PROGRESS IN PHYSICAL GEOGRAPHY

EFFECTS OF WATERSHED TOPOGRAPHY, SOILS, LAND USE, AND CLIMATE ON BASEFLOW HYDROLOGY IN HUMID REGIONS: A REVIEW

KATIE PRICE2011

Presented by: Jordan Martin

Article Overview

I. Introduction – Baseflow, Identification, QuantificationII. Geomorphic Controls

I. Geology

II. Surface Topography

III. Subsurface topography and Soils

IV. Combined Influences

III. Effects of Human Land UseI. Forest Removal

II. Urbanization

III. Agriculture

IV. Effects of Climate Change

V. Summary and Conclusion

Introduction – “A Cohesive Summary”

Baseflow: “the portion of streamflow that is sustained between precipitation events, fed to stream channels by delayed (usually subsurface) pathways”

Reason for this Study Timing, quantity, and quality of baseflow all can be

impacted by many factors Previous research mostly emphasizes flood response

to human pressures (not base flow) Conditions associated with baseflow merit special

attention

Introduction – Baseflow Overview

Baseflow = Low flow ? = Groundwater flow ?

Baseflow: deep subsurface and delayed shallow subsurface storage between precipitation and/or snowmelt events Groundwater flow is only one component of baseflow

Low flow: dry season minimum flows

Introduction – Methods of IdentifyingSources and Residence Times

1. Stable and Radioactive Environmental Isotopes• Stable: Hydrogen and Oxygen isotopes (water age)• Radioactive: Radon

2. Tracing “Injected” or Naturally Occurring Solutes• Injected: Cl or Br gas, highly saline water, etc.• Natural: ones that are known to originate in certain

areas of the watershed

3. End-Member Mixing Analysis (EMMA)• Uses ratios of multiple solutes characteristic of known

mineralogical and geological differences

Introduction – Methods of Quantifying Baseflow and Low Flow

Four Major Metric Categories Event-based low flow statistics

Applied to water quality and aquatic habitat management Environmental flow, waste-load allocations, point source

discharge permits, withdraw allowances 7Q10, 7Q2, average annual minimum flow

Flow-duration curve statistics Identification of exceedance probabilities Common interest in values such as 𝑄𝑄99, 𝑄𝑄95, or 𝑄𝑄75

Introduction – Methods of Quantifying Baseflow and Low Flow

Four Major Metric Categories Metrics that express the proportion of baseflow to total

flow Baseflow Index (BFI): the proportion of baseflow to total

streamflow over a continuous period of record Seven noted methods to separate event and pre-event water (i.e.

environmental isotope tracing)

Baseflow recession statistics Various computer programs / software developed Used for the regionalization of low flow distribution functions…

[and] to evaluate the relative impacts of climate change and land-use change

Emphasizes the need to establish a consistent set of baseflow metrics

Geomorphic Controls - Geology

Influential Characteristics Bedrock type Bedrock structure “Erodability”, Porosity, and Extent of FracturingFactors Influenced by Geology Groundwater “disconnected” storage volumes Surface water connectivity Level of low flow Channel formation and pedogenesis

Geomorphic Controls – Surface Topography

Topography Factors Influencing Baseflow Slopes influence where

Distribution of subsurface storage, stream network

Slopes influence when Stormwater delivery to stream, soil water retention

Slopes influence how much Subsurface storage volume, runoff volume

May mitigate or amplify other effects i.e. land use and climate change

Geomorphic Controls – Surface Topography

Topography Factors Influencing Baseflow Topography Index (TI) = ln(a/tan b)

a = specific contributing area to a given site b = local slope angle at that site Used along with transmissivity to estimate depth to shallow

water tables (accuracy is questioned) Catchment geometry shown to correlate with potential

discharge (related to baseflow?) Particularly in steep forested catchments “Flow path distribution is largely a function of catchment

geometry” Length of stream network per unit watershed area

may reduce baseflow levels

Geomorphic Controls –Subsurface Topography & Soils

What Subsurface Topography are we referring to: “pedogenically unaltered parent material” Flow paths created by tree roots, burrowing

animals, and other bioturbationImpact of Confining Subsurface Layers Prevent continued infiltration Directs shallow subsurface flow (usually laterally) Creates storage locations Amplified control on flow in low moisture conditions

Geomorphic Controls –Combined Influences

Topography and Soil Interplay Topography influences spatial variability of soil

moisture Topography can impact soil quantity, texture,

compaction, and thus water movement

In summary, geomorphic controls often cannot be isolated from each other in terms of their baseflow impact

Human Land Use - Summary Table

Human Land Use – Forest Removal

Initially, there seemed to be a negative relationship between watershed forest cover and baseflow volume However, studies show a significant positive

relationship between forest cover and baseflow discharge. Explanation? More permanent land use change from forest to non-forest

cover causes soil compaction, reduction in soil organic matter, increase in impervious surface, decreased recharge of basin subsurface storage

Human Land Use – Urbanization

Impacts of Urbanization on Baseflow Simply the reorganization of surface and

subsurface pathways Importation of water from previously disconnected

watersheds Infrastructure – increased impervious cover, soil

compaction, subsurface drainage networks Would urbanization, therefore, increase or decrease

baseflow??

Human Land Use – Urbanization

“The complete picture of hydrologic response to urbanization is extremely complex, with some factors acting to reduce recharge and others to increase recharge.”

Decrease in evapotranspiration

Human Land Use – Urbanization

Decrease Increase Inconsistent No Response

IIIII IIIII II IIII III

“it unfortunately appears that baseflow

response to urbanization cannot be predicted by a highly

simplified set of parameters”

Human Land Use - Agriculture

Similar to urbanization, baseflow response to agriculture can vary depending on various confounding factors:

“Watersheds that have been under agricultural land use for extended periods show baseflow increases in response to improved cropping and tillage practices”

Management Irrigation method Irrigation water source Tilling practices Drainage systems

Crop type Growing season

frequency Change from perennial

to seasonal cultivation

Climate Change – Temperature Rise

Local Scale Impact Increase in Evaporation Increase in Precipitation Baseflow reduction is offset

Regional/Global Scale Impact Dependent on the changes in circulation patterns Hard to isolate ET and P alteration impacts (mainly on

timing) from projected land-use changes (mainly on magnitude)

Highly variable impact based on region

Climate Change – Temperature Rise

What can we [almost] be sure of? Increased seasonality of hydrologic regimes

Dryer dry seasons and wetter wet seasons More extreme low flows, especially lowered baseflows in

late summer Colder regions will likely experience more

precipitation High latitude and high altitude regions will likely

experience higher baseflows – permafrost, infiltration, etc. Land-use and climate change effects will likely

combine to increase overland flow and reduce recharge

7 Key Needs for Future Research

1. Experimental studies specifically designed to evaluate the influence of subsurface topography on baseflow

2. Improvement of methods to determine distribution of shallow subsurface storage at scales relevant to policy and management

3. Comprehensive empirical comparisons that link soil hydrology and baseflows under land-use gradients that incorporate more detail than the broad categories of forest, agriculture, and urban land use

4. Study multiple aspects of watershed hydrology in a single study to better understand the watershed as a complete, interactive system – suggests modeling and empirical studies

7 Key Needs for Future Research

5. Modeling and empirical studies that explore baseflow response to varied landuse change, planned growth, and mitigation strategies6. Understanding analytical methods and strategies - do research conclusions differ with the specific baseflow metric analyzed? Are there optimal baseflow separation methods, recession statistics, and low flow statistics?7. Ensemble modeling studies that explore multiple working hypotheses of atmospheric feedbacks that will accompany warming, and various interactions between land-use and climate change, in order to ensure mitigation plans are in place for any scenario that is likely to occur

Summary and Conclusion

“Understanding how land-use and climate change will affect baseflow quantity, in the context of watershed geomorphology, will aid watershed managers and stream ecologists in the protection of adequate water supply for human needs and habitat availability for stream biota.”

Discussion Questions

Focusing on baseflow - Do you agree with the author’s belief that studying impacts on baseflow is particularly important? Justification?

Biota neglected – What role do you think various organisms could play in affecting baseflow?

Future Land-use – what trends do you predict we will see in land-use and how do you think that may impact baseflow?