Soil Water At the end of class, student are expected to have ability to: Discuss and describe soil-water-plant relationship. Analyze hydraulic properties of soils and flow of water into and through soil. Assess the equipment for soil water technique
Outline • Water and Plant Growth • Flow of water in soil • Permeability • Hydraulic conductivity • Capillarity • Soil Moisture Capacity • Field capacity • Permanent Wilting Point • Available Water • Soil Water Measurement
Roles of Water in Plant Growth Tissue composition50–90% water Photosynthesis Transpiration Nutrient availability Nutrient transport within plants Chemical reactions – as solvent Ease of root growth Microbe growth Water shortage causes plant growth inhibition and wilting. Excess water causes oxygen and nutrient deficiencies, susceptibility to fungi, toxin build-up, and root damage.
Flow of Water in Soil • Soil’s characteristics & engineering behaviour are greatly influenced by its water content • Cohesive soils tend to shrink when dry & swell when wet • Fine-grained soils are significantly weakened at high water contents • Such factors must be taken into considerations during geotechnical designs. • Effects of water movement within soil are also very important.
Permeability • The facility with which water flows through soil is an engineering property known as permeability or hydraulic conductivity. • As water movement within soil is through interconnected voids. • In general, the larger the void spaces, the greater the permeability.
Permeability water Dense soil - difficult to flow - low permeability Loose soil - easy to flow - high permeability
Permeability • Flow of water in soil between 2 points occurs as a result of a pressure (or hydraulic head)difference between 2 points. • Flows from higher to lower pressure • Velocity of flows varies directly with: • magnitude of the difference between the hydraulic heads • Permeability of soil
Permeability • Flow of water in soil can be analyzed quantitatively by Darcy’s Law:
Permeability • If a constant of proportionality, k, is supplied, the preceding proportionality becomes: • k is known as coefficient of permeability • Hydraulic gradient, i = h/L • Eqn 5.1 can be written as
Permeability • If the velocity of flow, v, is required as q = Av • The velocity is an average velocity because it represents flow rate divided by gross cross-sectional area of the soil (includes both solid & voids) • As water only travels through voids the actual velocity is
Example 5-1 • Water flows through the sand filter as shown in Figure 5-1 • The cross-sectional area & length of the soil mass are 0.250m2 & 2.00m, respectively. • The hydraulic head difference is 0.160 m • The coefficient of permeability is 6.90x10-4 m/s • Question: Determine the flow rate of water through the soil
Example 5-1 Solution • From Eq. 5.2
Example 5-2 • In a soil test, it took 16.0 min for 1508 cm3 of water to flow through a sand sample • The cross-sectional area was 50.3 cm3 • The void ratio of the soil sample was 0.68. • Determine • The velocity of water through the soil • Actual velocity
Example 5-2 Solution
Empirical relationships for k • For uniform sands in a loose state: • For dense or compacted sands
Coefficient of Permeability, k • If silts or clays are present in sandy soils, even in small amounts, k may change significantly. • k varies greatly among the types of soils encountered in practice, below table gives a broad classification of soils according to k.
Permeability in stratified soils • In the preceding discussion, soil was assumed to be homogeneous • In reality, natural soil deposits are often non-homogeneous, value of k varies • The general procedure is to find & use an average value of k • Value of k is different vertically & horizontally
Permeability in stratified soils • Consider the case where flow is in the y-direction • The water must travel successively through layers 1,2,3, … , n • Flow rate & velocity through each layer must be equal
Permeability in stratified soils • Similarly the flow in the horizontal direction, kx can be determined • In stratified soils, average kx is greater than average ky
Example 5-3 • A non-homogeneous soil consisting of layers of soil with different permeabilities as shown in figure below • Estimate kx & ky
Example 5-3 Solution
Example 5-3 Solution (cont’d)
Laboratory determination of permeability • For coarse grained soil can be determined by a constant head permeability test, and • In a fine grained soil, falling head permeability testworks the best.
Constant-head method • Suitable for granular soil • Utilises a constant-head permeameter as shown • To allow water to move through the soil specimen under a stable-head condition
Constant-head method • The coefficient of permeability, k, can be determined by measuring & recording the following • Volume (quantity) of water discharged, Q • Length of specimen, L • Cross-sectional area of specimen, A • Time required for Q to be discharged, t • Head, h
Falling-head method • Suitable for fine-grained & coarse-grained soils & granular soils • Utilises a permeameter as shown in diagram • Testing procedure is similar to constant-head • The specimen is first saturated with water • Water is then allowed to move through the soil specimen under falling-head condition
Falling-head method • The coefficient of permeability can be derived by measuring & recording • Time required for a certain quantity of water to pass through the specimen, t • Cross-sectional area of burette, A • Hydraulic heads at the beginning & end of test, h1 & h2 • As shown in figure, the velocity of fall in the burette is given by • V = dh/dt (-ve sign to indicate a falling head) • Flow of water into the specimen • Flow of water through & out the specimen
Capillarity • Refers to the rise of water in a small-diameter tube inserted into water, the rise being caused by both surface tension & adhesion of the water to the tube’s wall • Amount of rise of water above the surrounding water level is inversely proportional to the tube’s diameter
Capillarity • With soils, this occurs at the water table • Height of capillary rise is associated with mean diameter of soil’s voids, in turn, related to average grain size • The figure illustrates the capillary rise of water in a tube • In equilibrium • Weight of H2O = adhesion
Capillarity • With soils, capillarity occurs at the ground water table (GWT) when water rises from saturated soil below into dry or partially saturated soil above the GWT • Because the voids interconnect in varying directions & irregular in size & shape, accurate calculation of the height of capillary rise is impossible
Capillarity • Eq 5-39 is applicable only to the rise of pure water in clean glass
Example 5-4 • A clean glass capillary tube with a diameter of 0.5mm is inserted into water with a surface tension of 0.073 N/m • Determine the height of capillary rise in the tube.
What is Water Table? • the level beneath the soil which the soil is saturated with water • marshes develop where the water table is just below the ground surface • if the water table is not too low, dryness tends to correct itself through capillary movement
Soil Water Holding Capacity Analyze the soil water holding capacity Determinemethods of soil water measurements and their advantages and limitations,
Soil Moisture Capacity • Water in the soil is held by the forces of cohesion and adhesion in which surface tension, capillarity, and osmotic pressure play a significant role. • There are two types of forces acting on soil moisture. • Positive forces are those that enhance soil’s affinity for water (e.g., forces of cohesion and adhesion). • In contrast, some negative forces that take water away from soil include gravity, actively growing plant roots, and evaporative demand of the atmosphere. • At any given point in time, soil’s moisture content is the net result of these positive and negative forces.
How is soil water classified? • Hygroscopic Water is held so strongly by the soil particles (adhesion), that it is not available to the plants 2) Capillary Water is held by cohesive forces greater than gravity and is available to plants 3) Gravitational Water is that water which cannot be held against gravity as water is pulled down through the soil, nutrients are "leached" out of the soil (nitrogen)
Gravitational water is the water that fills macropores. This water drains quickly. • Capillary water fills the micropores and drains slowly. This makes it available to plants between precipitation events. • Hygroscopic water is water that forms a thin film around individual particles. These particles hold it tightly in place, so much of this water is not available to plants. Source: Kaufmann, Robert K. and Cleveland, Cutler J. 2007. Environmental Science (McGraw-Hill, Dubuque, IA).
Field Capacity • Field Capacity is greatest amount of water the soil can hold under drainage. • For most soils, it is obtained after two days of drainage after the soil was saturated by heavy rain or irrigation. • It is the optimum amount of water needed for agriculture. • After the drainage has stopped, the large soil pores are filled with both air and water while the smaller pores are still full of water. • At this stage, the soil is said to be at field capacity. • At field capacity, the water and air contents of the soil are considered to be ideal for crop growth
Permanent Wilting Point and Available Water • Below Field capacity, the plant finds it more and more difficult to extract water until the suction or tension reaches 15 atmospheres permanent wilting point is obtained, which is the maximum tension the plant can exert on the soil to extract water. • Available water is the difference between the moisture contents at field capacity and permanent wilting point. • Clay holds more water but the plants exert more tension to extract water more than sand. • Little by little, the water stored in the soil is taken up by the plant roots or evaporated from the topsoil into the atmosphere. • If no additional water is supplied to the soil, it gradually dries out.
Permanent Wilting Point • The dryer the soil becomes, the more tightly the remaining water is retained and the more difficult it is for the plant roots to extract it. • At a certain stage, the uptake of water is not sufficient to meet the plant's needs. The plant looses freshness and wilts; the leaves change colour from green to yellow. Finally the plant dies. • The soil water content at the stage where the plant dies, is called permanent wilting point. • The soil still contains some water, but it is too difficult for the roots to suck it from the soil
Available water content • Plant available water, AW, may be defined as the difference between field capacity, FC, and wilting point, WP. The formula is: AWC = FC − PWP • The soil can be compared to a water reservoir for the plants. • When the soil is saturated, the reservoir is full. • However, some water drains rapidly below the rootzone before the plant can use it.