• Definition: PMP is the theoretically greatest amount of precipitation that is meteorologically possible for a specific location and time.

• Significance:
– Used for hydrological safety design of critical structures (e.g., dams, nuclear plants).
– Basis for determining the Probable Maximum Flood (PMF).
– Helps in evaluating the risk and performance of infrastructure under extreme conditions.
– Ensures compliance with national and international safety standards.

PMP estimation depends on a variety of meteorological and geographic conditions:

1. Atmospheric Moisture Content: The capacity of the atmosphere to hold moisture.
2. Storm Efficiency: How effectively atmospheric moisture is converted into precipitation.
3. Topographic Features: Mountains, valleys, and elevation changes affect orographic lifting.
4. Storm Path and Duration: The trajectory and longevity of storm systems.
5. Temperature: Impacts both evaporation and the type of precipitation (e.g., snow or rain).

There are primarily three methods used to estimate PMP:

14.3.1 Statistical Method (Empirical Method)
• Based on historical rainfall records and extreme value analysis.
• Limited to regions with long and reliable rainfall data.
• Uses frequency analysis to extrapolate extreme precipitation values.
• Limitations: Assumes past maximum events can predict future extremes, which may not be sufficient for PMP estimation.

14.3.2 Hydrometeorological Method
• Moisture Maximization Approach:
– Based on actual storm events but scaled up using higher moisture content.
– Uses the formula:

\[ PMP = P \times \frac{PW_{max}}{PW_{storm}} \]

• Where:
– P = observed precipitation
– PW_{max} = maximum precipitable water at location
– PW_{storm} = precipitable water during the storm event.

• Transposition Technique:
– Involves applying the characteristics of extreme storms from one region to another.
– Storm data from areas with more extreme weather is 'transposed' geographically.

• Envelopment Curve Method:
– Uses upper limits of rainfall from multiple storms across regions.
– Draws envelope curves representing maximum limits for different durations and areas.

14.3.3 Numerical Weather Modelling
• Advanced method using mesoscale meteorological models.
• Simulates extreme storm events by inputting boundary conditions with maximum moisture.
• Requires supercomputing and expert input.
• Still under research and development in many countries.

• Design of Dams and Spillways:
– Critical for setting spillway capacity to prevent overtopping.

• Flood Hazard Mapping and Risk Assessment:
– Establishes PMF for emergency preparedness.

• Urban Drainage Systems:
– Helps in designing for rare, high-intensity events.

• Nuclear Facility Safety Design:
– Ensures cooling and containment systems are protected under extreme rainfall.

• Data Availability: Lack of high-quality rainfall and moisture data, especially in remote regions.

• Assumptions in Maximization: Meteorological assumptions may not hold under future climate conditions.

• Transposition Errors: Applying storms from one region to another may lead to inaccuracies.

• Climate Change Uncertainty: Future PMP may be different due to changing atmospheric moisture capacity.

• Warming climate leads to increased moisture-holding capacity in the atmosphere (as per Clausius-Clapeyron relation).

• Potential increase in extreme precipitation events globally.

• Current PMP values may need revision under future climate scenarios.

• Use of Regional Climate Models (RCMs) and Global Climate Models (GCMs) is being explored to project future PMP values.