UNIT 5: Water Resource Engineering - Short Notes

1. Introduction to Irrigation and Water Resources

• Irrigation: Artificial application of water to soil to supplement rainfall and ensure crop growth.

• Necessity:

  • Uneven rainfall distribution.

  • To grow high-water-requirement crops.

  • To increase agricultural productivity and ensure food security.

• Advantages:

  • Increases crop yield & multiple cropping.

  • Improves soil fertility (leaching).

  • Provides insurance against drought.

  • Enhances land value.

• Disadvantages:

  • High initial cost & maintenance.

  • Risk of waterlogging & salinity.

  • Potential for mosquito breeding & diseases.

  • May lead to groundwater depletion.

[!TIP]

Exam Focus: Be prepared to list specific examples of advantages/disadvantages. Link disadvantages to concepts like waterlogging (Unit 6) and irrigation efficiency (Unit 4).

2. Hydrology and Precipitation

2.1 Hydrological Cycle

• Components: Evaporation, Transpiration (together Evapotranspiration), Condensation, Precipitation, Interception, Infiltration, Runoff, Groundwater flow.

• Water Balance Equation for a catchment:

$$P = E + T + R + \Delta S$$

Where, $P$ = Precipitation, $E$ = Evaporation, $T$ = Transpiration, $R$ = Runoff, $\Delta S$ = Change in storage.

2.2 Precipitation

• Rain Gauges:

  • Non-recording: Symon's rain gauge (manual measurement).

  • Recording: Self-recording (tipping bucket, weighing type) for continuous intensity data.

• Areal Estimation Methods (for missing data):

  | Method | Principle | Best For | | :--- | :--- | :--- | | Arithmetic Mean | Simple average of station rainfall. | Homogeneous regions, uniform gauge distribution. | | Thiessen Polygon | Weighted average based on area of influence. | Irregular gauge distribution, varying orography. | | Isohyetal | Contour lines of equal rainfall; planimeter area between contours. | Accurate for large areas with many gauges. |

• Depth-Area-Duration (DAD) Curves:

  • Shows maximum average depth over an area for a given duration.

  • Significance: Used to derive Design Storm for flood estimation (higher duration for larger area). Curve shifts right (higher depth) for rarer (higher return period) storms.

2.3 Infiltration

• Definition: Entry of water into soil through the surface.

• Infiltration Capacity Curve (f-t curve): Decreases exponentially with time. Initial high rate ($$\displaystyle f_0 $$) reduces to steady rate ($$\displaystyle f_c $$).

• Factors Affecting:

  1. Soil Properties: Texture, structure, porosity.

  2. Vegetative Cover: Increases interception, improves structure.

  3. Slope: Steeper slope → less time for infiltration → lower cumulative infiltration.

  4. Antecedent Moisture Condition (AMC): Wetter soil → lower initial infiltration.

  5. Land Use/Management: Tillage, compaction.

• Infiltration Indices:

  • φ-index: Constant infiltration rate that produces runoff equal to actual runoff for a storm. (Total rainfall - φ-index × duration = runoff). Used for large storms where initial abstraction is negligible.

  • W-index: Weighted average infiltration rate over the entire storm duration where runoff occurs. More accurate than φ-index.

2.4 Evaporation & Evapotranspiration

• Lake Evaporation:

  • Energy Balance Method:

$$E = \frac{R_n - G - H - \lambda E}{\lambda}$$

Where $$\displaystyle R_n $$=net radiation, $G$=soil heat flux, $H$=sensible heat, $\lambda E$=latent heat. Most fundamental but complex.

*   **Aerodynamic Method**: Based on turbulent transfer. 

$$E = C \cdot (e_s - e_a) \cdot u_2$$

Where $$\displaystyle e_s $$=saturation vapor pressure, $$\displaystyle e_a $$=actual vapor pressure, $$\displaystyle u_2 $$=wind speed at 2m.

• Consumptive Use (CU): Water used by vegetation for transpiration + evaporation from soil/plant surfaces. Evapotranspiration (ET) is the preferred scientific term.

• Determination Methods:

  • Blaney-Criddle:

$$CU = K \cdot f$$

Where $$\displaystyle f = \frac{p}{40} \sum_{i=1}^{12} t_i $$ (temperature factor), $K$=crop coefficient.

*   **Penman**: Combines energy balance & aerodynamic principles. Most reliable but data-intensive.

*   **Hargreaves**: Simplified Penman using only temperature.

2.5 Runoff & Unit Hydrograph

• Unit Hydrograph (UH): Direct runoff hydrograph (DRH) from 1 cm (or 1 unit) of effective rainfall uniformly distributed over the catchment in a specified duration (e.g., 4-hr, 6-hr).

• Assumptions: Linearity & time invariance (system is linear & stationary). Rainfall is uniformly distributed.

• Derivation:

  1. Separate baseflow from observed storm hydrograph.

  2. Calculate effective rainfall (total rainfall - losses).

  3. Scale DRH ordinates to correspond to 1 cm of effective rainfall.

• Instantaneous Unit Hydrograph (IUH): UH for an infinitesimally small duration. Derived from S-curve (summation of UHs of same duration).

• Factors Affecting Hydrograph Shape:

  • Catchment Size: Larger → higher lag time, lower peak.

  • Slope: Steeper → lower lag, higher peak.

  • Rainfall Intensity/Distribution: Intense, concentrated rainfall → higher peak.

  • Soil & Land Use: Impervious area → higher peak, lower lag.

• Stream Types:

  • Perennial: Flow year-round (fed by groundwater).

  • Ephemeral: Flow only during/after rainfall (no baseflow).

2.6 Floods

• Flood Frequency Analysis (FFA):

  • Gumbel (EV1) Distribution:

$$P = 1 - e^{-e^{-y}}$$

where

$$y = \frac{x - \bar{x}}{S} - 0.5772$$

$x$ = flood magnitude, $\bar{x}$ & $S$ = mean & std. dev. of annual series.

*   **Log-Pearson Type III (LP-III)**: Recommended by USGS. Logs of flood peaks fit a Pearson Type III distribution. More flexible (skewness parameter).

• Peak Runoff for Small Catchments:

  • Rational Method:

$$Q_{peak} = \frac{C i A}{3.6}$$

(SI units: $Q$ in m³/s, $i$ in mm/hr, $A$ in ha, $C$=runoff coefficient). Used for urban drainage & small basins (< 200 km²).

*   **Curve Number (CN) Method** (SCS): Estimates direct runoff from rainfall. 

$$Q = \frac{(P - I_a)^2}{(P - I_a) + S}$$

$$\displaystyle I_a $$ = initial abstraction (≈0.2S), $S$ = potential maximum retention. CN depends on soil & land use.

*   **IDF Curves**: Intensity-Duration-Frequency curves from rainfall data. Used with Rational method to get $i$ for given $T$ and $$\displaystyle t_c $$ (time of concentration).

• Flood Routing - Muskingum Method:

  • Principle: Storage is a function of both inflow and outflow:

$$S = K[xI + (1-x)O]$$

    *   $K$ = storage time constant (hr), related to travel time.

    *   $x$ = weighting factor (0 ≤ x ≤ 0.5). $$\displaystyle x=0.5 $$ → linear reservoir; $$\displaystyle x=0 $$ → simple lag.

*   **Routing Equation**: 

$$O_{j+1} = C_0 I_{j+1} + C_1 I_j + C_2 O_j$$

    Where coefficients $$\displaystyle C_0, C_1, C_2 $$ depend on $K$, $x$, and routing interval $\Delta t$.

3. Soil-Water-Plant Relationships & Crop Water Requirements

3.1 Soil Moisture Characteristics

• Field Capacity (FC): Water content after excess drains under gravity (~2-3 days after saturation). θ_fc (vol. %).

• Permanent Wilting Point (PWP): Water content at which plants cannot extract water & wilt permanently. θ_pwp.

• Available Moisture (AM):

$$AM = \theta_{fc} - \theta_{pwp}$$

• Wilting Coefficient: Soil moisture tension (~15 atm) at PWP.

• Soil Properties:

  • Apparent/Bulk Density (ρ_b):

$$ρ_b = \frac{\text{Mass of oven-dry soil}}{\text{Total soil volume}}$$

*   **Porosity (n)**: 

$$n = 1 - \frac{ρ_b}{ρ_s}$$

($$\displaystyle ρ_s $$ = particle density ≈ 2.65 g/cm³).

*   **Specific Yield (Sy)**: Portion of water that can be drained by gravity (relevant for groundwater).

3.2 Crop Water Use

• Consumptive Use (CU): As defined earlier. Calculated using ** Blaney-Criddle** or Penman methods.

• Daily CU from soil data:

  • Moisture depletion:

$$D = (θ_{fc} - θ_{pwp}) × ρ_b × D_e$$

(mm), $$\displaystyle D_e $$ = effective root zone depth.

*   Irrigation frequency (f): 

$$f = \frac{D}{CU_{daily}}$$

(days).

3.3 Irrigation Scheduling

• Irrigation Frequency: Interval between irrigations, based on soil moisture depletion and crop CU.

• Irrigation Depth (d):

$$d = \frac{D}{η_a}$$

Where $$\displaystyle η_a $$ = water application efficiency.

• Kor Period: First critical growth stage when crop requires maximum water. Irrigation must be provided at the start of kor to avoid yield reduction.

3.4 Duty, Delta, and Base Period

• Definitions:

  • Duty (D): Area irrigated by 1 cumec of water continuously during the base period. Unit: ha/cumec.

  • Delta (Δ): Total depth of water required by a crop during its entire growth period. Unit: cm.

  • Base Period (B): Total time (days) between first and last watering of a crop in its growth season.

• Relationship:

$$Δ = \frac{8.64 \times B}{D}$$

**Derivation**: Volume of water = Duty × Base Period = Area × Delta.

$$1 \text{ cumec} \times B \text{ days} = D \text{ ha} \times \frac{Δ \text{ cm}}{100}$$

Convert units: 1 cumec-day = 8.64 ha-cm.

$$\boxed{Δ (\text{cm}) = \frac{8.64 \times B (\text{days})}{D (\text{ha/cumec})}}$$

• Factors Affecting Duty:

  • Canal losses (seepage, percolation) → Decreases duty.

  • Soil type (permeability) → Sandy soil → lower duty.

  • Crop type → High Δ crop → lower duty.

  • Method of irrigation → Surface (low efficiency) vs. Drip (high efficiency).

• Methods to Improve Duty:

  • Canal lining (reduce seepage).

  • Reduce canal length & number of distributaries.

  • Adopt efficient irrigation methods (drip/sprinkler).

  • Improve water management & reduce operational losses.

• Canal Discharge (Q):

$$Q = \frac{\text{CCA} \times Δ}{8.64 \times B \times η}$$

Where CCA = Culturable Command Area (hectares), $η$ = overall efficiency.

3.5 Crop Planning

• Crop Ratio:

$$\text{Crop Ratio} = \frac{\text{Area under Rabi crop}}{\text{Area under Kharif crop}}$$

Determines seasonal water demand.

• Crop Rotation: Sequential cultivation of different crops on same land to maintain soil fertility.

• Command Area Terms:

  • Gross Command Area (GCA): Total area that can be irrigated by a canal system (includes uncultivable, villages, etc.).

  • Culturable Command Area (CCA): Portion of GCA that is cultivable (GCA - Unculturable area).

  • Culturable Uncultivated Area: Part of CCA left fallow or under non-crop use in a season.

• Paleo Irrigation: Ancient/Historical irrigation systems (e.g., stepwells, tanks, karez).

4. Irrigation Methods and Systems

• Irrigation Efficiency:

  • Water Conveyance Efficiency (η_c):

$$\eta_c = \frac{\text{Water delivered to field}}{\text{Water diverted from source}} \times 100$$

*   **Water Application Efficiency (η_a)**: 

$$\eta_a = \frac{\text{Water stored in root zone}}{\text{Water delivered to field}} \times 100$$

*   **Overall Efficiency (η_o)**: 

$$\eta_o = \eta_c \times \eta_a$$

5. Canal Systems and Design

5.1 Canal Classification

• Based on Function:

  1. Main Canal: From headworks to branch canals. No direct irrigation.

  2. Branch Canal: Off-takes from main. May irrigate directly if discharge small.

  3. Distributary: Takes from branch/main. Supplies water to minors.

  4. Minor: Takes from distributary. Supplies water to field channels.

  5. Field Channel (Watercourse): Directly irrigates fields.

• Based on Discharge/Importance:

  • Primary (Main), Secondary (Branch), Tertiary (Distributary/Minor).

5.2 Canal Alignment & Layout

• Factors:

  • Topography: Align along ridge for gravity flow (minimizes crossings). Avoid valleys.

  • Soil: Stable, low permeability (seepage).

  • Drainage: Avoid intercepting natural drains; provide cross-drainage.

  • Command Area: Must cover entire area with minimal length.

  • Cost: Earthwork, structures, land acquisition.

• Main Canals: Aligned on ridge (watershed) to irrigate both sides.

• Distributaries: Aligned down the slope following natural drainage.

5.3 Canal Design Theories

$$f = 1.76 \sqrt{d_{50}}$$

(mm) |

| Design Equation |

$$V_0 = 0.55 m D^{0.63} S^{0.25}$$

(for alluvium) <br> $m$ = silt exponent (1.0-1.5), $D$ = depth, $S$ = slope. | Perimeter (P):

$$P = 4.75 \sqrt{Q}$$

<br> Area (A):

$$A = \frac{Q^2}{f^2}$$

<br> Slope (S):

$$S = \frac{f^{5/3}}{Q^{1/3}}$$

|

| Assumptions | Uniform silt, critical velocity keeps silt in suspension. | Channel in true regime (no degradation/aggradation), silt uniformly graded. | | Drawbacks | No equation for area/perimeter; $m$ is empirical; doesn't account for silt load explicitly. | Silt factor $f$ is empirical; regime conditions rarely achieved in new canals; ignores roughness. |

5.4 Regime Channel Design (Lacey)

Given: Discharge $Q$, Silt factor $f$, Side slope $z:1$. Steps:

1. Calculate Area (A):

$$A = \frac{Q^2}{f^2}$$

2. Calculate Wetted Perimeter (P):

$$P = 4.75 \sqrt{Q}$$

3. For trapezoidal section:

$$A = (b + zD)D$$

,

$$P = b + 2D\sqrt{1+z^2}$$

4. Solve simultaneous equations for Bed width (b) and Depth (D).

5. Check Mean Velocity (V):

$$V = \frac{Q}{A}$$

6. Calculate Slope (S) using Lacey's formula:

$$S = \frac{f^{5/3}}{Q^{1/3}}$$

7. Verify $V$ is within regime limits (1.5-2.5 m/s typically).

5.5 Canal Hydraulics

• Manning's Formula (most common):

$$V = \frac{1}{n} R^{2/3} S^{1/2}$$

*   $n$ = Manning's roughness coefficient (0.025-0.033 for earth, 0.013-0.017 for lined).

*   $R$ = hydraulic radius = $A/P$.

• Kutter's Formula: More complex, accounts for roughness & slope. Rarely used now.

• Design Procedure (Kennedy/Lacey often used for initial sizing, Manning for final check).

5.6 Canal Lining

• Importance:

  • Reduces seepage loss (up to 70%).

  • Increases canal capacity (reduced wetted perimeter).

  • Prevents erosion & weed growth.

  • Reduces maintenance cost.

• Types:

  • Rigid: Concrete, masonry, soil-cement. Durable, smooth ($n$ low).

  • Flexible/Plastic: Bituminous, geomembranes (HDPE, PVC). Good for expansive soils.

  • Clay/Soil: Puddled clay, compacted earth. Cheap but less effective.

• Selection Criteria: Cost, soil conditions, water chemistry, availability of materials, durability.

5.7 Canal Operation & Regulation

• Regulation Structures:

  • Head Regulator: At canal head; controls diversion from source.

  • Cross Regulator: Across canal; raises water level for off-takes.

  • Escapes: Safely discharge excess/surplus water.

• Outlets (Off-takes):

  • Modular: Discharge independent of upstream water level (e.g., ** orifice** outlet).

  • Non-Modular: Discharge varies with differential head (e.g., free outlet).

• Operation: Rotational (warabandi), continuous, or on-demand. Scheduling based on crop water requirement & availability.

6. Groundwater Engineering

6.1 Aquifers & Properties

• Types:

  • Unconfined (Water Table): Upper surface is water table; atmospheric pressure.

  • Confined (Artesian): Between impermeable layers; under pressure.

  • Semi-confined (Leaky): Confined but with leakage through overlying/underlying layer.

• Properties:

  • Porosity (n): Total void space.

  • Specific Yield (Sy): Volume of water drained per unit area per unit decline in water table (effective porosity). Key for unconfined.

  • Storativity (S): Volume of water released per unit area per unit decline in head.

    • Unconfined: $S \approx Sy$.

    • Confined: $$\displaystyle S = S_s \cdot b $$ ($$\displaystyle S_s $$ = specific storage, $b$ = thickness).

  • Transmissivity (T):

$$T = K \cdot b$$

($K$ = hydraulic conductivity, $b$ = saturated thickness). Rate of flow through aquifer.

*   **Hydraulic Conductivity (K) / Permeability**: Rate of water flow through soil/rock under unit hydraulic gradient.

6.2 Wells

• Types:

  • Open Well (Dugwell): Large diameter, shallow, low yield.

  • Tube Well: Deep, small diameter, high yield (drilled).

  • Artesian Well: Taps confined aquifer; water flows up.

  • Infiltration Gallery: Horizontal perforated pipe in shallow aquifer near surface.

• Well Components: Casing, Screen/Strainer, Gravel Pack.

• Discharge Calculation:

  • Unconfined (Dupuit's Equation):

$$Q = \frac{\pi K (h_1^2 - h_2^2)}{\ln(r_2/r_1)}$$

    $$\displaystyle h_1, h_2 $$ = head at radii $$\displaystyle r_1 $$ (well radius) & $$\displaystyle r_2 $$ (radius of influence).

*   **Confined (Theim's Equation)**: 

$$Q = \frac{2\pi K h (h_1 - h_2)}{\ln(r_2/r_1)}$$

    $h$ = aquifer thickness (constant).

• Radius of Influence ($$\displaystyle r_o $$): Distance from well where drawdown is zero. Empirical:

$$r_o \approx 3000 \times D \times \sqrt{K}$$

($D$ = drawdown).

• Well Interference: When cones of depression from adjacent wells overlap → reduced yield.

6.3 Groundwater Flow

• Darcy's Law:

$$Q = K \cdot i \cdot A$$

Where $i$ = hydraulic gradient ($\Delta h/L$), $A$ = cross-sectional area.

• Flow Nets: Graphical solution (flow lines & equipotential lines) for 2D steady flow. Used to calculate discharge ($$\displaystyle Q = k \cdot \Delta h \cdot \frac{N_f}{N_d} $$) and seepage velocity.

• Hydraulic Gradient: Determined from water table elevations in wells:

$$i = \frac{\Delta h}{\Delta l}$$

6.4 Groundwater Recharge

• Natural: Infiltration from rainfall, leakage from rivers/lakes.

• Artificial Methods:

  • Infiltration Galleries: Perforated pipes laid in permeable stratum near stream/river to capture floodwater.

    DiagramSEARCH: infiltration gallery schematic

  • Percolation Ponds/Tanks: Store surface water for infiltration.

  • Check Dams: Small barriers in streams to slow flow & increase recharge.

  • Recharge Wells: Direct injection into aquifer.

6.5 Water Logging & Salinity

• Waterlogging:

  • Causes: Over-irrigation, poor drainage, canal seepage, high water table.

  • Effects: Reduced soil aeration, root growth inhibition, crop yield decline.

  • Prevention: Drainage (surface/subsurface), canal lining, land leveling, controlled irrigation.

• Soil Salinity (Salt Efflorescence):

  • Causes: Capillary rise of saline groundwater (high evaporation), poor drainage, irrigation with saline water.

  • Effects: Ion toxicity, osmotic stress, soil structure degradation (dispersed clays).

• Reclamation:

  • Leaching: Apply excess water to flush salts below root zone.

  • Drainage Improvement: Lower water table.

  • Gypsum Application: Replace sodium with calcium in sodic soils.

  • Salt-Tolerant Crops (e.g., barley, sugar beet).

  • Soil Amendments (organic matter, sulfuric acid).

7. Hydraulic Structures in Irrigation

7.1 Cross-Drainage Works

• Purpose: Carry canal across a natural drain.

• Types:

  • Aqueduct: Canal over drain (most common). Drain flows freely below.

  • Syphon Aqueduct: Canal over drain, drain flows under pressure through syphon pipes.

  • Superpassage: Drain over canal.

  • Underpass: Canal under drain (tunnel/syphon).

• Selection Factors: Relative sizes of canal & drain, topography, foundation conditions, cost.

7.2 Canal Regulation Structures

• Head Regulator: At canal head; controls inflow from source. Often with head sluice.

• Cross Regulator: On main/distributary; maintains water level for off-takes. Has scouring sluice for silt removal.

• Escapes: Surplus water disposal. Channel escape (into drain) or surplus escape (into lower canal).

• Off-take Structures: Modular (orifice) or non-modular (open flume) outlets from distributaries.

7.3 Weirs & Barrages

• Weir: Low, overflow structure for flow measurement or small diversion. Crest at atmospheric pressure.

• Barrage: Low, gated diversion structure across a river. Gates control water level & diversion into canals. Crest is submerged.

• Design Aspects: Crest level (based on FSL in canals), gate size & operation, sill level, approach/exit conditions.

7.4 Distribution Structures (Outlets)

• Modular Outlet: Discharge constant for a range of upstream water levels (e.g., ** orifice outlet** with rigid module).

• Non-Modular Outlet: Discharge varies with differential head (e.g., free overfall outlet).

• Types: Submerged (always flowing), Free (nappe free fall), Orifice.

8. Flood Management and Control

8.1 Flood Control Measures

• Structural:

  • Reservoirs: Store flood peak, release gradually (detention/retention).

  • Levees/Embankments: Contain flow within channel.

  • Channel Improvement: Straightening, widening, deepening to increase capacity.

  • Floodways/Bypasses: Divert excess flow away from protected area.

  • Detention Basins: Temporarily store runoff & release slowly.

• Non-Structural:

  • Flood forecasting & warning systems.

  • Floodplain zoning & regulation.

  • Flood insurance.

  • Emergency preparedness & response plans.

  • Watershed management (afforestation, check dams).

8.2 Flood Frequency Analysis (FFA)

• Steps:

  1. Collect annual maximum series (AMS) or partial duration series (PDS).

  2. Fit probability distribution (Gumbel, LP-III).

  3. Estimate parameters (mean, std. dev., skewness for LP-III).

  4. Compute flood magnitude $$\displaystyle x_T $$ for return period $T$ (e.g., $$\displaystyle T=100 $$ years).

     • Gumbel:

$$x_T = \bar{x} + K_T \cdot S$$

        $$\displaystyle K_T $$ = frequency factor from Gumbel table.

    *   **LP-III**: Use log-transformed data, fit Pearson Type III, then back-transform.

5.  Include **plotting position** (e.g., Weibull: $$\displaystyle P = \frac{m}{N+1} $$) for empirical plotting.

8.3 Flood Routing - Muskingum

• Application: River routing (predict flood hydrograph downstream) or reservoir routing.

• Steps:

  1. Determine parameters $K$ (storage constant) & $x$ (weighting factor) from known inflow/outflow hydrograph or from channel characteristics.

  2. Choose routing interval $\Delta t$ (usually 1/4 to 1/3 of time-to-peak of inflow).

  3. Compute coefficients:

$$C_0 = \frac{-Kx + 0.5\Delta t}{K(1-x) + 0.5\Delta t}$$

$$C_1 = \frac{Kx + 0.5\Delta t}{K(1-x) + 0.5\Delta t}$$

$$C_2 = \frac{K(1-x) - 0.5\Delta t}{K(1-x) + 0.5\Delta t}$$

4.  Apply routing equation sequentially for each time step.

9. Advanced Hydrological Analysis

9.1 Depth-Area-Duration (DAD) Curves

• Construction:

  1. For a storm, plot isohyets.

  2. For each isohyet, calculate area enclosed.

  3. For each duration (e.g., 1-hr, 2-hr, 6-hr, 24-hr), find maximum average depth over various areas.

  4. Plot depth vs. area for fixed duration; or depth vs. duration for fixed area.

• Significance:

  • Design Storm: For a given catchment area & return period, select point on DAD curve (higher duration for larger area).

  • Flood Estimation: Used with unit hydrograph to compute peak flood.

  • Water Resources Planning: Reservoir capacity, spillway design.

9.2 Analytical Methods for Lake Evaporation

• Energy Balance Method (most accurate):

$$E = \frac{R_n - G - H}{\lambda}$$

Requires measurement of net radiation ($$\displaystyle R_n $$), soil heat flux ($G$), sensible heat flux ($H$).

• Aerodynamic Method:

$$E = C \cdot (e_s - e_a) \cdot u_2$$

$C$ = bulk transfer coefficient, $$\displaystyle e_s $$=saturation vapor pressure at water temp, $$\displaystyle e_a $$=vapor pressure of air, $$\displaystyle u_2 $$=wind speed at 2m.

• Pan Evaporation: Measured in Class A pan. Convert to lake evaporation using pan coefficient ($$\displaystyle K_p $$):

$$E_{lake} = K_p \times E_{pan}$$

($$\displaystyle K_p $$ ≈ 0.7 for open water).

9.3 Water Resources Planning

• Integrated Water Resources Management (IWRM): Process promoting coordinated development & management of water, land, & related resources to maximize economic & social welfare equitably without compromising ecosystem sustainability.

• Key Aspects:

  • Sustainability: Balance current use with future needs.

  • Climate Change: Incorporate variability & extremes in design.

  • Stakeholder Participation: Involve users in planning.

  • ** conjunctive use**: Surface & groundwater as a single resource.

• Case Study Focus: E.g., Indira Gandhi Canal Project (Rajasthan) – addressed water scarcity, used canal lining, introduced sprinkler/drip to control salinity. Or Hirakud Dam – multipurpose (flood control, irrigation, power).