Hydrology Review, New paradigms, and Challenges Intent quick introduction with emphasis on aspects related to watershed hydrochemistry and new paradigms
Watershed / Catchment Definition Portion of landscape that drain into a river, creek, stream, lake or any water body Typically 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 Forest canopy Forest floor Soil profile / rhizosphere
wetlands hillslopes 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 signatures Proportion 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 soils Liang 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 Definition Conversion 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 climates 400 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 moisture 2. 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.
Start of infiltration Soil pores are empty Infiltration occurs rapidly Latter stages of infiltration Soil pores are filled with Water infiltration rate Controlled by saturated conductivity
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 distribution Hydraulic 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 / interflow Definition - 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 flow Q = 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 Concept Conceptualized by Hewlett and Hibbert 1967 in US Cappus 1960 in France Tsukamoto 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 area Variable - because the areal extent changes depending on the wetness of the catchment.
T 2 T 1 Rain VSA at T2 VSA at T1 Storm-event expansion & contraction The variable source area concept (VSA) dynamic spatial extent
Role of topography in VSA hydrology Surface topography controls Convergence/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
So, what are the key factors that determine the wetness at a point? - Valley bottom wetness in Point Peter Brook watershed role of VSA dynamics
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 angle under 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 program Needed DEM (preferably 3m or less) http://www.es.lancs.ac.uk/hfdg/freeware/hfdg_freeware_top.htm
tidwi3 2-5 5-7 7-9 9-11 11-13 13-15 15-17 17-19 No Data
Comparing Topographic index derived saturation potential against actual wetness in the catchment Comparisons 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, 2006 Spence, 2010 Zehe and Sivapalan, 2009 Key examples Isolated hydrologic patches and their connection Isolated 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
Fill and spill hypothesis runoff will be low when patches of saturation/water accumulation are hydrologically isolated Sudden increase in runoff as isolated patches are connected hydrologically! A non-linear threshold response.
Patches of wetness could form due to various reasons
Changes with catchment scale Proportion of landscape units Response of runoff and its components