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

Where does our water come from Where does our water come from? Oceanic Sources of Continental Precipitation JJA DJF 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

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

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

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

General Water Cycle Hewlett (1982) 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

Water Balance S(t) = A R(t) – Q(t) – Qgw(t) – A ET(t) 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)

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

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

Overland flow generation Runoff occurs when R > I Or in words, rainfall intensity exceeds the infiltration rate Stream response to rainfall input. I decreases as voids are filled with water. R > I contributes to storm hydrograph. Lag to peak = time from pk of rainfall event to pk of runoff hydrograph = f( drainage density, infiltration = f(antecedent moisture, etc))

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

Fig. 5.3

A different form of overland flow R > I

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

Saturation Overland Flow Hortonian Overland Flow Storm Precipitation Saturation Overland Flow Hortonian Overland Flow Channel Precip. + Overland Flow Soil Mantle Storage Baseflow Overland Flow Subsurface Stormflow Interflow the idea that in forested watesheds v. little runoff is on the surface Basin Hydrograph Re-drawn from Hewlett and Troendle, 1975

Troendle, 1985

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)

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

Runoff Generation Mechanisms runoff generation: 3 mechanisms (can happen simultaneously) = f(place); infiltration excess overland flow (e.g. football field, compaction, Horton); subsurface storm flow (soil infiltration exceeds precip; infiltration down to an impeding layer; development of a shallow water table), SOF (opposite of overland flow, water infiltrates, fills storage. fill storage until WT rises to meet surface; saturation from below causes exfiltrating GW, seepage, return flow runoff mechanisms: variable source area (expands/contracts seasonally);

The Water Cycle: More Detail Gw is the saturated zone below the water table where pores are completely filled with water

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 infiltration is a function of above ground conditions (what’s growing; precip) and below ground (type of soil, antecedent soil conditions, where the water table is)

effec. water capacity: fraction of void spaces available for water storage. min infil rate: final rate that water passes through the soil profile under sat. conditions sand size < 4 mm, silt 63 microns to 1mm, clay < 63 microns

Porosity Ratio of void volume to total volume V = Va + Vw + Vs “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 Capacity of soil or rock to hold water At saturation,

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

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

S(t) = A R(t) – Q(t) – Qgw(t) – A ET(t) 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

GROUNDWATER

TABLE 3.1 Range of Porosity Soil Type Porosity, pt Unconsolidated deposits Gravel 0.25 - 0.40 Sand 0.25 - 0.50 Silt 0.35 - 0.50 Clay 0.40 - 0.70 Rocks Fractured basalt 0.05 - 0.50 Karst limestone Sandstone 0.05 - 0.30 Limestone, dolomite 0.00 - 0.20 Shale 0.00 - 0.10 Fractured crystalline rock Dense crystalline rock 0.00 - 0.05 Source: Freeze and Cherry (1979). The capacity of soil or rock to hold water is called porosity. the ratio of the pore volume to the total volume of a representative sample of the medium. Assuming that the soil system is composed of three phases -- solid, liquid (water), and gas (air) -- Saturated sand contains about 20% water; gravel, 25%; and clay, 48%. Saturated bedrock with few crevices commonly contains less than 1% water. Clay is not a good water source despite its high water content, or porosity, because the extremely small size of the openings between microscopic particles creates friction that effectively halts water movement. Saturated clay is virtually impermeable.

Specific yield (effective porosity): 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 Permeability is a measure of how fast water will flow through connected openings in soil or rock. Low permeability refers to soil or rock that restricts the movement of water through it . The specific yield is the actual amount of water that will drain out of saturated soil and rock by gravity flow. It does not drain out completely because some water forms a film that clings to soil and rock. Permeability is critical for water supply purposes; if contained in soil or rock will not drain out, it is not available to water wells.

area ab. wt where flow is due to tension rather than gravity

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

The void spaces between the soil particles are known as the soil pores The void spaces between the soil particles are known as the soil pores. Below the water table the pore spaces are filled with water. Above the water table the pore spaces are filled with varied amounts of air and water. These soil pores are interconnected amongst themselves. The capillary fringe is immediately above the water table. Under ideal conditions, when a well is installed in the soil, water will not rise above the level of the water table. There is no measurable rise of water in the well since the diameter of the well is large and has no capillary effect. However, the capillary effect of the small-diameter soil pore sizes will cause water to rise above the water table. This water is not free water. This water is essentially trapped within these small capillary pores.

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

GROUNDWATER

GROUNDWATER The vadose zone, also termed the unsaturated zone, is the portion of Earth between the land surface and the phreatic zone or zone of saturation ("vadose" is Latin for "shallow"). It extends from the top of the ground surface to the water table. Water in the vadose zone has a pressure head less than atmospheric pressure, and is retained by a combination of adhesion (funiculary groundwater), and capillary action (capillary groundwater). If the vadose zone envelops soil, the water contained therein is termed soil moisture.

GROUNDWATER