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Examensarbete vid Institutionen fr geovetenskaper

ISSN 1650-6553 Nr 228

Rainfall-runoff Model Application in

Ungauged Catchments in Scotland

Alexander Peter Anthony Fionda

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Abstract

Rainfall-runoff model application for ungauged catchments in Scotland

Alexander Peter Anthony FiondaDepartment of Earth Sciences, Uppsala University

Villavgen 16, SE-752 36 Uppsala, Sweden.

The conceptual rainfall-runoff model Hysim is used to estimate the flow in ungauged

catchments in Scotland by Scottish Water. However, there are non-quantified uncertainties

associated with the outcomes of the modelling strategy used. In order to identify and quantify

these uncertainties it was necessary to use the framework of proxy-basin validation in order

to evaluate the performance of different modelling strategies.

The proxy-basin validation test requires hydrologically analogous catchments for the

evaluation of models, a Region Of Influence regionalisation method was used in order group

selected catchments by Q95(%MF). Four groups of four catchments were established, which

covered Q95(%MF) 5-7%, 7-9%, 9-11% and 11-13%.

The allocation of donor catchment and target catchment for each Q95(%MF) group

was accomplished through discussion with Scottish Water with respect to existing Scottish

Water modelled catchments. A single donor catchment and three target catchments were

therefore indicated for each group.

Two modelling strategies were developed by the study; the first full transposition method

used the entire optimised parameter-set from the donor catchment with the exception of the

target catchments catchment area parameter. The second partial transposition method usedthe entire optimal parameter-set with the exception of the target catchments interception

storage, time to peak, rooting depth and catchment area parameters.

It was found that the full transposition method had the least uncertainty associated its use

for flow estimation when the parameter-set was derived from a donor catchment calibration

that was excellent. Contrarily, it was found that the partial transposition model method had

the least uncertainty associated with flow estimation for parameter-sets that were derived

from a relatively poor donor catchment calibration.

Encouraged by this testing framework, this study has suggested the use of catalogue of

donor parameter-sets that can be used to estimate flow for catchments that are hydrologicallysimilar. This strategy of hydrological modelling has been recommended to improve existing

Scottish Water Hysim methodology.

Keywords

ungauged catchment, proxy-basin validation, region of interest, transposition method, hysim,

rainfall-runoff model, sepa, scottish water, scotland.

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Referat

Anvndning av en avrinningsmodell i ett skotskt avrinningsomrde utan

vattenfringsmtningar

Alexander Peter Anthony Fionda

Institutionen fr geovetenskaper, Uppsala universitet

Villavgen 16, 752 36 UPPSALA.

Scottish Water anvnder den begreppsmssiga avrinningsmodellen Hysim fr att uppskatta

vattenfringen i skotska avrinningsomrden utan vattenfringsdata. Den valda

modelleringsstrategin har emellertid resulterat i icke-kvantifierade oskerheter i berknade

vattenfringar. Fr att identifiera och kvantifiera de oskerheter som r frbundna med olika

modelleringsstrategier var det ndvndigt att anvnda sig av information frn likartadeavrinningsomrden.

Den valda regionaliseringsmetoden anvnde hydrologiskt analoga avrinningsomrden som

definition p likhet. Analogin grundades p inflytanderegion (Region of Influence) som

erhlls genom att gruppera utvalda avrinningsomrden utefter Q95 (% medelflde). Fyra

grupper med fyra avrinningsomrden valdes ut grundat p fljande Q95-grnser (%

medelflde): 5-7%, 7-9%, 9-11% and 11-13%.

Frdelningen av analoga avrinningsomrden (omrden med vattenfringsmtningar vars

parametervrdesuppsttningar skulle verflyttas) och mlomrden (utan mtningar) fr varje

Q95-grupp erhlls efter diskussion med Scottish Water frn omrden dr Scottish Water

modellerat vattenfringen. Ett analogt omrde och tre mlomrden valdes ut fr varje grupp.

Studien anvnde tv modelleringsstrategier. Den frsta metoden, total verflyttning,

anvnde hela parametervrdesuppsttningen frn det analoga omrdet med undantag av

mlomrdets area. Den andra metoden, partiell verflyttning, anvnde hela

parametervrdesuppsttningen med undantag fr mlomrdets interceptionslager, tid till

hgflde, rotdjup och area.

Den totala verflyttningsmetoden hade lgst oskerhet nr parametervrdesuppsttningen

hrleddes frn ett omrde med utmrkt kalibrering. Den partiella verflyttningsmetoden hade,

andra sidan, lgst oskerhet nr parametervrdesuppsttningen hrleddes frn ett omrde

med dlig kalibrering.

Efter att ha provat de tv metoderna utmynnade studien i ett frslag till en katalog med

parametervrdesuppsttningar fr omrden som kan bedmas som hydrologiskt lika. Denna

strategi fr hydrologisk modellering har rekommenderats som frbttring av befintlig Hysim-

metodik hos Scottish Water.

Nyckelord

Avrinningsomrde utan vattenfringsdata, validering mot likartade omrden,

inflytanderegion, verflyttningsmetod, hysim, avrinningsmodell, SEPA, Scottish Water,

Skottland.

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Contents

1. Introduction ................................................................................................................. 1

1.1 Research objectives ................................................................................................. 1

1.1.2 Scottish Water and its resource systems .......................................................... 2

1.1.2 The role of hydrologic modelling in Scottish Water ........................................ 5

1.2 The use of Hysim rainfall-runoff modelling by Scottish Water............................ 12

1.2.1 Quantifying the uncertainty associated with parameterisation ...................... 13

1.2.2 Modelling flow in ungauged catchments ....................................................... 15

1.3 Key questions and summary of methods ............................................................... 16

2 Materials and Methods................................................................................................. 18

2.1 Analogue and target site selection from SEPA catchments .................................. 18

2.2 The Hysim conceptual rainfall-runoff model ........................................................ 21

2.3 Derivation of inputs ............................................................................................... 24

2.4 Hysim model calibration ....................................................................................... 26

2.5 Development of parameter transposition methods ................................................ 27

2.6 Evaluating model performance using the proxy-basin test ................................... 28

3 Results.......................................................................................................................... 29

3.1 Hydrological statistics of mega-zones and SEPA catchments .............................. 29

3.2 Calibration quality of donor catchments ............................................................... 31

3.3 Evaluating model performance with transposition method chosen....................... 39

3.4 Evaluating model performance with selection of target catchment ...................... 44

4 Discussion .................................................................................................................... 47

4.1 Hydrological statistics of mega-zones and SEPA catchments .............................. 474.2 Calibration quality of donor catchments ............................................................... 48

4.3 Uncertainty identified with selection of target catchment..................................... 53

4.4 A more pragmatic methodology for estimations of flow ...................................... 54

5 Conclusion ................................................................................................................... 56

6 Acknowledgements...................................................................................................... 58

7 References.................................................................................................................... 59

8 Appendices................................................................................................................... 63

Appendix A: Hysim operational notes ........................................................................... 63

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Appendix B: Parameter-set references ........................................................................... 64

Appendix C: Results of validation ................................................................................. 65

Definition of terms

MF - the mean flow.

Q95 - the 95th

percentile of mean flow; the flow exceeded or equalled 95 % of the time.

Q95(%MF) - the