19 Detection of Land Cover Change
421
hectares in Sweden (Ahlcrona, 1995) or a 25m raster in Great Britain (Fuller &
Brown, 1996).
Land Cover Changes and Hydrology. Changes in land cover include changes in
the hydrological cycle and in most of the mass and energy fluxes that sustain the
biosphere and geosphere. Due to agriculture, forest management and urbanisation
the physical characteristics of the landsurface and upper soil as well as the evapotranspiration process which is strongly related to the type of vegetation are
changed. As a result the amount of runoff, the soil moisture and groundwater recharge are strongly affected by land use changes. The evapotranspiration of crops
in the mid latitudes can e.g. be assessed with 40-50 % of the yearly precipitation
amount, but coniferous forests produce 70 % (Baumgartner, 1970). Measurements
of Robinson et al. (1991) show a strong increase in evapotranspiration and (as a
result) a reduction of the yearly runoff by 40 % for re-forested agricultural areas in
southern Gennany. Flood runoff from urbanized areas can be two or three times
greater than that stemming from the surrounding vegetated areas. As a result of the
large percentage of impervious areas and the artificial drainage network the flood
hydro graph from urbanized areas is much steeper than from natural areas. The
increase of runoff values coincides with a decrease of the groundwater recharge by
urbanisation. As the land use detennines not only the hydrological processes but
also the material flows, land use changes have also strong impacts on water quality. In many countries of the world the intensification of agriculture is coupled with
the utilization of chemical substances like pesticides, nutrients and phosphor. In
Gennany e.g. the amount of nitrogen fertiliser used per area was increased from 60
kg/ha in 1950 to 200 kg/ha in 1985 (Mehlhorn & Rohrle, 1990). These developments have a strong impact on water quality conditions, e.g. in fresh water reservoirs which are endangered by eutrophication.
In summary it is necessary to estimate the temporal and spatial development of
land use changes to assess their impact on water quality and quantity. In most of
the cases topographic maps are not sufficient to mirror the dynamics of land use
changes as they were not updated in sufficiently small time steps. The land use
statistic which is provided for taxation purposes by different state agencies gives
us only a lumped infonnation about the changes in general. As land use is only one
of many factors which detennine the local water balance its impact assessment
demands a consideration of other local characteristics as geomorphological, hydrometeorological, soil and geological parameters. The impact of land use changes on
catchment hydrology depends on interactions of all physical characteristics which
detennine the water and energy fluxes (e.g. Dunn & Mackay, 1995). The effects of
alterations in land use can be intensified or reduced in relationship to the other
physical characteristics of a river basin. The impact of urbanization for example on
the local water balance depends mostly on the soil type which is paved. If a soil
with low penneability is paved the effects of the newly built impervious areas will
be much smaller than if a penneable soil surface will be sealed. This example
shows that it is not sufficient to estimate land use changes in total. The temporal
and spatial highly detailed land use data which are needed for such analysis can be
421
hectares in Sweden (Ahlcrona, 1995) or a 25m raster in Great Britain (Fuller &
Brown, 1996).
Land Cover Changes and Hydrology. Changes in land cover include changes in
the hydrological cycle and in most of the mass and energy fluxes that sustain the
biosphere and geosphere. Due to agriculture, forest management and urbanisation
the physical characteristics of the landsurface and upper soil as well as the evapotranspiration process which is strongly related to the type of vegetation are
changed. As a result the amount of runoff, the soil moisture and groundwater recharge are strongly affected by land use changes. The evapotranspiration of crops
in the mid latitudes can e.g. be assessed with 40-50 % of the yearly precipitation
amount, but coniferous forests produce 70 % (Baumgartner, 1970). Measurements
of Robinson et al. (1991) show a strong increase in evapotranspiration and (as a
result) a reduction of the yearly runoff by 40 % for re-forested agricultural areas in
southern Gennany. Flood runoff from urbanized areas can be two or three times
greater than that stemming from the surrounding vegetated areas. As a result of the
large percentage of impervious areas and the artificial drainage network the flood
hydro graph from urbanized areas is much steeper than from natural areas. The
increase of runoff values coincides with a decrease of the groundwater recharge by
urbanisation. As the land use detennines not only the hydrological processes but
also the material flows, land use changes have also strong impacts on water quality. In many countries of the world the intensification of agriculture is coupled with
the utilization of chemical substances like pesticides, nutrients and phosphor. In
Gennany e.g. the amount of nitrogen fertiliser used per area was increased from 60
kg/ha in 1950 to 200 kg/ha in 1985 (Mehlhorn & Rohrle, 1990). These developments have a strong impact on water quality conditions, e.g. in fresh water reservoirs which are endangered by eutrophication.
In summary it is necessary to estimate the temporal and spatial development of
land use changes to assess their impact on water quality and quantity. In most of
the cases topographic maps are not sufficient to mirror the dynamics of land use
changes as they were not updated in sufficiently small time steps. The land use
statistic which is provided for taxation purposes by different state agencies gives
us only a lumped infonnation about the changes in general. As land use is only one
of many factors which detennine the local water balance its impact assessment
demands a consideration of other local characteristics as geomorphological, hydrometeorological, soil and geological parameters. The impact of land use changes on
catchment hydrology depends on interactions of all physical characteristics which
detennine the water and energy fluxes (e.g. Dunn & Mackay, 1995). The effects of
alterations in land use can be intensified or reduced in relationship to the other
physical characteristics of a river basin. The impact of urbanization for example on
the local water balance depends mostly on the soil type which is paved. If a soil
with low penneability is paved the effects of the newly built impervious areas will
be much smaller than if a penneable soil surface will be sealed. This example
shows that it is not sufficient to estimate land use changes in total. The temporal
and spatial highly detailed land use data which are needed for such analysis can be