river modelling keeping low flows in mind

Incorporating surfacegroundwater interactions into river modelling keeping low flows in mind David Rassam 11 February 2013 CSIRO LAND WATER; WATER FOR A HEALTHY COUNTRY FLAGSHIP

Outline of presentation Types of river models and their shortcomings Common approaches for modelling SW-GW interactions Review of SW-GW interaction processes Newly developed river models that incorporate SW-GW interactions

River models Water generation (rainfall-runoff) models River management-planning models River management models Catchment water generation models

What do conventional rainfall-runoff models lack? Usually not designed to model low flows and cease-to-flow conditions Significant variablility in river-aquifer connection type (both spatial and temporal) requires the model to handle losing conditions as well as gaining Do not model land use change Do not model climate change impacts on recharge

What do river management models lack? In most river system models GW-SW exchange is often treated simply as part of the unaccounted losses/gains component of the water balance for a given river reach; i.e., they are implicitly accounted for during a black-box calibration approach, thus ignoring their dynamic nature When groundwater processes with significant time lags are not modelled, the forecasting capacity of the river model is compromised 5

Groundwater exchange time lags and river model calibration problems

Accounting for surface-groundwater interactions in river modelling Fully coupled SW-GW models can adequately handle the interactions but... These are not always readily available Fully coupled seldom incorporate the complex river management rules Need a simplified physically-based approach whereby the primary design goal is the accurate representation of the significant time lags with the aim of enhancing the forecasting ability of river models especially during low flow conditions 7

Common approaches for modelling SW-GW interactions 8

River management models Adding capacity to model simple GW processes in river management models e.g.; Source Rivers and SW-GW Link model

River management models Water generation models Adding capacity to model simple GW processes in river management models e.g.; Source Rivers and SW-GW Link model Predict water yield from upland unregulated catchments Rainfall-runoff modelling with improved functionality to model GW processes e.g.; GWlag model

River management models Adding capacity to model simple GW processes in river management models e.g.; Source Rivers and SW-GW Link model GW models e.g.: MODFLOW River and Stream packages River is a boundary in the GW model; exchange fluxes can be estimated Water generation models Predict water yield from upland unregulated catchments Rainfall-runoff modelling with improved functionality to model GW processes Results in enhanced low flow predictions

River management models Adding capacity to model simple GW processes in river management models e.g.; Source Rivers and SW-GW Link model Hybrid model GW models e.g.: MODFLOW River and Stream packages River is a boundary in the GW model; exchange fluxes can be estimated Required data for the river boundary can be imported from a river model Used in MBDSY project Water generation models Predict water yield from upland un-regulated catchments Rainfall-runoff modelling with improved functionality to model GW processes Results in enhanced low flow predictions

Review of SW-GW interaction processes, why? The goal of modelling SW-GW interaction is estimating the magnitude and direction of the exchange flux between the two systems There are a number of processes that may contribute to this flux The state of connection between the river and the underlying aquifer dictates which processes actively contribute to the exchange flux

GW driven SW driven Climate driven Groundwater discharge Stream depletion Stream recharge Bank storage Overbank flooding/recharge Groundwater ET Diffuse recharge GW discharge Gaining stream

GW driven SW driven Climate driven Groundwater discharge Stream depletion Stream recharge Bank storage Overbank flooding/recharge Groundwater ET Diffuse recharge Stream recharge Losing stream

GW driven SW driven Climate driven Groundwater discharge Stream depletion Stream recharge Bank storage Overbank flooding/recharge Groundwater ET Diffuse recharge d R Stream recharge Losing stream

GW driven SW driven Climate driven Groundwater discharge Stream depletion Stream recharge Bank storage Overbank flooding/recharge Groundwater ET Diffuse recharge P Pumping Q d p d p (t, D, x 2 ) x D is aquifer diffusivity

GW driven SW driven Climate driven Groundwater discharge Stream depletion Stream recharge Bank storage Overbank flooding/recharge Groundwater ET Diffuse recharge ΔR Recharge Q Δ Q x Δ Q (t, D, x 2 ) D is aquifer diffusivity Δ Q

GW driven SW driven Climate driven Groundwater discharge Stream depletion Stream recharge Bank storage Overbank flooding/recharge Groundwater ET Diffuse recharge Q d un x Lateral delay in aquifer (of diffusivity D) R

GW driven SW driven Climate driven Groundwater discharge Stream depletion Stream recharge Bank storage Overbank flooding/recharge Groundwater ET Diffuse recharge ET a (ET p, d WT, S t, L c) d WT Depletes the stream ET p is potential ET S t is soil type, and L c is Land cover

GW driven SW driven Climate driven Groundwater discharge Stream depletion Stream recharge Bank storage Overbank flooding/recharge Groundwater ET Diffuse recharge

GW driven SW driven Climate driven Groundwater discharge Stream depletion Stream recharge Bank storage Overbank flooding/recharge Groundwater ET Diffuse recharge Flux in Cumulative flux response Time Flux out Flux response

ewater Source IMS initiative Groundwater-surface water interaction tools have been developed for two of the river planning and management tools in Source IMS: Source Catchments for predicting water yield and constituents from upland unregulated catchments Source Rivers for assessing long term impacts of water resource policy on system storages, flows and water shares in regulated rivers PERFECT- GWlag Model Source Rivers GW-SW Link Model Source Catchments

Adding GW processes into rainfall-runoff modelling This model provides output of daily flows at a gauging station of interest, it: Allows variation in land-use across the modelled area (i.e., models land use change) Incorporates groundwater processes Accounts for groundwater response times, and Accounts for stream losses to groundwater (i.e., handles losing, as well gaining streams) This leads to improved low flow predictions

Landscape disaggregation Topographic, divided into sub-catchments Water balance From 1-D water balance modelling daily time series. Lumped to sub-catchment Groundwater Single groundwater store per sub-catchment Capture variability between sub-catchments, but kept simple, using response times Calibration Calibrate to observed flows at a gauge GWlag Model Calibrate to observed Flow Duration Curve Put emphasis on low flows through weighting parameters

GWlag conceptual model

Calibration results

How does it compare to conventional lumped rainfall-runoff models?

Low flow analysis Predict river flow under various scenarios where recharge is scaled down (up to 40% with 5% increments) Assess impacts on flow using a number of low flow indices that include: (1) the slope of the low flow part of the Flow Duration Curve, is indicative of the sustainability of low flows whereby a steep curve indicates small and/or variable baseflow contribution (2) the percentage of zero-flow days in the record as the FDC has no regard to the sequence of occurrence, the % of zero-flow days illustrates the degree of stream intermittency

Impacts on low flows 18 120 % Zero-flow days 16 14 12 10 8 6 4 2 Zero-flow days Slope change 100 80 60 40 20 % Slope change of low-flow part of cc FDCc 0 0 5 10 15 20 25 30 35 40 % Reduction in recharge 0

Adding GW processes into river management models Reach-scale model that determines the exchange flux of water between a river and the underlying aquifer Applicable to regulated river systems Handles both gaining and losing (saturated and unsaturated) connections Dynamically models groundwater pumping, diffuse, irrigation and flood recharge, bank storage exchange, and evapotranspiration Allows fluxes or heads from external groundwater models (such as MODFLOW) to be passed to/from the model

Conceptualisation of SW-GW interaction processes in the Link Model Node1 (Upstream) We estimate the SW-GW exchange fluxes and heads due to individual processes then use superposition to evaluate the overall exchange flux at the end of every time step Link length L GW/SW exchange flux Processes depend on type of connection Node 2 (Downstream)

Cumulative Interaction Gaining river Processes modelled Overbank event Saturated connection Within bank event Infiltration, recharge, and discharge Bank storage Groundwater pumping Groundwater ET Irrigation recharge Node1 (Upstream) Losing river Groundwater table Link length L GW/SW exchange flux Losing river Unsaturated connection Processes depend on type of connection Recharge driven by difference in head between river and groundwater Node 2 (Downstream)

Explicit representation of SW-GW interactions results in a realistic calibration of the river model 10000 SW-GW fluxes 1000 100 Residual inflows without SW-GW fluxes Residual inflows with SW-GW fluxes Flow (ML/day) 10 1 0.1 0.01 0 10 20 30 40 50 60 70 80 90 100 % Time exceeded or equaled

Significance of groundwater extractions on river/aquifer interaction

In summary: Incorporating SW-GW interaction into river modelling: Enhances low flow predictions Enhances forecasting capacity in areas with significant groundwater development 37 Presentation title Presenter name

Thank you.. CSIRO LAND AND WATER WATER FOR A HEALTHY COUNTRY FLAGSHIP David Rassam Hydrological Modeller Phone (+61 7 3833 5586) Email david.rassam@csiro.au

GW-SW Link Model input data Pumping schedules, pump locations from river and boundaries were identified

GW-SW Link Model input data Diffuse recharge Rates, areas, and their centroids were located

GW-SW Link Model input data Irrigation recharge Rates, areas, and their centroids were located

Namoi Basin trial Narrabri to Boggabri reach

Variability in river-aquifer connectivity