climate they use the Standardized Precipitation and Evapotranspiration Index (SPEI) developed by VicenteSerrano et al. (2010). The SPEI factors in both precipitation and potential evapotranspiration to capture the ability of soil to retain water, and thus outperforms other indices in predicting crop yields. The SPEI is expressed in units of standard deviation from each grid-cell’s historical average (and thus has a mean of zero). HF measure yearly SPEI for each grid-cell as well as SPEI specific to the growing season of the main crop in a given grid-cell. Additionally, HF utilize data on conflict occurrence between 1997 and 2011 from the Armed Conflict Location and Event Dataset (ACLED), which records a wide range of conflict events such as protests, battles and rebel activities derived from war zone media reports, humanitarian agencies and research publications. As is standard in the conflict literature, HF code a grid-cell ( ) as a dummy equal to 1 if the grid-cell experienced any conflict event in a given year ( ). The data from HF on the occurrence of ii conflict and the yearly SPEI for the continent of Africa used by HF are presented in Figure A1 in the appendix. tt The analysis in section 5.1 also uses all time-invariant characteristics of grid-cells that HF also control for, taken directly from their data – these include cell-specific measures of elevation, roughness, area, presence of roads, distance to river, cell shared by multiple countries, border presence, presence of minerals, and ethnolinguistic fractionalization.

The next empirical approach presented in section 5.2 examines conflict breakout in West Africa more specifically, before and after the Arab Spring (circa 2011), covering the period between 1998 and 2017. This requires using data on conflict events directly provided by ACLED, since the HF analysis does not cover the post-2011 period. To focus the analysis on fragility arising from violent political clashes over control of irrigated territory, this section focuses on a narrower definition of conflict which captures only the occurrence of “battles” in ACLED, which is defined as “a violent interaction between two political organized armed groups at a particular time and location”, with a grid-cell being coded as a dummy equal to one if a battle occurred in year . Figure A2 in the appendix plots the spatial distribution of the frequency of battle ii events in the countries of Western Africa, and for the whole African continent, at the 50km x 50 km tt resolution, which is the resolution of the main analysis in section 5.2. Additionally, to measure the share of each grid-cell that is irrigated at the start of the analysis period gridded, gridded data on the average area equipped for irrigation between 1990 and 2005 is taken from the FAO. The construction of this data set is described in Siebert et al (2015). Figure A2 also plots this data at the 1km x 1km resolution at which the raw data is available. To conduct the analysis, this raw data is aggregated to 50km x 50km by taking the average percentage of area irrigated in all smaller cells that fall within a larger grid-cell.

In addition to these main data, the analysis detailed in section 5.2 also employs various other time-invariant grid-cell level characteristics to control for possible confounds of climate and irrigation availability that may also be correlated with conflict. These include: the share of area with cropland and pastureland taken from Ramankutty et al (2008); population density in the year 2000 derived from the Gridded Population of the World data set (GPW v4); the presence of wetlands from the Global Lakes and Wetlands Database (GWLD v3) from WWF; and the presence of ethnic homelands of ethnicities whose historic economic activity is identified as being agricultural (see Michalopoulos and Papaioannou, 2013 for data description).

# 5. Empirical Approach and Results

Building on our conceptual framework, this section analyzes the relationship between water and fragility in the context of the Sahel region. We first show evidence that the region is historically at a higher risk of conflict related to variations in water availability. We argue that this is due to the high degree of water 7