MONITORING SOIL DROUGHT
MONITORING SOIL DROUGHT
Assoc.Prof. Vesselin Alexandrov
1. INTRODUCTION 3
2. MEASUREMENT OF SOIL MOISTURE 3
2.1. Soil-water content 4
2.1.1. Direct measurements 4
2.1.2. Indirect methods 4
188.8.131.52. Radiological methods 4
184.108.40.206.1. Neutron attenuation 5
220.127.116.11.2. Gamma absorption 5
18.104.22.168. Soil-water dielectrics 6
22.214.171.124.1. Time-domain reflectometry 6
126.96.36.199.2. Microwave probe 7
2.1.3. Emerging technologies 7
188.8.131.52. Pulsed nuclear magnetic resonance (PNMR) 7
184.108.40.206. Remote sensing 8
2.2. Soil-water potential instrumentation 8
2.2.1. Tensiometers 9
2.2.2. Resistance blocks 9
2.2.3. Psychrometers 9
MONITORING AGRICULTURAL (SOIL) DROUGHT – STATE OF
3.1. Monitoring soil drought abroad 10
3.1.1. Soil moisture, indices and indicators 12
3.1.2. Remote sensed indices (Satellite monitoring) 17
3.1.3. Soil-water balance based models 19
3.2. Monitoring soil drought in Bulgaria 22
3.2.1. Direct soil moisture measurements and drought indices 23
3.2.2. Remote sensed indices 27
3.2.3. Soil-water balance based models 28
Monitoring (soil) drought in Bulgaria through international cooperation -
Drought Management Centre for South-Eastern Europe
4. REFERENCES 31
MONITORING SOIL DROUGHT
Drought is one of the most severe and extreme weather events affecting more people than
any other form of natural disaster (e.g. Wilhite, 2000). Given the consequences and pervasiveness of
drought, it is important to assess drought severity. However, the precise quantification of drought is
difficult as no universal drought estimation method (e.g. drought indices, hydrological or soil water
balance models) can be defined through the complexity of the problem. The American
Meteorological Society (1997) suggests that the time and space processes of supply and demand are
the two basic processes that should be included in an objective definition of drought, and thus in the
derivation of drought estimation methods. Common to all types of drought is a lack of precipitation
(e.g. WMO, 1993). From a meteorological standpoint, drought is associated with dry spells of
varying lengths and degrees of dryness. The basic measure of drought is inadequate precipitation
for a particular activity (i.e. crop growth, irrigation supply, reservoir level).
In the scientific literature four types of droughts are commonly distinguished:
meteorological or climatological, hydrological, agricultural, and socioeconomic (e.g. Rasmussen et
al., 1993, Wilhite and Glantz, 1985). Meteorological drought results from a shortage of
precipitation, while hydrological drought describes a deficiency in the volume of water supply (e.g.
Wilhite, 2000). Agricultural drought relates to a shortage of available water for plant growth, and is
assessed as insufficient soil moisture to replace evapotranspirative losses (e.g. WMO, 1975).
Agriculture is probably the most vulnerable economic sector to extreme weather events such
as drought and many other economic sectors of our society depend on agroecosystems, which is a
specific form of ecosystem adapted by humans for food production. As agriculture is an important
economic factor in many countries, drought can have a number of economic and socio-economic
consequences (e.g. CAgM, 1992, 1993; WMO, 1995, 2001) such as loss of income in agriculture
and food industry, significant higher costs for water and production techniques (e.g. irrigation
systems). Of all natural disasters, droughts globally occur most frequently, have the longest duration,
cover the largest areas, and cause the greatest loss to agricultural production (e.g. WMO, 1997).
Although drought is a natural component of climate, in arid and semi-arid climatic regions, it can
also occur in areas that normally receive adequate precipitation (e.g. Li and Makarau, 1994). Most
places in the world can be affected by agricultural droughts which reduce the availability of water
required in agricultural production, but duration and intensity vary greatly from one climatic zone to
another (e.g. Wilhite, 1993). Because of climate changes that could change climatic variability
including precipitation pattern, extreme weather events such as drought are likely to occur more
frequently in different spatial and time scales in future (e.g. IPCC, 2001b).
2. MEASUREMENT OF SOIL MOISTURE
One of the most significant factors influencing crop yield and watershed performance is the
amount of water stored in the soil mantle. This soil moisture information is essential for
determining irrigation schedules, for the evaluation of water and solute fluxes, and for partitioning
of net solar radiation into latent and sensible heat components. Determination of soil moisture is of
great concern to a number of agricultural disciplines. Soil moisture is the main indicator for the
agricultural drought phenomenon assessment. To satisfy the widespread need of determining soil
moisture status, a number of commercially-available instruments have been developed. The most
commonly used and a few state-of-the-art instruments as well as techniques used instruments are
2.1. Soil-water content
2.1.1. Direct measurements
The simplest and most widely used method for measuring soil-water content is the
gravimetric technique. Because this method is easy and based on direct measurements, it is the
standard with which all other procedures are compared. Gravimetric soil moisture, is typically
determined on a dry mass basis.
In order to determine soil-water content, soil samples are removed from the field with the
most convenient tool. Typical tools include shovels, spiral hand augers, bucket augers, as well as
power-driven coring tubes. The soil samples are then placed in a leak-proof, tare-weighed container
suitable for transporting to a laboratory and drying in an electrically heated oven. The samples and
container are weighed in the laboratory both before and after drying, the difference being the mass
of water originally in the sample. The drying procedure consists in placing the open container in an
electric oven at 105°C until the mass stabilizes at a constant value. The time required varies from 16
to 24 hours. However, if the soil samples contain considerable amounts of organic matter, excessive
oxidation may occur and some of the organic matter will be lost from the sample. Although the
specific temperature at which excessive oxidation occurs is difficult to specify, lowering the oven
temperature from 105 to 70°C seems to be sufficiently low to avoid significant loss of organic
Microwave oven drying for the determination of gravimetric water contents can also be used
effectively (Gee and Dodson, 1981). In this method, soil water temperature is quickly raised to
boiling point where it remains constant for a period of time due to the consumption of heat in
vaporizing water. However, the temperature rapidly rises as soon as the energy absorbed by the soil
water exceeds that consumed for vaporizing the water. Caution should be used with this method as
temperatures can become so high that they can melt plastic containers if stones are present in the
Although rarely used, there are other methods for the direct measurement of soil-water
content. However, they are limited to special purposes and emergencies. One of these methods
involves placing the soil in a tared container with a perforated bottom and weighing it to determine
the wet mass. The soil samples are irrigated with methanol, which will eventually displace the water.
The methanol is then ignited, and the procedure is repeated at least one more time. The sample is
then again weighed to determine its dry mass. The amount of methanol needed to displace the water
depends on a number of factors, such as the size of the sample, its water content, and its texture.
The latter method is very susceptible to error as volatile soil components may be lost.
2.1.2. Indirect methods
The capacity of soil to retain water is, among other variables, a function of soil texture and
structure. In removing a soil sample, the soil being evaluated will be disturbed, and its water-
holding capacity altered. Indirect methods of measuring soil water are beneficial as they allow
information to be collected at the same location for each observation without disturbing the soil-
220.127.116.11. Radiological methods
Two general radiological methods are widely used and available for measuring soil-water
content. One is the neutron scatter method, which is based on the inter-action of high-energy (fast)
neutrons and the nuclei of hydrogen atoms in the soil. The other method utilizes the attenuation of
gamma rays as they pass through soil. Both instruments use portable equipment for taking
measurements at permanent observation sites and require careful calibration, preferably with the
soil in which the equipment is to be used.
When using any radioactive emitting device, some precautions are necessary. All rules
regarding radiation hazard laid down by the manufacturers and health authorities must be observed.
When the guidelines and regulations are followed, there is no need to fear exposure to excessive
radiation levels, regardless of the frequency of use. None the less, whatever the type of radioactive
emitting device is used, the operator should wear some type of film badge that will enable the
exposure levels to be evaluated and recorded on a monthly basis.
18.104.22.168.1. Neutron attenuation
There are two types of neutron soil moisture detecting device a soil surface meter and a
depth probe. In both devices, high-energy (fast) neutrons are emitted and are eventually slowed
down upon their interaction with matter (resulting in neutron thermalization) (Visvalingam and
Tandy, 1972). The hydrogen nuclei, having about the same mass as neutrons, are by far the most
effective soil components in slowing down neutrons upon collision. As a result, the density of slow
neutrons in the vicinity of the neutron probe is nearly proportional to the volumetric soil-water
content. The slow or thermalized neutrons form a cloud around the neutron-emitting device where
its density and size represent an equilibrium between the emission rate of fast neutrons and those
thermalized. Within each neutron-emitting device is a thermalized neutron detector which
determines the density of the thermalized neutron cloud. Unfortunately, the volume encompassed
by the thermalized neutron cloud varies substantially with water content. For example, in wet soil,
the radius of influence may be only 15 cm, while in dry soil, the radius may increase to 35 cm.
Because the volume being measured varies with water content, this method lacks high resolution,
making it impossible to localize water-content discontinuities. A particular problem occurs at the
soil interface on account of the soil-air discontinuity. As a result, the neutron probe is not used in
the top 18 cm of soil. However, the neutron surface meter is used exclusively for measuring water
contents in the soil surface (0–30 cm). Unfortunately, where the soil surface is rough, precision falls
A neutron depth probe comprises a radioactive source of high-energy neutrons, and a
detector of slow thermalized neutrons, typically in a cylindrical form. The probe is attached by
cable to the main electronics so that the probe can be lowered into a previously installed access tube.
Although several arrangements of source-detector are possible, it is best to have a probe with a
double detector and a central source. This arrangement allows for a more spherical zone of
influence and leads to a more linear response with soil-water content. The neutron surface meter
usually has a thermalized neutron detector laid horizontally on the soil surface with a fast neutron
source behind it.
The access tube should be seamless and thick enough (typically 1.25 mm) to be rigid, but
not so rigid that the access tube itself is responsible for thermalizing neutrons. The access tube must
be made of a non- corrosive material, such as stainless steel, aluminium, or some plastics, but
polyvinylchloride should be avoided as it absorbs slow neutrons. The probe should be capable of
being inserted into the tube without risk of jamming; usually a 4-cm diameter tube is sufficient.
Care should be taken in installing the access tube to ensure that it is not bent.
Additionally, no air voids should exist between the access tube and the soil matrix.
Approximately 15 cm of the tube should extend beyond the soil surface as the box containing the
electronics fits on top of the access tube. All access tubes should be fitted with a removable cap to
keep rainwater from entering the tubes.
2.1.2. 1.2. Gamma absorption
Whereas the neutron-attenuation method measures the volumetric water content in a large
sphere, gamma absorption scans a 1-cm layer. Although it has a high degree of resolution, the small
soil volume evaluated will exhibit more spatial variation due to soil heterogeneities (Gardner and
Calissendorff, 1967). The single-probe gamma device measures attenuation by reflection and is no
longer widely used. However, the dual-probe gamma device which measures both soil density and
water content is still a widely accepted instrument.
Changes in gamma attenuation for a given mass absorption coefficient and absorber
thickness can be related to changes in total density. As the attenuation of gamma rays is due to mass,
it is not possible to determine water content unless the attenuation of gamma rays responding to dry
soil density is known. Additionally, the dry density of the soil must remain unchanged with
changing water content. If the dry soil density is known, then the soil-water content can be
determined from the difference between the total and dry density values.
Unlike neutron attenuation, gamma-ray attenuation enables a high spatial resolution.
Vertical measurements at 2.5 cm can be made with excellent precision. It also has the advantage of
making accurate measurements 2.5 cm below the air-surface interface.
Additional caution should be taken with the use of gamma-emitting devices as they are
potentially more dangerous than the neutron-emitting devices. The manufacturer will provide a
shield which should be used at all times. The only time the probe leaves the shield is when it is
lowered into the access tube.
22.214.171.124. Soil-water dielectrics
Because of the dramatic difference in the dielectric constants of water and dry soil
(approximately 80 and 3.5, respectively), theoretical and empirical relationships relating soil
volumetric water content to the dielectric constant of the soil-water system have been proposed.
This approach allows reliable, fast, non-destructive measurements of the volumetric water content,
without the potential hazard associated with radioactive emitting devices. In addition, these
methods lend themselves to being fully automated for large-scale data-acquisition programs. At
present, two newly developed instruments which evaluate soil-water dielectrics are commercially
available and are being used on an international scale. The first instrument utilizes time-domain
reflectometry (TDR) technology, while the other measures the dielectric constant at a specific
126.96.36.199.1. Time-domain reflectometry
Time-domain reflectometry is a relatively new method which determines the dielectric
constant of the soil by measuring the transmittal time of an electromagnetic pulse launched along a
pair of parallel rods of known length embedded in the soil. As the sampling area is essentially a
cylinder around the parallel probes, a large soil volume is examined. Theoretically, the dielectric
constant is sensitive to soil surface area; however, time-domain reflectometry does not appear to be
sensitive enough to require calibration for the range in surface areas typically found in soils.
Generally, the parallel probes are separated by 5 cm and can vary in length from a few to
over 30 cm. Additionally, the rods making the probe can be of any metallic substance; stainless
steel is most frequently used. Although some care should be taken to ensure that the probes are
parallel, slight deviations do not affect the resultant dielectric readings.
In theory, the attenuated time-domain reflectometry signal should be able to measure both
soil-water content and the salinity independently from a single reading; however, this work is still
in its infancy. Additional work is being evaluated which allows this technique to be automated by
examining the water content from a buried set of probes, each placed horizontally at a different
depth. The probes are then linked through a multiplexing device attached to a field data logger.
188.8.131.52.2. Microwave probe
The microwave dielectric probe utilizes an open-ended coaxial cable and a single
reflectometer at the probe tip to measure amplitude and phase at a particular frequency (typically in
the microwave region). Soil measurements are referenced to air, and typically calibrated with
dielectric blocks and/or liquids of known dielectric properties. One advantage of using the liquids
for calibration is that a perfect electrical contact between the probe tip and the material can be
maintained (Jackson, 1990).
As a single, small probe tip is used, only a small volume of soil is ever evaluated. As a result,
this method is excellent for laboratory or point measurements but is likely to be subject to spatial
variability problems if used on a field scale. Additionally, the probe evaluates a small soil volume;
therefore, soil contact is critical.
2.1.3. Emerging technologies
Due to recent engineering advances, new methods are being developed which allow for the
rapid measurement of soil moisture conditions. Two recent developments in soil moisture
measurements are the use of pulsed nuclear magnetic resonance and microwave remote sensing.
184.108.40.206. Pulsed nuclear magnetic resonance (PNMR)
Still in the research and development stage, the use of PNMR may have practical application
in the near future (Paetzold et al., 1987). This measurement approach focuses on the interaction
between hydrogen nuclear magnetic moments and a magnetic field. The sensor unit consists of an
electromagnetic, radio-frequency coil, and a tuning capacitor. Essentially, this method allows for
the instantaneous measurement of the volumetric water content in soil - independent of texture -
organic matter content, and soil density.
The magnetic moment of a nucleus which contains an odd number of protons/neutrons
behaves like a spinning bar magnet. When placed in a static magnetic field, the magnetic moment
precesses about an axis parallel to the applied magnetic field. If an oscillating magnetic field equal
to the precession frequency of a hydrogen atom is applied at right angles to the static magnetic field,
it will force the magnetic moments of hydrogen to precess in phase. The oscillating magnetic field
is produced by the radio-frequency generator. The amount of energy adsorbed by the sample can,
then, be measured, as well as the decay signal of the oscillating field. The analysis of the resultant
adsorption and decay signals yields information concerning the spin-spin and spin-lattice relaxation
times which, in turn, are used to calculate the amount of hydrogen in the sample.
A tractor fitted with a prototype PNMR device has already been built and tested. This device
could be used to determine soil-water content at the time of planting, or could be used to collect
ground data for calibrating remote sensing instruments. Although the tractor PNMR system can
accurately evaluate approximately 5 cm of surface soil moisture, precision drops off dramatically
with increasing depth. The magnetic field must be homogeneous for PNMR techniques to work
effectively, and obtaining a homogeneous magnetic field in undisturbed soil is the greatest
limitation of this technique.
Laboratory PNMR instruments can be purchased but they are generally too expensive for
220.127.116.11. Remote sensing
Measurements from space-borne instruments utilizing remote sensing techniques will be
available in the near future for evaluating soil-water content, estimation of evapotranspiration rates,
and evaluation of plant stress on a watershed scale (Jackson and Schmugge, 1989). Although
infrared and microwave energy levels have been widely studied, only the microwave region has the
potential for obtaining direct quantitative soil moisture measurements from a space platform.
Microwave techniques can be separated into passive (radiometric) and active (radon)
radiation. Passive microwave techniques focus on analysing the natural microwave emissions from
the Earth’s surface, while active radiation refers to measuring the attenuation of a radar
backscattering signal. Both approaches are based on the large differences that exist between the
dielectric properties of liquid water and dry soil, and both are conducive to monitoring surface soil-
water content over large areas of land.
2.2. Soil-water potential instrumentation
To date, only instruments capable of measuring the matrix potential are sufficiently
inexpensive and reliable to use in a field-scale monitoring programme. In each case, there are
severe limitations to the range in water potential in which the instrument functions properly. Care
must, therefore, be exercised if osmotic potentials are significant.
The most widely used and least expensive water-potential measuring device is the
tensiometer. Tensiometers are simple, generally consisting of a porous ceramic cup and a plastic
cylindrical tube connecting the porous cup to a recording device which seals the top of the cylinder.
In view of their universal availability and low cost, a detailed description of their construction is
The tensiometer establishes a quasi-equilibrium condition with the soil-water system. The
porous ceramic cup acts as a membrane through which water flows, and as such, must remain
saturated if it is to function properly. Consequently, all the pores in the ceramic cup and the
cylindrical tube are initially filled with de-aerated water. Once in place, the tensiometer will be
subject to negative soil-water potentials, causing water to move from the tensiometer into the
surrounding soil matrix. The water movement from the tensiometer will create a negative potential
or suction in the tensiometer cylinder which will register on the recording device. The recording
device can be a pressure transducer (Marthaler, et al., 1983), a Bourdon-type vacuum gauge, or a
simple U-tube filled with water and/or mercury. On the other hand, if the soil receives water, the
soil-water potential may increase to where water moves from the soil back into the tensiometer,
resulting in a less negative water potential reading. This exchange of water between the soil and the
tensiometer, as well as the tensiometer’s exposure to negative potentials, will cause dissolved gases
to be released by the solution, forming air bubbles. The formation of air bubbles will alter the
pressure readings in the tensiometer cylinder and will result in faulty readings. Consequently, the
cylinders occasionally need to be refilled and de-aired with a hand-held vacuum pump.
Before installation, but after the tensiometer has been filled with water and degassed, the
ceramic cup must remain wet. Wrapping the ceramic cup in wet rags or inserting it into a container
of water will keep the cup wet during transport from the laboratory to the field. In the field, a hole
of the appropriate size and depth is prepared. The hole should be large enough to be snug on all
sides of the cylinder and long enough for the tensiometer to extend several centimetres above the
soil surface. Since the ceramic cup must remain in contact with the soil, it is generally beneficial to
prepare a thin slurry of mud from the excavated site and to pour it into the hole before inserting the
tensiometer. Care should also be taken to ensure that the hole is backfilled properly, thus
eliminating any depressions that may lead to ponded conditions adjacent to the tensiometer. The
latter precaution will minimize any water movement down the cylinder walls, which would produce
unrepresentative soil-water conditions.
Tensiometers can measure only the matrix potential, because solutes can move freely
through the porous cup. However, tensiometers can be purchased with additional features such as
electrodes, placed either inside the ceramic cup or just above the ceramic chamber, thus allowing
electrical conductivity within the tensiometer to be determined simultaneously. Obviously, it may
take some time for these tensiometers to equilibrate with the soil environment. Another limitation is
that the tensiometer has a practical lower limit of about –80 kPa. Beyond –100 kPa, water will boil
at ambient temperature, forming water vapour bubbles which will destroy the vacuum inside the
The cylinder and recording portion of the tensiometer allow for appreciable changes in
volume. Under drought conditions, appreciable amounts of water can move through the tensiometer
to the soil. Thus, tensiometers can alter the very condition they were designed to measure. Typically,
when the tensiometer acts as an irrigator, so much water is lost through the ceramic cups that a
vacuum in the cylinder cannot be maintained, and the tensiometer gauge will be inoperative.
Additional support of this process comes from excavated tensiometers which have accumulated
large numbers of roots in the proximity of the ceramic cups.
The tensiometer is also sensitive to temperature. Although only a small portion of the
tensiometer is exposed to ambient conditions, the interception of solar radiation may induce thermal
expansion of the tensiometer cylinder. Additionally, temperature gradients from the soil surface to
the ceramic cup may result in thermal expansion or contraction of the cylinder, thus inducing false
water-potential readings. To minimize these effects, the tensiometer cylinder should be constructed
of non-conducting materials and readings should be taken at the same time every day, preferably in
the early morning.
2.2.2. Resistance blocks
Electrical resistance blocks, although insensitive to water potentials in the wet range, are
excellent companions to the tensiometer. Electrical resistance blocks consist of electrodes encased
in some type of porous material that will reach a quasi-equilibrium state with the soil. The most
common block materials are gypsum, nylon fabric, and fibreglass (Perrier and Marsh, 1958).
Resistance blocks are relatively inexpensive and are good for field investigations. However,
they do need to be calibrated before installation. This is generally accomplished by saturating the
blocks in distilled water and then subjecting them to a predetermined pressure in a pressure-plate
apparatus. After equilibration at a specific pressure, readings are taken, and the block is exposed to
successively greater pressure potentials. This procedure should be repeated for at least five different
pressures before installation. As the resistance-block calibration curves change with use, they need
to be calibrated both before installation and after each investigation.
Unfortunately, resistance blocks are slow to equilibrate with soil, generating water-potential
estimates that are more closely associated with the soil-drying curve. Consequently, this method is
subject to errors where soil hysteresis may be an important factor. There is also a problem with
shrinking and swelling soil which will break contact with the blocks. In addition, this approach
determines water potential as a function of electrical resistance, and is sensitive to soil salinity. If
saline conditions do exist, it must be remembered that added salts will decrease resistance, falsely
indicating a wetter soil. The gypsum blocks are less sensitive to salts because the electrodes are
consistently exposed to a saturated solution of calcium sulphate. However, gypsum blocks tend to
deteriorate faster than fibreglass blocks.
When installing the resistance blocks it is best to dig a small trench for the lead wires before
preparing the hole for the blocks. This will minimize water movement along the wires to the block,
which could result in erroneous readings.
Thermocouple psychrometers do not measure the soil-water potential directly, but measure
the vapour phase with which it is in equilibrium (Rawlins, 1972). As a result, psychrometers are
quick to equilibrate with the soil environment. As with electrical resistance blocks, this method is
not sensitive to wet conditions but is well suited to a dry soil environment. It also lends itself to
automated data acquisition.
Psychrometers consist of a miniature thermocouple placed within a small chamber. The
thermocouple is cooled off by the Peltier effect, condensing water on a wire junction. As water
evaporates from the junction, its temperature decreases and a current is produced which is measured
by a voltmeter. Consequently, these measurements are quick to respond to changes in soil-water
potential, but are very sensitive to temperature and salinity.
3. MONITORING AGRICULTURAL (SOIL) DROUGHT – STATE OF THE ART
3.1. Monitoring soil drought abroad
According to Wilhite and Svoboda (2000), the primary tasks for the monitoring processes
• To adopt an applicable definition for grading of drought. Many drought indices exist in
consequence of drought definition multiplicity. Therefore, it is important to evaluate the
performance of several drought indices in drought situation, for which comparative case studies
can be accomplished. The evaluation include the index potential for using in early warning
conditions and identifying the different effects of drought on hydrological features, ground
water table, yield and state economy.
• regionalization for drought management. Climatic, land use, hydrological, topographical factors
have to be considering to create homogenous areas for drought management.
• develop drought monitoring system.
In the Central European region the meteorological observations belong mostly to the
national meteorological services. This fact has a couple of positive influences: a. in the consequence
of the general automatization process at the meteorological institutes, fully or partly automatic
observation systems begun to be implemented. b. If the largest national meteorological network has
one owner, than there is a good possibility to have the same instruments, same measuring time,
observing rules, etc, resulting in a quite homogenous data. The same statement is valid for the
hydrological observations as well, even if the meteorological and hydrological networks may
belong to different institutes.
• inventory data qantity and quality from current observation networks.
Many indicators must be monitored beyond the meteorological and hydrological ones:
agricultural, industrial, etc.
• determine data needs of primary users. The effectivity of the new systems are the highest, when
primary users participate in the process of development.
The information delivery system has to take the information to the corresponding user on
time. The deliverer can be transmitting system (broadcasting, TV), telecommunication system
(phone, fax), but the most hopeful is probably the Internet. Many applications run on the Internet
already, and the number of requests is growing continuously from the user side, too.
During the past decade, there has been significant progress in drought monitoring strategies
in the United States (Svoboda and Hayes, 2004). Most of these developments have improved the
temporal and spatial resolution of monitoring drought conditions. Much of this development has
stemmed from the Internet, which has provided near real time access to data and improved
information sharing, as well as the development of satellite technology, Geographic Information
Systems (GIS), and super computing capabilities. Each of these technological developments has
helped improve our capability to monitor drought. Recently, decision support systems
incorporating drought monitoring have begun to be developed.
One of the best examples of a new drought monitoring tool is the U.S. Drought Monitor
map [http://drought.unl.edu/dm]. Produced on a weekly basis and operational since the summer of
1999, the Drought Monitor has become an accepted tool for drought assessment by the public,
media and decision makers. Using an integrated approach, the product is not an index itself, but a
composite indicator that uses several input parameters based on a ranking percentile in order to
gauge the severity of drought in a given area. Based on four drought classes and an abnormally dry
class, it utilizes information from snowpack, streamflow, soil moisture, precipitation, drought
indices, satellite-derived vegetation indices, and several others indicators. In addition, an
experimental effort has been underway between scientists in Canada, Mexico and the United States
to produce a monthly North American Drought Monitor
[http://www.ncdc.noaa.gov/oa/climate/monitoring/drought/nadm/index.html] map using the same
methodology as the U.S. Drought Monitor, only tailored to the data available on a near real-time
basis in Mexico and Canada. Canada has also a Drought Watch:
http://www.agr.gc.ca/pfra/drought/index_e.htm (Fig. 1)
Fig. 1. On-farm surface water supply in Canada
There has been a recent emphasis on improving the usefulness of the U.S. Drought Monitor
information for the users or decision makers, especially through decision support systems. States,
for example, are capitalizing on the improved spatial and temporal resolution of information to
develop state-based products critical to their specific needs. Oklahoma has developed a climate
Mesonet that provides detailed climate information and a variety of products that can be used for
drought monitoring. Within the Oklahoma Mesonet system, the users can decide which statewide
products are most valuable for their needs.
The National Drought Mitigation Center is involved in a couple of projects that provide
user-defined maps and data from a variety of drought indices [http://nadss.unl.edu] or satellite-
derived vegetation conditions [http://gisdata.usgs.gov/website/Drought_Monitoring/viewer.asp]. At
the gis.usgs.gov link, users can identify a variety of layers and resolutions for display, which can be
used to aid in their decision making.
In spite of these advancements, challenges in drought monitoring remain. There are still
temporal and spatial data resolution issues as decision makers struggle to respond to drought
conditions appropriately and in a timely fashion. In addition, the general lack of observed soil
moisture and groundwater measurements impair our ability to determine the true severity and
impacts of drought. Opportunities for cooperation and partnerships continue to be necessary in
order to improve data networks and their quality. Other challenges include the need for a better
understanding of the relationship between specific drought indicators and the severity of various
impacts, and improved drought prediction. Perhaps the greatest challenge is in finding money to
fund drought-related research.
It is recommended that European countries place greater emphasis on drought monitoring
and the development of proactive mitigation plans to reduce the impacts of future drought episodes
(Wilhite, 2004). The experiences of the United States and other countries can facilitate this process.
Sharing information on drought monitoring, planning, mitigation, and policy issues will be
beneficial for all parties and will likely stimulate greater progress on drought preparedness in the
An example of the automatically interactive, Internet based system is the automatical
irrigation advisory system, developed at the Hungarian Meteorological Service, by financial support
of Ministry for Agriculture and Rural Development. The system calculates daily evapotranspiration
(Penman-Monteith-formula) and uses this result and precipitation measurements to derive the
accumulated climatological water sortage. From water shortage the wat
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