Environ. Sci. Technol. 2009, 43, 7324–7330
Assessment of Land Use Impact on Water-Related Ecosystem Services Capturing the Integrated Terrestrial-Aquatic System WOUTER H. MAES,† GRIET HEUVELMANS,‡ AND B A R T M U Y S * ,† Katholieke Universiteit Leuven, Division Forest, Nature and Landscape Celestijnenlaan 200E10-2411, B-3001 Leuven, Belgium, and Vlaamse Milieumaatschappij, Koning Albert II Laan 20 bus 16, B-1000 Brussels, Belgium
Received February 27, 2009. Revised manuscript received July 22, 2009. Accepted August 4, 2009.
Although the importance of green (evaporative) water flows in delivering ecosystem services has been recognized, most operational impact assessment methods still focus only on blue water flows. In this paper, we present a new model to evaluate the effect of land use occupation and transformation on water quantity. Conceptually based on the supply of ecosystem services by terrestrial and aquatic ecosystems, the model is developed for, but not limited to, land use impact assessment in life cycle assessment (LCA) and requires a minimum amount of input data. Impact is minimal when evapotranspiration is equal to that of the potential natural vegetation, and maximal when evapotranspiration is zero or when it exceeds a threshold value derived from the concept of environmental water requirement. Three refinements to the model, requiring more input data, are proposed. The first refinement considers a minimal impact over a certain range based on the boundary evapotranspiration of the potential natural vegetation. In the second refinement the effects of evaporation and transpiration are accounted for separately, and in the third refinement a more correct estimate of evaporation from a fully sealed surface is incorporated. The simplicity and user friendliness of the proposed impact assessment method are illustrated with two examples.
1. Introduction Forests and Water. For a long time, forests were believed to act as sponges, absorbing large quantities of water and releasing them slowly, resulting in more constant water outflow, with more water becoming available for human use. The assumed positive hydrological effect, together with improved soil protection, timber production, and socio-economic development, formed the rationale behind several large-scale afforestation programmes in degraded areas in the second half of the twentieth century (1-3). Until recently, possible environmental side effects of these afforestation programmes were largely ignored, with all due consequences (4, 5). Contrary to earlier belief, although forest soils generally have a higher infiltration rate than soils of other land uses, * Corresponding author phone: +32 16 329726; fax: +32 16 329760; e-mail:
[email protected]. † Katholieke Universiteit Leuven. ‡ Vlaamse Milieumaatschappij. 7324
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total water use is likely to be higher for forests than for other land uses both in wet and in dry climates, as a consequence of the higher capacity of trees to evapotranspire (1, 6-8). This results in a reduced total water outflow and low flow of forests when compared with most other land uses, as confirmed by paired catchments studies (7). With rainfall and water availability likely to decrease in a context of climate change and rising human demand, particularly in the regions where water is already scarce (9), the pressure on water resources will dramatically increase in the next decades (10). This contributes to an environmental view on forests as big consumers of the already oversolicited local water budget, switching the enthusiasm for wide scale afforestation in the second half of the twentieth century to a present-day forest skepticism. The question arises whether this skepticism is scientifically better founded than the previous afforestation-enthusiasm. First of all, older plantations and indigenous forests, particularly when old-growth, generally consume less water than vigorously growing plantations (6, 7). Furthermore the expected suppression of plant and forest water use due to increased levels of atmospheric CO2 is already noticeable by increased freshwater levels at catchment level (11, 12). Additionally, forests also have positive effects on water flows. Apart from some exceptions (6, 8), erosion rates from forests are in general very low, and water quality is good (6). Thanks to the high infiltration rate, forests generally reduce peak flows and floods, though this effect is minimal during more extreme (and damaging) peak flows and at larger scale (1). Limitations of the Blue Water Approach. Approaching forests from a mere water consumption point of view arises from a conventional yet simplistic view on water flows, in which only the visible water resources, those that are immediately available for mankind, are considered useful and are accounted for. In this so-called “blue water approach” all water flows in rivers, lakes and aquifers are taken into account, while all water in water vapor flows, referred to as “green water”, is ignored (13). This blue water approach hampers a sustainable management of water flows (14). With only 20% of the world’s crop land irrigated, 5% in subSaharan Africa, green water flow on croplands is estimated to be 4 times that of consumptive blue water flow (14). Terrestrial ecosystems depend almost entirely on the green water flow for their productivity and functioning, and the annual green water flow (69.600 km3 · year-1 (15)) exceeds the blue water flow (42.000 km3 · year-1 (16)). Ecosystem Services of Green and Blue Water Flow. Green water flow is indispensable for the functioning and stability of terrestrial ecosystems (17). Apart from being an essential building block of biomass, water is lost as an inevitable consequence of stomatal opening for photosynthesis. Besides these obvious links between transpiration flow and production, transpiration, as medium for nutrient uptake and flow through the plants, is the key process in ecosystem control over water, nutrient, and sediment flows (17, 18). Trees owe their larger capacity to evapotranspire to their deep rooting depth and bigger leaf area (19). These two features and the green water flow they generate are the main drivers of life support processes and terrestrial ecosystem services. Deep rooting improves nutrient and water availability for the entire ecosystem through vertical uplifting (20) and hydraulic lift (21-23) and provides opportunities for carbon 10.1021/es900613w CCC: $40.75
2009 American Chemical Society
Published on Web 08/24/2009
sequestration (24). Likewise, the high leaf area and associated layered canopy structure drive ecosystem services as air filtering, habitat creation, nutrient redistribution, and erosion control (18, 25). The combined effect of canopy structure and evapotranspiration creates a steady microclimate (26, 27), providing the forest system the conditions needed to maintain forestrelated biodiversity (17, 28). Hence, green water flow is used to create a self-organizing complex system and facilitates the forests’ resilience to disturbances (17, 29). According to some authors, tropical forest systems self-sustain their climate at a much larger scale (30), by increasing the local precipitation through a change in roughness length and through higher transpiration (30-32). Valuing Ecosystem Services. There is growing awareness of the central role ecosystem services play in supporting the human well-being and of the need to incorporate these assets in the economy (e.g., refs 33-35). Costanza et al. (36) investigated the global value of forest ecosystem services and estimated that tropical and temperate forests deliver a mean of 2007 and 302 $U.S. · ha-1 · year-1 of ecosystem services, respectively. Estimates by other authors are of the same order (e.g., ref 37,) or higher (e.g., ref 38). On the other hand, freshwater ecosystems (rivers, lakes and most types of wetlands) and their biodiversity equally provide several ecosystem functions, including buffering against droughts and floods, biodegradation of organic waste, and a steady supply of fisheries and of water itself (39-41). The global value of these ecosystem services supplied by aquatic ecosystems is of the same order as those provided by forest ecosystems (4.9 × 1012 vs 4.7 × 1012 $U.S. · year-1 (36)). Furthermore, the freshwater ecosystems are currently experiencing far greater declines in biodiversity than most terrestrial ecosystems as a consequence of, among other reasons, flow modification and water pollution (41). Therefore, applying a “green-and-blue water” approach, in which the positive effects of green and blue water flows are accounted for, is a prerequisite for sustainable land management (17, 29, 42). However, up to now, this approach is not or poorly incorporated in impact assessment methods, such as life cycle assessment (LCA) (see Supporting Information for a short review of existing methods for water quantity impact assessment in LCA). The aim of this paper is to propose a simple and straightforward method to assess land use impact specifically related to water quantity, based on the ecosystem services green and blue water flows deliver. This method is designed for incorporation in land use impact assessment of LCA. In a LCA-context, land use impact consists of two types of intervention, i.e., land use occupation impact (the impact associated with the use of an area during a certain time for a certain human-controlled purpose) and land use transformation impact (the impact of a humaninduced change in use of a land area, sometimes referred to as land use change impact) (43, 44). Occupation impact is calculated as A*T *I, transformation impact as A*∆I, were A denotes the area occupied, T the time of occupation, I the environmental impact of the land, and ∆I the change in this impact following transformation (43). In the proposed method, we estimate the water quantity-related impact (I) of a land use occupation and its change following land use transformation (∆I).
(40). Hence, we use natural flow regime as the discharge level at which aquatic ecosystem services are maximal. The terrestrial ecosystem corresponding with the concept of absence of human disturbance is generally known as the shifting-mosaic steady state or potential natural vegetation (PNV). Even though biomass and control over nutrient flow are not necessarily maximal at this stage, the PNV can be interpreted as the stage in which ecosystem services are maximal, because it is the stage with the highest habitat diversity, resistance, and resilience of all succession phases (47). Impact Assessment Method. The indicator on which the impact assessment method is based is evaporative flow. The impact of a land use that evapotranspires the same amount of water as the PNV is minimal, and more transpiration than this level would not lead to better terrestrial ecosystem services. When green water flow is 0, no ecosystem services are fulfilled, and the impact on the terrestrial ecosystem is maximal. For the sake of simplicity, we use linear relationships to calculate impacts. The impact of a land use on the terrestrial (green) water flow is represented in Figure 1a, and is mathematically expressed as follows: If ETact < ETPNV: TWI )
ETPNV - ETact ETPNV
(1)
If ETact g ETPNV: TWI ) 0
(2)
Where TWI is the land use impact on the terrestrial (green) water flow (value between 0 for minimal and 1 for maximal impact), ETact and ETPNV the mean annual evapotranspiration (mm or l · m-2) of the actual land use system and of the potential natural vegetation. The impact on aquatic ecosystems of a land use system that evapotranspires the same amount of water as the PNV is minimal. If a land use provides more water, the impact remains minimal. If it consumes more water, the aquatic ecosystem functioning will become more disturbed, until a critical level is reached, passed which the aquatic ecosystem becomes irreversibly damaged. This critical level is calculated in environmental flow assessments and is known as minimal flow or environmental water requirement (EWR), mostly expressed as a proportion of the natural river discharge (denoted as xEWR, with xEWR < 1). Several strategies have been proposed for its calculation (46, 48). The corresponding threshold evapotranspiration level of the terrestrial ecosystem (ETEWR; (mm)) is then given as ETEWR ) P - xEWR · (P - ETPNV)
(3)
In eq 3, P represents the annual precipitation (mm). The impact of a land use on the aquatic (blue) water flow (AWI) is represented in Figure 1.b, and can be expressed as If ETact e ETPNV: AWI ) 0 If ETPNV < ETact < ETEWR: AWI )
(4)
ETact - ETPNV ETEWR - ETPNV
(5)
If ETact g ETEWR: AWI ) 1
(6)
2. A New Impact Assessment Method The natural flow regime is the natural river discharge in a river basin that would have occurred in the absence of human impacts in a river basin (40, 45). Its central role in sustaining biodiversity and ecosystem services is now widely recognized (46), as it represents the discharge level aquatic ecosystems are best adapted to and at which they have maximal resilience
TWI and AWI can be combined to yield the integrated terrestrial-aquatic water impact TAWI (Figure 1c). Combining eqs 1-6 yields If ETact e ETPNV: TAWI )
ETPNV - ETact ETPNV
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If ETPNV < ETact < ETEWR: TAWI )
ETact - ETPNV ETEWR - ETPNV (8)
If ETact g ETEWR: TAWI ) 1
(9)
If ETact is lower than ETPNV, there is an environmental impact because not all terrestrial ecosystem services are fulfilled, and this impact increases with decreasing evapotranspiration. If ETact is higher than ETPNV, there is also an impact because the aquatic terrestrial ecosystem functions are hampered, and the impact increases further with increasing water use, up to a threshold value. We aimed to keep the data requirements for the model as low as possible, allowing it to be incorporated in existing LCA methods or to be used on its own. Only two types of green water flow (ETact and ETPNV) and annual rainfall have to be measured or estimated. The fourth required variable, xEWR, can be estimated from Smakhtin et al. (40). In case of land use transformation, transformation impact ∆TAWI can be estimated as TAWI2 - TAWI1, with TAWI2 and TAWI1 the impact of the new and of the previous land use, respectively.
3. Discussion and Model Refinement 3.1. Underlying Assumptions of the Basic Model. The proposed model requires a minimum of input data, which implies that several (implicit) assumptions were made: • Linear relationships between the points with maximal and minimal impacts were assumed for the characterization functions. However, natural processes are rarely linear. In section 3.3, we propose using threshold values for the ETPNV to overcome this problem. • By using green flow (evapotranspiration) as a predictor of ecosystem services, it is implicitly assumed that transpiration and evaporation contribute equally to the ecosystem services. A correction for this will be discussed in section 3.4. • It was assumed that evapotranspiration of a fully sealed surface is 0, which is not correct. A way to account for this is presented in section 3.5. • The impact on aquatic ecosystem services was assumed minimal when fresh water flow is equal to or higher
than the natural river discharge (45). However, very high fresh water flows could impact aquatic ecosystem functioning. It is implicitly assumed in this model that this effect is negligible compared with the impact on terrestrial ecosystems when ETact < ETPNV. • The EWR-concept was used to couple green water flow, the basic indicator of our model, to blue water flow. xEWR is by origin an indicator at catchment or basin scale (40). The model implicitly assumes that xEWR is the same for the entire catchment, or that the considered land use covers the entire catchment. Furthermore, the impact on aquatic flows is restricted to ETEWR as a single indicator. This is a considerable simplification of the reality, in which the flow of a river is comprised of five key components, i.e., variability, magnitude, frequency, duration, timing, and rate of change (45). However, implying all these elements is impractical in a LCAapproach because of the large data input required (49). 3.2. Irrigated/Drained Cropland and Forest Plantations. When assessing the land use impact of irrigated cropland, irrigated forest plantations or of ecosystems taking up groundwater, the model does not need to be adapted. This can be illustrated with a fictitious example. Suppose that mean annual rainfall is 1000 mm, and that ETPNV and ETEWR are 600 and 800 mm. The impact will be minimal for an ecosystem evapotranspiring 600 mm, in which case 400 mm flows will flow to the aquatic ecosystem. If an additional 200 mm becomes available through irrigation or groundwater recharge, this can be interpreted as a rentless loan of 200 mm taken up from the catchment. If the system would use 600 mm, 600 mm would flow back to aquatic resources; 200 mm would payback the loan, and 400 mm would flow to the aquatic ecosystem; hence, total impact would be minimal. However, it is likely that the total evapotranspiration of an irrigated land use will be higher than ETPNV, and that TAWI will be higher than that of a nonirrigated land use. Although irrigation increases water availability, ETEWR still has to be calculated from the rainfall of the rainfed natural system. For drained systems, a similar reasoning prevails. The model does not need to be adapted, and ETPNV and ETEWR have to be calculated from the mean annual rainfall. Suppose that in the above example 300 mm is drained from the system. The water available to the drained system will be only 700
FIGURE 1. Model of water quantity impact a. on terrestrial ecosystems (TWI), b. on aquatic ecosystems (AWI), and c. on both aquatic and terrestrial ecosystems (TAWI) as a function of evapotranspiration (ET) of the potential natural vegetation (PNV) and a threshold ETEWR. Figure 1d demonstrates how linear functions starting from threshold values for ETPNV (ETPNV,min and ETPNV,max) can describe more accurately the actual characterization function, represented by the dotted line. 7326
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TABLE 1. Impact of Selected Forest Stands, Cropland and Grazing Land on Well-Drained Luvisols and Podzoluvisols and on Gleyic Cambisols in Flanders, Belgium (Data after Refs 55, 56 for the Basic (TAWI) and More Advanced Models (TAWI1 and TAWI2, See Text for Details). ETact and Tact Are Expressed As % of Annual Rainfall (P; (mm)) P Milio-Fagetum (PNV) Pedunculate oak Perennial ryegrass maize wheat Fago-Quercetum (PNV) Pedunculate oak
ETact
Tact
AWI
TAWI
(1) Ecosystems on Well-Drained Luvisols and Podzoluvisols 837 59 43 757 64 42 0 0.22 0.22 1120 33 23 0.45 0 0.45 889 49 30 0.17 0 0.17 899 58 43 0.3 0 0.03 773 972
(2) Ecosystems on Gleyic Cambisols 55 35 49 35 0.13 0
mm. If the system uses 600 mm for green water flow, the terrestrial ecosystem services are still maximally provided, and a total of 400 mm will flow to the aquatic ecosystem, hence the total impact is minimal; however, it is likely that the system would evapotranspire less, causing an impact on terrestrial ecosystem services. 3.3. Threshold Effect and the Potential Natural Vegetation PNV. Natural processes (50) and ecosystem services (51) are rarely linear. Instead, slight deviations from a reference value may not cause a significant impact, while the environmental impact may increase exponentially for larger deviations. Such relationships can be relatively well described by a linear function using a threshold value (see Figure 1d) (50). Threshold values of ETPNV can be estimated by using the standard error of ETPNV-estimates to define a confidence interval (ETPNV,min; ETPNV,max), or by estimating a minimal and maximal evapotranspiration based on the dynamic status of the potential natural vegetation. As a consequence of the biomass dynamics of the landscape patches constituting the PNV, its total biomass will fluctuate between a lower and an upper value (47). The evapotranspiration of the PNV will equally fluctuate within a range, with boundary levels ETPNV,min and ETPNV,max. The impact of the potential natural vegetation is always considered minimal, and it follows that all ecosystems whose evapotranspiration lies between these boundary levels will have a minimal impact. A model refinement based on ETPNV,min and ETPNV,max is illustrated in Figure 1d and is given by If ETact < ETPNV,min: TAWI )
ETPNV,min - ETact ETPNV,min (10)
If ETPNV,min < ETact < ETPNV,max:: TAWI ) 0 If ETPNV,max
TWI
(11)
ETact - ETPNV,max < ETact < ETEWR: TAWI ) ETEWR - ETPNV,max (12) If ETact g ETEWR: TAWI ) 1
(13)
3.4. Transpiration or Evapotranspiration? Green water flow is defined as the sum of transpiration and evaporation. Evaporation is the sum of soil evaporation and interception evaporation (42, 52) and can make up a substantial part of the water budget (53). In the basic model, it was implicitly assumed that the ecosystem services delivered by evaporative and by transpirative water flow were the same. However, the role of evaporative flow in providing ecosystem services is limited to climate regulation, and possibly control over nutrients and erosion, depending on the ecosystem, while the transpirative flow is an important driver of all ecosystem services. Hence, when calculating the impact of a land use
0.13
TAWI1
TAWI2
0.22 0.45 0.25 0.01
0.22 0.58 0.34 0.01
0.05
0.07
on the terrestrial water flow, the basic model tends to overestimate the terrestrial ecosystem services. If ETact < ETPNV and if data are available to estimate transpiration (T) and evaporation (E) of the actual and the potential natural vegetation separately, it is preferable to use characterization functions of the total green water flow (ETPNV,mod and ETact,mod): ETPNV,mod ) aPNV · TPNV + bPNV · EPNV
(14)
ETact,mod ) aact · Tact + bact · Eact
(15)
In this function, aPNV and aact are the weights of the transpirative flow and are always maximal (aact ) aPNV ) 1). The weights to be assigned to bPNV and bact depend on the ecosystem and land use. A possible approach could be to base the weights on the ecosystem services as listed by Costanza et al. (36). For instance, for temperate forests and if the contribution of evaporation to ecosystem services is only considered to be climate regulation, bPNV can be estimated as 86 × 302-1 or 0.28 (86 $U.S. · year-1 · ha-1 is the estimated contribution of climate regulation to the total ecosystem services, estimated to be 302 $U.S. · year-1 · ha-1). For the aquatic ecosystems only the total amount of water flow matters, not the underlying terrestrial processes. Hence, eqs 8 and 9 or 12 and 13 do not change when transpiration and evaporation are regarded separately. In eqs 7, 10, and 11, ETPNV and ETact can be substituted by ETPNV,mod and ETact,mod. 3.5. Evaporation of Sealed Surfaces. Evaporation of sealed surfaces is not zero. Even for a hypothetic fully sealed surface with an infinitely small storage capacity S, there will be a small amount of water evaporating during rainfall events. This amount can be estimated as (after Gash et al. (54), with lim(Sf0) ETmin )
E¯ · P j R
(16)
with Rj the mean rainfall rate (mm · h-1) and Ej the mean rate of evaporation from a saturated canopy (mm · h-1). Ej is a very conservative parameter, varying little with climate, and can be estimated as 0.22 mm · h-1. If estimates of Rj are available, eq 7 can be adapted to If ETact < ETPNV: TAWI )
ETPNV - ETact ETPNV - ETmin
(17)
4. Examples of Impact Calculation The model was first applied on a data set of a study in Flanders, Belgium, in which total rainfall, transpiration, and evaporation were measured separately (Table 1; data from refs 55, 56 - only the results of the year 2000, when measurements of the entire year were available, were used). VOL. 43, NO. 19, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Prolonged effect of clearfelling, burning and reseeding of a native mountain ash forest in Victoria, Australia (black rectangles, data from ref 60) and of clearfelling of a dry, sclerophyllous Eucalypt forest and replanting with Pinus radiata (white diamonds, data from ref 59) on a. the ETact (relative to the ETPNV, 100 · (ETact - ETPNV)/ETPNV) and b. on TAWI. TAWI was calculated for the basic model (ETact as input), for the advanced model with Eact and Tact used separately (TAWI1, a value of 0.28 for bact and bPNV was used, see section 3.4) and for the further advanced model in which also the evaporation of sealed surfaces is accounted for (TAWI2, rain was considered to occur in 6% of the total hours, after ref 57). xEWR was 0.44 (40). Measurements from two sites were used. The sites on well-drained luvisols and podzoluvisols had a Milio-Fagetum as PNV (a mesotrophic beech forest (58)), while actual land use consisted of oak forest, cropland (maize and wheat) and grazing land. The oak forest consumed more water than the PNV, resulting in a positive AWI (Table 1). The grazing land and the maize field, whose ETact were lower than ETPNV, had a positive TWI. Due to the higher E-fraction of these land uses, TWI further increased in the more advanced model (see Table 1a). The second site on Gleyic cambisols had a Fago-Quercetum as PNV (an oligotrophic oak-beech forest), while actual land use consisted of a monospecific oak stand. Lower interception caused ETact to be lower than ETPNV, resulting in relatively high TAWI, but this was corrected for in the more advanced models. As an illustration of how the model can be applied in a non-LCA-related context, the prolonged impact in time of clearfelling and replanting is shown in Figure 2. Two cases of paired catchment studies were examined. In the first study, a dry sclerophyllous Eucalypt forest (PNV) was cleared and replaced by a Pinus radiata plantation in New South Wales, Australia (Subtropical climate, xEWR ) 0.27, all data from ref 59). Immediately after clearfelling, ETact was much lower than ETPNV, and TAWI was high, attributable to TWI. However, after 8 years, ETact was already (slightly) higher than ETPNV. As in natural conditions only 6.5 ( 5% of P flows to the rivers, this has a profound impact on the aquatic water resources, and TAWI is maximal. Results of a second study on clearfelling, burning, and reseeding of a native mountain Ash forest in Victoria, Australia (all data from ref 60, xEWR ) 0.27, ETPNV uses 81 ( 2% of P) show a similar pattern, although a longer time is needed for recovery. 7328
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The above examples illustrate the user friendliness and limited data requirements of the models and furthermore demonstrate that the model is not limited to LCA, but can be useful in environmental impact assessment or any other type of land use impact study.
Acknowledgments W.H.M. was funded by a K. U. Leuven Ph.D. grant. We would like to acknowledge the Forecoman team for the inspiration when building the model, particularly Bert Reubens and Wouter Achten. We are grateful to three anonymous reviewers for their valuable comments.
Supporting Information Available A short review of existing methods for impact assessment of water quantity, with a focus on methods in the life cycle assessment (LCA) framework. This material is available free of charge via the Internet at http://pubs.acs.org.
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