Table 1.
Climate and environmental covariates, and their definitions
| Covariate | Data sources | Definition |
|---|---|---|
| Enhanced Vegetation Index (EVI) | Nigeria Demographic and Health Survey (NDHS) Spatial Analysis data | The average vegetation index value within the 2 km (urban) or 10 km (rural) buffer surrounding the DHS cluster at the time of measurement (year). The enhanced vegetation index was calculated by measuring the density of green leaves in the near-infrared and visible bands. |
| Proximity to waters (Coast/Large Lakes) | NDHS Spatial Analysis data | Straight-line distance to the nearest major water body. Based on the World Vector Shorelines, CIA World Data Bank II, and Atlas of the Cryosphere. |
| Population density | NDHS Spatial Analysis data | Estimates of human population density is the number of persons/km2 based on counts consistent with national censuses and population registers. |
| Precipitation | NDHS Spatial Analysis data | The average precipitation measured within the 2 km (urban) or 10 km (rural) buffer surrounding the DHS survey cluster each year. |
| Travel time to nearest settlement >50,000 inhabitants | NDHS Spatial Analysis data | The average time (min) required to reach a high-density urban center from the area within the 2 km (urban) or 10 km (rural) buffer surrounding the DHS cluster location, based on year 2015 infrastructure data. |
| Minimum temperature | NDHS Spatial Analysis data | The average annual maximum temperature within the 2 km (urban) or 10 km (rural) buffer surrounding the DHS cluster location. The maximum temperature is calculated from the modeled mean temperature and the modeled diurnal temperature range. |
| Maximum temperature | NDHS Spatial Analysis data | The average annual minimum temperature within the 2 km (urban) or 10 km (rural) buffer surrounding the DHS cluster location. The minimum temperature is calculated from the modeled mean temperature and the modeled diurnal temperature range. |
| Potential Evapotranspiration (PET) | NDHS Spatial Analysis data | The average annual PET within the 2 km (urban) or 10 km (rural) buffer surrounding the DHS cluster location, synthetic measurement that was calculated using a variation of the Penman–Monteith formula. |
| Cluster altitude | NDHS Spatial Analysis | Measure of surface altitude (m). The data were interpolated using a thin plate smoothing spline algorithm with altitude, longitude and latitude as independent variables. |
| Wet days | NDHS Spatial Analysis data | The average number of days receiving rainfall within the 2 km (urban) or 10 km (rural) buffer surrounding the DHS cluster location. It combines the number of observed days with rainfall from weather stations with the number of days that should have received rainfall. |
| Urban–rural settlement | NDHS Spatial Analysis data | This is urban–rural population classification of the area within the 2 km (urban) or 10 km (rural) buffer surrounding the DHS survey cluster location. It is the representation of the degree of urbanization concept obtained using urban-rural settlement classification model adopted by the Global Human Settlement Layer (GHSL) project. |