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Hydrochemistry of Forested Catchments - M. Robbins Church

• Defines hydrochemistry and provides an overall introduction to Catchment Hydrochemistry

• Discusses the processes and factors that may influence the fate, transport, and exports of solutes from catchments

• Approaches to studying watershed hydrochemistry

What is Hydrochemistry?

Study of evolution of water or runoff chemistry in catchments and the processes and factors that influence the fate and transport.

What are the key factors that dictate the fate, transport and exports of solutes from catchments?

• Stores/Pools of solutes in the catchment and their location • Hydrologic flow paths and their intersection with these pools • Physical/Chemical and biological processes that regulate the pools

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Figure 1 – conceptualizes the processes, hydrologic flow paths and transport mechanisms for solutes

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Key components coming into play –

• Atmosphere • Vegetation • Soils • Geology • Water bodies or aquatic ecosystems

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Hydrologic flow paths and processes

Key factors that influence the runoff chemistry along the hydrologic flow paths –

• Chemical composition of precipitation • Abiotic materials or biota that are intersected by flow paths • Reactivity of solutes • Contact time of water and solutes

Knowledge of flow paths and the contact time provide an idea of how watersheds “work”

Flow paths may vary dramatically between baseflow and storm event periods

Totally new sets of flow paths may be “activated” during storm events

Runoff in catchments can be generated by –

• Direct interception of runoff by the stream channel • Surface flow • Subsurface flow • Groundwater

The flow paths and the amounts of runoff generated via the various mechanisms will be influenced by – precipitation characteristics, topography, geology, soils, and vegetation type.

Various hydrologic processes and mechanisms – we will cover in our introduction to hydrology

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Concepts of Old (pre-event) and New (event) water

• Old water – water residing in the catchment before the storm event – has a greater contact time – will have a different chemistry

• New water – water introduced by precipitation – less contact time

• The relative amounts of these waters in streamflow will have a profound influence on stream water chemistry

• Interestingly, most studies show that the runoff that comes out during storms is primarily old water!

• So the key question is – how does the old water come out? • Many theories explaining the quick discharge of old water from catchments

Various methods have been used to characterize the sources and origins of runoff – Spatial or temporal origins

Techniques that have been used range from –

• Naturally-occurring chemical tracers • Naturally occurring Stable isotopes (especially 18O and 2H) • Other methods – such as numerical of graphical methods for hydrograph separations

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Chemical and Biological Processes

In addition to flow paths, it’s critical to know how the pools of solutes may change with time and the processes that affect these pools

Key processes –

• Physical processes of erosion and gas exchange • Chemical processes of weathering, chemical precipitation, cation exchange, and ion

sorption • Biological processes of uptake, respiration, decomposition, mineralization, oxidation

and reduction

These processes will continuously modify the pools at various time scales – annual and seasonal, event.

The article highlights key examples where such processes have come into play --

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I Acid Deposition and its influence on base cation leaching from soils

Acid deposition?? Base cations???

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Shallow flow paths Less contact time Low cations Low pH – closer to rainfall

Deeper flow paths, Greater contact time More cations Higher pH

Hydrologic flow paths pay an important role in catchment response

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II Forest growth and vegetative uptake on Nitrogen cycling and saturation in forests

Relates to the amount of NO3 that is available to be leached in catchment runoff

Inputs of N – atmosphereic, anthropogenic, internal cycling

The consumption/sequestration/removal of N in catchments is influenced by two key processes

• Vegetative uptake • Denitrification

If plants/forests are removed or if forest reach a stage of maturation – N consumption will decline. This will allow excess N (NO3) to accumulate in catchments and which will be leached with runoff.

There may be seasonal variation too in NO3 exports that are driven by seasonal patterns of forest growth!

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Figures highlighting this aspect

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Approaches to studying Catchment Hydrochemistry

Small Catchment Approach

• Small catchments < 500 ha • Very popular approach to understanding watershed functions and workings • Numerous studies across the world • Pioneered by studies at Hubbard Brook and Coweeta Hydrologic Laboratory • Convenient and manageable in scale • The small size allows for assumptions to be made • Some of the assumptions that have been made may not be correct – “closed” systems

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Coweeta Hydrologic Laboratory http://coweeta.uga.edu/

• Located in the Blue Ridge Physiographic province of North Carolina • 2185 hectares • Streamflow monitoring in 1934 • Stream chemistry monitoring – 1968

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PI – Dr. Wayne Swank

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Hubbard Brook Experimental Forest (HBEF) http://www.hubbardbrook.org/

• Established in 1955 in the White Mountains of New Hampshire • 3307 ha watershed • Stream chemistry monitoring started in 1963 • First watershed where budgets for element cycling were developed

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Dr. Gene E. Likens

Catchment Monitoring

• Measuring the response of catchments – small or large • Important insights into how watershed are behaving and responding to external

influences can be derived from studying watershed data • This data can be long-term or short-term • This data can be used to – test hypotheses, develop conceptual and numerical models of

watershed functioning • Critical that catchment monitoring should be driven or geared towards addressing

specific questions, OR should be designed with an end goal in mind. • Examples of lessons learnt from catchment monitoring!

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Catchment manipulations

• Direct experimentation – e.g., paired watershed studies • Direct evaluation of hypotheses • Typically involves watershed of similar characteristics and response • Manipulations have involved – forest removal, forest practices, fertilizer or chemical

additions

Hubbard Brook Clear cut – from http://www.ecostudies.org/people_sci_likens_experimental_manipulations.html

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Example of the Bear Brook Watershed Manipulation project in Maine – http://www.umaine.edu/drsoils/bbwm/AboutBBWM.htm

http://www.umaine.edu/drsoils/bbwm/Treatment.htm

• Intent – investigate catchment response to input of acidic or acidifying substances • Two 10 ha paired watersheds were selected which were similar in hydrologic and

biogeochemical response • West Bear Brook – dry ammonium sulfate was added – applied bimonthly at the rate of

1800 equivalents per ha per year • Tripled the sulfur loadings and quadrupled the annual loadings of nitrogen • Stream concentrations of treated watershed increased dramatically • Total N output doubled, mainly due to increases in NO3 • Responses indicate – that increase in atmospheric inputs can have a dramatic and rapid

effect on stream chemistry • The responses were consistent with accepted conceptual models • Losses of N during the summer growing season indicated that N had reached deeper soil

horizons and was being exported via deeper flow paths

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Use of Catchment Models

• Useful tool - Especially useful for future long-term predictions – e.g., determining how catchments will respond to decrease in acid deposition

• Models can vary in their complexity • Need to be careful – since predictions could be incorrect or uncertain • Various types of models with different purposes and philosophies – EMMA, TOPMODEL,

PnET-BGC, etc..