Conceptual model of drought intensity-duration-frequency     Type of climate Superarid Hyperarid Arid Semiarid Subhumid Humid Hyperhumid Super- humid Pma (mm) 100 200 400 800 1600 3200 6400 Pma / Pagt 0.125 0.25 0.5 1 2 4 8 Eap (mm) 3000 2400 2000 1600 1200 1200 1200 Eap / Pma 30 12 5 2 0.75 0.375 0.1875 Lrs (months) 1 2 3 4 6 9 12 Moderate intensity 0.25 0.5 1.0 1.5 1.0 0.5 0.25 Severe intensity 0.5 1.0 2.0 3.0 2.0 1.0 0.5 Extreme intensity 0.75 1.5 3.0 4.5 3.0 1.5 0.75 Duration (years) 1 2 4 6 4 2 1 Return period (years) 2 3 6 12 25 50 100