|
1. Definition
| Name |
DRAINAGE
DENSITY |
| Brief definition |
A measure of
the length of stream channel per unit area of drainage basin |
| Unit of measure |
km-¹ |
| Spatial scale |
Watershed |
| Temporal scale |
|
2. Position
within the logical framework DPSIR
3. Target and
political pertinence
| Objective |
The
measurement of drainage density provides a hydrologist or geomorphologist
with a useful numerical measure of landscape dissection and
run-off potential. |
| Importance
with respect to desertification |
Drainage density
is considered to be an important index; it is a measure of the
texture of the network, and indicates the balance between the
erosive power of overland flow and the resistance of surface
soils and rocks. Also, a good estimate of gullies development
can be determined in any similar lithologic formation using
daily rainfall data in any period of time starting from a known
drainage network length. Gully erosion contributes to various
problems including: the extension of badlands area, the loss
of the topsoil, the increase of the drainage network length,
and the accelerated decrease of the cropped lands. |
| International
Conventions and agreements |
The
American hydraulic engineer and hydrologist Robert E. Horton
was the first to establish a quantitative method for analysing
drainage networks. Stream order, developed in the early 1940s,
ranks streams hierarchically. In 1945 Horton developed statistical
"laws" of drainage network composition relating stream
order, number, length, and drainage area. Hortons laws, as they
became known, were subsequently modified and developed, most
notably by the American researchers. |
| Secondary
objectives of the indicator |
Drainage
density is one of the factors describing the drainage basin
morphometry in addition to basin area, length, shape, and relief
attributes. The pattern of natural drainage has been studied
in relation to the drainage density and drainage basin characteristics
which can be quantified and used in rainfall-run-off modelling
and in the interpretation of river discharge; to the nature
of the drainage network including the pattern of the drainage
and also the stream order; and to the evolution of the drainage
pattern. |
4. Methodological
description and basic definitions
| Definitions
and basic concepts |
The drainage density
is the measure of the length of stream channel per unit area
of drainage basin. Mathematically it is expressed as:
drainage density = stream
length / basin area
A drainage network
is a system of interconnected stream channels found in
a drainage basin.
A drainage basin
is a land surface region drained by a length of stream channel.
Closer investigations
of the processes responsible for drainage density variation
have discovered that a number of factors collectively influence
stream density. These factors include climate, topography,
soil infiltration capacity, vegetation, and geology.
|
| Benchmarks
Indication of the values/ranges of value |
Values range
from about 5 km of channel per sq km (8 mi per sq mi) on erosion-resistant,
permeable sandstones, to 500 km per sq km (810 mi per sq mi)
on highly erodible, impermeable clays. Run-off production and
peak flows increase markedly with drainage density.Other investigators
sustain that a highly permeable landscape, with small potential
for runoff, drainage densities are sometimes less than 1 kilometre
per square kilometre. On highly dissected surfaces, densities
of over 500 kilometres per square kilometre are often reported. |
| Methods
of measurement |
When calculating the stream
length one of the first attributes to be quantified is the
hierarchy of stream segments according to an ordering classification
system. In this system, channel segments are ordered numerically
from the headwaters to a point somewhere downstream. Numerical
ordering begins with the tributaries at the headwaters being
assigned the value 1. A stream segment that resulted from
the joining of two 1st order segments is given an order of
2. Two 2nd order streams formed a 3rd order stream, and so
on. Analysis of this data reveals some interesting relationships.
For example, the ratio between the number of stream segments
in one order and the next, called the bifurcation ratio, is
consistently around three. R.E. Horton called this association
the law of stream numbers.
 |
| Order |
No. of segments |
| 1 |
10 |
| 2 |
3 |
| 3 |
1 |
|
Example
of stream ordering and the calculation of bifurcation
ratio |
The current way of measuring
the basin area is through the planimetry of the plane surface
in the map. Because the accurate determination of the basin
depends on the precision of the cartography, and in order
not to omit the small streams that would change not only the
length but the surface as well, the recommended scale would
be around 1:10.000 and 1:50.000.
|
| Limits
of the indicator |
The
indicator lacks of homogeneity because it is not easy to establish
the order of streams for which information is to be provided.
It depends completely on the scale of the map on which the length
of the stream is calculated. It is possible that some of the
smaller streams might be ignored due to a lack of accuracy in
the maps. Therefore, it is very difficult to make comparisons
between different basins unless the working scale is the same.
|
| Linkages
with other indicators |
Infiltration
capacity, Rainfall run-off relationship, Rainfall
erosivity, Soil erosion
|
5. Evaluation
of data needs and availability
| Data
required to calculate the indicator |
Complete
scheme of the basin water. It is important to establish the
scale of the map. |
| Data sources |
The accurate
data needed could be provided by the River Basin Authorities,
and also the National Hydrological Plan in its different applications
for the various European basins. However, a good cartographic
background and support is enough for a basic calculation. |
| Availability
of data from national and international sources |
National
Cartographic Services. River Basin Authorities. |
6. Institutions
that have participated in developing the indicator
| Main institutions
responsible |
Dirección
General para la Biodiversidad. Ministerio de Medio Ambiente.
Spain. |
| Other contributing
organizations |
|
7. Additional
information
| Bibliography
|
LOPEZ CADENAS DE LLANO,F.
y MINTEGUI AGUIRRE J.A. (1986) Hidrología de Superficie,
Tomo I. Fundación Conde del Valle del Salazar, pp.399-405
PARACCHINI M.L. et al.
2004 Development of a pan-European database of rivers and
catchments: a GIS application in support to European water
monitoring activities. From " Workshop on 'Identification
of the current status and needs of GIS and databases technology
in the agricultural sector sector - GIS for analysis and monitoring
of land use and land/water quality'. On-Line: www.proland.iung.pulawy.pl/Materials/WP1/Paracchini.doc
FELFOUL M.S. et al. (1999)
Assessment of the influence of the lithology and rainfall
events on gully erosion in Oued Maiez - Watershed in central
Tunisia. From 2ND INTER-REGIONAL CONFERENCE ON ENVIRONMENT-WATER
99. On-Line: www.wca-infonet.org/cds_static/assessment_influence_lithology_rainfall_events__9377_25893.html
Fundamentals of Physical
Geography. Chapter 10: Introduction to the Lithosphere- Stream
Morphometry". 2004. Coordinator: Dr. Michael Pidwirny,
Department of Geography, Okanagan University College. On-Line:
www.geog.ouc.bc.ca/physgeog/contents/chapter11.html
BURNETTE, L.(2002) Effects
of Lithology on Elevation, Slope, and Drainage Density. Spatial
Analysis for Resource Management - Division of Resource Management
Davis College of Agriculture, Forestry and Consumer Sciences
West Virginia University. On-Line: www.nrac.wvu.edu/rm493-591/fall2002/students/Burnette/index.htm
|
| Other references |
Plan Hidrológico
Nacional. España. Ministerio de Medio Ambiente. Hydrological
Plans for the main Spanish Basins |
| Contacts
Name and address |
Leopoldo
Rojo Serrano
Dirección General para la Biodiversidad
Ministerio de Medio Ambiente
Gran Vía de San Francisco
428005 Madrid (Spain)
email: <lrojo@mma.es> |
|
