ATM 301 Lecture #6 Soil Properties and Soil Water Storage

Most (about 2/3 -3/4) of global land precipitation enters the soil

infiltration (~76% of land precip.) redistribution unsaturated soil saturated groundwater Overland flow

Why care about soil water? About 75% of precipitation over land “infiltrates” to become soil water Most vegetation gets its water from the soil (forests, crops, etc.) Precipitation that does not infiltrate can quickly become runoff and lead to flooding and erosion Groundwater is largely supplied from soil water Applications Irrigation strategies Flood forecasting Soil chemistry and composition (nutrients, contaminants, etc.) Ground water management ers-hit-record-levels-in-capital- region/2689/irene-flooding/ Important questions How much water is in the soil? How do we know? How quickly can water go into the soil? What are the limits? How hard plants need to work to pull water out of the soil? How does water move in and through the soil?

Properties of soil: What properties are hydrologically relevant / useful ? What is the soil made of? How big are the soil grains? How much space is there between the soil grains? How much water is in the soil? How much water can be in the soil? How easily does water enter and move through the soil?

Properties of soil: Some basic definitions: Soil volume V s = V m + V a + V w = V m + V v where V m = vol. for soil mineral, V a = vol. for air, V w = vol. for water, and V v = vol. for voids or pores in the soil. Soil bulk density: the dry density of the soil:  b =M m /V s = M m /(V a +V m ) = M m /(V v +V m ) constant in in time, but increases with depth. Porosity (  ): is the proportion of pore spaces in a volume of soil:  = V v /V s = (V a +V w )/V s = 1 -  b (kg/m 3 )/2650 VmVm VvVv

Ranges of Porosities of soil: Porosity decreases with grain size

We often want to classify fraction of various soil particle sizes (i.e., texture), as they affect the storage and movement of water We can quantify this using soil sieves Pass soil through a series of progressively finer meshes by shaking Each mesh stops all grains > some specified diameter Weighing contents of each sieve to get “%-finer” Can be done wet or dry Properties of soil: 1. Texture (particle size) Size classes (USDA): Gravel: >2mm Sand: mm Silt: mm Clay: <0.002mm

Properties of soil: 1.Texture (particle size) Can plot results as a “grain size distribution curve” Silt sand gravel Clay %- finer Diameter (mm)

Can plot results as a “soil-texture triangle” Dingman Fig. 7-5 Properties of soil: 1. Texture (particle size) ex. 53% sand 44% silt 3% clay

Properties of soil: 2. Composition Clay minerals are a particularly important component Aluminosilicate minerals contain alumina Al(OH) 6 & Silica SiO 3 Kaolinite, illlite, smectite, … Mostly form as a byproduct of weathering of other minerals…often present as clay grain sizes Has distinct hydrological properties Effective at absorbing and retaining water Classify the soil by chemical and biological composition Biological vs. mineral content (we’ll ignore biological component) Type of minerals present mageofthemonth/Imageofthemont h2008/cfam_february08.aspx

Water Storage in Soil: 1. Volumetric water content  (or water content or soil-moisture content): the ratio of water volume to soil volume:  = V w / V s where V w = vol. for water, and V s = soil vol. Soil water storage =  x Layer depth Measurement of  : Gravimetric method:  = (M swet – M sdry )/(  w V s ) A soil sample with V s vol. and M swet mass is oven-dried at 105 o C for 24 hrs. Other methods for measuring , see pp , Box 7.2

1 inch = 2.54 cm Distribution of Soil Depth in cm (Webb et al. 1993)

Water Storage in Soil: 2. Saturation or wetness  : is the proportion of pores that contain water:  = V w / V v = V w /(V a +V w ) =  /  where V w = vol. for water, V a = vol. for air and V v = vol. for pores Effective saturation  * : for practically available water  * = (  -  r ) / (  -  r ) where  r = the permanent residual water content (  0.05)

Forms of Soil Water Storage 17 Water is held in soil in various forms and not all is available to plants. Chemical water is part of the molecular structure of soil minerals and unavailable to plants. Gravitational water is held in large soil pores and rapidly drains out within a day or two after rain. Capillary water is held in small soil pores that does not drain out under gravity and is the main source of water for plants. At saturation, all pores are full of water, but after a day or so, all gravitational water drains out, leaving the soil at field capacity (or water holding capacity). Plants then draw water out of the capillary pores, readily at first and then with greater difficulty, until no more can be withdrawn and the water left is in the micro-pores. The soil is then at wilting point, and without water addition, plants die.

Changes in Soil Water Storage

Field Capacity (or Water Holding Capacity): Drainage due to gravity because negligible within a few days after saturation Field Capacity (  fc )is the soil water content at which the gravity-drainage rate becomes “negligible” (e.g.,  0.1mm/day). This happens when the vertical water pressure gradient balances the downward gravitational gradient: dp = -  g dz (similar to atmospheric hydrostatic Eq)

Field capacity y= log 10 T fc T fc = days drain to field capacity T fc = 10 y where n,  r,  rG are van Genuchten parameters, and Kh is the saturated hydraulic conductivity (see p. 347, Dingman)

Soil Water Holding Capacity Water holding capacity (mm/cm depth of soil) of main texture groups. Figures are averages and vary with structure and organic matter differences. TextureField CapacityWilting pointAvailable water Coarse sand Fine sand Loamy sand Sandy loam Light sandy clay loam Loam Sandy clay loam Clay loam Clay Self-mulching clay Source: Department of Agriculture Bulletin 462, % 10%

Permanent Wilting Point: Surface evaporation or plant uptake for transpiration can remove water from a soil that has reached field capacity. But there is a limit to the suction (negative pressure) by plants In hydrology, this pressure limit is usually -15 bar (=- 15,000cm of water =-1470kPa=-14.5 atm) The water content at this pressure limit is defined as the permanent wilting point:  pwp   (-15bar) Using the van Genuchten relations (Table 7.2 on p.336, Dingman), one gets: Wilting point: high for clay, low for silt and sand

θ ≈ 0 Available Water Content (  a ): The difference between the field capacity and the permanent wilting point is the available water content for plant use:  a =  fc   pwp Soil-water Status: θ = ϕ θ = θ fc θ = θ pwp

Hydrologic Soil Horizons (or layers): Phreatic Zone (saturated zone) Vadose Zone (unsaturated zone) p=  w g (z’-z’ o )

Root Zone Depth: varies with vegetation types and locations

SMOS=Soil Moisture and Ocean Salinity Satellite SMOS Satellite-derived Root Zone Soil Moisture Content (in fraction of soil volume)

Soil Moisture Content Patterns 27

Equilibium Soil-Water Profiles: the vertical water-content profile above the water table under the local mean recharge rate. Finer soil texture has higher water content Water content increase with depth A higher water table maintains a wetter soil

Soil Moisture Measurements 29 Direct methods: volumetric water content (  ) can be derived using the known volume of the soil sample and a drying oven. Most common method. Other measurements: various instruments (see pp ) - Tensiometer - time-domain reflectometry (TDR), neutron probe, frequency domain sensor, capacitance probe, amplitude domain reflectometry, electrical resistivity tomography, ground penetrating radar (GPR) time-domain reflectometryneutron probefrequency domain sensorcapacitance probe amplitude domain reflectometryelectrical resistivity tomographyground penetrating radar Satellite remote sensing method: Satellite microwave remote sensing is used to estimate soil moisture based on the large contrast between the dielectric properties (mainly emissivity) of wet and dry soils. Canopy water can contaminate the signal.

Tensiometer gauge readout