Effective rainfall and it's calculations

This topic about effective rainfall and its calculations.

1. Effective rainfall and its calculation

2. Presented to: Dr. Mohinder Singh Presented by: Rammehar (B2020A45BVI) Determination of the Effective Rainfall

3. Determination of the Effective Rainfall The amount, the intensity and the distribution of rainfall is important.
1. When rain water falls on the soil surface, some of it infiltrates into the soil, some stagnates on the surface, while some flows over the surface as runoff.
2. When the rainfall stops, some of the water stagnating on the surface evaporates to the atmosphere, while the rest slowly infiltrates into the soil.
3. From all the water that infiltrates into the soil, some percolates below the root zone, while the rest remains stored in the root zone. 3

4. In other words, the effective rainfall is the total rainfall minus runoff minus evaporation and minus deep percolation. EF = Total rainfall – Runoff – Evaporation – Deep percolation 4

5. Factors Affecting Effective Rainfall Soil type (e.g., sandy vs. Clay soils)
Topography (e.g., slope of the land)
Rainfall intensity and distribution
Crop type and rooting depth
Agronomic practices (e.g., mulching, bunding ) 5

6. Importance in Agriculture Helps in irrigation planning.
Determines crop water budgeting.
Influences crop selection in rainfed regions. 6

7. Only 2 simple formulae are provided to estimate the fraction of the total rainfall which is used effectively. These formulae can be applied in areas with a maximum slope of 4-5%: Pe = 0.8 P if P > 75 mm/month Pe = 0.6 P if P < 75 mm/month
with P = rainfall or precipitation (mm/month) Pe = effective rainfall or effective precipitation (mm/month)
NOTE: Pe is always equal to or larger than zero; never negative 7

8. Excercise 1. Calculate the effective rainfall for the following monthly rainfall figures: P = 35,90,116, 5, 260, 75 mm. For monthly rainfall:
If monthly rainfall (P) ≤ 75 mm, then Effective Rainfall = P × 0.8
If monthly rainfall (P) > 75 mm, then Effective Rainfall = 75 + 0.25 × (P − 75)
Let’s apply this to your data: Given monthly rainfall (in mm):
P = 35, 90, 116, 5, 260, 75
Step-by-step Calculation:
1. 35 mm → 35 × 0.8 = 28 mm
2. 90 mm → 75 + 0.25 × (90 − 75) = 75 + 0.25 × 15 = 75 + 3.75 = 78.75 mm
3. 116 mm → 75 + 0.25 × (116 − 75) = 75 + 0.25 × 41 = 75 + 10.25 = 85.25 mm
4. 5 mm → 5 × 0.8 = 4 mm
5. 260 mm → 75 + 0.25 × (260 − 75) = 75 + 0.25 × 185 = 75 + 46.25 = 121.25 mm
6. 75 mm → 75 × 0.8 = 60 mm 8

9. Total Effective Rainfall:
28 + 78.75 + 85.25 + 4 + 121.25 + 60 =
377.25 mm
Final Answer:
Effective Rainfall = 377.25 mm 9

10. Practical Problems in Indian Agriculture 10