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1. Definition
| Name |
WATER
STORAGE CAPACITY |
| Brief definition |
The water storage
capacity of the soil is the water that is available in the soil
for use by plants, excluding therefore water that moves quickly
though the soil under the influence of gravity as well as the
water bound by strong forces to mineral surfaces. The storage
capacity depends on both the volume of soil and the volume of
available pore space that can retain water against gravitational
forces. In the context of desertification this indicator is
related to the production and regulation functions of soil quality.
Changes in water storage capacity can be used as an early warning
of desertification. |
| Unit of measure |
It can be expressed
as a depth (mm) or volume. |
| Spatial scale |
Local |
| Temporal scale |
days to months |
2. Position
within the logical framework DPSIR
| Type of Indicator |
State and impact
|
3. Target and
political pertinence
| Objective |
Water
storage capacity is an excellent and appropriate desertification
indicator with a long history of use in the context of water
and soil management. It has been extensively used by hydrologists
in the calculation of the soil water balance and is used in
scheduling volumes of irrigation water. This is a valuable indicator
because it lies at the heart of soil quality and health. |
| Importance
with respect to desertification |
Changes in water storage
capacity can be attributed to soil degradation processes and
soil health. The water storage capacity influences the depth
to which rainfall penetrates the soil. The vertical differences
in storage capacity in the soil affect both the subsequent
evaporation of water and crop productivity. The erosion of
the soil results in a loss of storage capacity and this has
always been considered as a major factor of desertification.
Soil organisms produce
many of the pores in a soil that enable a soil to store water.
Organisms also produce substances that make pores able to
retain coherence when the soil is wet. A decrease in water
storage capacity may give an early warning that the resilience
or regenerative capacity of a soil is being affected by desertification.
If a soil cannot store
as much water and nutrients as before, it will produce less
biomass and there will be a positive feedback in which the
storage capacity decreases.
Rocks weather into regolith
and soils that have characteristic pore-size distributions
(soil texture). This parameter is very important in the concepts
and models being applied for the management of subhumid areas.
|
| International
Conventions and agreements |
This is a key
indicator in agro-meteorology and it has a long history. It
is relatively easy to measure and could be a headline indicator
of desertification. |
| Secondary
objectives of the indicator |
It
is a general indicator of soil fertility and soil quality. It
can be viewed as a sensitive indicator of land degradation. |
4. Methodological
description and basic definitions
| Definitions
and basic concepts |
Water
storage capacity is defined as the amount of water available
for plant growth in the soil. It therefore depends on the water
holding capacity and the depth or volume of soil. |
| Benchmarks
Indication of the values/ranges of value |
The water storage
capacity can be related to the critical water requirements of
crops and in this way provides a critical measure of suitability.
There is a huge data base kept by the FAO describing plant water
requirements and how these are related to soil water storage
capacity. Typically a 10 cm layer of soil might have a storage
capacity that ranges from 2 to 6 cm. This can be also measured
with respect to the energy holding water in the soil and in
this case the pF is a good measure. Available water is that
occurring in the range of suctions occurring between field capacity
and the wilting point. |
| Methods
of measurement |
Storage
capacity can be measured experimentally in situ. The soil is
saturated, covered by plastic to prevent evaporation and allowed
to drain against gravity so that the field capacity is known.
There are many procedures for measuring the energy with which
water is held in the soil, for example using tensiometers and
porous blocks. Soil moisture suction can be converted to a measure
of soil pore size distributions. Lysimeters can also be used.
Values can be calculated by keeping a soil water balance account.
This requires measuring or calculating the difference between
evaporation and precipitation. Extensive and very useful guidelines
are available from the FAO. Soil thin sections can be used and
pore size distributions measured automatically under the microscope.
There is no need to study the entire range of soil suction values.
It is possible to measure the high energy suction as a good
and sensitive indicator of soil structure stability (See Collis-George,
1984) . |
| Limits of the
indicator |
More methods
should be available for measuring this indicator in the field.
Conventions are needed to agree on some standards. |
| Linkages with
other indicators |
Erosion
risk (RDI), Infiltration
capacity, Parent material,
Soil crusting, Soil
erosion (USLE), Soil quality
index, Soil texture, Soil
permeability, Management
quality index, Tillage
operations, Grazing intensity,
Runoff water storage,
Water availability. |
5. Evaluation
of data needs and availability
| Data required
to calculate the indicator |
Field determinations
of soil moisture retention and soil depth. An alternative is
the data used to calculate the soil water balance (rainfall,
potential evaporation and soil depth). |
| Data sources |
Organisations
responsible for soil and water conservation, land owners and
land users. In particular, the FAO can provide much data. |
| Availability
of data from national and international sources |
Considerable
data must be available from Agricultural and Environmental Services
in most countries. |
6. Institutions
that have participated in developing the indicator
| Main institutions
responsible |
FSD |
| Other
contributing organizations |
Many
organisations such as the ESSC, CSIC, the Universities of Ghent,
Wageningen, Amsterdam and INRA and Valencia, and the Agricultural
University of Athens have contributed to the development of
this indicator. |
7. Additional
information
| Bibliography
|
FAO Crop evapotranspiration
- Guidelines for computing crop water requirements - FAO Irrigation
and drainage paper 56 http://www.fao.org/docrep/X0490E/X0490E00.htm
Collis-George, N. &
Figueroa, BS 1984, 'The use of high energy moisture characteristics
to assess soil stability', Aust. J. Soil Res. 22: pp349-356.
|
| Other references |
G. J. Levy
and A. I. Mamedov 2002 High-Energy-Moisture-Characteristic Aggregate
Stability as a Predictor for Seal Formation Soil Science Society
of America Journal 66:1603-1609 (2002) |
| Contact's name
and address |
A.C. Imeson
Foundation for Sustainable
Development (3D-EC), Netherlands
3de@hetnet.nl
|
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