Soil Water Balance Reading: Applied Hydrology Sections 4.3 and 4.4 Topics Soil water properties Soil water flux Soil water balance

Subsurface water Infiltration Soil moisture Subsurface flow Groundwater flow

Porous Medium Flow Groundwater Porous media All waters found beneath the ground surface Occupies pores (void space not occupied by solid matter) Porous media Numerous pores of small size Pores contain fluids (e.g., water and air) Pores act as conduits for flow of fluids The storage and flow through porous media is affected by Type of rocks in a formation Number, size, and arrangement of pores Pores are generally irregular in shape because of differences in the minerals making up the rocks geologic processes experienced by them.

Zones of Saturation Unsaturated zone Saturated zone Water table Zone between the land surface and water table Pore contains water and air Also called as vadose zone or the zone of aeration Saturated zone pores are completely filled with water Contains water at greater than atmospheric pressure Also called phreatic zone Water table Surface where the pore water pressure is atmospheric Divide between saturated and unsaturated zone Capillary fringe Zone immediately above the water table that gets saturated by capillary forces

Soil Water Three categories Field capacity Hygroscopic water Microscopic film of water surrounding soil particles Strong molecular attraction; water cannot be removed by natural forces Adhesive forces (>31 bars and upto 10,000 bars!) Capillary water Water held by cohesive forces between films of hygroscopic water Can be removed by air drying or plant absorption Plants extract capillary water until the soil capillary force is equal to the extractive force Wilting point: soil capillary force > plant extractive force Gravity water Water that moves through the soil by the force of gravity Field capacity Amount of water held in the soil after excess water has drained is called the field capacity of the soil.

Soil Sieves http://www.rtg.wa.edu.au/loanpool/belmont/sieves.jpg

Soil Particle Sizes (USDA Soil Classification System Table 1. Size limits (diameter in millimeters) of soil separates in the USDA soil textural classification system. Soil Particle Sizes (USDA Soil Classification System Name of soil separate Diameter limits (mm) Very coarse sand* 2.00 - 1.00 Coarse sand 1.00 - 0.50 Medium sand 0.50 - 0.25 Fine sand 0.25 - 0.10 Very fine sand 0.10 - 0.05 Silt 0.05 - 0.002 Clay less than 0.002 1 mm 0.1 mm 0.01 mm . 0.001 mm

http://www.uga.edu/srel/kidsdoscience/soils-planets/soil-particle-size.pdf

Soil Texture Triangle Source: USDA Soil Survey Manual Chapter 3

Soil Water Content Soil Water Content

Soil Water Flux, q q = Q/A

Energy fluxes at Freeman Ranch

Evaporation (mm/day) November Average = 1.1 mm/day August

Daily Rainfall Aug 6 – Aug 13 30 min Rainfall August 6, 2004

Soil Water Content Rainfall mm/30min

Soil Water Balance at Freeman Ranch P E 0 cm Net Exchange 10 cm 20 cm

Soil Water Tension, y Measures the suction head of the soil water Like p/g in fluid mechanics but its always a suction (negative head) Three key variables in soil water movement Flux, q Water content, q Tension, y Total energy head = h z=0 z1 q12 z2

Darcy’s Law K = hydraulic conductivity q = specific discharge V = q/n = average velocity through the area

Definitions Element of soil, V (Saturated) Pore with water solid air Element of soil, V (Unsaturated)

Continuity Equation z dx dy dz y x

Continuity (Cont.) Continuity Equation

Surface Tension Below surface, forces act equally in all directions At surface, some forces are missing, pulls molecules down and exert tension on the surface If interface is curved, higher pressure will exist on concave side Pressure increase is balanced by surface tension, s s = 0.073 N/m (@ 20oC) Interface air Net force inward water No net force

Capillary Action Capillary Pressure Related to saturation Suction Head Total Head Solid Water Air R

Pressure Distribution in Subsurface Clay Sand Top of capillary fringe Water table z = 0, P=0

Richard’s Equation Recall So Darcy becomes Continuity becomes Darcy’s Law Total head So Darcy becomes Continuity becomes Soil water diffusivity