Hydrology – Review, New paradigms, and Challenges

Intent – quick introduction with emphasis on aspects related to watershed hydrochemistry and new paradigms

Watershed / Catchment DefinitionPortion of landscape that drain into a river, creek, stream, lake or any water bodyTypically defined by surface topography – DEM, contours, etc.

Can be defined using a DEM – e.g., Archer Creek catchment in the Adirondacks, USA. – 3 m DEM

Assumed that no water beyond the boundaries of the catchment contributes to runoff in the catchment – very often this is violated and groundwaters beyond surficial boundaries may contribute.

Especially in cases of flat landscapes with deep aquifers.

Components or parts of a watershed –

• Atmosphere• Vegetation or Forest canopy• Forest floor or litter layer• Soils • Regolith • Bedrock • Streams• Water bodies

Soil profile / rhizosphere

Forest floor

Forest canopy

hillslopes

wetlands

Proportion of the watershed as well as the spatial location matters!

Lakes, ponds

Streams, riparian zones, alluvial zones

Hydrologic connectivity

• All these components/parts of the catchment play an important role in influencing runoff amounts, flowpaths, timing (residence time) and chemistry

• Each of these parts of the catchment have a unique influence on hydrology and biogeochemical response

• Relative influence of these parts will vary with location and scale of the catchment

• Streamflow response is an integrated (in many cases) sum of all these parts

• Our interest – look at the integrated response and determine how each of these parts make up the response

REVIEW - Hydrologic cycle and Hydrologic processes

Hydrologic budget

Water balance

P + Gin – Q – ET – Gout = ΔS

Assumptions in small catchment research?

Different flow paths have different transit times and therefore unique chemical signaturesProportion of “old” and “new” waters or water age may vary with flow paths

The various hydrologic components/processes affected by –

• Climate• Topography• Soils• Vegetation• Geology• Landuse

Precipitation

Rainfall, snowfall, ice storms

• Amount• Intensity• Duration • Frequency/return-period

• Timing/seasonal -- convective versus frontal storms; storms associated with hurricanes / marine influences

• Spatial variation – a factor in the response of large watersheds or catchments

• Antecedent moisture conditions• Antecedent freeze-thaw conditions• Rain on snow events

Snowpack / Snowmelt –

• Presence or absence of snow cover or snow pack - Critical factor in biogeochemical response of catchments -- Why?????

• Snowpack and snowmelt - Significant in the northeast, Mountainous West - Primary driver of hydrology and catchment response. Generates big differences in the annual discharge curve!

• Not as significant in Mid-Atlantic or the South USA

Interception – canopy or forest floor

Definition –

Water collected or intercepted by the vegetation canopy or the litter layer – water which does not reach the soil surface for infiltration or runoff.

Factors -Meteorological Factors• Intensity• Size• Duration• Wind speed• Air temperature

Vegetation Characteristics:• Vegetation (crown) form – conifer,

deciduous, grasses• Plant physiology• Density• Community structure

Will influence the response of streamflow at the watershed outlet – amount and timing

• Interception varies with vegetation type – high for conifers versus deciduous! (on an annual basis)

• Interception for conifers 10 to 50% of annual precip• For non-conifers = 10 to 35% of annual precip. (will vary with leaf cover – or LAI

– leaf area index)

Throughfall and stemflow

Throughfall and stemflow - play an important role in differential input of precipitation to the soil surface.

This differential input facilitates preferential flow in soilsLiang et al’s 2011 (Water Resources Research) figures – showing throughfall and stemflow input in the soil

Which trees have the highest amounts of stemflow and why?

Stemflow

Photo credit – Delphis Levia

Evapotranspiration

DefinitionConversion of water to vapor and its transport away from the evaporating surface.

Solar radiation – main driving source

Losses occur from:• water accumulated on plant surfaces - evaporation• loss from plant stomata – transpiration • loss from water and soil surfaces -evaporation

Significance:ET losses could be as high as 90% in arid climates - typically around 40 to 70% in humid climates400 to 900 mm/yr

Strong controls on hydrologic response in arid climates

ET –• Losses maximum during summer• Max during late noon• Can vary spatially – based on aspect and soil depth

ET –• Losses maximum during summer• Max during late noon• Can vary spatially – based on aspect and soil depth

Primary meteorological factors affecting ET:1. Radiation (MJ m-2 day-1 or W m-2 or Langleys day-1)2. Vapor Pressure (kPa)3. Wind speed (m/s)4. Air temperature (degrees C)5. Relative Humidity (%)Plant-water factors:1. leaf area index (leaf cover)2. root depth (availability of water to the transpiring surface)

Soil-water factors:1. soil moisture2. hydraulic conductivity (rate at which water moves through the soil medium to the surface or the root)

Mean Annual Lake Evaporation

Infiltration

Definition - entry of water into the soil surface as a distinct wetting front

Some soil definitions first!• Porosity • Saturation • Bulk density • Field capacity• Drainable porosity• Wilting point

• Porosity • Saturation • Bulk density and porosity • Field capacity• Drainable porosity• Wilting point

A typical infiltration pattern in non-sandy soils.

Latter stages of infiltration –Soil pores are filled withWater – infiltration rate Controlled by saturated conductivity

Start of infiltration –Soil pores are empty –Infiltration occurs rapidly

Latter stages of infiltration –Soil pores are filled withWater – infiltration rate Controlled by saturated conductivity

Start of infiltration –Soil pores are empty –Infiltration occurs rapidly

• Infiltration rates initially are very high but gradually decrease with time• Ultimately, infiltration will approach the saturated hydraulic

conductivity• Infiltration rate is regulated by either the -• rainfall rate (when rainfall rate is less that the saturated hydraulic

conductivity of the soil)• soil related factors (when rainfall rate exceeds the saturated hydraulic

conductivity)

Factors -Soil properties – hydraulic conductivity, porosity, depth of soil, hydrophobicity, rainfall intensity, preferential flow

Factors affecting Infiltration:Soil properties

Pore size distributionHydraulic conductivity

Layered soils

Presence or absence of the liter layer can have an important impact. Water can pond over the litter surface – “thatched roof” effect.

Vertical drainage

Definition – vertical movement of water through the unsaturated soil profile.

Combination of Soil Matrix flow and preferential flow via macropores and soil pipes

When will vertical preferential flow occur?

Subsurface flow / interflowDefinition - Generated when water perched over an impeding or restricting layer. A saturated layer is formed in which water moves downslope due to gravity

Factors –

• Slope gradient• Soil depth or depth to the restricting layer• Soil hydraulic conductivity• Soil porosity• Preferential flow paths and macropores -- while macropores may

constitute a small fraction of the porosity – they may contribute to a large extent of the subsurface flow

• Antecedent moisture conditions• Rainfall intensity, amount and duration

Models, equations to characterize flowQ = k * S * d

Determining the depth of subsurface saturation????

Rapid subsurface flow can occur as a combination of –• Displacement• Preferential flow

Infiltration-excess surface runoff (or Hortonian overland flow)

Definition – when rainfall input exceeds the infiltration rate of the soil surface.

Typically observed on soil surfaces with low hydraulic conductivity –compacted soils, agricultural and urban surfaces. Very rarely observed on forested soils/landscapes.

Saturation overland flow

Definition – when the soil profile is completely saturated and cannot accept any more rainfall input.

Conditions and factors for occurrence• Where soils thin out• Soils with low storage• Base of hillslopes where the slope changes from steep to mild• Base of converging hillslopes

Return flow

Definition – when subsurface flow is forced to the surface.

Average Annual Runoff

Variable Source Area ConceptConceptualized by –Hewlett and Hibbert 1967 in USCappus 1960 in FranceTsukamoto 1961 in Japan

when shallow groundwater intersects the soil surface• the areal extent of intersection determines saturation

over land flow and return flow• the extent of intersection varies – with moisture and

amount of groundwater

The areal extent of the near-stream saturated area – characterized as the variable source areaVariable - because the areal extent changes depending on the wetness of the catchment.

Rain

T 1

T 2

VSA at T2

VSA at T1

Storm-event expansion & contractionThe variable source area concept (VSA) – dynamic spatial extent

Role of topography in VSA hydrology

Surface topography controlsConvergence/divergence

Anderson and Burt, 1978 – topographic hollows were key hotspots for runoff generation Work of Troch et al., 2009 –

• Convergent slopes – because of storage at the bottom yielded – bell shaped runoff hydrographs

• Divergent slopes – displayed peaked response

Valley bottom wetness in Point Peter Brook watershed – role of VSA dynamics

So, what are the key factors that determine the wetness at a point? -

Topographic Index

Based on the Variable Source Area Concept! -- A catchment scale representation

Potential for saturation at a point in the watershed or hillslope – dependent on

• Contributing area – a – determines the runoff volume at a point• Local slope – that determines the ability to move that runoff volume through

the soil

Potential for wetness characterized by - ln (a/tanB)a – contributing upslope area; B – local slope angleunder uniform recharge, steady state conditions -• high a – more runoff• low a – less runoff

• high tanB – higher hydraulic gradient – less backup of water• low tan B – lower hydraulic gradient – greater backup of water

high ln(a/tanB) – more wetness -- values 9 – 16 low ln(a/tanB) – less wetness or drier areas – values 2 - 5

upper slope areas – low ln(a/tanB); near stream areas – high ln(a/tanB).

Topographic index computations can be performed using programNeeded – DEM (preferably 3m or less)http://www.es.lancs.ac.uk/hfdg/freeware/hfdg_freeware_top.htm

tidwi32 - 55 - 77 - 99 - 1111 - 1313 - 1515 - 1717 - 19No Data

Comparing Topographic index derived saturation potential against actual wetness in the catchmentComparisons for Point Peter Brook (Inamdar et al., 2004)

Valley-bottom wetland in the catchment

Value of Topographic Index maps –

Hydrologic

Biogeochemical – moisture and wetness a driver of many biogeochemical processes!

Why topographic indices do not match field observed wetness?

Bedrock topography controls

Soil moisture distribution and catchment wetness may not necessarily be determined by surface topography in all catchments – especially the case in mild or moderate slopes.

• Soil thickness, bedrock features may come into play!• Bedrock not impermeable in most cases• Bedrock permeability can influence hillslope drainage and the length of the

recession curve.• Exfiltration and downslope leakage of water from bedrock may also be an

important factor• Bedrock topography may be an important control during medium to large

storms• During small storms – the soil depth may have greater controls.

Jim Freer paper and work --

Regional Groundwater & Runoff

Key Challenges in Hydrology:

• Characterizing spatial and temporal variability, unified theory, and catchment classification

• Non-linearity and thresholds

• Scale issues – measurements and modeling

• Water age – measurements and modeling

Non linearity, Threshold responses, and Hydrologic connectivity

Recent work by Tromp-van Meerveld and McDonnell, 2006Spence, 2010Zehe and Sivapalan, 2009

Key examples –Isolated hydrologic patches and their connectionIsolated moisture patches can form due to –

Variations in surface and bedrock topography Filling of depressions in surface and bedrock before runoff begins

Spatially variable precipitation input – concentrated inputs due to throughfall, stemflow, preferential flow paths

Differences in soil depth and porosity

Sudden increase in runoff as isolated patches are connected hydrologically!A non-linear threshold response.

“Fill and spill” hypothesis – runoff will be low when patches of saturation/water accumulation are hydrologically isolated

Patches of wetness could form due to various reasons

Changes with catchment scale

Proportion of landscape units –Response of runoff and its components –