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| some are demonstrably better than others, hence the use of the word “art”. There appears to be no standard classification of forecasting methods (Makridadis et al., 1998; Armstrong, 2001a). Roughly speaking, forecasting methods can be subdivided into various categories according to the relative share of judgement, statistics, models and data used in the process. Armstrong identifies 11 types of methods that can be grouped as | some are demonstrably better than others, hence the use of the word “art”. There appears to be no standard classification of forecasting methods (Makridadis et al., 1998; Armstrong, 2001a). Roughly speaking, forecasting methods can be subdivided into various categories according to the relative share of judgement, statistics, models and data used in the process. Armstrong identifies 11 types of methods that can be grouped as | ||
| * Judgemental, based on stakeholders’ intentions or on the forecasters' or other experts’ opinions or Intentions. Some applications of this approach exist in agrometeorological forecasting, especially when other variables such as economic variables play a part (for instance the “Delphi expert forecasting method” for coffee, Moricochi et al., 1995); | * Judgemental, based on stakeholders’ intentions or on the forecasters' or other experts’ opinions or Intentions. Some applications of this approach exist in agrometeorological forecasting, especially when other variables such as economic variables play a part (for instance the “Delphi expert forecasting method” for coffee, Moricochi et al., 1995); | ||
| - | * Statistical, including univariate (or extrapolation), multivariate (statistical “models”) and theory-based methods. This is the category where most | + | * Statistical, including univariate (or extrapolation), multivariate (statistical “models”) and theory-based methods. This is the category where most agrometeorological forecasting belongs; |
| - | agrometeorological forecasting belongs; | + | |
| * Intermediate types include expert systems, basically a variant of extrapolation with some admixture of expert opinion, and analogies, which Armstrong places between expert opinions and extrapolation models. This is also covered in the present chapter. | * Intermediate types include expert systems, basically a variant of extrapolation with some admixture of expert opinion, and analogies, which Armstrong places between expert opinions and extrapolation models. This is also covered in the present chapter. | ||
Revision as of 09:55, 22 September 2006
Partly taken from Jan Jansonius, 2003 Crop Forecasting Methods
Various disciplines make a contribution to crop forecasting and all over the world different methods are used. Several methods of crop forecasting are briefly reviewed here, but a detailed treatment is beyond the scope of this manual. Reference is made to several technical publications that may be consulted. In practice, the choice of a specific methodology or mix of methodologies depends also on cost and available budget.
Forecasting and monitoring
We should mention the links between forecasting and monitoring. Traditionally, monitoring is implemented by direct observation of the stage and condition of the organisms being monitored (type 1), or by observing the environmental conditions that are conducive (or not) to the development of organisms (type 2). The second type allies mostly to pests and diseases. Surprisingly, type 1 monitoring, which involves the direct observation of organisms, is often more expensive than type 2 because of elevated labour costs. On the other hand, when data are collected to assess environmental conditions, we are relatively close to forecasting as data requirements naturally overlap between type 2 monitoring and forecasting.
Micro and Macro level Crop Forecasting
It is important to note that operational forecasting is done for different spatial scales (Gorski and Gorska, 2003). At the lowest end, the “micro-scale”, we have the field or the farm. Data are usually available with good accuracy at that scale, for instance the breed or the variety are known, and so are the yield and the environmental conditions: soil type, soil depth, rate of application of inputs. The micro-scale is the scale of on-farm decision making by individuals, irrigation plant managers, etc. The macro-scale is the scale of the region, which is why forecasting for a district, or a province is usually referred to as “regional” forecasting. Regional forecasts are at the scale of agricultural statistics. Regional forecasts are relevant for a completely different category of users, including national food security managers, market planners and traders, etc. At the macro-scale, many variables are not known and others are meaningless, such as soil water holding capacity. Needless to say, the real world covers the spectrum from macro- to micro-scales, but the two extremes are very well defined in terms of customers and methods. Several applications are at an intermediate scale. They would include, for instance, certain types of crop insurances, the “livelihood analysis” that is now applied in many food security monitoring systems, fire monitoring systems, etc.
Forecasting techniques in general
There are a variety of generic forecasting methods, of which most can somehow be applied to agrometeorological forecasting as well (Petr, 1991). According to Armstrong (2001b), “judgement pervades all aspects of forecasting”, which is close to a definition which one of the authors has frequently applied to crop yield forecasting, which can be seen as “the art of identifying the factors that determine the spatial and inter-annual variability of crop yields” (Gommes, 2003a). In fact, given the same set of input data, different experts frequently come up with rather different forecasts of which, however, some are demonstrably better than others, hence the use of the word “art”. There appears to be no standard classification of forecasting methods (Makridadis et al., 1998; Armstrong, 2001a). Roughly speaking, forecasting methods can be subdivided into various categories according to the relative share of judgement, statistics, models and data used in the process. Armstrong identifies 11 types of methods that can be grouped as
- Judgemental, based on stakeholders’ intentions or on the forecasters' or other experts’ opinions or Intentions. Some applications of this approach exist in agrometeorological forecasting, especially when other variables such as economic variables play a part (for instance the “Delphi expert forecasting method” for coffee, Moricochi et al., 1995);
- Statistical, including univariate (or extrapolation), multivariate (statistical “models”) and theory-based methods. This is the category where most agrometeorological forecasting belongs;
- Intermediate types include expert systems, basically a variant of extrapolation with some admixture of expert opinion, and analogies, which Armstrong places between expert opinions and extrapolation models. This is also covered in the present chapter.
Many countries still apply a system whereby each Agric. Extension Worker actually produces area and production figures for all crops in his area, simply based on his judgement. Such data are added up to the next administrative level and finally aggregated into provincial and national crop estimates. Quantitative crop reporting by Extension Workers is not recommended, as the figures will normally be unreliable.
On the other hand, experienced agricultural field staff are normally well qualified to make useful reports on crop conditions in qualitative terms and to relate the current situation to last years crop or to “normal” conditions. They can also highlight the specific critical factors affecting the crop.
Crop reporting is normally a regular, ongoing activity during the 12 months of the year. In each country the MOA will have its own priorities for the reporting system. During the growing cycle of the main crops the reporting should be an important tool for crop forecasting. Reporting later in the agricultural year may focus more on indicators of actual crop production, such as marketing activity, prices and food supply conditions, in addition to monitoring of growing conditions during a second crop season. All major food crops and HVC’s should be covered, with the exception probably of industrial crops like rubber and sugarcane.
The Officer in-charge of crop monitoring at Provincial level (PCO) is a key figure in the process. He will report on crop conditions, normally monthly and separately for each of the Crop Zones (strata) of his Province. His report will be based partly on the reports from his staff at District level and below, partly on other sources.
For this purpose a network of informers need to be built up, including large farmers, crop collectors/ assembly traders, millers, representatives of seed, fertilizer and pesticides companies, suppliers of agricultural credit. Their information on the crop situation needs to be tapped regularly by either visits on the spot or by telephone.
A small annual budget is needed to cover maintenance and operation of a few vehicles and motorcycles, travel allowances and modest requirements for office equipment and stationary.
The required background information, that should be at the finger tips of all field staff includes:
- the delimitation and description of Crop Zones (see chapter 3)
- crop calendar, showing planting and harvesting period of crops
- crop varieties and their characteristics
- a description of the main pests and diseases in the area
- a list of most used fertilizers, pesticides and other inputs
Variables to be monitored at the start of the planting season and during the early months of the crop cycle:
- Weather conditions: early or late rains, rains insufficient, normal or excessive
- Availability of seed and fertilizer as compared to last year and, if possible, actual quantities of inputs distributed or sold
- Availability of tractors and fuel and/or draught animals for ploughing
- Availability of labour, agricultural equipment, credit
- Last years crop prices and current input prices acting as an incentive or disincentive for planting
- Impact of population displacement, migration or civil strife on planting performance
- Crop area planted as compared to last year (reasons for any significant change)
- Crop varieties planted: short or long cycle, high or low yield potential, any changes compared to last year
Variables to be monitored, on monthly basis during the whole crop cycle:
- Development of crops in the field (approximate distribution of crop area by crop stage, if planting is staggered over several weeks or months)
- Crop condition as compared to normal (reasons for any significant deterioration)
- Rainfall distribution as compared to normal and compared to crop water requirements
- Degree of damage by pests and diseases
- Effect of adverse weather conditions, dry spell, excessive rain, hailstorm, extreme temperatures
- Area replanted after crop loss due to drought, severe pest attack or natural catastrophes
- Water availability for irrigation
During the last month of the crop cycle an assessment is made of:
- Crop area ready for harvest, i.e. planted area minus crop area lost, as compared to last year
- The average expected yield level in the Crop Zone, as compared to the previous year
Standardized reporting forms are often used. Crop stage for example is reported as a code from 1 - 5 (1=emerging and early vegetative stage, 2= late vegetative stage etc.). Planted area and crop condition are also coded from 1 - 3 or 1 - 5 to indicate percentage change compared to normal or last year. Damage from insect pests and other adverse conditions is reported as “slight”, or ‘serious”.
A standardized format has both advantages and disadvantages In principle it is a convenient way to summarize crop conditions and makes it easy to compare the situation in all Districts of a Province or all Provinces of the Country. On the other hand it may lead field staff to fill out their forms superficially, crossing the box “normal” in most cases.
Therefore the reporting form should provide ample space for remarks and additional information in free format. The rule should be enforced that convincing reasons have to be given for any significant change in crop area and condition that is reported. On the other hand, if no change is reported at all, this will have to be justified as well.
Complete enumeration A very different approach to crop reporting is followed in India. This is possible because most States, accounting together for over 95 percent of the cropped area, maintain a system of land records. Statistics on land utilization and crop area are collected as part of updating of land records. This is done regularly by complete field enumeration, carried out by the village revenue agency. The area statistics collected at village level are summarized and submitted to the next administrative levels, finally reaching the State level. A number of shortcomings of this method have been recognized, one of which is excessive delays before all data have been aggregated.
For crop yield estimates a very large number of random crop cuts is carried out in most States of India.
Crop zones and extension areas
The District Crop Officer (DCO) should be fully aware of the stratification of the Province in Crop Zones. In fact he should have participated in the original delineation of those zones and should have an input for their eventual adjustment or sub-division, if new developments occur (for example new crops coming up or increased production in remote areas after road access has been improved).
The District is normally divided in Extension Areas (EA), one such area being the responsibility of one Agric. Extension Worker (AEW). Ideally each EA is situated fully within the limits of one Crop Zone. If this is not yet the case, adjustments of the operational units should be made to this effect, if possible. In practice it is often found that in the existing organization individual Extension Workers are specialized in particular crops and are already assigned to the relevant crop zones.
Recommended reporting procedure
1) The District Crop Officer (DCO) calls a monthly meeting of his extension workers. AEW’s report on crop conditions in their EA, in order of Crop Zones. AEW’s are not required to come up with actual crop area and yield figures, as this is considered unrealistic. In stead they will give their reasoned assessment of the variables to be monitored, as mentioned in the previous section (crop stage, trend in crop area, crop condition, adverse factors etc.).
2) After discussion and cross-checking with other sources available at District level, the DCO summarizes the information on a standardized reporting form, supplemented by much additional explanations, which is submitted to the Provincial level.
3) The PCO uses his other sources, to cross-check the District reports. He will make a monthly tour of all Crop Zones of the province for direct observation of crop conditions and interviews with farmers and other knowledgeable persons of his network of informers.
4) The PCO consolidates all information into one monthly report for the Province, containing: • A summary table in standardized format, showing one row for each variable and one column for each Crop Zone (as one Crop Zone may extend over parts of two or more Districts, information from several District reports may need to be combined). • The general part of the report highlights all significant developments and problems encountered. • A separate section is added concerning the Commercial Sector.
5) Reports are submitted to the Crop Forecasting Centre, located probably in MOA Headquarters.
6) This Centre will critically review and aggregate the Provincial reports and issue monthly situation reports at National level. These reports are also an important input for the forecasts to be issued by the Centre, considered together with other methods and information sources.
Rapid appraisal methods
Damage assessment
Natural calamities such as devastating crop disease, pest attack (locust, army worm), severe drought, floods, typhoons may strike standing crops and cause partial or total loss of crop production. It will be necessary to assess the damage rapidly. The crop area in ha. that suffered total crop failure or partial crop losses and the number of villages and population affected will need to be determined. The affected area can in most cases also be identified from satellite imagery or aerial photography and it may also be possible to assess the degree of damage using remote sensing methods.
A sample of farmers affected by the calamity may be interviewed or the damage to their fields assessed by an eye estimate of the investigators. The size of the sample will probably be small, it may be by reasoned choice, not necessarily a random sample.
Crop assessment mission
In many countries it is an established practice to organize once or twice-yearly a crop assessment missions by a team of experts in various fields. The purpose is to produce an up-to-date picture of crop conditions and all related information. Rough and ready methods are used. The team travels through extensive cropping areas for direct observation of crop conditions. Every opportunity will be taken to interview farmers. Meetings will be held with staff of MOA, Local Government, other Departments or Services, Agricultural Research Institutions, NGO’s, Traders and Processors representatives.
This approach has often proven itself to be very effective in mobilizing locally available information and identifying important aspects and new developments that otherwise would have gone undiscovered. The reliability of such assessments will improve, if the same persons participate in the mission several years in a row, allowing them to compare their current observations with the situation seen during previous years.
Agro-meteorological crop monitoring
Rainfall monitoring
Rainfall is one of the most critical factors to be monitored, especially in conditions of rainfed agriculture, but indirectly also for irrigated crop production (water availability). Many crops are highly sensitive to water stress, for example paddy during the whole growing cycle and maize during flowering stage.
Monitoring rainfall distribution in rainfed crop production areas will provide “early warning” indications rather than a quantitative forecast of crop production. Nevertheless it plays an important role in forecasting in conjunction with other methods.
In most countries it is the Meteorological Service that operates a network of stations at national level, the major stations often being located at airports and in the bigger towns. MOA often operates its own network of cheap raingauges with a better representation of important cropping areas. It has to be seen if in each of the Crop Zones, which are the basis of the crop reporting system, rainfall records are available from at least two and preferably more stations. If not, additional raingauges should be installed.
It is recommended that direct decentralised use is allowed to be made of rainfall data recorded in the Provinces and Districts. This is in addition to the regular system of meteorological data collection and publication by the National Meteorological Service, which may result in delays in data availability. In many countries the Met. Service claims a monopoly on all meteorological and climatic data and its interpretation and will only give access to its database against payment.
Officers in charge of crop forecasting may simply record weekly or 10-daily rainfall records for each available station and compare with normal values, possibly also with estimated crop water requirements. This information will be used to detect dry spells and potential damage from excessive rainfall. It is a check on and complement to the crop reporting mentioned in 4.1
Weather models
More sophisticated agro-meteorological monitoring is done with the help of models. India and various other countries have developed models based on weather data. It involves identification of the most significant weather parameters during different stages of crop growth, which are then used in a regression model with or without trend.
These models require a long series of weather data and reliable crop yield data for calibration of the model. The data available for this purpose are often insufficient. A practical problem is also scarcity of current data. While in most countries there is a large number of rainfall stations, the number of climatic stations that record also other variables such as temperature, sunshine hours and relative humidity is much lower. The data of one station is then used for large areas, while there may be in fact considerable variability in weather conditions.
In China a 30-year series of meteorological data has been compiled in six regions. The meteorological variables most relevant for each crop stage were studied by region, by crop and by month, taking into account the growing cycle of each crop. On this basis, the weather conditions in the current year and those in the previous years are weighed and clustered so as to identify the year in which the weather conditions were most similar to the current year.
In Japan it was found that the yield of paddy is highly correlated with weather conditions. A weather index representing temperature, rainfall, sunshine hours and wind speed has been used successfully to forecast paddy yield.
Most models do not respond well to extreme conditions in weather. They work best when the weather conditions are within the range of the data that were used for fitting the models.
The FAO Water Balance Model
This model has been used for many years, especially by countries relying on rain-fed agriculture where inadequate availability of water to the crop is the main constraint. The model calculates a running water requirement satisfaction index (WRSI) during the growing season of the crop. The final value of this index is correlated with crop yield level. The correlation will be high, if all other factors affecting crop yield are fairly constant from year to year. Historical data on rainfall and crop yield are required, to establish a regression relationship between Index and yield. Unfortunately such data is in practice often not available.
Constraints of the model:
- A fairly dense network of raingauges and reliable records is required, because of high spatial variability in rain distribution in many countries
- The model is not reliable in case of extreme weather conditions
- The effects of excessive rainfall can not easily be accommodated
- Water holding capacity of the soil is a key variable in the model, for which there is often insufficient information and the variability may be high from place to place
- The input in the model should in principle be effective rainfall rather than the recorded rainfall. Therefore runoff as a proportion of total rainfall should be taken into consideration. Runoff depends on average slope of farmland, soil conditions and farming practices and again there is little data available.
To summarize, the Water Balance method provides a very useful “early warning” indicator for yield reduction due to water stress. However, in most circumstances, it can not be expected to produce a reliable quantitative yield forecast. The method will be used together with other indicators.