Hillslope hydrology and intro to groundwater

1. Hillslope hydrology and intro to groundwater (with many slides borrowed from Jeff McDonnell/Oregon State)

3. JJA DJF Where does our water come from? Oceanic Sources of Continental Precipitation • Global evaporation: 500,000 km3/yr of water • 86% oceans, 14% continents • 90% of water evaporated from oceans goes back to ocean • 10% to continents • 2/3 of the 10% is recycled on the continents • 1/3 of the 10% runs off directly to ocean • Isotopic analysis to determine source of water • relative proportions of isotopes of hydrogen and oxygen Sources: EOS, 7 June 2011 and Gimeno et al., 2010b

4. Implications of Climate Change • Changing atmospheric circulation patterns => changing precipitation patterns • Convergence and transport from regions of high water vapor => extreme floods • Absence of moisture transport => extreme drought • Regions getting water from multiple oceanic source regions are less susceptible to shifts in circulation patterns

5. How does the water come from the ocean? • “Atmospheric river” (Zhu and Newell, 1998) • 90% of the poleward atmospheric water vapor transport through the midlatitudes is concentrated in 4-5 narrow bands • <10% of the Earth circumference • Transport 13-26 km3/day of water vapor • =7.5-15 times Qavg of Miss. R at New Orleans • Land interactions • forced up/over mountains • cool, condense, produce precipitation (rain or snow) • Major source of precip in coastal regions

6. Fig. 1. Analysis of an atmospheric river (AR) that hit California on 13–14 October 2009. (a) A Special Sensor Microwave Imager (SSM/I) satellite image from 13–14 October showing the AR hitting the California coast; color bar shows, in centimeters, the amount of water vapor present throughout the air column at any given point if all the water vapor were condensed into one layer of liquid (vertically integrated water vapor). Source: EOS, 9 August 2011

7. Hewlett (1982) General Water Cycle Water Balance: accounting of water conservation of water volume Input (I) – Output (O) = DS (changes in storage) Inputs: rain and snow Output: stream discharge (Q), evapotranspiration (ET), groundwater/infiltration Storage: Soil moisture, groundwater, snow, ice, lakes

8. Water Balance A, drainage basin area stream discharge HYDROGRAPH evapotranspiration storage S(t) = A R(t) – Q(t) – Qgw(t) – A ET(t) also infiltration rainfall groundwater discharge Over long periods (> 1yr), changes in storage can be neglected S(t) = 0 Groundwater flow is very small compared to the other terms Qgw(t) Q(t) = A[ R(t) – ET(t)] For example, CONUS average annual precipitation: 76 cm Q = R – ET 23 = 76 – 53 (cm)

9. A whole litany of controls on runoff or discharge (Q) generation Broad conceptual controls • Rainfall intensity or amount • Antecedent conditions • Soils and vegetation • Depth to water table (topography) • Geology

10. Overland Flow Occurrence • On road surfaces and other impermeable areas • bedrock outcrops, city parks, lawns • On hydrophobic soils (fire and seasonality) • On trampled and crusted soils • On low permeable soils • Silt-clay soils without macropores • On saturated soils (SOF) • Riparian zone • Waterlogged soils

11. Overland flow generation • Runoff occurs when • R > I • Or in words, rainfall intensity exceeds the infiltration rate

12. Horton Overland Flow Qho(t) = w(t) - f(t) where: w(t) is the water input rate f(t) is the infiltration rate

13. Fig. 5.3

14. A different form of overland flow R > I

15. Runoff Pathways Slide from Mike Kirkby, University of Leeds, AGU Chapman Conference on Hillslope Hydrology, October 2001

17. Storm Precipitation Saturation Overland Flow Hortonian Overland Flow Channel Precip. + Overland Flow Soil Mantle Storage Baseflow Overland Flow Subsurface Stormflow Interflow Basin Hydrograph Re-drawn from Hewlett and Troendle, 1975

18. Troendle, 1985

19. Dominant processes of hillslope response to rainfall Thin soils; gentle concave footslopes; wide valley bottoms; soils of high to low permeability Direct precipitation and return flow dominate hydrograph; subsurface stormflow less important Horton overland flow dominates hydrograph; contributions from subsurface stormflow are less important Variable source concept Subsurface stormflow dominates hydrograph volumetrically; peaks produced by return flow and direct precipitation Topography and soils Steep, straight hillslopes; deep,very permeable soils; narrow valley bottoms Arid to sub-humid climate; thin vegetation or disturbed by humans Humid climate; dense vegetation Climate, vegetation and land use (Dunne and Leopold, 1978)

20. The old water paradox “…streamflow responds promptly to rainfall inputs, but fluctuations in passive tracers are often strongly damped. This indicates that storm flow in these catchments is mostly ‘old’ water” Kirchner 2003 Hydrological Processes

21. Runoff Generation Mechanisms

22. The Water Cycle: More Detail

23. Infiltration • “the entry of waters into the ground” • rate and quantity of infiltration = f( • soil type • soil moisture • soil permeability • ground cover • drainage conditions • depth to water table • intensity and volume of precip

28. “Hillslopes consist of soils and regolith overlying rock. Both have a definable porosity.” Porosity • Ratio of void volume to total volume • V = Va + Vw + Vs • Voids are spaces filled with air and water • Range of porosity values • granular mass of uniform spheres with loose packing, n=47.6% • granular mass of uniform spheres with tight packing, n = 26% • unconsolidated material like sandstones and limestones, n = 5-15% • Vv = Va + Vw Volumetric water content At saturation,

29. Horton’s eqn. f = infiltration rate at some time t, cm/hr or in/hr fo = initial infiltration rate at time zero fc = final constant infiltration capacity, analogous to soil permeability beta = recession constant, hr-1

31. Rate of Infiltration (velocity of flow through unsaturated media) • Green/Ampt eqn. f = infiltration rate or velocity, (in/hr) Ks = hydraulic conductivity, (in/hr) h = pressure head, (in or ft) z = vertical direction, (in or ft) Infiltration is a function of time because as the ground/soil becomes more saturated, there is less infiltration

32. Calculate the steady state water discharge at the base of a hillslope. The hillslope is 150 m long, the rainfall rate is 7 mm/hr and the rain has been falling for long enough that the hydrology of the slope may be taken as steady, with a uniform steady infiltration rate of 1.5 mm/hr. Provide the answer both in m3/s per m length of the bounding stream, and in cubic ft per second (cfs) per linear foot of channel. S(t) = A R(t) – Q(t) – Qgw(t) – A ET(t) At steady state the inputs of water to the hillslope must equal the outputs Q = L[ R – I] 1 cf = .3 m3

33. GROUNDWATER

35. n = Sy + Sr Specific yield (effective porosity): measure of gw that drains by gravity; storage characteristics of aquifer Specific retention: measure of gw that doesn’t drain under gravity

37. http://www.uiowa.edu/~c012003a/14.%20Groundwater.pdf

39. Hyetograph of rainfall Hydrograph of streamflow Initially, there is little runoff => b/c more rain goes into infiltration Later, there is more runoff => less infiltration due to saturated ground

40. GROUNDWATER

41. GROUNDWATER

42. GROUNDWATER