rule-based soil erosion modeling in gis
Eva Švandová
Department of geography
Masaryk University Brno
Kotlárská 2
611 37 Brno
Czech Republic
tel.:+420-5-42 128 317
evas@ porthos.geogr.muni.cz
Summary:
This paper deals with evaluation of susceptibility of the cadaster of the commune Kobylí to soil erosion. The rule-based modeling in geographical information systems (GIS) - MGE - is used. Parameters of relief (slope, aspect), soil (texture), geology (character of bedrock) and human utilization parameter (management and landuse) were taken into account. Relief parameters were calculated by using digital terrain model. Rules for classification of parameters and index overlay were based on general knowledge of soil erosion and reflect the conditions of the cadastre. Index map overlay was performed at three stages: 1. susceptibility to soil erosion based on relief parameters and soil-geology parameters, 2. susceptibility based on parameters of potential erosion (both relief and soil-geology), 3. susceptibility based on both potential and human utilization parameters.
Key words: soil erosion, GIS, digital terrain model, rule-based modeling, index map overlay
INTRODUCTION
Soil erosion process effects mainly the agriculture land with intensive utilization. Its extend increased in the last decades due to collectivization being carried out without respect to nature.
New technology of data collection and processing - especially geographic information systems and remote sensing - influence the methods of soil erosion research. The aim of this paper is presentation of the landscape evaluation method executed within GIS. Not only potential erosion is the object of research but also human utilization (mostly the main erosion factors) is taken into account.
SOFTWARE
Data processing was performed within GIS - MGE (Modular GIS Environment). The main modules - MGE/SX (MGE Base Mapper, MGE Base Administrator, MGE Base Nucleus) and advanced ones (MGE Terrain Analyst, MGE Analyst) were used. MicroStation (the graphics environment of MGE), IRASB, IRASC were applied for production of digital thematic maps.
STUDY AREA
Study area - cadastre of the community Kobyli (21 km2) - is located in the Trkmanka catchment in the Middlemoravian Carpathians. Intensive agricultural utilization is distinctive for this catchment - arable land, vineyards and orchards often situated on artificial terraces dominate. High susceptibility to soil erosion is caused also by the character of bedrock - loess, sandstone and claystone prevail.
DEFINITION OF SOIL EROSION
Total erosion is delimited as a function of potential erosion parameters (natural conditions) and human utilization parameters (crop management, erosion control practice). The natural conditions include morphological, soil-geological and climatic situation. Soil-geological are expressed by soil and bedrock susceptibility to erosion, morphological by slope, and aspect express through relief climatic condition. Human utilization parameters include susceptibility of vegetation cover, crop management, slope length and technical arrangements (fig.1).
A = f (K, G, S, E, C )
Fig. 1: Total susceptibility to soil erosion (A) as a function of potential erosion factors and human utilization factors (fig. ).
METHODOLOGY
The rule-based modeling (Meijerink in Solín, Lehotský, 1996) was used for evaluation of susceptibility to soil erosion.
Good knowledge of the relation between landscape components and soil erosion process is the substance of this method. The character of soil erosion process is one of the inputs into the model. Due to lack of the laboratory analysis this character was assessed only on the base of the general assumptions, field survey and the aerial photos interpretation.
Thematic and topographic maps are the other inputs into the model. These were used:
Fig. 2: Structure diagram of rule based modeling for evaluation of susceptibility to soil erosion in GIS.
The map of landuse resulted from the field survey and interpretation of aerial photos.
Following digital maps were constructed:
Digital terrain model was created from contours (equidistance of 2 m). Clearing and complexing of contours was followed by tagging and import to the model. TIN model was chosen for better approximation of terrain and direct transfer of the analysis result into MGE project. Breaklines were automatically inferred to avoid generating of flat triangles.
Two relief parameters were assessed:
This background preparation - digital thematic map creation, field survey, aerial photos interpretation, DTM analyses and recapitulation of theoretic knowledge - was followed by the decision stage. On the base of the preparing stage it was necessary to chose parameters entering the model and to formulate decision rules.
These parameters were chosen:
Decision rules were formulated to determine erosion indexes of each parameter. Erosion indexes are as follows (tab.1).
Tab. 1: Erosion indexes of parameters entering modeling ( *1-the smallest, *4 - the biggest susceptibility to soil erosion)
param. |
class, criterion |
eros. index |
texture |
sand |
tex1 |
loamy sand |
tex2 |
|
sandy loam |
tex2 |
|
loam |
tex3 |
|
clay loam |
tex3 |
|
clay |
tex2 |
|
bedrock |
fluvial, deluvio-fluvial and deluvial sediments |
geo2 |
loess |
geo3 |
|
sandy limestone |
geo1 |
|
sandstone and claystone |
geo2 |
|
slope |
0 - 3o (plateau) |
slo1 |
3 - 6o (moderate slope) |
slo2 |
|
6 - 12o (middle slope) |
slo3 |
|
12o + (distinct slope) |
slo4 |
|
aspect |
N |
asp1 |
NE |
asp1 |
|
E |
asp2 |
|
SE |
asp3 |
|
S |
asp3 |
|
SW |
asp3 |
|
W |
asp2 |
|
NW |
asp1 |
|
human utilization |
mild susceptibility to soil erosion - crop on flat position - nonagricultural utilization |
util1 |
middle susceptibility - permanent crop on terraces - contour cultivation |
util2 |
|
high susceptibility to soil erosion - slope-direction cultivation on middle slope - slope-direction cultivation on long middle steep slopes |
util3 |
|
extremely high susceptibility to soil erosion - vineyards on distinct slope with slope-direction cultivation - arable land on distinct slope |
util4 |
Decision rules are applied on thematic maps and index maps are created:
Index maps enter overlay analyses. Decision rules define weight of influence of each parameter evaluating these overlays.
Index map overlays were performed in three stages:
I. Evaluation of susceptibility to soil erosion from the viewpoint of morphological (A) and soil-geology (B) parameters.
A: slope (slo) + aspect (asp) = relief (rel)
B: texture (tex) + bedrock (geo) = soil-geology (sg)
II. Evaluation of susceptibility to soil erosion from view point of natural condition (potential erosion)
soil-geology (sg) +relief (rel) = natural condition (fg)
III. Evaluation of the total susceptibility to soil erosion based on parameters of potential erosion and human utilization parameters.
natural condition (fg) + human utilization (util) = total erosion (erosion)
Index map overlays were evaluated according to following formula:
I = (x1 . v1) + (x2 . v2)
I - index overlay value
x1 - erosion index of the first parameter
x2 - erosion index of the second parameter
v1 - weight of influence of the first parameter
v2 - weight of influence of the second parameter
Association of weights with each parameter, evaluation of index map overlays and resultant index association is shown in table 2:
Tab. 2: Index map overlay valuation
stage of overlay |
weights of param. |
erosion index combin. |
index overlay value (I) |
result. index |
characterization |
|
I.A |
tex 50% geo50% |
tex1 + geo1 tex1 + geo2 tex2 + geo1 |
2 3 3 |
sg1 |
favorable soil-geological conditions (minimum susceptibility to soil erosion) |
|
tex1 + geo3 tex2 + geo2 tex 3 + geo1 |
4 4 4 |
sg2 |
rather favorable soil-geological conditions (moderate susceptibility to soil erosion) |
|||
tex2 + geo3 tex3 + geo2 |
5 5 |
sg3 |
rather unfavorable soil-geological conditions (middle susceptibility to soil erosion) |
|||
tex 3 + geo3 |
6 |
sg4 |
high unfavorable soil geological conditions (high susceptibility to soil erosion) |
|||
I.B |
slo 66% asp 34% |
slo1 |
rel1 |
very favorable morphological conditions (minimum susceptibility to soil erosion) |
||
slo2 + asp1 slo2 + asp2 |
1,66 2 |
rel2 |
favorable morphological conditions (moderate susceptibility to soil erosion) |
|||
slo2 + asp3 slo3 + asp1 |
2,34 2,32 |
rel3 |
rather unfavorable morphological conditions (middle susceptibility to soil erosion) |
|||
slo3 + asp2 slo4 + asp1 slo3 + asp3 |
2,66 2,98 3 |
rel4 |
unfavorable morphological conditions (high susceptibility so soil erosion) |
|||
slo4 + asp2 slo4 + asp3 |
3,32 3,66 |
rel5 |
high unfavorable morphological conditions (very high susceptibility so soil erosion) |
|||
II. |
rel 70% sg 30% |
rel1+sg1 rel1+sg2 rel1+sg3 rel2+sg1 rel1+sg4 |
1 1,3 1,6 1,7 1,9 |
fg1 |
high favorable natural conditions (no susceptibility to soil erosion) |
|
rel2+sg2 rel2+sg3 rel3+sg1 |
2 2,3 2,4 |
fg2 |
favorable natural conditions (moderate susceptibility to soil erosion) |
|||
rel2+sg4 rel3+sg2 |
2,6 2,7 |
fg3 |
rather unfavorable natural conditions (middle susceptibility to soil erosion) |
|||
rel3+sg3 rel4+sg1 rel3+sg4 rel4+sg2 |
3 3,1 3,3 3,4 |
fg4 |
unfavorable natural conditions (high susceptibility to soil erosion) |
|||
rel4+sg3 rel5+sg1 |
3,7 3,8 |
fg5 |
high unfavorable natural conditions (very high susceptibility to soil erosion) |
|||
rel4+sg4 rel5+sg2 rel5+sg3 rel5+sg4 |
4 4,1 4,4 4,7 |
fg6 |
extremely unfavorable natural conditions (maximum susceptibility to soil erosion)
|
Objective evaluation of the human utilization parameter was impossible and so for the third stage of overlays the weights were not valuated. Only four resulting indexes were determined (table 3).
Tab. 3: Evaluation of index map overlay for total erosion
stage of overlay |
erosion index combination |
resultant index |
characterization |
III. |
h1+FG1, h1+FG2, h1+FG3, h1+FG4, h1+FG5, h1+FG6, h2+FG1, h3+FG1, h4+FG1, h2+FG2 |
erosion1 |
minimum susceptibility to soil erosion |
h2+FG3, h2+FG4, h2+FG5, h3+FG2 |
erosion2 |
moderate to middle susceptibility to soil erosion |
|
h2+FG6, h3+FG3, h3+FG4, h4+FG2 |
erosion3 |
high susceptibility to soil erosion |
|
h3+FG5, h3+FG6, h4+FG3, h4+FG4, h4+FG5, h4+FG6 |
erosion4 |
extremely high susceptibility to soil erosion |
Tools of MGE Analyst were used to perform index map overlays. Topo files were created over index maps entering each overlay and spatial querying followed (table 4). Query results were transferred to graphic and following synthetics maps were created:
Tab. 4: Spatial querying structure
query name |
structure of query |
slo1 |
slope where mini=0 |
slo2 |
slope where mini=3 |
slo3 |
slope where mini=6 or 9 |
slo4 |
slope where mini 9 |
asp1 |
aspect where mini=22 or 292 or 337 |
asp2 |
aspect where mini=67 or 247 |
asp3 |
aspect where mini=112 or 157 or 202 |
tex1 |
texture where erozní_index=1 |
tex2 |
texture where erosion_index=2 |
tex3 |
texture where erosion_index=3 |
geo1 |
bedrock where erosion_index=1 |
geo2 |
bedrock where erosion_index=2 |
geo3 |
bedrock where erosion_index=3 |
rel1 |
slo1 |
rel2 |
(slo2 intersect asp1) union (slo2 intersect asp2) |
rel3 |
(slo2 intersect asp3) union (slo3 intersect asp1) |
rel4 |
(slo3 intersect asp2) union (slo4 intersect asp1) |
rel5 |
(slo3 intersect asp3) union (slo4 intersect asp2) union (slo4 intersect asp3) |
sg1 |
(tex1 intersect geo2) union (tex2 intersect geo1) |
sg2 |
(tex1 intersect geo3) union (tex2 intersect geo2) union (tex3 intersect geo1) |
sg3 |
(tex2 intersect geo3) union (tex3 intersect geo2) |
sg4 |
tex3 intersect geo3 |
FG1 |
(rel1 intersect sg1) union (rel1 intersect sg2) union (rel1 intersect sg3) union (rel1 intersect sg4) union (rel2 intersect sg1) |
FG2 |
(rel2 intersect sg2) union (rel2 intersect sg3) union (rel3 intersect sg1) |
FG3 |
(rel2 intersect sg4) union (rel3 intersect sg2) |
FG4 |
(rel3 intersect sg3) union (rel4 intersect sg1) union (rel3 intersect sg4) union (rel4 intersect sg2) |
FG5 |
(rel4 intersect sg3) union (rel5 intersect sg1) |
FG6 |
(rel4 intersect sg4) union (rel5 intersect sg2) union (rel5 intersect sg3) union (rel5 intersect sg4) |
util1 |
utilization where erosion_index = 1 |
util2 |
utilization where erosion_index = 2 |
util3 |
utilization where erosion_index = 3 |
util4 |
utilization where erosion_index = 4 |
erosion1 |
(util1 intersect FG1) union (util1 intersect FG2) union (util1 intersect FG3) union (util1 intersect FG4) union (util1 intersect FG5) union (util1 intersect FG6) union (util2 intersect FG1) union (util2 intersect FG2) union (util3 intersect FG1) union (util4 intersect FG1) |
erosion2 |
(h2 intersect FG3) union (h2 intersect FG4) union (h2 intersect FG5) union (h3 intersect FG2) |
erosion3 |
(h2 intersect FG6) union (h3 intersect FG3) union (h3 intersect FG4) union (h4 intersect FG2) |
erosion4 |
(h3 intersect FG6) union (h3 intersect FG6) union (h4 intersect FG3) union (h4 intersect FG4) union (h4 intersect FG5) union (h4 intersect FG6) |
SUMMARY AND DISCUSION
Universal soil loss equitation (USLE) (Kirkby, Morgan, 1980) is still the most used method in soil erosion research. It brings the total amount of eroded material (absolute value) on the spot. The method discussed (rule-based modeling of soil assessment using GIS) enables to determine relative values of erosion in the study area.
Map of susceptibility of natural conditions to soil erosion (Map7) shows the spatial variation of factors positively affecting surface runoff and contributing soil erosion process. In this way it determines natural limits which should be respected in the landuse planning.
Map of total susceptibility to soil erosion (Map 8) evaluate human activity in the relationship to natural condition. It points out areas where landuse is in the contrast with the natural landscape character and where the antierosion arrangements should be of the main importance.
The results of the model are applicable in the regional scale. Precision of the model reflects the quality of input data. The better quality and more input data, the better precision of the model.
This study is about to show the method of soil erosion research. Commonly accessible data are processed. To get the better results by using this method I suggest following adjustment of input data:
REFERENCES:
Buzek, L. (1983): Eroze pùdy. Pedagogická fakulta v Ostravì, Ostrava, 257s.