Ch 18. Hydrologic Cycle and streams. Tom Bean

Transcription

1 Ch 18. Hydrologic Cycle and streams Tom Bean

2 Wednesday s outline 1. the hydrologic cycle reservoirs cycling between them Evaporation and the atmosphere 2. Surface hydrology infiltration and soil moisture streamflow ET Snow/ice 3. water balance Water balance equation control volume a drainge basin water balance example units

3 Why do we care about the hydrologic Cycle and streams Water resources Floods = hazards Rivers shape Earth s surface

4 1. Hydrologic Cycle: continuous circulation of earth s water, between oceans, atmosphere and continents

5 Major reservoirs of water, in kg Atmosphere 13.5 Land 16,000 Cryosphere (ice and snow) 43,000 Oceans 1,400,000 70% of all fresh water 97% of all water (salty) 29% of fresh water as GROUNDWATER Lithosphere (LOTS) hydrosphere

6 Moving water between reservoirs Atmosphere 13.5 Cryosphere 43,000 precipitation: rain and snow Evaporation Land 16,000 rivers Oceans 1,400,000 Lithosphere

7 Evaporation and hydro cycle Water in atmosphere 1. Gas (you cannot see water vapor) Condensation Gas to liquid or ice Water in atmosphere 2. Clouds: liquid and ice Evaporation liquid to gas Ocean LAND

8 Surface hydrology: land or terrestrial branch of the hydrologic cycle

9 Infiltration: movement of water from the soil surface into the soil Overland flow to streams Soil s Sediment/Rock

10 Evapotranspiration (ET) is the combination of: 1. Evaporation of water from soil (liquid to gas) 2. Transpiration: water evaporates inside leaves; moves into atmosphere as gas Transpiration Evaporation Soil s Sediment/Rock

11 Streamflow (or runoff) movement of water downhill in a channel Sources: 1. From groundwater 2. Overland flow 3. Precipitation on channel Evaporation

12 Snow and ice: water is temporarily stored as a solid

13 Water Balance Change in vol. of water equals inputs minus outputs Similar to 1.Balancing a bank account 2.Inventory at a store

14 1. Need a control volume A user defined compartment Need not be spatially connected for example, the earth s glaciers

15 Drainage basin or watershed An area of the earth s surface that contributes the water passing a point on a stream the boundary is called a divide Mississippi River basin 6th largest river ~4000 km long 41% of U.S.

16 Drainage Basin of the Colorado River Fig

17 Catchment water budget Change in volume of water = input minus output change in volume p et r Mississippi River basin Inputs? Precipitation (P) Outputs? Evapotranspiration (ET) Runoff (R)

18 Catchment water budget change in volume p et r 0 p et r et r p Precipitation that falls on a basin, leaves the basin via the sum of ET and runoff

19 Catchment water balance example: In a year, how much ET from Boulder creek watershed et p r 1. P measured as a depth/time mm/year 2. R measured as a volume/time m 3 /second What percent of precipitation leaves basin via ET? A) 20% B) 40% C) 60% D) 80%

20 Raingauge depth per time (length per time)

21 Streamgauge; measure discharge volume per time

22 What is stream discharge? 3 m/s Area = height * width 2 m 2 m DISCHARGE = area x AVERAGE velocity Discharge = 4 m 2 x 3 m/s = 12 m 3 / s

23 Volume = area * depth km 3 [=] km 2 * km Map of Boulder Creek drainage basin Area

24 Does it rain more in Rhode Island or New Mexico? By volume: more rain falls in New Mexico. By depth: more rain falls in Rhode island volume = area * depth depth = volume / area

25 1. Boulder Creek example P = 1 m/year Basin area = 500 km 2 =500,000,000 m 2 What is volume of precipitation in Boulder creek basin? Either: 500,000,000 m 3 /year or 0.5 km 3

26 2. Boulder creek example Runoff = 3.2 m 3/ s What is volume of runoff (per year) 3 m yr 3 m sec x 3.15x10 1yr 7 sec R x10 s 8 m m 3.2 x 1.01x10 100,000,000 1yr yr yr 3 3 km 0.1 yr 3

27 Catchment water balance example: In a year, how much ET from Boulder creek watershed et p r P = 0.5 km 3 R = 0.1 km 3 What percent of precipitation leaves basin via ET? A) 20% B) 40% C) 60% D) 80% Answer: ET = 0.4 km 3 or 80% of p

28 Catchment water balance example: In a year, how much ET from Boulder creek watershed et p r P = 1.0 m / yr or 1000 mm/yr R = 0.2 m / yr or 200 mm/yr ET = 0.8 m/yr or 800 mm/yr 20% of precipitation ends up as runoff 80% of returns to atmosphere via ET

29 Water balance by continent (units = mm yr -1 ) Continent P (mm) R (mm) ET (mm) % runoff Globally, only 40% of precipitation is runoff Africa Asia Majority (60%) goes to ET! Australia Europe N. America S. America Antarctica All land All oceans

30 Water Balance Component 108,000 (62.7%) Evapotranspiration Precipitation 46,000 (95.8%) In southwestern US: Nearly all P ET 48, ,000 CA NV 73,000 (97.3%) 75,000 67,000 (98.5%) NM 68,000 AZ Data in Million Gallon/Day. Source: USGS Water Use Report 1990

31 Water flowing in streams Tom Bean

32 What is a stream : flow of water within any natural channel bed: floor of the channel. floodplain: the flat region adjoining the channel occupied in times of flood

33 How does water get into a stream? Sources: 1. From groundwater 2. Overland flow 3. Precipitation on channel

34 Streams flow downhill gravity Continual conversion of: Potential energy kinetic energy 1. Where does energy come from? 2. How come water doesn t keep going faster? The sun is the energy source for the hydrologic cycle

35 Streams are Turbulence turbulent flows Energy loss to friction in turbulent flows so, water doesn t keep moving faster Even a quiet, gentle stream is a turbulent flow, not laminar

36 Drainage basins and SUB-BASINS A basin can be divided into a series of sub-basins Political boundaries rarely follow basin boundaries

37 Boulder Creek Every basin has a stream network Pattern of stream channels in a basin

38 Some drainage basins are VERY VERY BIG Some drainage basins are VERY VERY small

39 Water flowing in stream channels What controls how water flows down a channel?

40 Channel controls on flow 1. cross sectional AREA 3 m/s Area = height * width 2 m 2 m DISCHARGE = area x AVERAGE velocity Discharge = 4 m 2 x 3 m/s = 12 m 3 / s

41 Velocity varies throughout a channel cross-section

42 Channel controls on flow: 2. Gradient or slope run rise Gradient or slope = rise/run Low gradient High gradient 2 m 2 m

43 Channel controls on flow: 3. material on bed Boulders All else equal: Smooth bed = high velocity Sand Concrete channels

44 Meandering River Point Bar Peter Kresan Fig

45 Braided River Tom Bean Fig

46 Rivers and Geology Erosion Transportation Deposition

47 EROSION: Waterfall Retreating Upriver stream is eroding bedrock Donald Nausbaum Fig. 13.7

48 Carr Clifton/Minden Pictures EROSION Pebbles Caught in Eddies Form Potholes

49 Transport: Streams move material in three forms Dissolved load Suspended load Bed load

50 Bedload -> Saltation Fig. 13.3

51 Deposition 1: Mississippi Delta Delta: sediments deposited where river meets the ocean or a lake Landsat 2 image annotated by Moore, 1979 Fig

52 Shifting Mississippi River Delta Over the Past 6000 Years Fig

53 Deposition 2. Formation of Natural Levees Levees built up further for flood control

54 Xie Jiahua/China Features/Sygma FLOODING

55 Discharge varies through time Boulder Creek, near Orodell Mean daily streamflow, ft 3 /sec Date

56 Flooding Discharge exceeds capacity of channel Interval between floods depends on climate, channel type, basin size and stream network Flooding is natural, but human activities affect flood frequency and magnitude

57 Recurrence interval - Average time between occurrences of a given event The 100-year flood: a flood of this size is expected every 100 years The 500-year flood: a flood of this size is expected every 500 years ZONING and INSURANCE

58 The floodplain in Boulder

59 Annual Flood Frequency Curve Fig. 13.1