419 chap 1

Dr A.W. Salami CVE 419: SURFACE AND GROUNDWATER HYDROLOGY Land Mark University

CVE 419: SURFACE AND GROUNDWATER HYDROLOGY Course Outline 1.0 Introduction

1.1 Definition of Hydrology 1.2 Engineering Hydrology 1.3 Hydrologic Cycle 1.4 Element of Hydrology

1.4.1 Precipitation 1.4.2 Infiltration 1.4.3 Subsurface Percolation 1.4.4 Evapotranspiration

2.0 Collection and Analysis of Precipitation Data 2.1 Collection of precipitation data 2.2 Analysis of precipitation data

2.2.1 Estimation of missing precipitation data 2.2.2 Estimation of consistency of precipitation data 2.2.3 Estimation of a real mean precipitation

3.0 Collection of Stream Data Flow 3.1 Collection of stream flow data 3.2 Methods of stream flow measurement

4.0 Analysis of Stream Data Flow

4.1 Statistical analysis/probability analysis • Graphical method • Probability distribution function

4.2 Hydrograph 4.3 Flood Routing

5.0 Groundwater

5.1 Occurrences of groundwater and distribution 5.2 Well hydraulics/factors that affect water movement 5.3 Flow patterns in aquifers/factors that affect aquifer yield 5.4 Principles of groundwater investigation/exploration


1.0 INTRODUCTION 1.1 Definition of Hydrology Hydrology is the science which deals with the occurrence, distribution and movement of water on earth, in the atmosphere and below the surface of the earth. It also concerned with their chemical and physical properties as related with the environment and the living things. 1.2 Engineering Hydrology Engineering Hydrology includes those parts of the hydrology related to the design and operation of engineering projects for the control and use of water. Hence, hydrology is used in engineering mainly in connection with the design, operation of hydraulic structures. However, the typical questions that hydrologists is expected to answer include;

i. what flood flows can be expected at a spillway or highway culvert in a city drainage. ii. what reservoir capacity is expected to ensure adequate water for irrigation or municipal

water supply. In order to understand better, the occurrence, distribution and movement of the water on the earth, it is best explained using hydrologic cycle presented as follows; 1.3 Hydrologic Cycle The earth water circulatory system is known as the hydrologic cycle. Hydrologic cycle is the continuous process by which moisture is transported from the atmosphere to the earth in the form of precipitation, conveyance of rainfall-runoff by streams and rivers to the ocean and evaporation back to the atmosphere. This natural circulation of water is presented in Figure 1.

Figure 1: Hydrologic Cycle


The driving force of the circulation is derived from the radiant energy received from the sun. The bulk of earth’s water is stored on the surface in oceans (see Table 1). Hence, it is logical to consider the hydrological cycle as the beginning with the direct effect of the sun’s radiation on the largest reservoir. Heating of the sea surface causes evaporation, the transfer of water from the liquid to the gaseous state, to form part of the atmosphere. The water vapour changes back to the liquid state again through the process of condensation to form clouds and with favourable atmospheric condition, precipitation (rainfall or snow) is produced either to return directly to the ocean storage or via the land surface through runoff. Snow may accumulate in polar regions or on high mountains and consolidate into ice, in which may be stored naturally for very long period (Table 1). In more temperate lands, rainfall may be intercepted by vegetation from which the intercepted water may return at once to the air, by evaporation. Rainfall reaching the ground may gather to form surface runoff or it may infiltrate into the ground. The liquid water in the soil then percolates through the unsaturated layers to reach the water table where the groundwater becomes saturated or it is taken up by vegetation from which it may return to atmosphere by the process of transpiration. The surface runoff and groundwater flow and join together in surface streams and rivers which may be held up temporarily in lakes but finally flow into the oceans. The land phases of the hydrological cycle have an enhanced importance in nature since evaporation is a purifying process, the salt sea water is transferred into fresh precipitation water and therefore water sources and storages on the continents consists largely of fresh water. The exceptions include groundwater storages with dissolved salts (brackish water) and surface water polluted by man or natural suspended solids. Table 1 gives the estimates of the world’s water. Occurrence Volume

106 km3 Percentage (%)

Rate of Exchange

Oceans 1370 94.2 3000 Groundwater 60 4.13 5000 Ice sheets and glaciers

24 1.65 8000

Surface water on land

0.28 0.019 7

Soil moisture 0.08 0.0055 1 Rivers 0.0012 0.00008 0.031 Atmospheric vapour

0.014 0.00096 0.027


1.4 Element of Hydrology 1.4.1 Precipitation Precipitation can be defined as the fall of moisture from the atmosphere to the earth surface in any form. Liquid precipitation is known as rainfall. Frozen precipitation may be in form of snow, hail, sheet, freezing rain. If the precipitation is greater than infiltration rate, runoff occurs. The moisture in the atmosphere, although forming one of the smallest storages of earth’s water, is the most vital source of freshwater for mankind. Water is present in the air in its gaseous, liquid and solid states as water vapour, cloud droplets and ice crystals respectively. All forms of precipitation are normally measured with rain gauge. Methods of obtaining readings vary from manual to autographic (charts), signed by telemetry and RADAR (remote sensing). There are two types of rain gauge:

(i) Non-recording i.e standard 8 inches ppt gauge. (ii) Recording type: weighing bucket gauge Tipping bucket gauge Siphon or flow type gauge Assignment Distinguish between the three types of recording rain gauges. Difficulties encountered in the accurate measurement of precipitation.

1. Any suitable device for use as rain gauge extends above the earth surface and creates eddy currents which affect the amount of catch.

2. Wind affects the amount of catch. The rain gauge should be sheltered from the wind to minimize the wind effect and at the same time clear of obstruction.

3. The measurement of precipitation is never subject to check by repetition and seldom by duplication.

4. The samples constituting the measurement are always very small compared to the total quantity of precipitation over the area.

The number of observation may be decided by the following consideration: 1. For area up to 250km2, one station per 125km2 area is sufficient. 2. For area above 250km2, one additional station is provided for every additional area of

250km2 or part of it. Table below gives a rough idea of the number of gauges required in a catchment of various areas Table 2: Number of rain gauge for specified areas Area (km2) No of Rain gauge station 0 – 80 1 80 – 160 2 160 – 320 3 320 – 560 4 560 – 800 5 800 – 1200 6


Site selection for Rain gauge While selecting the site for installation of rain gauge, the following should be observed,

1. The rain gauge should be set in an open space. 2. The distance between the rain gauge and the nearest object should not be less than twice

the height of the object. In no case, this distance should be less than 30m. 3. If a suitable level ground is available rain gauge should not be installed on top of a hill. 4. In hilly areas where level ground is difficult to find, the rain gauge should be located

where it is best shielded from high velocity winds and where wind couldn’t cause eddy. 5. In case a fence is provided around the rain gauge to protect it from intruders, the distance

between the rain gauge and the fence should not be less than the height of the fence. Assignment Write short note on the following

1.4. 2 Infiltration/infiltration rates 1.4. 3 Subsurface percolation 1.4. 4 Evaporation 1.4 .5 Evapotranspiration 1.4. 6 Transpiration 1.4. 7 Groundwater 1.4. 8 Surface runoff.

419 chap 1

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