Botswana faces two major environmental problems, drought and desertification, which are heavily linked. Three-quarters of the country's human and animal populations depend on groundwater due to drought. Groundwater use through deep borehole drilling has somewhat eased the effects of drought. Surface water is scarce in Botswana and less than 5% of the agriculture in the country is sustainable by rainfall. In the remaining 95% of the country, raising livestock is the primary source of rural income. Approximately 71% of the country's land is used for communal grazing, which has been a major cause of the desertification and the accelerating soil erosion of the country.