ATM 301 Lecture #6 Soil Properties and Soil Water Storage

Download ATM 301 Lecture #6 Soil Properties and Soil Water Storage

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ATM 301 Lecture #6 Soil Properties and Soil Water Storage Slide 2 Most (about 2/3 -3/4) of global land precipitation enters the soil Slide 3 infiltration (~76% of land precip.) redistribution unsaturated soil saturated groundwater Overland flow Slide 4 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? Slide 5 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? Slide 6 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 Slide 7 Ranges of Porosities of soil: Porosity decreases with grain size Slide 8 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: 0.05-2mm Silt: 0.002-0.05mm Clay:


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