Jawaharlal Nehru Technological University Hyderabad B.Tech III WATER RESOURCES ENGINEERING - I Previous Question Paper (February - 2022 - R13)

The average depth of rainfall over a basin is computed using three primary methods:

1. Arithmetical Mean Method: This is the simplest method, calculated by taking the arithmetic average of rainfall values recorded at all rain gauge stations within the basin. It is suitable for flat areas with uniformly distributed gauges.
2. Thiessen Polygon Method: This method assigns weights to each station based on the area of the polygon surrounding it. The basin is divided into polygons by drawing perpendicular bisectors of the lines connecting adjacent stations. The average rainfall is the weighted average of the rainfall at each station, where weights are the areas of the respective polygons.
3. Isohyetal Method: This is the most accurate method. Isohyets (lines of equal rainfall) are drawn based on point measurements. The average rainfall is calculated by determining the area between successive isohyets, multiplying each area by the average rainfall of that interval, summing these products, and dividing by the total basin area.

Run-off is the portion of precipitation that makes its way towards streams, rivers, or lakes after accounting for losses such as evaporation, transpiration, and infiltration. It represents the water flowing over the land surface (surface run-off) or through the soil (sub-surface run-off) to reach a drainage channel.

Factors affecting run-off from a catchment area include:

• Precipitation Characteristics: Intensity, duration, and distribution of rainfall significantly influence the volume and rate of run-off.
• Catchment Characteristics: Size, shape, and topography of the basin; a steep slope promotes faster run-off, while a flat area encourages infiltration.
• Soil Type and Geology: Soil permeability and the underlying rock structure determine the infiltration capacity; sandy soils allow more infiltration, reducing surface run-off.
• Land Use: Vegetation cover intercepts rainfall and increases infiltration, whereas urbanized or paved surfaces increase the run-off coefficient.
• Antecedent Moisture Condition: If the soil is already saturated from previous rainfall, its infiltration capacity is reduced, leading to higher run-off.
• Climatic Factors: Temperature, humidity, and wind speed affect the rate of evaporation and evapotranspiration, thereby reducing the net run-off.