Water Sciences Unit 1

WATS 281 UNL

Distribution of All Water on Earth

Ocean: 97%, Freshwater: 3%

Distribution of Freshwater on Earth

Ice caps/Glaciers: 69%, GW: 29%, Accessible Surface Freshwater: 1%

Distribution of Accessible Surface Freshwater

Lakes: 52%, Soil Moisture: 38%, Water Vapor: 8%, Rivers: 1%, Living Organisms: 1%

Water Balance Equation

P + SWin + GWin = E + T + SWout +GWout + ΔS

Watershed

Area that appears on the basis of topography to contribute all the water that passes thru a given cross-section of a stream

Divide

Surface trace of the boundary delimiting a watershed

Drainage Area

Horizontal projection of the area of a watershed

• At or above the cross-section

4 Types of Uplift

1. Orographic Uplift

2. Convective Uplift

3. Frontal Uplift

4. Convergent Uplift

Areal Estimation (3)

1. Arithmetic Average

2. Thiessen Polygons

3. Isohyetal Method

Orographic Uplift

Convective Uplift

Frontal Uplift

Convergent Uplift

Arithmetic Method

Areal Estimation

• For multiple stations in a watershed

• Add the rainfall amounts at each station within the basin, and divide this total by the number of stations to get the average value

Thiessen Method

Areal Estimation

• For multiple stations in a watershed

• Connect stations with dashed lines

• Bisect each dashed line with a solid line

• Connect the lines to form the polygons

• Determine the area of each polygon

• Determine the weighted precipitation total for each polygon

• Sum the data

Isohyetal Method

Areal Estimation

• For multiple stations in a watershed

• Contour the stations using lines of equal intensity (1 in, 2 in, etc)

• Determine the area of each contour band

• Determine the weighted precipitation total for each polygon

• Sum the data

Snow Water Equivalent (h_m)

Amount of water the snow actually contains and yields when melted

h_m ≡ V_m / A

  h_m = (r_s  / r_w) · h_s  

Factors Affecting Runoff (3)

1. Climate

2. Phys Characteristics of Watershed (Drainage Basin)

3. Land Use (Human Alterations)

Physical Characteristics of a Watershed (4)

1. Elevation and Orientation of Basin

2. Topography

3. Veg and Soil Type

4. Geology

Effects of Geology on Watersheds - Drainage Types (5)

1. Dendritic drainage

2. Centripetal drainage

3. Rectangular drainage

4. Radial drainage

5. Trellis drainage

Discharge (Q)

The volume rate of flow [L³/T] thru a stream cross-section at right angles to the flow direction

• Process of measuring stream discharge = stream gaging

ET Formula

ET = P - SWout - GWout

Energy Balance

Hydrologic, Energy, and Radiation Balance

Aquifer

Subsurface geologic unit that stores and transmits water

2 Types:

1. Unconfined (Water table)

2. Confined (Potentiometric Surface)

Henry Darcy

• Found that Discharge (Q) increased when the change in head (H1 - H2) was increased, and decreased when the head was decreased

• Q decreased when the length (L) btn the 2 points where the head was measured was increased, and increased when L was decreased

• Q increased when the cross-sectional area (A) of the column (cylinder) was increased, and decreased when A decreased

• If (H1-H2) and L were constant, Q was larger for some soils and smaller for others

  • Accounted for this relationship with a coefficient for each different soil type that he put in his column - Hydraulic Conductivity of a material (K)

• Darcy measured:

  • High K’s for very porous loose soils

  • Low K’s in very tight, compacted soils

• Q is a function of: f(H1-H2, L, Length, Area, Material)

Henry Darcy Equation/Darcy’s Law

Q = Discharge

K = Hydraulic conductance

A = Area

H1-H2 = Hydraulic gradient

L = Length

Water Balance Concept - Conservation of Mass

Inputs = Outputs ± Change in Storage

I = O ± ΔS

REV

Representative Elementary Volume

• Ex: soil, lake, watershed

Precipitation and ET as:

Water Depth

• mm or in

Stream Flow and GW as:

Rate or Velocity or Discharge

• m/sec; ft/day; m³/day)

Storage as:

Volume

• Liters, Gallons, Acre, Ft

Potential ET

Amount of water demanded by atm conditions (ie input rad)

Actual ET

Actual amount of ET that the land surface exchanges with the atm

• For plants: PET is roughly AET in well-watered conditions where there is enough water in the root zone to meet the atm demand

Primary factors controlling the rate at which water will infiltrate into the ground following a rain event:

1. Rainfall amount and rate

2. Antecedent moisture conditions

3. Slope

4. Soil type

5. Soil hydraulic conductivity

Confined Aquifer

Energized - the pressure exerted on an open well would result in the height of the well being above the water table under atm conditions

• Have potentiometric surfaces

Potentiometric Surface

Height at which the water in a well would rise due to pressure forces

Hydrology

Predicts the occurrence, circulation, and distribution of water of the earth and its atm

• Global cycle includes: Terrestrial, Oceanic, and Atm

Inputs - Water Balance

1. Precipitation

   1. Rain, Snow/ice, Fog/mist

2. Surface water

   1. Runoff (overland flow), Soil water (interflow)

3. Groundwater

Outputs - Water Balance

1. Evaporation

   1. Open water, Bare soil, Leaf/Plant surfaces)

2. Transpiration

3. Ground water

4. Surface Water

   1. Runoff (overland flow)

   2. Soil water (interflow)

5. Storage

Storage - Water Balance

• Atmosphere/Clouds

• Lakes/Rivers/Reservoirs

• Glaciers

• Canopy/Biomass

• Soil Moisture

• Aquifers (GW)

• Ocean