Water Resources Research

An evaluation of the impacts of energy tree plantations on water resources in the United Kingdom under present and future UKCIP02 climate scenarios

Authors


Abstract

[1] The Hydrological Land Use Change model was used to assess the range of water resource impacts associated with four potential energy tree species (Eucalyptus nitens, Eucalyptus gunnii, Nothofagus sp., and Fraxinus excelsior) at eight United Kingdom locations under present and future, Environment Agency Rainfall and Weather Impacts Generator, climate scenarios generated using UK Climate Impacts Programme 2002 (UKCIP02). Parameter values were derived using expert opinion and interpolation because of limited data. For Fraxinus excelsior, there are questions concerning the unusual, in a world context, published findings that evaporation from a tree crop is less than that from grass. Model predictions indicated that under the present climate all tree species, excepting Fraxinus excelsior, at all sites have greater mean annual evaporation, (8 to 84%) and reduced water yields (−6 to −97%) compared with grass. The predicted increase in tree evaporation arises from parameter values reflecting both increased rainfall interception and higher transpiration due to deeper rooting depths. Under future climate scenarios, (1) “potential annual yield” (difference between actual rainfall and potential evaporation) will decrease, becoming negative at all studied sites in England and Wales by 2080; (2) at drier sites and for species with highest evaporation rates, E. nitens and Nothofagus, evaporation rates will decrease; (3) at wetter sites and for all species, evaporation rates will increase; (4) at all sites and for all species, water yields will decrease; (5) differences between species remain the same, with evaporation rates increasing and water yield decreasing in the order Fraxinus excelsior, grass, E. gunnii, Nothofagus, and E. Nitens; and (6) there is an overall trend through time toward convergence in water yields from trees and grass. If higher water yield predictions for Fraxinus excelsior are proved correct, this would represent an attractive land use option for water and energy production. Field research is required to validate these predictions. Assuming future climate changes match those predicted, soil moisture deficits will occur for longer periods during the year and will become increasingly limiting for evaporation. The monitoring of soil moisture may then provide one of the most sensitive methods of both determining model parameter values and testing predictions of differences in evaporation between species and changes in evaporation over time.

1. Introduction

[2] There is increasing interest throughout the world in the use of energy tree plantations to meet renewable energy requirements. This is in line with global targets (Kyoto) and European directives on reducing carbon dioxide emissions, and the resultant attempt by the energy industry to find cost-effective alternatives to burning fossil fuels. However, concerns have been raised about a number of adverse impacts of large-scale energy plantations. Reports by both the European Environment Agency [2007] and UK Woodland Policy Group [Lawson and Hemery, 2007] warn of the potential to damage the environment, disrupt existing timber markets and create future wood resource shortages.

[3] An earlier study [Hardcastle et al., 2006] highlighted the threat that extensive areas of energy plantations could pose to water resources, particularly when considered in the context of climate change predictions of shorter, warmer, wetter winters and longer, warmer, drier summers [Hulme et al., 2002]. A number of studies have demonstrated the high potential transpiration rates of short rotation coppice (SRC) crops of willow and poplar [Hall et al., 1996] leading to the development of planting guidelines [Hall, 2003], but little work has been done on the impact of short rotation forestry energy plantations (SRF). SRF differs from SRC in terms of tree species, stand structure and rotation length, all of which are likely to influence crop water use. Most of the high-yielding tree species suitable for SRF, such as Eucalyptus nitens, are known to have a high potential water use due to deep rooting sustaining high transpiration rates [Calder et al., 1997].

[4] The aim of this study was to investigate, through the use of the Hydrological Land Use Change (HYLUC) model [Calder, 2003; I. R. Calder and J. A. Harrison, Can parsimonious parameter evaporation models help estimate forest and short crop evaporation in a changing climate — Will forest impacts on water resources increase or decrease with climate change?, submitted to Water Resources Research, 2007], the range of possible water resource impacts that different energy crop tree species grown as SRF, might have at different United Kingdom locations under future climate change prediction scenarios. This information will assist the identification of tree species and sites most suitable for plantations to meet wood energy targets without compromising water resources.

2. Location and Species

[5] Four tree species with potential to be suitable for future energy plantations in the United Kingdom were selected for this investigation. Three of these are exotic species, Eucalyptus nitens and gunnii, and Nothofagus sp., and the fourth is native, Fraxinus excelsior (common ash). Eight sites were selected for the study spanning east-west and north-south climate gradients (Figure 1).

Figure 1.

Location of the eight case study sites in United Kingdom: location 1 shows Aberdeen in northeast Scotland, location 2 shows Eskdalemuir in south Scotland, location 3 shows Clipstone in the Midlands, location 4 shows Shawbury on the Welsh border, location 5 shows Thetford in east England, location 6 shows Alice Holt, location 7 shows Black Wood in southern England, and location 8 shows Exeter in southwest England.

3. Methodology

3.1. HYLUC Model

[6] The parsimonious parameter Hydrological Land Use Change (HYLUC) model [Calder, 2003] was selected for this study. HYLUC incorporates the principal limiting processes determining forest and short crop evaporation: radiation, advection, plant physiology and soil water availability. These processes are described in the model by three principle parameters: the available soil water parameter “aw” that specifies the “available water,” the maximum amount of soil water accessible to a particular vegetation for a given soil type; the “β” parameter that governs the ratio of actual to potential transpiration in non water limited conditions; and the “γ” parameter which determines the maximum interception loss that can occur in a day. The Penman [1948] potential transpiration estimate for short grass is used as the reference value against which actual transpiration rates, using the β parameter, are calculated. The β and γ parameters may vary seasonally, particularly for deciduous species, in which case β and γ represent summer values and wβ and wγ represent winter (leafless) values.

[7] The climate data required by the HYLUC model are daily values of rainfall and Penman potential transpiration for short grass. These have been derived from measured series and from the use of a weather generator. The evaporation response of the four energy tree species together with a “reference” grassland cover are explored under the “present,” or baseline, climate together with seven future climate scenarios.

3.2. Environment Agency Rainfall and Weather Impacts Generator (EARWIG)

[8] The Environment Agency Rainfall and Weather Impacts Generator (EARWIG) [Kilsby et al., 2007] produces internally consistent series of meteorological variables which can be generated for present or future climates at sites across the United Kingdom on a 5-km resolution grid. Present-day scenarios are based on the observation period 1961–1990, while future climate scenarios are consistent with national scenarios from the United Kingdom Climate Impacts Programme (UKCIP02).

[9] EARWIG calculates potential evaporation using either the Met Office Rainfall and Evaporation Calculation System or Food and Agriculture Organization (FAO) formulations from generated meteorological data comprising daily mean temperature, daily temperature range, vapor pressure, sunshine duration and wind speed. The FAO-based values are considered to be the closest approximates to the original Penman potential transpiration estimate for short grass and have been used as such in the HYLUC model. The EARWIG generator has the option of either generating totally synthetic rainfall and potential evaporation sequences or using measured rainfall series to derive related “conditioned” potential evaporation series. Figures 2a and 2b show the variation of the EARWIG-generated data between sites and scenarios, while Figures 3a and 3b demonstrate seasonal variation for contrasting sites.

Figure 2.

Changes in (a) rainfall and (b) potential evaporation (PE) for all sites for the Low Emission climate scenarios.

Figure 3.

Seasonal variation in mean daily potential PE and rainfall for (a) Thetford (dry southern United Kingdom) and (b) Aberdeen (wetter north United Kingdom) for Low Emission climate scenarios.

3.3. Present Climate Scenario

[10] Daily rainfall data were available from field measurements for half of the sites. For the remainder a rainfall record was obtained from the nearest site in a “400 gauge national network” (British Atmospheric Data Centre, http://badc.nerc.ac.uk/home/index.html). Potential evaporation data were lacking for all sites except Clipstone, where extensive field studies of the evaporative characteristics of lowland forest and grassland carried out as part of the “Trees And Drought Project On Lowland England” [Calder et al., 2003] provided an uninterrupted 30 year daily record derived from the nearby Agricultural Development and Advisory Service site. EARWIG was used to generate baseline potential evaporation estimates for the other seven sites, which were conditioned with the measured (network) rainfall data. For comparisons with future climate change scenarios an EARWIG baseline scenario was established on the basis of 1960–1990 measurements. A potential annual yield value for each site was calculated from the difference between annual rainfall and the EARWIG baseline annual potential evaporation (Figure 4).

Figure 4.

“Potential yield” expressed as the difference between rainfall and potential evaporation at all sites under Low Emission climate scenarios.

3.4. Future Climate Scenarios

[11] The EARWIG weather generator produced 30 year sequences of synthetic daily rainfall and potential evaporation for all sites for the 2020, 2050 and 2080 Low Carbon Emission climate change scenarios. Data were also generated for these years for a subset of sites (Clipstone and Eskdalemuir) using the Medium-High Emission scenarios. The difference between Low and Medium-High Emission scenarios of average rainfall and potential evaporation is shown in Figure 5. This gave a total of seven synthetic climate scenarios, including the present-day climate.

Figure 5.

Seasonal variation in (a) mean daily PE and (b) mean daily rainfall for Low and Medium-High Emission scenarios for Clipstone and Eskdalemuir.

3.5. HYLUC Parameterization

[12] As there is little information on the evaporative characteristics of the selected energy tree species, particularly in a United Kingdom context, model parameter values were derived by a combination of expert opinion and interpolation of published values for analog tree species. Some data were available for Fraxinus excelsior but this was limited to one field study in southern England. These results have been questioned since they are unusual in a world context by displaying less annual evaporation from forest compared to grass as a result of the low β values reported by Harding et al. [1992].

[13] The derivation of the HYLUC parameters; aw, β, wβ, γ, and wγ for the different tree species is described below. While recognizing that there is a paucity of data specific to these tree species, and the assignment of parameter values is in some cases arbitrary, the parameter values representing the relatively sparse canopy of deciduous Fraxinus excelsior and those assigned to the relatively dense evergreen canopy of Eucalyptus nitens, probably demarcate the likely range of water use by energy species in the United Kingdom.

3.5.1. Available Water Parameter, aw

[14] The available water parameter values were derived through interpolation of published values relating to soil moisture availability controls on transpiration for grass and tree species in the United Kingdom. By making use of neutron probe observations of soil moisture depletion patterns under grass, “layer model root constant” values have been defined for a range of soil types (Table 1) [Calder et al., 1983]. More recently, HYLUC aw values for grass, pine, oak, and heath were determined at Clipstone Forest on a sand soil (Table 2) [Calder et al., 2003].

Table 1. Layer Model Root Constant Parameter Values for Grass on Various Soil Typesa
 Site
ThetfordGrendonCamPlynlimonBridgets Farm
  • a

    In mm.

Soil typeSandClayLoamPeatChalk
Grass577774Not defined119 (lower limit)
Table 2. HYLUC aw Values for Grass, Pine, Oak, and Heath at Clipstonea
Coveraw
  • a

    In mm.

Grass166
Pine173
Oak288
Heath157
Soil typeSand

[15] To convert the original layer model formulation root constant values to the equivalent aw value it is assumed that the maximum available water that can be extracted from the soil is 2.5 times that of the threshold moisture content below which no soil moisture related reductions in transpiration occur. On the basis of this and the additional assumptions, proposed in the absence of field data, that all broadleaf species have a similar available water for a particular site and that the ratio of aw values for broadleaf species to those for grass remains the same for different sites, allows the interpolation of aw values between soil types and species as shown in Table 3. However, it is recognized that eucalypt species in particular have been associated with very deep rooting, even in compacted soils, in countries such as India, South Africa and Australia [Calder et al., 1997], so the values for aw specified in Table 3 could conceivably represent significant underestimates. Field studies are required to determine actual values in United Kingdom conditions.

Table 3. Derived HYLUC aw Values for E. Nitens, E. Gunnii, Nothofagus, and Fraxinus excelsiora
 ClipstoneThetfordGrendonCamPlynlimonBridgets Farm
  • a

    In mm.

  • b

    Published values or those directly calculated from published values.

  • c

    Derived and interpolated values.

Soil typeSandSandClayLoamPeatChalk
Grass166b142.5b192.5b185b150b297.5b
Pine173b138c186c179c145c288c
Oak288b229c310c298c241c479c
Heath157b125c169c162c132c261c
E. nitens288c229c310c298c241c479c
E. gunnii288c229c310c298c241c479c
Nothofagus288c229c310c298c241c479c
Fraxinus excelsior288c229c310c298c241c479c

3.5.2. Transpiration Fraction Parameter, β

[16] The derived β parameter values for E. nitens, E. Gunnii, and Nothofagus are based on the assumption that, as fast growing and thus probably hydraulically very efficient species [Tyree, 2002], their values will be equivalent to at least those of the highest recorded for broadleaved species at lowland sites in the United Kingdom. They are therefore taken to be equivalent to those recorded for oak at Clipstone (Table 4).

Table 4. Derived and Published HYLUC β Parameter Values
Coverβwβ
Grass11
Pine0.76a0.89a
Oak0.99a0.82a
Heath0.82a0.78a
E. nitens0.990.99
E. gunnii0.990.99
Nothofagus0.990.82
Fraxinus excelsior0.810.11

[17] The values for Fraxinus excelsior were derived from studies on this species by Harding et al. [1992]. They reported foliated and unfoliated β values of 0.71 and 0.1 on the basis of comparisons with potential evaporation data from an automatic weather station mounted above the forest canopy. Assuming that the measured potential evaporation above the forest will be around 10% greater than the reference potential evaporation values for short grass that are used in the HYLUC model, the published β values have been increased by 10% to compensate. The reported foliate period of day 90 to 270 for Fraxinus excelsior has also been used in the HYLUC model.

3.5.3. Interception Parameter, γ

[18] Interception parameters for the four energy crop species have been derived from published values for tree species with similar characteristics in the United Kingdom or overseas (Table 5). Details for each species are as follows:

Table 5. Derived and Published HYLUC γ (Foliated) and wγ (Unfoliated) Parameter Values
Coverγwγ
Grass00
Pine3.5a4.1a
Oak3a0.7a
Heath2.3a1.4a
E. nitens3.343.34
E. gunnii1.111.11
Nothofagus2.231.84
Fraxinus excelsior1.10.66

[19] 1. For E. gunnii, the γ parameter value for this species is based on the assumption that the expected sparse nature of the canopy in United Kingdom conditions approximates to that of the canopy of E. Camaldulensis in southern India, for which a value of 1.11 has been reported by Hall et al. [1992].

[20] 2. For E. nitens, a γ value 3 times that of E. Gunnii has been assumed because of the much denser canopy of E. nitens in United Kingdom conditions, comparable to that of a pine canopy.

[21] 3. For Nothofagus, the γ values for Nothofagus were derived from consideration of values reported by Harding et al. [1992] using an earlier two parameter interception model: γ 2.23; wγ 1.84; δ 0.21; wδ 0.108. The nearest equivalent one parameter representation of the two parameter model has values: γ 1.9; wγ 1.2. However, on consideration that Nothofagus in ideal conditions within the United Kingdom may have a denser canopy than native beech, it is suggested that γ and wγ values of 2.23 and 1.84 (the same as those reported using the two parameter model for native beech) would be more appropriate.

[22] 4. For Fraxinus excelsior, the γ values for Fraxinus excelsior were derived from Price [2005] and Harding et al. [1992] using a one parameter interception model: γ 1.1; wγ 0.66.

3.5.4. Summary of Parameter Values

[23] The derived HYLUC parameter values for all vegetation and soil types are summarized in Table 6. In total, the HYLUC model was used to calculate annual evaporation and water yield for each combination of site (8), soil type (4), species (5), climate scenario (7) and year in the record (30), giving a total of 38,400 years of model runs.

Table 6. HYLUC Parameters Used for the Different Species and Soil Type Combinationsa
 Aw (mm)Bwβγwγ
  • a

    Summer and winter start days, relating to foliate conditions, are considered to be day number 168 and 311 for all species except Fraxinus excelsior, for which 90 and 270 are used.

Grass
Sand166.51100
Peat1851100
Clay192.51100
Chalk297.51100
 
E. nitens
Sand2880.990.993.343.34
Peat2980.990.993.343.34
Clay3100.990.993.343.34
Chalk4790.990.993.343.34
 
E. gunnii
Sand2880.990.991.111.11
Peat2980.990.991.111.11
Clay3100.990.991.111.11
Chalk4790.990.991.111.11
 
Nothofagus
Sand2880.990.822.231.84
Peat2980.990.822.231.84
Clay3100.990.822.231.84
Chalk4790.990.822.231.84
 
Fraxinus excelsior
Sand2880.810.111.10.66
Peat2980.810.111.10.66
Clay3100.810.111.10.66
Chalk4790.810.111.10.66

4. Results and Discussion

4.1. Future Climate

[24] The predicted annual rainfall, potential evaporation and potential water yield (expressed as the difference between mean annual rainfall and potential evaporation) values for the 2020, 2050 and 2080 Low Emission scenarios are displayed in Figures 2 and 4. For all sites, there is a consistent pattern of rainfall progressively decreasing and potential evaporation increasing through time. The percentage increase in potential evaporation greatly exceeds the percentage decrease for rainfall, ranging between 34.5 to 43.2% and −0.9 to −11.1%, respectively, across the eight sites by 2080. As a result, all sites show a marked decline in potential water yield, with values for sites in England and Wales becoming negative by 2080. Thetford in eastern England experiences the largest reduction, changing from +107 mm to −205 mm, a decline of 291%. This represents a major shift from potential water surplus to deficit, with major implications for water resources.

[25] Changes in winter (beginning of September to end of February) and summer (beginning of March to end of August) rainfall and potential evaporation for the Low Emission scenario are presented for Thetford and Aberdeen in Figure 3. There is a similar marked rise in potential evaporation across both seasons and sites, while rainfall displays a variable response. Summer rainfall remains constant through time at Aberdeen but declines at Thetford. Winter rainfall increases at both sites by 2080, with the largest rise occurring at Aberdeen.

[26] The seasonal changes for the Low and Medium-High Emission scenarios are compared for Clipstone and Eskdalemuir in Figure 5. As expected, the seasonal rise in potential evaporation is greatest for the higher emission scenario. In contrast, there is little difference in the rainfall trends between the two scenarios.

4.2. Assessment of Uncertainty in EARWIG-Generated Climate Data

[27] Before analyzing the effect of the different energy tree species, the uncertainties introduced in the evaporation and yield calculations by using weather-generated, rather than actual measured, climate data were assessed. Figure 6 compares mean annual evaporation totals calculated for Clipstone for all “species” using: measured rain and potential evaporation; measured rain and EARWIG potential evaporation conditioned by measured rain; and EARWIG-generated rain and potential evaporation for the baseline climate. The results indicate particularly good agreement between the measured and site-conditioned values for both modeled evaporation and water yield. The match is less good using entirely EARWIG-generated data, which is thought to be mainly because of the differences, or “error,” between the EARWIG synthetically generated rainfall and measured site rainfall. For those tree species where the annual average evaporation is very close to the average rainfall, small percentage differences in the rainfall will result in much larger percentage differences in water yield. This sensitivity to error in the EARWIG-generated rainfall is illustrated in Figure 6b.

Figure 6.

Mean annual (a) evaporation and (b) water yield for the Clipstone site derived using different climate data sources showing measured, site conditioned, and baseline. The mean annual rainfall and potential evaporation for the different data sources are also shown for comparison in Figure 6a.

[28] It is notable that the finite length (30 years) of the synthetic data introduces a random error associated with the value of the mean annual rainfall. The standard error for the low rainfall sites (<1000 mm) is typically around 20 mm, which is reflected as “noise” in the evaporation and yield calculated results when comparing sites and scenarios.

4.3. Impact of Tree Species on Evaporation and Water Yield

[29] The predicted mean annual evaporation and water yield for each species and site combination are shown in Figures 7 and 8 and Tables 7 and 8, for the Low Emission climate change scenarios. Under the present (baseline) climate scenario all three exotic tree species are predicted to have higher evaporation losses than grass at all sites. The increases compared with grass range from 8% for E. gunnii at Black Wood to 84% for E. nitens at Eskdalemuir. In contrast, evaporation from Fraxinus excelsior is expected (on the basis of the specified parameterization) to be lower than grass by between 16% (Eskdalemuir) to 35% (Clipstone).

Figure 7.

Mean annual evaporation calculated for all sites and species for the Low Emission climate change scenarios.

Figure 8.

Mean annual water yield calculated for all sites and species for the Low Emission climate change scenarios.

Table 7. Predicted Mean Annual Evaporation and Water Yield for Different Sites and Vegetation Covers for the Low Emission Scenarios
 Annual Average Evaporation (mm)Annual Average Yield (mm)
Baseline202020502080Baseline202020502080
Eskdalemuir
Grass451514575621124611651071986
E. nitens829859895920880832761697
E. gunnii524572624665117511091023943
Nothofagus61464568371210881038966898
Fraxinus excelsior3784094424661316126711991135
 
Black Wood
Grass578638663661292163158129
E. nitens7967988167918515168
E. gunnii6247007477532481038241
Nothofagus649709755759223947436
Fraxinus excelsior436491533565429305284222
 
Aberdeen
Grass490545576602371287265255
E. nitens741773803827128705040
E. gunnii541596639673321237204186
Nothofagus588629665696276205179164
Fraxinus excelsior383417446471474411392382
 
Exeter
Grass558615616624295270233202
E. nitens78582780881076685224
E. gunnii625691704717231196154111
Nothofagus651708714726206180144107
Fraxinus excelsior430478498515419403347307
 
Alice Holt
Grass541576587575296240197180
E. nitens76879376875373352312
E. gunnii6126716926842261559578
Nothofagus6406927107001981347864
Fraxinus excelsior432479513522390334269231
 
Shawbury
Grass52056856958622515513298
E. nitens74273170968913233
E. gunnii587643660672160825215
Nothofagus62567067768012456368
Fraxinus excelsior416456490510326263209170
 
Clipstone
Grass530548549549177140127104
E. nitens7116876806556546
E. gunnii599637645636110533621
Nothofagus62565366164386382115
Fraxinus excelsior342377407426359301265223
 
Thetford
Grass5215455565471551018257
E. nitens6796486436088434
E. gunnii6016226316028126118
Nothofagus626630637605561875
Fraxinus excelsior347387421434324250212172
Table 8. Predicted Mean Annual Evaporation and Water Yield for Different Sites and Vegetation Covers for the Medium-High Emission Scenarios
 Annual Average Evaporation (mm)Annual Average Yield (mm)
Baseline202020502080Baseline202020502080
Eskdalemuir
Grass451525594703124611671048961
E. nitens829870898986880833754687
E. gunnii524582634753117511111009913
Nothofagus61465468878310881042957883
Fraxinus excelsior3784154425221316127311961136
 
Clipstone
Grass53054653956517713611481
E. nitens7116876586466344
E. gunnii59963363364111053278
Nothofagus6256516466438637157
Fraxinus excelsior342383422451359293228195

[30] The impact of the tree species on present water yield reflects that for evaporation, with the three exotic species reducing mean annual water yield from −6% for E. gunnii at Eskdalemuir to as much as −97% for E. nitens at Clipstone, compared to grass. Reductions are greatest for E. nitens across all sites and increase with decreasing site rainfall. For Fraxinus excelsior, yield increases range from 6% at Eskdalemuir to 108% at Thetford and tend to be inversely related to site rainfall.

[31] Climate change is predicted to increase evaporation losses for most species at all sites (Figure 7 and Tables 7 and 8), with the percentage changes from the baseline values ranging between +3% and +38% by 2080. The main exception is E. nitens, which displays a decline in evaporation loss at five of the eight sites. This reflects the species high water use and the near parity between rainfall and evaporation totals, with the latter limited by the former rather than by potential evaporation. The only other species to show a decline in evaporation is Nothofagus at the driest site (Thetford).

[32] Water yields are predicted to decline through time for all sites and species in both absolute and percentage terms (Figure 8). Reductions are least at the wettest site (Eskdalemuir), ranging between −14% to −21% for all species, and greater, −27% to −93%, at the five drier sites. The three exotic tree species generally display the largest reductions, ranging from −40% to −93%, except at Eskdalemuir. Fraxinus excelsior is associated with the smallest decrease at all but Alice Holt (range −14% to −48%). It is notable that, as rainfall and evaporation totals converge through time with climate change, there is a similar trend toward convergence in water yield between the different vegetation covers. This convergence is consistent with expectations of what would ultimately happen under hypothetical conditions of ever increasing potential evaporation: evaporation from all land uses will tend asymptotically toward the rainfall limit as water yields from all land uses converge and ultimately tend toward zero.

[33] For all sites and for all climate scenarios the predicted ordering of the species in terms of increasing evaporation and reducing water yield remains the same: Fraxinus excelsior, grass, E. gunnii, Nothofagus, E. nitens. Extensive planting of E. nitens would have serious implications for water resources at all but Eskdalemuir, with mean annual water yield reducing to <10 mm at the driest three sites and <40 mm at the other four (Figure 9). E. gunnii and Nothofagus would also threaten water supplies at the three driest sites (water yields reducing to <21 mm by 2080). In contrast, the planting of Fraxinus excelsior is predicted to significantly enhance water resources compared to grass at all sites. At four of the eight sites, including the two driest, mean annual water yields by 2080 for this tree species would still exceed those for grass under the baseline climate. The enhancement in water yield compared to grass in 2080 ranges between +15% at the wettest site to as high as +202% at the driest. If these predictions are proved correct this would represent a very attractive land use option for mitigating the impacts of climate change in terms of both water and energy production. The greatest benefit could be achieved by targeting woodland creation to those surface and groundwater bodies at highest risk from water abstraction.

Figure 9.

Mean annual water yield calculated for all species and land covers for all sites for the Low Emission climate change scenarios.

[34] The differences in model predictions between Low and Medium-High Emission climate scenarios can be compared for Eskdalemuir (Tables 7 and 8) and Clipstone (Tables 7 and 8). Evaporation losses are greater by 7 to 13% for the higher emission scenario at Eskdalemuir, whereas there is little change at Clipstone (−1% to +6%). This difference in response can be explained by actual evaporation being generally more limited by potential evaporation at the wetter site whereas at the drier site evaporation is more limited by rainfall.

4.4. Limitations of the Present Study and Future Work

[35] This study provides an indication of the likely magnitude and range of effects of climate change on evaporation and water yield at sites across Britain. However, it is recognized that these predictions are based on very limited data and in the case of the trees, parameter values mainly drawn from analog species. Further research is needed to validate these values and check model predictions.

[36] In particular, there is concern that the parameter values published by Harding et al. [1992] may represent an underestimate of the actual evaporation rates for this native species. The parameter values were derived from measurements taken during a very dry period (1990–1991) when the woodland may have been subject to soil water stress. If this was the case, then the β values might be expected to be greater for average rainfall years. The HYLUC predictions reported in this study used these dry period values and may therefore represent a significant underestimate of the actual evaporation rates in wetter periods. The reported interception ratios of around 10% are also at the lower end of the range for broadleaved species, although this may be partly explained by the low leaf area index of ash. However, even if the water yield predictions have been overestimated, Fraxinus excelsior may still represent a good land use option if water use is proved to be significantly less than that from the other energy species.

[37] Other limitations include the inability of the current version of the HYLUC model to address various climatic feedback effects on vegetation water use. For example, it is possible that climate warming and rises in carbon dioxide concentrations will increase water use efficiencies, with different vegetation types and individual species responding to different degrees [Nisbet, 2002]. Similarly, the expected lengthening of future growing seasons will influence annual evaporation rates, especially for broadleaved species which may remain in leaf for longer periods. Vegetation management is another factor to be considered, with overgrazing of grass and harvesting of trees reducing mean annual losses.

[38] This study has only investigated the range of possible water resource impacts, in space and time, associated with four potential tree species for energy plantations. It has not considered the growth rates associated with these species or, for that matter, whether climate conditions would be viable for the survival of all species at all sites. A full evaluation of the energy benefit/water resource tradeoffs associated with each site and species combination would require associated information on site yields, as well as a consideration of other benefits and costs.

5. Conclusions

[39] This study provides an indication of the likely magnitude and range of effects of climate change on evaporation and water yield for four potential SRF plantation tree species (E. nitens, E. gunnii, Nothofagus and Fraxinus excelsior) versus grass at eight sites across Britain. Climate change according to the studied Low and Medium-High Emission scenarios is predicted to increase evaporation losses for most species at all sites, with the rise in evaporation by 2080 ranging between +3% and +38%. Water yields are predicted to decline through time for all sites and species in both absolute and percentage terms, with the exception of E. nitens at Clipstone. Reductions are greatest at the drier sites and range between −27% and −93% for all species. Changes in evaporation and water yield are only slightly greater for the Medium-High compared to the Low Emission scenario.

[40] For all sites and for all climate scenarios the predicted ordering of the species in terms of increasing evaporation and reducing water yield remains the same: Fraxinus excelsior, grass, E. gunnii, Nothofagus, E. nitens. Extensive planting of any of the three exotic tree species at the three driest sites (rainfall < 800 mm) would have potentially serious implications for water resources, with mean annual water yield reducing by 2080 from a mean of 86 mm under grass to a mean of 9 mm under the exotic species. In contrast, the planting of Fraxinus excelsior is predicted to significantly enhance water resources compared to grass at all sites by 2080, by a margin of +15% to +202%. At four of the eight sites, including the two driest, mean annual water yields by 2080 for this tree species would still exceed those for grass under the present climate. If these predictions are proved correct this would represent a very attractive land use option for mitigating the impacts of climate change in terms of both water and energy production.

[41] It is recognized that the model predictions are based on very limited data and in the case of the trees, parameter values mainly drawn from analog species. Further research is needed to validate these values and check model results, particularly for Fraxinus excelsior where previously published results can be regarded as unusual in a world context. If future climate changes match those predicted, soil moisture deficits will occur for longer periods during the year and will become increasingly limiting for evaporation. The monitoring of soil moisture may then provide one of the most sensitive methods of both determining model parameter values and testing predictions of differences in evaporation between species and changes in evaporation over time.

Acknowledgments

[42] The authors wish to acknowledge funding provided by the Forestry Commission under the project “Scenario study evaluation of the water resource impacts of energy plantations in the United Kingdom using HYLUC modeling.”

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