topography and the spatial distribution of groundwater recharge and evapotranspiration

Topography and the Spatial Distribution of Groundwater Recharge and Evapotranspiration:

A Need to Revisit Distributed Water Budget Analysis when

Assessing Impacts to Ecological Systems.

By M.A. Marchildon, D.C. Kassenaar, E.J. Wexler, P.J. Thompson

Groundwater Recharge: The rate at which water is added to the groundwater system

Evapotranspiration: The combination of evaporation from all surfaces and any additional water loss from the physiological processes of vegetation.

Soil Zone

𝑷 = 𝑬𝑻 + 𝑸 + 𝑮 + ∆𝑺

Simple Waterbalance

Runoff (𝑸)

Groundwater Recharge (𝑮)

Evapotranspiration (𝑬𝑻)

∆𝑺

Precipitation (𝑷)

Change in Storage

Surface Water Modeling Strategies

10:1 vertical exaggeration


Streamflow monitoring location

Lumped Model By assuming homogeneity, model parameters are effectively “lumped” together. Works very well at predicting outflows from the catchment. Computationally efficient. Need to consider timing.


Semi-Distributed Model Adds a degree of heterogeneity, lumping model parameters in a distributed fashion. Works very well at predicting outflows from the catchment where some catchment heterogeneity is present.

Streamflow monitoring location


Semi-Distributed Model Adds a degree of heterogeneity, lumping model parameters in a distributed fashion. Works very well at predicting outflows from the catchment where catchment heterogeneity is present. Multiple lumped models in series and parallel. Still need to consider timing.


Distributed Model Considers heterogeneity, model parameters are distributed in a gridded fashion. Can predict flows at any point within the catchment. Less computationally efficient, greater data demands.


Lumped Semi-Distributed (Fully-) Distributed

𝑷 = 𝑬𝑻 + 𝑸 + 𝑮

𝑷 = 𝑬𝑻 + 𝑸 + 𝑮

𝑷 = 𝑬𝑻 + 𝑸 + 𝑮

𝑷 = 𝑬𝑻 + 𝑸 + 𝑮

𝑷 = 𝑬𝑻 + 𝑸 + 𝑮

𝑷 = 𝑬𝑻 + 𝑸 + 𝑮


Lumped Semi-Distributed (Fully-) Distributed

Increased computation time Increased data demands Increased heterogeneity

Low resolution High resolution

Hydrostratigraphy

Soil Zone

Hydrostratigraphy

Saturated Zone

Bedrock

Soil Zone

Hydrostratigraphy

GSFLOW — Coupled Ground-Water and Surface-Water Flow Model Based on

the Integration of the Precipitation-Runoff Modeling System (PRMS) and the Modular Ground-Water Flow Model (MODFLOW)

(Markstrom et.al., 2008)

• Free • Open source • Modular design

Climate variables 10:1

Surface hydrology 1:1

Sub-surface 5:1 hydrogeology

Stream networks Treated as linear segments independent of grid resolution

GSFLOW: Multi-Resolution

Recall timing

causality

GSFLOW: Cascade flowpaths

Evapotranspiration ranges from 300 mm/yr on the peaks to 480 mm/yr in the valleys. Distribution is strongly related to topography.

Cascade: Implications to Evapotranspiration

Groundwater recharge ranges from 380 mm/yr on the peaks to 760 mm/yr in the valleys (Kv = 4x10-8 m/s). Distribution is strongly related to topography.

Cascade: Implications to Recharge

(Franks and Beven, 1997)

Konza Prairie Natural Area, near Manhattan Kansas, USA. Image taken on August 15, 1987.

Valleys

Increased Evapotranspiration potential in valleys

Latent Heat Flux Latent Heat Flux = Energy

consumed by evaporation

Inversion of the Topographic Influence

Groundwater Discharge: The rate at which water is added to the soil zone from the groundwater system as the water table approaches the surface.

Soil Zone

(Hewlett & Hibbert, 1963) Variable Source Area Concept

Please note that this is a place-holder for an animation that is too large to upload. Please see if the animation on slide 23 works; if so, this animation should work.

GSFLOW: Variable Source Area

Daily Statistics Min 4.6% Max 28.4 % Difference ~64 km²

Monthly Average

GSFLOW: Variable Source Area

When do we need Distributed Integrated Modeling? • Where there’s an interest in analysing natural systems integrated modeling

cannot be avoided. • groundwater discharge to coldwater streams, • impacts to wetland hydroperiod under a changing climate, • identifying recharge areas that contribute significant volumes of water

to wetland systems, • etc.

• Longer run times; but increased targets • Greater data requirements is an overstated issue:

• no difference in data requirements from a lumped approach, except that extra information is gained (i.e., digital elevation models) by imposing topography on the runoff process.

• By looking at the problem from a top-down approach, topography is a first-order process.

References Franks, S.W., K.J. Beven, 1997. Estimation of evapotranspiration at the landscape scale: A fuzzy disaggregation approach. Water Resources Research 33(12). pp. 2929-2938. Hewlett, J.D., A.R. Hibbert, 1963. Moisture and energy conditions within a sloping soil mass during drainage. J. Geophys. Res. 68(4). Pp. 1081-1087. Markstrom, S.L., Niswonger, R.G., Regan, R.S., Prudic, D.E., and Barlow, P.M., 2008, GSFLOW—Coupled ground-water and surface-water flow model based on the integration of the Precipitation-Runoff Modeling System (PRMS) and the Modular Ground-Water Flow Model (MODFLOW-2005): U.S. Geological Survey Techniques and Methods 6-D1, 240 p.

𝑄 =1

𝑛𝐴𝑅2/3𝑆𝑓

1/2

𝑄 = ℎ ∙ 𝑎𝑆𝑏 ASSUMPTIONS Steady uniform flow 𝑆 = 𝑆𝑓

Wide channel 𝑤 ≫ ℎ (𝑤 = 50ℎ), 𝑅 → ℎ

Recharge Presentation • Our Work

• Highlight project areas • GW resource manager

• My work • Prediction transient recharge

• Typical recharge prediction techniques (read recharge book) • Lumped to semi-distributed to distributed

• Hillslope cross-section • Show extent of soil zone (distinguish from unsat modelling) • Use duffins

• First show no detail • Then introduce geology illustrating complexity

• Our conceptualization: • GSFLOW (cascade, GW feedback, etc.)

• go through PRMS cascade slides • By introducing this level of detail, our clients to raise questions on:

• Wetland water budgets and hydroperiod • Groundwater discharge to streams • Significant recharge areas

• Not overkill… • NEXRAD: show animations

• Recharge • Spatial distribution (animation); seasonal groundwater variation • Variable source area

• Old Water Paradox • Find Freeze quote on the feasibility of a Darcian explaination • Show recent data (compiled chart, Ken Howard urbanized example)

• What has changed: Laser spec, smaller-scale hillslope studies • “no longer shall we differentiate between surface water and groundwater, it’s all just water”

• Topography • Most certain, most ubiquitous, most readily available • 𝑄 ∝ 𝑎𝑆𝑏

• GSFLOW • Multiscale: climate (NEXRAD)surfacedrainage featuresunsatsaturated zone • Cascade: “assuming water flows downhill” • MC recharge and ET • Cross-section: point to soil zone (not talking about unsat zone, just the thin surface layeractive recharge zone) • Differentiate lumped/semi-/distributed • Modular design: briefly mention add-ons (LIDs, SCS CN, Green-Ampt, etc.)

• (Potential) Limitations • Long run times: BUT more potential calibration targets.

• Split sample spatially, rather than temporally • May not be necessary for deep systems, BUT required when ecological systems are of concern (cold water streams,

ecohydraulics, hydroperiod, etc.) • Data Requirements:

• Top-down approach • “Over-stated issue”

• No difference in data requirements from a lumped approach; except I gained extra information by imposing topography on the conceptual model

• Identifiability analysis shows topography is a first-order process • Additional Issues:

• Monitoring: Spot flows?? cross-sectional info, sediment info • Rooting depth

• Topography: longer rooting depth • Drier conditions longer rooting depth • Varying water level varying water depth • Annuals/biennials/perennials : short deep

topography and the spatial distribution of groundwater recharge and evapotranspiration

Engineering