# Soil Water Introduction The amount of water associated with a given volume or mass of soil ("soil water" or "soil moisture") It is a highly variable property.

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Soil Water Introduction The amount of water associated with a given volume or mass of soil ("soil water" or "soil moisture") It is a highly variable property. It can change on time scales of minutes to years. Water use efficiency in Jordan is a very important issue.

Particle size & pore space Large Particle Pore 2 x 2 x 2 = 8 radius = 4r 16 r

Particle size & pore space Medium Particle Pore 4 x 4 x 4 = 64 radius = 2r 16 r

Particle size & pore space Small Particle Pore 8 x 8 x 8 = 512 radius = r 16 r

Fig. 8.5. Classification of soil water (after Heaney, Crown and Palylyk, 1995). Credit: Pedosphere.comPedosphere.com

Soil Water zSaturated soil is when that soils pores are full of water. zGravitational water is that water that moves out of the soil due to gravity. This water is generally in the larger Macro-pores. zCapillary water is that water that is held in the soil due to adhesion and cohesion against the pull of gravity. This water is generally held in the smaller Micro-pores and as a film around soil particles.

Soil Water zAfter a major rain event, once the gravitational water has left the soil, the soil is at Field Moisture Capacity. zThe wilting point is reached when soil water levels decline to the point that all remaining water is held too tightly by soil particles to be removed by the plant.

Soil Water

Available Water Capacity zAvailable water capacity is a measure of the water available to plants. zCommonly defined as the difference between the amount of water at field moisture capacity and the amount at the wilting point. zThis is the water a plant has a chance of utilizing.

Available Water Capacity

Soil water content How to quantify soil moisture content? Soil Moisture: Mass basis: 1. Gravimetric water content, θ w (P w ) = mass of water/mass of dry soil = Mw/Ms = %, or gm/gm

Soil water content, Volume basis 2. Volumetric water content, θ v (P v ) = volume water/bulk volume of soil = Vw/V = %, or mm/m or in/ft ranges from 0 to 60% θ v = θ w x ρ b / ρ w

Expressing soil water in depth units, dw

Note: θv is a better term for quantifying the amount of moisture of the soil, as it can be expressed as an equivalent depth of water per depth of soil. * This is necessary when determining appropriate depths of irrigation water to apply to replenish depleted soil moisture.

Example A soil sample is taken with a soil container. Sample volume is 100 cm 3 Total wet mass (weight) including container = 188 g Total oven dried mass including container = 155 g Container weight = 21 g

1- What is the bulk density? 2- soil moisture, mass basis? 3- soil moisture, volume basis? 4- total moisture in mm/m and in/ft?

Wet mass of the soil = 188 - 21 = 167 g Dry mass of the soil = 155 – 21= 134 g ρb = mass of dry soil/bulk volume = 134/100=1.34 g/cm 3 θ w=(Mw/Ms) = (167-134)/134 = 33/134 = 0.246 or 24.6 θv = (Vw/V) = (33/1)/100 = 0.33 or 33% θv = θw x (ρb/ρw) = 24.6 (1.34/1) = 33%

Fig. 8.10. Saturated and Unsaturated Flow

Fig. 8.4. Capillary rise and capillary retention Credit: Brady & Weil, 1996; Kohnke, 1968

Fig. 8.6. Interaction of water molecules with clay surfaces, and cations and anions in soil Credit: Pedosphere.comPedosphere.com

Matric Potential zMatric Potential: Adhesion of water to surfaces through adsorption and capillarity; markedly reduces the energy state of adsorbed water molecules zMatric potential is universally important and is used in calculations of water movement

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