Soil Water Balance Reading Applied Hydrology Sections 4

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 – 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 – 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 1. 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!) 2. 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 3. 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 Name of soil separate 1 mm 0. 01 mm . 0. 001 mm Very coarse sand* Diameter limits (mm) 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 Clay 0. 05 - 0. 002 less than 0. 002

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 Flux, q q = Q/A

Energy fluxes at Freeman Ranch

Evaporation (mm/day) Average = 1. 1 mm/day November August Average = 3. 1 mm/day

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

Soil Water Content Rainfall mm/30 min

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 z 1 q 12 z 2

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 Pore with 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 (@ 20 o. C) Interface water air Net force inward No net force

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

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

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