Homework I will be ed It is also posted on the website

Characterizing Soil Water

Three Potential Energies: Gravitational Potential Capillary or Matric Potential Submergence Potential

Gravitational Potential 1. Gravitational potential energy is due only to the height of an object (water) above some reference point. 2. Gravitational potential energy is independent of soil properties. We will use gravitational potential energy per unit weight of water (cm).

Porous block Suction (capillarity) Matric or Capillary Potential 100 cm Dry soil Ψm = -100 cm (suction) Vertical distance between the surface of the water and the porous cup.

Submergence Potential (ψ s ) Equal to the distance below a free water surface Water Table 10 cm Sand Clay

Total Potential Energy is the sum of the gravitational, submergence, and matric potential energies. Ψ g + ψ m + ψ s = ψ T

Gravitational Potential + Matric Potential = Total Potential Reference level Ψ g = 0 Height (cm) a 10 Ψm = -95 cm Ψg = 50 cm Ψ T = -45 cm

Gravitational Potential + Matric Potential = Total Potential Reference level Ψ g = 0 Height (cm) a b 10 Ψm = -95 cm Ψm = -25 cmΨg = 10 cm Ψg = 50 cm Ψ T = -45 cm Ψ T = -15 cm Ψ Ta – Ψ Tb = (- 45cm) - (-15cm) = -30 cm

Quantifying Water Movement

Gradient The difference in potential divided by the Distance between the two points considered total potential at point A – total potential at point B distance between points A and B The driving force for water flow. The stronger the gradient, the greater the driving force for water movement.

Reference level Ψ g = 0 Height (cm) a b 10 Ψ Ta = -20 cm Ψ Tb =-100 cm Difference in total potential = 80 cm = 2 Distance between the points 40 cm = Gradient Difference in potential energy = -20 cm – (-100 cm) = 80 cm Gradient = Distance between points A and B = 40 cm

Distance (cm) 0 Height (cm) a b 10 Difference in total potential (-200) = 100 cm = 5 Distance between the points 20 cm 20 cm = 5 25 Ψma = -100 cm Ψga = 0 cm Ψmb = -200 cm Ψgb = 0 cm Ref.

The stronger the gradient, the greater the driving force for water movement.

Characterizing Soil Moisture Status

Water Content Based

Soil Water Content Water content by weight Moist weight – Dry weight Dry soil weight = Water weight Dry soil weight Multiply by 100 to yield % water by weight V = Πr 2 h Water content by Volume Volume Water Volume Soil Multiply by 100 to yield % water by volume

Example: You collect a 200 cm 3 soil sample. Its moist weight is 150 g. After drying, the dry weight is 100 g. Gravimetric water content: Moist weight – Dry weight Dry weight = Water weight Dry weight 150 g - 100g 100g = 50 g = 0.5 or 50% 100g

Example: You collect a 200 cm 3 soil sample. Its moist weight is 150 g. After drying the dry weight is 100 g. Volumetric water content: 150 g - 100g 200 cm 3 = = 50 cm 3 water = 0.25 or 25% 200 cm 3 soil Volume Water Volume Soil Density of water 1 g/cm 3 50 g 200 cm 3

Energy-Based Characterizing Soil Moisture Status Relating water content and matric potential (suction)

suction porous plate Soil Core Characterizing Soil Water

Suction applied in discrete increments. Water Remaining In soil Suction applied (cm) 0 10,000 One soil saturated * Soil Core Moisture Release Curve

Texture, Density Water Remaining In soil Suction applied (cm) 0 10,000 saturated * A B Two Soils coarser finer

Pore Size Distribution Water Remaining In soil Suction applied (cm) 10,000 saturated *

Soil Moisture Status

Field Capacity:Water content of soil after drainage from saturation by gravity Suction equivalent: bars (or –0.10 bars) - 33 KPa cm water Permanent:Water can no longer be accessed by plants Wilting pointSuction equivalent: -15 bars KPa - 15,000 cm water Saturation:Water content of soil when all pores are filled Suction equivalent: 0 bars 0 KPa 0 cm water Plant Available water: Field Capacity - PWP

Texture Field Capacity Perm. Wilting Point Sandy Loam 179 Loam2411 Clay3620 Heavy Clay 5728 Energy and Texture Smaller particles and pores Water Content (%) at

Practical Measures Water Remaining In soil Suction applied (cm) 0 10,000 saturated *

Direct Methods Soil Resistance Blocks Time Domain Reflectometry

The Rate of Water Movement

Hydraulic Conductivity Strongly responsible for water distribution within the soil volume. Determines the rate of water movement in soil. Texture Density Structure Water content The ease with which water moves through soils

Coarse uncompacted Fine compacted Hydraulic Conductivity

h L A Volume time ⃗ h * A L WATERWATER Determining Saturated Hydraulic Conductivity Volume time = h * A L K K = V * L h * A * t Soil

Approximate Ksat and Uses Ksat (cm/h) Comments 36 Beach sand/Golf Course Greens 18 Very sandy soils, cannot filter pollutants 1.8 Suitable for most agricultural, recreational, and urban uses 0.18 Too slow for most uses <3.6 x Extremely slow; good if compacted material is needed Saturated hydraulic conductivity

Determining Saturated Flow

Darcy’s Equation Volume flow Area * time = Q A = Ksat * gradient

Reference level Ψ g = 0 Height (cm) a b 10 Ψ Ta = -20 cm Ψ Tb =-100 cm Difference in total potential = 80 cm = 2 Distance between the points 40 cm = Gradient Difference in potential energy = -20 cm – (-100 cm) = 80 cm Gradient = Distance between points A and B = 40 cm

Darcy’s Equation Volume flow Area * time = Q = Ksat * gradient (Q) = 5 cm/hr * 2 = 10 cm/hr Difference in total potential = 80 cm = 2 Distance between the points 40 cm = Gradient =

Distance (cm) 0 Height (cm) a b 10 Difference in total potential (-200) = 100 cm = 5 Distance between the points 20 cm 20 cm = 5 25 Ψma = -100 cm Ψga = 0 cm Ψmb = -200 cm Ψgb = 0 cm Ref. If Ksat = 5 cm/hr, then the flow (Q) = 5 cm/hr * 5 = 25 cm/hr

Exam is Friday, May 22 in class Review session: Thursday Study Guide: Wednesday