JNTU B.Tech II HYDROLOGY Previous Question Paper (November/December - 2020 - R13)

Precipitation is the primary source of water in the hydrological cycle, occurring when atmospheric water vapor condenses and falls to the Earth's surface. It is classified based on the mechanism of lifting and the physical state of the water.

Types of Precipitation (based on lifting mechanisms):

1. Cyclonic Precipitation: Caused by the lifting of air masses due to pressure differences. It is divided into frontal precipitation, where warm air rises over cold air at a front, and non-frontal precipitation, occurring in low-pressure areas (cyclones) where air converges and rises.
2. Convective Precipitation: Occurs when air near the ground is heated intensely, causing it to rise rapidly as a localized convection current. This often results in heavy, short-duration showers, typical of thunderstorms.
3. Orographic Precipitation: Caused by moist air masses being forced to rise over mountain barriers. As the air rises, it cools and condenses, leading to heavy rainfall on the windward side and a 'rain shadow' effect on the leeward side.
4. Turbulent (or Convergence) Precipitation: Results from the friction of the Earth's surface causing air to rise, often occurring when air moves from a smooth surface (like the ocean) to a rough surface (like land).

Forms of Precipitation (based on physical state):

• Rain: Liquid water droplets with a diameter greater than 0.5 mm.
• Drizzle: Fine water droplets with a diameter less than 0.5 mm and low intensity.
• Snow: Ice crystals formed by sublimation, often occurring in hexagonal shapes.
• Glaze (Freezing Rain): Rain that freezes upon contact with cold surfaces, creating a coating of ice.
• Sleet: Frozen raindrops or refrozen melted snow, appearing as small, translucent ice pellets.
• Hail: Large, irregular lumps of ice formed in intense thunderstorms where water droplets are carried into freezing atmospheric layers by strong updrafts.

The average depth of rainfall over a basin is determined using three primary methods, depending on the distribution and density of rain gauges within the catchment area.

1. Arithmetical Mean Method: This is the simplest method, calculated by taking the arithmetic average of the rainfall depths recorded at all stations within the basin. It is expressed as:

where P_1, P_2, ..., P_n are the rainfall depths at n stations. This method is only accurate when the basin is relatively flat and the rain gauges are uniformly distributed.

2. Thiessen Polygon Method: This method assigns a weight to each station based on the area of the basin it represents. The steps are:

1. Plot all rain gauge stations on a map of the basin.
2. Connect adjacent stations with straight lines to form a network of triangles.
3. Draw perpendicular bisectors to these lines to create polygons around each station.
4. Calculate the area of each polygon (A_i) and multiply it by the rainfall at the enclosed station (P_i).
5. The average rainfall is the weighted average: P_avg = (Σ P_i * A_i) / Σ A_i.

This method is superior to the arithmetic mean as it accounts for non-uniform gauge distribution.

3. Isohyetal Method: This is considered the most accurate method. It involves drawing isohyets (lines of equal rainfall) based on point rainfall data. The steps are:

1. Draw isohyets using interpolation between stations.
2. Calculate the area between successive isohyets (a_1, a_2, ...).
3. Determine the average rainfall between these isohyets (p_1, p_2, ...), typically taken as the mean of the two bounding isohyets.
4. Calculate the total volume of rainfall as the sum of the products of these areas and their corresponding average rainfall depths.
5. Divide the total volume by the total area of the basin to obtain the average depth.

The formula is: P_avg = (Σ a_i * p_i) / A_total. This method effectively accounts for the orographic effects and spatial variability of rainfall across the basin.