Irrigation & Drainage management
Measurement of Irrigation Water
|
Unit |
Equivalent |
Remarks |
|
1 Cusec (Cubic foot per second) |
28.3 litres/sec |
Used for small water flow measurement |
|
1 Cumec (Cubic metre per second) |
1000 litres/sec |
Used for large canal discharges |
|
1 hectare-centimetre (ha-cm) |
100,000 litres |
Volume of water required to cover 1 ha area to 1 cm depth |
|
1 hectare-metre (ha-m) |
10,000,000 litres |
Volume of water to cover 1 ha area to 1 m depth |
Measurement of Water Height
|
Unit |
Equivalent Height of Water |
|
1 Atmosphere (atm) |
1036 cm of water |
|
1 Bar |
1023 cm of water |
Conversion Units
|
Unit |
Equivalent |
|
1 Acre |
0.405 ha |
|
1 ha |
2.47 acres |
Classification of Ploughing (CRIDA, Hyderabad 1985)
|
Type |
Depth (cm) |
Use |
|
Shallow |
5–6 cm |
For seedbed preparation, interculturing |
|
Medium |
15–20 cm |
General purpose ploughing |
|
Deep |
25–30 cm |
To break hardpan, improve aeration and root penetration |
Watershed Management
🔹 Definition: A watershed is a natural unit of land where runoff collects and drains through a common outlet like a stream or river.
🔹 Classification (based on area):
|
Type |
Area (ha) |
|
Macro-watershed |
> 50,000 ha |
|
Sub-macro watershed |
10,000 – 50,000 ha |
|
Milli watershed |
1,000 – 10,000 ha |
|
Micro watershed |
100 – 1,000 ha |
|
Mini watershed |
10 – 100 ha |
Irrigation
Definition: Artificial application of water to the soil to meet crop evapotranspiration (ET) needs.
Methods of Irrigation
Surface Irrigation (Most common in India)
Includes several methods:
|
Method |
Description |
Suitable For |
Key Points |
|
a. Flood irrigation |
Entire field flooded |
Rice |
High water loss, low efficiency |
|
b. Check basin |
Field divided into basins surrounded by ridges |
Wheat, Groundnut |
Most common, labor-intensive |
|
c. Ring basin |
Water applied in circular basins |
Fruit crops |
Efficient for orchards |
|
d. Border strip |
Field divided into long strips |
Wheat, Bajra |
For 0.5–1% slope soils |
|
e. Furrow method |
Water flows through small channels between rows |
Maize, Potato, Sugarcane |
Best for row crops, avoids water contact with stems |
|
f. Corrugation |
Small shallow furrows |
Close-growing crops |
Low cost method |
Surge Irrigation: Intermittent ON–OFF flow under gravity to minimize deep percolation & runoff.
Cablegation: Automatic surface irrigation method (less used in India).
Sub-surface Irrigation
- Water applied below the soil surface through pipes.
- Used in Kerala (coconut) and Gujarat, Kashmir (vegetables).
- Suitable for sandy loam soils.
- Reduces evaporation losses.
Sprinkler Irrigation
💧 Simulates natural rainfall.
Pressure: 2.5–4.5 kg/cm²
Discharge rate: >1000 lit/hr
Water saving: 25–50%
WUE (Water Use Efficiency): ~60%
Advantages:
- Suitable for undulating lands.
- Prevents soil erosion.
- Reduces labor by 40–60%.
- Can protect crops from frost and heat stress.
- Can apply fertilizer (Fertigation).
Not suitable for: Heavy clay soils & rice cultivation.
Drip (Trickle) Irrigation
💧 Introduced from Israel.
Definition: Slow, precise application of water directly to root zone through emitters.
Pressure: 1.5–2.5 kg/cm²
Discharge rate: 1–8 lit/hr
Water saving: 50–70%
Advantages:
- Saves water and fertilizers (Fertigation possible).
- Reduces weed growth and soil erosion.
- Suitable for orchards, vegetables, greenhouses.
- Efficient for undulating and saline areas.
Special Type:
Typhoon System of Drip Irrigation (used in sugarcane).
Drainage
Definition: Drainage is the removal of excess surface or subsurface water to maintain favorable soil conditions for plant growth.
Types of Drainage
Surface Drainage
- Removal of surface water by open ditches.
- Simple and low-cost.
- Common in India.
- Drainage of Flat Areas: Slope < 2%.
b. Broad Bed and Furrow (BBF) System:
- Beds 120–150 cm wide, 15 cm high.
- Furrows 45 cm wide.
- Suitable for groundnut, pulses in heavy soils.
- 0.5% slope for easy drainage.
Subsurface Drainage
- Removes water below root zone to lower water table.
Methods:
|
Method |
Description |
Suitable for |
|
Tile drains |
Perforated pipes buried underground |
Alluvial soils |
|
Mole drainage |
Unlined channels formed by mole plough |
Clay soils |
|
Vertical drainage |
Using wells or boreholes to pump out water |
Coffee plantations, high rainfall zones |
Methods of Measuring Flow
|
Device |
Use |
Formula |
|
Orifices |
Small streams or furrows |
Q = a √(2gh) |
|
Weirs |
Large channels |
Q = CLH³/² |
|
V-notch (90°) |
Small flows |
Q = 0.0138 H²·⁵ |
|
Parshall Flume (Venture Flume) |
Combines weir and orifice |
Used for flat gradient channels |
Soil Moisture Constants & Matric Potential
Soil Moisture Potential
It is the energy status of soil water which determines its availability to plants.
Types of Soil Water Potential:
- Matric Potential (Ψm): Due to adhesion and capillary forces. Always negative. Dominant in unsaturated soil.
- Osmotic Potential (Ψo): Due to dissolved salts in soil water. Affects water uptake under saline conditions.
- Gravitational Potential (Ψg): Due to gravity; affects drainage.
- Pressure Potential (Ψp): Positive in saturated soil or turgid cells.
🌾 Soil Moisture Constants
|
Constant |
Tension (bars) |
Description |
|
Saturation |
0 |
All pores (macro + micro) filled with water |
|
Field Capacity (FC) |
–0.1 to –0.3 bars |
Water left after gravitational drainage; maximum available for plants |
|
Permanent Wilting Point (PWP) |
–15 bars |
Plants permanently wilt and cannot recover |
|
Hygroscopic Coefficient |
–31 bars |
Thin film of water tightly bound to soil particles |
|
Available Soil Moisture (ASM) |
FC – PWP |
Water usable by plants |
|
Capillary Water |
Held between FC and PWP |
Available to plants |
|
Gravitational Water |
Held above FC |
Drains quickly, unavailable |
|
Hygroscopic Water |
Below PWP |
Unavailable to plants |
Key Competitive Points
- Matric potential is negative in unsaturated soil.
- Soil moisture at saturation = 0 bar.
- Plant available water = FC – PWP.
- Field capacity attained 2–3 days after irrigation/rain.
- Wilting coefficient (PWP) determined using sunflower test.
- Soil moisture units: 1 bar = 10⁶ dynes/cm² = 1023 cm of water.
- Tensiometers measure soil moisture up to 0.85 bar tension.
