Ithnin, H., and N. Sakke. “Precipitation Indices As Indicators of Water Resource Sustainability: A Study on Developed States of Peninsular Malaysia”. Perspektif Jurnal Sains Sosial Dan Kemanusiaan, vol. 5, no. 1, May 2013, pp. 97-106, https://ojs.upsi.edu.my/index.php/PERS/article/view/1627.