|
1. Definition
| Name |
SLOPE
GRADIENT |
| Brief
definition |
Slope gradient refers
to the angle which any part of the earth's surface makes with
an horizontal datum.
 |
Sloping
land in which vegetation has been removed subjected to
high erosion rates (photo C. Kosmas) |
|
| Unit of measure |
%, degrees
|
2. Position
within the logical framework DPSIR
3. Target and
political pertinence
| Objective |
Contribution
to (a) the definition and mapping of ESAs and (b) assessment
of the desertification risk of an area. |
| Importance
with respect to desertification |
Soil erosion
is considered as the main process of land degradation and desertification
in hilly Mediterranean areas. Generally speaking, soil sediment
loss can be estimated by the product of the amount of surface
water run-off times the slope gradient times a constant related
to soil surface characteristics. As the slope becomes steeper,
the runoff coefficient increases, the kinetic energy and carrying
capacity of surface water flow becomes greater, soil stability
and slope stability decreases, soil sediment loss increase.
Therefore, slope gradient is undoubtedly considered as one of
the most important determinants of soil erosion and desertification. |
| International
Conventions and agreements |
The CCD emphasizes
that combating desertification must be tackled within the general
framework of actions to promote sustainable development. |
| Secondary objectives
of the indicator |
Within the
ESA model investigation of the individual processes linked to
land degradation and desertification.
|
4. Methodological
description and basic definitions
| Definitions
and basic concepts |
Slope gradient greatly
affects amount of surface water run-off and soil sediment
loss. Soil erosion rates becomes acute when slope angle exceeds
a critical value and then increases logarithmically. The slope
gradient can have variable effect in different climatic zones,
depending mainly on annual rainfall. Measurements conducted
in different areas with natural vegetation in the Mediterranean
region have shown that severely eroded soils prevail in semi-arid
climatic conditions with slopes greater than 12%, while slightly
to moderately eroded soils are found in dry sub-humid climatic
zones under similar slopes.
The amount of sediment
transported after each rainfall event is a function of climate,
vegetation, topography and soil which can be estimated by
the equation:
S=kq(**m) L (**n)
where: S is the sediment
loss (t ha-1), k is soil erodibility, q is overland flow discharge
per unit width, L is local slope gradient, and m, n, are empirical
exponents to be determined. Except slope gradient, slope length
is also important affecting soil loss due to surface water
runoff. Tillage erosion caused by tillage implements is greatly
affected by slope gradient. As the following equation shows,
soil erosion is propotionally relted to slope gradient. The
flux of soil in the direction of ploughing (Qs, in kg m-1)
per tillage operation can be determined by the equation:
Qs = D*BD*G*B
where, D is the ploughing
depth (m), BD is the bulk density of the soil (kg m-3), G
is slope gradient (tan), and B is coefficient, corresponding
to plough depth D.
|
| Benchmarks
Indication of the values/ranges of value |
- <6 %
- 6-18 %
- 18-35 %
- >35 %
|
| Methods
of measurement |
Slope
gradient can be easily measured (a) by using topographic maps,
and (b) in the field by using a clinimeter or by rough estimation. |
| Limits of the
indicator |
The quality
of the indicator depends on the scale of measurement. |
| Linkages with
other indicators |
Soil
depth, Slope aspect, Rainfall,
Vegetation cover. |
5. Evaluation
of data needs and availability
| Data required
to calculate the indicator |
A topographic
map. |
| Data
sources |
Necessary
data are usually available and accessible and the cost/benefit
ratio is reasonable. |
| Availability
of data from national and international sources |
Data can be
obtained from various regional, national or international institutions
involved in collecting topographic data. |
6. Institutions
that have participated in developing the indicator
| Main
institutions responsible |
Agricultural
University of Athens
|
| Other contributing
organizations |
Universities
of Lisbon, Murcia, Basilicata, Amsterdam, Leeds |
7. Additional
information
| Bibliography
|
Kosmas,
C., Kirkby, M. and Geeson, N. 1999. Manual on: Key indicators
of desertification and mapping environmentally sensitive areas
to desertification. European Commission, Energy, Environment
and Sustainable Development, EUR 18882, 87 p. |
| Other references |
Kirkby, M., & Cox,
N.J., 1995. A climatic index for soil erosion potential (CSEP)
including seasonal and vegetation factors. Catena, 25: 333
- 352.
Kirkby, M., 1998. Modelling
across scales: The Medalus family of models. In: J. Boardman
and D. Favis-Mortlock (Editors), Modelling Soil Erosion by
Water. NATO ASI Series, Vol. 155, pp. 161 - 173.
Kirkby, M.J., Le Bissonais,
Y., Coulthard, T.J., Daroussin, J., & McMahon, M.D., 2000.
The development of Land Quality Indicators for Soil Degradation
by Water Erosion. Agriculture, Ecosystems and Environment,
81: 125 - 136
Kosmas, C., Danalatos,
N.G, and Gerontidis, St. 2000. The effect of land parameters
on vegetation performance and degree of erosion under Mediterranean
conditions. Catena, 40:3-17.
Lindstrom, M.J., Nelson,
W.W, Schumacher, T.E., 1992. Quantifying tillage erosion rates
due to moldboard plowing. Soil and Tillage Research 24, 243-264.
|
| Contacts Name
and address |
Agricultural University
of Athens, Laboratory of Soils and Agricultural Chemistry,
Iera Odos 75, Athens 11855, Greece
Dr Constantinos Kosmas
email: lsos2kok@aua.gr
|
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