Module EVE203: Unit I - Comprehensive Study Notes on Irrigation Engineering

Introduction to Irrigation Engineering

• Essentiality of Water: Water is the most essential commodity on earth, required for the survival of both human and animal life.

• Natural Cycle: The chief source of potable water for humans and animals is rain, which occurs through evaporation from oceans driven by appropriate wind forces.

• Available Water Sources:

  • Ice on high mountain peaks.

  • Dew on open ground.

  • Groundwater located below the earth's surface.

• Climatic Variability: Heavy rains can lead to floods that endanger crop yields, while a scarcity of rain leads to famine across the globe.

• Resource Management: Mankind has developed methods to store water during periods of excessive rain to ensure its availability during periods of "less rain" or "no rain."

• Definition of Irrigation: Irrigation is defined as the science of artificial application of water to the land in accordance with the crop requirement throughout the crop period for the full-fledged requirement of the crop.

Irrigation Status in Bhutan

• Agrarian Context: Bhutan is predominantly an agrarian society. The agriculture sector is the primary driver for growth in the combined sector of agriculture, livestock, and forestry.

• Water Resource Availability: Bhutan possesses one of the highest per capita water resource availabilities in the world, recorded at $94,500\,\text{m}^3/\text{capita/annum}$ (Bhutan Water Fact 2018).

• Accessibility Challenges: Despite the high availability of resources, accessibility to water for irrigation and drinking at the community level remains a challenge due to geographical terrains and climatic conditions.

• Policy Framework:

  • National Irrigation Policy (NIP): Developed by the Ministry of Agriculture and Livestock (MoAL) to envision a stable and productive agriculture sector with dynamic and sustainable irrigation systems.

  • National Irrigation Master Plan (NIMP): A 15-year action plan for developing climate-adaptive irrigation systems and irrigated agriculture in the country.

• Agriculture Land Statistics (2019):

  • Total Wetland: Approximately $56,069\,\text{acres}$, which constitutes $21\%$ of the total arable land in Bhutan.

  • Rice Cultivation: Of the total wetland area, $31,574\,\text{acres}$ are currently under rice cultivation.

• Management Types (NIMP 2016):

  • Community Managed Irrigation System (CMIS): The oldest form of irrigation in Bhutan. Constructed and managed by the community using local knowledge and skills. The government (RGoB) or donor projects support repair, maintenance, and expansion.

  • Agency-built Community Managed Irrigation System (ACMIS): Systems constructed by the government and subsequently handed over to the community for management.

• Irrigation Methods in Bhutan (RNR Census 2019):

  1. Surface Irrigation ($89\%$): Water is applied and distributed across the field surface by gravity. This is the primary method for rice cultivators.

  2. Sprinkler Irrigation ($9\%$): Water moves under pressure through pipe networks and is delivered via nozzles.

  3. Localized Irrigation ($2\%$): Water is distributed under low pressure through a piped network in a predetermined pattern (e.g., drip and micro-irrigation).

Micro-Irrigation Systems

• Overview: A modern irrigation method that uses drippers, sprinklers, foggers, or other emitters on the land surface or subsurface. It is popular due to its low cost and water efficiency.

• Drip Irrigation (Trickle Irrigation):

  • Mechanism: Water is applied drop by drop near the plant's root zone.

  • Suitability: Most suitable for crops with wider spacing.

  • Emitters: Use vents, twisters, or long flow paths to optimize water pressure, allowing only a limited amount of water to pass. They can be placed on the ground or buried deep in the soil.

• Sprinkler Irrigation:

  • Mechanism: Water is pumped through a pipe system and distributed through sprinklers that split the water into tiny drops to fall like natural rainfall.

  • Coverage: Spray heads at terminals distribute water over the entire soil surface.

  • Efficiency: Provides water efficiently and ensures improved crop yield by supplying the exact amount needed for optimum growth.

Pros and Cons of Micro-Irrigation Systems

Drip Irrigation

• Pros:

  • Water saving and higher yields.

  • Increased fruit size and high quality.

  • Suitable for all soil types.

  • Facilitates easy fertigation (applying fertilizer) and chemigation (applying chemicals).

  • Reduction in labor and field preparation costs.

• Cons:

  • High initial investment cost.

  • Emitters are prone to clogging.

  • System components are susceptible to damage by animals.

Sprinkler Irrigation

• Pros:

  • Affordable and easy to set up.

  • Requires minimal field area for installation.

  • Allows frequent water application.

  • Supports fertigation and chemigation.

  • Can be used for environmental cooling during high temperatures.

  • Ensures equal water distribution across all soil types.

• Cons:

  • Requires a constant water supply for even spraying.

  • High risk of evaporation in windy or high-humidity environments.

  • Nozzles can become clogged with debris or sediments.

  • Requires a continuous power supply for operation.

Technical Terminologies and Crop Seasons

• Kharif Season:

  • Duration: Middle of June to middle of October.

  • Base Period: $120\,\text{days}$.

  • Major Crops: Rice, maize, millets, groundnuts.

• Rabi Season:

  • Duration: Middle of October to middle of February.

  • Base Period: $120\,\text{days}$.

  • Major Crops: Wheat, gram, potatoes, pulses. Rabi crops generally require less water than Kharif crops.

• Catchment Area: The area where rainfall flows by gravity toward a river or reservoir for storage. It represents the source of water.

• Command Area: The area to which stored water is distributed by a canal network for irrigation. It represents the utilization of water.

• Crop Rotation: The practice of growing different types of crops in the same area across a sequence of seasons.

• Base Period ($B$): The time interval between the first watering at the time of sowing and the last watering before harvesting, expressed in days.

• Duty ($D$): The irrigating capacity of water. It is the number of hectares of land irrigated by one cumec ($\text{m}^3/\text{s}$) of water supplied throughout the base period.

  • Unit: $\text{hectare/cumec}$ (Area/Discharge).

  • Significance: Helps determine water requirements for crop maturity, canal capacity, and reservoir size.

• Delta ($\Delta$): The total depth of water required from seeding to harvesting, expressed in centimeters ($\text{cm}$) or meters ($\text{m}$).

• Crop Period: The time interval between actual sowing and harvesting, expressed in days.

Mathematical Relationships and Formulas

• Relationship between Base Period ($B$), Duty ($D$), and Delta ($\Delta$):

  • If $\Delta$ is in meters: $\Delta = \frac{8.64 \times B}{D}$

  • Where $B = \text{base period in days}$ and $D = \text{Duty in hectare/cumec}$.

Numerical Examples

• Example 1: Rice has a base period ($B$) of $120\,\text{days}$ and duty ($D$) of $900\,\text{Ha/cumec}$.

  • $\Delta = \frac{8.64 \times 120}{900} = 1.152\,\text{m}$

• Example 2: A head regulator releases $5\,\text{cumecs}$. Field duty is $100\,\text{Ha/cumec}$ and transit losses are $30\%$. Find the irrigated area.

  • Net discharge at field = $5 \times (1 - 0.30) = 3.5\,\text{cumecs}$.

  • Area = $D \times Q = 100 \times 3.5 = 350\,\text{Ha}$.

• Comparison Example:

  • System 1: Channel discharge $2.25\,\text{cumec}$, area $1620\,\text{ha}$, intensity $50\%$, $B = 140\,\text{days}$.

    • Actual area = $1620 \times 0.50 = 810\,\text{ha}$.

    • $D_1 = \frac{810}{2.25} = 360\,\text{ha/cumec}$.

    • $\Delta_1 = \frac{8.64 \times 140}{360} = 3.36\,\text{m}$.

  • System 2: Distributary discharge $0.6\,\text{cumec}$, area $240\,\text{ha}$, same $B$.

    • $D_2 = \frac{240}{0.6} = 400\,\text{ha/cumec}$.

    • $\Delta_2 = \frac{8.64 \times 140}{400} = 3.024\,\text{m}$.

  • Conclusion: System 2 is more economical because its duty is higher, meaning it consumes less water for the same volume of supply.

Canal Systems and Duty Variations

• Canal Hierarchy: Water moves from the Main Canal $\rightarrow$ Branch Canal $\rightarrow$ Distributary $\rightarrow$ Minor $\rightarrow$ Field Channels $\rightarrow$ Agricultural Fields.

• Transit Losses: Also known as transmission or conveyance losses, these occur due to evaporation and percolation as water travels through channels.

• Spatial Variation of Duty:

  • Duty increases as water moves downstream from the head of the main canal toward the fields.

  • Duty at the head of a water course is called the outlet discharge factor.

  • Duty is lowest at the head of the main canal because it must account for losses throughout the entire system. Duty at the field is highest because the water has already undergone those losses.

Classification of Irrigation based on Water Availability

1. Flow Irrigation System

Developed where water is available at a high level and distributed by gravity flow.

• Direct Irrigation System: Used for perennial rivers with year-round flow. Water is diverted into canals using a diversion weir or barrage without a storage reservoir.

• Storage Irrigation System: Used for non-perennial rivers. A dam is constructed to create a large storage reservoir from which the canal originates. These systems are larger than direct systems.

• Combined Direct and Storage System: Used for non-perennial rivers when a dam site is far from the command area.

  • A dam is built upstream (often in mountains) to store water.

  • Water is released into the river and then captured downstream near the plains.

  • A pick-up weir is built downstream to divert the released water into canals.

2. Lift Irrigation System

Used when the water source is lower than the ground surface of the fields.

• Mechanism: Water is lifted using pumps or other mechanical means.

• Characteristics: Higher cost compared to flow irrigation; used only when high-level gravity sources are unavailable.