APPLIED WATERSHED MANAGEMENT (LWR 406)
MODULE NOTES
By DR N. M. ZHOU
INTRODUCTION
This is a core module for the B Sc Honours degree in Land and Water Resources
Management. Delivery of this module will be based on presentation of a series of formal
lectures, practicals and seminars during a semester. Assessment will be based on
coursework (30%) and final end of semester examination (70%) of three hours paper
usually with 5 questions to choose four. It is advisable to attend all lectures in order to
benefit fully from discussions that arise from pertinent issues in watershed management.
This module draws much from field experience in watershed and environmental
management and from research.
WHAT ARE YOU EXPECTED TO KNOW?
1. An understanding of the concept of applied watershed management and why it is
important
2. What techniques are available for assessing environmental potential is semi arid areas?
3. The importance of vegetation in environmental management.
4. The concept of environmental gradients and their significance in applied watershed
and environmental management in semi arid areas
5. The concept of ecosystems and their management.
6. The major types of land degradation, as well as some idea of the nature and extent of
land degradation; e.g. how spatially continuous is one type, or how dynamic is another?
7. Soil degradation management including an appreciation of soils data showing degrees
of degradation
8. Field evidence of land degradation: what would you look out for?
9. How might land degradation be avoided in environmental management.
10. What approaches can be designed for sustainable management in watersheds by local
population
These and other points will be dealt with during the lectures, but within the time limit of a
lecture only cursory attention will be given to some.
REFERENCES
Apart from standard texts such as N. Hudson as well as the several books looking at soil
erosion, there are very few works on soil degradation that are available.
Much of the lectures will centre around land degradation work in Zimbabwe such as by
Elwell and Stocking and Whitlow as well as FAO’s work on soil degradation.
Unfortunately, the references are not generally available (but they may be borrowed from
me). You are therefore encouraged to consult Environmental Management Agency
Libraries in Harare or provinces where they may likely have some of these references.
FAO 1978 Methodology for Assessing Soil Degradation. Report on FAO/UNEP Expert
Consultation. Rome
Rafiq, M. 1978. The Present Situation and Potential Hazard of Soil Degradation in Ten
Countries of the Near East Region. FAO Regional Officer for the Near, East, Cairo
FAO 1979. A Provisional Methodology for Soil Degradation Assessment. FAO, Rome
Haynes R. (ed) (1982). Environmental Science Methods Chapman and Hill, Cambridge
Dent, D and A. Young (1980). Soil Survey and Land Evaluation, George Allen and
Unwin, London
Zhou, N.M. (2006) Land system based land degradation monitoring for environmental
management in Zimbabwe. Paper presented at the Geographic Biannual Conference, UZ
THE CONCEPT OF APPLIED WATERSHED MANAGFEMENT
The historical concept of watershed management is exclusively related to the erosion
process, their physical effects on land, vegetation and agriculture and t he consequent
economic, social and human welfare aspects of the erosion problem. Early American soil
conservationists regarded erosion as the major cause for rendering watersheds
unproductive and they pleaded for the preservation of the soil.
Nowadays, the concept of watershed management has expanded to include:
(i) Assessment of environmental quality and potential
(ii) Protection of soil against physical, chemical and biological degradation and
(iii) A combination or deterioration by natural or accelerated means.
The use of the term applied watershed management therefore includes the preservation of
the remaining good watershed qualities and the sustainable restoration of these qualities
where they have been destroyed by man-made and/or natural forces.
What does watershed management involve and why are we interested in it?
Our present production or life supporting or land use systems are disastrous to the
environment. They cause a number of environmental problems related to anthropogenic,
technogenous systems or both
Some examples:
Continuous cultivation
Intensive agricultural systems, use of chemicals
Irrigation
Deforestation, bush clearing for land preparation
Use of fire in land management
Gold panning
Water development, dams
Etc
Major dimensions of concern
Spatial, Temporal, Technocratical
Certainly, it has been noticed that the expansion of population, agriculture and industrial
development have caused natural resources to diminish at a fast rate ultimately leading to
environmental degradation. As a result the depletion of the watersheds and resources
have started to affect us in one-way or another.
For example the land use/cultivation intensities in many watershed in communal areas
has risen to unacceptable and unsustainable levels that cause irreparable damage on the
natural resource.
R = C/ (C+F+L) x 100
Where R = Cultivation factor
C = Years of cultivation
F = Years of fallow
L = Years of lay or other non-arable use
R smaller than 30% (Shifting cultivation)
30% greater than R smaller than 70% (semi-permanent cultivation)
R greater than 70% (Permanent cultivation)
In rangelands, livestock intensities exceed the carrying capacities of the areas
We are interested in watershed management to reduce the “debt” our future generation
will inherit because our livelihoods are almost entirely dependent on the resources around
there. We want to reduce land degradation, soil erosion, deforestation and other forms of
desertification.
We are interested in the application of environmental management principles to improve
the productive potential of watershed environments in a sustainable way. These views
have earlier been expressed by many countries of the world in Stockholm (Sweden) at a
United Nations (UN) conference on Human and Environment in 1972. The same views
were expressed more recently in 1990 again by many countries of the world at a United
Nations Conference on Environment and Development (UNCED) called the Rio Summit
WHAT IS A WATERSHED?
A watershed has been defined as an area draining rainwater into a stream. It is a small
catchment from which precipitation, rainfall as well as snowfall flow into a single stream.
It forms naturally to dispose off the runoff of rain as efficiently as possible (Murty,
1995). A watershed is synonymous to catchments or basin of a river. Rainfall carves or
shapes each watershed depending on the prevailing environmental qualities and
characteristics such as vegetation, geology, soils and land use. These qualities and
characteristics are responsible for heterogeneity in the watershed. An assessment of these
environmental variables helps in the understanding of variability in each watershed as
well as various ways by which each (watershed) can be managed.
A watershed is natural scientific unit. It provides a limited surface area within which
physical processes pertinent to hydrology, soil morphology, vegetation distribution and
other changes could be appreciated.
Characteristics of watersheds
Each watershed shows distinct characteristics, which are so much variable that two
watersheds are identical. Al the characteristics affect the disposal of water. Several
characteristics namely size, shape, slope, drainage, vegetation, geology, soil, climate and
land use are considered as important. A few more can be added in consideration to their
role in disposing the rainwater into a stream and managing a watershed.
Size
The size of a watershed forms a basis for further classification categories. Bali (1980)
gave the nomenclature Sub-Watershed (100-500sq-km), Milli-Watershed (10-100 sq.
km), Micro-Watershed (1-10sq. km) and Mini-Watershed (Less than sq. km). The size
helps in computing many parameters like precipitation received, retained and drained off.
Larger the watershed, more the heterogeneity of the other characteristics.
Shape
Watersheds differ in their shape based on morphometric parameters like geology and
structure. The general shapes are pear, elongated, triangular, circular etc. Shape
determines the length –width ratio, which affects the runoff characteristics like runofftime.
Physiography
Type of land, its altitude and physical disposition immensely speak about a watershed as
to the climate and planning the activities in greening. For example a hilly tract could be
useful mainly for forestry and plains of populated areas could be utilised only for crops.
Slope
It controls the rainfall distribution and movement, land utilisation and watershed
behaviour. The degree of slope affects the velocity of overland flow and runoff,
infiltration rate and thus soil transportation.
Climate Meteorological parameter like precipitation, temperature, wind velocity,
humidity and evaporation decide a quantitative approach for arriving at water availability
in a watershed. Climate is a determining factor for the management of all aspects of
watershed. For example the entire planning of greenery depends on climate.
Drainage
The order, pattern and density of drainage have a profound influence on watershed as to
runoff, infiltration, land management etc. It determines the flow characteristics and Eros
ional behaviour.
Land Use
Land use pattern is vital for planning, programming and implementing a management
project on a watershed. I t is an important statistic for ascertaining the background,
appreciating the status and planning the programmes in management. It portrays man’s
impact on the specific watershed and forms a basis for categorising the land for
formulation of a pragmatically action plan.
Vegetation
Detailed information on vegetation helps in choosing type, mode and manner of greening
the watershed. Information on local species gives a sure ground for selecting plants and
crops. It confirms authoritatively what greenery can be grown where. With care, soil
capabilities could be analysed, compared and profitably confirmed for management.
Geology and soils
Rock and their structure control the formation of a watershed itself because their nature
determines size, shape, physiography, drainage and groundwater conditions. Soils,
derivate of rocks, are basic to greening. Soil parameters as to depth, nature, moisture and
fertility determine crops. Rocks and soils, together influence water storage, movement
and infiltration.
Hydrology
Availability, quality and distribution of surface water is basic to the final goal of growing
greenery in a watershed. Hydrological parameters help in quantification of water
available, utilised and the additional exploitable resources for greening the area. They
determine the location and design of conservation structures.
Hydrogeology
The demand for groundwater is ever on the increase. As such the appreciation of
groundwater resources for determining their further availability in the context of
conjunctive use of water resources for greening the specific watershed is a logical
prelude. The information should not only include nature, thickness and characteristics of
aquifers but also contain quantity available for additional exploitation through specific
number of wells.
Socio-economics
Statistics on people and their health, hygiene, wants, wishes cattle and farming practices
and share of participation are equally important in managing a watershed.
Why are we interested in these factors?
We are interested in the environmental characteristics because they influence populations
and processes in the watershed which in turn affects environmental potential..
These factors interact with one another and a dominant integrator as well as a sensitive
indicator of all these factors is vegetation because it (vegetation) experiences conditions
(joint effects) provided by these variables. To assess the impact of environmental
relationships in species – environmental relationships is indispensable. Ecologists attempt
to uncover the various relationships between plants and environmental from data
collected from field surveys in order to come up with appropriate knowledge and
techniques that can be used to dertemine environmental potential and manage it in a
sustainable way
.
TECHNIQUES FOR ENVIRONMENTAL MANAGEMENT
(i) Lands systems approach
(ii) Land capability classification..
(iii) Land suitability classification for irrigation
Land systems concept for environmental planning and management in watersheds
The need for a system with a good environmental basis leads to development of
techniques for environmental assessment and management. At the moment in Zimbabwe
and perhaps the re4st of the SADC region, techniques for doing this are not yet clearly
developed. The use of the agro ecological classification as the basis for environmental
planning that is still in use is technically and environmentally inadequate because of their
restrictive nature.
Environmental resources inventories are intended to provide a broad and integrated view
of the land resources of an area, to serve as a guide for planning and development, as
framework for environmental monitoring management and for more detailed surveys.
They are by no means confirmed to single environmental factors or characteristics such
as soils, vegetation, climate e.t.c. but cover the whole environment to the extent that it
affects environmental potential.
In order to conduct and present results of environmental surveys a concept that can be
used for this purpose to come up with ecological land classification with a universal
application is the land systems or integrated approach, meaning that factors of the
physical environment are assessed and mapped simultaneously, can be used. Although
the approach has been used in empty and sparsely settled areas of developing countries in
the past, however, to environmental planners they have (land systems) shown different
types of environmental resources or problems present, their location and extent. In local
environmental action planning, however, the nature of environmental resources and
problems will probably be known already by communities locally but their extent or
dimension is not.
The major purpose of land systems assessment will adequately be served only if as much
effort is devoted to evaluation of environmental factors as to their mapping, description
and interpretation.
Process of air photo interpretation
Air photo interpretation is an initial step in land systems surveys. The process of air photo
interpretation involves:
(a) Detection
(b) Recognition and identification
(c) Analysis or delineation of objects or areas
(d) Classification
(e) Idealization
From the air photographs and mosaics (or x2 enlargements) units of landscape are
identified within which there is a similar pattern of environmental factors such as
topography, drainage, vegetation or land use. These units are known as land systems. A
land system can therefore be defined as an area with a recurring pattern of topography,
soils and vegetation, and with a relatively uniform climate. Land systems may be of any
size and each has a unique combination of features observable on the photographs or
mosaics and even topographic maps. Quite often land systems are distinguished primarily
on topographic characteristics such as flood plain, escarpment, hills, foothills, river valley
e.t.c. but vegetation patterns become more significant in areas of low relief and
depositional landforms.
As a result of the reliance on topographic features and characteristics topographic maps
can sometimes be used for analysis and as base maps in place of photographs and
mosaics depending on the experience of the planner or surveyor.
Within each land system, smallest areas that can be recognised as homogeneous from the
point of view of features shown on photographs or topographic maps. These areas are
called land facets. Land facets within a land system are not a random collection of
contiguous areas but are often linked by geomorphological process e.g. ground water
movement. Examples of land facts are slope facets that can be identified along a criteria.
The photo interpretation phase therefore involves mapping of landforms and vegetation
in their own right. The identification and delineation of land systems and facets is nearly
always done by judgement and their description is based on photo and/or map
interpretation on qualitative terms e.g. gentle slopping, broad interfluves and ridge crests,
close-dendritic drainage without preferred orientation etc.
Fieldwork
Fieldwork is carried in catenary’s transverses planned in advance to visit all or most of
the land systems identified by photo or topographic map interpretation. A
multidisciplinary team together with local communities normally does this.
Production of map and legend.
The output of land system assessment is a map/or derived maps of environmental factors,
their interpretations and evaluations.
Advantages and disadvantage of the land systems approach in environmental planning,
and monitoring
Advantages
(a) Speed and relative cheapness
(b) Integration of different factors of environment and multidisplinery
(c) Clear communication of results
(d) High correlation with community perspectives
(e) An ecological land classification
(f) Versatile and can be used for environmental monitoring and management
(g) Can be used as a framework for participatory planning and environmental monitoring
Disadvantages
(h) High degree of generalisation
(i) Variable and somewhat ill defined mapping units
(j) Static nature of information presented
(k) Weakness of evaluation stage
(l) Requires experienced surveyors
Land systems assessments are very effective when using photo and or map scales of 1: 25
000 to 1: 100 000 where rapid coverage of large areas is required. It can also be applied
to satellite imagery e.g. LANDSAT. Although based on air photography: one of the least
sophisticated forms of remote sensing, land system or slope facet maps provide
considerable promise to supplement and guide planning, management and monitoring of
environmental resources.
VEGETATION IN APPLIED WATERSHED MANAGEMENT
Applied watershed management and its biomass-oriented activities have a significant role
in restoring a good environment. A deeper understanding of the qualities and
characteristics that affect watersheds as well as relationships among them is needed to be
able to manage them.
In this section, we will look at:
- the importance of vegetation or forests in watershed management
- determinants of vegetation / factors that cause vegetation variability and
- how this vegetation variability can be used for land management
Why vegetation in applied watershed management?
It is generally agreed that vegetation plays a major role in ameliorating environmental
problems by
- arresting soil erosion
- controlling floods
- storing ground water
- modifying temperatures
- slowing down build up of carbon dioxide
- makes available energy and conserves it
- providing basic needs
Vegetation rectifies negative effects in the environment by maintaining a good balance
What causes vegetation changes in the watershed
Determinant analysis of vegetation types
- Climate e.g. rainfall regimes micro and macro
- Topography e.g. the catena concept and environmental gradients
- Soils e.g. soil moisture, nutrient, pH, etc regimes
- Fire climax vegetation
- Herb ivory, from insects to elephant
- Humans, land use
These factors occur as interactions. None of the factors work alone.
Because of the, there is high vegetation spatial variability. If we can detect this
variability, then we will be halfway towards sustainable management.
In what ways can this variability be useful in watershed management?
- veld planning and management
- soil conservation
- fire control
- biodiversity conservation
- site condition assessment and management
- research and development
Many techniques can be used to assess this variability. We will deal with these in the next
sections, but within the time limit only cursory attention will be given to some, the less
common used ones.
What are savannas?
These are typical vegetation physiognomic types in which ecological processes such as
primary production, hydrology and nutrient cycling are strongly influenced by woody
plants and grasses.
Because of variety within savanna structure, we usually classify and describe vegetation
according to type of wood vegetation and how much of each physiognomic type is preset.
Research in granitic soils with the semi-arid savanna vegetation type has shown that
vegetation and species composition are influenced by erosion, drainage, sodicity and clay
content.
Savannas occupy part of a continuum of vegetation types found in any watershed with
varying proportions of woody plants and grasses.
Arid savanna Vegetation
Watersheds in arid and semi-arid tropics are characterised by tropical vegetation types
which range from pure grasslands to woodlands. Savanna types cover over 65% of these.
THE CONCEPT OF ENVIRONMENTAL GRADIENTS IN APPLIED WATERSHED
MANAGEMENT
In this unit we are interested in environmental gradients as they help to understand more
about factors of the environment, how they are related, what they show and how we can
approach management of the watershed. We will look at
- Environmental gradients, what they are,
- Important characteristics of watersheds
- And how we can manage them
What is an environmental gradient?
An environmental gradient is a term that is used to describe a range in environmental
conditions that affect distributions of plant species and communities in a particular area.
Plant species occur in a limited range of habitats. They are usually most abundant where
environmental conditions are optimum. Thus species composition changes along
environmental gradients and the significance of these is realised in the explanation of the
distribution of vegetation types and species in space and time. The study of vegetationenvironmental relations along gradients have therefore formed an important basis for
applied watershed management.
There are two types of environmental gradients namely direct and indirect gradients.
Direct Gradients: those that impose a direct influence on vegetation growth e.g. soil pH,
nutrients, soil moisture, temperature etc.
Indirect Gradients: those that do not impose a direct influence on vegetation growth e.g.
topography, slope altitude etc.
Gradient analysis thus includes both direct gradient analysis in which vegetation species
abundance (or probability of occurrence) is described as a function of measured
environmental variables; and indirect gradient analysis in which vegetation species
(samples) are displayed along axes of variation that can subsequently be interpreted in
terms of environmental gradients.
Methods for gradient analyses relate species abundance to environmental variables on the
basis of (species and/or environmental) data form the same set of sample plots.
Examples of common environmental gradients in semi-arid watersheds
Uplands Lowlands
Well drained (drainage gradient) Poorly drained
Light texture (texture gradient) Heavy texture
Low AWC (soil moisture gradient) High AWC
Acidic (pH gradient) Basic
Environmental gradients (also referred as) ecological gradients are important in land and
soil management because:
1. They give an indication of land and soil management problems that are likely to be
found at a site e.g. drainage, sodicity, acidity, drought etc.
2. They provide an indication of land potential for a specified land use e.g. potential for
wetland rice, tobacco etc.
3. They serve as a preliminary classification of areas
4. Decision-making – what to do, where?
5. Cultivation systems e.g. continuous, rest/fallow, rotation
6. Identification of suitability suitability e.g. for forestry, crops, grazing (similar to 2)
1 Uplands (granitic)
(i) Mainly covered by miombo vegetation composed of Brachystegia spiciformis and
Julbemadia globiflora with root systems capable of mining water and nutrients from
deeper layers of the soil.
(ii) Soils are mainly coarse to medium grained, well drained and infertile sandy soils of
66 soil family. They have low % clay, %BS and CEC 0.6-11 g c mol c/kg. They are
acidic (pH values 4.3 -5.8), non-sodic (ESP values <4). Aeration is food (no mottles) and
AWC is in the range 15-25% (see soil water retention curves)
(iii) The major land management problems associated with these watershed sites are
mainly acidity due to rapid leaching, poor soil fertility and erosion. Continuous
cultivation leads to soil degradation and low yields.
(iv) Land management in these areas should include liming and fertilization. Dolomitic
lime, Aluminium and manganese and copper toxicities, which are common at pH<4.7. On
liming H-ion is replaced by metallic cations. Liming materials are oxides and hydroxides
of carbonates of calcium and magnesium.
Benefits of Lime
· H + decreases and OH - increases
· Solubility of Aluminium and Iron decrease
· Availability of phosphorus and Molybedenum increases
· Exchangeable Ca and Mg increases
· %BS increases
· biological activity and soil structure improves by flocculation and formation of
aggregates
· In leguminous plants, nodule formation is promoted
2 DRY LOWLAND AREAS
(i) In Zimbabwe, soils with high ESP>9 within the first 80cm depth have been classified
as sodic soils. The soils are dry, compact, alkaline and poorly drained. These soil are
derived from granite rich plagioclase feldspar rich in sodium releasing mineral called
albite. In these soils under wet conditions, clay particles repel one another (deflocculate)
which impedes water flow, but promotes clay movement. In conjunction with the
differentiable mobility of sodium and calcium in the soil, this mechanism is responsible
for occurrence of sodic soils in dry savanna landscapes. Due to clay movement, erosion
of surface layer occurs leading to bare ground devoid of vegetation matter.
(ii) The commonest vegetation types in these areas are bush savannah or bush scrub
associated with shallow rooted trees and grasses with root systems that coincide with
zone of maximum moisture retention vegetation species such as mopane, Acacia and
certain Combretums that can withstand repeated root pruning in the soil are able to
survive in these harsh conditions where other species die.
(iii) The major soil management problems in these areas are sodicity and salinity
Classification of salt –affected soils
SOILS Ecw saturation extract (m s/cm) ESP pH Description
Saline soils >4 <15 <8.5 Sufficient soluble salts to interfere with crop growth
Saline –sodic soils >4 >15 <8.5 Sufficient ESP plus an appreciable amount of soluble
salts to interfere with growth of most crops. Unstable soil structure.
Saline –sodic soils >4 >15 (>9 for Zimbabwe) <8.5 Sufficient ESP but without soluble
salts to cause harm to plants. Unstable soil structure.
(a) ESP = exchangeable Na x 100/
CE
(b) ECW = Salinity (measured in the field using a portable conductivity meter)
OR
= Salinity (interpretations of crop response based on electrical conductivity of saturation
extract – a soil solution obtained from saturated soil paste).
= Total soluble salts (Na+, CI and So2 4
(c) SAR = Na/(Ca+Mg)/2
Plant growth is impired by toxicity of ions (high EC) in solution mostly Na +.CI and
SO2-4. Sodicity causes unstable soil structure which inhibits drainage and leaching.
These effects are high in smectites than is kandite clays.
In addition, soils are hard and compact leading to poor rooting conditions and poor
workability when wet.
Accumulation of salts in the soil lead to ‘White’ alkalis mainly sodium chloride, sodium
sulphide or sodium nitrate. Accumulation of organics matter leading to ‘black alkalis’
mainly carbonate and bicarbonate of soda.
The high concentration of these alkalis causes toxicity and poor plant growth.
(iv) Management of Reclamation
Reclamation of this soil involves removal or reduction of the ESP.
(a) Physical Methods
These involve removal of alkali by scrapping using machinery followed by flooding and
draining. This method can be expensive and therefore any suitable for research, small
fields and gardens.
(b) Chemical Methods
These involve conversion of salts to less injuries forms by addition of beneficial ion of
calcium from gypsum.
For example:
*Ca. Mg SO4 + 2 Na [micelle] Ca[micelle] + Na2 SO4 leached
*Ground sulphur but much slowly because S2 combines with oxidized and combined
with H2O to form H2SO4
*Other soluble sulphates of A1 and Fe can be used but these are more expensive
(c) Biological Methods
These involve use of bioremedial plants e.g. triplex species can be grown on sodic soils.
These can accumulate sodium in their structures such as leaves and stems. Up to
0.5t/ha/yr of sodium Ca be removed when plant is harvested. These slowly reduce
sodicity. These methods are only suitable for experimental purposes.
3 WETLAND AREAS/WETLANDS (VLEIS/OR DAMBOS)
(i) These are seasonally waterlogged bottom lands or vleis. In Zimbabwe, they cover
approximately 2.8 million ha(about 3% of total area of country of which 25% are found
in communal areas)
(ii) What are the functions of wetlands?
(iii) The formation of wetlands has occurred under conditions of clay and organic matter
eluviations with the formation of a dark surface soil rich in organic matter and nutrients.
Due to the imperious clay layer infiltration and drainage are reduced. As a result the soils
that are predominant are classified as hydromophic (5G) soils which are seasonally
waterlogged, with high pH and anaerobic conditions (low Redox / Eh potential) in the
rooting from the fowl smell produced) which cause sudden wilting and mortality of
plants.
(iv) How are these wetlands destroyed (see fact sheet)
(v) The most common land management problems include drainage, inundation and
sulphide toxicity/
(vi) Wetlands management
There are many facets of wetland management and some of these include
· Water management
· Improved soil conservation and drainage system
· Improved soil technology – adoption e.g. bunds, raised beds, broad based broad furrow
yields increase 3t/ha rice 7t/ha maize
· Community participation management community identify own needs and problems
and then become actively involved
· Identification of the wetlands
· Legalise wetland cultivation
Present Water Act and EMA do not provide adequate framework for effective and
sustainable uses of wetlands.
· Research in wetlands
SOIL DEGRADATION AND CONSERVATION MEASURES IN SEMI-ARID
WATERSHEDS.
Deforestation, overgrazing and inadequate land management techniques have led to
widespread soil degradation watersheds have largely remained unprotected. Soil
degradation in all its many forms is a critical constraint both to existing agriculture and
any future developments. We shall look at the forms soil degradation takes, a
classification of processes, and methods of detecting soil degradation.
Definitions of Soil Degradation
(as used by FAO in their Soil Degradation Assessment)
Soil degradation is a process that lowers the current and/or the potential capability of a
soil to produce (quantitatively or qualitatively) goods or services. Soil degradation is not
necessarily continuous. It may take place over just a short period between two states of
ecological equilibrium.
Processes of soil degradation
FAO distinguish six, although many overlap and interact. Note that desertification will
include many of the processes together:
(i) Water erosions subdivided into
· Sheet and rill erosion
· Gully erosion
· Various types of mass movements e.g. landslides, mudflows
(ii) Wind erosion
(iii) Excess of salts
· Salinization
· Alkalinization
(iv) Chemical degradation
· Leaching of nutritive elements and acidification
· Toxicities other than excess of salt
(v) Physical degradation
· Loss of structure
· Sealing and crusting of soil surface
· Reduction impermeability
· Compaction in-depth
· Decrease of macroporosity
· Limitations to rooting
(vi) Biological degradation
· Mineralization of humus
· Increase in activity of micro-organisms responsible for organic decay
· Decrease in CO2 release if organic decay and humification does not compensate for the
mineralization of humus
LIST OF SOIL DEGRADATION CLASSES
Water erosion (E)None to slight ModerateHigh Very high Soil losst/ha/year<1010-5050200>200 or mm/year<0.60.6-3.33.3-13.3>13.3
Wind erosion (W)Class limits for soil as for water erosion.
Excess of salts (S) Sanilization (Sa) Increase in conductivity 0-60cm layermmho/cm2
/year Alkalinization (Sa) Increase in ESP 0-60 cm layer percent/year
none to slight moderatehighvery high <11-33-5>5 <11-33-7>7
Chemical degradation (P) Decrease of pH 0-30cm layer pH units/year Decrease of base
saturation 0-30cm layer percent/year
None to slightModerateHighVery high <0.10.05 –0.20.2 – 0.5>0.5 <2.52.5-55-10>10
Physical degradation (D) Increase in apparent density 0-60cm layer g/cm³/year Decrease
in total porosity 0-60cm layerpercent/year Decrease in permeability 0-60cm layer
cm/h/year
None to slightModerateHighVery high <0.10.1–0.20.2 – 0.3>0.3 <11-33-5>5 <0.50.5 –
55 – 20>20
Biological degradation (B) Decrease in organic matter 0-30cm layerpercent/year
None to slightModerateHighVery high <11-1010-20>20
Classification of soil degradation
Soil erosion as one example of soil degradation can be defined as a process whereby
removal of soil surface materials by water or wind take place through detachment,
transportation and deposition. Many factors which include soil erodibility, rainfall
erosivity, vegetation type and management cause these processes to take place either
naturally or man/animal induced.
Soil degradation is prevalent everywhere, nowhere more so that in semi-arid areas. In
Zimbabwe up to 500t/ha/yr has been reported to be lost from farmland through soil
erosion. Accelerating degradation makes it all worse.
FAO classifies soil degradation into three categories
(1) Physical
This includes soil erosion, sheet wash gulling, deposition and siltation.
(2) Biological
This includes loss of biodiversity, weed infestation, bush encroachment and
deforestation.
(3) Chemical
Included under this category are processes that of acidification and leaching, salinisation,
sodification eutrophication and chemical toxicities
Impact of soil degradation
Soil erosion does not only cause loss of soil and water but also nutrients as well.
(i) Soil and Productivity Loss
Eroded sediments contain about 2.5 times more nutrients and fertile particles than soil
from which it formed
Table: Soil and nutrients loss from a red silty soil in Zimbabwe
Soil Erosion t/ha/yr Loss of N (K/ha) Loss of P (kg/ha)
5 9 0.6
15 26 1.8
50 84 5.5
75 124 8.1
Symptoms of soil degradation
1. Loss of nutrients
Table: Soil and Nutrient Loss from a Zimbabwean red silty soil
Soil Erosion t/ha/yr Loss of N (K/ha) Loss of P (kg/ha)
5 9 0.6
15 26 1.8
50 84 5.5
75 124 8.1
2. Yield decline
There is a negative exponential relationship between erosion (t/ha/yr) and yield (t/ha)
There are large incremental losses in yield due to a small increase in erosion
3. Gullies
These make land preparation difficult and interfere with normal drainage and stream
flow.
4. Loss of Rain Water (due to run off)
This is very significant in serious arid areas. Runoff accounts for about 20% of annual
rainfall in Zimbabwe
5. Degraded soil particles provide less available water to plant. For typical erosion rates
in Zimbabwe e.g. 10t/ha/yr – this equivalent to moving a farm from a 650mm rainfall
zone to 450mm.
6. Depositional effects
Eroded soil deposited elsewhere causes problems such as siltation of rivers, dams,
irrigation channels or even cause mad flows in severe cases.
7. Eutrophication
Poisoning of water supplies by runoff washing pesticides, herbicides, fertilisers,
industrial wastes in rivers, lakes and dams.
8. The financial cost of degradation is potentially ruinous e.g. 50t/ha soil loss contains
90kg/ha N, 8kg/ha p and 1400kg/ha organic matter lost (Zimbabwe data)
DIMENSION OF THE SOIL DEGRADATION PROBLEM
The primary symptoms of the problem of land degradation in Zimbabwe and causes have
already been documented. Possible causes are related to increasing population pressure
on land resources. The result has been soil loss and extensive deforestation brought about
mainly as a result of clearance of land for arable farming
and stream bank cultivation, irrational land use, overgrazing and periodic droughts.
Soil Erosion Estimation
There is a general paucity in information on recent erosion research in Zimbabwe.
Concerns about erosion risk and hazards particularly in communal and resettlement areas
are outstanding and have expressed themselves in previous studies on the nature and
distribution of erosion and on erosion hazard assessment from an analysis of factors that
control erosion processes (Stocking and Elwell, 1973; Whitlow, 1978). Quantitative
assessments of erosion for example using USLE or SLEMSA have been done in the past,
however, these are open to criticism and therefore require re assessment( Anderson et al,
1993). Small scale air photography that is widely available is not suitable for identifying,
by stereo examination, the presence of erosion features other than gullies. The degree of
erosion is therefore more meaningfully estimated visually at observation points on an
ordinal scale (Table 1) as used by field officers in the Department of Research and
Extension (AREX)
Table 1: Ordinal scale estimation of soil erosion intensity
Type of erosion Class Description
Sheet erosion 1 No apparent sheet erosion
2 Slight erosion
3 Moderate erosion
4 Severe erosion
Gully erosion 1 No gully erosion
2 Slight
3 Moderate
4 Severe
(Modified from Ivy, 1978)
Methods of erosion in watersheds
Methods
Mechanical controls Biological controls Cultural controls
Contour ridges TerracesPlough along contourNets across gulliesGabions FurrowsBunds
Strip croppingGrass stripsPlanting vetiverOrganic mulchesCover cropsAgro
forestryHedgingReafforestation (Combinations)Conservation tillageMulch rippingCrop
rotationsLand rotationsBush controlFire control
Other techniques
1. Integrated methods/Approaches or strategies
2. Indigenous methods
Indigenous methods of controlling soil degradation and erosion.
Western trained observers would seem to see behaviour of small scale subsistence
farmers in Africa is irrational. But a careful study will reveal a complex rationality
behind each spectacular practice.
Such practices have evolved and developed over many years and are logical and sensible
to the smaller farmer.
Major features of farming household in Zambia
1. Small scale subsistence traditional
89%
Farms size 0.25 –5 sulphur, millet maize, cassava
2. Small scale commercial
10%
0.25-5 maize, millet, sorghum, cassava
3. Medium scale commercial
0.75%
10-40 maize
4. Larger scale commercial
0.06%
40ha + maize + poles
Measures have been launched to control soil erosion
Soil erosion + soil degradation through destocking e.g. cattle in the Wasakuma of
Tanzania – failed
*An aerial song if Shinyanga region in November 1981 indicated that the true number of
cattle was more than 2 x the official population
Mixed Cropping
This is traditional agriculture strategy – mixed cropping and associated inter planting
with the sequential planting on the same parcel of land.
Spreading risk – maximum changes obtaining a food supply.
Mixed cropping among other advantages has conservation benefits of – plant cover and
soil protection.
Objectives and criteria in design of approaches.
(1) Prime objective – provide for permanent maintenance of soil’s productive potential –
though a package of physical and biological conservation within the context of the people
that they will effect.
(2) 2nd objective – to provide food or hope to improving living standard for people – the
integration of technical measures into socio –political and economic circumstances.
Approaches have disadvantages and advantages but three criteria are important.
(i) Efficiency – not necessarily most efficient
(ii) Acceptability
(iii) Cost