Soil Water: Characteristics & Behaviour PDF

CHAPTER FIVE: SOIL WATER: CHARACTERISTICS & BEHAVIOUR Literature source: Title: The Nature and Properties of Soils, I led Authors: Nyle C.Brady & Raymond R. Weil. Page Reference: pg 143-146,...

CHAPTER FIVE: SOIL WATER: CHARACTERISTICS & BEHAVIOUR Literature source: Title: The Nature and Properties of Soils, I led Authors: Nyle C.Brady & Raymond R. Weil. Page Reference: pg 143-146, 149, 157, 159, 160, 162, 166-168. 5.1 INTRODUCTION: Water is a vital component of everything living. Water influences every aspect of soil deyelopment, from the weathering of minerals to the decomposition of organic matter, from the growth of plants to the pollution of groundwater. We are all familiar with water. We drink it oyasmyyjtwjt swim in it, and Irrigavour_crops with it. Water in the soil is different from water in a drinking glass. Water &ausessoil particles to shrink and swell to egdhere to each other, and to form structuralggregates. It partakes in many chemical reactions that releases or tie up nutrients. Water Vaffects the balance of air, temperature, metabolism sojl organisms, rate of leaching, and rate of evapotranspiration. 5.2 Structure & Related Properties Of Water: 5.2.1 Polarity: The property of water helps explain how water molecules interact with each other. The hydrogen (positive) end of one molecule attracts the oxygen (negative) end of another, resulting in a chainlike (polymer) grouping. This explains why water is attracted to clay surfaces. The negatively charged clay surfaces attract water through the hydrogen (positive) end of the molecule. When water molecules are attracted to clay surfaces, they are more closely packed than in pure water. In this packed state their freedom of movement is restricted and their energy status is lower than in pure water. 5.2.2 Hydrogen Bonding: The_phenomenon by which hydrogen moleculesjs called hydrogen bonding.  bonding accounts for 5.2.3 Cohesion versus Adhesion: Hydrogen responsible for water retention and movement two basic forces for each other in soils: the attraction of water molecules for solid (cohesion) and the attraction of water molecules adsorption), surfaces (adhesion). By adhesion (also called surfaces. some water molecules are held rigidly at the soil solid In turn, these tightly bound water molecules hold by cohesion other water molecules further removed from the solid surfaces. Together, the forces of adhesion and cohesion make it possible for the soil solids to retain water and control its movement and use. Adhesion & cohesion make possible the property of plasticity possessed by clays. 5.2.4 Surface Tension: Surface tension is commonly evident at liquid-air interfaces and results from the greater attraction of water molecules for each other (cohesion)than for the air above. The net effect is an inward force at the surface that causes water to behave as if its surfaces were covered with a stretched elastic membrane. Because of the high attraction of water molecules for each other, water has a high surface tension. Surface tension is an important property, especially as a factor in the phenomenonof capillarity which determines how water moves and is retained in soil. (refer to fig 5.2 pg 145) 5.3 Capillary Fundamentals & Soil Water: The movement of water up a wick when the lower end is immersedin water exemplifies the phenomenon of capillarity. Two forces cause capillarity: 1. The attraction of water for solid walls of channels through which it moves (adhesion or adsorption) 2. The surface tension of water, which is due largely to the attractionof water molecules for each other (cohesion). (fig 5.4 & 5.5 pg 147)  5.3.1 Capillary Mechanism: Capillarity can be demonstrated water (fig 5.4). The water by placing one end of a fine glass tube in rises in the tube, and the smaller thetube bore, the higher the water rises. The water moleculesare attractedto the sides of the tube (adhesion) and start moving up the tube in response to this attraction. The cohesive force between individualwater moleculesensures that water not directly in contact with the sides walls is also pulled up the tube. This will continue until the weight of water in the tube counter balances the cohesion and adhesive forces. 5.3.1.1 Height of Rise in Soils: Capillary forces are at work in all moist soils. Soil pores determinethe rate of water movement and how high the water will rise. This is because of the following: u Soil pores are not straight. u Soil pores does not have uniform openings like glass tubes. u Are filled with air, which may slow down or prevent water movement. Height of rise is greater in fine-texturedsoils, providingpores are not too small. In sandy soils (large pore space) the movement is rapid, but the height of rise is small. Capillarity is often referred to as an upward movement,but it must be remembered that soil pores are horizontaland vertical, and therefore movement takes place in any direction. 5.4 SOIL WATER ENERGY CONCEPTS: 5.4.1 Forces Affecting Potential Energy: There are three important forces affecting the energy levels of soil water: l. Adhesion or the attraction of the soil solids (matrix) for water, provides a matric force (responsible for adsorption & capillarity) that markedlyreduces the energy state of the adsorbed water molecules, and to a lesser degree of those held by cohesion. This force give rise to matricpotential.  and solutes for water, resulting in osmotic 2. The attraction of ions the soil solution. forces, tends to reduce the energy level of water in This force gives rise to osmotic potential. 3. Gravity, which tends to pull the water downward. This force gives rise to gravitationalpotential. 5.4.2 Soil Water Potential: The differencein energy level of water at one site or one condition (e.g, wet) from that at another site or condition (e.g.,dry) will determinethe directionand the rate of water movement in the soils and in plants. Water always moves from a point where it has a high energy level to where it has a lower one. Since water in wet soils is not held very tightly by soil solids (matrix), the water molecules have considerablefreedom of movement,so the energy level is not much lower than that of pure water not influencedby soil. In a drier soil, however, the water that remains is very tightly held by soil solids, the water molecules have little freedom of movement, and the energy of the water is much lower than that of the water in wet soil. If the wet and dry samples are brought in touch with each other, water will move from the wet soil (higher energy state) to the drier soil (lower energy state). 5.4.3 Field Tensiometer: The tenacity with which water is held in soils is an expression of the soil water potential. The field tensiometer measures this attraction. The tensiometeris filled with water then placed in the soil. Water in the tensiometeris drawn through a fine porous cup into the adjacent soil until equilibrium is reached, at which time the potential in the soil is the same as that in the tensiometer. Tensiometers are used successfully in determining the need fro irrigation when the soil is to be kept well suppliedwith water. Their range of usefulness is between 0 and —80kPapotential. (Fig 5.16 pg 159) 5.5 TYPES OF SOIL WATER MOVEMENTS: Three types of movements in the soil are recognised: 5.5.1 Saturated flow. 5.5.2 Unsaturated flow. 5.5.3 Vapour flow. 5.5.1 Saturated Flows: Under some conditions, at least part of a soil profile may be completely saturated, that is, all pores (including pores in the upper zones), large and small, are filled with water. The lower horizons of poorly drained soils are often saturated,as are portions of well-drained soils. This usually happens immediately following a heavy rain or irrigation. 5.5.2 Unsaturated Flow: In saturated soils essentially all pores are filled with water, although rapid movement may still occur through large pores. But in unsaturated soils, these macropores are filled with air leaving only the finer pores to accommodatewater movement. The water content and the tightness with which water is held varies from time to time and place to place in unsaturated soils. 5.5.3 Water Vapor Movement: Two types of water vapour movement in soils: Internal and External. Internal movement takes place within the soil, i.e. in the soil pores. External movement occurs at the land surface, and water vapor is lost by surface evaporation. 5.6 RETENTION OF SOIL WATER IN THE FIELD: Keeping in mind the energy-soil water relations covered in the previous section, we now turn to some more practical considerations. We shall start by following the moisture and energy relations of soil during and after a heavy rain or the application of in-igation water. 5.6.1 Vaximum Retentive Capacity: When all soils are filled with water from rainfall or irrioation the soil is said to be saturated with res ect to water and at its maximum retentive capacity. The matric potential is high, almost the same as that of pure water. Maximum retentive capacities in soils are useful in predicting how much rain water be stored in the soil temporarily, thus avoiding floods.  downward in 5.6.2 Field Capacity: irrigation, some of the water will drain downward Following rain or three days, this rapid responseto gravity. After one to soil is said to be at its field tnovetnent will become negligible.The its water has moved out of the macropores and capacity. At this time or capillary pores are place has been taken by air. The micropores plants with needed water. still filled with water and will supply the to —30kPa. Water The tnatric potential generally range from —10 tnovetnent will continue to take place at a slow rate. 5.6-3 Permanent ilting Percentage or WiltingCoefficient: As plants absorb water from the soil they loose most of it through evaporation at the leaf surfaces (transpiration).Some water is lost by evaporation directly from the soil surface. These two loses occur sitnultaneously, and the combined loss is termed evapotranspiration. As the soil dries plants begin to wilt to conserve moisture during the daytime. At first the plant will regain vigor at night, but ultimately thay will retnain wilted night and day. Although not dead the plants are now in a permanent wilted condition and will die if water is not provided. Under this condition the soil water potential generally range from —15bars (-1500kPa) for most crop plants. Xerophytes (desert- type) plants can continue to remove water at even more negligible potentials. The water content of the soil at this stage is called the wilting coefficient or perrnanent wilting percentage. (fig 5.26 pg 167) 5.6.4 Ilygroscopic Coefficient: When soil moisture is further lowered below the wilting point, the water molecules that remain are very tightly held, mostly being absorbed by colloidal surfaces. This state is approximatedwhen the atrnosphereabove a soil sample is essentially saturated with water vapor (9800 relative humudity) and equilibrium is established. The water is held so tightly (-3100kPa) that much of it is considered nonliquid and can move only in the vapor phase. The moisture content of the soil at this point is termed the hygroscopic coefficient. Soils high in organic matter and clay will hold Inore water than sandy soils low in organic matter and clay.