Methods

Our suitability mapping work utilizes six environmental parameters to determine the suitability of growing a given crop for a given site:

• Rainfall
• Air temperature
• Soil pH
• Soil depth
• Soil drainage
• Surface slope

Crop growth ranges

For each crop, the “optimum (or ideal)” and “absolute” growth ranges of those environmental parameters were first obtained from the FAO EcoCrop database and then modified with local expert knowledge from CTAHR researchers and extension agents. The optimum growth range for a crop defines the range where the optimum growth of the crop is expected. The absolute growth range for a crop defines the range outside of which the crop is not expected to produce viable yields.

Determining suitability

The site suitability for a crop was determined for every environmental parameter by comparing the parameter value of the site to the optimum and absolute growth ranges of the crop. When the site parameter value was within the optimum range, the site was rated as Suitable for the environmental parameter. When the site parameter value was outside the optimum range and less than half way to the absolute growth range, the site was rated as Conditionally Suitable for the parameter. Any parameter value less than half the optimum, the site was rated as Unsuitable. The overall suitability of the site was determined conservatively by taking the lowest rating out of the six suitability values.

Perennial vs. annual crops

For perennial crops, mean annual rainfall, and mean annual maximum and minimum temperatures were used to determine the rainfall and temperature suitability ratings, respectively. Seasonal variations in rainfall and temperature were not considered in the suitability determination for perennial crops.

For annual crops, mean monthly rainfall, and mean monthly maximum and minimum temperatures were used to determine the rainfall and temperature suitability ratings, respectively, for the maximum growing period. We added this new variable, the maximum growing period, defined as the number of months for an annual cropping cycle from the seeding or planting to the end of harvest (for example, six months for cucumber and four months for carrot). The maximum growing periods of annual crops were first obtained from the FAO EcoCrop database and then revised with local expert knowledge from CTAHR researchers and extension agents for every annual crop.

Since the optimum and maximum rainfall ranges were based on annual rainfall, we simply divided them by twelve to obtain the optimum and maximum monthly rainfall ranges. We further adjusted the optimum and absolute monthly rainfall ranges by applying the weight of two to the first month and one to the rest of the growing period as the crops have a higher water demand after the seeding/planting in general. The rainfall suitability was determined by first comparing monthly precipitations to the rainfall ranges of the corresponding months and then taking the lowest rating out of those within the maximum growing period.

Temperature suitability values were determined for each of the maximum and minimum temperatures for every month within the maximum growing period. The lowest out of them was taken as an overall temperature suitability of the site.

Irrigation

The effect of irrigation was incorporated by setting the rainfall suitability to Suitable even when the site’s rainfall amount was lower than the optimum minimum. No adjustment was made when the site’s rainfall was higher than the optimum maximum.