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Re-thinking Recharge

IAH 2012

Dirk Kassenaar, Mason Marchildon, EJ Wexler Earthfx Inc.

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The “Old Water Paradox”

► Hydrologists are re-evaluating the concept of runoff

Jeff McDonnell, 2011 Birdsall-Dreiss Lecture

Rainfall Event Increase in Streamflow Deuterium isotope profile shows that streamflow is predominantly “old” water (i.e. water that has been subject to ET processes) Conclusion: Storm event streamflow is primarily mobilized shallow groundwater!!!

Time

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Revised Conceptual Model

P ET Ro

Rch

P ET

Event Mobilized GW

Rch

Base of soil, weathering,

bedrock contact or watertable

Old Conceptual Model New Conceptual Model

Ro = Special cases: - Dunnian processes - Impervious/paved surfaces

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Old Water Paradox: Implications

► Old concepts (and emphasis) on overland runoff should be discounted

Sheet flow is not the dominant precipitation event response process

The unsaturated zone is not dominated by 1D vertical (“Richards equation”) type flow

► Too simplistic a model in areas where there is GW/SW response

Simple HRU (Hydrologic Response Unit) water budget modelling strategies are inadequate

► Fail to recognize shallow zone configuration and spatial relationships

► We need to re-think/re-conceptualize recharge and the shallow zone flow system as a set of 3D processes

The concept of event mobilized groundwater response is analogous to interflow, but the mobilized water is primarily “old” groundwater

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Old Water Paradox: What does this mean to hydrogeology/modelling?

► The shallow groundwater system is responding much faster, and with greater volumes, than we might ever have thought

► We need to spend more time understanding and representing the soil/weathered zone and shallow geologic layers Storage and mobilization of soil zone water and shallow groundwater need to be

simulated if we are to truly understand both recharge and streamflow response

► Concepts of focused recharge (hummocky topography, potholes, etc.) and groundwater feedback (Dunnian processes and the contributing area concept) are broadly more important

► We already have 3D models – we need 3D recharge

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Modelling 3D Recharge

► Fully-distributed models such as GSFLOW model can be used to simulate and understand shallow event mobilized groundwater

► Key aspects of modelling the shallow zone in GSFLOW include:

1. Soil zone storage and cascading soil zone interflow

2. GW feedback and Dunnian rejected precipitation

3. Aquifer/aquitard saturated interface flow

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Soil Zone Storage and Cascade

► GSFLOW can simulate the storage and 3D movement of soil zone water using a cascading inter-cell network

Till uplands

Flow accumulates in swales

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Storage and 3D movement of water in the Soil Zone

► Soil zone moisture content

Beach Deposits

Till Upland - Till uplands drain both vertically and downslope - Lateral drainage to the beach deposits from the till uplands enhances recharge - Soil zone storage helps supply rate limited GW recharge to the lower layers Click for Animation

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Soil Zone Drainage (GW Recharge) ► When moisture is available (winter months) there is a near constant, but rate

limited, drainage from the soil zone

Click for Animation Beach Deposits Till Upland

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Focused/3D GW Recharge

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Topographically driven soil zone flow recharges at a geologic transition contact

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3D Soil Zone Drainage

Higher recharge at the geologic contact due to re-infiltration of runoff

Till uplands

Coarser grained beach deposits

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2. GW Feedback: Dunnian Processes

► Precipitation can be rejected from fully saturated soils

Occurs when the water table is at or near surface

Not sensitive to surficial material K

► Can result in runoff from saturated gravels

Spatially controlled:

► Tends to occur in stream valley areas

Seasonally controlled: ► Tends to occur in spring when water table is higher

► Runoff does occur, but in natural systems it is a more variable process

U n s a t u r a t e d z o n e

S t r e a m S t r e a m

G r a v i t y d r a i n a g e

R e c h a r g e

GW Discharge to the Soil Zone

Precipitation Rejected

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GW Discharge to the Soil Zone (Daily) Click for Animation

Daily GW discharge to soil zone

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Time-varying GW Feedback

► The “contributing area” that generates true runoff depends on the time-varying position of the water table

► Example: Dunnian process response area varies seasonally between 5 and 25% of the study area

► Runoff occurs, but it is a groundwater dependent process!

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3. Shallow Layer Interface Flow

► The “Old Water Paradox” suggests that the water table must fluctuate through multiple shallow layers

As the water table rises into higher K layers the flow and discharge rapidly increase to provide the event water

► This results in a “Fill and Spill” (McDonnell, 2011) Basin response

Highly variable discharge depends on basin configuration and saturation

► This shallow, rapid GW flow system response is difficult to simulate due to the numerical instability and wet/dry cell problems GSFLOW’s new NWT solver, and total flow 3D routing, provides a highly stable

means to simulate flow within shallow partially saturated, highly variable layers

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Water Table Fluctuation across Layers ► Water Table rises and falls through multiple thin, variably saturated, layers

Highly variable response to precipitation events

► GSFLOW NWT Solver – no dry cell problems!

Click for Animation

Water Table

GW Discharge

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Summary

► New isotope analysis requires that we consider a significant portion of streamflow response as “event mobilized groundwater”

Overland runoff is not the dominant process

Old simplifications of the shallow unsaturated zone water budget with predominantly vertical flow need to be discarded

Simple HRU modelling approximations fail to recognize the 3D processes and spatially inter-related response

► Modelling suggests that the key processes are:

Soil zone storage and cascading soil zone interflow

Aquifer/aquitard saturated interface flow

► Simulations suggest that runoff response in natural basins is highly variable and actually groundwater controlled

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Conclusions

► The ball is in our court – the hydrologists, in finding the “Old Water Paradox”, have told us that event driven GW discharge is key

We need to update our conceptual models of the unsaturated zone and recharge processes

SW = fast, shallow GW

► Truly integrated models are needed to simulate:

Soil zone storage and 3D drainage

Dunnian processes, including rejected precipitation and GW discharge to both the soil zone and the wetland/stream network

Highly complex shallow layer geometry with transient variably saturated layers

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Thanks

► Jeff McDonnell, 2011 Birdsall-Dreiss Lecturer

► Conservation Halton

► Town of Seaton Land Development Partners