surveys due to extreme weather conditions, recent violence against Western nationals, and its selfcontained migration patterns. According to the ERSS, 55 percent of respondent households in the Afar region that own livestock indicated that they had taken their livestock outside of the village to graze for at least one night during the previous season, and 41 percent indicated that they had similar plans for the upcoming dry season. Most respondents (56 percent) made only one trip in the previous year, with an additional 24 percent making two trips. Of those making trips with their livestock, less than one percent travelled outside of Afar and no one reported migrating to Zone 2. These results also support our choice of Afar for this pilot project. See figure 1 in the appendix for further details on traditional migration routes.

# **3. Study Design**

## **3.1 Stratification**

We divided Afar into five strata before selecting points to improve the statistical and cost efficiency of the project. The five strata were defined by the expected likelihood of finding herders and livestock, based on an assumption that herds congregate around limited water sources and available pasture in the driest part of the year. Spatial datasets describing land cover, land use, and other geographic features were used as input to delineate five discrete, mutually exclusive strata.

The first stratum consisted of land in or near towns, defined by population density measures in the AfriPop dataset (Tatem, 2010). The second stratum consisted of permanent agriculture, under the assumption that livestock would be largely excluded from these areas. Boundaries were defined based on the interpretation of five meter resolution SPOT Imagery from 2006 from the CSA’s Land Ccover Mapping project, and included commercial agriculture as well as some small individual farms. Area placed in the first two strata was then excluded from remaining strata definitions.

The third stratum consisted of land within two kilometers of a major water source, including the Awash River and its permanent tributaries, and which also met criteria for pasture based on the average annual mean and range of the long-term normalized difference vegetation index (USGS Earth Resources Observation and Science Center, 2012). This stratum was considered to be the most likely to contain livestock. The fourth stratum consisted of land between two and ten kilometers from a major water source which met criteria of pasture land. The remainder of the land was placed into the lowest probability stratum. See Figure 2 in the appendix for a map of the five strata.

A total of 125 points were selected from these five strata for the survey. The total number of points selected and the allocation between strata was based on sample size calculations from the previously collected data from the 2008/2009 Agricultural Sample Survey, the expected number of households to be found and interviewed in each stratum based on the results of the pretests, and the available budget for the pilot project. The number of selected points was higher in the strata where we expected the highest concentrations of potentially nomadic households and livestock (stratum 3), and lower in areas of lower expected density (stratum 5). Points were selected in areas with low-likelihood of finding pastoralists, towns and settled agricultural areas, because excluding these areas would bias the total livestock populations. The radii for the circles also varied across the strata. In areas where we expected higher densities, we drew smaller circles to keep the workload reasonable. In areas where we expected few or no livestock, we expanded the circle radius to the largest feasible dimensions to maximize the probability of finding animals. See Table 1 for the definition, sample size, and radius used in each of the five strata.

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