Chapter 2 hydrologic cycle


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Chapter 2 hydrologic cycle

  1. 1. Chapter 2 The Hydrologic Cycle and hydrologic processes Prof. Dr. Ali El-Naqa Hashemite University June 2013
  2. 2. INTRODUCTION  HYDROLOGY and HYDROGEOLOGY  Scope of Hydrogeology  Historical Developments in Hydrogeology  Hydrologic Cycle  groundwater component in hydrologic cycle,  Hydrologic Equation  HYDROLOGY and HYDROGEOLOGY  HYDROLOGY:  the study of water. Hydrology addresses the occurrence, distribution, movement, and chemistry of ALL waters of the earth.  HYDROGEOLOGY: includes the study of the interrelationship of geologic materials and processes with water,  origin  Movement  development and management
  3. 3. Hydrologic Cycle  Saline water in oceans accounts for 97.2% of total water on earth. Land areas hold 2.8% of which ice caps and glaciers hold 76.4% (2.14% of total water)  Groundwater to a depth 4000 m: 0.61%  Soil moisture .005%  Fresh-water lakes .009%  Rivers 0.0001%.  >98% of available fresh water is groundwater. Hydrologic CYCLE has no beginning and no end Water evaporates from surface of the ocean, land, plants.. Amount of evaporated water varies, greatest near the equator. Evaporated water is pure (salts are left behind).
  4. 4. When atmospheric conditions are suitable, water vapor condenses and forms droplets. These droplets may fall to the sea, or unto land (precipitation) or may evaporate while still aloft Precipitation falling on land surface enters into a number of different pathways of the hydrologic cycle: some temporarily stored on land surface as ice and snow or water puddles (depression storage) some will drain across land to a stream channel (overland flow). If surface soil is porous, some water will seep into the ground by a process called infiltration (ultimate source of recharge to groundwater).
  5. 5. Below land surface soil pores contain both air and water: region is called vadose zone or zone of aeration Water stored in vadose zone is called soil moisture Soil moisture is drawn into rootlets of growing plants Water is transpired from plants as vapor to the atmosphere Under certain conditions, water can flow laterally in the vadose zone (interflow) Water vapor in vadose zone can also migrate to land surface, then evaporates Excess soil moisture is pulled downward by gravity (gravity drainage) At some depth, pores of rock are saturated with water marking the top of the saturated zone.
  6. 6. Top of saturated zone is called the water table. Water stored in the saturated zone is known as ground water (groundwater) Groundwater moves through rock and soil layers until it discharges as springs, or seeps into ponds, lakes, stream, rivers, ocean Groundwater contribution to a stream is called baseflow Total flow in a stream is runoff Water stored on the surface of the earth in ponds, lakes, rivers is called surface water  Precipitation intercepted by plant leaves can evaporate to atmosphere
  7. 7. Groundwater component in the hydrologic cycle  Vadose zone = unsaturated zone  Phreatic zone = saturated zone  Intermediate zone separates phreatic zone from soil water  Water table marks bottom of capillary water and beginning of saturated zone
  8. 8. Distribution of Water in the Subsurface
  9. 9. Units are relative to annual P on land surface 100 = 119,000 km3/yr)
  10. 10. Hydrologic Equation  Hydrologic cycle is a network of inflows and outflows, expressed as  Input - Output = Change in Storage (1)  Eq. (1) is a conservation statement: ALL water is accounted for, i.e., we can neither gain nor lose water.  On a global scale  atmosphere gains moisture from oceans and land areas E  releases it back in the form of precipitation P.  P is disposed of by evaporation to the atmosphere E,  overland flow to the channel network of streams Qo,  Infiltration through the soil F.  Water in the soil is subject to transpiration T, outflow to the channel network Qo, and recharge to the groundwater RN.
  11. 11.  The groundwater reservoir may receive water Qi and release water Qo to the channel network of streams and atmosphere.  Streams receiving water from groundwater aquifers by base flow are termed effluent or gaining streams.  Streams losing water to groundwater are called influent or losing streams
  12. 12.  A basin scale hydrologic subsystem is connected to the global scale through P, Ro , equation (1) may be reformulated as P - E - T -Ro = S (2) S is the lumped change in all subsurface water. All terms have the unit of discharge, or volume per unit time.  Equation (2) may be expanded or abbreviated depending on what part of the cycle we are interested in. for example, for groundwater component, equation (2) may be written as RN + Qi - T -Qo = S (3)
  13. 13.  Over long periods of time, provided basin is in its natural state and no groundwater pumping taking place, RN and Qi are balanced by T and Qo, so change in storage is zero. This gives: RN + Qi = T + Q0 (4)  => groundwater is hydrologically in a steady state.  If pumping included, equation (4) becomes RN + Qi - T -Qo - Qp = S (5) Qp= added withdrawal.
  14. 14.  As pumping is a new output from the system,  water level will decline  Stream will be converted to a totally effluent,  transpiration will decline and approach zero.  Potential recharge (which was formerly rejected due to a wt at or near gl) will increase.  Therefore, at some time after pumping starts, equation (5) becomes: RN + Qi - Qo - Qp = S (6)
  15. 15.  A new steady state can be achieved if pumping does not exceed RN and Qi.  If pumping exceeds these values, water is continually removed from storage and wl will continue to fall over time. Here, the steady state has been replaced by a transient or unsteady state.  In addition to groundwater being depleted from storage, surface flow has been lost from the stream.
  16. 16. Example groundwater changes in response to pumping Inflows ft3/ s Outflows ft3/s 1. Precipitation 2475 2. E of P 1175 3. gw discharge to sea 725 4. Streamflow to sea 525 5. ET of gw 25 6. Spring flow 25
  17. 17. Example, contd.  Write an equation to describe water balance. SOLUTION: Water balance equation: Water input from precipitation – evapotranspiration of precipitation – evapotranspiration of groundwater – stream flow discharging to the sea – groundwater discharging to the sea – spring flow = change in storage P –ETp – ETgw –Qswo – Qgwo –Qso = ∆S
  18. 18. Example, contd Is the system in steady state? Substitute appropriate values in above equation: 2475 – 1175 -25 -525 -25 = ∆S
  19. 19. 1. Basic Hydrology Concept  Water is vital for all living organisms on Earth.  For centuries, people have been investigating where water comes from and where it goes, why some of it is salty and some is fresh, why sometimes there is not enough and sometimes too much. All questions and answers related to water have been grouped together into a discipline.  The name of the discipline is hydrology and is formed by two Greek words: "hydro" and "logos" meaning "water" and "science". 1.1. Introduction
  20. 20.  What is Hydrology?  It is a science of water.  It is the science that deals with the occurrence, circulation and distribution of water of the earth and earth’s atmosphere.  A good understanding of the hydrologic processes is important for the assessment of the water resources, their management and conservation on global and regional scales.
  21. 21. In general sense engineering hydrology deals with  Estimation of water resources  The study of processes such as precipitation, evapotranspiration, runoff and their interaction  The study of problems such as floods and droughts and strategies to combat them
  22. 22. 1.2 Hydrologic Cycle  Water exists on the earth in all its three states, viz. liquid, solid, gaseous and in various degrees of motion.
  23. 23. Hydrologic cycle….  Water, irrespective of different states, involves dynamic aspect in nature.  The dynamic nature of water, the existence of water in various state with different hydrological process result in a very important natural phenomenon called Hydrologic cycle.
  24. 24. Hydrologic cycle….  Evaporation of water from water bodies, such as oceans and lakes, formation and movement of clouds, rain and snowfall, stream flow and ground water movement are some examples of the dynamic aspects of water.
  25. 25.  Evaporation from water bodies  Water vapour moves upwards  Cloud formation  Condensation  Precipitate  Interception  Transpiration  Infiltration  Runoff–streamflow  Deep percolation  Ground water flow Hydrologic cycle….
  26. 26. Hydrologic cycle….  The hydrologic cycle has importance influence in a variety of fields agriculture, forestry, geography, economics, sociology, and political scene.  Engineering application of the knowledge are found in the design and operation of the projects dealing with water supply, hydropower, irrigation & drainage, flood control, navigation, coastal work, various hydraulic structure works, salinity control and recreational use of water.
  27. 27. 1.3 Water Budget Equation  The area of land draining in to a stream or a water course at a given location is called catchment area / drainage area / drainage basin / watershed.  A catchment area is separated from its neighbouring areas by a ridge called divide / watershed. Catchment area
  28. 28. 1.3 Water Budget Equation  A watershed is a geographical unit in which the hydrological cycle and its components can be analysed. The equation is applied in the form of water-balance equation to a geographical region, in order to establish the basic hydrologic characteristics of the region. Usually a watershed is defined as the area that appears, on the basis of topography, to contribute all the water that passes through a given cross section of a stream. Catchment area….
  29. 29. Watershed and watershed divide Watershed/ catchment Watershed/ catchment
  30. 30.  If a permeable soil covers an impermeable substrate, the topographical division of watershed will not always correspond to the line that is effectively delimiting the groundwater. Catchment area….
  31. 31. Watershed characteristics
  32. 32. Water Budget Equation  For a given catchment, in an interval of time ∆t, the continuity equation for water in its various phases can be given as: Mass inflow – Mass outflow = change in mass storage  If the density of the inflow, outflow and storage volumes are the same: Vi - Inflow volume in to the catchment, Vo - Outflow volume from the catchment and ∆S - change in the water volume i o V V S
  33. 33. Water Budget Equation…  Therefore, the water budget of a catchment for a time interval ∆t is written as: P – R – G – E – T = ∆S P = Precipitation, R = Surface runoff, G = net ground water flow out of the catchment, E = Evaporation, T = Transpiration, and ∆S = change in storage  The above equation is called the water budget equation for a catchment NOTE: All the terms in the equation have the dimension of volume and these terms can be expressed as depth over the catchment area.
  34. 34. Components of hydrologic cycle Precipitation Infiltration Evapo transpiration Inter flow Groundwater flow Base flow Stream flow (Runoff)
  35. 35. 1.3 World Water Budget Total quantity of water in the world is estimated as 1386 M km3  1337.5 M km3 of water is contained in oceans as saline water  The rest 48.5 M km3 is land water  13.8 M km3 is again saline  34.7 M km3 is fresh water  10.6 M km3 is both liquid and fresh  24.1 M km3 is a frozen ice and glaciers in the polar regions and mountain tops
  36. 36. Estimated World Water Quantitites 96% 1% 1% 2% Ocean-saline Land - saline Fresh - Liquid Fresh - Frozen
  37. 37. Global annual water balance SN Item Ocean Land 1 Area (km2) 361.3 148.8 2 Precipitation (km3/year) (mm/year) 458,000 1270 119,000 800 3 Evaporation (km3/year) (mm/year) 505,000 1400 72,000 484 4 Runoff to ocean Rivers (km3/year) Groundwater (km3/year) 44,700 2,200 Total Runoff (km3/year) (mm/year) 47,000 316
  38. 38. Water Balance of Continents Area (M km^2) 30.3 8.7 9.8 20.7 17.8 45 0 10 20 30 40 50 Africa Asia Australia Europe N.America S.America Precipitation (mm/yr) 686 736 734 670726 1648 0 500 1000 1500 2000 Africa Asia Australia Europe N.America S.America
  39. 39. Water Balance ……. Precipitation (mm/yr) 686 736 734 670726 1648 0 500 1000 1500 2000 Africa Asia Australia Europe N.America S.America Evaporation (mm/yr) 547 510 415 383 1065 433 0 200 400 600 800 1000 1200 Africa Asia Australia Europe N.America S.America Total Runoff (mm/yr) 139 226 319 287293 583 0 100 200 300 400 500 600 700 Africa Asia Australia Europe N.America S.America Drop of water ….. Matter…..
  40. 40. Water Balance of Oceans 107 12 75 167 780 240 1010 1210 1040 120 1380 1140 0 200 400 600 800 1000 1200 1400 1600 Atlantic Arctic Indian Pacific Area M km^2 Precp (mm/yr) Evap. (mm/yr) Water flow in Ocean 200 230 70 60 350 -300 130 -60 -400 -200 0 200 400 Atlantic Arctic Indian Pacific Continental Inflow (mm/yr) water exch. with ocean(mm/yr)
  41. 41. 1.4 Application in Engineering  Hydrology finds its greatest application in the design and operation of water resources engineering projects  The capacity of storage structures such as reservoir  The magnitude of flood flows to enable safe disposal of the excess flow  The minimum flow and quantity of flow available at various seasons  The interaction of the flood wave and hydraulic structures, such as levees, reservoirs, barrages and bridges
  42. 42. Chapter Headings  The hydrologic cycle  Precipitation  Runoff  Surface and groundwater storage  Evaporation  Condensation  Climate and weather  Climate  Monitoring climate change  Weather  Weather modification  Floods  Drought
  43. 43. Groundwater Storage Fetter, Applied Hydrology
  44. 44. Groundwater Storage  Groundwater recharge  Water added to groundwater usually through percolation down through the soil to the water table  Groundwater discharge  Water lost from groundwater usually through springs, streams, and rivers
  45. 45. Groundwater Storage Fetter, Applied Hydrology
  46. 46. Introduction  Precipitation is any form of solid or liquid water that falls from the atmosphere to the earth‟s surface. Rain, drizzle, hail and snow are examples of precipitation.  Evapotranspiration is the process which returns water to the atmosphere and thus completes the hydrologic cycle. Evapotranspiration consists of two parts, Evaporation and Transpiration.  Evaporation is the loss of water molecules from soil masses and water bodies. Transpiration is the loss of water from plants in the form of vapour.
  47. 47. Precipitation types  The can be categorized as.  Frontal precipitation  This is the precipitation that is caused by the expansion of air on  ascent along or near a frontal surface.  • Convective precipitation  Precipitation caused by the upward movement of air which is  warmer than its surroundings. This precipitation is generally  showery nature with rapid changes of intensities.  • Orographic precipitation  Precipitation caused by the air masses which strike the mountain  barriers and rise up, causing condensation and precipitation. The  greatest amount of precipitation will fall on the windward side of the  barrier and little amount of precipitation will fall on leave ward side.
  48. 48. Measurement of rainfall  One can measure the rain falling at a place by placing a measuring cylinder graduated in a length scale, commonly in mm. In this way, we are not measuring the volume of water that is stored in the cylinder, but the „depth‟ of rainfall.  The cylinder can be of any diameter, and we would expect the same „depth‟ even for large diameter cylinders provided the rain that is falling is uniformly distributed in space.  In practice, rain is mostly measured with the standard non- recording rain gauge the details of which are given in Bureau of Indian Standards code IS 4989: 2002. The rainfall variation at a point with time is measured with a recording rain-gauge, the details of which may be found in IS 8389: 2003. Modern technology has helped to develop Radars, which measures rainfall over an entire region
  49. 49. Variation of rainfall  Rainfall measurement is commonly used to estimate the amount of water falling over the land surface, part of which infiltrates into the soil and part of which flows down to a stream or river. For a scientific study of the hydrologic cycle, a correlation is sought, between the amount of water falling within a catchment, the portion of which that adds to the ground water and the part that appears as streamflow. Some of the water that has fallen would evaporate or be extracted from the ground by plants.
  50. 50. Variation of rainfall  In Figure 1, a catchment of a river is shown with four rain gauges, for which an assumed recorded value of rainfall depth have been shown in the table. It is on the basis of these discrete measurements of rainfall that an estimation of the average amount of rainfall that has probably fallen over a catchment has to be made. Three methods are commonly used, which are discussed in the following section.
  51. 51. Average rainfall depth  Average rainfall depth  The time of rainfall record can vary and may typically range from 1 minute to 1 day for non – recording gauges, Recording gauges, on the other hand, continuously record the rainfall and may do so from 1 day 1 week, depending on the make of instrument. For any time duration, the average depth of rainfall falling over a catchment can be found by the following three methods.  The Arithmetic Mean Method  The Thiessen Polygon Method  The Isohyetal Method  Arithmetic Mean Method  The simplest of all is the Arithmetic Mean Method, which taken an average of all the rainfall depths as shown in Figure 2.
  52. 52. Average rainfall depth  Average rainfall as the arithmetic mean of all the records of the four rain  gauges, as show in below:  The Theissen polygon method  This method, first proposed by Thiessen in 1911, considers the representative area for each rain gauge. These could also be thought of as the areas of influence of each rain gauge, as shown in Figure 3.
  53. 53. Average rainfall depth
  54. 54.  These areas are found out using a method consisting of the following three steps:  1. Joining the rain gauge station locations by straight lines to form  triangles  2. Bisecting the edges of the triangles to form the so-called “Thiessen polygons”  3. Calculate the area enclosed around each rain gauge station  bounded by the polygon edges (and the catchment boundary,  wherever appropriate) to find the area of influence corresponding to  the rain gauge.  For the given example, the “weighted” average rainfall over the catchment is determined as Average rainfall depth
  55. 55. Average rainfall depth  The Isohyetal method  This is considered as one of the most accurate methods, but it is dependent on the skill and experience of the analyst. The method requires the plotting of isohyets as shown in the figure and calculating the areas enclosed either between the isohyets or between an isohyet and the catchment boundary.  The areas may be measured with a planimeter if the catchment map is drawn to a scale.
  56. 56. Average rainfall depth
  57. 57. Average rainfall depth  For the problem shown in Figure 4, the following may be assumed to be the  areas enclosed between two consecutive isohyets and are calculated as  under:  Area I = 40 km2  Area II = 80 km2  Area III = 70 km2  Area IV = 50 km2  Total catchment area = 240 km2  The areas II and III fall between two isohyets each. Hence, these areas may  be thought of as corresponding to the following rainfall depths:  Area II : Corresponds to (10 + 15)/2 = 12.5 mm rainfall depth  Area III : Corresponds to (5 + 10)/2 = 7.5 mm rainfall depth  For Area I, we would expect rainfall to be more than 15mm but since there is  no record, a rainfall depth of 15mm is accepted. Similarly, for Area IV, a  rainfall depth of 5mm has to be taken. Hence, the average precipitation by the isohyetal method is calculated to be
  58. 58. Average rainfall depth  Please note the following terms used in this section:  Isohyets: Lines drawn on a map passing through places having equal amount of rainfall recorded during the same period at these places (these lines are drawn after giving consideration to the topography of the region).  Planimeter: This is a drafting instrument used to measure the area of a graphically represented planar region.
  59. 59. Class A evaporation pan
  60. 60. Evaporation  Evaporation – loss of liquid water from land and water surfaces as it is converted to a gas (water vapor)  Transpiration – liquid water moving from soil through a plant and evaporating from the leaves  Evapotranspiration (ET) – combination of evaporation and transpiration