Assessing the Impacts of Climate Change on Irrigation Water Use in the Cambridge University Botanic Garden

7/30/2019 Assessing the Impacts of Climate Change on Irrigation Water Use in the Cambridge University Botanic Garden

1/66

CRANFIELD UNIVERSITY

Daro Lorenzo Daz

Assessing the Impacts of Climate Change on Irrigation Water Use

in the Cambridge University Botanic Garden

SCHOOL OF APPLIED SCIENCES

MSc THESIS


2/66

CRANFIELD UNIVERSITY

SCHOOL OF APPLIED SCIENCES

MSc THESIS

Academic Year 2006-2007

Daro Lorenzo Daz

Assessing the Impacts of Climate Change on Irrigation Water Use

in the Cambridge University Botanic Garden

Supervisor: Dr Jerry W. Knox

September 2007

This thesis is submitted in partial fulfilment of the requirements

for the degree of Master of Science in Water Management

Cranfield University 2007. All rights reserved. No part of this publication may bereproduced without the written permission of the copyright owner.


3/66

i

Abstract

Gardening is not only an essential part of the UK cultural heritage and style of life, but

also a direct contributor to the economy. Climate change will vary temperature and

precipitation patterns, increasing water demand for irrigation, and will put pressure into

water resources, reducing water availability for the different users, including the

gardening sector. In this context, the impacts of climate change on irrigation water use

were assessed for the Cambridge University Botanic Garden.

Using the UKCIP02 scenarios, four future climate data sets were derived for the 2050s

and 2080s time slices and for the Low and High scenarios. Then, future volumetric

water demand (m 3) in the garden was estimated on the basis of historical irrigation

trends and climate patterns, through the agroclimatic indicator PSMD. On a next step,

future irrigation water needs (mm) were estimated for four different irrigated plant

species displayed in the garden using WaSim model. Finally, an adaptation survey was

carried out with the intention of identifying possible future strategies to cope with water

shortages.

Recent dry conditions, such as those in 1976 or 1990, will become more common in the

future, rising volumetric irrigation water demand in the garden by 21-53% for the 2050s

and by 37-80% for the 2080s. WaSim estimations confirm that climate change will

increase irrigation requirements for the four plant species studied by 33-176%. The

model also proves that impacts will be more significant over perennial plant speciesthan over annuals. Finally, responses to the adaptation survey reveal that the garden

accounts already for several measures to deal with water scarcity problems, such as

rain-water harvesting structures, drought resistant plants or improved soil structure.

However, repairing leakage in the water system, improving irrigation scheduling or

switching to more efficient irrigation methods, are still key gaps to be covered if water

use efficiency in the Cambridge Botanic Garden is to be maximized.

Keywords: Climate change, gardening, irrigation, water use efficiency.


4/66

ii

Acknowledgements

I would like to thank Dr Jerry Knox for wisely guiding me throughout the work. I also

would like to express my gratitude to the managers of the Cambridge University

Botanic Garden, Peter Atkinson and Tim Upson, for their inestimable collaboration. For

supporting me on the field work, much thanks to the staff on the garden, especially John

Kapor, always ready for a chat about the weather.

Finally I would like to show my most sincere appreciation to my loving mother, father

and sister, who overall give me all the support I need to reach my most elevated goals.


5/66

Cranfield University Dario Lorenzo Diaz, 2007

iii

Table of contents

1 INTRODUCTION .................................................................................................. 1

1.1 Climate change ................................................................................................. 11.2 Importance of gardens in UK............................................................................ 3

1.3 Need for further research .................................................................................. 3

1.4 Aim and Objectives ..........................................................................................4

1.4.1 General Aim.............................................................................................. 4

1.4.2 Specific Objectives ................................................................................... 4

1.5 Case study: Cambridge University Botanic Garden .........................................4

1.5.1 Soil ............................................................................................................5

1.5.2 Climate...................................................................................................... 6

1.5.3 Water sources............................................................................................8

2 LITERATURE REVIEW ......................................................................................9

2.1 Water supplies and water demand ....................................................................9

2.2 Plant responses to water stress ........................................................................ 10

2.3 Impacts on soils and water regime..................................................................10

2.4 Impacts on water bodies ................................................................................. 11

2.5 Impacts on irrigation .......................................................................................11

2.6 CO 2 impacts on plant water use...................................................................... 12

3 METHODOLOGY ............................................................................................... 13

3.1 Current volumetric water demand correlated against PSMD ......................... 14

3.2 WaSim modelling irrigation needs .................................................................16

3.2.1 Soil input data .........................................................................................17

3.2.2 Crop input data........................................................................................ 18

3.2.3 Irrigation scheduling input data ..............................................................21

3.2.4 Weather input data .................................................................................. 21

3.3 Future irrigation water demand modelling ..................................................... 22

3.3.1 Future volumetric irrigation water demand (m 3) .................................... 25

3.3.2 Future irrigation needs (mm) ..................................................................25

4 RESULTS AND ANALYSIS ............................................................................... 26

4.1 Historical pattern of irrigation water demand (m 3)......................................... 26

4.2 Assessing current irrigation needs (mm) ........................................................ 284.3 Future irrigation water requirements ..............................................................30


6/66


iv

4.3.1 Future irrigation water abstraction (m 3)..................................................32

4.3.2 Future irrigation needs (mm) ..................................................................34

5 DISCUSSION ........................................................................................................ 39

5.1 Climate change uncertainties .......................................................................... 395.2 Modelling limitations......................................................................................40

5.2.1 Water sources validation ......................................................................... 40

5.2.2 Crop modelling limitations ..................................................................... 41

5.2.3 Overall limitations of the approach......................................................... 41

5.3 Adaptation options ..........................................................................................42

5.3.1 Licensed water use .................................................................................. 45

6 CONCLUSIONS AND RECOMMENDATIONS .............................................. 47

7 REFERENCES ...................................................................................................... 50

8 APPENDICES ....................................................................................................... 54

8.1 Appendix A: Survey of climate change impacts over water use in theCambridge University Botanic Garden....................................................................... 54


7/66


v

List of Tables

Table 1-1: IPCC Climate change evidence indicators. .................................................... 1

Table 1-2: Monthly averages for the Cambridge Botanic Garden weather station. ............................................................................................................................... 6

Table 3-1: Weather stations coordinates. ....................................................................... 14

Table 3-2: Summary of soil characteristics used for modelling the water balance. ........................................................................................................................... 18

Table 3-3: Selected representative species to calculate irrigation needs usingWaSim. ........................................................................................................................... 18

Table 3-4: Crop characteristics for each plant species used in the WaSimmodel. ............................................................................................................................. 20

Table 3-5: Irrigation scheduling input data for each plant entered in WaSimmodel. ............................................................................................................................. 21

Table 3-6: Socio-economic, temperature (C) and CO 2 concentration (ppm)changes for the 2080s time slice for the different UKCIP02 scenarios.......................... 22

Table 3-7: Percentage Change factors (%) for mean monthly rainfall for theselected UKCIP02 scenarios. ..........................................................................................24

Table 3-8: Percentage Change factors (%) for mean monthly referenceevapotranspiration (ETo) for the selected UKCIP02 scenarios......................................24

Table 4-1: Yearly water abstraction from Borehole 1 with irrigation purposesfor the baseline (1970-2006). ..........................................................................................26

Table 5-1: Summary of responses of the water shortage adaptation optionssurvey. ............................................................................................................................. 42


8/66


vi

List of Figures

Figure 1-1: Soil strata at Cambridge University Botanic Garden.................................... 5

Figure 1-2: General and detailed views of the weather station at CambridgeUniversity Botanic Garden. .............................................................................................. 6

Figure 1-3: Mean monthly data for rainfall and reference evapotranspiration(1970-2006) (mm/month). ................................................................................................ 7

Figure 1-4: Cambridge University Botanic Garden map. The Systematic BedsArea is surrounded with a red circle. Water sources are also indicated. .......................... 8

Figure 3-1: Ranked maximum PSMD for the Cambridge University BotanicGarden (1970-2006) (mm/year). .....................................................................................15

Figure 3-6: UKCIP02 50 km resolution cell size grids for the UK and for thestudy area. ....................................................................................................................... 23

Figure 4-1: Annual correlation (1970-2006) between PSMDmax (mm) andirrigation water abstraction (m 3). ....................................................................................27

Figure 4-2: WaSim annual theoretical irrigation needs (mm) for representative

perennial species. ............................................................................................................28Figure 4-3: WaSim annual theoretical irrigation needs (mm) for representativeannual species. ................................................................................................................29

Figure 4-4: Comparison of mean monthly rainfall (mm/month) for CambridgeUniversity Botanic Garden for the baseline and UKCIP02 scenarios. ........................... 30

Figure 4-5: Comparison of mean monthly reference evapotranspiration (ETo)(mm/month) for Cambridge Botanic Garden for the baseline and UKCIP02scenarios.......................................................................................................................... 31

Figure 4-6: Comparison between ranked annual PSMDmax (mm) for the baseline and for the UKCIP02 scenarios. ....................................................................... 32

Figure 4-7: Comparison between ranked annual volumetric irrigation water use (m 3) for the baseline (1970-2006) and for the UKCIP02 scenarios. ........................ 33

Figure 4-8: Comparison of irrigation needs (mm) for the selected plantspecies, for the baseline and for the UKCIP02 scenarios. .............................................. 34

Figure 4-9: Comparison of volumetric irrigation demand (m 3) for the selected plant species, for an average bed size of 25.42 m 2, for the baseline and for theUKCIP02 scenarios......................................................................................................... 35


9/66


vii

Figure 4-10: Annual correlation (1970-2006) between WaSim irrigationdepths (mm) and PSMDmax (mm) for Ligularia. .......................................................... 36

Figure 4-11: Annual correlation (1970-2006) between WaSim irrigationdepths (mm) and PSMDmax (mm) for Marrow. ............................................................36

Figure 4-12: Annual correlation (1970-2006) between WaSim irrigationdepths (mm) and PSMDmax (mm) for Tobacco. ........................................................... 37

Figure 4-13: Annual correlation (1970-2006) between WaSim irrigationdepths (mm) and PSMDmax (mm) for Rhubarb. ........................................................... 37

Figure 5-1: Mulching techniques at Cambridge University Botanic Garden. ...............43

Figure 5-2: Drought tolerant species at Cambridge University Botanic Garden.In the picture, rosemary, pine tree and lavender. ............................................................44

Figure 5-3: Rain-water harvesting structure and underground water storagetank.................................................................................................................................. 44

Figure 5-4: Traditional irrigation methods at the garden. Hose-reel to applysurface irrigation. ............................................................................................................44

Figure 5-5: Lake at the Rock Garden, the main leakage source. ................................... 45

Figure 5-6: Percentage (%) of licensed irrigation water volume abstracted for the baseline and for the UKCIP02 scenarios. .................................................................45


10/66



11/66


1

1 INTRODUCTION

The aim of this section is to set the context of the study, providing some generalinformation to introduce the covered topic and allow the reader to get a better

understanding of the issues developed in this thesis. Firstly a quick overview about

climate change is provided, especially its implications and trends for the UK climate.

Secondly, the importance of gardening in UK is highlighted. Then, the aim and

objectives of the thesis are presented. Finally, the location and characteristics of the

study site, the Cambridge University Botanic Garden, are introduced.

1.1 Climate change

Even though taking into account natural climatic variability, the overwhelming majority

of scientific opinion is confident about the fact of global warming and climate change

due to human activities (Houghton, 2004). Since the beginning of the Industrial

Revolution in 1750, global atmospheric concentrations of carbon dioxide, methane and

nitrous oxide have increased markedly as a consequence of human activities (IPCC,

2007a). That increase in green-house gases brings about warming of the earths

temperature, leading to a change in global climate. This is currently supported by

several evidences, which the IPCC (2001) develops under the form of different

indicators (Table 1-1).

Table 1-1: IPCC Climate change evidence indicators.

Indicator Observed Changes

Atmospheric concentration of CO 2 280 ppm for the period 1000-1750 to368ppm in year 2000

Global mean surface temperature Increased by 0.6+0.2C over the 20 th century

El Nio eventsBecame more frequent, persistent andintense during the last twenty to thirtyyears compared to the previous 100 years

Source: Adapted from IPCC (2001).


12/66


2

The table above displays observed changes attributable to human actions for different

evidence indicators. An increment in atmospheric CO 2 concentration of about 90ppm

for the period 1750-2000 has been demonstrated by the IPCC. Also, anthropogenic

activities have lead to an increase in temperature of approximately 0.6C during the 20 th

century. Besides, higher recurrence of extreme events such as El Nio evidences the

effects of a changing climate.

Gathered climatic data concerning the UK confirms that the last decade has been the

warmest in over 300 years, and 0.5C warmer than the average 1961-90 climate

(MAFF, 2000). Bisgrove and Hadley (2002) affirm that, since the 18 th century, while

the frequency of hot days in the UK has doubled during the last decade respect to thelong term average, in contrast, the number of cold days has fallen from 15-20 days per

year to approximately 10 days per year. These are no other than evidences that climate

change is a reality also in the UK. The prospects for future climate change in the UK

sets that both temperature and precipitation patterns will vary across the country, with

the greatest changes and greatest extremes occurring in the south east (Bisgrove and

Hadley, 2002), including Cambridge, the location for this case study.

All these, above mentioned, variations in earths climate and likely future changes, not

only present a serious threat to human society in general (HM Government, 2006), but

more in particular to the water sector, bringing consequences and implications that will

affect the future management of water resources. Climate change, among others, put

pressure into water resources and become water scarcer, therefore reducing its

availability for the different water users, including the gardening sector. Hence that a

growing awareness of the reality of long term climate change has led to concern for the

future of UK gardens (Bisgrove and Hadley, 2002).


13/66


3

1.2 Importance of gardens in UK

The UK has a remarkable history of gardens and gardening spanning a thousand years,

ranging from rich heritage gardens to lively traditional domestic gardens (Bisgrove and

Hadley, 2002). This is supported by the statement: England is a nation of gardeners,cited in more than one occasion by different authors (Bisgrove and National Trust.,

1990; DEFRA, 2006). Moreover, gardens make a substantial contribution to the UK

economy of about 300 million per year (Bisgrove and Hadley, 2002). This fact sheds

light on gardening being not only an essential part of the UK cultural heritage and style

of life, but also a direct contributor to the economy.

UK Historic gardens and traditional planting schemes are, among many other aspects of

the historic environment, potentially at risk from climate change (English Heritage,

2006). Also, due to global warming, gardens are expected to play an increasingly

important role since more access to outdoor natural spaces will be demanded in the

future (London Climate Change Partnership, 2002; London Climate Change

Partnership, 2005). In addition, botanic gardens in particular, will face even more

elevated challenges due to climate change, such as securing plant species conservation

or conveying important environmental messages (BGCI, 2006).

1.3 Need for further research

In the preceding section, not only has just been highlighted the great significance of

gardening in the UK, but also its increasing importance under the threat of climate

change. As a consequence of this, some general reports have been published to date

relating climate change and gardening in the UK, of whom the most relevant is that by

Bisgrove and Hadley, (2002), Gardening in the Global Greenhouse. However, in

many studies concerning climate change (Bisgrove and Hadley, 2002; IPCC, 2007a), it

is pointed out the need for regional analysis if the real scope of climate change impacts

in a particular place is to be known. What is more, the magnitude of climatic changes to

which a garden is likely to be subject will depend on its local setting and on the

characteristics of the garden on itself (Bisgrove and Hadley, 2002). Also in Bisgrove

and Hadley (2002), one of the points arranged in the future research agenda claims for further analysis of the potential demand for water in gardens on a regional basis in order


14/66


4

to highlight future water management problems. In this context, a study concerning

climate change in relation with water use on garden, carried out in a specific site like the

Cambridge University Botanic Garden, would be covering, for a concrete location, the

gap specified in the research agenda and, therefore, contributing to asses the real future

impacts of climate change on UK gardens. This is precisely the objective of this thesis.

1.4 Aim and Objectives

1.4.1 General Aim

The principal aim of this thesis is to asses the impacts of climate change over water use,

focusing on irrigation, on the Cambridge University Botanic Garden.

1.4.2 Specific Objectives

To establish a climate and irrigation water abstraction baseline for the

Cambridge University Botanic Garden.

To create a set of future climatic conditions for the Cambridge University

Botanic Garden by using the UKCIP02 scenarios.

To estimate the quantity of water needed for irrigation in the garden under future

climate conditions.

To asses the impacts of climate change on irrigation needs over different species

displayed in the garden.

To review the strategies in the garden to cope with water shortages and identify

potential adaptation measures to deal with climate change in the Cambridge

University Botanic Garden.

1.5 Case study: Cambridge University Botanic Garden

The site of the study, the Cambridge University Botanic Garden, is located in the city

centre of Cambridge, in Eastern England. The garden spreads over a surface of 16 ha.

Nevertheless, this study is focused on only the irrigated part of the garden, named the

Systematic Beds Area, with an extension of 0.4 ha. In this region of the garden, more


15/66


5

than 80 families of plants are displayed in different beds, constituting a really

heterogeneous area (Figure 1-4).

Since it was opened to the public in 1856, the Cambridge University Botanic Garden

has grown several collections of trees, shrubs and herbs, both native from England and

from other world habitats. Nowadays, its tree collection is regarded as the finest of all

East of England (Cambridge University Botanic Garden, 2006). Other relevant data

showing the high importance of the garden is its elevating number of visitors, rising

about 100,000 every year. Furthermore, the garden is currently running a wide range of

activities and events linked to local schools and associations. This makes the Cambridge

University Botanic Garden not only a wonderful place for tourists, but also an essentialmeeting point for the local community.

1.5.1 Soil

By contrasting the Soil Survey and Land Research Centre map corresponding to the

Cambridge area (Palmer, 1988) with a geological exploration carried out in the garden

for drilling purposes in 1966, it is concluded that the soil in the Botanic Garden is amoderately loamy over clayey soil, with an Organic Carbon percentage between 2-4%.

Drilling ground exploration shows that strata in the Cambridge Botanic Garden are

allocated as following:

Surface

3.7m

43m

49m

GAULT CLAY

SAND

LOAM

Surface

3.7m

43m

49m

GAULT CLAY

SAND

LOAM

Figure 1-1 : Soil strata at Cambridge University Botanic Garden.Source: Adapted from Forbes (1967).


16/66


6

Gault Clay acts as an impervious layer, which limits severely root development beyond

3.7m. According to the Aquifer Classification type, the soil leaching class is low, which

reveals that almost no percolation problems may take place in this site.

1.5.2 Climate

Cambridge University Botanic Garden has a weather station (Figure 1-2), where data

has been collected and transmitted to the Met Office since 1904. Cambridge is in the

driest region of Britain and its climate is more continental than most of the rest of the

country (Cambridge University Botanic Garden, 2006). Climate data collected for the

period 1970-2006 is presented in Table 1-2.

Figure 1-2: General and detailed views of the weather station at Cambridge University BotanicGarden.

Table 1-2: Monthly averages for the Cambridge Botanic Garden weather station.

Month Tmax C Tmin C Wind m/s RH % SUN h Rain mm ETo mmJan 7.2 1.3 3.3 88.5 1.8 46.1 12.9Feb 7.5 1.0 3.3 86.9 2.7 33.9 17.9Mar 10.3 2.6 4.2 81.3 3.4 40.3 38.9Apr 13.1 4.1 4.0 74.9 4.9 41.8 61.1May 17.0 7.0 4.5 71.5 6.1 44.1 90.8Jun 20.1 9.9 4.3 73.0 6.2 49.3 99.6Jul 22.6 12.1 4.2 74.9 6.1 44.2 107.4Aug 22.7 11.9 3.9 75.3 6.0 50.1 93.7Sep 19.5 9.9 4.3 79.5 4.8 51.2 62.1Oct 15.3 7.0 4.2 85.0 3.7 54.3 34.4

Nov 10.5 3.7 3.4 87.8 2.3 55.2 15.0Dec 7.9 2.0 3.1 89.1 1.6 47.9 10.4Total - - - - - 558.4 644.1


17/66


7

The previous table confirms that August is the warmest month, with an average

maximum temperature of 22.7C. By the contrary, February is the coldest month with

an average minimum temperature of 1.0C. Also, figures demonstrate that, while May is

the windiest month, with an average wind speed of 4.5 m/s, June is the month

accounting for more sunshine hours, with an average of 6.2 hours/day, and December is

the most humid, with an average relative humidity of 89.1%.

Data concerning precipitation and reference evapotranspiration are illustrated in Figure

1-3.

Figure 1-3: Mean monthly data for rainfall and reference evapotranspiration (1970-2006)(mm/month).

Precipitation is evenly distributed throughout the year, being November the wettest

month with 55.2 mm. The data shows that precipitation for the summer period (Jun, Jul

and Aug) exceeds that for the winter season, in which February accounts for the lowest

rainfall value with only 33.9 mm on average. Though, evapotranspiration rises notablyduring the summer months and then goes down considerably during the winter. This

fact sheds light on the need for irrigating during the summer season (highlighted in red

in Figure 1-3). Yearly average figures for the 37 year period are 558 mm/year for

rainfall and 644 mm/year for reference evapotranspiration.

0

20

40

60

80

100

120

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

m m

/ m o n

t h

Rainfall Reference Evapotranspiration (ETo)


18/66


8

1.5.3 Water sources

There are four main water sources in the garden (Figure 1-4). The first is an open

channel diverting water from the public supply at Hobsons Conduit, whose principal

aim is topping up the lake in the Rock Garden area. The second are rain water harvesting structures. Collected rain water is stored in underground tanks and mostly

used for general gardening purposes and yard cleaning. Thirdly is Borehole 2, currently

abstracting only small amounts of water for specific tasks. Finally there is Borehole 1,

from where water is withdrawn in large amounts for irrigation.

Figure 1-4: Cambridge University Botanic Garden map. The Systematic Beds Area issurrounded with a red circle. Water sources are also indicated.

Borehole 2

Hobsons Conduit

Borehole 1

Rainwater Harvesting


19/66


9

2 LITERATURE REVIEW

This chapter includes a review of the available literature, including an assessment of climate change impacts on water supplies and demand, plant responses to water stress,

soils, water bodies and irrigation. In addition, the effects of CO 2 over plant water use

are also analyzed.

2.1 Water supplies and water demand

SDRT (2003) regional climate change modelling for East of England concludes that,within the next hundred years, temperature is likely to rise between 1 and 4.5C and

overall precipitation is likely to decrease between 10 and 20%. Rising temperatures

combined with less precipitation will suppose meeting higher evapotranspirative

demands with a reducing amount of rainfall. This reduction in natural water supply, and

therefore in the water available for the plant, can be expressed in terms of an increase in

potential soil moisture deficit, which, particularly for the case of Cambridge will range

between 61 and 112% (Downing et al ., 2003).

Cambridge is not only experiencing significant population growth and urban

development, but is likely to do so in the future (SDRT, 2003), which will put even

more pressure on local water resources. The increase in water use will claim for a water

demand management strategy in the area, probably provoking reductions in the supply

and higher water prices (Bisgrove and Hadley, 2002).

Then, impacts on the local gardens will come as a result of both, directly a decrease in

natural water supply and indirectly a reduction in water availability at the public supply

system (Bisgrove and Hadley, 2002).These reductions in water consumption could

bring as a consequence plant stress, whose effects are explained in the following

section.


20/66


10

2.2 Plant responses to water stress

The most obvious effect of even water stress is growth reduction (Hsiao et al ., 1976). In

addition, due to plants show a marked capacity for acclimation to stress, water deficits

can greatly modify plant development and morphology (Jones, 1992). These adaptationsare described by Bisgrove and Hadley (2002):

In the longer term (days, weeks, months), early flowering, compact plants with

thicker leaves and resources orientated to root development.

In the very long term (centuries, millennia), plants will develop adaptive

mechanisms such as hairiness, waxiness, water storage tissues or specialized

metabolisms.

However, the effects of water stress are dependent on the extent of the water deficit and

are dissimilar for the different plant species (Griffiths and Parry, 2002).

The issues above mentioned will provoke changes in the ornamental value of the plants,

therefore affecting the aesthetic composition of the garden.

2.3 Impacts on soils and water regimeTemperature, precipitation and atmospheric CO 2 changes due to climate change will

affect soil ecology and organic matter, in turn affecting soil structure and ultimately

water regime and plant growth (MAFF, 2000). Hence that modification of soil

properties by climate change is taken into account when assessing water use on garden.

The main effects of climate change on soils will be to accelerate loss of soil organic

matter and to release nutrients in higher quantities (Bisgrove and Hadley, 2002).

Decreasing amounts of organic matter provoke negative effects on the structural

stability of the soil, reducing its water-holding capacity (Bisgrove and Hadley, 2002;

IPCC, 2001). This, together with the facts of more concentrated precipitation patterns

and increasing incidence of extreme events, will cause:

In well drained soils, more drainage water flowing through the water table and,consequently, groundwater pollution.


21/66


11

In heavy, poorly drained soils, more water-logging and surface run-off with

associated flooding and erosion problems.

Therefore, special attention will be required in the management of the gardens soil in

order to, firstly, keep an acceptable soil water retention level that support adequate plant

growth, and, secondly, avoid water waste and environmental problems that may

negatively affect the garden.

2.4 Impacts on water bodies

From the eighteen century, when English gardening shifted towards a more naturalistic

approach, water features have played an increasingly important role in the landscape of

the English garden (Paul and Rees, 1986). Bisgrove and Hadley, (2002) state that the

main impact of climate change on all water bodies will be the fluctuation of water

throughput. This will vary from falling water levels during the summer months due to

high surface evaporation, to overflow in excess supply periods originated by the

concentration of rainfall events. In addition, higher temperatures will make oxygen in

the water less available, which together with higher nitrate concentrations due toaccelerated soil breakdown, will provoke algal blooms and will increase the risk of

eutrophication on water bodies (Bisgrove and Hadley, 2002). This will be a detriment to

the gardens aesthetic qualities.

2.5 Impacts on irrigation

Changes in local weather, particularly in rainfall and evapotranspiration patterns, willaffect the soil water balance and hence the irrigation needs (Downing et al ., 2003). A

study by Herrington (1996) shows that an increase in UK temperature by 1.1C, would

bring a 35% increase in water use for lawn sprinkling. Furthermore, a specific study for

Cambridge (Harte et al ., 1995) demonstrated that a 3C rise in soil temperature would

entail a 25% decrease in soil moisture, which consequently would enlarge irrigation

needs. This is proved by Dll (2002), who confirms that the net irrigation requirement

(mm/year) for South East England will switch from 77 (current baseline) to 129 in the2020s scenario. However, irrigation to reduce the impact of water deficits will be


22/66


12

subject both, to the availability of sufficient water resources, and to higher supply prices

originated by an increasing water demand by the different sectors of the society

(Bisgrove and Hadley, 2002).

2.6 CO 2 impacts on plant water use

Sections above have shown how climate change may cause water stress and its effects.

However, when including in the analysis the effect of rising atmospheric CO 2

concentrations, the picture changes significantly. Studies looking at the interaction of

elevated CO 2 concentrations and water stress showed that plants growing in elevated

CO 2 were able to withstand stress better and often showed a delay in the onset of stress

(Allen et al ., 1990). This is due to decreased stomatal conductance and transpiration in

response to elevated CO 2 concentrations, leading to an increase in water use efficiency

(Eamus, 1991; Johnson et al ., 2002). This means that for the same growth rate, water

consumption by the plant would be smaller. However, Eamus (1991) concludes that

substantially local and regional further studies are required before reliable predictions

about the effects of CO 2 can be made.


23/66


13

3 METHODOLOGY

In order to make a detailed assessment of such a complex phenomenon, different toolsand methods of analysis must be carried out (Downing et al ., 2003). In this study, a

three stage methodology was developed.

Firstly, current volumetric irrigation water demand (m 3) in the Cambridge University

Botanic Garden was assessed on the basis of historical irrigation trends and climate

patterns. For this purpose, the climate and irrigation water abstraction baselines were

correlated using an agroclimatic indicator.

Secondly, irrigation needs (mm) were estimated using a computer model (WaSim)

(Hess and Counsell, 2000) to calculate plant water requirements. However, irrigation

needs were not calculated globally for the whole garden, but for four different plant

species. This allows knowing how different plants will be affected by climate change in

relation to water needs. The species chosen for this purpose are representative for those

groups of plants that are most sensitive to water stress, and therefore which would need

to be monitored more intensively under drier conditions under climate change.

However, in order to cover the whole range of irrigated plants in the garden, species

representing low water consumption plants were also modelled.

Finally, through using the selected agroclimatic indicator, comparison between the

baseline and the future in terms of climate and water use, gives an estimation of futurevolumetric irrigation water demand (m 3) and irrigation needs (mm) in the Botanic

Garden. For this purpose, a set of future climatic conditions was modelled by applying

four defined climate change scenarios to the historical climate data set.

In addition, once the effects of climate change over the Cambridge University Botanic

Garden were recognized, a survey was carried out with the intention of identifying

possible adaptation options in terms of improving water use efficiency. Also, an


24/66


14

assessment contrasting actual volumetric water use with licensed water volumes was

completed so as to identify potential future changes in the quantity of water allowed for

abstraction.

3.1 Current volumetric water demand correlated against PSMD

If future water demand is to be calculated on the basis of historical climate and

irrigation patterns, a baseline representing the present conditions must be defined.

Baseline data are then used as a reference when predicting future conditions and when

establishing comparisons.

A climate baseline for the study site was created by gathering meteorological data from

the BADC service for the period 1970-2006. The main body of the data comes from the

Cambridge Botanic Garden weather station. The rest of the data were collected from the

Cambridge NIAB weather station, also considered as representative since it is located

within Cambridge city. Reference evapotranspiration was calculated with WaSim-ET

(Hess and Counsell, 2000) using the Penman-Monteith equation 1 using the parameters

wind speed, relative humidity, sunshine hours and temperature.

Table 3-1: Weather stations coordinates.

Weather Station Longitude (decimal degrees) Latitude (decimal degrees)

Cambridge Botanic Garden 52.193 0.132

Cambridge NIAB 52.225 0.103

Most studies involved in assessing climate change impacts in relation with future water

demand require the computation of a water balance (Knox et al ., 2005). In order to do

so, the agroclimatic indicator PSMD was used. This parameter was selected because it

requires input data relating to rainfall and reference evapotranspiration, variables which

1 See FAO 56 for the Penman-Monteith method to calculate reference evapotranspiration.


25/66


15

together are able to generate a simple water balance. The PSMD was calculated monthly

according to the following method suggested by Knox et al. (2005):

PSMD i = PSMD i-1 + ET i P i

Where

PSMD i potential soil moisture deficit in month i, mm

The PSMD in the Cambridge University Botanic Garden for the period 1970-2006 is

illustrated in Figure 3-1.

Figure 3-1: Ranked maximum PSMD for the Cambridge University Botanic Garden (1970-2006) (mm/year).

The long term average (LTA) PSMD was 263mm, which corresponds to the year 1997.

The driest year of the baseline was 1990, showing a PSMD close to 500 mm. By the

contrary, the years 1987and 1988 were particularly wet. This is reflected by their lowPSMD values (88 mm and 139 mm respectively).

Then, the PSMD was correlated against the volumetric irrigation water demand for the

period 1970-2006.

Volumetric irrigation water demand baseline was produced from collecting water use

records displayed in the Abstraction Licence corresponding to the main borehole at the

Cambridge University Botanic Garden, principally used for irrigation purposes. This

0

100

200

300

400

500

600

8 7 8 8 0 0 8 2 8 1 8 5 9 9 8 6 9 3 0 1 8 3 0 5 9 2 8 4 9 8 7 7 7 9 9 1 7 4 7 8 0 4 9 7 7 1 0 2 7 3 8 0 7 2 7 5 0 6 7 0 9 4 8 9 9 6 9 5 0 3 7 6 9 0

Ranked Years

P S M D ( m m

)

LTA


26/66


16

was corroborated when reading through the boreholes water abstraction contract, where

spray irrigation is displayed as the main authorized purpose of the licence. Once

collected the data, irrigation water abstractions were ranked yearly in order to show the

years with higher water consumptions. To do so, water use for each particular year was

derived from the monthly records written down in Boreholes 1 Abstraction Licence.

3.2 WaSim modelling irrigation needs

WaSim software is a one-dimensional model developed by Cranfield University which

simulates changes in soil water content in response to weather and water management

(Hess, 2000). Relevant parameters used in the model concerning soil water content, and

therefore irrigation needs, are easily available water capacity (EAWC) and soil water

deficit (SWD). The equations used by the software to calculate these parameters are

displayed below:

EAWC = TAWC * p

Where

EAWC easily available water capacity of root zone, mm

p fraction of total available water that is easily available, dimensionless

TAWC total available water capacity of root zone, mm

TAWC = ( FC PWP ) * r i* 1000

Where

FC volume water fraction at field capacity, dimensionless

PWP volume water fraction at permanent wilting point, dimensionless

r i root zone at day i, m


27/66


17

SWD = ( FC ) * r * 1000

Where

SWD soil water deficit of root zone, mm

r root depth, m

FC volume water fraction at field capacity, dimensionless

volume water fraction of root zone, dimensionless

Other important assumptions utilized when running WaSim were the following:

The initial water content in the unsaturated zone is field capacity. The model was run with the water-table and salinity options off.

No capillary rise was assumed to be taking place.

In order to run the model, input data on soil, crop and weather are required. Input data

for the Cambridge University Botanic Garden is explained below.

3.2.1 Soil input data

According to the map from the Soil Survey and Land Research Centre for South

Cambridgeshire (Palmer, 1988), the soil in the Botanic Garden is classified as Loam

over clay. Due to WaSim is a two layered model (topsoil and subsoil) (Knox et al .,

1997), a first intuitive approach would consider appropriate to represent both layers by

choosing a Clay Loam soil type. However, WaSim model works between an upper boundary represented by the soil and the impermeable layer acting as lower boundary

(Hess et al ., 2000). For this reason, taking into account that Gault Clay forms an

impervious layer in the gardens soil profile, a Loamy soil type with a depth profile of

3.7 m was the most suitable for the existing conditions. In addition, since the soil in the

Cambridge University Botanic Garden is receiving constant maintenance and

improvements, it should be better represented by choosing a soil with a higher value of

water holding capacity. Soil characteristics used in WaSim are summarized in Table

3-2.


28/66


18

Table 3-2: Summary of soil characteristics used for modelling the water balance.

Soil Type Saturation (%) FC (%) PWP (%) AWC(mm/m)

Hydraulic

Conductivity

(m/d)

Loam 46.3 27.9 11.7 162 1.0

3.2.2 Crop input data

Calculation of irrigation requirements using WaSim covered four different groups of

plants from the Systematic Beds area in the Cambridge University Botanic Garden. Two

criteria were set in order to form the groups. The first was to select mainly species withmedium or high water needs. This is because plants with elevated water requirements

are consuming the most water within the garden and because precisely these ones will

be the most affected under future water scarcity conditions. However, as pointed out

earlier, a low water use plant was also modelled so as to complete the analysis. The

second criterion was making a difference between annual and perennial species. This

standard was picked because, since annual and perennials do not remain the same time

on the field, their water consumption is dissimilar.

Finally, four species were selected as representative for the different groups of irrigated

plants (Table 3-3). Selection of suitable model plants was made relying on local

knowledge from the gardens staff and supported by a review of general literature

(FAO, 2007; Sanders, 1997).

Table 3-3: Selected representative species to calculate irrigation needs using WaSim.

Plant Species Representative group

Ligularia (Ligularia japonica) Perennials with high water requirements

Rhubarb (Rheum palmatum) Perennials with low water requirements

Marrow (Cucurbita pepo) Annuals with high water requirements

Tobacco (Nicotiana rustica) Annuals with medium water requirements


29/66


19

Pictures of the selected model plants are shown below:

Figure 3-4 : Ligularia japonica at theCambridge University Botanic Garden.

Figure 3-3: Cucurbita pepo at theCambridge University Botanic Garden.

Figure 3-5: Nicotiana rustica at the CambridgeUniversity Botanic Garden.

Figure 3-2: Rheum palmatum at theCambridge University Botanic Garden.


30/66


20

For each plant, the characteristics used to run WaSim are summarized in Table 3-4.

Table 3-4: Crop characteristics for each plant species used in the WaSim model.

Crop Characteristics Ligularia Rhubarb Marrow Tobacco

Planting date 1 Mar 1 Mar 1 May 15 May

Emergence date 15 Mar 15 Mar 15 May 29 May

20% cover date 30 Mar 30 Mar 30 May 13 Jun

Full cover date 19 May 19 May 15 Jul 2 Aug

Maturity date 7 Aug 7 Aug 15 Jul 2 Aug

Harvest date 15 Nov 15 Nov 1 Sep 1 Sep

Maximum root depth date 19 May 19 May 15 Jul 2 Aug

Crop cycle duration (days) 260 260 124 110

Maximum crop cover (%) 100 100 100 100

Crop coefficient at full cover (Kc) 1.0 1.0 1.0 1.15

Planting depth (m) 0.1 0.1 0.1 0.1

Maximum root depth (m) 1.5 1.5 0.8 0.8

p-fraction (%) 0.45 0.45 0.5 0.6

2

Values for crop parameters were taken from a review of literature. Plant dates, lengths

of crop development stages, depletion fractions and crop coefficients have been

extracted from FAO 56 (Allen et al ., 1998) and Vegetable Crop Irrigation (Sanders,1997). Rooting depth was taken from Root Development of Vegetable Crops (Weaver

2 Even though Ligularia and Rhubarb are perennial species; planting dates have been used in their

modelling. This is because both Rhubarb and Ligularia, after following a growth season their crowns

become dormant and is not till next spring, once bud break have been stimulate, that subsequent

vegetative growth happens. Therefore, to monitor these plants in terms of water consumption, it is

regarded as most sensitive to consider only the growth season, which is actually the period when these

plants demand water.


31/66


21

and Bruner, 1927). Values for ornamental specimens could not be found in general

literature, so they were obtained by comparison with agricultural crops with similar

characteristics. This was the case of ligularia, compared to the perennial vegetable

artichoke.

3.2.3 Irrigation scheduling input data

Irrigation scheduling and irrigation periods used in WaSim to calculate irrigation needs

for the model plant species are shown in Table 3-5.

Table 3-5: Irrigation scheduling input data for each plant entered in WaSim model. Plant Irrigation period Irrigation scheduling:

Irrigate at fixed depletion (%AWC) Return to fixed deficit (%AWC)

Ligularia 1 Mar-15 Nov 30% 5%

Rhubarb 1 Mar-15 Nov 80% 5%

Marrow 1 May1 Sep 30% 5%

Tobacco 15 May1 Sep 60% 5%

Guidelines for irrigation scheduling of the different species was taken from general

literature (FAO, 2007; Sanders, 1997). Irrigation was scheduled returning the soil to a

fixed deficit of 5% to allow possible contributions from rainfall.

3.2.4 Weather input data

WaSim runs on daily basis using rainfall and reference evapotranspiration data (Hess

and Counsell, 2000). Therefore, daily data corresponding to the Cambridge Botanic

Garden and Cambridge NIAB weather stations for the period 1970-2006 were used to

calculate baseline irrigation requirements. Simulations up to 30 years duration can be

undertaken using WaSim (Hess and Counsell, 2000). Then, the 37 years data set (1970-

2006) was split into two and the model was run twice for each simulation.


32/66


22

3.3 Future irrigation water demand modelling

In order to generate future climatic conditions, the UKCIP02 scenarios were used

(Hulme et al ., 2002). These scenarios, the latest version prepared for the Tyndall Centre

for Climate Change Research, represent future climatic projections for three thirty-year

time slices 2020s (2011-2040), 2050s (2041-2070) and 2080s (2071-2100) and four

equally possible scenarios of greenhouse gas emissions (Low, Medium-Low, Medium-

High and High). The characteristics for the different UKCIP02 scenarios are

summarized in Table 3-6.

Table 3-6: Socio-economic, temperature (C) and CO 2 concentration (ppm) changes for the2080s time slice for the different UKCIP02 scenarios.

Emissionsscenarios

UKCIP02climate change

scenariosStoryline description

Increase in

globaltemperature

(C)

AtmosphericCO 2

concentration(ppm)

B1 Low

Local solutions tosustainability,

increasing population atlow rates, slowtechnological change

2.0 525

B2 Medium-Low

Global solutions tosustainability,

population peaks mid-century, clean andefficient technologies

2.3 562

A2 Medium-High

Economic growth onregional scales,increasing population,

preservation of localidentities

3.3 715

A1 High

Very rapid economicgrowth, population

peaks mid-century,market mechanismsdominate

3.9 810

Source: Adapted from Hulme et al., 2002.


33/66


23

These predicted changes on climate are reported across the UK in 50 km resolution cell

size grids (UK Climate Impacts Programme, 2007). Since the alternative scenarios

result from uncertain future conditions (UK Climate Impacts Programme, 2007), thewider the range of scenarios chosen, the more robust and reliable the results obtained.

However, for this study, in order to simplify the analysis only the scenarios deriving

higher contrasts were considered. Therefore only the Low and High scenarios were

modelled. Furthermore, only the 2050s and 2080s time slices were used in order to

come out with more marked figures, which provide more significant results.

It was determined that the UKCIP02 grid corresponding to Cambridge University

Botanic Garden was number 376 (Figure 3-6).

Figure 3-6: UKCIP02 50 km resolution cell size grids for the UK and for the study area.


34/66


24

For the selected grid, it were extracted from the UKCIP02 dataset archive a 50km

resolution modelled baseline (1961-1990) and its correspondent 50km resolution future

climate change scenarios for the 2050s and 2080s. Then, by comparing future climate

with baseline data the percentage changes were calculated (Table 3-7 and Table 3-8).

Finally, by applying these change factors into the observed climate baseline (1970-

2006), a set of future climatic conditions was derived for the Cambridge University

Botanic Garden.

Table 3-7: Percentage Changes (%) expressed as a proportion of the baseline, for mean monthlyrainfall for the selected UKCIP02 scenarios.

Baseline (mm) 2050L 2050H 2080L 2080H

Jan 46.1 10 25 14 28Feb 33.9 8 21 11 22Mar 40.3 4 10 5 10Apr 41.8 -2 -4 -2 -4May 44.1 -7 -18 -11 -21Jun 49.3 -14 -33 -19 -38Jul 44.2 -18 -51 -26 -51Aug 50.1 -18 -58 -26 -50Sep 51.2 -13 -40 -19 -36

Oct54.3 -6 -15 -9 -17

Nov 55.2 2 4 2 4Dec 47.9 8 18 11 22

Table 3-8: Percentage Changes (%) expressed as a proportion of the baseline, for mean monthlyreference evapotranspiration (ETo) for the selected UKCIP02 scenarios.

Baseline (mm) 2050L 2050H 2080L 2080H

Jan 12.9 38 60 53 106Feb 17.9 24 40 35 72

Mar 38.9 16 26 23 49Apr 61.1 13 21 19 39May 90.8 14 23 20 42Jun 99.6 16 27 24 50Jul 107.4 18 29 26 51Aug 93.7 14 21 19 29Sep 62.1 9 9 9 -7Oct 34.4 8 10 10 2Nov 15.0 17 29 25 53Dec 10.4 34 56 50 102


35/66


25

3.3.1 Future volumetric irrigation water demand (m 3 )

The first step to calculate future irritation water demand was to work out the design dry

year PSMD for the selected UKCIP02 scenarios. The concept dry year is introduced in

the following paragraph.

UK summer weather unpredictability provokes marked variations on irrigation demand

between years. It is not economically feasible to design for the extreme dry year or for

the average one. As an alternative, irrigation capacity is designed for a designed dry

year, which is this year whose supply is sufficient to meet the demand with 80%

probability of exceedance or the demand of 4 out of 5 years (Downing et al ., 2003). In

this study, the dry year has been set according to a raking (Hess, 2001) to calculate

effective rainfall, but applied for the parameter PSMDmax. Following this, annualmaximum PSMD data were ranked and annual PSMD with the rank n x 80% was

selected, where n is the number of years of data available, in this case 37.

Then, using the correlation graph for PSMDmax and irrigation water abstraction, future

volumetric irrigation water demands (m 3) were estimated for the design dry year.

3.3.2 Future irrigation needs (mm)

Climate change impacts on water depths applied over the selected model plant specieswere estimated using WaSim. Future irrigation needs (mm) were calculated using daily

climate input data corresponding to the different UKCIP02 scenarios and for the

different time slices. Soil and plant characteristics remained the same as for calculating

baseline irrigation needs.

Furthermore, irrigation needs for the studied plant species were also calculated in

volumetric terms (m 3). For this purpose, irrigation depths (mm) from WaSim were

multiplied by the average plant bed size of the studied area.

VIN = IN * Abed / 1000

Where

V IN Volumetric Irrigation Demand, m 3

IN Irrigation Depth, mm

Abed Average plant bed area, m 2


36/66


26

4 RESULTS AND ANALYSIS

In this chapter the results obtained are presented and analyzed. The structure followed inthis section corresponds to the three main work areas of the study: assessing current

volumetric irrigation water demand, assessing current irrigation needs for the selected

plant species and calculating future irrigation requirements through modelling future

climatic conditions.

4.1 Historical pattern of irrigation water demand (m 3)

Historical pattern of irrigation water demand shows how much water was withdrawn

each year for irrigation purposes from Borehole 1 at Cambridge University Botanic

Garden (Table 4-1).

Table 4-1: Yearly water abstraction from Borehole 1 with irrigation purposes for the baseline(1970-2006).

Year Irrigation abstraction(m 3) Year Irrigation abstraction (m 3)

1970 4845 1989 36751971 3116 1990 75751972 4626 1991 25221973 2957 1992 33101974 1991 1993 21751975 3721 1994 55601976 6598 1995 103011977 1078 1996 125541978 1073 1997 93791979 1514 1998 44851980 1477 1999 3438

1981 1113 2000 37301982 2000 2001 18461983 1445 2002 48881984 1521 2003 89561985 1092 2004 66461986 361 2005 78041987 989 2006 41641988 1084 Average 3935


37/66


27

The historical data shows an enormous variation in water abstractions between the

different years, ranging from 361 m 3 abstracted in 1986 to 12554 m 3 in 1996. The

average quantity of water withdrawn from Borehole 1 is 3935m 3. Weather variability is

the main source of variability in water use between years, but not the only one. Other

possible factors affecting the quantity of water withdrawn from Borehole 1 are outlined

next in this section.

To calculate future volumetric irrigation water demand on the basis of historical climate

and irrigation patterns, the parameters irrigation water use and PSMD were correlated,

proving certain relation (Figure 4-1).

Figure 4-1: Annual correlation (1970-2006) between PSMDmax (mm) and irrigation water abstraction (m 3).

The relation between irrigation water abstraction and PSMD displays a logical trend,

showing higher water use for the years with higher PSMD. However, it is demonstrated

that years with peak PSMD are not necessarily the ones consuming the more water, or

that in years with medium or low PSMD values, excessive quantities of water are

abstracted. In order to validate the PSMD as a suitable indicator to predict future

irrigation requirements, reasons for these inaccuracies were pursued through

interviewing the gardens staff. Results from the investigation showed that, contrary to

that assumed in this study, not all the water coming from Borehole 1 was being used for its licensed purpose, spray irrigation. Depending on concrete conditions for specific

y = 22.391x - 1958.9R2 = 0.5071

0

2000

4000

6000

8000

10000

12000

14000

0 100 200 300 400 500 600 700

Annual PSMD max (mm)

A n n u a l w a t e r a b s t r a c t

i o n

( m 3 )


38/66


28

moments during the baseline period, and without responding to a fixed pattern, in some

of the years water abstracted from the borehole was also utilized for domestic use,

gardening purposes or to top up the gardens lake suffering from leakage. In addition, it

was discovered that during some years, presumably 1996 and 1997, the other main

source of water for the garden, the Hobsons Conduit, was cut off due to road works in

Cambridge City. Replacement of this water source was solved through additional

abstraction from Borehole 1, which explains the apparently unreasonable high water use

for these two years in relation with its PSMD (these years are highlighted in red).

All these reasons justify a correlation which is not excessively strong R 2=0.5071 and

suggest that results obtained express water use values higher than those which should

have been obtained by analyzing exclusively irrigation water use. Going further, takinginto account all these non-licensed water uses in the analysis is a way of including

possible future unexpected events in the Botanic Gardens water balance, providing

more real and robust results.

4.2 Assessing current irrigation needs (mm)

Baseline theoretical irrigation requirements for reference plants were calculated usingWaSim (Hess and Counsell, 2000). Output from the model is shown in Figure 4-2 and

Figure 4-3.

0

50

100

150

200250

300

350

400

7 0 7 1 7 2 7 3 7 4 7 5 7 6 7 7 7 8 7 9 8 0 8 1 8 2 8 3 8 4 8 5 8 6 8 7 8 8 8 9 9 0 9 1 9 2 9 3 9 4 9 5 9 6 9 7 9 8 9 9 0 0 0 1 0 2 0 3 0 4 0 5 0 6

I r r i g a t

i o n

D e p t

h ( m m

)

Ligularia Rhubarb

Figure 4-2: WaSim annual theoretical irrigation needs (mm) for representative perennialspecies.


39/66


29

According to WaSim output data, the species ligularia japonica, which represents

perennial plants with high watering needs, requires to be irrigated every year during the

baseline period 1970-2006. On the contrary, rhubarb, which represents perennial plants

with low irrigation requirements, only needs to be irrigated in the driest years. Long

term average theoretical irrigation requirements are 206 mm/year and 68 mm/year for

ligularia and rhubarb respectively.

0

50

100

150

200

250

300

7 0

7 1

7 2

7 3

7 4

7 5

7 6

7 7

7 8

7 9

8 0

8 1

8 2

8 3

8 4

8 5

8 6

8 7

8 8

8 9

9 0

9 1

9 2

9 3

9 4

9 5

9 6

9 7

9 8

9 9

0 0

0 1

0 2

0 3

0 4

0 5

0 6

I r r i g a t i o n D e p t h ( m m

)

Tobacco Marrow

Figure 4-3: WaSim annual theoretical irrigation needs (mm) for representative annual species.

The theoretical LTA irrigation needs for marrow, which represents annuals with high

irrigation requirements, is 115 mm/year. For tobacco, identified with medium water

consumption annuals, the LTA irrigation need is 87 mm/year. As expected, tobacco was

proved to be more resistant to water stress than marrow. Furthermore, output data from

the model shows that, while the marrow should have been irrigated every year during

the period 1970-2006, tobacco did not require irrigation in the wettest years.


40/66


30

4.3 Future irrigation water requirements

Future climatic conditions for the Cambridge University Botanic Garden were estimated

by applying the corresponding climate change factors to the observed climate baseline.

Rainfall and ETo, for the 2050s and 2080s time slices and for the Low and HighUKCIP02 scenarios, are illustrated in Figure 4-4 and Figure 4-5.

0

10

20

30

40

50

60

70

Ja n Feb Mar Apr Ma y Jun Jul Aug Se p Oct Nov Dec

M e a n m o n t h

l y r a

i n f a l l ( m m

)

Baseline 2050L 2050H 2080L 2080H

Figure 4-4: Comparison of mean monthly rainfall (mm/month) for Cambridge UniversityBotanic Garden for the baseline and UKCIP02 scenarios.

Comparison between baseline and UKCIP02 scenarios shows that rainfall decreases

from April to October, and increases from November to March. Annually, a total

reduction in rainfall takes place as a consequence of climate change, since the decrease

in rainfall for the summer months is higher than the increase for the winter months. The

most significant changes happen for the High scenario, during the months of January

and August. It is also worthy to point out that, contrary as expected, summer rainfall at

some points is higher for the 2080-High scenario than for the 2050-High. Although this

fact does not look reasonable, it is supported by the Hadley Centre (2005), which

demonstrates that rainfall in South-East England for the 2080Med-High scenarios may

not necessarily decrease. This is because when doing regional or local climate change

assessments, like in this case, and especially for the longest term scenarios, the

uncertainty is even bigger (Hadley Centre for Climate Prediction and Research, 2005).


41/66


31

0

20

40

60

80

100120

140

160

180

Ja n Fe b Ma r Apr Ma y Jun Jul Aug Se p Oct Nov De c

M e a n m o n

t h l y E T o

( m m

)

Baseline 2050L 2050H 2080L 2080H

Figure 4-5: Comparison of mean monthly reference evapotranspiration (ETo) (mm/month) for Cambridge Botanic Garden for the baseline and UKCIP02 scenarios.

As a consequence of climate change, evapotranspiration is predicted to rise throughout

all months of the year. Higher ETo increments occur during the summer season, being

July the peak month with an increment of almost 60% for the 2080s High scenario.

On the one hand, higher evapotranspiration and less rain during the summer season

suggest an increase in the net irrigation demand. On the other hand, wetter winters offer

greater extent for conservation of winter rainfall, through rainwater harvesting

structures or winter storage reservoirs (Downing et al ., 2003).


42/66


32

4.3.1 Future irrigation water abstraction (m 3 )

In this section, predicted future changes in PSMD are compared with the historical

records for the Cambridge University Botanic Garden (Figure 4-6).

Figure 4-6: Comparison between ranked annual PSMDmax (mm) for the baseline and for theUKCIP02 scenarios.

The design dry year PSMD is 337 mm, which corresponds to the year 1970. Estimateddry year PSMD for the 2050-Low (449 mm) and 2080-Low (490 mm) scenarios

correspond roughly to the years 1976 and 1990 respectively. For the 2050-High and

2080-High scenarios, dry year PSMD projections are, in that order, 539 and 620 mm,

which exceed the PSMD even for 1990, the driest year in the 37 year baseline.

Therefore, as a consequence of climate change, recent dry conditions such as those in

1976 or 1990, and even drier ones, will become more common in the future (2050s and

2080s).

PSMD and water use for irrigation were linked by using baseline figures as input data.

Once correlated both parameters (correlation details were presented in section 4.1),

volumetric irrigation water demand for the future UKCIP02 scenarios was estimated.

Irrigation water abstraction is presented in Figure 4-7.

0

100

200

300

400

500

600

700

8 7 8 8 0 0 8 2 8 1 8 5 9 9 8 6 9 3 0 1 8 3 0 5 9 2 8 4 9 8 7 7 7 9 9 1 7 4 7 8 0 4 9 7 7 1 0 2 7 3 8 0 7 2 7 5 0 6 7 0 9 4 8 9 9 6 9 5 0 3 7 6

2 0 5 0

L

2 0 8 0 L 9 0

2 0 5 0

H

2 0 8 0 H

Ranked Years

P S M D ( m m

)

80% Dry year


43/66


33

Figure 4-7: Comparison between ranked annual volumetric irrigation water use (m 3) for the baseline (1970-2006) and for the UKCIP02 scenarios.

For the present design dry year at Cambridge University Botanic Garden, 1970,

irrigation water abstraction account for 4845 m 3. For future design dry years, irrigation

water demand for the 2050s time slice are 8000 m 3 and 10110 m 3 respectively for the

Low and High scenarios, matching approximately with the irrigation water abstraction

of the years 2005 and 1995. As expected, for the 2080s the design dry year irrigationdemand is higher, reaching 9017 m 3 for the Low scenario, which roughly equates to the

year 2003, and 11923 m 3 for the High, which is slightly lower than water use in 1996

(the highest water consumption year). Therefore, elevated volumetric irrigation water

abstractions, such as the ones experienced in highly water consumption years (1995,

1996), will be more typical in forthcoming years as a consequence of climate change.

0

2000

4000

6000

8000

10000

12000

14000

8 6 8 7 7 8 7 7 8 8 8 5 8 1 8 3 8 0 7 9 8 4 0 1 7 4 8 2 9 3 9 1 7 3 7 1 9 2 9 9 8 9 7 5 0 0 0 6 9 8 7 2 7 0 0 2 9 4 7 6 0 4 9 0 0 5

2 0 5 0

L 0 3

2 0 8 0 L 9 7

2 0 5 0

H 9 5

2 0 8 0 H 9 6

Ranked Years

W a t e r

U s e

( m 3 )

80% Dry year


44/66


34

4.3.2 Future irrigation needs (mm)

In this section, future irrigation needs (mm) are estimated. For each plant species, the

dry year irrigation needs, for the baseline and for the UKCIP02 scenarios are illustrated

in Figure 4-8.

0

50

100

150200

250

300

350

400

450

500

Ligualria Rhubarb Marrow Tobacco

I r r i g a t i o n

N e e d s

( m m

)

LTA Baseline DY(Dry Year) 2050L(DY) 2050H(DY) 2080L(DY) 2080H(DY)

Figure 4-8: Comparison of irrigation needs (mm) for the selected plant species, for the baselineand for the UKCIP02 scenarios.

Figures show that the present design dry year irrigation needs are 266 mm/year for

ligularia, 124 mm/year for rhubarb, 157 mm/year for marrow and 126 mm/year for

tobacco. This suggests that, at present conditions, plant water use in the Botanic Garden

is ranked as following:

High IN Perennials > High IN Annuals > Medium IN Annuals > Low IN perennials

However, the picture changes concerning future scenarios. Analyzing the most extreme

case, the 2080-High scenario, design dry year irrigation needs are 497 mm /year for

ligularia, 348 mm /year for rhubarb, 317 mm /year for marrow and 269 mm /year for

tobacco. This suggests that, in the future (2050s, 2080s), plant water use in the Botanic

Garden will be ranked as following:

High IN Perennials > Low IN Perennials > High IN Annuals > Medium IN Annuals

WaSim estimations confirm that climate change will increase irrigation requirements for

the four plant species studied by 33-176%, depending on the climate change scenariosand time slices.


45/66


35

These results explain, firstly, that future conditions will involve increasing irrigation

needs and, secondly, that climate change will have greater impact on irrigation needs

over perennial species than over annuals. Reasons for this are further explained next in

this section.

In addition, volumetric irrigation demand (m 3) was worked out for the selected model

plants assuming an irrigated area of 25.42 m 2, corresponding to the average bed size of

the Systematic Beds Area. For each plant species, the dry year volumetric irrigation

demand for the baseline and for the UKCIP02 scenarios is illustrated in Figure 4-9.

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

Ligualria Rhubarb Marrow Tobacco

V o

l u m e

t r i c I r r i g a

t i o n

D e m a n

d ( m 3 )

LTA Baseline DY(Dry Year) 2050L(DY) 2050H(DY) 2080L(DY) 2080H(DY)

Figure 4-9: Comparison of volumetric irrigation demand (m 3) for the selected plant species, for an average bed size of 25.42 m 2, for the baseline and for the UKCIP02 scenarios.

The picture above shows the amount of water required for 25.42 m 2 of land planted with

each one of the different model pants for the different scenarios. Then, for the 2080-

High scenario, the average bed size planted with ligularia will require 12.64 m 3 for the

design dry year. The same bed will require 8.05 m 3/year of irrigation if planted with

marrow, 8.85 m 3/year if planted with rhubarb and 6.83 m 3/year if cropped with tobacco

(again design dry year).

In order to validate future water needs modelling, irrigation output data from WaSimwere correlated with the agroclimatic indicator PSMD for the baseline period.


46/66


36

Correlation for the different model species are illustrated in Figure 4-10, Figure 4-11,

Figure 4-12 and Figure 4-13.

L.Japonica y = 0.8148x - 8.1875R2 = 0.8732

050

100150200250300350400450

0 100 200 300 400 500 600PSMD (mm)

I r r i g a t

i o n

N e e

d s

( m m

)

Figure 4-10: Annual correlation (1970-2006) between WaSim irrigation depths (mm) andPSMDmax (mm) for Ligularia.

Marrow y = 0.5649x - 33.668R2 = 0.7508

0

50

100

150

200

250

300

0 100 200 300 400 500 600

PSMD (mm)

I r r i g a t

i o n

N e e

d s

( m m

)

Figure 4-11: Annual correlation (1970-2006) between WaSim irrigation depths (mm) andPSMDmax (mm) for Marrow.


47/66


37

Tobacco y = 0.5094x - 47.347R2 = 0.6919

-50

0

50

100

150

200

250

300

0 100 200 300 400 500 600

PSMD (mm)

I r r i g a t

i o n

N e e

d s

( m m

)

Figure 4-12: Annual correlation (1970-2006) between WaSim irrigation depths (mm) andPSMDmax (mm) for Tobacco.

Rhubarb y = 0.7844x - 138.07R2 = 0.6285

-100

-50

0

50

100

150

200

250

300

0 100 200 300 400 500 600

PSMD (mm)

I r r i g a t

i o n

N e e

d s ( m m

)

Figure 4-13: Annual correlation (1970-2006) between WaSim irrigation depths (mm) andPSMDmax (mm) for Rhubarb.

The graphs prove a clear relationship between irrigation depths and PSMD,

demonstrating that the higher the deficit of water in the soil, the higher the irrigation

needs. Correlation values displayed in the graphs are ranked as following:

Ligularia (R 2=0.8732) > Marrow (0.7508) > Tobacco (0.6919) > Rhubarb (0.6285)

Species showing higher correlations are those whose water use is more sensitive toweather variations, and therefore that might be more affected by changing conditions


48/66


38

under climate change. Then, for example, future irrigation requirements should increase

more for marrow than for rhubarb, which is proved to be more insensitive to water

stress (as it is evidenced by the linear trend of the scatter showed in Figure 4-13).

However, this statement was proved to be false (see Figure 4-8), since in the majority of

the future scenarios contemplated the rhubarb consumes more water than the marrow.

Apparent source of error arises from the relationship showed on these graphs, only

representing current conditions, but not capable to predict future changes. Indeed, the

key issue is that peak PSMD does not take place when annual plants are over the field,

but it does when the perennials are. Since the most significant changes as a consequence

of climate change are those happening for the peak PSMD, perennial species, such as

the rhubarb, will subsequently be more affected in terms of irrigation needs than annual

ones, such as the marrow.

Therefore, when considering the scope of climate change over different plants in terms

of water use, not only its sensitivity to climate and water must be assessed, but also its

annual or perennial nature.


49/66


39

5 DISCUSSION

In this section, the key assumptions for this study case and the main uncertaintiessurrounding climate change modelling are explained, giving way to carry out a further

analysis of the results. Also, possible solutions to cope with water shortages are

assessed through the adaptation options survey and a contrast of licensed water use

against actual water use.

5.1 Climate change uncertainties

The basic physics of climate change is well-understood and not controversial (Heal and

Kristrm, 2002). However, there are many sources of uncertainty regarding climate

change science. The UKCIP02 scenarios are alternative descriptions about future trends

and behaviour in terms of population growth, socio-economic development and

technological progress, and how these might influence future global emissions of

greenhouse gases (UK Climate Impacts Programme, 2007). Since all these factors

depend on human behaviour, impossible to forecast, results coming from climate

change studies must not be presented as exact future predictions, but just as estimations.

Fur

Documents

Assessing the Impacts of Climate Change on Irrigation Water Use in the Cambridge University Botanic Garden