Introduction to Hydrology PDF

CE 2221 Module 1: Introduction to Hydrology PRESENTED BY: ENGR. DANIELA R. SIBUG Topic Outline 1.1 H y d r o l o g i c C ycl e 1.2 H y d r o l o g i ca l Sy s t e m 1.3 C o n ce p t o f Wa t e r Ba l a n ce...

CE 2221 Module 1: Introduction to Hydrology PRESENTED BY: ENGR. DANIELA R. SIBUG Topic Outline 1.1 H y d r o l o g i c C ycl e 1.2 H y d r o l o g i ca l Sy s t e m 1.3 C o n ce p t o f Wa t e r Ba l a n ce 2 1.1 Hydrologic Cycle 3  Definition Hydrology Hudor - “water ”; Logy - “study of ” “Hydrology is the science that treats the waters of the earth, their occurrence, circulation and distribution, their chemical and physical properties, and their reaction with their environment, including their relation to living things. The domain of hydrology embraces the full life history of water on the earth” Water Management Revolves around how water resources is used sustainably. It’s manipulating the hydrologic cycle to design hydraulic structures, water supply and water treatment facilities, wastewater treatment & disposal facilities, irrigation systems, hydropower generation, flood control, etc. 4  Hydrology as a Profession A profession is a “calling requiring specialized knowledge, which has as its prime purpose the rendering of a public service” What hydrologists do: Water use – water withdrawal and instream uses Water Control – f lood and drought mitigation Pollution Control – point and nonpoint sources 5 Global Water Resources 6 Global Water Withdrawal & Consumption Wa t e r w i t h d ra wa l r e f e r s t o t h e p r o c e s s o f t a k i n g wa t e r f r o m n a t u ra l s o u r c e s l i ke r i v e r s , l a ke s , a q u i f e r s , o r r e s e r vo i r s f o r va r i o u s u s e s s u c h a s a g r i c u l tu ra l i r r i g a ti o n , i n d u s tr i a l p r o c e s s e s , o r d o m e s ti c u s e. T h i s wa t e r m a y o r m a y n o t b e returned to the original source after use. Wa t e r c o n s u m p t i o n r e f e r s t o t h e p o r t i o n o f t h e w i t h d ra w n wa t e r t h a t i s n o t r e t u r n e d t o t h e s o u r c e. T h i s wa t e r i s u s e d i n a wa y t h a t m a ke s i t u n a va i l a b l e f o r i m m e d i a te r e u s e w i t h i n t h e s a m e wa t e r s h e d o r s y s t e m— s u ch a s wa t e r t h a t e va p o ra te s , i s i n c o r p o ra te d i n t o products, or is consumed by p l a n ts o r a n i m a l s. 7 Global Water Use In essence, water withdrawal is about how much water is taken , while water consum ption is about how much of that water is permanently removed from the system. 8 Where are the World’s Freshwater Resources? The Philippines has abundant freshwater resources , suppor ting its rich ecosystem s and communities. 9  Definition Hydrologic Cycle Also known as the water cycle, is the continuous, global process by which water circulates through the Earth's atmosphere, surface, and subsurface. Components of the Hydrologic Cycle Evaporation. The process by which liquid water is converted into water vapor as water is heated by the sun and it's surface molecules become sufficiently energized to break free of the attractive force binding them together. Condensation. The process by which water vapor condenses back into liquid af ter it rises and cools in the atmosphere. When condensation occurs at the ground level, the resulting water droplets are called dew. Precipitation. The discharge of water out of the atmosphere, generally onto land or water surface. It is also commonly used to designate the quantity of water that is precipitated and is the primary input quantity to the hydrologic cycle. 10  Definition Interception. The process by which precipitation is caught and held by foliage, twigs, and branches of trees, shrubs, and other vegetation, and lost by evaporation, never reaching the surface of the ground. Inf iltration. The movement of water through the soil surface into the soil which is controlled by soil texture, soil structure, vegetation and soil moisture status. Transpiration. The process by which water vapor is emitted into the atmosphere from plant surfaces. Evapotranspiration is the combination of water released to the atmosphere by evaporation and transpiration. Surface Runoff. The portion of water which does not infiltrate the soil but flows over the surface of the ground to a stream channel. Surface runoff is also known as overland flow. Interf low. Lateral movement of infiltrated water. Some water that is precipitated seeps through soil and continues to follow the slope. This water is eventually discharged into rivers, streams, and lakes. 11  Definition Percolation. Precipitation that moves downwards, percolates or infiltrates through cracks, joints and pores in soil and rocks until it reaches the water table where it becomes groundwater. Groundwater Flow. A body of water found in a deep aquifer zone that flows laterally and eventually merges with rivers, streams, lakes, and oceans. 12 Schematic Diagram of Hydrologic Cycle 13 Global Water Cycle 14 1.2 Hydrologic System 15  Definition A hydrological system refers to the interconnected components and processes within a specific area that govern the movement, distribution, and storage of water. This system includes elements like rivers, lakes, groundwater, watersheds, and man-made structures like dams and reservoirs. It encompasses both the physical components (e.g., water bodies, soil, vegetation) and the processes (e.g., precipitation, infiltration, evaporation) that influence water flow and storage in a particular region. Essentially, it is the framework through which water moves and is managed in a specific area. 16 Definition A Watershed is an area of land that drains to a stream (at a given location), lake or ocean and is separated from other watersheds by a watershed divide. Figure: Delineation of watershed boundary 17  Definition Synonyms: watershed, catchment, river basin, drainage area. Watershed : Think of it as a small area of land where all the rainwater drains into a single stream or river. Imagine a valley with hills on both sides —any rain that falls within that valley f lows into the same stream at the bottom. Catchment: is ver y similar to a watershed. It’s the area where rainwater "catches " and drains into a specif ic river, stream , or lake. The terms watershed and catchm ent are of ten used interchangeably. River Basin: A river basin is much bigger. It's like a giant version of a watershed. It includes many watersheds or catchment s that all drain into one large river. For example, all the small rivers and stream s that f low into the Pampanga River make up the Pampanga River Basin. Drainag e Area: This is a general term for any land area where water drains toward a cer tain point, whether it's a small stream or a huge river basin. It can be any size, from a small hill to an entire continent. 18  Pasig- Marikina-Laguna Lake River Basin RIVER BASIN EXAMPLE This is because it encompasses a large area with multiple smaller watersheds and catchments that all drain into a major river system—the Pasig River, Marikina River, and eventually into Laguna Lake. It includes the entire network of rivers, streams, and lakes that contribute to the flow of water within this system. 19  Marikina River Basin RIVER BASIN EXAMPLE The Marikina River is best described as part of a river basin. This includes several smaller watersheds and catchments within its reach. 20  Pampanga River Basin RIVER BASIN EXAMPLE Figure: Assembly of 1:50,000 Scale NAMRIA Topographic Maps which Covers the Entire Pampanga River Basin. 21  Pampanga River Basin RIVER BASIN EXAMPLE Figure: Pampanga River Basin and River Network with 11 Major Sub-Basins and 3 Major River Systems 22  Manila Bay N W E S Watershed or R-1 R-6 Catchment R-5 R-7 Example FIGURE: PILAR RIVER Legend CATCHMENT IN BATAAN R-4 Basin Boundary R-3 Main River It’s a smaller, localized area where rainwater drains Tributary River (1st Deg) R-2 Tributary River (2nd Deg) Tributary River (3rd Deg) into the Pilar River. 1000 0 1000 2000 3000 4000 Meter s 23  Watershed or Catchment Example FIGURE: SA N ROQUE WATERSH ED IN PA N GA SIN AN The San Roque Watershed is part of the Agno River Basin, which is a large river basin in the Philippines that covers a significant portion of northern Luzon. 24  Summary The concept of a watershed is basic to all hydrologic designs. Large watersheds are made up of smaller watersheds. It is necessary to define the watershed in terms of a point, referred to as the outlet. This is the location at which the design is being made. With respect to the outlet, the watershed consists of all land area that sheds water to the outlet during a rainfall event. A watershed is defined by all points enclosed within an area from which rain falling at these points will contribute to the outlet. Given the definition of watershed, the delineation of a watershed is very important to hydrologic design. It is necessary to decide which points in a region will contribute water to the outlet. The most extreme of these points represents the watershed boundary. 25  Watershed Example: A Swimming Pool T h e b l u e cove r s y m b o l i ze s t h e wa t ers hed , t h e a r e a w h e re p r e ci p i t a t i on f l o w s " d o w n - g ra d i e n t " t o wa rd t h e b a s i n ' s l o w e st p o i n t , w h i ch , i n t h i s ca s e , i s t h e ce n t er o f t h e p o o l cove r. S i m i l a r l y, i n n a t u r e , wa t er f l o w s t o wa rd va l l e y s , r i ve rs, o r l a ke s. A l a ke fo r m s a t t h e p o o l ' s ce n t er fo r t h e s a m e r e a s on i t fo r m s i n a l a n d s ca p e — i t i s t h e l o w e st p o i n t , a n d wa t er co l l e ct s t h e re t h r o u g h r i ve r i n f l o w, s e ep a ge, o r r e d u ced e va p o rat i o n. In m o s t va l l e y s , i f t h e l a n d s l o p e s d o w n wa rd , a r i ve r n a t u ra l l y fo r m s a s t h e wa t er s e ek s t h e l o w e s t pa th. 26 Watershed Delineation Wa t e r s h e d d e l i n ea t i o n i s t h e p r o c e s s o f i d e n t i f y i ng a n d m a p p i n g t h e b o u n d a r i es o f a w a t e r s h ed , w h i c h i s a n a r e a o f land where all surface water c o n ve r g e s t o a s i n g l e p o i n t , usually at the outfall of a s t r e a m , r i ve r, o r o t h e r b o d y o f w a t e r. T h i s p r o c e s s i n vo l ve s d e t e r mi n i n g t h e t o p o g ra p h i c high points, or divides, that d e f i n e t h e p e r i m e t er o f t h e w a t e r s h ed. 27  Watershed Delineation Manual Delineation o involves using topographic maps and elevation data to identify watershed boundaries by hand. Automated Delineation o uses Geographic Information Systems (GIS) and digital elevation models (DEMs) to perform the process quickly and efficiently. 28 Understanding Contour Line Formations and How to Read Topographic Maps To p o g rap h i c m a p s u s e a co m b i n a t i o n o f co l o r s , s h a d i n g , a n d co n t o u r l i n e s t o r e p resen t ch a n ge s i n e l e va ti o n a n d t e rra i n s h a p e. E s sen t i a l l y, t o p o g ra p h i c m a p s r e p resen t t h e t h r e e - d i m e n s i o n a l l a n d s ca p e o f E a r t h w i t h i n t h e t w o -d i m en si o n a l s p a ce o f a m a p. co n t o u r l i n e s m a r k p o i n t s o f e q u a l e l e va ti on o n a ma p 29 Manual Delineation Steps 1.Draw a circle at the outlet or downstream point 2.Put small "X's" at the high points along both sides of the watercourse, working your way upstream towards the headwaters of the watershed. 3.Starting at the circle that was made in step one, draw a line connecting the "X's" along one side of the watercourse (Figure E-5, below left). This line should always cross the contours at right angles (i.e., perpendicular to each contour line it crosses). 30 Manual Delineation Steps 4. Continue the line until it passes around the head of the watershed and down the opposite side of the watercourse. Eventually, it will connect with the circle from which you started. At this point you have delineated the watershed of the desired outlet being evaluated. 31 Manual Delineation Example 32 Manual Delineation Example 33 Manual Delineation Example 34  Automated Delineation Steps Watershed Drainage Line Flow Direction and Stream Definition and Data Preparation Delineation and Delineation and Accumulation Segmentation Polygon Processing Polyline Processing 1. Data Preparation: Import a projected Digital Elevation Model (DEM) into the application. 2. Create Flow Direction Grid: Utilize the "Flow Direction" tool to establish flow directions. 3. Create Flow Accumulation Grid: Employ the "Flow Accumulation" tool to compute flow accumulation values. 4. Define Stream Network: Utilize tools like "Stream Definition" and "Stream Segmentation" to identify stream channels. 5. Delineate Watershed Boundary: Utilize tools such as "Catchment Grid Delineation" and "Catchment Polygon Processing" to delineate watershed boundaries and create polygons. 6. Delineate Stream Network: Use tools like "Drainage Line Processing", "Drainage Point Processing", and "Longest Flow Path" to delineate the stream network accurately and create polylines. 35 Sample DEM 36 Sample DEM 37  Flow Direction Flow Accumulation Automated Delineation Steps Stream Definition Stream Segmentation 1 2 7 8 DEM Reconditioning Fill Sinks Catchment Grid Delineation Catchment Polygon Processing 3 4 9 10 Drainage Line and Point Processing Longest Flow Path Flow Direction Flow Accumulation 5 6 Stream Definition Stream Segmentation 38 Automated Delineation Example o The result is a delineated watershed boundary and stream network. o Other GIS tools can then be used to calculate physical properties of the watershed and streams, such as area, stream lengths, slope, and drainage density 39 Automated Delineation Example 40 Parts of a Watershed D ra i n a g e N e t w o r k : T h e i n t e r c o n n e c t e d s y s t e m o f s t r e a m s , r i v e r s , a n d c h a n n e l s t h a t t ra n s p o r t water through the watershed. D i v i d e o r Wa t e r s h e d B o u n d a r y : T h e h i g h p o i n t s o r r i d g e l i n e s t h a t s e p a ra t e o n e wa t e r s h e d f r o m a n o t h e r, d i r e c t i n g t h e f l o w o f w a t e r. 41  Parts of a Watershed o He a dwaters : The s o ur ce re gio n s of s trea m s or ri ve r s, o f ten fo u nd in hi gh er eleva tio n s wit hin the wa te r s h ed. o Su bbas i n s : Sm alle r, di stinct a re a s within a water she d, e a ch contrib uti ng r unof f to a p ar ticul ar s tr e a m o r r i ve r. o Tri b u t aries : Smalle r stre am s o r ri ver s th at f low i nto a l ar ge r rive r, co n tri bu ting a ddi tion al water t o th e m a i n ch a n n e l. o Rea ches : Se cti on s o f a rive r o r s tr ea m wi thi n a water s h ed, ch a ra cteri zed b y rela tively u nif orm f l o w co n d i tio n s a n d p hy s i c a l ch a ra cte ris tics. o Ma i n C h a n nel : Th e p rimar y water cou r se t ha t colle cts a n d ch an nel s wa te r f r om s u b ba sin s a n d r e a che s towa r d s t h e wa t e r s h e d o u tl e t. o F l o o d p la i n : Th e f lat a rea adj a ce nt to a river or s tre am tha t i s p erio di cally in un da te d by f l o o dwate rs. o O u t let : The p oin t w he re water e xits th e water s he d, of ten ma rked by the co nf lu en ce of stre am s o r a r i ve r m outh. 42  Watershed Properties 1. Drainage Area (D.A.) ▪ Probably the most impor tant watershed characteristic for hydrologi c design. ▪ It ref lects the volume of water that can be generated from rainfall. ▪ The D.A. is req ui red as i np ut to m od els rangi ng fro m simpl e l inear p redi cti on equations to complex computer models. ▪ The D.A. of a watershed requires the delineati on of the watershed boundar y. ▪ Geog rap hical i nform ati on systems (GIS) are commonl y used to d eli neate watershed boundari es in which areas are computed automaticall y. ▪ The area can also b e computed manually using planimeter and the stone age method of counting squares. 43 Watershed Properties 44  Watershed Properties 2. Watershed Length (L) The length of a watershed is the second watershed characteristi c of interest. It is imp or tant i n hydrol ogi c computati ons , i.e., it is used in tim e-of -concent ration computat ions. Def ined as t he distance measured along the m ain channel f rom the watershed outlet to the basin divide. Leng th i s measured al ong the p ri ncipal f l ow p ath. Thus, usuall y l abeled the hydrol ogi c length. The length is usually used in computing a time parameter, which is a measure of the travel time of water through the watershed. 45  Watershed Properties Figure: Delineation of watershed and channel lengths 46  Watershed Properties 3. Watershed Slope (S) Fl ood m ag nitud es ref lect the m om entum of the runoff. Sl ope is an im por tant factor i n the momentum. Watershed slope ref lect s the rate of change of elevation with respect to distance along the principal f low path. Where the design work requires th e watershed to be subdivided, i t wil l be necessar y to compute the slopes of each sub-area. I t m ay also be necessar y to com put e the channel slopes f or the individual sections of the stream s that f low through the sub-areas. 47 Watershed Properties 48 Watershed Properties Table: Lengths and slopes of watersheds 49  Watershed Properties 4. Wa t e r she d S h a p e Bas i n s ha pe i s no t us u al l y used d irect l y in hy dro l o g i c des ig n me t ho ds ; howe ver, p a ra meters th a t ref le ct ba s i n s h ape are used o cca s i o n a l l y a n d h ave a co n ce p t u a l b a s i s. A c ircu lar water sh ed wo uld r e sult in r unof f f rom vario u s p ar ts of the water s he d r ea ching th e o u tl et a t th e s a m e t i m e. A n e l l i pt i ca l waters hed h avi ng the ou tlet at o ne end of the maj or a xi s a n d h avin g th e sa me a re a a s the cir cula r waters h ed w o uld ca u se t he r u nof f to be s p rea d ou t ove r time , thu s p r od u cin g a sm aller f loo d p ea k th an tha t of th e ci r cul ar wa te r shed. 50  Watershed Properties Watershed parameters that reflect basin shape: a) Length to the center of area (Lca): the distance in miles measured along the main channel from the basin outlet to the point on the main channel opposite the center of the area. b) Shape factor (Ll): Ll = (LLca ) 0.3 c) Circularity ratio (Fc): P Fc = (4A)0.5 where, P and A are the perimeter and area of the watershed respectively d) Circularity ratio (Rc): A Rc = Ao where, Ao is the area of a circle having a perimeter equal to the perimeter of the basin e) Elongation ratio (Re): Elongation Ratio = DC / LLFP DC 51  Watershed Properties 5. Land Cover and Use Land cover and use serve as inputs for problems in hydrologic analysis and design. A qualitative description of land cover is transformed into a quantitative index of runoff potential such as the runoff coefficient, c, to reflect the runoff potential of a watershed. The Soil Conservation Service (SCS) uses a different land cover/use index in their models which is called the runoff curve number (CN). It is an integration of the hydrologic effects of land use, soil type, and antecedent moisture. Urban land covers are especially important in hydrology in which the percentage of imperviousness is a commonly used index of the level of urban development. High-density residential areas have percentages of imperviousness from 40% to 70%. Commercial and industrial areas are characterized by impervious cover often from 70% to 90%. 52  Watershed Properties 6. Surface Roughness Roughness implies unevenness of texture. Surface roughness is also important in hydrologic design, for example, grass retards flow to a greater extent than the roof since grass is hydrologically rougher than the roof. Manning’s roughness coefficient, n, is the most frequently used index of surface roughness. In general, its dimensions are TL-1/3. 53  Watershed Properties Table: Manning’s roughness coefficient, n, for overland flow surfaces 54  Watershed Properties Soil Characteristics a. Soil Profile o The hydrologic characteristics of soil vary not only spatially but with depth as well. o Layers of soil vary in composition, structure, texture, and color. These characteristics and the number of layers may also vary from site to site. o Soil horizons are referenced as follows: 55  Watershed Properties Soil Characteristics b. Soil Texture o Soil texture is a physical characteristic of a soil which refers to the size of the mineral particles and the fraction of the particles in different size classes. 56  Watershed Properties Particles of soil can be separated into three classes on the basis of the mean diameter, d, in millimeters: Clay: d < 0.002 mm Silt: 0.002 < or = d < 0.02 mm Sand: 0.02 < or = d < or = 2 mm The texture is further divided on the basis of the percentage of each of these soil classes. Mineral particles have diameters that typically fall into size ranges. The U.S. System for Texture Designations proposed a classification system in which 11 texture groups are classified based on the mixture of the three soil classes. Soil texture is an important factor in determining the water-holding characteristics of the soil and therefore the infiltration capacity of a soil layer. As the diameter of the soil particles increase, the pore spaces increase in size, which increases the capacity of the soil to transmit and store infiltrating water through the soil profile. 57  Watershed Properties Figure: Guide for textural classification by the U.S. System for Textural Designations 58  Watershed Properties c. Soil Structure Soil structure is another property of a soil that affects the hydrologic response of a watershed. It refers to the tendency of soil particles to aggregate into lumps and clods. The structure is a function of the soil texture, the texture and type of minerals present, and the amount of biological activity in the soil column. The structure of the soil influences the amount of pore space in the soil column, which in turn, affects infiltration, soil moisture retention, and other water movement in the soil. The average soil column may have pore space of about 45%, with a variation from 30% to 70%. 59 Watershed Properties Table: Characteristics of soils assigned to soil groups and their estimated minimum infiltration rates 60  Watershed Properties Table: Runoff Curve Numbers [Average Watershed Condition] 61  Watershed Properties Table: Runoff Curve Numbers [Average Watershed Condition] 62  Watershed Properties Table: Runoff Curve Numbers [Average Watershed Condition] 63 Watershed Properties Table: Antecedent Soil Moisture Condition Condition I: Soils are dry but not to wilting point; satisfactory cultivation has taken place Condition II: Average conditions Condition III: Heavy rainfall, or light rainfall and low temperatures occurred within the last 5 days; saturated soil 64  Watershed Properties Table: Adjustment Curve Numbers for Dry (condition I) and Wet (condition III) Antecedent Moisture Conditions. 65  Watershed Properties d. Volumetric Characteristics of Soil o Porosity: Measures the total volume of void spaces within the soil, but does not alone determine water transmission ability. o Permeability: Indicates how well soil or geologic formations allow water to flow through it, factoring in pore size and connectivity. o Hydraulic Conductivity: Describes the rate at which water can move through soil pores, providing a more complete understanding of water movement. o Pore Size Distribution: Affects how water is retained and transmitted within the soil; larger pores generally facilitate faster water movement. 66  Watershed Properties d. Volumetric Characteristics of Soil o Soil Structure: Influences the connectivity and arrangement of pores, impacting the ease with which water can flow through the soil. o Degree of Saturation: Affects water movement, as water flow is influenced by how much pore space is occupied by water versus air. o Specific Yield: Refers to the volume of water that can be drained from a saturated soil by gravity, expressed as a ratio of the volume of water released to the total volume of soil. It provides insight into the soil's ability to supply water for extraction purposes. 67  Watershed Properties o Porosity increases with grain size, as larger grains create more void space. o Specific yield is higher in coarse-grained soils due to larger pores allowing more water to drain. o Specific retention is higher in fine-grained soils, as smaller pores retain more water Figure: Variation of specific retention, specific against gravity. yield, and porosity with the grain size 68  Main Channel and Reach Main Channel: The primar y watercourse that carries water from tributaries and subbasins toward the watershed outlet. Re a c h : A s p e c i f i c s e g m e n t o f t h e r i v e r o r s t r e a m w i t h r e l a t i ve l y u n i f o r m f l o w a n d p h y s i c a l c h a ra c t e r i s t i c s w i t h i n t h e m a i n c h a n n e l. REACH MAIN CHANNEL 69 Channel Properties 70  Channel Properties 1. Channel Length (LC) Channel length is also used frequently in hydrologic computations. It is defined as the distance measured along the main channel from the watershed outlet to the end of the channel as indicated on the map. The definition for channel length, like for watershed length, also involves a measure of subjectivity because the end point of the channel is dependent on the way the map was drawn. The location of the end of the channel may depend on the level of flow at the time the map was compiled. This subjective assessment may introduce an unknown degree of inaccuracy into the final design. 71  Channel Properties 3. Drainage Density (D) The drainage density is the ratio of the total length of streams within a watershed to the total area of the watershed, thus D has units of the reciprocal of length. A high value of the drainage density would indicate a relatively high density of streams and thus a rapid storm response. LT where, D= LT is the total length of streams D.A. D.A. is the watershed area 72  Channel Properties 4. Stream Numbers (Ni ) The concept of stream order is used to compute other indicators of drainage character. The bifurcation ratio (Rb) is defined as the ratio of the number of streams of any order to the number of streams of the next higher order. Values typically range from 2 to 4. 73 Channel Properties 74 Channel Properties 75 Channel Properties 1 1 1 2 1 2 1 1 1 1 2 2 2 1 3 3 1 1 3 2 3 1 2 1 1 1 4 2 1 1 4 1 76  Channel Properties 5. Horton’s Stream Lengths (Li ) The law of stream lengths relates the average length of stream order i (Li) to the stream length ratio (rL) and the average length of first-order streams (L1). The stream length ratio is defined as the average length of streams of any order to the average length of streams of the next order. Li = L1rLi −1 6. Horton’s Stream Areas (Ai ) The law of stream areas relates the mean tributary area of streams of order i (Ai) to the mean drainage area of first-order basins (A1) and the stream area ratio (ra). The stream area ratio is the average basin area of streams of one order to the average area of basins of the next lower order. Ai = A1rai −1 77  Channel Properties 7. Horton’s Stream Slopes (Si ) The law of stream slopes relates the average slope of streams of order i (Si) to the average slope of first-order streams (S1) and the stream slope ratio (rs). The stream slope ratio is the average slope of streams of order j to the average slope of streams of the next higher order, j+1. Si = S1rsi −1 8. Channel Cross Sections Shape (x & y coordinates), area, wetted perimeter, slope, roughness, and average velocity are important channel parameters. 9. Channel Roughness 78  Channel Properties Table: Recommended design values of Manning Roughness coefficients, n  Channel Properties Table: Recommended design values of Manning Roughness coefficients, n  Channel Properties Table: Recommended design values of Manning Roughness coefficients, n Channel Properties Table: Roughness Coefficient Modifier: Basic Coefficient, (n1) Channel Properties Table: Roughness Coefficient Modifier: Correction for Channel Irregularity, (n2) Channel Properties Table: Roughness Coefficient Modifier: Correction for Cross Section Variation, (n3) Channel Properties Table: Roughness Coefficient Modifier: Correction for Obstructions, (n4)  Channel Properties Table: Roughness Coefficient Modifier: Correction for Vegetation, (n5) Channel Properties Table: Roughness Coefficient Modifier: Correction for Channel Meandering, (n6)  Channel Properties Table: Computation Sheet for Manning’s Roughness Coefficient  Channel Properties 10. Travel Time The velocity method is based on the concept that the travel time (Tt) for a particular flow path is a function of the length (L) and the velocity (V). L Tt = V Velocity is a function of the type of flow, roughness of the flow path, and the slope of the path V = kS 0.5 1 2 / 3 1/ 2 V = Rh S n  Channel Properties Table: Coefficients of velocity versus slope relationship for estimating travel times with the velocity method  Channel Properties Table: Velocities for upland method of estimating tc 1.3 Water Balance 92  Definition Water Budget or Water Balance is the accounting or allocation of water to each of the components of the hydrologic cycle. The hydrologic continuity equation for any system is: dS I −Q = dt where, I = inflow in vol/time Q = outflow in vol/time dS/dt = change in storage in vol/time 93  Definition Applying the continuity equation to a basin or watershed, for a given period of time, the overall water budget, in units of depth (mm or cm) over the watershed will be: The hydrologic continuity equation for any system is: P − O − G − E − T = S where, P = precipitation E = evaporation O = runoff or outflow T = transpiration G = groundwater flow S = change in storage 94  Sample Problem A watershed with an area of 2500 km2 received 130 cm of precipitation in a given year. The average rate of flow measured in a river draining the watershed was 30 m3/s. Estimate the amount of water lost (in cm) due to the combined effects of evaporation, transpiration, and infiltration to ground? What is the runoff coefficient? Assumption: No change in storage from t = 0 to t = 1yr. Required: Water lost due to combined effects of E, T and I, runoff (in cm), runoff coefficient. Given: Area 2500 km2 2500000000 m2 Rainfall (P) 130 cm 1.3 m Outflow (Q) 30 m3/s 30 m3/s Time (T) 1 yr 31536000 s 95 Sample Problem Solution: Given: Area 2500 km2 2500000000 m2 Water Balance Equation: Rainfall (P) 130 cm 1.3 m Outflow (Q) 30 m3/s 30 m3/s P−Q−E−T−I = 0 Time (T) 1 yr 31536000 s (E + T + I ) = P − Q Runoff coefficient is a dimensionless factor that is used to convert the rainfall amounts  m 3  31,536,000 s  (E + T + I) = 1.3 m − 30   2  to runoff.  s  2,500,000,000 m  Runoff Coefficient, c = Q/P (E + T + I ) = 0.921568 m or 92.1568 cm 92.1568 cm = 130 cm - Q Q = 130 cm – 92.1568 cm = 37.8432 cm c = 37.8432 / 130 = 0.2911 96 Key Points 1. H y d r o l o g y i s t h e S t u d y o f w a t e r m ove m e n t , d i s t r i b u t i o n , and quality on Earth. 2. The Hydrologic Cycle includes key processes like p r e c i p i t a t i o n , e va p o ra t i o n , t ra n s p i ra t i o n , i n f i l t ra t i o n , a n d r u n o f f. 3. T h e h y d r o l o g i c c yc l e c a n b e a n a l y z e d w i t h i n a w a t e r s h e d t o u n d e r s t a n d w a t e r m ove m e n t , d i s t r i b u t i o n , a n d t ra n s f o r m a t i o n. 4. Wa t e r B a l a n c e r e q u i r e s u n d e r s t a n d i n g w a t e r i n p u t s ( p r e c i p i t a t i o n ) a n d o u t p u t s ( e va p o ra t i o n , r u n o f f, t ra n s p i ra t i o n ) t h a t d e t e r m i n e s c h a n g e s i n s t o ra g e. 5. U n d e r s t a n d i n g t h e s e c o n c e p t s i s c r u c i a l fo r e f fe c t i v e water resource management, flood control, and e nv i r o n m e n t a l p r o t e c t i o n. 97 Thank you 98