Watershed Hydrology and Water Resources Science Teacher Education Program (STEP) Presented by Amy Tidwell Water and Environmental Research Center/ Institute of Northern Engineering University of Alaska Fairbanks July 2007 STEP July, 2007: Hydrology Page No. 1
Watershed Hydrology and Water Resources Outline Watershed Hydrology Watershed Delineation (Exercise) Water Budget Precipitation Evapotranspiration Infiltration Runoff Groundwater (Demonstration/Activity) Wetlands (Hand out) Climate Change Considerations Water Resources Water Resources Planning and Management (Hand out) Water Supply Water as a Hazard Water Management: Health, Safety, and the Environment Climate Change and Water Resources Additional Resources Online Activities (time permitting) STEP July, 2007: Hydrology Page No. 2
Watershed Hydrology STEP July, 2007: Hydrology Page No. 3
Watershed Hydrology Watersheds We will apply some of what you ve learned about the global hydrologic cycle to watersheds. As you will see, a watershed is a logical accounting unit in hydrology and water resources. What is a watershed? Where is a watershed? And how large is a watershed? Delineating watersheds Topographic maps, contour lines and slopes where does the water flow? Begin with a point of interest, usually along a stream. Trace the outline of the watershed, beginning at one side of the stream, by following the steepest slope (gradient). Recall that the steepest gradient occurs at a right angle to contour lines. Then begin tracing the outline from the other side of the stream until your second trace meets up with the first. Check your work: Consider a rain drop falling over your delineated watershed. Pick several points around and even outside of your watershed and trace the downhill flow path of the rain drop. Is it consistent with your drawn watershed boundary? STEP July, 2007: Hydrology Page No. 4
STEP July, 2007: Hydrology Page No. 5
USGS, 2001 STEP July, 2007: Hydrology Page No. 6
USGS, 2001 STEP July, 2007: Hydrology Page No. 7
Watershed Hydrology Hydrologic Cycle and the Water Budget P ET I Q G Where, P = precipitation ET = evapotranspiration I = Infiltration G=Groundwater Q= Runoff ΔWatershed Storage = P ET G R The Hydrologic Cycle and its Role in Arctic and Global Environmental Change, 2001. STEP July, 2007: Hydrology Page No. 8
Precipitation STEP July, 2007: Hydrology Page No. 9
Precipitation Introduction Precipitation is the primary driver for the land phase of the hydrologic cycle Total precipitation over land gets partitioned into different components: some soaks into the ground (infiltration), some evaporates from the surface of leaves and soil, some is taken up into plant roots and released back into the atmosphere (transpiration), some is stored at the surface as snow/ice. For a given watershed, how precipitation is partitioned depends on a number of environmental factors: Temperature Soil moisture Intensity of rainfall Vegetation (seasonal effects) Furthermore, the state of precipitation (liquid/solid) is very important for seasonal (and sometimes interannual) partitioning. STEP July, 2007: Hydrology Page No. 10
C USGS, 2001 STEP July, 2007: Hydrology Page No. 11
Precipitation Mean Areal Precipitation Precipitation measurements are point data; however, models often require the amount of rainfall over the watershed or area of interest. As a result, several methods have been developed to determine the average rainfall over the watershed - called the mean areal precipitation (MAP). MAP = Total Precipitation Volume Watershed Area G3 G4 Examples of methods include: Arithmetic Average (simple average of all stations) Thiessen Polygon (weighted average based on area of influence) Hypsometric (weighted average based on basin topography and location of stations) G1 G2 G6 G7 G5 STEP July, 2007: Hydrology Page No. 12
Precipitation Where to Obtain Precipitation Data (and Other Surface Observations) In the United States the National Weather Service (www.nws.noaa.gov) has a network of precipitation gages 278 primary stations - staffed full time by paid technicians (~20 AK) 8,000 cooperative stations - mostly volunteer stations (~70 AK) Historical data for these stations may be downloaded at the National Climate Data Center website (www.ncdc.noaa.gov) STEP July, 2007: Hydrology Page No. 13
Precipitation Precipitation gage networks Primary Stations Dingman, 2002 STEP July, 2007: Hydrology Page No. 14
Precipitation and Rainfall Climatology Precipitation gage networks Cooperative Stations Dingman, 2002 STEP July, 2007: Hydrology Page No. 15
Evapotranspiration STEP July, 2007: Hydrology Page No. 16
Evapotranspiration Overview Evaporation + Transpiration = Evapotranspiration Evaporation (E) occurs when water is converted into water vapor. This may occur from an open water surface or through exfiltration of soil moisture. Methods for estimating evaporation include: Water budget Energy budget Mass transfer techniques Pan evaporation measurements Transpiration occurs when water vapor is lost to the atmosphere through small openings in the leaves of plants. Potential Evapotranspiration (PET) is a combined estimate of the maximum potential evaporation + transpiration over an area. When there is limited water (open surface or soil moisture) actual rates of evapotranspiration (ET) are less than the potential rate. Transpiration Translocation soil surface Absorption STEP July, 2007: Hydrology Page No. 17
Evapotranspiration Estimating Evaporation: Water Balance Method Mass Balance for Water body: ΔV = P + SW in + GW in E SW out GW out Solve for E E = P + SW in + GW in SW out GW out ΔV Where, E = Evaporation P = Precipitation SW in = Surface Water Inflow SW out = Surface Water Outflow GW in = Groundwater Inflow GW out = Groundwater Outflow ΔV = Change in storage SW out P GW out ΔV E GW in SW in The water balance method is computationally simple. However, gathering the data for implementation of this method may be difficult. Each of the quantities in the equation above are measured or estimated, which results in uncertainty. Thus the calculation of evaporation includes the sum of the errors related to each component. STEP July, 2007: Hydrology Page No. 18
Evapotranspiration Estimating Evaporation: Penman Method Δ E = ( K + L) + γ K ρ λ * E w v va ea (1 ra ρw λv ( Δ + γ ) Where, Δ = 2508.3 17.3 T exp a with T a in ºC ( T + 237.3) 2 T + 237.3 a a γ = psychrometric constant = ( c P) (0.622 a λ v ) c a =heat capacity of air=1.0x10-3 [MJ/kgK] P = pressure [kpa] λ v = latent heat of vaporization [MJ/Kg] = 2.50 2.36x10-3 T s, T in ºC K = net short-wave radiation input = I o (0.803-0.34c-0.458c 2 ) (1- a) I o = solar insolation at the top of the atmosphere [MJ/m 2 day] a = albedo c = Cloud cover K E = coefficient reflecting the efficiency of vertical transport of water vapor by turbulent eddies of the wind = 1.69 10-5 A -0.05 L A L = water surface area [km 2] ρ w = density of water = 1000 [kg/m 3] ) Required input data 1. A L used in K E 2. P or Altitude 3. T s used in λ v and L 4. T a used in Δ, L and e * a 5. e a 6. v a 7. c used in K 8. I o used in K 9. a used in K L = net long wave radiation input 4 = εw εat σ ( Ta + 273.4) εw σ ( T s + 273.2) T a = temperature of atmosphere, in ºC T s = temperature of surface, in º C σ=stefan-boltzmann const = 4.90x10-9 [MJ/m 2 dayk 4 ] ε w = effective emissivity of water = 0.97 ε at = effective emissivity of atmosphere 1/ 7 ea 2 = 1.72 (1 0.22 C ) T 273.2 + a + v a = wind speed [km/day] r a = relative humidity = e a /e* a e* a = saturation vapor pressure at the air temperature = 17.3 T a 0.611 exp [kpa] T a + 237. 3 STEP July, 2007: Hydrology Page No. 19 4
Infiltration STEP July, 2007: Hydrology Page No. 20
Infiltration Soil Properties The properties of a homogeneous soil matrix include: Porosity, φ = Water content, θ = Volume Air & Water V = VolumeAir, Water& Minerals V Field capacity, θ fc = water content at which further drainage due to gravity is negligible Permanent wilting point, θ pwp = water content at which plants are unable to extract additional water void VolumeWater V = Volume Air, Water & Minerals V tot w t ot Sand grains Clay particles Dingman, 2002 If a soil is saturated and then allowed to drain, its water content will decrease indefinitely in a quasi-exponential manner, with the drainage rate negligible within a few days to a week Dingman, 2002 STEP July, 2007: Hydrology Page No. 21
Infiltration Hydrologic Horizons Ground-water zone: Saturated, positive pressure; in absence of ground-water flow pressure is hydrostatic p( z) = γ w ( z1 z2) ; z1> z2, where p is the pressure, z is the height above the datum, and γ w is the specific weight of water Tension-saturated zone (capillary fringe): Saturated zone above the water table due to capillary rise through the pore spaces; pressure is zero at the top of the water table and negative in the capillary fringe Intermediate zone: Water enters as percolation from above and leaves by gravity drainage Root zone: Layer from which plant roots can extract water, bounded by the surface above and an indefinite and irregular lower bound; water enters by infiltration and leaves via transpiration and gravity drainage Dingman, 2002 STEP July, 2007: Hydrology Page No. 22
Infiltration The Infiltration Process Infiltration is the process by which water arriving at the soil surface enters the soil column. The maximum rate that a soil can accept water is called the infiltration capacity, f(t) *. At a given point the infiltration rate, f(t), changes systematically with time and is influenced by: The rate at which water arrives from above, w(t), or the depth of ponding on the surface, H(t) The hydraulic conductivity of the soil, K * h Antecedent soil moisture Three general conditions during infiltration may be distinguished No ponding: In this case the infiltration rate equals the water-input rate and is less than or equal to the infiltrability H ( t) = 0, f ( t) = w( t) f * ( t) Saturation from above: Ponding is present because the water-input rate exceeds the infiltrability in which case the infiltration rate equals the infiltrability H ( t) > 0, f ( t) = f * ( t) w( t) Saturation from below: Ponding is present because the water table has risen to or above the surface in which case the infiltration rate is zero H ( t) 0, f ( t) = 0 STEP July, 2007: Hydrology Page No. 23
Runoff STEP July, 2007: Hydrology Page No. 24
Runoff Basic Aspects of Stream Response Definitions Watershed response to an input event is characterized by stream discharge at a single point that defines the outlet of the watershed Hyetograph A graph of water input vs. time can be constructed from spatially averaged precipitation measurements and is called a hyetograph Rain (depth/time) A graph of stream discharge vs. time is a streamflow hydrograph Hydrograph A storm hydrograph is the time trace made by an observer at a fixed point of a flood wave moving downstream Discharge (volume/time) Dingman, 2002 STEP July, 2007: Hydrology Page No. 25
Runoff Basic Aspects of Stream Response Streamflow Streamflow is a spatially and temporally integrated response determined by Spatially and temporally varying input rates (precipitation, snow melt, glacial melt) Time required for each drop of water to travel from where it strikes the watershed surface to the stream network (determined by length, slope, vegetative cover, soils, and geology of hillslopes) Time required for water to travel from its entrance into the channel to the point of measurement Flow may enter the stream at the surface, from overland flow and channel precipitation, and as subsurface flow, from groundwater and interflow Flow in the stream takes the form of a flood wave that moves downstream through the stream network Dingman, 2002 STEP July, 2007: Hydrology Page No. 26
Runoff Response Hydrographs Effective Rainfall Only a fraction of water input to the watershed actually appears in the response hydrograph, with the remainder leaving the watershed as Evapotranspiration Streamflow that is realized too long after the input event to be associated with that event (baseflow) Groundwater outflow (other than baseflow) Depending on the type of model, it is often necessary to estimate the effective rainfall from the hyetograph of water input There are several approaches used for this estimation as shown here a) Losses equal to a constant fraction of water input for each time period b) Losses equal a constant rate throughout event c) Losses given by an initial abstraction followed by a constant rate d) Losses given by an approximation to an infiltration-type curve Dingman, 2002 STEP July, 2007: Hydrology Page No. 27
Runoff Response Hydrographs Hydrograph Separation Event flow is streamflow resulting from the effective rainfall Hydrograph separation divides the hydrograph into a portion attributed to event flow and a portion attributed to baseflow Gaging station measurements of streamflow cannot distinguish event flow from flow originating from a previous event Therefore, graphical hydrograph separation is often used as a convenient delineation in order to analyze and model event responses and the factors influencing them Graphical separation does not actually identify flow from different sources After Linsley and Franzini, 1979 STEP July, 2007: Hydrology Page No. 28
Flow statistics of three rivers near the headwaters of the Yukon River. USGS, 2001 STEP July, 2007: Hydrology Page No. 29
Runoff Where to Obtain Streamflow Data (and Groundwater Data) US Geological Survey http://water.usgs.gov/ Select Alaska Select Real Time Data Table Select station Select data product (Daily data is usually best) STEP July, 2007: Hydrology Page No. 30
Runoff Simple Runoff Model Rational Method Regional equations suitable for assessing the impact of developing on peak discharge are not generally available for small watersheds One widely used method, intended for use on small watersheds, is the Rational Method, which relates the peak discharge of an area, q p (ft 3 /s), to Drainage area, A (acres), Rainfall intensity, i (in/hr), Runoff coefficient, C q = CiA Rainfall intensity is obtained from an intensity-duration-frequency (IDF) curve using a specified return period Primary use of Rational Method: design problems for small urban areas (small drainage areas, short times of concentration) STEP July, 2007: Hydrology Page No. 31
Runoff Sophisticated Model: Sacramento Soil Moisture Accounting Model The model states consist of the contents of various conceptual reservoirs identified in the upper and lower soil zones Water fills and spills over in a cascade of reservoirs based on parameters that represent average soil characteristics in each reservoir This movement of water between compartments is governed by the precipitation rate, the capacities of each reservoir, evapotranspiration, and the rates at which water can transfer between compartments (infiltration, interflow, or percolation) While an infinite number of layers could be established, the goal of parameterization is to use no more than necessary to effectively describe the physical system STEP July, 2007: Hydrology Page No. 32
Runoff Sophisticated Runoff Model STEP July, 2007: Hydrology Page No. 33
Groundwater STEP July, 2007: Hydrology Page No. 34
Groundwater Snow Infiltration Basic Groundwater Characteristics Pumped well The groundwater portion of the hydrologic cycle is rather complex Water enters at the surface (infiltration), Redistributes under forces of gravity, energy gradients, capillary rise, and evapotranspiration Water percolates to lower water reservoirs (aquifers) Groundwater flows under the influence of energy gradients Groundwater may flow into or receive recharge from surface water bodies Pumping of groundwater alters the region around the well by drawing down either the water table or the piezometric surface (cone of depression) Unsaturated zone Percolation Influent stream (seepage from stream) Water table Cone of depression Zone of saturation Unconfined aquifer Piezometric surface Spring Lake Ground water flow Effluent stream (seepage into stream) Perched water table Perched aquifer Spring Water table Marsh Confining layer Ocean Confined (artesian) aquifer Bedrock Artesian well Saltwater intrusion Reproduced from McCuen, 1998 STEP July, 2007: Hydrology Page No. 35
Groundwater Basic Groundwater Characteristics Definitions Water that enters the soil is considered soil moisture while in the unsaturated zone and is called groundwater once in the saturated zone. Within the saturated zone water occupies all pore space and is under hydrostatic pressure Aquifer- groundwater-bearing formations sufficiently permeable to transmit and yield usable quantities of water Unconfined Aquifer- permeable underground formation having a surface at atmospheric pressure Confined Aquifer- confined (or artesian) aquifers form between layers of very low permeability material If the layers are essentially impermeable they are called aquicludes If the layers are permeable to transmit water vertically to or from the confined aquifer, but not permeable enough for lateral transport, they are called aquitards Bouwer, 1978 STEP July, 2007: Hydrology Page No. 36
Wetlands STEP July, 2007: Hydrology Page No. 37
Handout: What are wetlands and why are they important? STEP July, 2007: Hydrology USGS, 2001 Page No. 38
Watershed Hydrology Climate Change Considerations 6000 5000 30-Year Monthly Mean Flow Base Max Base Min Base Mean Future Max Future Min Future Mean 4000 Ways that climate change might affect hydrology: (class suggestions- recall the components of the water budget) Flow (mcm) 3000 2000 1000 Atbara Example from the Nile basin 0 1 2 3 4 5 6 7 8 9 10 11 12 Month How are Alaska and the Arctic different from lower latitudes? 18000 16000 14000 30-Year Monthly Mean Flow Base Max Base Min Base Mean Future Max Future Min Future Mean 12000 Evapotranspiration- temperature, soil moisture, vegetation Flow (mcm) 10000 8000 6000 Blue Nile 4000 Glacial fed streams Continuous permafrost regions Discontinuous permafrost Groundwater Storm frequency/intensity Flow (mcm) 2000 0 3000 2500 2000 1500 1000 500 0 1 2 3 4 5 6 7 8 9 10 11 12 Month 30-Year Monthly Mean Flow Sobat Base Max Base Min Base Mean Future Max Future Min Future Mean 1 2 3 4 5 6 7 8 9 10 11 12 Month STEP July, 2007: Hydrology Page No. 39
USGS, 2001 STEP July, 2007: Hydrology Page No. 40
Water Resources STEP July, 2007: Hydrology Page No. 41
Water Resources Planning and Management Introduction to Water Resources Handout: The development of Dryville STEP July, 2007: Hydrology Page No. 42
Water Resources Planning and Management Role of Hydrologic Analysis Hydrologic analysis for water resources management can be categorized according to the following assessments: 1. Present and future supply of water available from surface and/or ground water sources; 2. Present and future quality of surface and/or ground water; 3. Present and future frequency with which human activities will be subject to floods; and 4. Present and future frequencies of low streamflows and drought. Relationships among water resources management goals, purposes, and types of analyses: Goals Purpose Economic Development Environmental Quality Social Well- Being Hydrologic Analysis Public water supply X X WS, D, Q Industrial water supply X X WS, D, Q Irrigation X X WS, D, Q Hydroelectric power X X WS WS (D, F) Navigation X X WS Waste transport and treatment X X X WS, Q Recreation X X WS, Q Wildlife habitat X X WS WS (Q, F) Reduction of flood damages X X F WS = water supply; D = drought; Q = water quality; F = flood magnitude-frequency. After Dingman, 2002 STEP July, 2007: Hydrology Page No. 43
Water Supply Water in Our Communities Water Users Water Uses Environmental needsusers, uses, minimum requirements? Infrastructure Resource allocation and competing objectives STEP July, 2007: Hydrology Page No. 44
Water as a Hazard STEP July, 2007: Hydrology Page No. 45
USGS, 2001 STEP July, 2007: Hydrology Page No. 46
Water as a Hazard Planning, Management and Risk There s not enough Storage: Reservoirs, tanks, groundwater Redistribute water for year round availability, according to demand Mitigate the effects of drought (period of consecutive dry years) Emergency supply: fire flows require greater water volumes and much higher pressures There s too much Storms and runoff Seasonal high flows Excessive wet years (storage plays an important role here as well) Wetlands?? It s dirty Sediment and mineral loads (water quality, water treatment) Naturally occurring and introduced contaminants (suitability, pollution) Flooding and water quality (contamination) STEP July, 2007: Hydrology Page No. 47
Water as a Hazard Impacts of Water Management Activity Magnitude-Frequency Timing Duration Rate of Change Damming Reduced variability (WS, FC); Reduced peak flows (FC) Altered (WS, FC, HP) Reduced periods of inundation (FC) Rapid fluctuations (HP) Diversion Reduced flows; Reduced variability Altered Urbanization and drainage Increased variability; Increased peak flows Reduced periods of floodplain inundation due to stream entrenchment Levees and channelization May increase downstream peak flows Reduced periods of floodplain inundation Groundw water pumping Reduced low flows Deforestation Increased variability; Increased peak flows; Reduced low flows WS = water supply; FC = flood control; HP = hydropower After Dingman, 2002 STEP July, 2007: Hydrology Page No. 48
Water Management: Health, Safety, and the Environment Pollution Point source versus Non-point source (NPS) Types of pollutants: Oxygen-demanding material Nutrients Pathogens Suspended solids Toxic metals and organic compounds Heat Typical water quality concerns for types of water resources: Water Resource ph Dissolved Solids Precipitation X X Suspended Solids Dissolved Oxygen Organics and Petrleum Compounds Pathogenic Organisms Excess Heat Ground Water X X X Streams X X X X X X X Lakes X X X X "X" Indicates that a given type of water-quality constituent is typically of concern in a given type of water resource. After Dingman, 2002 Pollution management: control the discharge of pollutants so that water quality is not degraded to an unacceptable extent below the natural background level. Davis and Cornwell To do so, we need to: Measure pollutants, predict impacts of pollutants, know background (natural) water quality, decide on acceptable water quality level for a water body. STEP July, 2007: Hydrology Page No. 49
Water Management: Health, Safety, and the Environment Pollution Biological Oxygen Demand (BOD): dissolved oxygen consumed during the oxidation of an organic compound. Consumption of dissolved oxygen poses a threat to fish and other higher forms of aquatic life that must have oxygen to live. Davis and Cornwell, 1998 STEP July, 2007: Hydrology Page No. 50
Climate Change and Water Resources Potential Impacts of Climate Change Tools for understanding potential consequences of change (for better or worse) Global Climate Models (GCMs), historical climate records Hydrologic Models (& other environmental models) Water Resources Systems Models Verification requires observations and an understanding of uncertainty Climate Change Impact Assessments: Climate change and water supply Climate change and infrastructure Climate change and ecology Sustainability What are the appropriate time and spatial scales for these assessments? How do we experience the change? At what scales do we have confidence in projections and models? Hand-out: Climate Change and Wetlands STEP July, 2007: Hydrology Page No. 51
Water Science Additional Resources The Water Source Books by the Environmental Protection Agency http://www.epa.gov/safewater/kids/wsb/index.html Water Science for Schools by the US Geological Survey http://ga.water.usgs.gov/edu/ Watershed Game at the Bell Museum site http://www.bellmuseum.org/distancelearning/watershed/watershed2.html Rivers 2001 by the National Geographic Society http://www.nationalgeographic.com/geographyaction/rivers/ga22.html Alaska Wildlife Curriculum Teacher s Guide by the Alaska Dept of Fish and Game http://www.wildlife.alaska.gov/index.cfm?adfg=education.awc STEP July, 2007: Hydrology Page No. 52
References Davis, M. and D. Cornwell, 1998. Environmental Engineering. 3 rd Ed. McGraw-Hill, Boston, 919 pp. Dingman, L., 2002. Physical Hydrology. 2nd Ed. Prentice Hall, Upper Saddle River, New Jersey, 646 pp. McCuen, R., 1998. Hydrologic Analysis and Design. 2nd Ed. Prentice Hall, Upper Saddle River, New Jersey, 814 pp. Thompson, S., 1999. Water Use, Management, and Planning in the United States. Academic Press, San Diego, 371. STEP July, 2007: Hydrology Page No. 53