1. Gibbs, W.J. (1975). Drought—Its Definition, Delineation, and Effects, World Meteorological Organization. Special Environmental Report No.5.
2. Estrela, T., Menéndez, M., Dimas, M., Marcuello, C., Rees, G., Cole, G., Weber, K., Grath, J., Leonard, J., and Ovesen, N.B. (2001). Part 3: Extreme hydrological events: Floods and droughts, Sustainable Water Use in Europe, Environmental issue report No 21.
3. Understanding: The drought phenomenon: The role of definitions;Wilhite;Water Int.,1985
4. American Meteorological Society (1997). Meteorological drought-policy statement. Bull. Am. Meteorol. Soc., 78, 847–849.
5. Tallaksen, L.M., and Van Lanen, H.A.J. (2004). Hydrological Drought. Processes and Estimation Methods for Streamflow and Groundwater, Elsevier Science B.V.. Developments in Water Science.