What is the Vadose Zone? Why Do We Care? Tiffany Messer Assistant Professor Biological Systems Engineering School of Natural Resources University of Nebraska-Lincoln
Learning Objectives History of Vadose Zone Hydrology What is the vadose zone? Why do we care? Soil Basics Are there differences between soils? If so, how do we identify those differences? Water Movement What is preferential flow? What is a vadose lag time? Nitrate Movement Are common assumptions correct? Is groundwater more vulnerable dependent on depth or rainfall timing?
History of Vadose Zone Hydrology
What is the vadose zone? The vadose zone, also termed the unsaturated zone, extends from the top of the ground surface to the water table. The word Vadose means "shallow in Latin.
How far does soil science go back? A. 1700 BC B. 460 BC C. 1500 AD D. 180 AD
History of the Soil Science Samaria (1700 BC) Instructions on the preparation of land and planting of grain crops in the Euphrates River valley Democritus (460-370 BC) plant growth involved the cycling of indestructible elements Aristotle (384-322 BC) Taught that plants absorbed their nutrients from humus through the root system Homer to Odysseus using manure as fertilizer The Bible 1 Sam. 13:20- Israelites to sharpen what would be considered their agricultural implements Deut. 11:10-11- irrigation practices
1) Drought mitigation 2) Water contaminant movement 3) Food production 4) Flood prevention Why do we care about the vadose zone?
Soils Basics
Slide By Trenton Franz Particles & Pores
Size of Soil Particles SAND: 0.05 2.0 mm SILT: 0.002 0.05 mm CLAY: < 0.002 mm
Soil Texture Determined by the proportion of sand, silt and clay in a soil. Soil-texture Triangle Slide By Trenton Franz
Where is the soil profile likely located in Nebraska? A. Table Land B. Sandhills 15 ft Rz = Loamy Sand Vadose Zone Sand & Gravel T = 3-9 months C. River Valley D. None of the Above 50 ft Aquifer Sand and Gravel C = 7 yrs RZ = Root Zone T = Travel time to aquifer (years) C= Time from initial pollution to increase average concentration in aquifer by 10 ppm (years)
Where is the soil profile likely located in Nebraska? A. Table Land Rz = Fine Sand B. Sandhills C. River Valley 80 ft Vadose Zone Fine Sand T = 8 10 yrs D. None of the Above RZ = Root Zone T = Travel time to aquifer (years) C= Time from initial pollution to increase average concentration in aquifer by 10 ppm (years) 100 ft Aquifer Sand and Gravel C = 14 yrs
Where is the soil profile likely located in Nebraska? A. Table Land Rz = Silty Clay Loam B. Sandhills C. River Valley 80 ft Vadose Zone Silt Silt Loam T = 25-30 yrs D. None of the Above RZ = Root Zone T = Travel time to aquifer (years) C= Time from initial pollution to increase average concentration in aquifer by 10 ppm (years) 100 ft Aquifer Sand and Gravel C = 14 yrs
https://websoilsurvey.sc.egov.usd a.gov/app/homepage.htm
https://websoilsurvey.sc.egov.usd a.gov/app/homepage.htm
https://websoilsurvey.sc.egov.usd a.gov/app/homepage.htm
https://websoilsurvey.sc.egov.usd a.gov/app/homepage.htm
Common Nebraska Soils Potential for: Holdrege Nora Wymore Valentine Openland wildlife habitat Good Fair Fair Fair Nitrate leaching Moderate Low Moderate High Dryland crop yield potential Good Poor Good Poor Pesticide runoff Low High Moderate Low Sanitary facilities/septic system Recreational development campground Fair Fair Poor Good Good Moderate Fair Poor * Taken from UNL Plant and Soil Science Libraryhttp://passel.unl.edu/pages/informationmodule.php?idinformationmodu le=1130447038&topicorder=7&maxto=10
Water Movement
Water Movement Exfiltration Percolation Unsaturated Zone Infiltration Redistribution hydrat symbol Plant uptake Overland flow Interflow Saturated Zone Water table Recharge Groundwater Capillary rise Slide By Trenton Franz
Porosity vs Hydraulic Conductivity Porosity: Measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0 and 100%. Hydraulic Conductivity: Describes the ease with which water can move through pore spaces or fractures in the soil.
Which has the greater porosity? A B Beaker of Marbles Beaker of Beads A. Jar A B. Jar B C. Same in both jars Slide By Trenton Franz
Porosity vs Hydraulic Conductivity Porosity is the same in each beaker! Porosity 48% Porosity 48% Beaker of Marbles Beaker of Beads Slide By Trenton Franz
Which has the greater hydraulic conductivity (permeability)? A B Beaker of Marbles A. Jar A B. Jar B Slide By Trenton Franz Beaker of Beads C. Same in both jars
Porosity vs Hydraulic Conductivity Slide By Trenton Franz Which has the greatest hydraulic conductivity (permeability)? Beaker of marbles! WHY: The pore spaces are more connected and thus water flows more easily from the marble beaker. So for well sorted, spherical grains, with the same porosity, larger grain size means greater K values.
Porosity vs Hydraulic Conductivity HOWEVER: This relationship is not true for non-spherical, platy, or layered sediments such as clays, nor does it hold true for poorly sorted sediments. For example, clays have very high porosities, but very low hydraulic conductivities. Slide By Trenton Franz
Porosity vs Hydraulic Conductivity Slide By Trenton Franz
Pore Connectivity & Straightness of Path (Tortuosity) Soil Water or Groundwater Flow Well Sorted Gravel Clay Well Sorted Sand Sand & Gravel Slide By Trenton Franz
Slide By Trenton Franz Macropores
Slide By Trenton Franz
Slide By Trenton Franz
Infiltration Process by which water arriving at the soil surface enters the soil. At a given point, the rate of infiltration generally changes systematically with time during a given water-input event. Conditions affecting infiltration change between water-input events. Slide By Trenton Franz
Infiltration Event t=0 t=t w Water-Input rate, is the rate at which water arrives at the surface due to precipitation or irrigation. Depth of ponding, is the depth of water standing on the surface Infiltration rate, is the rate at which water enters the soil from the surface [L/T]. Infiltrability (infiltration capacity), is the maximum rate at which infiltration can occur [L/T]. This rate is NOT constant but changes during the infiltration event. Slide By Trenton Franz
Infiltration Conditions No Ponding: Infiltration rate = water-input rate and is infiltrability Infiltration is said to be supply-controlled. Slide By Trenton Franz
Infiltration Conditions Saturation From Above: Ponding is present because the waterinput rate exceeds the infiltrability Infiltration rate = infiltrability Rate of infiltration is determined by soil type and wetness and is said to be profile-controlled. Slide By Trenton Franz
Infiltration Conditions Saturation from below: Ponding is present because the water table has risen to or above the surface, and the entire soil column is saturated. Infiltration rate = zero Slide By Trenton Franz
How Do We Measure Infiltration? Infiltrometer a device for direct field measurement of infiltrability over a small area (0.02 to 1 m 2 ) Area defined by an impermeable boundary (typically a ring) Ponding due to saturation from above created by direct flooding at surface or by simulated rainfall Slide By Trenton Franz
Single-Ring Infiltrometer Constant Water Level Slide By Trenton Franz Drive a ring into the soil and supply water in the ring with a constant water level. The infiltration capacity of the soil is the maximum infiltration rate, and if infiltration rate exceeds the infiltration capacity, ponding and runoff will be the consequence. Therefore maintaining constant head means the rate of water supplied corresponds to the infiltration capacity or infiltrability. The supplying of water is done with a Mariotte bottle.
Single Ring Infiltrometer Falling Water Level Slide By Trenton Franz Drive a ring into the soil Fill with water to allow the water level inside the ring to drop with time. Records how much water goes into the soil for a given time period. The rate of which water goes into the soil is a measure of the soils ability to conduct water, called its permeability or hydraulic conductivity.
But There is a Problem! Water infiltrating into an unsaturated soil is influenced by BOTH gravitational and capillary (pressure) forces. So, the water moves BOTH vertically and horizontally. Thus, the measured infiltration rate exceeds the rate that would occur if the entire surface were ponded. Solution? - Use a double-ring infiltrometer. Slide By Trenton Franz
Double-Ring Infiltrometer Two concentric rings are ponded Area between inner and outer rings acts as a buffer zone Measurements on the inner ring only are used to compute infiltration rate Suggested ring diameters Inner 100 cm Outer 120 cm Slide By Trenton Franz
Factors Affecting Infiltration Rate Saturated Hydraulic Conductivity of the Soil Profile & Surface Organic Surface Layers Frost Swelling-Drying Rain Compaction Inwashing of Fine Sediment Human Modification of Soil Surface Water Content of Surface Pores Rising water table Low permeability layer at depth Soil Surface Characteristics Waxy organic coatings Other Hydrophobic compounds Surface Slope & Roughness Ponding increased until sufficient to overcome hydraulic resistance of the surface Overland Flow Depth of Ponding Physical & Chemical Properties of Water Surface tension Density Viscosity Temperature sensitive Slide By Trenton Franz
Nitrate-N Movement
Plant and Microbial Assimilation (Temporary Removal) Plant uptake NO 3- & NH 4 + Decomposition/ Mineralization
Groundwater Leaching
Denitrification (Permanent Removal) N 2 N 2 NO 3 - O 2 O 2 NO 3 - NO 3- & Organic Carbon NO 3 - NO 3 - O 2 NO 3 - O 2 NO 3 -
What is the vadose zone? A. Unsaturated zone B. Part of Earth between the land surface and groundwater C. Does not include the are that is still saturated above the water table (capillary fringe) D. All of the above
Why is the vadose layer A. Drought Mitigation B. Water Contamination Movement C. Food Production and Flood Prevention D. All of the above important?
A. Process by which water arriving at the soil surface enters the soil B. Measure of the void spaces in a material C. Ease water can move through pore spaces in the soil D. Rate at which water arrives at the surface due to precipitation or irrigation What is porosity?
What is the shortest flow path? A. B. C. D. Well Sorted Gravel Clay Well Sorted Sand Sand & Gravel
What impacts the vadose lag time? A. Viscosity B. Frost C. Rain Compaction D. All of the above
Thank you! Tiffany Messer Assistant Professor Biological Systems Engineering School of Natural Resources University of Nebraska-Lincoln Tiffany.Messer@unl.edu