Growing Seasons define the period of time when temperature and moisture conditions are suitable for crop growth. Understanding when these periods of growth occur helps farmers, researchers, and policy makers better manage their land and water resources and better understand how variability in climate affects the ability of farmers to plant, grow and harvest specific crops. Measuring Growing Seasons has consistently been a challenge to farmers, agricultural researchers and others, including those concerned with the effects of climate change. Historically LGP (length of growing period) has been calculated based on time series climate data from existing rainfall stations. Determining growing seasons using satellite data provides a more accurate surface since it is based on actual reflectance values. This method also relies on the statistical analysis of time series data but since the satellite data are available continuously across space there is no need to estimate values using sparsely available point data. There is, thus, added confidence that the growing period data for each individual cell is a measure of truth. With the increased availability of satellite data and the growing periods can be more easily measured for multiple points in time.

Growing Seasons - Map 1

Growing seasons define the period of time when temperature and moisture conditions are suitable for crop growth. Understanding when these periods of growth occur helps researchers, policymakers, and farmers to better manage their land and water resources and to better understand how variability in climate affects the ability of farmers to plant, grow, and harvest specific crops. The concept of growing seasons takes into account the seasonality and length of potential growing periods during the year. The growing periods are determined based on the start of the rainy season, potential evapotranspiration, and temperature. Some areas of the world are not suitable for crop growth at any time, whereas others are suitable year round; still others are defined by multiple growing seasons. The HarvestChoice growing seasons surfaces were derived specifically for Sub-Saharan Africa (SSA). Season A refers to the primary growing season. If a region is bi-modal (defined by two growing seasons), it is identified as growing season B.

Defining Growing Seasons using Satellite Data

Measuring Growing Seasons has consistently been a challenge to farmers, agricultural researchers, and those concerned with the effects of climate change. Historically LGP has been calculated based on time series climate data from existing rainfall stations. The data are collected from the stations in the study area for a minimum of 20 years (FAO 1996), and then extrapolated from and analyzed across space to determine the start and end dates, and ultimately create a continuous LGP surface.

Determining growing seasons using satellite data provides a more accurate surface since it is based on actual reflectance values. This method also relies on the statistical analysis of time series data but since the satellite data are available continuously across space there is no need to estimate values using sparsely available point data. There is, thus, added confidence that the growing period data for each individual cell is a measure of truth. With the increased availability of satellite data and the growing periods can be more easily measured for multiple points in time.

Use of the Enhanced Vegetation Index

In order to improve our definition of growing seasons for input into the crop growth model and ultimately to redefine the Agroecological Zones surface, we explored the use of 1 kilometer (km) resolution greening up/down data derived from MODIS satellite images. These data are available for 4 years (2001-2004) and provide a comprehensive picture of the start and end days of the growing season for each year based on the Enhanced Vegetation Index (EVI). The EVI is a refined vegetation index that ‘de-couples’ the canopy background signal and reduces atmospheric influences (Huete et al., 1999). This index is considered a compliment to, but also an improvement over, NDVI since it uses both the red, NIR and blue reflectance values. The greening up/down data are presented in days based on a value of 1 for Jan 1, 2000 up to a value of 1826 for December 31, 2004. The value for each cell represents the date of greenup and the date of senescence (greening down). The processing of the satellite data was done at UMD. For most years there are four surfaces of start days and four surfaces of end days (two each forJan.-Dec.and two each for July-June of the following year). For HarvestChoice the data sets were analyzed together to determine the start and end dates for each calendar year and whether the pixel represents a bimodal area. The annual values were then compared to determine a ‘best guess’ representation of the start and end dates for a given pixel. The source data are 1x1km resolution. The analysis was done at 10x10km.

The graph to the right illustrates how the onset, senescence (decline), and thus length of the growing season are determined based on the satellite derived EVI. The bottom axis represents time (in days or weeks) and the side axis represents reflectance values. The growing season begins at the point that the reflectance value indicates a ‘greening up’ of the given cell.
A detailed description of how these data were used to determine the start, end, length, and modality of the growing season is available in Creating Growing Period Start and End Dates using satellite derived data – notes on methodology v1.2.

Data Gaps

The greening up/down images only provide data for cells that have a change in phenology throughout the year. Thus cells that are never green (desert, ice, urban, and other bare areas) or always green (evergreen forests) are left null. This is problematic for a number of reasons: 1) Because the documentation is no longer readily available, we do not know how these cells were identified. Did they eliminate certain cells based on their classification in ancillary land cover data set(s) or did they eliminate these areas based strictly on the reported EVI? 2) These cells do not naturally coincide with areas that we (IFPRI/HC) define as non-vegetated or evergreen forests based on GLCCD, GLC2000, MODIS so we need to decide how to best ‘fill-in’ values for these cells.

To test the extent of the problem we summarized the GLC2000 & MODIS (2001 data) land cover classes into six summary classes: Evergreen forest; deciduous or mosaic forest; dhrub or grassland; croplands or cropland mosaic; non-vegetated areas; and water. The above map shows the deciduous forest, shrub/grassland, and cropland classes as ‘other’ in order to better isolate the evergreen and non-vegetated classes. This is overlayed with a Boolean surface showing whether a cell has a growing season or not (yes = 1; no = 0). The GLC2000 and MODIS aggregations were overlayed, and the extent of growing season (for SSA) into one surface (ssa-grseasonxlndcov) and created a map showing areas that had growing season data and if not, how the cells were classified in the GLC2000 and MODIS data. In the map, the evergreen and non-vegetated classes took precedence over the other classes, ie. if either of the datasets had a cell classified as evergreen, we classified it as evergreen and did the same for non-veg & water (in that order). The rest fell into ‘other’.

DATA

Start of Growing Season A (Version 1.0)

The start weeks for the primary growing season (A) are based on values dating from 2001-2004. The source data contains a value in days (0-365), indicating the greening up day for the given year. These values were reclassified to weeks (values 1-52) in order to decrease file size and processing time. The ‘greening up’ data for all years were combined and the minimum and maximum start dates were identified for each year. The growing season start date was determined based on the median of the minimum start dates for the four years of data. Medians were used as a quick means of avoiding the assignment of anomalous values for the start date.

• Frequency distribution by start week A by country
• Growing season reporting tool by country & HarvestChoice administrative unit
• Download data

Start of Growing Season B (Version 1.0)

The start weeks for the secondary growing season (B) are based on values dating from 2001-2004. The source data contains a value in days (0-365) indicating the greening up day for the given year. These values were reclassified to weeks (values 1-52) in order to decrease file size and processing time. The ‘greening up’ data for all years were combined and the minimum and maximum start dates were identified for each year. The growing season start date for the primary growing season was determined based on the median of the minimum start dates for the four years of data. The growing season start date was determined based on the median of the minimum start dates for the four years of data. If there was a gap of 10 weeks or greater between the earliest and latest start dates within a calendar year for 2 or more years then it was assumed that the cell has a bi-modal growing season. This cell was then flagged and the start date for the second growing season was determined using the median of the later dates for each year. Medians were used as a quick means of avoiding the assignment of anomalous values for the start date.

• Growing season reporting tool by country & HarvestChoice administrative unit
• Download data

Bi-modal Regions (Version 1.0)

Bimodal areas are those with two or more distinct growing seasons. For this analysis, regions were defined as bimodal if there was a gap of 10 weeks or greater between the earliest and latest start dates within a calendar year for two or more years. This measure was taken on a cell by cell basis and the qualifying cells were then flagged as bimodal.

• Growing season reporting tool by country & HarvestChoice administrative unit
• Download data

End of Growing Season A (Version 1.0.beta)

The end weeks for the primary growing season (A) are based on values dating from 2001-2004. The source data contains a value in days (0-365) indicating the end of scenesence (greening down) for the given year. These values were reclassified to weeks (values 1-52) in order to decrease file size and processing time. The ‘greening down’ data for all years were combined and the end date was determined based on an analysis of the latest dates for each year. Cells identified as bi-modal were analyzed separately since the latest date would more than likely correspond to the second growing season so this had to be accounted for. The growing season start date was determined based on the median of the minimum start dates for the four years of data. Medians were used as a quick means of avoiding the assignment of anomalous values for the start date.

• Growing season reporting tool by country & HarvestChoice administrative unit
• Download data

Length of Growing Season A (Version 1.0.beta)

The length of the primary growing season (A) was determined by subtracting the calculated start week from the calculated end week (see start and end date sections for a more detailed description of these surfaces and how they were derived).

• Frequency distribution by start week by country
• Growing Season reporting tool by country & HarvestChoice administrative unit
• Download Data

Download Data

Reporting Tools

Location-specific Start Week of Seasons A and B

• Drag and drop the marker to retrieve the site-specific growing seasons information
• Overlaid map indicates the start week of the growing season A (see the Map 1 for legend)
• The map looks better on Firefox/Chrome/Safari than on Internet Explorer

Frequency distribution by start week for select countries in Sub-Saharan Africa

Frequency distribution by start week for the agroecological zones within the cool, Tropics in Sub-Saharan Africa

Frequency distribution by start week for the agroecological zones within the cool, Subtropics in Sub-Saharan Africa

Frequency distribution by start week for the agroecological zones within the warm, Subtropics in Sub-Saharan Africa

Earliest start week – Season A (col 1) & Season B (col 2)

Coming Soon

• Comparison to cropping calendar by country
• Updated and revised end weeks and LGP data
• Improved Agroecological Zones dataset using satellite derived LGP surface

References

• Huete, A, C Justice, W van Leeuwen. April 1999. MODIS VEGETATION INDEX (MOD 13). Algorithm Theoretical Basis Document. Version 3. Available at: http://modis.gsfc.nasa.gov/data/atbd/atbd_mod13.pdf

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