A system for drought monitoring and severity assessment
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Date
1995-02
Authors
Lourens, Uys Wilhelm
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
English: The objectives of this study were:
(i) to develop a near real-time crop-specific drought
monitoring system that delimits drought stricken areas and
assesses the severity of droughts in these areas,
(ii) to produce products from the system which can be used for
decision support by decision makers, and,
(ii) to test the system for maize production using historical
production seasons.
Objectives (i) and (ii)
An agricultural drought monitoring system was designed, which
combined crop growth modelling and a Geographic Information
System (GIS) . The use of crop models made it possible to assess
the drought damage suffered by crops, in relation to their growth
stage. As drought is a spatially related phenomenon, a GIS was
used to present the geographic distribution of a drought
situation.
A grid based, spatially distributed, system was designed. The
map units of the South African 1.:250 000 map series were used as
the base units on which to present information. Each base unit
was divided into cells covering an area of 2' by 2' minutes of
latitude and longitude. There were thus 1.800 grid cells in one
such unit. The models were run for each of these cells.
The data inputs required by the crop models therefore had to be
spatially distributed. Methods of creating spatially distributed
weather data bases, were implemented or developed. Existing
interpolation techniques were used to create the rainfall and
temperature data bases. A technique developed for determining
daily irradiance, from the Japanese Geostationary Meteorological
Satellite, was adapted for use on METEOSAT data obtained over
South Africa. A spatially distributed soil data base was also
created. Maize was chosen as the crop to monitor in the initial evaluation
of the system. Drought monitoring was undertaken at fortnightly
intervals from the beginning of the crop production season. At
each interval, observed weather data was used up to the present
date, and the season completed with surrogate data. Three
surrogate scenarios were used: a below normal rainfall year, a
normal rainfall year, and, an above normal rainfall year.
Surrogate data were created for each homogeneous climate zone
(HCZ) within the study area. The HCZ within which the cell lay
was determined and its data used to complete the season. A
rainfall data generator, the accuracy of which had been proved,
was used in establishing the surrogate data.
The cumulative probability distribution function (CDF) of
seasonal yield, was used as the norm against which to measure
current season performance at the conclusion of each monitoring
session. CDF's were established for all combinations of soil,
climate, and planting dates used within the bounds of a
particular 1:250 000 map unit.
The yield simulated for each cell was compared with the
appropriate CDF, and the probability range within which it lay,
determined. A drought index value was assigned based on this
comparison. The indices were:
1- Extreme Drought (CDF probability range 0- 10%),
2- Severe Drought (>10- 20%),
3- Moderate Drought (>20- 30%),
4- Mild Drought (>30- 40%), and,
5 - No Drought (>40 - 100%).
Maps showing the distribution, and tables providing the extent
of area classified, were produced.
Objective (iii)
The drought monitoring system was tested for three maize
production seasons. The accuracy of the system was determined by comparing the average maize yield per magisterial district
with measured yield data. Individual farm records were also
evaluated. The system accurately portrayed the general maize
production trends during a severe drought (91/92}, while an r 2 of
0.59 was obtained for the individual yields.
The crop modelling approach to drought assessment takes the
interaction of the soil, plant and atmosphere into account and
is crop specific. The important influence of both the amount and
timing of rainfall in relation to crop growth stages is therefore
reflected in the drought index.
Description
Keywords
Corn -- Water requirements, Environmental monitoring, Crops and climate, Arid regions agricultur, Thesis (Ph.D. (Agrometeorology))--University of the Free State, 1995