1. Subsurface Water unit volume of subsurface consists of soil/rock, and pores which may be filled with water and/or air total porosity= volume voids/total volume water content=volume water/total volume saturation=volume water/volume voids degree of saturation delineates various zones of subsurface water

2. Definitions soil water - Ground surface to bottom of root zone depth depends on soil type and vegetation. May become saturated during periods of rainfall otherwise unsaturated (soil pores partially filled with air). Plants extract water from this zone. Evaporation occurs from this zone. intermediate vadose zone - Between soil water zone and capillary fringe. Unsaturated except during extreme precipitation events. Depth of zone may range from centimeters to 100s of meters.

3. Definitions Continued capillary zone - Above saturated zone. Water rises into this zone as a result of capillary force. Depth of this zone is a function of the soil type. Fractions of a meter for sands (mm) to meters for fine clays. All pores filled with H 2 O, p < 0. Effect seen if place bottom of dry porous media (soil or sponge) into water. Water will be drawn up into media to a height above water where soil suction and gravity forces are equal. saturated zone - All pores filled with water, p > 0. Formations in this zone with ability to transmit water are called aquifers.

4. Unsaturated Zone Water can exist in all its phases in the unsaturated zone. Liquid water occurs as: –hygroscopic water - adsorbed from air by molecular interaction (H-bonds) –capillary water - held by surface tension due to viscosity of liquid –gravitational water-water in unsaturated zone in excess of field capacity which percolates downward due to gravity ultimately reaching saturated zone as recharge.

5. Unsaturated Zone Hygroscopic and capillary waters are held by molecular electrostatic forces (between polar bonds and particles -- surface tension) in thin films around soil particles  drier soil, smaller pores  hygroscopic and capillary forces Hygroscopic water - held at -31 to -10,000 bars. Water is unavailable to plants or for recharge to groundwater. Capillary water - Held at -0.33 to -31 bars. More water filling pores but discontinuous except in capillary fringe. This water can be used by plants.

6. Definitions Permanent wilting point: tension (suction, negative pressure) below which plant root system cannot extract water. Depends on soil and type of vegetation. Typically -15 bars (-15x10 5 Pa, - 15000cm Field capacity: tension (suction, negative pressure) below which water cannot be drained by gravity (due to capillary and hygroscopic forces) Depends on soil type. Typically about -0.33 bars

7. Typical Moisture Profiles rain after a long dry period direction of moisture movement moisture content depth root zone hygroscopic wilting point field capacity saturation

8. Typical Moisture Profiles Drying process moisture depth field capacity saturation 1 - Drying in upper layers by ET. 2 - Bottom part of wetting front continues . Upper part continues to dry. 3 - At some point  and  movement results in no moisture gradient 4 - Dry front established. Lower zones are being depleted to satisfy PET at surface. Drying continues until capillary forces are unable to move water to surface.

9. Dacry-Buckingham law Flow in unsaturated porous media governed by a modified Darcy’s law called Darcy-Buckingham law :  - suction head (capillary head) or negative pressure head. Energy possessed by the fluid due to soil suction forces. Suction head varies with moisture content,   n,  0,  < n,  is negative. K(  ) - hydraulic conductivity is a function of water content  ,  K(  ) because more continuously connected pores, more space available for water to travel through, until at  = n, K(n) = K sat

10. Measuring Soil Suction Soil Suction (  ) head measured with tensiometers, an airtight ceramic cup and tube containing water. Soil tension measured as vacuum in tubes created when water drawn out of tube into soil. Comes to equilibrium at soil tension value. Tensiometers often used to schedule irrigation.

11. Estimating water flux from tensiometer measurements h = z +  h 1 = 100 cm - 65 cm = 35 cm h 2 = 50 cm - 50 cm = 0 cm z 1 = 100 cm z 2 = 50 cm h 1 = z 1 +  1 h 2 = z 2 +  2  1 (negative) (-65 cm)  2 (negative) (-50 cm) z = 0

12. Look at components of flux: Capillary gradient cause upward flow, gravitational gradient causes downward flow. Net flux is down. What would it take to get net flux upward?