Groundwater: a global assessment of scale and significance - Shah, Tushaar; Burke, J.; Villholth, Karen G.; Angelica, M.; Custodio, E.; Daibes, F.; Hoogesteger, J.; Giordano, Mark; Girman, J.; Gun, Jac van der; Kendy, E.; Kijne, Jacob W.; et al. (2007)