Engineering Hydrology

 👉The hydrological budget equation is:

                          P-R-E-T-G = △S

Where, P = total precipitation

             R = Net runoff = R2-R1 = surface runoff flow- surface runoff inflow

            E = Total evaporation

            T = total Transpiration

          G = Net ground water flow

         △S = Total storage increase

 👉 Hydrological cycle is the cycle in which water is transported from the oceans to the atmosphere as vapours, from the atmosphere to the land as precipitation and back from land to oceans as runoff.   

👉Drizzle:  A fine sprinkle of numerous water droplets of size less than 0.5 mm and intensity less than 1 mm/h

Rain: The precipitation of liquid water with drops of sizes larger than 0.5 mm 

Hail: It is showery precipitation in the form of irregular pellets or lumps of ice of size more than 5 mm.

👉 Average rainfall over a catchment area can be estimated as

1. Arithmetic mean method: 

• every rain gauge station is given equal weightage regardless of its location.
• Rainfall is almost uniformly distributed over the whole catchment area which rarely occurs in nature.
• method is fast but does not give accurate results.

2. Theissen polygon method: 

• In this method weightage to the various rainfall datas based on area close to the rain gauge station 
• Method is fast when once the weights are known
• It does not take care of the variability in rainfall due to elevation difference(topographical influence are not take care of)
• New polygon is required to be drawn when due to addition or deletion of raingauges to the network, weight of each station changes.

3. Isohyetal method :

• This is most accurate method
• It utilizes all relevant data's and properly interpret them.
• method is slow and laborious

4. Normal ratio method: 

• It is used to estimate missing rainfall data.