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 Why do we care about the vadose zone? 1) Drought mitigation 2) Water contaminant movement 3) Food production 4) Flood prevention Soils Basics Particles & Pores Slide By Trenton Franz 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 Rz = Loamy Sand 15 ft Vadose Zone T = 3-9 months B. Sandhills Sand & Gravel 50 ft C. River Valley Aquifer C = 7 yrs Sand and Gravel 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) Where is the soil profile likely located in Nebraska? A. Table Land Rz = Fine Sand B. Sandhills 80 ft Vadose Zone T = 8 – 10 yrs C. River Valley Fine Sand D. None of the Above RZ = Root Zone T = Travel time to aquifer (years) 100 ft Aquifer C = 14 yrs Sand and Gravel 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 = Silty Clay Loam B. Sandhills 80 ft Vadose Zone T = 25-30 yrs C. River Valley Silt – Silt Loam D. None of the Above RZ = Root Zone T = Travel time to aquifer (years) 100 ft Aquifer C = 14 yrs Sand and Gravel C= Time from initial pollution to increase average concentration in aquifer by 10 ppm (years) 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 Fair Fair Poor Good system Recreational development Good Moderate Fair Poor campground * 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 Overland flow Infiltration Redistribution Interflow Unsaturated Zone hydrat symbol Plant uptake Water table Recharge Capillary rise Saturated Zone Groundwater 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 Beaker of Beads A. Jar A B. Jar B C. Same in both jars Slide By Trenton Franz 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 Macropores Slide By Trenton Franz 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 Water-Input rate, is the rate at which t=0 t=tw 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 water- input 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 m2) • 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 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. Slide By Trenton Franz Single Ring Infiltrometer Falling Water Level • 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. Slide By Trenton Franz 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 Slide By Trenton Franz • Outer 120 cm Factors Affecting Infiltration Rate • Saturated Hydraulic Conductivity of the Soil Profile & Surface • Surface Slope & Roughness • Organic Surface Layers • Frost • Ponding increased until sufficient to overcome • Swelling-Drying hydraulic resistance of the • Rain Compaction surface • Inwashing of Fine Sediment • Overland Flow • Human Modification of Soil Surface • Depth of Ponding • Water Content of Surface Pores • Physical & Chemical Properties of • Rising water table Water • Low permeability layer at • Surface tension depth • Density • Soil Surface Characteristics • Viscosity • Waxy organic coatings • Temperature sensitive • Other Hydrophobic compounds Slide By Trenton Franz Nitrate-N Movement Plant and Microbial Assimilation (Temporary Removal) Plant uptake - + Decomposition/ NO3 & NH4 Mineralization Groundwater Leaching Denitrification (Permanent Removal) N2 N2 - NO3 - O2 NO3 O2 - NO3 - NO3 & Organic Carbon - O2 NO3 O2 - - NO3 NO3 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 important? A.