Irrigation Hydrology: Rainfall Measurement

Irrigation hydrology is a crucial branch of civil engineering that deals with the study of water, particularly its occurrence, movement, and distribution in relation to agriculture and irrigation. A fundamental aspect of this study is understanding rainfall, as it is the primary source of water for most irrigation systems. Accurately measuring rainfall is essential for designing irrigation infrastructure, managing water resources, and predicting crop yields.

Rainfall Measurement Devices

The most common instrument used to measure rainfall is the rain gauge. There are several types of rain gauges, each with its advantages and applications.

1. Non-recording Rain Gauges (Ordinary Rain Gauges)

These are the simplest type of rain gauges and require manual reading. They are suitable for areas where continuous recording is not essential or where power supply is an issue.

• Symon's Rain Gauge: This is a widely used type. It consists of a funnel with a known area (typically 100 cm² or 0.02 m²) that collects rainwater and directs it into a measuring cylinder. The diameter of the funnel is usually 20 cm. The rim of the funnel is sharp and mounted horizontally. The collected water is then measured using a graduated cylinder.
• Construction and Installation: The gauge is usually installed on a concrete or masonry pedestal to a height of about 1 meter above the ground. This prevents splash into the funnel and ensures accurate collection. The surrounding area should be open, with no obstructions like trees or buildings that could affect rainfall distribution.
• Reading the Gauge: Readings are typically taken once or twice a day at fixed times (e.g., 8:00 AM IST). If the amount of rain exceeds the capacity of the measuring cylinder, it is emptied and refilled with a known quantity of water, and the process is repeated. The total rainfall is the sum of all measurements.
• Advantages: Simple to operate, low cost, no power required.
• Disadvantages: Requires manual reading, prone to errors due to evaporation or leakage, not suitable for measuring snowfall or intensity.

2. Recording Rain Gauges (Automatic Rain Gauges)

These gauges automatically record the amount of rainfall over time, providing a continuous record. This is useful for determining rainfall intensity and duration, which are important for hydrological analysis and flood forecasting.

• Tipping Bucket Rain Gauge: This is the most common type of recording rain gauge. It consists of a funnel that collects rain and directs it into a small bucket mounted on a pivot. When a specific amount of rain (e.g., 0.2 mm or 0.5 mm) has collected in one half of the bucket, it tips over, emptying the water and triggering a mechanism that records an 'event' (a tip). Each tip corresponds to a known amount of rainfall. The record is usually transmitted electronically to a data logger or computer.
• Weighing Bucket Rain Gauge: In this type, the rainwater is collected in a bucket placed on a weighing mechanism. As the water accumulates, the weight increases, and this change in weight is recorded over time. The record is typically made on a chart driven by a clock.
• Float Type Rain Gauge: Rainwater is collected in a float chamber. As the water level rises, a float rises with it, moving a pen or stylus that records the rainfall on a rotating drum. A siphoning mechanism empties the chamber when the water level reaches a certain height.
• Advantages: Provides continuous record, measures intensity and duration, reduces manual error, suitable for remote locations with telemetry.
• Disadvantages: Higher cost, requires maintenance, potential for mechanical failure, some types require power.

Rainfall Measurement Network and Data Processing

For effective hydrological analysis, a network of rain gauges is established in a catchment area. The density of the network depends on factors like topography, rainfall variability, and the required accuracy.

• Network Density: The World Meteorological Organization (WMO) recommends a general guideline of 1 gauge per 600-1000 km² for plains and 1 gauge per 250-400 km² for mountainous regions or areas with high rainfall variability.
• Areal Average Rainfall: When rainfall is measured at multiple points in a catchment, it is often necessary to calculate the average rainfall over the entire area. Common methods include:
  • Arithmetic Average Method: This is the simplest method, where the average rainfall is the sum of rainfall recorded at all stations divided by the number of stations. This method is suitable when the gauges are uniformly distributed and rainfall is relatively uniform across the area.
  • Thiessen Polygon Method: This method assigns a weight to each rain gauge based on the area it represents. Polygons are constructed around each gauge such that any point within a polygon is closer to its corresponding gauge than to any other gauge. The average rainfall is calculated as the weighted average of the rainfall at each station.
  • Isohyetal Method: This method uses lines of equal rainfall (isohyets) drawn on a map. The average rainfall is calculated by taking the weighted average of rainfall amounts corresponding to the areas between successive isohyets. This method is generally considered the most accurate, especially in areas with significant rainfall gradients.
• Data Quality Control: Rainfall data must be checked for errors, inconsistencies, and missing values before being used for analysis.

Irrigation Hydrology: Runoff

Runoff is the portion of precipitation (rain or snowmelt) that flows over the land surface and through stream channels. It is a critical component of the hydrological cycle, as it represents the water available for surface water resources like rivers and reservoirs. Understanding runoff is vital for designing dams, bridges, flood control structures, and for estimating water supply for irrigation.

Factors Affecting Runoff

Several factors influence the amount and rate of runoff from a watershed. These can be broadly categorized as meteorological and physiographical factors.

1. Meteorological Factors

• Rainfall Intensity and Duration: Higher intensity rainfall, especially when it exceeds the infiltration capacity of the soil, leads to greater runoff. Longer duration storms can saturate the soil, increasing runoff over time.
• Type of Precipitation: Rain directly contributes to surface runoff. Snowmelt, however, is a slower process and its contribution to runoff depends on temperature, solar radiation, and snowpack conditions.
• Antecedent Precipitation: The amount of rainfall or moisture present in the soil before a storm significantly impacts runoff. A wet soil will produce more runoff than a dry soil because its infiltration capacity is reduced.

2. Physiographical Factors

• Topography (Slope): Steeper slopes result in faster surface flow and less time for infiltration, leading to higher runoff volumes and peak flows.
• Soil Type and Permeability: Soils with low permeability (e.g., clay soils) allow less water to infiltrate, thus increasing runoff. Permeable soils (e.g., sandy soils) absorb more water, reducing runoff.
• Land Cover and Use: Vegetation cover (forests, grasslands) intercepts rainfall, enhances infiltration, and reduces runoff velocity. Impervious surfaces like paved areas, roads, and buildings significantly increase runoff. Urbanization typically leads to higher runoff rates.
• Channel Characteristics: The size, shape, slope, and roughness of stream channels influence how quickly runoff is conveyed downstream.
• Drainage Density: A higher drainage density (more stream channels per unit area) means water can be collected and transported more efficiently, leading to faster runoff response.

Runoff Calculation Methods

Calculating runoff is complex, and various methods are employed depending on the data availability and desired accuracy.

1. Empirical Methods

These methods use empirical formulas based on observed data and are often used for preliminary estimates.

• Rational Formula: This is a widely used empirical formula for estimating peak runoff rate (Q) for small drainage areas (typically less than 20 km²).

  Q = (C * I * A) / 3.6

  Where:
  • Q = Peak runoff rate in cubic meters per second (m³/s)
  • C = Runoff coefficient (dimensionless), depends on land use, soil type, and slope.
  • I = Average rainfall intensity for a duration equal to the time of concentration (in mm/hr).
  • A = Drainage area in square kilometers (km²).
  • The factor 3.6 is used to convert units (mm/hr * km² to m³/s).
  Runoff Coefficient (C) values:

  Land Cover	C
  Asphalt/Concrete	0.70 - 0.95
  Shingle roofs	0.60 - 0.85
  Lawns (sandy soil)	0.10 - 0.20
  Lawns (clay soil)	0.25 - 0.40
  Paved areas	0.70 - 0.95
  Forest/Meadows	0.10 - 0.35

  The 'Time of Concentration' (Tc) is the time it takes for runoff from the farthest point of the watershed to reach the outlet. It is crucial for determining the appropriate rainfall intensity (I).
• SCS Curve Number (CN) Method: Developed by the USDA Soil Conservation Service (now NRCS), this method estimates direct runoff (Q) based on rainfall (P) and a Curve Number (CN) that represents the watershed's characteristics.

  Q = (P - 0.2S)² / (P + 0.8S) for P > 0.2S
  Q = 0 for P ≤ 0.2S

  Where:
  • Q = Direct runoff (mm)
  • P = Antecedent rainfall (mm)
  • S = Potential maximum retention after runoff begins (mm)
  • S = (1000 / CN) - 10
  The Curve Number (CN) ranges from 0 to 100, with higher values indicating greater runoff potential. CN values depend on soil type, land cover, and antecedent moisture condition.
  SCS Curve Number (CN) Factors: CN values are tabulated based on soil groups (A, B, C, D), land cover types, and treatment. Group A soils have the lowest runoff potential (high infiltration), while Group D soils have the highest runoff potential (low infiltration).

2. Hydrological Models

More complex hydrological models (e.g., HEC-HMS, SWAT) are used for detailed watershed analysis, incorporating various hydrological processes like infiltration, evapotranspiration, and channel routing. These models require more extensive data but provide more accurate and comprehensive results.

Components of Runoff

Total runoff can be divided into different components:

• Surface Runoff (Overland Flow): Water that flows over the ground surface.
• Interflow (Subsurface Flow): Water that infiltrates the soil and moves laterally through the unsaturated zone before reaching a stream.
• Groundwater Runoff (Base Flow): Water that percolates into the groundwater system and then seeps into stream channels.

The hydrograph of a stream, which plots discharge against time, typically shows a sharp rise due to surface runoff and a sustained flow due to interflow and base flow.

Irrigation Hydrology: Crop Water Requirements

Understanding the water needs of different crops is fundamental to efficient irrigation. Crop water requirement refers to the total amount of water needed by a crop for its normal growth and development, excluding precipitation. This is a crucial parameter for designing irrigation systems and scheduling irrigation applications.

Factors Affecting Crop Water Requirements

The water needs of a crop are not constant and vary depending on several factors:

• Crop Characteristics: Different crops have different rooting depths, leaf area, and growth stages, all of which influence their water uptake. For example, deep-rooted crops can access water from deeper soil layers, reducing their immediate irrigation needs.
• Climate: This is the most significant factor.
  • Temperature: Higher temperatures increase evapotranspiration rates.
  • Humidity: Lower humidity increases the vapor pressure deficit, leading to higher evapotranspiration.
  • Wind Speed: Higher wind speeds increase evapotranspiration by removing moist air from the vicinity of the leaves.
  • Solar Radiation: Higher solar radiation provides the energy needed for photosynthesis and also increases evapotranspiration.
• Soil Properties: Soil type influences the water-holding capacity and the ease with which roots can extract water.
• Growth Stage: Crop water requirements vary significantly throughout the crop's life cycle. Seedling stages require less water, while flowering and fruiting stages often have the highest water demands.
• Irrigation Method: The efficiency of the irrigation method used can affect the net amount of water required at the field level.

Evapotranspiration (ET)

The total water loss from a cropped field is primarily due to evapotranspiration (ET). ET is the sum of evaporation (water loss from soil and plant surfaces) and transpiration (water loss from plant leaves).

• Reference Crop Evapotranspiration (ET_o): This is the evapotranspiration from a reference surface, not short of water. It represents the atmospheric demand for water. The most commonly used reference crop is a hypothetical grass surface with specific characteristics (height, albedo, etc.). ET_o is primarily influenced by meteorological factors.
• Actual Crop Evapotranspiration (ET_c): This is the evapotranspiration from a specific crop under specific conditions. It is calculated using the reference crop evapotranspiration and a crop coefficient (K_c).

  ET_c = K_c * ET_o

  Where:
  • ET_c = Crop evapotranspiration (mm/day)
  • K_c = Crop coefficient (dimensionless), which varies with crop type and growth stage.
  • ET_o = Reference crop evapotranspiration (mm/day)
  Crop Coefficient (K_c): Kc values are determined empirically and are available for various crops at different growth stages (initial, development, mid-season, late-season). For example, K_c for wheat might be 0.3 in the initial stage, rising to 1.1 during the mid-season (peak water demand), and falling to 0.4 in the late season.

Methods for Estimating ET_o

Several methods are used to estimate ET_o, ranging from simple empirical formulas to complex energy balance equations.

• Penman-Monteith Equation: This is the most widely accepted and physically based method. It combines energy balance and aerodynamic factors to estimate ET_o. It requires detailed meteorological data (temperature, humidity, wind speed, solar radiation).
• Blaney-Criddle Method: An older, simpler empirical method based on mean monthly temperature and daylight hours. It is less accurate than Penman-Monteith but useful when data is limited.
• Pan Evaporation Method: This method uses the evaporation rate from a standardized evaporation pan (e.g., Class A pan) and applies a pan coefficient (K_p) to estimate ET_o.

  ET_o = K_p * E_pan

  Where E_pan is the pan evaporation.

Calculating Net Irrigation Water Requirement

Once ET_c is known, the net irrigation water requirement can be calculated. This is the amount of water that needs to be applied through irrigation to meet the crop's water needs.

Net Irrigation Water Requirement (NIR) = ET_c - Effective Rainfall (P_eff)

• Effective Rainfall (P_eff): This is the portion of total rainfall that is actually available for crop use. It excludes runoff and deep percolation losses. Various methods exist to estimate P_eff, often based on empirical relationships with total rainfall.

Calculating Gross Irrigation Water Requirement

The gross irrigation water requirement accounts for the inefficiencies in the irrigation system (e.g., losses during conveyance, application, and storage).

Gross Irrigation Water Requirement (GIR) = NIR / Irrigation Efficiency (η)

• Irrigation Efficiency (η): This is the ratio of the water beneficially used by the crop to the total amount of water applied. It varies significantly with the type of irrigation system (e.g., surface, sprinkler, drip). Drip irrigation is the most efficient (up to 90-95%), while traditional surface irrigation can be as low as 30-50%.

Irrigation Hydrology: Duty, Delta, and Base Period

These three terms are fundamental concepts in irrigation engineering, particularly in the context of canal irrigation design. They help in quantifying the water requirements of crops and designing the capacity of irrigation channels.

1. Base Period (B)

The Base Period of a crop is the total number of days the crop is continuously present in the field, from the day it is sown to the day it is harvested. It includes the period during which the crop requires water.

• Importance: The base period is crucial because it determines the duration over which the crop's water demand needs to be met. It influences the design of irrigation channels and the overall water allocation strategy.
• Factors Affecting Base Period:
  • Crop Type: Different crops have different growth durations. For example, paddy requires a longer base period than wheat.
  • Variety of Crop: Different varieties of the same crop may have slightly different durations.
  • Climate and Soil Conditions: Local climatic and soil conditions can sometimes influence the optimal growing period.
• Example: A typical base period for wheat is around 120 days, while for paddy it can be around 130-150 days, and for sugarcane, it can be 300-360 days.

2. Delta (Δ)

Delta is the total depth of water, expressed in centimeters or meters, required by a crop during its entire base period for its full growth. It represents the cumulative water requirement of the crop over its lifespan.

• Relationship with ET_c: Delta is essentially the integrated value of the crop's evapotranspiration (ET_c) over its base period. If ET_c is expressed in cm/day, then:

  Δ (in cm) = ET_c (in cm/day) * Base Period (in days)

• Units: Delta is usually expressed in centimeters (cm) or meters (m).
• Factors Affecting Delta: Delta is directly influenced by the factors affecting crop water requirements, primarily climate (which determines ET_c) and the crop type (which influences its water needs and base period).
• Example: For wheat, with a base period of 120 days and an average daily water requirement of 0.5 cm, the Delta would be 0.5 cm/day * 120 days = 60 cm.

3. Duty (D)

Duty is defined as the area of land (in hectares) that can be irrigated with a unit volume of water flowing continuously throughout the base period of the crop. Alternatively, it is the quantity of water required to irrigate 1 hectare of land continuously throughout the base period.

• Units: Duty is typically expressed in hectares per cumec (ha/cumec). A 'cumec' is a flow rate of one cubic meter per second (m³/s).
• Relationship with Delta and Base Period: The duty is inversely related to the water requirement (Delta) and the base period. A crop requiring more water (larger Delta) or having a longer base period will have a lower duty (i.e., less area can be irrigated with the same amount of water). The relationship is derived from the concept of water flow:

  One cumec of water flowing for the entire Base Period (B) can irrigate an area 'A'. The total volume of water supplied is (1 m³/s) * (B days) * (24 hours/day) * (3600 seconds/hour) = 86400 * B m³.
  This volume of water is required to provide a depth of Delta (converted to meters) over an area 'A' (in m²).
  Volume = Area * Depth = (A * 10000 m²/ha) * (Δ in meters)
  Equating the two volumes: 86400 * B = A * 10000 * Δ_m
  So, A = (86400 * B) / (10000 * Δ_m) = (8.64 * B) / Δ_m
  Since Duty (D) = A / 1 (where 1 is the unit flow rate in cumecs),
  D (in ha/cumec) = (8.64 * B) / Δ (where Δ is in meters)

  If Delta is in centimeters, the formula becomes:

  D (in ha/cumec) = (864 * B) / Δ (where Δ is in cm)

• Factors Affecting Duty:
  • Crop Type: Crops with higher water requirements have lower duty.
  • Climate: Arid regions with high ET rates have lower duty compared to humid regions.
  • Soil Type: Permeable soils lead to higher water losses, reducing duty.
  • Method of Irrigation: Efficient irrigation methods (like drip) improve duty.
  • Method of Application: Continuous flow irrigation generally has lower duty than rotational irrigation.
  • Purity of Water: If irrigation water contains silt, it may reduce the need for subsequent irrigation, potentially affecting duty calculations over longer periods.
  • Field Slope and Conditions: Proper leveling and slope improve water distribution and efficiency.

Interrelation and Importance

These three parameters are intrinsically linked and are essential for designing efficient irrigation systems.

• The Base Period defines the duration of water need.
• Delta quantifies the total volume of water needed over that duration.
• Duty translates this water requirement into the command area that can be irrigated by a specific flow rate from a canal.

For instance, if an irrigation department plans to supply water from a canal with a discharge capacity of 10 cumecs, and the duty for a particular crop (say, wheat) is known to be 1500 ha/cumec, then the total area that can be irrigated by this canal for wheat cultivation is 10 cumecs * 1500 ha/cumec = 15000 hectares.

Irrigation Hydrology: Canal Types and Canal Lining

Canals are artificial channels constructed to convey water for various purposes, primarily irrigation. The design and type of canal depend on factors like the source of water, topography, soil conditions, and the command area to be irrigated. Canal lining is a crucial aspect of canal construction that significantly impacts water efficiency and canal performance.

Types of Canals

Canals can be classified based on several criteria:

1. Based on Source of Water

• Perennial Canals: These canals are fed by reservoirs or rivers that have a perennial source of water, ensuring a continuous supply throughout the year. They provide a reliable source for irrigation.
• Non-Perennial Canals (or Escaped Canals): These canals are fed by rivers that swell only during the monsoon season or heavy rainfall. They are used for irrigation only during the periods of high flow in the river.
• Inundation Canals: These canals draw water directly from a river without any head regulator or dam. They function only when the river level is high enough to flow into the canal, typically during flood seasons. Their discharge varies significantly with the river's water level.
• Bandh or Tank Canals: These canals draw water from tanks or reservoirs formed by constructing a bandh (dam) across a river or stream.

2. Based on Position with respect to Ground Contour

• Ridge Canals (or Watershed Canals): These canals are aligned along the watershed or ridge line separating two drainage areas. They are ideal because irrigation can be provided on both sides of the canal, maximizing the command area. They generally do not require cross-drainage structures as they don't intersect natural drainage paths.
• Contour Canals: These canals are aligned roughly parallel to the natural ground contours. They can irrigate land only on one side (the lower side) because the ground slopes away from the canal on the other side. They often require numerous cross-drainage works (aqueducts, super passages) to pass natural streams across them.
• Side Slope Canals: These canals are aligned roughly perpendicular to the natural ground contours. They can irrigate land on both sides, but they usually require cross-drainage works if they intersect natural drainage.

3. Based on Purpose

• Irrigation Canals: Primarily designed to convey water for agricultural purposes.
• Navigation Canals: Designed for the passage of boats and ships.
• Hydel Canals: Constructed to carry water to powerhouses for generating hydroelectricity.
• Combination Canals: Canals designed for multiple purposes, such as irrigation and navigation.

4. Based on Design (Unlined vs. Lined)

• Unlined Canals: These are the traditional earthen canals without any protective lining. They are cheaper to construct but suffer from significant water losses.
• Lined Canals: These canals have a protective layer or lining constructed on their bed and sides to prevent seepage, erosion, and weed growth.

Canal Lining

Canal lining is the process of constructing a protective layer on the inner surface of a canal. This lining can be made of various materials like concrete, brick, stone, tiles, or plastic films.

Objectives of Canal Lining

• Reduce Seepage Losses: Unlined canals lose a significant amount of water through seepage into the surrounding soil. Lining minimizes these losses, making more water available for irrigation.
• Prevent Scouring and Erosion: The flow of water in unlined canals can erode the bed and sides, especially at higher velocities. Lining provides a smooth and stable surface, preventing erosion and allowing for higher permissible velocities.
• Reduce Weed Growth: Unlined canals are prone to weed growth, which obstructs flow, reduces the canal's carrying capacity, and requires frequent desilting. Lining prevents weed growth.
• Increase Water Carrying Capacity: Due to smoother surfaces and higher permissible velocities, lined canals can carry more water than unlined canals of the same cross-section.
• Reduce Maintenance Costs: Although initial construction costs are higher, lining reduces the long-term costs associated with desilting, repairing bank erosion, and controlling weeds.
• Improve Water Logging Conditions: By reducing seepage, lining helps to prevent the rise of the water table in the areas adjacent to the canal, thus mitigating waterlogging problems.

Materials Used for Lining

• Concrete Lining: Most common and durable. Can be plain concrete or reinforced concrete.
• Brick Lining: Bricks laid in cement mortar. Suitable for smaller canals and areas where bricks are readily available.
• Stone Paving: Stones laid in mortar. Used in areas with abundant stone resources.
• Kankar or Clay Puddling: Traditional methods using compacted clay or kankar (nodular limestone) layers. Less durable and effective than concrete.
• Plastic Films/Geomembranes: Modern synthetic materials used for lining, offering excellent impermeability.

Permissible Velocity in Lined and Unlined Canals

The maximum velocity of water flow that a canal can sustain without causing scouring (erosion) or silting (deposition) is known as permissible velocity.

Lining significantly increases the permissible velocity, allowing for narrower and shallower canals to carry the same discharge, or a larger discharge in a canal of the same dimensions.