Meteorology: Precipitation and Rain Gauges

Watershed Hydrology (SWC-201) Lecture Notes

• Course: Watershed Hydrology (SWC-201)
• Instructor: Dr. R. Suresh, Professor SWE
• Department: Soil & Water Engineering
• College: College of Agricultural Engineering
• University: R.P.C.A.U., Pusa (Samastipur)-848 125

Lecture 1: Introduction

• Hydrology: The science of water, its occurrence, distribution, circulation on and below the earth, and its interaction with earth materials.
• Watershed: A land area in which all precipitation drains to the same point, a basin or catchment.
• Importance of Hydrology: Understanding water production from catchments; relationship between surface and groundwater; flood forecasting and design of hydraulic structures; water supply design; crop planning.
• Divisions of Hydrology: Engineering hydrology (design and operation of water projects), Applied hydrology (study of hydrological cycle, precipitation, runoff), Chemical hydrology (chemical characteristics of water), Eco-hydrology (interaction between organisms and the hydrological cycle), Hydrogeology (groundwater), Hydro-informatics (information technology in hydrology), Hydrometeorology (water-energy transfer), Isotope hydrology (origin and age of water).

Lecture 2: Watershed- Concepts and Laws

• Watershed Components:
• Boundary: Defines the size and shape of the watershed, typically ridges or elevated areas.
• Stream Network: Interconnected stream system draining the watershed to the outlet. Drainage density is the ratio of total stream length to total area.
• Watershed Soils/Land: Varied soil types and their characteristics affect water movement and availability.
• Land Use System: Different land uses (agriculture, forests, etc.) influence the hydrological behavior of the watershed.

Lecture 3: Clouds and Precipitation

• Clouds: The source of precipitation, including nimbostratus (continuous rain) and cumulonimbus (thunderstorms).
• Precipitation: Moisture from clouds falling as rain, snow, hail, sleet impacting water resources and affecting various activities.
• Forms of Precipitation: Liquid forms (rain, drizzle), solid forms (snow, sleet, hail).
• Factors affecting precipitation: Evaporation, Condensation, Sublimation, Precipitation

Lecture 4: Rainfall Measurement

• Rain Gauges: Used to measure rainfall, Non-recording (measuring rainfall at a particular interval) type and recording (continuous) type.
• Non-recording: Simplest type, using a funnel and bottle to collect rainfall.
• Recording: More complex types, automatically recording rainfall events.
• Float: Water level rise causes a float to move, recording on a chart.
• Weighing: An increasingly heavier float/lever system indicates rainfall.
• Tipping bucket: Each 0.25 mm rainfall causes a bucket to tip, recorded mechanically or electronically.

Lecture 5: Mean Areal Rainfall

• Computing Mean Areal Rainfall: Method used to determine average rainfall over a large area.
• Arithmetic Average Method: Simple average of collected rainfall data at individual stations.
• Thiessen Polygon Method: Area-weighted average using polygons around stations.
• Isohyetal Method: Plotting isohyets (lines of equal rainfall), determining average precipitation in each area between the isohyets.

Lecture 6: Rainfall Analysis

• Mass Curve: Accumulated rainfall over time.
• Double Mass Curve: Used to check consistency of data, to identify abrupt changes in rainfall records.
• Hyetograph: Rainfall intensity versus time, showing rainfall distribution over a period.

Lecture 7: Rainfall Abstractions and Initial Loss

• Initial Loss: Water losses due to interception, evaporation, infiltration, and depression storage prior to runoff.
• Interception: Precipitation intercepted by vegetation and then evaporated.
• Depression Storage: Water stored in surface depressions that evaporates or infiltrates.
• Infiltration: Water entering the soil.
• Runoff coefficient: The proportion of rainfall that becomes runoff.

Lecture 8: Runoff and its Computation

• Runoff: Excess water flowing over the land surface into channels, rivers or other watercourses.
• Runoff Classification:
• Direct Runoff: Runoff generated directly from rainfall.
• Indirect Runoff: Runoff generated from groundwater or interflow.
• Factors Affecting Runoff: Rainfall intensity, duration, distribution and direction, watershed characteristics (size, shape, slope, soil type, vegetation, and land use) as affecting factors.

Lecture 9: SCS method of Runoff Computation

• Curve Number (CN): Represents the retention capacity of the soil.
• Antecedent Moisture Conditions (AMC): Indicates the previous moisture level of the soil.
• Soil hydrologic Groups: The classifications of the soil that determines the retention and infiltration properties.
• Equation: (P-02.S)2 Q = (P+0.8S)
• where P is rainfall , Q is Runoff, S is storage/ retention

Lecture 10: Cook's Method of Runoff Computation

• Method: Four watershed characteristics: relief, infiltration rate, vegetation cover, and surface storage.
• Numerical Values (W): Assigned to each characteristic based on its influence on runoff.
• Runoff rate: Determined using the selected curve and ∑W value.
• Equation: Qpeak = prfs

Lecture 11: Hydrograph

• Hydrograph Definition: A graph plotting stream discharge against time, showing rise and fall of water.
• Characteristics of a Hydrograph: Rising limb (increasing discharge), peak (maximum discharge), falling limb (decreasing discharge). Base flow (continuous, slow flow).
• Factors affecting the hydrograph: Rainfall intensity, duration, watershed characteristics and time of the year.

Lecture 12: Unit Hydrograph

• Unit Hydrograph Definition: A standardized hydrograph response to a unit depth of rainfall.
• Assumptions of the Unit Hydrograph: Uniform distribution of rainfall, duration of rainfall, time and linearity.
• Limitations: Not valid for all rainfall patterns or areas, not applicable for large areas or areas with temporal and spatial variation in intensity and duration of rainfall events.

Lecture 13: Unit Hydrograph for Different Durations

• Superposition Method: Used when the duration of effective rainfall is a multiple of the basic UHG.
• S-curve Method: Used when the duration of effective rainfall is not a multiple of the basic UHG.

Lecture 14: Synthetic Unit Hydrograph (SUHG)

• Purpose: Estimate the streamflow response for un-gauged or ungaged watersheds.
• Key Parameters: Basin lag (time between center of rainfall excess and peak of hydrograph), and standard duration of effective rainfall.
• Computation procedure

Lecture 15: Stream Flow Measurement

• Methods: Various techniques like staff gauge, chain/tape gauge, current meter, Pressure transmitter, crest stage indicator, and automatic stage recorder.

Lecture 16: Manning's Formula

• Mannings Formula: Estimate flow velocity in open channels like rivers or natural streams. V = 1.49R^(2/3)S^(1/2)/n. Where V is the flow velocity, R is the hydraulic radius, S is the slope of the stream and n is Manning's roughness coefficient.
• Factors affecting flow velocity: Hydraulic radius, slope and roughness (including vegetation).
• Different types of channels: Including straight, irregular, rocky, weedy etc
• Manning's roughness coefficient (n): Values for various channel conditions are provided in a table.

Lecture 17: Dilution Method

• Purpose: To estimate stream discharge.
• Principle: A known amount of tracer substance (e.g., salt) is added to the stream, and the concentration is measured downstream at a fixed rate.
• Chemical Properties: Chemically conservative, easily measurable, soluble, non-toxic

Lecture 18: Stream- Discharge Relationship

• Relationship: Plotted on log-log graph paper. The relationship is between discharge (Q) and stage (G).
• Equation: Log Q = β log (G – a) + log C, where a, β, and C are constants determined from the data.
• Procedure: Plot discharge versus stage.
• Determination of constant "a": Determine the gauge height corresponding to zero discharge using the graph.

Lecture 19: Stream Flow and Reservoir Routing

• Flood Routing: Determines the timing and magnitude of a flood wave at a point on a stream from known data at an upstream point.
• Continuity Equation: dS/dt = I – O, where S is storage, I is inflow, and O is outflow.

Lecture 20: Routing Procedure

• Muskingum Method: One of the method to compute the stream flow downstream of a specified point.
• Calculation of Constants: The method involves calculating constants (Co, C1 and C2) based on the storage constant (K) and weighing factor (x).