1st EGU General Assembly25-30 April 2004, Nice, France

HS30 - Session onEnvironmental Flow Requirements

A modelling framework to assess the impact of streamflow regulation on floodplain vegetation ecosystem

P. Burlando, P. Molnar, W. Ruf, L. Foglia and P. Perona

Institute of Hydromechanics and Water Resources Management, ETH Zurich, Switzerlandpaolo.burlando@ethz.ch

MotivationMotivation

• Alpine river basins generally contain a permeable alluvial valley fill which promotes strong river-aquifer exchange.

• The riverine ecosystems in these mountain valleys are to a large extent dependent on the resulting patterns of surface and subsurface water flow.

• In addition, many Alpine basins are being utilised for hydropower production. The diversion and storage of water in most cases leads to significant changes in the natural hydrological regime.

• It is yet unclear how these changes affect riverine vegetation in the long term.

• There is a need to identify eco-oriented criteria aimed at defining environmental flow requirements, which overcome limitations of the statistically based ones.

Motivation (evidence)Motivation (evidence)Example of water stress on Alnus Incana

(grey alder) root growth[Hughes et al., Gl.Ecol.Biog. Let., 6, 1997]

1 cm/day

3 cm/day

0.5 cm/day

Drawdown rates

3 cm/day + 1 cm rain/week

0 cm/day

Example of change in streamflow regime(Maggia River @ Bignasco, Switzerland)

long-term interaction mechanismswithin the riverine corridor

•river aquifer interaction has been mainly investigated at small scales

• impact on aquatic ecosystems has been mainly based on holistic investigations

• pre-dam Qd=16.4 m3/s• post-dam Qd=4 m3/s• current EFs: 1.2 m3/s W, 1.8 m3/s S

Motivation (evidence)Motivation (evidence)

VEG11

2

3

4

6

7

WaterSedimentsHerbaceous areasWoodFNAZNA

WaterSedimentsHerbaceous areasWoodFNAZNA

VEG11

2

3

4

6

7

VEG11

2

3

4

6

7

WaterSedimentsHerbaceous areasWoodFNAZNA

WaterSedimentsHerbaceous areasWoodFNAZNA

Alluvial vegetation mosaics(Maggia River, Switzerland; Favre, 2004)

A

A

B

B

C

C

D

D

E

E

F

F

G

G

H

H

I

I

J

J

K

K

10 10

9 9

8 8

7 7

6 6

5 5

4 4

3 3

2 2

1 1

1989, about 25 years post-dam

A

A

B

B

C

C

D

D

E

E

F

F

G

G

H

H

I

I

J

J

K

K

10 10

9 9

8 8

7 7

6 6

5 5

4 4

3 3

2 2

1 1

1995

A

A

B

B

C

C

D

D

E

E

F

F

G

G

H

H

I

I

J

J

K

K

10 10

9 9

8 8

7 7

6 6

5 5

4 4

3 3

2 2

1 1

1962, shortly post-dam200 0 200100

Meters

vegetation cover dynamics and alternation results frommodified streamflow regime in both low flows and floods

overall loss of vegetation dynamics

EFR aimed riverEFR aimed river--aquifer interaction analysisaquifer interaction analysis

– Transient nature of river-aquifer exchangeit is the transient nature of both processes, and of the exchange flux, that is extremely important for determining the variability in time and space of the hydraulic head (Swain, 1997)

– Spatial distribution of surface and subsurface water flowriver-aquifer interactions are heavily dependent on lateral inflows and antecedent moisture conditions (e.g., Freeze, 1972; Vekerdy and Meijerink, 1998).

– Hydraulic connection between river and aquiferIn streams with highly variable streamflow regimes, it is specially important to evaluate aquifer response (e.g., Ackerer et al., 1990)

– Effects of the river-aquifer interaction and water dynamics on riverine ecosystems.

ecosystems in the riverine corridor are heavily dependent on water availability and variability, on flow velocity, water stage, the duration of inundation, etc. (e.g., Ward et al., 1999)

Relevant issues in floodplain of mountain rivers

EcoEco--accounting EFR estimationaccounting EFR estimation

• explicitely account for the dynamics of – streamflow, as dependent on natural variability (basin response)

and dam regulation;– water table, as dependent on streamflow regime, and

exploitation;– riparian and floodplain vegetation, as dependent on surface and

subsurface flows (nutrients are not a limiting factor);

• address the impact of man-induced disturbances at the floodplain scale

• virtual laboratory to assess the long-term impact of scenarios generated by natural changes in the hydrological regime or by anthropogenic influences

Hyp. the riverine corridor flora determines the fauna habitatconcentrate on vegetation dynamics

Methodology requirements

MULTILEVEL NESTED HYDROLOGICAL MODELLING SYSTEM

Time and space (modelling) scalesTime and space (modelling) scales

time scale: continuous space scales: nesteddistributed

Steep valley margingeological control

RIVERINE CORRIDOR

Floodplain Channel system and riparian zone

Surface water and groundwater interactions

Groundwatertable

Alluvial fill

Surface runoff

Surface waterprofile

Evapotranspiration

Infiltration

Subsurface runoff

WATERSHED

Intake station

Surge chamber

Penstock

Power house

Tunnel canal

Altstafel

Robiei

Sambuco

Naret

Zöt

Peccia

Cavergno

Bavona

Verbano

Cavagnoli

Palagnedra

EFRs assessment “MaVal” pilot projectEFRs assessment “MaVal” pilot project

• 1949 and 1953 green light to hydropower exploitation of the Maggia Valley and the Blenio Valley

• Conflictual evolution of the identification of the residual flows: 1966, first decision; (1975, evaluation report); 1982, new residual flows; (1991, Federal Law); 1997, new evaluation in view of the restoration of areas included in the national inventory (2007)

• From 1953 on (beginning of construction) one section of the river reach in the flood plain dried progressively out, thus suggesting an excess of seepage likely due to the lowering of the water table as a consequence of the modification of the natural regime.

VislettoCevio

Someo

GiumaglioCoglio

Aurigeno

Moghegno

Lodano

Bignasco(455 m a.s.l.)

Maggia

Gordevio

Avegno

Tegna(255 m a.s.l.)

Fondovalle dellaValle Maggia

5 km

Bignasco

Giumaglio

Moghegno

Tegna

on going

MaVal evidenceMaVal evidence

A

A

B

B

C

C

D

D

E

E

F

F

G

G

H

H

I

I

J

J

K

K

10 10

9 9

8 8

7 7

6 6

5 5

4 4

3 3

2 2

1 1

variation of total vegetation cover 1962-1977

decrease

maintain

increase

4/15/02 4/18/02 4/21/02 4/24/02 4/27/02 4/30/02 5/3/02 5/6/02 5/9/02 5/12/02 5/15/02 5/18/02Data

303.5

305.5

307.5

309.5

304.5

306.5

308.5

Pie

zom

etric

hea

d (m

a.s

.l.)

Piezometer 819/1Piezometer 819/2Piezometer 819/3Precipitation at Locarno Monti

4/15/02 4/18/02 4/21/02 4/24/02 4/27/02 4/30/02 5/3/02 5/6/02 5/9/02 5/12/02 5/15/02 5/18/02

0

20

40

60

80

100

Precipitation m

m/h

Piezometers 819/1, 819/2 and 819/3, Moghegno, from 13.04.02 to 17.05.02

example of river-aquifer connectivityduring the flood of 2002 shows substantial dynamics in the response

MULTILEVELNESTED HYDROLOGICALMODELLING SYSTEM

MaVal modelling approachMaVal modelling approach

CLIMATE

VEGETATION

SOIL

Precipitation, temperature, radiation, flooding, etc.

Texture, porosity, etc.

Physiology, rooting depth, leaf area, etc.

Soil Water Balance

Vegetation stress

V(s,t)

surface water module

RIV

ERIN

E C

OR

RID

OR scale ~20 km

Groundwater model

MODFLOW

O.S. Finite Difference, 3D, unsteady, modified to account for groundwater-river interactionfluxes

groundwater module

ecosystem module

WA

TER

SHED

RIV

ERIN

E C

OR

RID

OR

REA

CH

scale ~300÷900 km2

Distributed watershed rainfall-runoff modelTOPKAPI [Zhiyou and Todini, 2002]

modified to account for icemelt and reservoir operation

scale ~20 km

Hydrodynamic routing model

FESWMS-2DH (version 1)

O.S. Finite Element, 2 dimensional, unsteady, modified to account for groundwater-river interaction fluxes

scale ~100 m

validation of the coupled groundwater-surface model

Data

Surface Water Module

Groundwater Module

Ecosystem Module

Impacts ModuleEFRs

MaVal model layoutMaVal model layout

Alluvial Plane

Groundwater Model

SVAT ModelVegetation Model

Hydrodynamic modelinflitrationexfiltrationin / exfiltrationinfiltration to fans

Watershed hydrological modelincluding water abstraction by hydropower system ( )

lateral flowriverhillslope

Alluvial plane

Watershed (Model)

Hydropower System

River network and tribuaries

MaVal instrumented area (1)MaVal instrumented area (1)

Existing measurements

precipitationstreamflowpiezometric levelsdam inflows/outflows

Additional measurements

new piezometersnew streamflow gaugesenvironmental tracerstracer experiments

MaVal instrumented area (2)MaVal instrumented area (2)

Watershed Model (WM)Watershed Model (WM)TOPKAPI (TOPographic Kinematic APproximation and Integration)

[Zhiyou and Todini, HESS, 2002]

Icemelt Modellateral flow input to groundwater model

EFRs scenarios

Watershed Model (2)Watershed Model (2)

0

200

400

600

800

1000

1200

09.2000 10.2000 11.2000 12.2000

Q [m

³/s]

ObservedSimulated

Flood event 2000, Maggia @ Solduno

Hydrodynamic Model (HD)Hydrodynamic Model (HD)

Features

• open source code• 2D – depth averaged flow

(vertically integrated equations of motion and continuityto obtain depth-averagd velocities and flow depths)

• (St-Venant-Equation)• accounting for bed friction and turbulence stresses• finite elements• conservative form (sub- and supercritical flow)• triangles and quadrilaterals (quadratic interpolation

function)• capability of drying and wetting cells• steady and time-dependent simulations• internal boundary nodes

why 2D?

large change of wetted area during flood events2D pattern of in/exfiltration affecting vegetation onset and developmentbraided reach

Finite Element Surface-Water Modeling System (FESWMS-2DH) rel. 1[U.S. Department fo Transportation; Federal Highway Administration]

GOVERNING EQUATIONS

• vertically-integrated momentum equation

• mass conservation

FLOODPLAIN

HD model domainHD model domain

2D FE mesh

BRAIDED REACH

zoom of the BRAIDED AREA withwet (blue) anddry (white) elements

Features

MODFLOW is (an open source code) designed to simulate aquifer systems in which

1. saturated-flow conditions exist, 2. Darcy's Law applies, 3. the density of ground water is constant, and 4. the principal directions of horizontal hydraulic

conductivity or transmissivity do not vary within the system.

Groundwater Model (GW)Groundwater Model (GW)MODular three-dimensional finite-difference ground-water FLOW model

MODFLOW 2000[USGS, http://water.usgs.gov/pubs/fs/FS-121-97/]

MODFLOW for MaVal (1st stand-alone modelling)

Two confined aquifers, water table is considered iterativelyTwo steady-state conditions, with and without recharge to the aquifer from the hillslopes ⇒ simultaneous calibrationTransient model implemented for selected flood periods and with recharge varying accordingly with the results of the hydrological model

Advantages• robust code, widely used• many code options already

available • customization (for

coupling with HD model) possible

GW Model: GW Model: 1st calibration/validation

• Parameters defined to represent: areal recharge, hydraulic conductivity of the aquifer (up to 5 classes), and streambed hydraulic conductivity

• Several conceptual models were developed by changing the number of hydraulic conductivity classes

Values of estimated parameters are reasonable and in agreement with measurements

Based on fit to observations and realistic parameters estimation, the “best” model has been identified

Better knowledge of aquifer geology necessary

Conductivity map

Measured value

Quaternarymap

F.E. mesh

F.D. gridh

ModFlow

FESWM 1h i,j

h j

q i,j

q j

Information of the piezometric head hi,j from all the FEM nodes belonging to the j node of the FD grid must be conveniently transferred, and viceversa for q (exfiltrating or inflitrating flow) into qi,j.

Fluss

Ufer

glatter_Fluss

Coupling HD and GW models (1)Coupling HD and GW models (1)

To establish a one-to-one correspondence between the nodes of the FD grid and the

FEM mesh

Advantages: rapid, flexible, calculated just once, can be used in both directions i.e., for h and q, good reliability on average.

Disadvantages: is a static relation, indipendent on the water surface profile, memory consuming

To estimate a function h=h(x,y) that fits the current value of water surface

elevation.

Advantages: dynamic and able to catch the spatial gradient in the water surface elevation, immediate computation of the water depth, no memory consuming

Disadvantages: ‘fitting’ at each time step is required, questions may arise about fitting stability, reliability, and fitting time requirements

Coupling HD and GW models (2)Coupling HD and GW models (2)Constraints:• element mass balance (FEM FD) • different temporal resolution of HD and GW models (mass balance in time)• computational efficiency

Premisethe most important controls on the successful establishment of riparian vegetation are• lack of disturbance• availability of moisture

VGM is fully distributed and coupled to• groundwater model

– through unsaturated zone– through saturated zone

• hydraulic model– through floodplain inundation– through transient flow properties

MaVal ecosystem moduleMaVal ecosystem moduleStudy the interconnections between water and vegetation dynamics, with the aim to evaluate potential impacts of changes in the streamflow regime on floodplain vegetation in the Maggia Valley

Vegetation Growth Model (VGM)to simulate the space-time evolution of vegetation V(s,t)

Vegetation Growth Model (VGM)

Primary state variables• plant biomass• soil moisture / water table• flooding level / flow properties

VGM DEVELOPMENT• based on existing models (PATTERN as a basis)

WORK STEPS• vegetation mapping and characterisation• soil description (type, texture, conductivity, evaporation)• VGM parameterisation and validation

• definition of growth parameters• mortality parameters under water stress

• VGM calibration to current conditions• VGM coupling to the MaVal Modelling System

TIME SCALE (local model)

Plant Growth Module• biomass evolution model• plant succession under water stress• physically-based ET model

SPACE SCALE (extension model)

Vegetation Spatial Dynamics Module• spatial communication model (e.g.

percolation theory, cellular automata)

• vegetation destruction by floods• germination and vegetation

expansion

• Physiological models• Quasi-empirical mechanistic models• Dynamic plant growth models

e.g. PATTERN [Mulligan, 1996, 1998]

• production efficiency model (roots, shoots, leaves)

• simulates plant growth, respiration, allocation, death, dormancy, germination

• time and space• plant destruction by erosion

Ecosystem module (2)Ecosystem module (2)

VGM focus during floods• flood magnitude and resulting damage to vegetation• flow velocity and bed shear and resulting uprooting of

vegetation• duration of flood inundation• waterlogging of the soil profile and its impacts on plant

growth

VGM focus in low flow periods• water table decline and its impacts on seedlings and young plants• water availability in the unsaturated zone and uptake by plants• spatial distribution of the water table along the floodplain

VGM and Impact Scenarios (dynamic and long-term)• analysis of historical (natural) hydrological regime• analysis of water management scenarios (streamflow regulation)• analysis of climate change driven scenarios

Ecosystem and Impact moduleEcosystem and Impact module

EFRs

http://www.maggia.ethz.chhttp://www.maggia.ethz.ch

The MaVal project is funded bythe Swiss National Science Foundation

the Swiss Federal Office for the EnvironmentCanton Tessin

We acknowledge the local support ofthe Institute of Earth Sciences of SUPSI