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Assessment of land use and climate change impacts on the mesoscale
phw. Ckrn.
Pergamon
PII:
Assessment
Earth (B), Vol. 26, NO. 7-8, pp. 565-575, 2001 0 2001 Elsevier Science Ltd. All rights reserved 1464-1909/01/$ - see front matter
S1464-1909(01)00051-X
of Land Use and Climate Change Impacts on the Mesoscale
W. Lahmer’, B. Pfiitzner’ and A. Becker’ ‘Potsdam Institute for Climate Impact Research, Telegrafenberg, D- 144 12 Potsdam, Germany 2Bureau for Applied Hydrology (BAH), Wollankstr. 117, D- 13 187 Berlin, Germany Received 24 April 2000; accepted 6 October 2000
of Global Change impacts on the regional water cycle induced by a changing climate or by land use changes belong to the urgent issues of today’s hydrologic research. Since the most important sources and drivers of Global Change are located at the regional scale, a stronger emphasis is needed at this scale, where political and technical measures can be taken in order to avoid critical developments for the environment and society. With respect to climate changes, studies indicate considerable regional vulnerabilities against changes of both temperature and precipitation. Land use or land cover changes, on the other hand, represent another anthropogenic ‘system disturbance’ which directly or indirectly influences many hydrologic processes. The present study outlines a methodology to derive and analyse scenarios, which allow to assess the influences of both climate and land use changes in a region. The results of a high resolution modelling approach demonstrate the chances and problems in such types of studies. The climate change impact studies performed in the German state of Brandenburg and in the Stepenitz river basin show that some of the water balance components may undergo a considerable change. Due to their high evapotranspiration potential, wetlands and open water bodies have turned out to be the most sensitive areas. The results of the land use change impact studies performed in two mesoscale river basins demonstrate that moderate land use changes result in only small changes of various water balance components. For the effects of land use changes climatic characteristics in a region Seem to play a Crucial role. 0 2001 Elsevier Science
AbstractStudies
Ltd. All rights reserved
1 Introduction
Global Change issues and human impacts on the hydrologic cycle play a growing role in today’s hydrologic research. Though these phenomena and processes take place at all spatial scales, from local to global, a stronger emphasis is needed on regional and local scales, where the most imporCorrespondence
to: Werner Lahmer
tant sources and drivers of Global Change are located. It is primarily at these scales that political and technical measures can and must be taken in order to avoid critical developments and to reduce negative effects on the environment and society. Thus, the regional scale is crucial for an improved understanding of both causes and impacts of Global Change and it’s contributing processes. The understanding of global climate systems has considerably increased in recent years, as well as human concern about future climatic changes. Along with these changes important consequences are expected in regional hydrologic cycles and subsequent effects on regional water resources. Since hydrologic processes directly depend on climate conditions, influences of possible climatic changes on these processes will differ from region to region. The magnitude and spatial distribution of climatic changes in combination with hydrologic characteristics of the study region determine which effects will be most relevant on the regional scale. Though the direction or magnitude of many important changes are not yet fully clear, studies in the last years have shown important regional vulnerabilities against changes of both temperature and precipitation patterns. They suggest that climatic changes will alter basic components of the hydrologic cycle like soil moisture, groundwater availability, magnitude and timing of runoff, and water quality (see e.g. reviews on water resources impacts of global warming scenarios given by Gleick, 1989; Dooge, 1992; Tegart and Sheldon, 1993). This would induce dramatic environmental dislocations and widespread implications for future water resources planning and management. Land use or land cover is the second of the main boundary conditions which directly or indirectly influences many hydrologic processes. Under certain circumstances land use changes as another important component of Global Change may induce comparable effects on water quantity and quality. Like climatic changes, changes of actual land use have far reaching impacts on the regional water cycle. Besides the given physical characteristics of a region, socio-economic constraints (like interests of the land owners or reforms due to the AGENDA 2000 of the European Union)
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strongly influence measures of land use changes. Therefore, the challenge in modelling such changes is to take into account as much influencing factors as possible. River basins are preferred land surface units for such regional studies, because they represent natural spatial integrators of water and associated material transports at the land surface. In order to forecast possible effects of climate and/or land use changes at these scales, appropriate models and high resolution data, spatial as well as hydro-meteorological, are necessary. Only by a sufficient density of such data all necessary input parameters can be appropriately taken into account in simulation calculations. Since water balance components calculated for meso- and macroscale river basins depend strongly on the accuracy of such data, quantitative and qualitative data deficits represent a major problem in assessing the actual hydrologic situation of a river basin. On the other hand. this assessment represents the basis for the evaluation of changes due to various scenarios of climate or land use change. The main objectives of the study were to investigate the direction and magnitude of possible climate change impacts on the hydrologic cycle at the regional scale to identify the main causes for the observed changes to identify specifically vulnerable landcover types in the study region and provide a scientific basis to derive possible response strategies to compare the impacts induced by climate changes to those from land use changes, as basis for medium term decision making by political stakeholders.
The results presented here are part of the WaStor (water and material retention in the Elbe river lowland) project (Bork, 1997) a regional sub-project of the interdisciplinary research project ‘Elbe-Ecology’ funded by the German Ministry for Education and Research (BMBF). Basic aim of this project is to study the influence of various measures of land use change on the regional water balance and the river runoff. Though the primary scale of the Elbe-Ecology project is the entire German part of the Elbe basin, special studies in vulnerable sub-regions were performed where more detailed data were available.
2 Methodology 2,l Study Regions In the present study, results for the influences of Global Change impacts on the regional water balance are given for two mesoscale tributary river basins of the German part of the Elbe basin, which is the driest of the five largest German river basins and covers an area of nearly 100.000 km’ (Fig. 1). The Stepenitz basin, situated in the state of Brandenburg, is characterized by water stress and water deficiencies, which occur earlier and more frequently in the case of droughts than in other parts of Germany. On the other hand, the Upper St& basin, situated in Northern Germany, is characterized by a mean annual precipitation which is by 240 mm/year higher than in the Stepenitz basin. With a maximum elevation of about 160 m and 100 m a.s.l., respectively, both basins are part of the Pleistocene Elbe
1.158 km* prectpitatton: ca. 890 mm/year Fig. 1: Overview of the German part of the Elbe river basin. the state of Brandenburg
and the two subbasins Stepenitz and Upper St0r
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lowland representative Europe.
2.2 The modelling
for humid/semihumid
landscapes
Assessment
in
approach
To study the impacts of climate or land use changes on the hydrologic cycle, appropriate modelling approaches are necessary. These must allow an effective simulation of the regional hydrologic cycle and enable studies, which in detail describe both the natural spatial variabilities and the anthropogenic impacts at various temporal and spatial scales. Medium to large scale applications of fully distributed physically based hydrologic models are constrained by the availability of required input data (Gleick, 1986; Beven. 1993). In addition, their use in prediction at the regional level is often very limited. Therefore, simplified (conceptional) models with a reduced number of physically meaningful parameters are needed which can be applied at these scales. Such models should be able to use directly information provided by digital maps and to handle different temporal and spatial discretization levels. Problems in using these models generally result from deficits in the generally available spatial data as well as in the time series data needed to provide the necessary meteorological input and to validate the model results. The modelling approach used in the present study applies methods of a GIS (Geographic Information System)-based modelling concept. It is based on variable spatial disaggregation and aggregation techniques and consequently uses the GIS-based derivation of model parameters from generally available spatial data. A key element of the modelling approach is the modelling system ARC/EGMO (Pfiitzner et al. 1997), which includes the hydrologic model EGMO (Becker, 1975; Becker and Pfiitzner, 1986; Becker and Pfiitzner, 1987). It was successfully applied in several former investigations (i.e. Lahmer, 1998; Lahmer and Becker, 1998; Becker and Lahmer, 1999; Lahmer et al., 1999). The modelling approach allows to take into account any type of land surface units, from very small spatial units up to larger landscape units like, e.g. biotopes, Hydrological Response Units (HRU) or subbasins. A schematic view of the model is given in Fig. 2. Besides the direct GIS coupling, an important feature of the approach is the clear separation of two domains, that of the vertical processes and that of the horizontal flows. By this, the spatial discretization of both domains can be easily adapted to the available data, which usually show a rather distinct spatial resolution. The model includes conceptual submodels with physically interpretable parameters for all hydrological processes taking place in complex landscapes and river basins. Usually, the model is validated by the discharge measured at the outlet of a river basin.
2.3 Spatial disaggregation
to elementary
units
One basic challenge in meso- and macroscale hydrologic modelling is the disaggregation of the study region into ar-
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eas characterized by a significantly different hydrologic behaviour. Therefore, methods of spatial disaggregation and aggregation play an important role at theses scales. In general, the degree in spatial disaggregation depends on the resolution of the available basic maps and must be determined for each study region.
vertical
pocessises
l
-•
lateral processes
Fig. 2: Schematic view of the processes at the landsurface-atmosphere-soil interface taken into account by the hydrological modelling system ARC/EGMO.
Basis for all simulation calculations with ARC/EGMO is the so called ‘elementary unit map’, generated by a GIS from all necessary digital maps (land use, vegetation cover, soil characteristics, topography, ground water level, river net, subbasins etc.). This map consists of ‘elementary units’ (EUs), which represent the smallest modelling units and can be considered homogeneous with respect to their hydrologic behaviour. Each of these units is characterized by the parameters of the underlying basic maps which are provided by so called ‘relate tables’. The shape and size of the EUs can vary considerably. Simulation runs can be performed on the level of EUs or on spatially aggregated area1 units like hydrotopes, subbasins, or the basin as a whole.
2.4 Spatial aggregation
to hydrotope classes
Though calculations based on elementary units represent the most precise approximation to reality, it is often more effective in case of meso- or macroscale applications to aggregate the EUs to larger spatial units, to minimize the number of EUs and the simulation time. This is an important aspect in climate or land use change impact studies, where long time periods (50 to 100 years or even more) must be simulated using different scenarios. On the basis of earlier studies (Lahmer et al., 1999; Becker and Labmer, 1999), the EU maps of the Stepenitz and Upper St& basin were aggregated to hydrotopes and hydrotope classes which refer to essential (hydrologic, geographic) characteristics of the study region. Hydrotopes represent spatially connected
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Hydrotope Classes classes a&e land, DOT, flat arable land, DOT, Inclined arable land, SOT, Rat .a arable land, SOT, inclined meadow, DOT, ftat meadow, DGT, Inclined w meadow, SOT, Rat m meadow, SOT, inclined forest, DQT, flat i forest, DOT, inclined * for@ SW, flat m forest, SOT, Inclined swamp bare land m urbanized areas m open water bodies hVdrOtODe
0
Fig. 3: Aggregation of the Upper Star basm into 16 hydrotopes processing of various spatial maps (left).
(nght). based on the disaggregation
EUs characterized by an unique hydrologic regime, while hydrotope classes are location independent combinations of similar hydrotopes within a larger area1 unit (‘semi-distributed modelling approach’). Since different EU-characteristics can be used for their definition, simulations based on hydrotope classes are flexible with respect to the definition of each class and the number of classes. To define a reasonable and adequate hydrotope classification is an important task, as this classification should take into account dominant characteristics of the study region and separate areas differing considerably in their evapotranspiration and runoff behaviour. In case of the St& basin, a classification into 16 hydrotope classes was chosen. For the dominant land use classes arable land, forests and meadows areas with shallow (SGT) and deep groundwater table (DGT) were distinguished. Based on the topographic information, each of these classes were further divided into subclasses of less (‘flat’) or more than 2 % gradient (‘inclined’). In addition, the classes swamp, bare land, urbanized areas and open water bodies were classified. The hydrotope classification is indicated in Fig. 3 together with a section of about 11 x 7 km2, which demonstrates the variability in size and shape of both the hydrotopes and the quasi-homogeneous elementary units (2 1.08 1 for the Upper St& basin as compared to 30.675 in the Stepenitz basin). The EU-map emphasises one of the advantages of the polygon-based disaggregation approach, which results in larger spatial units in homogeneous parts of the basin and in smaller units in areas of higher heterogeneity. This aspect is important in simulating land use changes, which normally is restricted to rather small and widely distributed parts of the study region.
Kilometers
mto 21.081 elementary
2.5 Spatial interpolation
units by a GIS-based
of meteorological
pre-
data
Due to it’s high influence on the calculated water balance components, the spatial distribution of meteorological input variables plays a key role in meso and large scale hydrologic modelling. In order to include ‘realistic’ spatial distributions of these variables in the simulation calculations, an appropriate interpolation method must be used. Since the interpolation is performed for every time step of the simulation period (usually one day), this method must be fast, excluding too sophisticated and time consuming algorithms. The interpolation algorithm used in ARC/EGMO (‘extended quadrant method’) is characterized by i) the selection of stations by the quadrant method, ii) a linear inverse distance interpolation of the respective variables, and iii) the possible inclusion of other dependencies (elevation, exposition or slope). This method has turned out to be very effective for large scale modelling purposes, producing rather realistic spatial distributions for all meteorological input variables (precipitation, mean temperature, relative humidity, insolation etc.) for every time step of the simulation runs. As an example, Fig. 4 shows the spatial distribution of precipitation in the state of Brandenburg, calculated on the basis of 40 climate and 33 precipitation stations for November 1985.
2.6 Calculation
of water balance components
Based on the aggregated spatial maps and the meteorological information from various meteorological stations, water balance calculations were performed for the State of Bran-
W. Lahmer et al.: Assessment
denburg and in the Stepenitz and Upper St& river basins on a daily basis for the period 1951-1990 and 1982-1993, respectively. For the reference state (either the actual meteorological conditions or the actual land use) the model AFXXGMO was validated by the measured discharge at the basin outlet. Among the various hydrologic processes taking place in complex landscapes, evapotranspiration, percolation and surface runoff are most essential. In general, the results obtained for these water balance components are characterized by the spatial distribution of the meteorological input variables and the heterogeneities of the underlying spatial units due to land use, soil, and groundwater level. In case of impact studies (climate or land use change) the results obtained in the scenario runs are more or less strongly influenced by the assumptions made in these scenarios.
Fig. 4: Spatial distribution of precipitation in the state of Brandenburg, calculated on the basis of 40 climate and 33 precipitation stations for November 1985.
3 Climate Change Impact Studies 3.1 Climate scenarios In order to provide accurate forecasts of regional hydrologic processes, quantitative estimates of changes in major long term climatic variables such as temperature, precipitation, and evapotranspiration are needed. Influences of climate change on the regional water balance may result from both spatial and temporal precipitation shift, changes of the actual evapotranspiration due to temperature increases, and an increase of extreme meteorological events (prolongation of dry periods, draughts, high intensity precipitation events etc.). General Circulation Models (GCMs) do not provide valuable detailed information on regional impacts on water supplies. Moreover, they simulate precipitation in considerably less detail than circulation or temperature. One of the reasons is the lack of spatial detail in the parameterisation of small scale convective precipitation events. Unless reliable
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meteorological forecasts can be provided, the predictive value of hydrologic assessments is, therefore, necessarily limited. Alternative approaches must be used to increase the understanding of climatic vulnerabilities, including the application of regional hydrologic models to explore a range of possible climate change scenarios. One way to study how discharge patterns and water availability in a region may change in coming decades is based on physically consistent regional scenarios which include general trends of GCM model calculations. Different types of regional climate scenarios are currently used in the scientific community. Among the three most important types the first method tries to embed a regional climate model into a global model, feeding the regional model by the large scale information of the global model. However, the advantage of physically coupling of large and small scale processes is reduced by an inaccurate reproduction of processes at the smaller scale, which reduces the use of such methods within small scale impact studies. The second method is based on the transformation of global climate model results to smaller scales by using statistical methods (statistical downscaling). A disadvantage is the fact that inaccuracies of the global model are directly transmitted into the regional scenarios. The third method is based on the assumption that GCM results are more suitable for large scales (globally or continentally) than for a defined region (covered by only a few grid points). Therefore, long term observed time series can be prepared by statistical methods to reflect the changes calculated by GCMs. By this, the regional deficits of GCM results can be reduced to a minimum and the consistency between the various meteorological parameters can be ensured. A disadvantage is the missing physical connection between the GCM results and the climate scenario.
3.2 Climate impact study in Brandenburg penitz river basin
and in the Ste-
Based on the increasing concentration of carbon dioxide in the atmosphere, the current GCMs calculate a global warming of between 1.5 and 4.5K by the year 2100, which is stronger over the continents and the regions near the poles than over the oceans and the equatorial zones. For Central European regions a temperature increase of about 24 K can be assumed within the next 100 years. Two scenarios derived by the third method mentioned above were applied in the present study both for the whole state of Brandenburg and for the Stepenitz basin. They are based on a temperature increase of 1.5K or 3K in the period 19962050, which was imposed on the values of this climate variable measured at 40 meteorological stations in Brandenburg in the period 1951-1990 (reference scenario). The other observed meteorological variables like precipitation or humidity were adapted consistently to these changes, maintaining the statistical characteristics of the observed data, i.e. interannual variability, annual course and persistence.
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Water balance calculations on a daily basis were performed for the reference state (period 195 l-1990) and for the two climate change scenarios assuming a temperature increase of 1.5K and 3K (period 1996-2050). For the Stepenitz basin, the calculations were based on 557 hydrotopes (10 hydrotope classes) in 64 subbasins. From the 40 meteorological stations available in the state of Brandenburg four stations next to the basin were used. Figure 5 shows the results of the scenario calculations in form of differences between the 1.5K and 3K climate change scenario (period 1996-2050) and the reference state (period 195 l1990), derived for the mean annual sums of various water balance components. 30 0 l.SK-reference w 3K-reference 20 15 s s v) 8 s ti
10
g
-10
5 0
d_ s
-5
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more dramatic changes. when this period is compared, e.g. to the period 198 l-1990 of the reference state. As an example for the basin wide calculated spatial distributions of water balance components in the Stepenitz basin. Fig. 6 shows the distribution of percolation in form of the differences of mean annual sums between the 3K scenario and the reference state. For the whole basin the percolation under climate change conditions is up to 100 mm lower than for the reference scenario. Due to the high evapotranspiration, the reductions are especially high for areas with shallow groundwater table. They are comparatively small for urbanized areas (cities, streets etc.) where evaporation and percolation are of less importance. As already indicated in Fig. 5. the calculated differences of evaporation, percolation and basin discharge are generally higher for the more extreme 3K scenario as compared to the more moderate 1.5K scenario. In summary, the impacts of climate change on the water balance in a region depend strongly on the assumptions made (temperature increase, changes in precipitation) and on the specific characteristics of that region. Sensitive areas are those characterized by already high evapotranspiration rates under temporary conditions, i.e. areas with a shallow groundwater table (lowlands, riperian areas etc.).
Fig. 5: Differences of various water balance terms (mean annual sums) between the 1SK and 3K climate change scenario (period 1996-2050) and the reference state (per& 1951-1990) calculated m the Stepenitz river basin.
-50
- -40
40-o 0
Due to the increasing mean temperatures, evapotranspiration slightly increases by 8 mm and 24 mm in case of the 1.5K and 3K scenario, respectively, but does by far not reach the values of potential evaporation calculated following Turc (1961). Percolation reduces considerably by 41 mm and 48 mm as compared to the reference state. The absolute changes of 3.9 mm (1.5K scenario) and 4.4 mm (3K scenario) for surface runoff formation, which is generally induced by extreme precipitation events, are comparatively small. Basin discharge, on the other hand, shows dramatic changes with reductions of 50 mm and 58 mm as compared to the reference scenario. For all water balance components the last decade (2041-2050) dominates the calculated changes, since both precipitation reductions and temperature increases are much higher than in the decades before. Therefore, the results given in Fig. 5 show much
8
Kilometers
4 Fig. 6: Spatial distribution of percolation in the Stepenitz river basin. given m the form of differences of mean annual sums between the 3K climate change scenario (period 1996-2050) and the reference scenario (permd 1951-1YYO).
Similar simulation runs were performed for the whole state of Brandenburg using 40 climate stations. From the variety of results only two maps are given in the following. Figure 7 shows the spatial distribution of the mean annual sums of evapotranspiration for the state of Brandenburg, given in the form of differences between the two climate change scenarios and the reference state. On the long term average evapotranspiration increases only by 1.O % (1.5K scenario) and 3.7 % (3K scenario) for the whole study re-
W. Lahmer et al.: Assessment
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Fig. 7: Spatial distributions of the mean annual sums of evapotrampiratlon for the state of Brandenburg, given m the form of differences between the two climate change scenarios assuming a 1SK (left) and a 3K (right) temperature Increase (penod 19962050) and the reference scenario (period
1951-1990).
However, the Due to the reduced precipitation and the increased temperatures, the actual evapotranspiration calculated in the climate change scenario runs is reduced in some areas suffering from water deficit already under the present conditions (negative values in Fig. 7). Most areas of Brandenburg show, however, an increase in evapotranspiration. The highest values of up to 100 mm/year are found for open water bodies and areas with shallow groundwater table. In general, the increase is much more pronounced for the 3K scenario, which is characterized by a lower precipitation reduction and a higher temperature increase as compared to the 1.5K scenario (see Fig. 5). gion as compared to the reference state. changes vary considerably within the region.
4 Land use Change Impact Studies 4.1 Scenario development The analysis of land use changes in a region is a rather complex task, since many different aspects must be taken into account. The question, which areas of the actual land use will be converted into which other type depends both on the physical properties of the specific region and on various socio-economic factors like the national and international legislation. In general, the land use change scenarios depend strongly on the specific aims of the investigation, the model used, the spatial scale, and the natural and socio-economic characteristics and constraints of the study
region itself. In the present case, the scenarios of change were developed according to the following steps: 1. Comprehensive
2. 3.
4.
5.
hydrologic modelling and analysis of the current state of the river basin, as a basis for the quantitative assessment of influences of various land use alternatives Development and analysis of the impacts of extreme land use changes Identification of sub-regions which - due to various landscape characteristics - are primary candidates for a possible land use change Inclusion of socio-economic aspects like management planning of the local government, concerns of various stakeholders and interest groups, and projections of the national and international legislation Implementation and analysis of ‘realistic’ scenarios, including the evaluation and comparison of different management strategies.
It should be emphasised that the development and analysis of extreme scenarios represents an important step in studying the effects of a changed land use on the regional water cycle, as such studies cover the possible range of hydrologic basin response and demonstrate the sensitivity limits of the used model. The results of such sensitivity studies show that the modelling approach is appropriate to study even the impacts of land use changes restricted to relatively small fractional areas. Following the above steps, a catalogue of land use change scenarios was developed and applied, which covers a con-
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572
siderable range of alternatives. In the first development phase, this catalogue includes just various physical characteristics (indicators) of the study region, since the inclusion of additional socio-economic aspects might strongly influence both the specific areas to be converted and the measures to be performed finally. One basic aim of the simulation calculations was to investigate whether and how land use changes may induce positive effects on the regional water balance. Finally, these scenarios might result in recommendations which can be used, e.g. by political stakeholders.
4.2 Results of land use change scenarios and the Upper St& river basins
in the Stepenitz
In the presence of a wide variety of possible measures to change the actual land use (especially arable land), a concentration on the most probable conversion alternatives is necessary, resulting in a restriction of the corresponding scenario catalogue to be defined. The scenarios covered by this catalogue result from the general principle to abandon less suited agricultural land in favour of an intensive use of good quality areas. This means that unprofitable areas should be abandoned, either totally (without any further management) or in the form of, e.g. unmanaged grassland. Thus, arable land should be converted where areas are used unprofitably and the existing conditions do not fit the actual use. These natural conditions or indicators include topography, soil properties, groundwater table depth, or the area1 structure. In addition, local climatic conditions define, whether and where arable land might be forested, changed into pasture or completely abandoned.
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table depth (GWTD), the slope and the ‘soil number‘ SN. Though the soil number is based on rather old German investigations (1934) and is relevant only for a few cultivation types, it still represents a basic indicator for the valuation of the agricultural use of a region. For Brandenburg, SN covers a range of 13-69. The area affected by each of the four conversion types is characterized in table 1 by it’s size and percentage as compared to the total area of arable land and the total basin area. Water balance calculations on a daily time step were performed for the period 1981 to 1993, both for the reference scenario (actual land use) and the four land use change scenarios. The reference scenario is based on a digital land use map of 30 x 30 m resolution, provided by the ESRI company in 1993. The changes of the calculated water balance components evapotranspiration, groundwater recharge, surface runoff formation and basin discharge due to the four assumed conversion types are given in Fig. 8 as differences between the reference state and the four conversion types given in table 1. The values of scenario 4 result from the simultaneous application of all four conversion types. 8 conversion
8
into
W dry pasture n meadow n set aside 0 forest EI scenario 4
Table 1: Overview on the conversion types and the derived scenarios for changing arable land mto four other land use types m the Stepemtz basin, The scenarios are based on the indicators groundwater table depth (GWTD), topography (slope) and productivity (soil number SN). conversion type
1 2 3 4
scenario
-
1?
conversion of arable land with slope >= 4% into dry pasture with GWTD c= 0.75m into meadow with SN <=29 and GWTD >= 4.5m into forest with SN C= 29 and 0.75m cGWTDc 4.5m into set aside sum
area km2 16,8
arable land % 4,4
basin area % 2,9
100,6
26,5
17,5
39,7
105
6,9
36,3
9,6
6,3
193,4
51,0
33,6
Among numerous scenarios studied in the Stepenitz river basin only those outlined in table 1 will be presented here. These more complex scenarios consist of the successive conversion of arable land into four other land use types, taking into account the specific meteorologic and agricultural situation in the state of Brandenburg. The natural indicators used to define conversion areas are the groundwater
evapotranspiration
groundwater recharge
surface runoff formation
basin discharge
Fig. 8: Changes of the mean annual sums of various water balance components calculated in the Stepenitz river basin for the period 1981-1993 in converting arable land in four other land use types. Scenano 4 Includes all of these conversion types.
The conversion of arable land into grassland or ‘dry pasture‘ (conversion type l), which is favoured on areas with deep GWTD and considerable slope (due to erosion and nutrient reduction), does not induce considerable changes of any water balance term. This is partly due to the relatively small size of the converted area. The conversion of arable land into meadows, which is restricted to areas with shallow GWTD, gives a comparable small impact, though the size of the converted area is about five times larger. The forestation of arable land (conversion type 3), which is the preferred land use change mode on areas with deep GWTD
W. Lahmer et al.: Assessment
and soils with soil numbers less than 30, on the other hand. leads to remarkable impacts for all water balance components. Due to sandy soils, which make up a major area in the flatlands of the Stepenitz basin, the conversion of arable land into set aside is the favoured conversion type in the state of Brandenburg, especially for heterogeneous or low productivity areas. However, also this conversion type does almost not contribute to the accumulated changes of scenario 4, which are given in Fig. 8 as well. For all conversion types the impact on the surface runoff formation is extremely small, since this water balance component does not play a dominant role for any of the land use classes mentioned here. In summary, the results in the Stepenitz basin show that moderate land use changes result in only small changes of various water balance components. In case of scenario 4, which covers 51 % of the available arable land or almost 34 % of the total basin area, these changes amount to about 7.2 mm (+1.5 %), -7.0 mm (-4.6 %), and -6.8 mm (-3.5 %) for the mean annual sums of evapotranspiration, groundwater recharge and basin discharge as compared to the actual land use. Larger reductions of the basin discharge and therefore a considerable increase of water retention in the basin would occur only in the case of a forestation of large areas. Due to the high evapotranspiration rates of forests this, however, seems to be an unrealistic option in view of the already existing water deficit in the region. In order to compare the results for the Stepenitz basin with those obtained in a region characterized by different climatic and land use conditions, similar impact analyses were performed in the Upper St& basin. Due to the lack of soil numbers, the indicators used for the definition of land use change scenarios were limited to groundwater table depth and slope. Table 2 gives an overview on a set of four scenarios similar to those analysed in the Stepenitz basin.
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type 4, however, results in a scenario 4, which covers almost half of the basin area. Though this scenario is unrealistic. it gives an impression.of extreme changes of water balance components that can be achieved by land use change in this basin. Figure 9 shows the changes of the mean annual values of evapotranspiration, groundwater recharge, surface runoff formation and basin discharge calculated in the period 1982-1993, which result from the assumed conversion measures. Again, the results are given in the form of differences between the reference state and the four conversion types. Like in the Stepenitz basin, the conversion of arable land into dry pasture (restricted to areas with considerable slope) and meadows (restricted to areas with shallow GWTD) almost does not change the water balance components, though the fractional areas for these conversion types sum up to about 5 % and 23 % of the total arable land in the basin, respectively. In contrast to these small changes, the impacts due to the conversion of arable land into set aside (medium GWTD) and forests (deep GWTD) result in considerable changes of all water balance components with the exception of surface runoff formation (see discussion of Fig. 8). Nevertheless, in case of scenario 4, which again includes all four conversion types, the impacts on the water balance components are rather small, though about half of the total basin area is assumed to be converted. The changes for the mean annual sums of evapotranspiration, groundwater recharge and basin discharge amount to only +3.5 %, -4.4 % and -3.4 %, respectively. conversion
20 15 10
Table 2: Overview on the converston types and the derived scenarios for changmg arable land into four other land use types in the Upper Star basm. The scenarios are based on the indicators groundwater table depth (GWTD) and topography (slope).
conversion
scenario
conversion of arable land
area
with slope >= 4% into dry pasture with GWTD c= 0.75m into meadow with 0.75m < GWTD < 3.2m into set aside with GWTD >= 3.2m into forest sum
2
into
n dry pasture n meadow q set aside 0 forest q scenario 4
5
Ll ‘5 E -5
km2 29,6
arable land % 5,l
basin area % 2.6
133.7
23.0
11,6
-15
98,3
16,9
85
-20
-10 i
I 319,6
55,O
27,6
581,l
100,O
50,2
The land use change scenarios 1, 2 and 3 include about 5.1 %, 28.1 % and 45.0 % of the total arable land in the basin and, therefore can still be considered rather ‘realistic’ with respect to the planned reforms of the AGENDA 2000 initiative of the European Union. The additional forestation of the remaining arable land (55 %) due to the conversion
evapotrsnsplratlon
groundwater recharge
surface runoff formatlon
basln discharge
Fig. 9: Changes of the mean annual sums of various water balance components calculated in the Upper Stbr river basin for the period 1982-1993 in converting arable land in four other land use types. Scenario 4 includes all four conversion types.
This means that the impacts of land use change measures in the Upper St& basin are even smaller than those in the Stepenitz basin, taking into account the different size of the
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areas affected by the conversion measures in both basins. The main reason for this are the different climatic conditions in both basins which are summarized in Fig. 10 together with the resulting water balance components for the periods 1982-1987, 1988-1993 and 1982-1993. The differences show that for the whole period 1982-1993 the mean annual precipitation in the Upper St& basin is almost 240 mm (or 36 %) higher than in the Stepenitz basin. In combination with the slightly lower mean daily temperatures (ca. 0.14 “C) the mean annual climatic water balance is about 245 mm higher in the St& basin, which therefore does not suffer from water deficit. Due to these climatic differences, evapotranspiration and surface runoff formation are only about 12 mm (2.5 %) higher and 7 mm (21 %) lower as compared to the Stepenitz basin. On the other hand, the differences for percolation and basin discharge amount to +235 mm (+157 %) and +223 mm (+119 %) for the Upper St& basin. These differences are more pronounced for the rather warm period 1988- 1993 than for the colder and wetter period 1982-1987. 350
1
q
1982-1987
0 1988.1993
1
a’ P
TV00
PET
WB
ET
PER
SR
Q
Fig. 10: Differences of mean annual climattc input and water balance components between the Upper Stiir and the Stepenitz basm. calculated for the periods 1982-1987. 1988-1993 and 1982-1993. Results are given m mm/year for precipnation (P). potenttal evapotranspiration (PEP). climatic water balance (WB). evapotranspirahon (ET). surface runoff formation (SR) and basm discharge (Q). mean daily temperature (T) m “CxlOO.
The results obtained in both basins indicate that the impacts of land use changes strongly depend both on the morphological and the meteorological situation of the region under study. The stronger response of the Stepenitz basin is mainly due to the lower precipitation rates and the higher temperatures in central Germany (continental climate), which is less influenced by the Northern Sea climate representative for the Upper St& basin.
5 Conclusions The application of a high resolution GIS-based modelling approach in two mesoscale river basins in the lowland of the Elbe river basin and in the state of Brandenburg has shown that the disaggregation of a study region into subareas of similar hydrologic behaviour represents an effective modelling approach to study the impacts of Global Change
of Land Use
on the hydrologic cycle. The results demonstrate the quality of the applied GIS-based modelling concept, which directly uses model parameters derived from generally available spatial data and provides spatial and temporal results of various water balance components. The climate change impact studies performed in Brandenburg and in the Stepenitz river basin show that some of the water balance components may undergo a considerable change. Areas characterized by a shallow groundwater table (lowlands, wetlands, riperian areas) as well as open water bodies (lakes etc.) have turned out to be the most sensitive areas, due to their high evapotranspiration potential. The percentage of such areas is relatively high in these regions, which in addition suffer from general water deficit. Since the climate change scenarios used in the present study can be characterized as relatively ‘conservative’, the impacts on the regional water balance might be even more severe. The results of the land use change impact studies performed in the Stepenitz and in the Upper St& river basins demonstrate the impacts of converting arable land into other land use types. The complex scenarios studied in detail show, which conversion measures are generally available in these regions, to what extent they influence the regional water cycle, and in which part of the basin they have the largest effect. The results obtained in both basins clearly demonstrate that moderate land use changes result in only small changes of various water balance components. Larger effects could be achieved only by the reforestation of larger agricultural areas. Due to the high evapotranspiration of forests this, however, seems to be an unrealistic option in view of the already existing water deficit in the region. Climatic characteristics seem to play a crucial role for the effectiveness of land use changes in a region. Even extreme conversion measures will have comparatively small impacts on the regional water balance in regions characterized by a positive climatic water balance (high precipitation, relatively moderate temperatures). On the other hand, such measures may introduce considerable changes in regions already suffering from water deficit, resulting in a considerable increase of evapotranspiration and in large reductions of basin discharge. If reforestation is considered a reasonable conversion mode at all, this kind of land use change should be realized in the lower part of a river course, where the effects are less severe than in the upper parts. Comparing the impacts of climate and land use changes in the Stepenitz basin shows that possible climate changes on the medium term play the dominant role. The impacts on the basin discharge, for example, show a reduction of about 23 % and 27 % assuming a temperature increase of 1.5K and 3K until 2050, respectively. Such a reduction is comparable only to the results of an extreme land use change scenario including the reforestation of all arable land in the basin, which would reduce the basin discharge by about 25 % as well. In any case, the combined effects of climate and land use change impacts on the regional water balance should be taken into consideration by local and regional political decision makers already in the planning stage of new waterrelated projects. Based on detailed analyses, an ‘integrated
W. Lahmer ef al.: Assessment
protection plan’ is proposed to be developed in such cases, including the prevention of causes, a reduction of impacts and an adaptation to unavoidable consequences. This plan must also include the implementation of socio-economic aspects, which represents a major challenge in long term Global Change scenario calculations.
Acknawledg~men6.Most of the results were derived in the framework of the WaStor project, a regional sub-project of the interdisctplmary research project ‘Elbe-Ecology’ funded by the German Ministry for Education and Research (BMBF).
References Becker. A. (1975). The integrated Hydrological Catchment Model EGMO. Bratislava: Int. Symp. and Workshop Appl. Math. Models in Hydrology and Water Res. Syst. In: Hydrol. Sri. Bull. 21: 1 Becker. A. and Pfiitzner, B. (1986). Identification and modelling of river Bow reductions caused by evapotranspiration losses from shallow groundwater areas. In: Conjunctive Water Use. IAHS-Publ. No. 156: 301-311
Becker. A. and Pfutzner. B. (1987). EGMO - system approach and subroutines for river basin modelling. Acta Hydroph,vsica JZ: 125-141 Becker, A. und Lahmer. W. (1999). GIS-basierte groBskahge hydrologtsthe Modellierung. In: Kleeberg, H.-B.. Mauser. W.. Peschke. G. und Streit. U. (Hrsg.). Hydrologie und Regionalisierung - Ergebnisse eines Schwerpunktprogramms (1992 bis 1998). Forschungsbericht. Deutsche Forschungsgemeinschaft (DFG). Wiley-VCH. Weinheim. 1999. ISBN 3527-27145-7: 115-129
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Beven. K. J. (1993). Prophesy, reality and uncertainty in distributed hydrological modelling. Adv. War Resour. 16: 41-51 Bork, H.-R., (1997). Wasser- und Stoffriickhalt im Tiefland des Elbeeinzugsgebietes” (WaStor). Project proposal to the BMBF (Federal Ministry for Education, Science, Research and Technology) Dooge, J.C.I. (1992). Hydrologic models and climate change. Journal of Geophysical
Research, 97, 03: 2677-2686
Gleick. P.H. (1986). Methods for evaluating the regional hydrologic im pacts of global climatic changes. J. Hydrol. 88: 99-116 Gleick. P.H. (1989). Climate change, hydrology, and water resources. Rewiews of Geophysirs.
27. 3: 329-344
Lahmer. W. (1998). Macro- and Mesoscale Hydrological Modelling in the Elbe River Basin. In: Proceedings of the International Conference ‘Catchment Hydrological and Biochemical Processes in Changing Environment’ in Ltblice. Czech Republic. September 22-24, 1998: 57-61 Lahmer. W. and Becker, A. (1998). Dynamic multi-scale hydrological and water quality modelling in regions of the Elbe Pleistocene lowland - test region Stepenitz. Second Intermediate Report to the BMBF project ElbeEcology, October 1998, unpublished Lahmer. W.. Becker. A.. Muller-Wohlfeil. D.-I.. and Pftitzner, B. (1999). A GIS-based Approach for Regional Hydrological Modelling. In: B. Diekkrtiger. M.J. Kirkby. U. Schroder (Ed.%): Regionalization in Hydrology. IAHSpublication no. 254, ISSN 0144-7815: 33-43 Pftitzner, B.. Lahmer, W., and Becker. A. (1997). ARC/EGMO - a prcgram system for GIS-based hydrological modelling. Short documentation to version 2.0. Potsdam Institute for Climate Impact Research (in German, unpubl.) Tegart W.J.McG. and Sheldon G.W. (eds.) (1993). Climate Change 1992: The supplementary Report to the IPCC Impacts Assessment. Australian Government Pubhshing Servtce. Canberra Turc, L. (1961). Evaluation des besoin en eau d’irrigation Cvapotranspiration potentielle. Ann. Agron. 12: 113 -149
Pergamon
PII:
Assessment
Earth (B), Vol. 26, NO. 7-8, pp. 565-575, 2001 0 2001 Elsevier Science Ltd. All rights reserved 1464-1909/01/$ - see front matter
S1464-1909(01)00051-X
of Land Use and Climate Change Impacts on the Mesoscale
W. Lahmer’, B. Pfiitzner’ and A. Becker’ ‘Potsdam Institute for Climate Impact Research, Telegrafenberg, D- 144 12 Potsdam, Germany 2Bureau for Applied Hydrology (BAH), Wollankstr. 117, D- 13 187 Berlin, Germany Received 24 April 2000; accepted 6 October 2000
of Global Change impacts on the regional water cycle induced by a changing climate or by land use changes belong to the urgent issues of today’s hydrologic research. Since the most important sources and drivers of Global Change are located at the regional scale, a stronger emphasis is needed at this scale, where political and technical measures can be taken in order to avoid critical developments for the environment and society. With respect to climate changes, studies indicate considerable regional vulnerabilities against changes of both temperature and precipitation. Land use or land cover changes, on the other hand, represent another anthropogenic ‘system disturbance’ which directly or indirectly influences many hydrologic processes. The present study outlines a methodology to derive and analyse scenarios, which allow to assess the influences of both climate and land use changes in a region. The results of a high resolution modelling approach demonstrate the chances and problems in such types of studies. The climate change impact studies performed in the German state of Brandenburg and in the Stepenitz river basin show that some of the water balance components may undergo a considerable change. Due to their high evapotranspiration potential, wetlands and open water bodies have turned out to be the most sensitive areas. The results of the land use change impact studies performed in two mesoscale river basins demonstrate that moderate land use changes result in only small changes of various water balance components. For the effects of land use changes climatic characteristics in a region Seem to play a Crucial role. 0 2001 Elsevier Science
AbstractStudies
Ltd. All rights reserved
1 Introduction
Global Change issues and human impacts on the hydrologic cycle play a growing role in today’s hydrologic research. Though these phenomena and processes take place at all spatial scales, from local to global, a stronger emphasis is needed on regional and local scales, where the most imporCorrespondence
to: Werner Lahmer
tant sources and drivers of Global Change are located. It is primarily at these scales that political and technical measures can and must be taken in order to avoid critical developments and to reduce negative effects on the environment and society. Thus, the regional scale is crucial for an improved understanding of both causes and impacts of Global Change and it’s contributing processes. The understanding of global climate systems has considerably increased in recent years, as well as human concern about future climatic changes. Along with these changes important consequences are expected in regional hydrologic cycles and subsequent effects on regional water resources. Since hydrologic processes directly depend on climate conditions, influences of possible climatic changes on these processes will differ from region to region. The magnitude and spatial distribution of climatic changes in combination with hydrologic characteristics of the study region determine which effects will be most relevant on the regional scale. Though the direction or magnitude of many important changes are not yet fully clear, studies in the last years have shown important regional vulnerabilities against changes of both temperature and precipitation patterns. They suggest that climatic changes will alter basic components of the hydrologic cycle like soil moisture, groundwater availability, magnitude and timing of runoff, and water quality (see e.g. reviews on water resources impacts of global warming scenarios given by Gleick, 1989; Dooge, 1992; Tegart and Sheldon, 1993). This would induce dramatic environmental dislocations and widespread implications for future water resources planning and management. Land use or land cover is the second of the main boundary conditions which directly or indirectly influences many hydrologic processes. Under certain circumstances land use changes as another important component of Global Change may induce comparable effects on water quantity and quality. Like climatic changes, changes of actual land use have far reaching impacts on the regional water cycle. Besides the given physical characteristics of a region, socio-economic constraints (like interests of the land owners or reforms due to the AGENDA 2000 of the European Union)
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strongly influence measures of land use changes. Therefore, the challenge in modelling such changes is to take into account as much influencing factors as possible. River basins are preferred land surface units for such regional studies, because they represent natural spatial integrators of water and associated material transports at the land surface. In order to forecast possible effects of climate and/or land use changes at these scales, appropriate models and high resolution data, spatial as well as hydro-meteorological, are necessary. Only by a sufficient density of such data all necessary input parameters can be appropriately taken into account in simulation calculations. Since water balance components calculated for meso- and macroscale river basins depend strongly on the accuracy of such data, quantitative and qualitative data deficits represent a major problem in assessing the actual hydrologic situation of a river basin. On the other hand. this assessment represents the basis for the evaluation of changes due to various scenarios of climate or land use change. The main objectives of the study were to investigate the direction and magnitude of possible climate change impacts on the hydrologic cycle at the regional scale to identify the main causes for the observed changes to identify specifically vulnerable landcover types in the study region and provide a scientific basis to derive possible response strategies to compare the impacts induced by climate changes to those from land use changes, as basis for medium term decision making by political stakeholders.
The results presented here are part of the WaStor (water and material retention in the Elbe river lowland) project (Bork, 1997) a regional sub-project of the interdisciplinary research project ‘Elbe-Ecology’ funded by the German Ministry for Education and Research (BMBF). Basic aim of this project is to study the influence of various measures of land use change on the regional water balance and the river runoff. Though the primary scale of the Elbe-Ecology project is the entire German part of the Elbe basin, special studies in vulnerable sub-regions were performed where more detailed data were available.
2 Methodology 2,l Study Regions In the present study, results for the influences of Global Change impacts on the regional water balance are given for two mesoscale tributary river basins of the German part of the Elbe basin, which is the driest of the five largest German river basins and covers an area of nearly 100.000 km’ (Fig. 1). The Stepenitz basin, situated in the state of Brandenburg, is characterized by water stress and water deficiencies, which occur earlier and more frequently in the case of droughts than in other parts of Germany. On the other hand, the Upper St& basin, situated in Northern Germany, is characterized by a mean annual precipitation which is by 240 mm/year higher than in the Stepenitz basin. With a maximum elevation of about 160 m and 100 m a.s.l., respectively, both basins are part of the Pleistocene Elbe
1.158 km* prectpitatton: ca. 890 mm/year Fig. 1: Overview of the German part of the Elbe river basin. the state of Brandenburg
and the two subbasins Stepenitz and Upper St0r
W. Lahmer et al.:
lowland representative Europe.
2.2 The modelling
for humid/semihumid
landscapes
Assessment
in
approach
To study the impacts of climate or land use changes on the hydrologic cycle, appropriate modelling approaches are necessary. These must allow an effective simulation of the regional hydrologic cycle and enable studies, which in detail describe both the natural spatial variabilities and the anthropogenic impacts at various temporal and spatial scales. Medium to large scale applications of fully distributed physically based hydrologic models are constrained by the availability of required input data (Gleick, 1986; Beven. 1993). In addition, their use in prediction at the regional level is often very limited. Therefore, simplified (conceptional) models with a reduced number of physically meaningful parameters are needed which can be applied at these scales. Such models should be able to use directly information provided by digital maps and to handle different temporal and spatial discretization levels. Problems in using these models generally result from deficits in the generally available spatial data as well as in the time series data needed to provide the necessary meteorological input and to validate the model results. The modelling approach used in the present study applies methods of a GIS (Geographic Information System)-based modelling concept. It is based on variable spatial disaggregation and aggregation techniques and consequently uses the GIS-based derivation of model parameters from generally available spatial data. A key element of the modelling approach is the modelling system ARC/EGMO (Pfiitzner et al. 1997), which includes the hydrologic model EGMO (Becker, 1975; Becker and Pfiitzner, 1986; Becker and Pfiitzner, 1987). It was successfully applied in several former investigations (i.e. Lahmer, 1998; Lahmer and Becker, 1998; Becker and Lahmer, 1999; Lahmer et al., 1999). The modelling approach allows to take into account any type of land surface units, from very small spatial units up to larger landscape units like, e.g. biotopes, Hydrological Response Units (HRU) or subbasins. A schematic view of the model is given in Fig. 2. Besides the direct GIS coupling, an important feature of the approach is the clear separation of two domains, that of the vertical processes and that of the horizontal flows. By this, the spatial discretization of both domains can be easily adapted to the available data, which usually show a rather distinct spatial resolution. The model includes conceptual submodels with physically interpretable parameters for all hydrological processes taking place in complex landscapes and river basins. Usually, the model is validated by the discharge measured at the outlet of a river basin.
2.3 Spatial disaggregation
to elementary
units
One basic challenge in meso- and macroscale hydrologic modelling is the disaggregation of the study region into ar-
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eas characterized by a significantly different hydrologic behaviour. Therefore, methods of spatial disaggregation and aggregation play an important role at theses scales. In general, the degree in spatial disaggregation depends on the resolution of the available basic maps and must be determined for each study region.
vertical
pocessises
l
-•
lateral processes
Fig. 2: Schematic view of the processes at the landsurface-atmosphere-soil interface taken into account by the hydrological modelling system ARC/EGMO.
Basis for all simulation calculations with ARC/EGMO is the so called ‘elementary unit map’, generated by a GIS from all necessary digital maps (land use, vegetation cover, soil characteristics, topography, ground water level, river net, subbasins etc.). This map consists of ‘elementary units’ (EUs), which represent the smallest modelling units and can be considered homogeneous with respect to their hydrologic behaviour. Each of these units is characterized by the parameters of the underlying basic maps which are provided by so called ‘relate tables’. The shape and size of the EUs can vary considerably. Simulation runs can be performed on the level of EUs or on spatially aggregated area1 units like hydrotopes, subbasins, or the basin as a whole.
2.4 Spatial aggregation
to hydrotope classes
Though calculations based on elementary units represent the most precise approximation to reality, it is often more effective in case of meso- or macroscale applications to aggregate the EUs to larger spatial units, to minimize the number of EUs and the simulation time. This is an important aspect in climate or land use change impact studies, where long time periods (50 to 100 years or even more) must be simulated using different scenarios. On the basis of earlier studies (Lahmer et al., 1999; Becker and Labmer, 1999), the EU maps of the Stepenitz and Upper St& basin were aggregated to hydrotopes and hydrotope classes which refer to essential (hydrologic, geographic) characteristics of the study region. Hydrotopes represent spatially connected
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Hydrotope Classes classes a&e land, DOT, flat arable land, DOT, Inclined arable land, SOT, Rat .a arable land, SOT, inclined meadow, DOT, ftat meadow, DGT, Inclined w meadow, SOT, Rat m meadow, SOT, inclined forest, DQT, flat i forest, DOT, inclined * for@ SW, flat m forest, SOT, Inclined swamp bare land m urbanized areas m open water bodies hVdrOtODe
0
Fig. 3: Aggregation of the Upper Star basm into 16 hydrotopes processing of various spatial maps (left).
(nght). based on the disaggregation
EUs characterized by an unique hydrologic regime, while hydrotope classes are location independent combinations of similar hydrotopes within a larger area1 unit (‘semi-distributed modelling approach’). Since different EU-characteristics can be used for their definition, simulations based on hydrotope classes are flexible with respect to the definition of each class and the number of classes. To define a reasonable and adequate hydrotope classification is an important task, as this classification should take into account dominant characteristics of the study region and separate areas differing considerably in their evapotranspiration and runoff behaviour. In case of the St& basin, a classification into 16 hydrotope classes was chosen. For the dominant land use classes arable land, forests and meadows areas with shallow (SGT) and deep groundwater table (DGT) were distinguished. Based on the topographic information, each of these classes were further divided into subclasses of less (‘flat’) or more than 2 % gradient (‘inclined’). In addition, the classes swamp, bare land, urbanized areas and open water bodies were classified. The hydrotope classification is indicated in Fig. 3 together with a section of about 11 x 7 km2, which demonstrates the variability in size and shape of both the hydrotopes and the quasi-homogeneous elementary units (2 1.08 1 for the Upper St& basin as compared to 30.675 in the Stepenitz basin). The EU-map emphasises one of the advantages of the polygon-based disaggregation approach, which results in larger spatial units in homogeneous parts of the basin and in smaller units in areas of higher heterogeneity. This aspect is important in simulating land use changes, which normally is restricted to rather small and widely distributed parts of the study region.
Kilometers
mto 21.081 elementary
2.5 Spatial interpolation
units by a GIS-based
of meteorological
pre-
data
Due to it’s high influence on the calculated water balance components, the spatial distribution of meteorological input variables plays a key role in meso and large scale hydrologic modelling. In order to include ‘realistic’ spatial distributions of these variables in the simulation calculations, an appropriate interpolation method must be used. Since the interpolation is performed for every time step of the simulation period (usually one day), this method must be fast, excluding too sophisticated and time consuming algorithms. The interpolation algorithm used in ARC/EGMO (‘extended quadrant method’) is characterized by i) the selection of stations by the quadrant method, ii) a linear inverse distance interpolation of the respective variables, and iii) the possible inclusion of other dependencies (elevation, exposition or slope). This method has turned out to be very effective for large scale modelling purposes, producing rather realistic spatial distributions for all meteorological input variables (precipitation, mean temperature, relative humidity, insolation etc.) for every time step of the simulation runs. As an example, Fig. 4 shows the spatial distribution of precipitation in the state of Brandenburg, calculated on the basis of 40 climate and 33 precipitation stations for November 1985.
2.6 Calculation
of water balance components
Based on the aggregated spatial maps and the meteorological information from various meteorological stations, water balance calculations were performed for the State of Bran-
W. Lahmer et al.: Assessment
denburg and in the Stepenitz and Upper St& river basins on a daily basis for the period 1951-1990 and 1982-1993, respectively. For the reference state (either the actual meteorological conditions or the actual land use) the model AFXXGMO was validated by the measured discharge at the basin outlet. Among the various hydrologic processes taking place in complex landscapes, evapotranspiration, percolation and surface runoff are most essential. In general, the results obtained for these water balance components are characterized by the spatial distribution of the meteorological input variables and the heterogeneities of the underlying spatial units due to land use, soil, and groundwater level. In case of impact studies (climate or land use change) the results obtained in the scenario runs are more or less strongly influenced by the assumptions made in these scenarios.
Fig. 4: Spatial distribution of precipitation in the state of Brandenburg, calculated on the basis of 40 climate and 33 precipitation stations for November 1985.
3 Climate Change Impact Studies 3.1 Climate scenarios In order to provide accurate forecasts of regional hydrologic processes, quantitative estimates of changes in major long term climatic variables such as temperature, precipitation, and evapotranspiration are needed. Influences of climate change on the regional water balance may result from both spatial and temporal precipitation shift, changes of the actual evapotranspiration due to temperature increases, and an increase of extreme meteorological events (prolongation of dry periods, draughts, high intensity precipitation events etc.). General Circulation Models (GCMs) do not provide valuable detailed information on regional impacts on water supplies. Moreover, they simulate precipitation in considerably less detail than circulation or temperature. One of the reasons is the lack of spatial detail in the parameterisation of small scale convective precipitation events. Unless reliable
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meteorological forecasts can be provided, the predictive value of hydrologic assessments is, therefore, necessarily limited. Alternative approaches must be used to increase the understanding of climatic vulnerabilities, including the application of regional hydrologic models to explore a range of possible climate change scenarios. One way to study how discharge patterns and water availability in a region may change in coming decades is based on physically consistent regional scenarios which include general trends of GCM model calculations. Different types of regional climate scenarios are currently used in the scientific community. Among the three most important types the first method tries to embed a regional climate model into a global model, feeding the regional model by the large scale information of the global model. However, the advantage of physically coupling of large and small scale processes is reduced by an inaccurate reproduction of processes at the smaller scale, which reduces the use of such methods within small scale impact studies. The second method is based on the transformation of global climate model results to smaller scales by using statistical methods (statistical downscaling). A disadvantage is the fact that inaccuracies of the global model are directly transmitted into the regional scenarios. The third method is based on the assumption that GCM results are more suitable for large scales (globally or continentally) than for a defined region (covered by only a few grid points). Therefore, long term observed time series can be prepared by statistical methods to reflect the changes calculated by GCMs. By this, the regional deficits of GCM results can be reduced to a minimum and the consistency between the various meteorological parameters can be ensured. A disadvantage is the missing physical connection between the GCM results and the climate scenario.
3.2 Climate impact study in Brandenburg penitz river basin
and in the Ste-
Based on the increasing concentration of carbon dioxide in the atmosphere, the current GCMs calculate a global warming of between 1.5 and 4.5K by the year 2100, which is stronger over the continents and the regions near the poles than over the oceans and the equatorial zones. For Central European regions a temperature increase of about 24 K can be assumed within the next 100 years. Two scenarios derived by the third method mentioned above were applied in the present study both for the whole state of Brandenburg and for the Stepenitz basin. They are based on a temperature increase of 1.5K or 3K in the period 19962050, which was imposed on the values of this climate variable measured at 40 meteorological stations in Brandenburg in the period 1951-1990 (reference scenario). The other observed meteorological variables like precipitation or humidity were adapted consistently to these changes, maintaining the statistical characteristics of the observed data, i.e. interannual variability, annual course and persistence.
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Water balance calculations on a daily basis were performed for the reference state (period 195 l-1990) and for the two climate change scenarios assuming a temperature increase of 1.5K and 3K (period 1996-2050). For the Stepenitz basin, the calculations were based on 557 hydrotopes (10 hydrotope classes) in 64 subbasins. From the 40 meteorological stations available in the state of Brandenburg four stations next to the basin were used. Figure 5 shows the results of the scenario calculations in form of differences between the 1.5K and 3K climate change scenario (period 1996-2050) and the reference state (period 195 l1990), derived for the mean annual sums of various water balance components. 30 0 l.SK-reference w 3K-reference 20 15 s s v) 8 s ti
10
g
-10
5 0
d_ s
-5
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more dramatic changes. when this period is compared, e.g. to the period 198 l-1990 of the reference state. As an example for the basin wide calculated spatial distributions of water balance components in the Stepenitz basin. Fig. 6 shows the distribution of percolation in form of the differences of mean annual sums between the 3K scenario and the reference state. For the whole basin the percolation under climate change conditions is up to 100 mm lower than for the reference scenario. Due to the high evapotranspiration, the reductions are especially high for areas with shallow groundwater table. They are comparatively small for urbanized areas (cities, streets etc.) where evaporation and percolation are of less importance. As already indicated in Fig. 5. the calculated differences of evaporation, percolation and basin discharge are generally higher for the more extreme 3K scenario as compared to the more moderate 1.5K scenario. In summary, the impacts of climate change on the water balance in a region depend strongly on the assumptions made (temperature increase, changes in precipitation) and on the specific characteristics of that region. Sensitive areas are those characterized by already high evapotranspiration rates under temporary conditions, i.e. areas with a shallow groundwater table (lowlands, riperian areas etc.).
Fig. 5: Differences of various water balance terms (mean annual sums) between the 1SK and 3K climate change scenario (period 1996-2050) and the reference state (per& 1951-1990) calculated m the Stepenitz river basin.
-50
- -40
40-o 0
Due to the increasing mean temperatures, evapotranspiration slightly increases by 8 mm and 24 mm in case of the 1.5K and 3K scenario, respectively, but does by far not reach the values of potential evaporation calculated following Turc (1961). Percolation reduces considerably by 41 mm and 48 mm as compared to the reference state. The absolute changes of 3.9 mm (1.5K scenario) and 4.4 mm (3K scenario) for surface runoff formation, which is generally induced by extreme precipitation events, are comparatively small. Basin discharge, on the other hand, shows dramatic changes with reductions of 50 mm and 58 mm as compared to the reference scenario. For all water balance components the last decade (2041-2050) dominates the calculated changes, since both precipitation reductions and temperature increases are much higher than in the decades before. Therefore, the results given in Fig. 5 show much
8
Kilometers
4 Fig. 6: Spatial distribution of percolation in the Stepenitz river basin. given m the form of differences of mean annual sums between the 3K climate change scenario (period 1996-2050) and the reference scenario (permd 1951-1YYO).
Similar simulation runs were performed for the whole state of Brandenburg using 40 climate stations. From the variety of results only two maps are given in the following. Figure 7 shows the spatial distribution of the mean annual sums of evapotranspiration for the state of Brandenburg, given in the form of differences between the two climate change scenarios and the reference state. On the long term average evapotranspiration increases only by 1.O % (1.5K scenario) and 3.7 % (3K scenario) for the whole study re-
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Fig. 7: Spatial distributions of the mean annual sums of evapotrampiratlon for the state of Brandenburg, given m the form of differences between the two climate change scenarios assuming a 1SK (left) and a 3K (right) temperature Increase (penod 19962050) and the reference scenario (period
1951-1990).
However, the Due to the reduced precipitation and the increased temperatures, the actual evapotranspiration calculated in the climate change scenario runs is reduced in some areas suffering from water deficit already under the present conditions (negative values in Fig. 7). Most areas of Brandenburg show, however, an increase in evapotranspiration. The highest values of up to 100 mm/year are found for open water bodies and areas with shallow groundwater table. In general, the increase is much more pronounced for the 3K scenario, which is characterized by a lower precipitation reduction and a higher temperature increase as compared to the 1.5K scenario (see Fig. 5). gion as compared to the reference state. changes vary considerably within the region.
4 Land use Change Impact Studies 4.1 Scenario development The analysis of land use changes in a region is a rather complex task, since many different aspects must be taken into account. The question, which areas of the actual land use will be converted into which other type depends both on the physical properties of the specific region and on various socio-economic factors like the national and international legislation. In general, the land use change scenarios depend strongly on the specific aims of the investigation, the model used, the spatial scale, and the natural and socio-economic characteristics and constraints of the study
region itself. In the present case, the scenarios of change were developed according to the following steps: 1. Comprehensive
2. 3.
4.
5.
hydrologic modelling and analysis of the current state of the river basin, as a basis for the quantitative assessment of influences of various land use alternatives Development and analysis of the impacts of extreme land use changes Identification of sub-regions which - due to various landscape characteristics - are primary candidates for a possible land use change Inclusion of socio-economic aspects like management planning of the local government, concerns of various stakeholders and interest groups, and projections of the national and international legislation Implementation and analysis of ‘realistic’ scenarios, including the evaluation and comparison of different management strategies.
It should be emphasised that the development and analysis of extreme scenarios represents an important step in studying the effects of a changed land use on the regional water cycle, as such studies cover the possible range of hydrologic basin response and demonstrate the sensitivity limits of the used model. The results of such sensitivity studies show that the modelling approach is appropriate to study even the impacts of land use changes restricted to relatively small fractional areas. Following the above steps, a catalogue of land use change scenarios was developed and applied, which covers a con-
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572
siderable range of alternatives. In the first development phase, this catalogue includes just various physical characteristics (indicators) of the study region, since the inclusion of additional socio-economic aspects might strongly influence both the specific areas to be converted and the measures to be performed finally. One basic aim of the simulation calculations was to investigate whether and how land use changes may induce positive effects on the regional water balance. Finally, these scenarios might result in recommendations which can be used, e.g. by political stakeholders.
4.2 Results of land use change scenarios and the Upper St& river basins
in the Stepenitz
In the presence of a wide variety of possible measures to change the actual land use (especially arable land), a concentration on the most probable conversion alternatives is necessary, resulting in a restriction of the corresponding scenario catalogue to be defined. The scenarios covered by this catalogue result from the general principle to abandon less suited agricultural land in favour of an intensive use of good quality areas. This means that unprofitable areas should be abandoned, either totally (without any further management) or in the form of, e.g. unmanaged grassland. Thus, arable land should be converted where areas are used unprofitably and the existing conditions do not fit the actual use. These natural conditions or indicators include topography, soil properties, groundwater table depth, or the area1 structure. In addition, local climatic conditions define, whether and where arable land might be forested, changed into pasture or completely abandoned.
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table depth (GWTD), the slope and the ‘soil number‘ SN. Though the soil number is based on rather old German investigations (1934) and is relevant only for a few cultivation types, it still represents a basic indicator for the valuation of the agricultural use of a region. For Brandenburg, SN covers a range of 13-69. The area affected by each of the four conversion types is characterized in table 1 by it’s size and percentage as compared to the total area of arable land and the total basin area. Water balance calculations on a daily time step were performed for the period 1981 to 1993, both for the reference scenario (actual land use) and the four land use change scenarios. The reference scenario is based on a digital land use map of 30 x 30 m resolution, provided by the ESRI company in 1993. The changes of the calculated water balance components evapotranspiration, groundwater recharge, surface runoff formation and basin discharge due to the four assumed conversion types are given in Fig. 8 as differences between the reference state and the four conversion types given in table 1. The values of scenario 4 result from the simultaneous application of all four conversion types. 8 conversion
8
into
W dry pasture n meadow n set aside 0 forest EI scenario 4
Table 1: Overview on the conversion types and the derived scenarios for changing arable land mto four other land use types m the Stepemtz basin, The scenarios are based on the indicators groundwater table depth (GWTD), topography (slope) and productivity (soil number SN). conversion type
1 2 3 4
scenario
-
1?
conversion of arable land with slope >= 4% into dry pasture with GWTD c= 0.75m into meadow with SN <=29 and GWTD >= 4.5m into forest with SN C= 29 and 0.75m cGWTDc 4.5m into set aside sum
area km2 16,8
arable land % 4,4
basin area % 2,9
100,6
26,5
17,5
39,7
105
6,9
36,3
9,6
6,3
193,4
51,0
33,6
Among numerous scenarios studied in the Stepenitz river basin only those outlined in table 1 will be presented here. These more complex scenarios consist of the successive conversion of arable land into four other land use types, taking into account the specific meteorologic and agricultural situation in the state of Brandenburg. The natural indicators used to define conversion areas are the groundwater
evapotranspiration
groundwater recharge
surface runoff formation
basin discharge
Fig. 8: Changes of the mean annual sums of various water balance components calculated in the Stepenitz river basin for the period 1981-1993 in converting arable land in four other land use types. Scenano 4 Includes all of these conversion types.
The conversion of arable land into grassland or ‘dry pasture‘ (conversion type l), which is favoured on areas with deep GWTD and considerable slope (due to erosion and nutrient reduction), does not induce considerable changes of any water balance term. This is partly due to the relatively small size of the converted area. The conversion of arable land into meadows, which is restricted to areas with shallow GWTD, gives a comparable small impact, though the size of the converted area is about five times larger. The forestation of arable land (conversion type 3), which is the preferred land use change mode on areas with deep GWTD
W. Lahmer et al.: Assessment
and soils with soil numbers less than 30, on the other hand. leads to remarkable impacts for all water balance components. Due to sandy soils, which make up a major area in the flatlands of the Stepenitz basin, the conversion of arable land into set aside is the favoured conversion type in the state of Brandenburg, especially for heterogeneous or low productivity areas. However, also this conversion type does almost not contribute to the accumulated changes of scenario 4, which are given in Fig. 8 as well. For all conversion types the impact on the surface runoff formation is extremely small, since this water balance component does not play a dominant role for any of the land use classes mentioned here. In summary, the results in the Stepenitz basin show that moderate land use changes result in only small changes of various water balance components. In case of scenario 4, which covers 51 % of the available arable land or almost 34 % of the total basin area, these changes amount to about 7.2 mm (+1.5 %), -7.0 mm (-4.6 %), and -6.8 mm (-3.5 %) for the mean annual sums of evapotranspiration, groundwater recharge and basin discharge as compared to the actual land use. Larger reductions of the basin discharge and therefore a considerable increase of water retention in the basin would occur only in the case of a forestation of large areas. Due to the high evapotranspiration rates of forests this, however, seems to be an unrealistic option in view of the already existing water deficit in the region. In order to compare the results for the Stepenitz basin with those obtained in a region characterized by different climatic and land use conditions, similar impact analyses were performed in the Upper St& basin. Due to the lack of soil numbers, the indicators used for the definition of land use change scenarios were limited to groundwater table depth and slope. Table 2 gives an overview on a set of four scenarios similar to those analysed in the Stepenitz basin.
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type 4, however, results in a scenario 4, which covers almost half of the basin area. Though this scenario is unrealistic. it gives an impression.of extreme changes of water balance components that can be achieved by land use change in this basin. Figure 9 shows the changes of the mean annual values of evapotranspiration, groundwater recharge, surface runoff formation and basin discharge calculated in the period 1982-1993, which result from the assumed conversion measures. Again, the results are given in the form of differences between the reference state and the four conversion types. Like in the Stepenitz basin, the conversion of arable land into dry pasture (restricted to areas with considerable slope) and meadows (restricted to areas with shallow GWTD) almost does not change the water balance components, though the fractional areas for these conversion types sum up to about 5 % and 23 % of the total arable land in the basin, respectively. In contrast to these small changes, the impacts due to the conversion of arable land into set aside (medium GWTD) and forests (deep GWTD) result in considerable changes of all water balance components with the exception of surface runoff formation (see discussion of Fig. 8). Nevertheless, in case of scenario 4, which again includes all four conversion types, the impacts on the water balance components are rather small, though about half of the total basin area is assumed to be converted. The changes for the mean annual sums of evapotranspiration, groundwater recharge and basin discharge amount to only +3.5 %, -4.4 % and -3.4 %, respectively. conversion
20 15 10
Table 2: Overview on the converston types and the derived scenarios for changmg arable land into four other land use types in the Upper Star basm. The scenarios are based on the indicators groundwater table depth (GWTD) and topography (slope).
conversion
scenario
conversion of arable land
area
with slope >= 4% into dry pasture with GWTD c= 0.75m into meadow with 0.75m < GWTD < 3.2m into set aside with GWTD >= 3.2m into forest sum
2
into
n dry pasture n meadow q set aside 0 forest q scenario 4
5
Ll ‘5 E -5
km2 29,6
arable land % 5,l
basin area % 2.6
133.7
23.0
11,6
-15
98,3
16,9
85
-20
-10 i
I 319,6
55,O
27,6
581,l
100,O
50,2
The land use change scenarios 1, 2 and 3 include about 5.1 %, 28.1 % and 45.0 % of the total arable land in the basin and, therefore can still be considered rather ‘realistic’ with respect to the planned reforms of the AGENDA 2000 initiative of the European Union. The additional forestation of the remaining arable land (55 %) due to the conversion
evapotrsnsplratlon
groundwater recharge
surface runoff formatlon
basln discharge
Fig. 9: Changes of the mean annual sums of various water balance components calculated in the Upper Stbr river basin for the period 1982-1993 in converting arable land in four other land use types. Scenario 4 includes all four conversion types.
This means that the impacts of land use change measures in the Upper St& basin are even smaller than those in the Stepenitz basin, taking into account the different size of the
514
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areas affected by the conversion measures in both basins. The main reason for this are the different climatic conditions in both basins which are summarized in Fig. 10 together with the resulting water balance components for the periods 1982-1987, 1988-1993 and 1982-1993. The differences show that for the whole period 1982-1993 the mean annual precipitation in the Upper St& basin is almost 240 mm (or 36 %) higher than in the Stepenitz basin. In combination with the slightly lower mean daily temperatures (ca. 0.14 “C) the mean annual climatic water balance is about 245 mm higher in the St& basin, which therefore does not suffer from water deficit. Due to these climatic differences, evapotranspiration and surface runoff formation are only about 12 mm (2.5 %) higher and 7 mm (21 %) lower as compared to the Stepenitz basin. On the other hand, the differences for percolation and basin discharge amount to +235 mm (+157 %) and +223 mm (+119 %) for the Upper St& basin. These differences are more pronounced for the rather warm period 1988- 1993 than for the colder and wetter period 1982-1987. 350
1
q
1982-1987
0 1988.1993
1
a’ P
TV00
PET
WB
ET
PER
SR
Q
Fig. 10: Differences of mean annual climattc input and water balance components between the Upper Stiir and the Stepenitz basm. calculated for the periods 1982-1987. 1988-1993 and 1982-1993. Results are given m mm/year for precipnation (P). potenttal evapotranspiration (PEP). climatic water balance (WB). evapotranspirahon (ET). surface runoff formation (SR) and basm discharge (Q). mean daily temperature (T) m “CxlOO.
The results obtained in both basins indicate that the impacts of land use changes strongly depend both on the morphological and the meteorological situation of the region under study. The stronger response of the Stepenitz basin is mainly due to the lower precipitation rates and the higher temperatures in central Germany (continental climate), which is less influenced by the Northern Sea climate representative for the Upper St& basin.
5 Conclusions The application of a high resolution GIS-based modelling approach in two mesoscale river basins in the lowland of the Elbe river basin and in the state of Brandenburg has shown that the disaggregation of a study region into subareas of similar hydrologic behaviour represents an effective modelling approach to study the impacts of Global Change
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on the hydrologic cycle. The results demonstrate the quality of the applied GIS-based modelling concept, which directly uses model parameters derived from generally available spatial data and provides spatial and temporal results of various water balance components. The climate change impact studies performed in Brandenburg and in the Stepenitz river basin show that some of the water balance components may undergo a considerable change. Areas characterized by a shallow groundwater table (lowlands, wetlands, riperian areas) as well as open water bodies (lakes etc.) have turned out to be the most sensitive areas, due to their high evapotranspiration potential. The percentage of such areas is relatively high in these regions, which in addition suffer from general water deficit. Since the climate change scenarios used in the present study can be characterized as relatively ‘conservative’, the impacts on the regional water balance might be even more severe. The results of the land use change impact studies performed in the Stepenitz and in the Upper St& river basins demonstrate the impacts of converting arable land into other land use types. The complex scenarios studied in detail show, which conversion measures are generally available in these regions, to what extent they influence the regional water cycle, and in which part of the basin they have the largest effect. The results obtained in both basins clearly demonstrate that moderate land use changes result in only small changes of various water balance components. Larger effects could be achieved only by the reforestation of larger agricultural areas. Due to the high evapotranspiration of forests this, however, seems to be an unrealistic option in view of the already existing water deficit in the region. Climatic characteristics seem to play a crucial role for the effectiveness of land use changes in a region. Even extreme conversion measures will have comparatively small impacts on the regional water balance in regions characterized by a positive climatic water balance (high precipitation, relatively moderate temperatures). On the other hand, such measures may introduce considerable changes in regions already suffering from water deficit, resulting in a considerable increase of evapotranspiration and in large reductions of basin discharge. If reforestation is considered a reasonable conversion mode at all, this kind of land use change should be realized in the lower part of a river course, where the effects are less severe than in the upper parts. Comparing the impacts of climate and land use changes in the Stepenitz basin shows that possible climate changes on the medium term play the dominant role. The impacts on the basin discharge, for example, show a reduction of about 23 % and 27 % assuming a temperature increase of 1.5K and 3K until 2050, respectively. Such a reduction is comparable only to the results of an extreme land use change scenario including the reforestation of all arable land in the basin, which would reduce the basin discharge by about 25 % as well. In any case, the combined effects of climate and land use change impacts on the regional water balance should be taken into consideration by local and regional political decision makers already in the planning stage of new waterrelated projects. Based on detailed analyses, an ‘integrated
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protection plan’ is proposed to be developed in such cases, including the prevention of causes, a reduction of impacts and an adaptation to unavoidable consequences. This plan must also include the implementation of socio-economic aspects, which represents a major challenge in long term Global Change scenario calculations.
Acknawledg~men6.Most of the results were derived in the framework of the WaStor project, a regional sub-project of the interdisctplmary research project ‘Elbe-Ecology’ funded by the German Ministry for Education and Research (BMBF).
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Becker. A. and Pfutzner. B. (1987). EGMO - system approach and subroutines for river basin modelling. Acta Hydroph,vsica JZ: 125-141 Becker, A. und Lahmer. W. (1999). GIS-basierte groBskahge hydrologtsthe Modellierung. In: Kleeberg, H.-B.. Mauser. W.. Peschke. G. und Streit. U. (Hrsg.). Hydrologie und Regionalisierung - Ergebnisse eines Schwerpunktprogramms (1992 bis 1998). Forschungsbericht. Deutsche Forschungsgemeinschaft (DFG). Wiley-VCH. Weinheim. 1999. ISBN 3527-27145-7: 115-129
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Beven. K. J. (1993). Prophesy, reality and uncertainty in distributed hydrological modelling. Adv. War Resour. 16: 41-51 Bork, H.-R., (1997). Wasser- und Stoffriickhalt im Tiefland des Elbeeinzugsgebietes” (WaStor). Project proposal to the BMBF (Federal Ministry for Education, Science, Research and Technology) Dooge, J.C.I. (1992). Hydrologic models and climate change. Journal of Geophysical
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Gleick. P.H. (1986). Methods for evaluating the regional hydrologic im pacts of global climatic changes. J. Hydrol. 88: 99-116 Gleick. P.H. (1989). Climate change, hydrology, and water resources. Rewiews of Geophysirs.
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Lahmer. W. (1998). Macro- and Mesoscale Hydrological Modelling in the Elbe River Basin. In: Proceedings of the International Conference ‘Catchment Hydrological and Biochemical Processes in Changing Environment’ in Ltblice. Czech Republic. September 22-24, 1998: 57-61 Lahmer. W. and Becker, A. (1998). Dynamic multi-scale hydrological and water quality modelling in regions of the Elbe Pleistocene lowland - test region Stepenitz. Second Intermediate Report to the BMBF project ElbeEcology, October 1998, unpublished Lahmer. W.. Becker. A.. Muller-Wohlfeil. D.-I.. and Pftitzner, B. (1999). A GIS-based Approach for Regional Hydrological Modelling. In: B. Diekkrtiger. M.J. Kirkby. U. Schroder (Ed.%): Regionalization in Hydrology. IAHSpublication no. 254, ISSN 0144-7815: 33-43 Pftitzner, B.. Lahmer, W., and Becker. A. (1997). ARC/EGMO - a prcgram system for GIS-based hydrological modelling. Short documentation to version 2.0. Potsdam Institute for Climate Impact Research (in German, unpubl.) Tegart W.J.McG. and Sheldon G.W. (eds.) (1993). Climate Change 1992: The supplementary Report to the IPCC Impacts Assessment. Australian Government Pubhshing Servtce. Canberra Turc, L. (1961). Evaluation des besoin en eau d’irrigation Cvapotranspiration potentielle. Ann. Agron. 12: 113 -149