EASC 325 Fundamentals of Hydrogeology and Hydrology Lecture 2021 PDF

EASC 325: FUNDAMENTALS OF HYDROGEOLOGY AND HYDROLOGY Earth, a watery planet! Anyone who has accepted, believed in and confessed Jesus as his/her Lord and personal Saviour is righteous! 1  Today, 12 th April....

EASC 325: FUNDAMENTALS OF HYDROGEOLOGY AND HYDROLOGY Earth, a watery planet! Anyone who has accepted, believed in and confessed Jesus as his/her Lord and personal Saviour is righteous! 1  Today, 12 th April. INTRODUCTION: –Distinguish Hydrology and Hydrogeology –Distribution of water on Earth –Components of the hydrologic cycle. –The hydrogeologic Equation  Objectives Distinguish hydrology from hydrogeology Learn about distribution of water on earth Define and describe the elements of hydrologic cycle. Discuss the methods and give examples of equipment used to measure the elements of the water cycle Write an equation that represents the hydrogeologic cycle  Hydrology vrs. Hydrogeology? Hydrology is the science of water occurrence, movement and transport on the Earth’s surface. Hydrogeology is that branch of geology that deals with the study of the occurrence, movement and quality of water beneath the Earth's surface (groundwater) Hydrogeology examines the relationships of geologic materials and flowing water underground Geohydrology deals with the hydraulics or the engineering aspects of groundwater flow.  Objectives Distinguish hydrology from hydrogeology Learn about distribution of water on earth Define and describe the elements of hydrologic cycle. Discuss the methods and give examples of equipment used to measure the elements of the water cycle Write an equation that represents the hydrogeologic cycle Earth – The Water Planet is unique. Water covers three fourths of Earth’s surface. Water is stored as ice in high mountains and in Earth’s polar regions. Water makes the clouds in Earth’s atmosphere.  Most of Earth’s water (about 97.2%) is in the oceans and salty. Distribution of water on the earth  Distribution of water on the earth contd Groundwater constitutes about 95% of the useable freshwater on Earth Lakes, swamps, rivers, streams account for about 3.5% soil moisture accounts for about 1.5% Global Distribution of Sources of water 10  Objectives Distinguish hydrology from hydrogeology Learn about distribution of water on earth Define and describe the elements of hydrologic cycle. Write an equation that represents the hydrogeologic cycle Discuss the methods and give examples of equipment used to measure the elements of the water cycle  THE HYDROLOGIC CYCLE It conceptually describes the continuous circulation of water on, over and in the Earth, and the processes by which the water moves between reservoirs. The reservoirs include Oceans (98% of all water) Ice caps (1.8%) Groundwater (0.6%) Lakes and rivers (0.01%) 5) Atmosphere (0.001%)  Processes in the hydrologic cycle Water is recycled between each of the reservoirs via the processes of: Evaporation, Sublimation and Transpiration (liquid to gas) Condensation (gas to liquid; e.g., clouds) Precipitation Infiltration (liquid water flows into the ground) Runoff ü Commonly known as stream or channel flow ü Has 3 components: overland flow, interflow and baseflow  Evaporation Oceans – large water reservoirs that supply almost all the evaporated water in the atmosphere  EVAPOTRANSPIRATION Total amount of water loss due to the combined effects of transpiration from plants and evaporation These two are inseparable under field conditions Phreatophytes are plants which have long tap root system extending into the water table to transpire huge quantities of subsurface water especially during their production season  EVAPOTRANSPIRATION Potential evapotranspiration is the amount of water that would have been lost to the atmosphere due evapotranspiration if water was available in excess throughout the year Actual evapotranspiration is the real amount of water that is lost to the atmosphere under prevailing conditions. Soil moisture depletion Soil moisture recharge  EVAPOTRANSPIRATION Potential evapotranspiration is almost always not satisfied especially in arid environments: actual evapotranspiration is lower than the potential evapotranspiration. Actual evapotranspiration is close to the potential evapotranspiration in areas where precipitation is high.  Condensation Water Vapour – primary form of water in the atmosphere (atmospheric moisture)  Precipitation Precipitation falls (as rainfall, sleet, hail, snow etc.) to the earth and distributed in four main ways.  Infiltration Water moves from the surface into the soil (subsurface) as soil moisture and percolates to recharge the groundwater. INFILTRATION AND RECHARGE Infiltration capacity of a soil drops as the capillary spaces in the soil get filled up with water: infiltration capacity reduces as soil get wet infiltration capacity of soils reduces exponentially as the moisture content increases Conditions that favor high infiltration rate: coarse soils, vegetated land, low soil moisture, porosity induced by insect and worm burrows in soils  Runoff Also known as total stream flow (runoff) is the sum of 3 components. So Runoff = overland flow + interflow + baseflow  Objectives Distinguish hydrology from hydrogeology Learn about distribution of water on earth Define and describe the elements of hydrologic cycle. Write an equation that represents the hydrogeologic cycle Describe the methods and give examples of equipment used to measure the elements of the water cycle  THE HYDROLOGIC EQUATION Inflow (Input)= outflow (output) +/- storage Or Inflow - Outflow = change in storage This is based on the law of conservation of mass, which assumes that all the water in the system is accounted for  THE HYDROLOGIC EQUATION Inflow = precipitation, surface inflow, loss from a body of surface water, subsurface inflow, overland flow, groundwater discharge into a body of surface water. Outflow = evaporation, transpiration, surface outflow, soil evaporation, loss of water through a stream bed, loss of water to a body of surface water, loss to vegetation, percolation to the water table, pumpage, removal to a water supply. Change in storage = increase or decrease in subsurface storage, increase or decrease in surface storage.  THE HYDROLOGIC EQUATION For the groundwater system, this equation becomes RN + Qi − ET − Q0 = ΔS Where RN = groundwater recharge from precipitation, Qi = groundwater inputs from streams or other surface water reservoirs, ET = evapotranspiration, Q0 = groundwater output into streams and ∆S = change in storage.  Objectives Distinguish hydrology from hydrogeology Learn about distribution of water on earth Define and describe the elements of hydrologic cycle. Write an equation that represents the hydrogeologic cycle Describe the methods and give examples of equipment used to measure the elements of the water cycle Quantifying (Measuring) Precipitation Measured in many ways; rain gauge, radar, satellites etc. Commonest is a rain gauge for measuring rainfall Different types of rain gauges: measure and record rainfall amounts automatically and others operate manually. Accuracy of the results depend on wind intensity and the positioning of the rain gauge Quantifying (Measuring) Precipitation In Ghana, rainfall is measured in mm/day or mm/month or mm/year Average rainfall in a basin within a specified time period is computed using various methods Quantifying (Measuring) Precipitation Usually the depth of precipitation within a basin is determined at several locations to provide the spatial distribution of rainfall at the end of an event, a day, week, month or year. At the end of the year, results are required from each measuring station in order to carry out informed analysis of the annual rainfall events Quantifying (Measuring) Precipitation Missing data for any station can always be obtained in several ways: If measurement for any rain/storm event is missing in station M, values from three surrounding stations (X, Y, Z) and the mean annual rainfall for all four stations can be used to calculate the missing data: 1 ⎡ NM NM NM ⎤ PM = ⎢ PX + PY + PZ ⎥ 3 ⎣ NX NY NZ ⎦ where P, and N are respectively depth of the precipitation event in question, and the mean annual precipitation at the four stations Quantifying (Measuring) Precipitation Isohyetal maps can also be used to estimate depths of precipitation at ungauged locations. Isohyetes are lines of equal depths precipitation. Y X 20 15 10 35 25 40 30  PRECIPITATION (CONT.) Y X 20 15 10 35 25 40 30  THE THIESSEN METHOD FOR ESTIMATING DEPTHS OF PRECIPITATION Thiessen polygons can be used to estimate depths of precipitation at ungauged locations or areas where values are missing. All the gauged stations are plotted on a map of the drainage basin. Polygons are produced by connecting close stations with lines. Thiessen method uses a weighting factor to estimate depths of precipitation at ungauged stations  THE THIESSEN METHOD The weighting factor is based on the area of the basin closes to the gauged station. Lines should normally be drawn between closest stations. Perpendicular lines are drawn at midpoints of the connecting lines Polygons are created by connecting these bisectors The area of each polygon is measured  THE THIESSEN METHOD A weighted average of each station’s precipitation is used to determine the effective uniform depth (EUD) of precipitation. The use of the Thiessen method in mountainous areas where rainfall amounts can vary vastly over short distances can lead to errors if the effects of the elevation is not taken into account.  EXAMPLES 2.2 1.8 1.6 1.4 2.5 1.7 2.1 1.2 1.5  EXAMPLES 2.2 1.5 2.8 2.2 1.8 1.1 1.6 1.4 2.5 1.7 2.7 2.1 1.2 1.5 1.7 1.8 1.3 1.6  EXAMPLES 2.2 1.5 2.8 2.2 1.8 1.1 1.6 1.4 2.5 1.7 2.7 2.1 1.2 1.5 1.7 1.8 1.3 1.6  EXAMPLES 2.2 1.5 2.8 2.2 1.8 1.1 1.6 1.4 2.5 1.7 2.7 2.1 1.2 1.5 1.7 1.8 1.3 1.6  EXAMPLES 2.2 1.5 2.8 2.2 1.8 1.1 1.6 1.4 2.5 1.7 2.7 2.1 1.2 1.5 1.7 1.8 1.3 1.6  Example Area of p Fraction of Precipitation (cm)(P) Pxf olygon(km2) total area (f) 1.2 15 0.163043 0.196 1.5 10 0.108696 0.163 1.4 8 0.086957 0.122 1.7 8 0.086957 0.148 1.8 17 0.184783 0.333 1.6 8 0.086957 0.139 2.1 10 0.108696 0.228 2.5 8 0.086957 0.217 2.2 8 0.086957 0.191 1.737 92 EUD  In summary, the EUD of a basin can be estimated using three methods: the arithmetic mean; the isohyetal method; the Thiessen method. The arithmetic mean = sum of precipitation depth at all gauged stations/ total number of stations  Quantifying Evaporation Various methods are employed Use evaporimeters Analytical methods Empirical equations  Reading Assignment Read Chapter 2 of Applied Hydrogeology by C. W. Fetter, 4th Edition, 2001, Prentice Hall