades of Oregon, at our study sites located within the H.J. Andrews Experimental Forest LTER site.

This work serves as an initial step towards improving our ability to predict and assess the extent of stream-groundwater interaction in steep, narrow headwater streams with the use of experimental field water elevation and solute transport data.  It also probes the potential of using temporal dynamics in a network of riparian water elevation records to detect spatial patterns in the riparian zone that may be indicative of similar behavior in other headwater riparian zones.  Lastly, it is part of a larger body of experimental work performed at two wellstudied headwater streams in the H.J. Andrews Experimental Forest (HJA), and will complement with new data what has already been learned there, while providing a clearer conceptual understanding of how these systems work that can inform and aid the planning of future work at these sites.

1.7   Scope and Objectives

The scope of this work consists of high-resolution temporal measurement of groundwater-level and stream-stage elevations in spatially dense monitoring networks in two steep, second-order headwater stream reaches in western Oregon, USA.  Interpretation of water table elevation data is limited to evaluation of hydraulic gradients based on the 3dimensional phreatic water surface as measured in wells and the open stream channel.  No vertical hydraulic gradients are analyzed at point locations in the stream to study upwelling and downwelling of water.  We also use fluid electrical conductivity (EC) data collected from the monitoring well networks during a series of constant-rate salt tracer injections into both streams conducted throughout the summer of 2010.  Our research objectives can be divided into two primary categories:

1. Hydraulic gradients. Although many researchers have acknowledged the down-valley gradient in their studies of floodplain and riparian water table dynamics, many continue to assume, often implicitly through 2-dimensional elevation-view diagrams (i.e. lateral cross-sections), that groundwater in riparian areas flows perpendicularly towards or away from stream channels. Additionally, while numerous researchers have studied nutrient cycling and retention along groundwater flowpaths from upland agricultural or other sites through riparian areas to streams, relatively fewer have examined transport of solutes and nutrients from the stream to the riparian zone, and of these few have assessed how the spatial extent of this exchange zone varies through time and with changing flow conditions.  Therefore, the research objectives for this first part of our work are:

1a.  To gain better understanding of how down- vs. cross-valley dominance in hydraulic gradients varies in space across headwater riparian zones and in time across both long (seasonal baseflow recession) and short (storm, daily) time scales

1b.  To assess the impact of seasonal baseflow recession on the extent of stream water intrusion into the riparian areas adjacent to streams receiving the salt tracer injections

2. Spatial and Temporal Water Table Dynamics. A significant volume of work exists that documents and analyzes various aspects of diurnal fluctuations in groundwater levels, stream stage, and discharge, but few studies focus on small, spatially dense networks of water table elevation measurements.  Additionally, there is utility in work aimed at characterizing hydraulic behavior of different areas of a riparian zone by specifically seeking to identify small-scale spatial patterns in relationships between different measures of water table dynamics over seasonal time scales and in view of proximity to sources of evapotranspiration (ET) demand and the position within the valley bottom

(i.e. distance from stream thalweg).  Our objectives in this second part of the work are:

2a.  To evaluate how diurnal water table fluctuation magnitudes are organized spatially and how they change throughout the baseflow recession period

2b.  To track how, if at all, the timing of the daily peak and trough water levels change throughout the baseflow recession season

2c.  To assess whether the magnitude  of the diurnal water table fluctuations (maximum minus minimum value)correlates with proximity of wells to nearby trees, which may act as localized sources of ET water withdrawal

2d.  To assess whether the proximity of a well to the stream thalweg correlates with the following aspects of water table dynamics:

1. Magnitude of diurnal water table fluctuations ii. Seasonal drop in water level over the summer baseflow recession iii. Total rise in water level in response to a 1.25-year storm event

2e.  To assess whether any correlation exists in comparing the following water table dynamics to each other:

1. Magnitude of diurnal water table fluctuations vs. seasonal drop ii. Magnitude of diurnal water table fluctuations vs. storm rise

 

RIPARIAN HYDRAULIC GRADIENT AND WATER TABLE  DYNAMICS IN TWO STEEP HEADWATER STREAMS

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