CROP PRODUCTION AND ROAD CONNECTIVITY IN SUB-SAHARAN AFRICA This paper is organized as follows. Section 1 describes the databases used and presents basic descriptive statistics. Section 2 discusses the econometric methodology. Section 3 presents results of regressions of agricultural output and choice of technology for various Sub-Saharan crops and regions. Section 4 presents results of a simulation of improvements in road quality. The final sections, 5 and 6, summarize the paper, present policy implications, and discuss possible further work.

# 1   Data

Three major types of spatial data are used in the analysis: agroecological zones and crop potentials as derived by FAO and IIASA, estimates of the spatial allocation of crops from the IFPRI SPAM, and various measures of economic distance constructed on the basis of estimated travel times and market size. This section presents a brief description of this data and details of the SPAM.

## Agroecological zones

The agroecological zone (AEZ) methodology developed by FAO and the IIASA combines georeferenced data on land resources (climate, soil, and terrain) with a mathematical model for the calculation of potential biomass and yields per crop and management system. Land resource data include an adjustment for estimated land requirements for housing and infrastructure, based on population and population density. The FAO-IIASA model produces biomass and yield estimates for 154 different land utilization types (LUTs) comprising various rainfed and irrigated crops, and fodder and grassland land uses, each at three generic levels of inputs and management (high, intermediate, and low).<sup>1</sup> The resolution of the model is 0.5-degree latitude/longitude cells. Crop agronomic potential in each location is summarized in the Crop Suitability Index (SI), which is defined as: SI = VS*0.9 + S*0.7 + MS*0.5 + NS*0.3, where VS, S, MS, and NS denote percentages of the grid cell with attainable yields that are 80 percent or more, 60–80 percent, 40–60 percent, and 20–40 percent of maximum potential yield. SI is essentially a measure of quality adjusted land area (the adjusted share of land suitable for cultivation of a particular crop or group of crops), with a maximum value of 0.9 and a minimum value of 0.<sup>2</sup>

## IFPRI SPAM

The IFPRI SPAM (You and others 2007b) is designed to estimate the spatial distribution of crop production in Sub-Saharan Africa. The model combines data on national and subnational crop area and production (three-year average, 1999–2001), land cover, and land suitability using a cross-entropy regression to estimate the spatial allocation of crops at a resolution of 5x5 minutes (approximately 9x9 kilometers on the equator).<sup>3</sup> Specifically, the model takes as inputs (a) crop area (the total physical area cultivated at least once per year from subnational estimates, generally from official government statistics), (b) total crop land (from 1 For irrigated land, only high and intermediate levels of inputs are defined. 2 For more information, consult FAO (2003, 1981); FAO, IFPRI, SAGE (2006); and Fischer and others (2001).

3 The number of pixels per country is given in table A.1.

2