Hydrology and Irrigation Water Resources

Introduction

Hydrology is the study of water, its occurrence, movement, distribution and properties on and below the Earth’s surface. For a civil draftsman working on irrigation schemes, canals, culverts, reservoirs, drainage works and bridges, hydrology provides the data needed to prepare reliable hydraulic drawings and construction details. A drawing showing a canal section, spillway, outlet, floodway or bridge opening is only as sound as the hydrological assumptions behind it.

Irrigation planning requires two complementary assessments. The first is the water-supply assessment: how much rainfall, river flow, groundwater or reservoir storage is available? The second is the water-demand assessment: how much water is required by crops, conveyance systems, villages and environmental flows? The relationship between supply and demand is commonly expressed through a water balance.

This chapter develops practical methods for interpreting rainfall, estimating runoff, reading hydrographs, selecting design floods, understanding groundwater, interpreting reservoir data and preparing water-balance information for irrigation planning.

Learning Objectives

After studying this chapter, the student should be able to:

1. Explain the hydrologic cycle and distinguish rainfall, runoff, infiltration, evaporation and transpiration.
2. Interpret rainfall records using depth, intensity, duration and frequency.
3. Estimate runoff using the rational method and the SCS Curve Number method under suitable conditions.
4. Construct and interpret a storm hydrograph, including base flow, peak discharge and time to peak.
5. Distinguish ordinary, design and extreme floods and select hydrologic inputs for hydraulic structures.
6. Describe groundwater occurrence, wells, recharge and aquifer behaviour.
7. Interpret reservoir storage–elevation–area data and calculate changes in storage.
8. Prepare a basic irrigation water balance and identify losses and uncertainty.
9. Convert hydrological results into useful inputs for civil and hydraulic drawings.

1. The Hydrologic Cycle and Catchment Concept

The hydrologic cycle describes the continuous circulation of water among the atmosphere, land surface, soil and groundwater. Solar energy drives evaporation from water bodies and transpiration from vegetation. Water vapour condenses into clouds and returns as precipitation. A portion is intercepted by vegetation, a portion infiltrates the soil, and the remainder becomes surface runoff. Infiltrated water may be stored in the soil, move laterally as interflow, or percolate to groundwater. Groundwater and surface water eventually discharge to streams, lakes or the sea.

A catchment or drainage basin is the land area contributing water to a specified outlet. Its boundary is normally a topographic divide. Catchment area, slope, soil, land use, drainage density, geology and storage strongly influence runoff response. The outlet may be a reservoir site, bridge location, canal intake or watershed boundary.

A simplified catchment water balance is:

where P is precipitation, Q is runoff leaving the catchment, ET is evapotranspiration, I is deep infiltration or groundwater loss from the defined system, and ΔS is change in water stored in soil, groundwater, lakes and channels. All terms must refer to the same period and use consistent units, commonly millimetres over the catchment or cubic metres.

The equation is a conservation statement, not a prediction by itself. Reliable use requires clearly defined boundaries and careful treatment of groundwater exchanges.

2. Rainfall Measurement and Interpretation

2.1 Rainfall characteristics

Rainfall is measured as an equivalent depth of water, normally in millimetres. Important descriptors are:

• Depth: total rainfall accumulated during an event or period.
• Intensity: rate of rainfall, commonly millimetres per hour.
• Duration: length of the rainfall period.
• Areal distribution: variation of rainfall across a catchment.
• Frequency or return period: statistical measure of how often a magnitude may be equalled or exceeded.

An intensity of 30 mm/h for 2 hours does not necessarily mean that 60 mm falls uniformly. Real storms vary in intensity, and the temporal pattern affects peak runoff.

2.2 Rain gauges and data quality

A standard rain gauge measures point rainfall. For a catchment, several gauges are preferable because rainfall is spatially variable. Common methods for estimating mean areal rainfall include the arithmetic mean, Thiessen polygon method and isohyetal method.

The arithmetic mean is suitable where gauges are reasonably uniform in distribution and rainfall variation is small:

P̄ is mean rainfall, Pᵢ is rainfall at gauge i, and n is the number of gauges. The method is simple but gives every station equal weight.

For Thiessen polygons:

where Aᵢ is the portion of the catchment represented by gauge i, Pᵢ is its rainfall and A is total catchment area. The weights must satisfy ∑Aᵢ = A.

Before use, records should be checked for missing values, changes in gauge location, changes in observer practice and transcription errors. A double-mass curve can identify inconsistency by comparing cumulative rainfall at one station with the cumulative mean of surrounding reliable stations. A change in slope may indicate a non-climatic change in the record.

2.3 Missing rainfall data

If a station has a missing value and normal annual rainfall at the target station is within about 10% of the average normal rainfall of surrounding stations, the normal-ratio correction is often used:

Pₓ is estimated rainfall at station x, Nₓ and Nᵢ are normal annual rainfalls, and Pᵢ is observed rainfall at surrounding station i. The formula assumes a stable relationship between stations and requires representative neighbouring data.

2.4 Intensity–duration–frequency relationships

An IDF relationship estimates rainfall intensity for a selected duration and return period. A commonly used local form is:

where i is rainfall intensity, t is duration, a, b and c are fitted regional constants. The constants and units are location-specific and must come from an approved national or regional analysis. An IDF equation must not be transferred blindly between climates.

3. Infiltration, Evaporation and Losses

Infiltration is the entry of water into the soil. Its rate depends on soil texture, initial moisture, compaction, vegetation and rainfall intensity. When rainfall intensity exceeds the infiltration capacity, excess water can form surface runoff. Infiltration capacity generally decreases during a storm as soil pores become filled.

Evaporation is conversion of liquid water to vapour. Transpiration is water vapour loss through plants. Their combined effect is evapotranspiration (ET). Reference evapotranspiration, crop coefficients and effective rainfall are used in irrigation planning.

A basic crop water requirement relation is:

ETc is crop evapotranspiration, Kc is the crop coefficient for growth stage and crop condition, and ETo is reference evapotranspiration. Values should be obtained from locally accepted agronomic guidance. Irrigation requirement is not simply ETc because rainfall, soil storage, application efficiency and leaching requirements also matter.

4. Estimating Runoff

Runoff is the part of precipitation that reaches a drainage channel, river or reservoir as surface flow and, depending on the time scale, rapid subsurface flow. The runoff coefficient is affected by imperviousness, soil, slope, antecedent moisture, storm intensity and catchment size.

4.1 Rational method

For small catchments with a reasonably uniform design storm, the rational method is:

Qₚ is peak discharge in m³/s, C is dimensionless runoff coefficient, i is rainfall intensity in mm/h for a duration approximately equal to the time of concentration, and A is catchment area in km².

The factor 0.278 converts mm·km²/h to m³/s. The method assumes that the selected intensity applies over the catchment, runoff response is sufficiently uniform, and the catchment is small enough that a single concentration time is meaningful. It estimates peak discharge, not runoff volume or a complete hydrograph. The coefficient must be selected from appropriate land-use and soil evidence rather than chosen merely to produce a desired answer.

Worked example. A 0.80 km² developed catchment has C = 0.55. The 20-minute rainfall intensity is 90 mm/h. If 20 minutes is the time of concentration:

Qₚ = 0.278 × 0.55 × 90 × 0.80 = 11.0 m³/s approximately.

This result is a peak estimate. It should not be used as the total storm discharge or automatically applied to a large rural basin.

4.2 SCS Curve Number method

For event runoff depth, the Curve Number method is commonly expressed as:

and Q = 0 when P ≤ 0.2S. Here Q and P are runoff and storm rainfall depths, in the same units, and S is potential maximum retention after runoff begins. In metric practice:

S is in millimetres and CN is the dimensionless Curve Number, normally between 0 and 100. CN depends on land use, hydrologic soil group and antecedent moisture. The method estimates event runoff depth; converting it to a hydrograph requires an additional distribution and routing procedure. Its assumptions are empirical, so local calibration is desirable.

5. Hydrographs and Streamflow Records

A hydrograph is a graph of discharge against time at a specified river section. Its rising limb shows increasing flow, the peak is maximum discharge, and the recession limb shows drainage of catchment storage. A storm hydrograph commonly includes base flow, direct runoff, time to peak and base time.

Figure: Principal features of an illustrative storm hydrograph.

The area under a discharge hydrograph represents volume:

V is flow volume in m³, Q is discharge in m³/s and t is time in seconds. For discrete observations, the trapezoidal approximation is:

The area between the total hydrograph and an estimated base-flow line is direct-runoff volume. Base-flow separation is not unique; straight-line, fixed-base and recession methods can produce different results. The selected method should be documented.

A stage–discharge rating curve converts observed water level (stage) to discharge. It is developed from measured pairs of stage and discharge and may be represented locally by:

where h is stage, h₀ is gauge datum or effective zero-flow stage, and a and b are fitted constants. The relation can change during floods because of channel scour, sediment deposition, backwater or control changes. Extrapolation beyond measured high flows is a significant source of uncertainty.

6. Design Floods and Frequency Analysis

A design flood is a selected flood discharge used to design a structure for a defined safety objective. Examples include a canal cross-drainage work, road culvert, bridge opening, diversion weir or reservoir spillway. The appropriate design flood depends on consequences of failure, service importance, regulation and economic considerations.

The return period T is related to annual exceedance probability p by:

Thus a 100-year event has an annual exceedance probability of 0.01, under the statistical model. It does not mean that the event occurs only once every 100 years, nor does it prevent two such events occurring in successive years.

For an annual maximum series, the probability that a specified event is not exceeded in n independent years is approximately:

The probability of at least one exceedance is 1 − (1 − p)ⁿ. Frequency distributions such as Gumbel or Log-Pearson Type III may be used where required by standards, but the data record, stationarity, independence, outliers and regional information must be considered. Climate change and land-use change can violate stationarity; historical frequency estimates should therefore be reviewed rather than treated as permanent truths.

For design, the hydrologist may provide a peak discharge, design hydrograph, flood volume, sediment estimate and uncertainty range. The draftsman must ensure that the drawing identifies the design water level, freeboard, flow direction, datum and relevant hydraulic control dimensions.

7. Groundwater Resources

Groundwater occupies interconnected pores and fractures below the water table. An aquifer transmits and stores useful quantities of water. An aquitard transmits water slowly, while an aquiclude transmits negligible quantities for practical purposes. Unconfined aquifers have a water table open to atmospheric pressure; confined aquifers are bounded by low-permeability layers and may have potentiometric levels above the aquifer roof.

The principal properties are:

• Porosity, n: ratio of void volume to total volume.
• Hydraulic conductivity, K: ability of the material to transmit water.
• Hydraulic gradient, i: change in hydraulic head per unit distance.
• Specific yield, Sy: drainable water released by gravity from an unconfined aquifer.
• Transmissivity, Tₕ: hydraulic conductivity multiplied by saturated thickness.

Darcy’s law for one-dimensional saturated flow is:

Q is groundwater discharge, K is hydraulic conductivity, A is cross-sectional area normal to flow and i is hydraulic gradient. The law assumes saturated, laminar flow and an adequately representative porous medium. For anisotropic or fractured formations, direction-dependent properties may be required.

Groundwater development must consider recharge, pumping, nearby wells, stream interaction, saltwater intrusion and water quality. A well field should not be designed only from initial yield. Long-term pumping can lower water levels, increase energy costs and reduce base flow to rivers. Monitoring water levels and abstraction is both a technical and governance requirement.

8. Reservoir Data and Storage Analysis

Reservoir planning uses relationships among elevation, surface area and storage. A typical reservoir table contains:

Storage between two surveyed elevations can be estimated using the trapezoidal prismoidal approximation. The trapezoidal form is:

where A₁ and A₂ are water-surface areas and z₂ − z₁ is elevation difference. A prismoidal estimate, when a reliable intermediate area is available, is:

where Aₘ is area at the mid-elevation and Δz is the total elevation interval. Areas must be in m² and elevation in m, giving volume in m³. Surveyed stage–area–storage curves are preferred to rough geometric assumptions.

Reservoir storage is divided conceptually into dead storage, active or conservation storage and flood-control storage. Sedimentation gradually reduces capacity, so original storage curves should not be used indefinitely. Evaporation from the reservoir surface, seepage, releases, abstractions and inflows must be included in operational studies.

9. Irrigation Water Balance

An irrigation water balance compares inflows, storage changes, demands and losses over a defined time period. A simplified reservoir balance is:

S₁ and S₂ are beginning and ending storage, I is inflow, R is controlled release or irrigation withdrawal, E is evaporation loss, Se is seepage loss, W represents other withdrawals, and Δ represents any additional measured or estimated terms. All terms must be volumes for the same period.

At field level, net irrigation requirement may be approximated by:

where NIR is net irrigation requirement, ETc is crop evapotranspiration, Pe is effective rainfall, ΔS(root zone) is useful change in root-zone soil-water storage, and Lr is water required for leaching where applicable. Gross irrigation requirement is:

where Ea is application efficiency expressed as a decimal. If conveyance efficiency Ec is also considered, diversion requirement is:

Efficiencies must describe the actual system and the same location in the water-delivery chain. Applying a field efficiency to a canal intake would underestimate the required diversion.

Worked irrigation example

Suppose a monthly crop requirement is ETc = 150 mm, effective rainfall is 45 mm, useful root-zone storage contribution is 15 mm and no leaching is required. Then:

NIR = 150 − 45 − 15 = 90 mm.

If field application efficiency is 0.75 and canal conveyance efficiency is 0.80:

DR = 90 ÷ (0.75 × 0.80) = 150 mm.

For a 20-hectare command area, where 1 mm over 1 hectare equals 10 m³:

Volume = 150 × 20 × 10 = 30,000 m³ for the month.

This is a planning estimate. Crop area, irrigation scheduling, rainfall reliability, soil variation, seepage and operational constraints may change the actual diversion.

10. Application to Hydraulic Drawings

Hydrological information must be translated carefully into drawing inputs. A professional drawing package may require:

• catchment boundary and drainage direction;
• design rainfall duration and intensity;
• design flood discharge and flood level;
• reservoir operating levels and storage zones;
• canal design discharge and command area;
• groundwater table, well depth and pumping level;
• datum, benchmark and survey reference;
• freeboard, erosion protection and sediment allowance;
• outlet, spillway, culvert and bridge opening dimensions.

The draftsman should not alter hydrological values for drawing convenience. Units, return period, design event, source, date and assumptions should be stated in notes or design schedules. A mismatch between hydrological datum and survey datum can cause serious construction errors. In plan drawings, north direction, chainage, cross-drainage locations and catchment flow paths should be clear. In sections, design water level, bed level, bank level and freeboard should be distinguishable.

11. Illustrative Case Study: Irrigation Diversion and Bridge Crossing

Background

This illustrative case concerns a proposed diversion weir supplying a 600-hectare irrigation command. A rural road crosses the supply canal downstream of the intake. A small natural drainage channel also crosses the road near the canal system.

Problem

The design team must provide preliminary inputs for the weir, canal, road crossing and drainage opening. Available information includes rainfall records, a catchment estimate, reservoir-like pond storage data and crop demand.

Analysis

The hydrology team estimates the design flood for the natural drainage catchment using a regional method supported by observed streamflow. The canal demand is calculated from crop evapotranspiration, effective rainfall and system efficiencies. Survey staff establish a common vertical datum. Storage data are used to assess whether the off-stream balancing pond can meet weekly demand during short dry periods.

The draftsman receives the following controlled inputs:

Approach and solution

The plan shows the catchment boundary, drainage line, canal alignment, road crossing and access for maintenance. The longitudinal section labels bed levels, normal water level, maximum water level and bank crest. The bridge detail identifies the design flood level, soffit clearance, abutments, wing walls and erosion protection. The irrigation schedule records the adopted discharge and command area, while notes identify the source and return period of the flood data.

A review detects that the initial road survey used a local benchmark unrelated to the canal survey. The levels are transformed to the common datum before final drafting. This prevents a false difference in water levels and avoids incorrect bridge clearance.

Outcome

The resulting drawings are hydraulically interpretable, traceable to design data and suitable for review. The project team also identifies the need for sediment monitoring, seasonal reservoir surveys and post-flood inspection of the drainage opening.

Learning points

1. Hydrological inputs must be linked to a stated location, datum, period and design criterion.
2. Canal demand and natural flood discharge are different design quantities.
3. A drawing should show operating levels as well as extreme levels.
4. Survey consistency is as important as hydrological calculation.
5. Uncertainty should be documented rather than hidden.

12. Limitations, Governance and Contemporary Practice

Hydrological estimates are uncertain because rainfall records may be short, stations may be unevenly distributed, catchments may change and extreme events are rare. Model parameters such as runoff coefficient, Curve Number, infiltration rate and efficiency are not universal constants. Sensitivity testing should identify which assumptions most affect the design.

Water allocation also involves governance. Irrigation abstractions may require permits; groundwater pumping may be regulated; environmental flows, downstream users and drinking-water supplies may have legal priority. Data should be documented, quality-controlled and protected from unauthorised alteration. Ethical practice requires that safety-critical assumptions and uncertainty be communicated clearly.

Remote sensing, geographic information systems, automated weather stations, telemetry and digital elevation models now support catchment analysis. Artificial intelligence can help detect anomalous rainfall or predict flows when trained on adequate local data, but it does not remove the need for physical reasoning, independent validation or professional responsibility. A model output should never be accepted merely because it is computationally sophisticated.

Best practice is to use multiple evidence sources: rainfall records, stream gauges, soil and land-use maps, field inspection, survey data and local knowledge. Keep a clear audit trail of data source, units, transformations, calibration, assumptions and approval status.

Chapter Summary

Hydrology supplies the physical basis for irrigation planning and hydraulic drafting. A catchment water balance relates precipitation to runoff, evapotranspiration, infiltration and storage change. Rainfall must be interpreted by depth, intensity, duration, spatial distribution and frequency, with careful attention to gauge quality and missing records.

Runoff may be estimated using the rational method for suitable small catchments or the Curve Number method for event runoff depth. Hydrographs show the timing and magnitude of stream response; their areas represent volumes. Design floods require an explicit safety objective and return-period interpretation. Groundwater analysis depends on aquifer properties, hydraulic gradients, recharge and sustainable abstraction.

Reservoir stage–area–storage data support capacity and operating-level decisions. Irrigation water balances combine inflows, storage, crop demand, rainfall, efficiencies and losses. Finally, hydrological results must be transferred accurately to plans, profiles and structural details using consistent units, datums, design levels and documented assumptions.

Key Terms

Hydrology: Study of water occurrence, movement and distribution.

Catchment: Area contributing runoff to a defined outlet.

Precipitation: Water falling from the atmosphere as rain, snow, hail or similar forms.

Rainfall intensity: Rainfall depth per unit time.

Infiltration: Entry of water into the soil surface.

Evapotranspiration: Combined evaporation and plant transpiration.

Runoff: Water flowing from a catchment toward a channel or outlet.

Runoff coefficient: Ratio or factor representing the portion of rainfall becoming rapid runoff.

Time of concentration: Approximate time for runoff from the hydraulically most distant point to reach the outlet.

Hydrograph: Graph of discharge against time.

Base flow: Sustained streamflow supplied mainly by delayed subsurface sources.

Design flood: Flood selected for design of a specified structure and safety objective.

Return period: Reciprocal of annual exceedance probability.

Aquifer: Geological formation capable of storing and transmitting useful groundwater.

Hydraulic conductivity: Ability of a material to transmit water under a hydraulic gradient.

Recharge: Addition of water to groundwater storage.

Reservoir storage: Volume of water held between specified levels.

Effective rainfall: Portion of rainfall contributing to meeting crop water demand.

Conveyance efficiency: Ratio of water delivered to the field system to water diverted into the conveyance system.

Freeboard: Vertical distance between design water level and the crest or top of a bank or structure.

Review Questions

Short Answer Questions

1. Define a catchment and explain why its boundary is important in a water balance.
2. Distinguish rainfall depth from rainfall intensity.
3. What information does a storm hydrograph provide?
4. Define aquifer, aquitard and recharge.
5. Why must a hydraulic drawing state its vertical datum?

Long Answer Questions

1. Explain the hydrologic cycle and develop a catchment water-balance interpretation.
2. Describe the rational method, including its equation, assumptions and limitations.
3. Explain how a hydrograph is separated into base flow and direct runoff and how runoff volume is obtained.
4. Discuss return period, annual exceedance probability and the selection of design floods.
5. Explain how reservoir storage data and irrigation efficiencies are used in irrigation planning.

Analytical / Application Questions

1. A 2.5 km² catchment has C = 0.40 and a design intensity of 75 mm/h for its concentration time. Estimate the peak discharge using the rational method and state whether the result represents volume or peak flow.
2. A crop has ETc = 180 mm for a period, effective rainfall is 60 mm and useful soil storage contributes 20 mm. If application efficiency is 0.70 and conveyance efficiency is 0.85, calculate the diversion depth.
3. Explain how a draftsman should convert design flood, normal operating level, maximum water level and freeboard into information shown on a canal-and-road-crossing drawing.

Multiple Choice Questions with Answers

Question 1

Which quantity is represented by the area under a discharge hydrograph?

A. Rainfall intensity

B. Flow volume

C. Hydraulic gradient

D. Catchment slope

Correct Answer: B. Flow volume

Question 2

In the rational method Qₚ = 0.278CiA, what is the unit of Qₚ when i is in mm/h and A is in km²?

A. m³/s

B. mm/s

C. km³/h

D. m²/s

Correct Answer: A. m³/s

Question 3

A 0.80 km² catchment has C = 0.55 and rainfall intensity i = 90 mm/h. The rational-method peak discharge is approximately:

A. 5.5 m³/s

B. 9.0 m³/s

C. 11.0 m³/s

D. 19.8 m³/s

Correct Answer: C. 11.0 m³/s

Question 4

A 100-year return-period event has an annual exceedance probability of:

A. 0.1

B. 0.01

C. 0.001

D. 100

Correct Answer: B. 0.01

Question 5

Which expression is Darcy’s law for one-dimensional saturated groundwater flow?

A. Q = KAi

B. Q = CA/i

C. Q = i/(KA)

D. Q = K/(Ai)

Correct Answer: A. Q = KAi

Question 6

If ETc = 150 mm, effective rainfall = 45 mm and root-zone storage contribution = 15 mm, the net irrigation requirement is:

A. 60 mm

B. 90 mm

C. 120 mm

D. 210 mm

Correct Answer: B. 90 mm

Question 7

The SCS Curve Number method primarily estimates:

A. Event runoff depth

B. Groundwater hydraulic conductivity

C. Reservoir evaporation temperature

D. Bridge scour depth directly

Correct Answer: A. Event runoff depth

Question 8

Which reservoir quantity is calculated by integrating or accumulating surface area over elevation?

A. Rainfall intensity

B. Storage volume

C. Runoff coefficient

D. Return period

Correct Answer: B. Storage volume

Question 9

If net irrigation requirement is 90 mm, application efficiency is 0.75 and conveyance efficiency is 0.80, the required diversion depth is:

A. 67.5 mm

B. 90 mm

C. 120 mm

D. 150 mm

Correct Answer: D. 150 mm

Question 10

What is the principal purpose of freeboard in an irrigation canal or reservoir embankment?

A. To increase soil porosity

B. To provide vertical allowance above the design water level

C. To measure groundwater conductivity

D. To define annual rainfall probability

Correct Answer: B. To provide vertical allowance above the design water level