1. Introduction to Hydrogeology
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1-1 GEOL473 Hydrogeology 1. Introduction to Hydrogeology Why is water so important? - Most common liquid on the planet - Unique properties (best solvent) - Every living organisms needs water Classification of water - Meteoric (from atmosphere) - Juvenile = new to hydrogeologic cycle (from magmatic, cosmic, volcanic) - Rejuvenated (returned by compaction and metamorphism) - Connate (stagnant water trapped in interstices) Modern groundwater hydrology - Provided an elaboration on the relation between geology and occurrence of groundwater - Developed equations & solutions to describe groundwater flow - Groundwater modeling → Groundwater hydrology = Hydrogeology = Geohydrology ← Current studies: - Geothermal activity - Hydrogeochemistry and fluid‐rock interactions - Water supply and resource allocation/management - Protection and remediation of contaminated aquifers Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-2 GEOL473 Hydrogeology How much water do we need? One person: Flushing toilet: Personal use: Industrial, Municipal, Commercial: Salt water Energy and food production: Where is the water (%by volume)? - Salt water (oceans) % - Fresh water % o Ice caps/Glaciers % o Groundwater % Groundwater o Surface water % Surface water o Soil moisture % o Atmosphere % Soil moisture Atmosphere Ice caps/Glaciers The hydrologic cycle: Hydrologic cycle describes how water is recycled in the planet which is exchange between hydrologic compartments (atmosphere, hydrosphere, and lithosphere) Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-3 GEOL473 Hydrogeology Within any system, water budget must be balanced! In − Out = ± Storage In Rate (volume/time; L3/T) or amount (mass, M; or volume L3) of water entering the system Out Rate (volume/time; L3/T) or amount (mass, M; or volume L3) of water exiting the system ±Storage Change in amount of water within a system Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-4 GEOL473 Hydrogeology Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-5 GEOL473 Hydrogeology Water balance does not only apply to the entire earth, but also to the “drainage basin” Drainage basin = area surrounded by a topographic divide Groundwater basin = area surrounded by a water table divide Water table = a surface below which all pores/cracks/interstices are filled with water If topographic divide and water table divide do not correspond, there will be an inter‐basin flow! DRAINAGE BASIN WATER TABL E GROUNDWATER BASIN For balance to be meaning ful, all inputs, outputs and changes in storage must be identified. For practical problems, a rough check on the balance is essential to ensure that you have approached and evaluated the problem correctly. Components of the basin water budget (usually exclude atmospheric water, why!?!?) In − Out = ±∆S Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-6 GEOL473 Hydrogeology In P + SW inflow + GW inflow + Imported Out: ET + EV + SW outflow + GW outflow + Exported + Consumption ∆S: (±∆SW storage) + (±∆GW storage) ET Evapotranspiration EV Evaporation P Precipitation SW Surface water GW Groundwater Types of storage: 1. intercepted water 2. surface water 3. ice/snow 4. temporary depression storage 5. soil moisture 6. groundwater Units: The above quantities (In’s, Out’s and Storage) are usually measured in terms of volume per time (L3/T) Volume = Area × Height Must understand the accuracy of each term!!! 1) Precipitation – rain, snow, hail, etc. Measured from “rain gage” “P” has a unit of length (L) or length per time (L/T) 2) Stream flow – main source of surface water inflow/outflow Measured using “stream gage” Records from meteorological department Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-7 GEOL473 Hydrogeology 3) Evaporation – loss of water from surface water Measured using evaporation pan Records from meteorological department 4) Evapotranspiration – loss of water from plants and soil directly to atmosphere Measured using lysimeters Calculated using formulae 5) Change in storage of SW (rivers, lakes, ponds, etc.) Measured by the change in water level in surface water bodies 6) GW inflow & outflow Calculated from Darcy’s law Using groundwater models 7) GW change in storage Calculated from water level, porosity, and aquifer behavior Before discussing how to calculate each term of the water budget equation, we need to know how to calculate volume of water from different unit of measurements. Unit conversion The following tables (appendices 7,8,9, and 10) show how to convert length (L), area (L2), volume (L3) and time from one unit to the others. Example I‐1 Convert 1.0 acre‐foot to cubic kilometer. Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology GEOL473 Hydrogeology 1-8 Example I‐2 Convert a flow rate of 1.0 inch3/min to m3/day. Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-9 GEOL473 Hydrogeology Precipitation Use open container to make measurement (rain gage) Locations of the gage is very critical We need an “average” precipitation over the entire basin which must be computed from data measured at discrete points What is a good precipitation data? o Long enough record o Proper distribution of gages over the basin For water budget calculation, use average depth (“effective uniform depth” or EUD) of precipitation over the basin. If precipitation data at any station is missing, use the following formula to calculate the missing station. Px = Ax ⎛ P1 P2 PN ⎞ ⎜ + +L+ ⎟ N − 1 ⎝ A1 A2 AN ⎠ 144424443 N −1 Stations Px precipitation at the “unknown” or missing station Ax average annual precipitation at the unknown station Pi precipitation at the i th station Ai annual precipitation at the i th station N total number of stations Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-10 GEOL473 Hydrogeology Example I‐3 In the following table, calculate precipitation at year 7 of station “B”. STATIONS YEAR A B C D 1 11 22 14 11 2 10 21 13 13 3 12 23 8 3 4 6 23 11 4 5 4 20 12 5 6 5 21 17 6 7 7 PX 10 8 AVERAGE 7.857143 21.66667 12.14286 7.142857 Px = 21.667 ⎡ 7 10 8 ⎤ + + = 20.471 ⎢ 4 − 1 ⎣ 7.857 12.143 7.143 ⎥⎦ Once you are confident in the data, now “EUD” can be computed. There are three common methods to calculate “EUD” 1. Arithmetic mean - For spatially uniform rain gage network (or for simple and fast approximation) P= P1 + P2 + L + PN ∑ Pj = N N 2. Theissen polygons - Connect each station j with straight line to nearest neighbors - Draw perpendicular bisectors to each line - Extend bisectors, three bisectors at a point - Yields polygon for each station with area Ai - Calculate percentage of area: Atot = ∑ Aj , then calculate the average precipitation using the following formula Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-11 GEOL473 Hydrogeology P= A1P1 + A2P2 + L + AN PN ∑ Aj Pj = A1 + A2 + L + AN ∑ Aj 3. Isohyetal method - Draw contour lines of equal precipitation (isohyets) - Measure area between isohyets - Average precipitation is calculated using the same equation shown above (Theissen method) Example I‐4 Calculate effective uniform depth (EUD) of precipitation of the following measurements using all three methods described above (measurements are made in centimeters). (1) Arithmetic average EUD = 1.03 + 0.65 + 1.46 + 1.75 + 4.81 + 3.45 + 5.76 = 2.70 7 (2) Theissen polygons Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-12 GEOL473 Hydrogeology (3) Isohyetal method Evaporation and Evapotranspiratoin Evaporation = loss of water from reservoirs, rivers, lakes directly to atmosphere Transpiration = loss of water directly to atmosphere by plants Evapotranspiration = total water loss from vegetated lands, and soil moisture evaporation Units of these values are either length (L) or length per time (L/T) which is similar to precipitation To quantify EV, we need information on atmospheric conditions (such as, wind speed, ambient temperature, dew‐point temperature, and solar radiation) Absolute humidity = grams of water per cubic meter of air Saturation humidity = maximum moisture content at a given temperature Relative humidity = absolute humidity ÷ saturation humidity at a given temperature Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-13 GEOL473 Hydrogeology Evaporation stops if relative humidity = 100% Condensation occurs at relative humidity = 100% Dew point = temperature where condensation occurs Measurement of evaporation Use evaporation pan (usually, “class A” pan) o Shallow pan o Monitor volume o class A pan coefficient < 1.0 Example I‐5 During the month of April, if the depth of water in the class A pan decreases by 1.0 inch, what is the actual value of EV? EV = (1.0 inch)×(0.77) = 0.77 inch use nomograph to find EV o Developed from U.S. National Weather Service o Need the following information to estimate EV ¾ Mean temperature ¾ Mean dew‐point temperature ¾ Solar radiation ¾ Wind movement Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-14 GEOL473 Hydrogeology Example I‐6 Find the average evaporation rate with given the following data Mean temperature 76 °F Solar radiation 500 Ly/dy Mean dew point temp. 50 °F Wind speed 200 mile/day From nomograph, EV ≈ 0.25 inch/day Evaportranspiration (ET) PET (potential ET) = amount of water that plants can transpire and air can carry away (if there is no water deficiency) o PET depends on temperature, wind speed, solar radiation, vegetation types, dew point, soil texture and permeability. Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-15 GEOL473 Hydrogeology AET (actual ET) = usually less than PET because soil doesn’t always have all the water required to achieve PET precip. Varies over the year precip. is relatively constant How to measure ET? ET is measured directly using lysimeters but this rechnique has numerous problems. From lysimeter experiment, ET can be calculated using the following formula: ET = Si + P + I − S f − D ET ET for the period Si initial volume of moisture Sf final volume of moisture P precipitation in to lysimeter I irrigation water added to lysimeter D excess moisture drained from soil In the present day, ET is usually obtained from calibration of a groundwater flow model Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology GEOL473 Hydrogeology 1-16 What happens during precipitation? Canopy interception Interflow Infiltration Percolation Overland flow Direct precipitation Runoff Groundwater flow (or baseflow) Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology GEOL473 Hydrogeology 1-17 Relationship between precipitation and infiltration No depression storage or overland flow Filling of depression storage after time t1 Depression storage = static body of water stored/trapped in soils Water held in depression storage will infiltrate fp Infiltration capacity (L/T) fc Steady‐state infiltration capacity (L/T) f0 Initial infiltration capacity (L/T) Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-18 GEOL473 Hydrogeology Interflow – develop where a highly permeable but thin layer of weathered rock overlies bedrock Baseflow – groundwater discharges to streams/rivers **amount of baseflow is proportional to the hydraulic gradient toward streams** Low gradient (small baseflow) High gradient (large baseflow) Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-19 GEOL473 Hydrogeology Surface water runoff – can be estimated from stream hydrographs ¾ Hydrograph shows the discharge (Q) of river at any point as a function of time Q Stream hydrograph t OUTLET ¾ Stream hydrograph is used to determine the amount of groundwater discharge or baseflow to streams Good approximation D = A0.2 D = number of days beween peak and end of overland flow A = drainage basin area (mi2) Baseflow recession begins Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-20 GEOL473 Hydrogeology Typical storm hydrograph 3 ¾ discharge, Q (L /T) = volume of water flowing pass a point over a period of time ¾ total stream flow can be separated into several compartments (baseflow, precipitation, interflow, and overland flow) QTOT = QOF + QBF + QDP + QIF QTOT QOF QBF QDP QIF Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-21 GEOL473 Hydrogeology How to estimate baseflow from hydrograph? Baseflow recession = duration that groundwater recharges streams/rivers (or GW flows into rivers) BASEFLOW ≠ 0 BASEFLOW = 0 Baseflow recession is a function of o topography o drainage pattern o soil types o geology baseflow = difference in stream flow rates at two points along stream (if there are no other contributions to stream) Usually, a hydrograph with no overland flow effect will exhibit an “exponential decay” Q = Q0 eat Q Discharge at time “t” Q0 Discharge at start of recession a Recession constant for basin t Time since recession starts Dr. Schradh Saenton Problems!!! → What are “a” and “Q0”? Q0 are flowrate at time t = D (where D = A0.2) t in equation Q = Q0e‐at starts at t = D a obtained from regression (curve fitting) Topic 1: Introduction to Hydrogeology 1-22 GEOL473 Hydrogeology Example I‐7 The following table shows data (Q vs. t, hydrograph) of a hypothetical drainage basin with an area of 16,807 mi2, determine the amount of baseflow (groundwater discharge) starting from day 0 to day 10 of baseflow recession duration. ◊ Q (m3/day) 2 2.1 2.07 2.15 2.8 3 4 5 7.3 7.58 7.64 7.4 6.7 6.1 5 4.5 Time (day) 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Q (m3/day) 4 3.3 3.1 2.9 2.87 2.8 2.76 2.63 2.6 2.58 2.43 2.31 2.28 2.23 2.21 2.2 HYDROGRAPH 10 8 Q (m3/day) Time (day) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Baseflow recession starts here (day 17) 6 4 2 0 0 5 10 15 20 25 30 35 Time (day) Step #1: calculate starting point of baseflow recession D = A0.2 = (16807)0.2 ≈ 7 days Since the peak flow occurs at time = 10 days and D = 7, the starting point of baseflow recession therefore occurs at day 17. ◊ In this case, the value of Q0 = 3.3 m3/day. Step #2: linearize baseflow equation; we will have straight line equation Q = Q0 e − at Q ln 0 = at Q 3 Time (day) Q (m /day) 0 3.3 3.1 1 2 2.9 3 2.87 4 2.8 5 2.76 2.63 6 7 2.6 8 2.58 9 2.43 10 2.31 11 2.28 12 2.23 13 2.21 14 2.2 Dr. Schradh Saenton ln (Q0/Q) 0 0.06252 0.129212 0.13961 0.164303 0.178692 0.226939 0.238411 0.246133 0.306031 0.356675 0.369747 0.391921 0.40093 0.405465 plot ln(Q0/Q) vs. t, then, find a slope of the best‐ fit line to obtain “a” BASEFLOW RECESSION 0.6 y = 0.0328x 0.4 ln(Q0/Q) 0.2 0 0 5 10 15 Tim e (day) [DAY 0 FOR BASEFLOW IS DAY 17 FROM ORIGINAL DATA] Topic 1: Introduction to Hydrogeology 1-23 GEOL473 Hydrogeology We now have completed equation for baseflow, which is a = 0.0328 day‐1 Q(t ) = 3.3e −0.0328t m3/day ◊ Step #3: determine the amount of baseflow from day 0 to day 10 10 V = ∫ Q(t )dt = ∫ ⎡⎣3.3e −0.0328t ⎤⎦dt = 28.1 m3 0 Gaining stream Losing stream Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-24 GEOL473 Hydrogeology Exercise 1. A farmer has a reservoir with vertical sides and a surface area of 2.5 acres. Following the rainy season, the reservoir is filled to a depth of 3.0 m. During the dry season, the reservoir loses 2.5 inches of water per week to evaporation. If the average irrigation demand during the dry season is 0.23 acre‐foot per day, how many weeks can the farmer irrigate from the reservoir? [Ans: 12 weeks] 2. How long must a pump with a capacity of 12 gallon per minute pump to fill a tank with a capacity of 37.0 m3. [Ans: 14 hours] 3. A small urban watershed has an area of 16.34 mi2. A summer storm drops an average of 1.50 inch of rain over the entire watershed. If 50% of the rainfall runs off the watershed into surface water bodies, what is the volume of runoff in inch3, ft3, and m3? [Ans: 4.92×1010 in3, 2.85×107 ft3,8.06×105 m3 ] r s tain un Mo ve Ri 4. Given the hypothetical basin in the diagram below, list the sources of water flowing into and out of the basin and calculate a water balance for the basin. Use the following information for your work. Mo unt ain Basin area 100 mi2 Reservoir s 2 Vegetated area 90 mi Reservoir area 10 mi2 Change in reservoir level ‐2 ft Evapotranspiration 12 in/yr Precipitation 36 in/yr Streams Stream flow from mountains 20,000 ac‐ft/yr Canal flow 40,000 ac‐ft/yr Ca na l EV from reservoir 48 in/yr River flow 50,000 ac‐ft/yr OUTLET If “IN” and “OUT” in your calculation does not match, indicate what is missing in your calculation. 5. A swimming pool has a length of 50 m and a width of 25 m. During July, the Class A land pan evaporation is 17.0 cm. If the pan coefficient is 0.80, what is the monthly water loss from the pool due to evaporation? [Ans: 170 m3] 6. A reservoir has a surface area of 690 acres. The following table shows the monthly inflow of surface water, outflow as releases from the reservoir via the spillway, direct precipitation into the reservoir, and evaporation from the reservoir. The reservoir elevation was 701 ft on January 1st. Compute the reservoir elevation at the end of each month. Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-25 GEOL473 Hydrogeology MONTH DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC INFLOW (ac-ft) OUTFLOW (ac-ft) 1732 1755 872 955 708 312 102 37 175 575 1250 1875 175 190 232 375 525 955 1720 2250 1575 550 175 125 Precipitation (inches) (ac-ft) 2.75 3.05 3.76 4.11 2.70 1.05 0.75 1.25 1.55 3.79 4.53 5.01 158.0 Actual Evaporation (inches) (ac-ft) 1.05 1.55 2.05 2.80 3.75 4.25 5.15 5.76 4.92 3.02 1.75 0.60 60.0 Net Change (ac-ft) (ft) +1655 +2.4 Elevation (ft) 701.0 703.4 7. A pond has a surface area of 22 acres. If the mean daily air temperature is 70 °F, the mean daily dew point temperature is 55 °F, the solar radiation is 450 langleys per day, and the daily wind movement is 70 mi/day, what is daily lake evaporation? [Ans: 0.13 in/day] 8. A reservoir wit ha surface area of 43 mi2 lies in a desert valley with a mean daily air temperature of 80 °F, mean daily dew point temperature of 45 °F, solar radiation of 650 ly/day, and a mean wind movement of 180 mi/day. 8.1. Compute mean daily lake evaporation in inches. 8.2. What is the volume of water in ft3 lost per day from lake due to evaporation? [Ans: 0.33 in/day, 3.4×107 ft3/d] 9. Figure below is a map of drainage basin and the rainfall amounts during a storm at a number of precipitation stations both within and outside the drainage basin. The exact location of the station on the map is the “decimal point” of the displayed number. Estimate the effective uniform depth (EUD) of precipitation over the drainage basin using all three methods (Arithmetic, Theissen, Isohyetal). One box has an area of 1 km2. [Ans: Arithmetic=1.30,Theissen=1.34, Isohyet=1.39] Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology 1-26 GEOL473 Hydrogeology Dr. Schradh Saenton Topic 1: Introduction to Hydrogeology
