CE 118 Hydrology - Infiltration PDF

CE 118 HYDROLOGY Chapter 4 INFILTRATION Objectives: By the end of this lesson, students will be able to:  Understand the fundamental process of evaporation and what controls its rate.  Have a knowledge of the techniques for measuring evaporation dir...

CE 118 HYDROLOGY Chapter 4 INFILTRATION Objectives: By the end of this lesson, students will be able to:  Understand the fundamental process of evaporation and what controls its rate.  Have a knowledge of the techniques for measuring evaporation directly.  Have a knowledge of the techniques used to estimate evaporation.  Understand the importance of vegetation in determining regional evaporation. DEFINITION Infiltration is the process of water entry into a soil from rainfall, or irrigation. Soil water movement (percolation) is the process of water flowing from one point to another point within the soil. Infiltration rate is the rate at which the water infiltrates through the soil during a storm, and it must be equal to the infiltration capacities or the rainfall rate, whichever is lesser. Infiltration capacity is the maximum rate at which soil in any given condition is capable of absorbing water. The rate of infiltration is primarily controlled by the rate of soil water movement below the surface and the soil water movement continues after an infiltration event, as the infiltrated water is redistributed. Infiltration and percolation play a key role in surface runoff, groundwater recharge, evapotranspiration, soil erosion, and transport of chemicals in surface and subsurface waters. 1  CE 118 HYDROLOGY I. FACTORS AFFECTING INFILTRATION Infiltration rates vary widely. It is dependent on the condition of the land surface (cracked, crusted, compacted, etc.), land vegetation cover, surface soil characteristics (grain size & gradation), storm characteristics (intensity, duration & magnitude), surface soil and water temperature, chemical properties of the water and soil. Surface and Soil Factors The surface factors are those that affect the movement of water through the air-soil interface. Cover material protect the soil surface. Bare soil leads to the formation of a surface crust under the impact of raindrops or other factors, which breakdown the soil structure and move soil fines into the surface or near-surface pores. Once formed, a crust impedes infiltration. Figure 4.1 illustrates that the removal of the surface cover (straw or burlap) reduces the steady-state infiltration rate from approximately 3 to 4 cm/hr. to less than 1 cm/hr. Figure 4.2 illustrates the difference between crusted, tilled, grass cover soil on the infiltration curve. The bare tilled soil has higher infiltration than a crusted soil initially; however, its steady-state rate approaches that of the crusted soil because a crust is developing. Also, the grass-covered soil has a higher rate than a crusted soil partially because the grass protects the soil from crusting. Natural processes such as soil erosion or man-made processes such as tillage, overgrazing and deforestation can cause change in soil surface configurations. 2  CE 118 HYDROLOGY The soil properties affecting soil water movement are hydraulic conductivity (a measure of the soil’s ability to transmit water) and water-retention characteristics (the ability of the soil to store and release water). These soil water properties are closely related to soil physical properties. II. MEASUREMENTS OF INFILTRATION Infiltration is a very complex process, which can vary temporally and spatially. Selection of measurement techniques and data analysis techniques should consider these effects, and their spatial dimensions can categorize infiltration measurement techniques. A brief introduction of infiltration measurement techniques is described below. A) Areal measurement Areal infiltration estimation is accomplished by analysis of rainfall-runoff data from a watershed. For a storm with a single runoff peak, the procedure resembles that of the calculation of a  index (see further index method). The rainfall hyetograph is integrated to calculate the total rainfall volume. Likewise, the runoff hydrograph is integrated to calculate the runoff volume. The infiltration volume is obtained by subtracting runoff volume from rainfall volume. The average infiltration rate is obtained by dividing infiltration volume by rainfall duration. 3  CE 118 HYDROLOGY B) Point measurement Point Infiltration measurements are normally made by applying water at a specific site to a finite area and measuring the intake of the soil. There are four types of infiltrometers: the ponded-water ring or cylinder type, the sprinkler type, the tension type, and the furrow type. An infiltrometer should be chosen that replicates the system being investigated. For example, ring infiltrometers should be used to determine infiltration rates for inundated soils such as flood irrigation or pond seepage. Sprinkler infiltrometers should be used where the effect of rainfall on surface conditions influences the infiltration rate. Tension infiltrometers are used to determine the infiltration rates of soil matrix in the presence of macropores. Furrow infiltrometers are used when the effect of flowing water is important, as in furrow irrigation. Ring or Cylinder Infiltrometers -These infiltrometers are usually metal rings with a diameter of 30 to 100 cm and a height of 20 cm. The ring is driven into the ground about 5 cm, water is applied inside the ring with a constant-head device, and intake measurements are recorded until a constant rate of infiltration is attained. To help eliminate the effect of lateral spreading use a double ring infiltrometer, which is a ring infiltrometer with a second larger ring around it. Sprinkler infiltrometer - Rain simulator - With the help of a rain simulator, water is sprinkled at a uniform rate more than the infiltration capacity, over a certain experimental area. The resultant runoff R is observed, and from that the infiltration of using f = (P-R)/t. Where P = Rain sprinkled, R = runoff collected, and t = duration of rainfall. Sample Problem No. 1: A USGS rain-simulator infiltrometer experiment was conducted on sandy loam soil. Rainfall was simulated at the rate of 20 cm/hr. The rainfall and runoff data are given in Table. (a) Find and plot the mass-infiltration curve from the experimental data. (b) Plot an infiltration rate curve. Elapsed t (Time in Simulated Measured F=P-R f = F/t time hrs.) rainfall runoff (Cumulative (Infiltration (mins) (cm) (cm) infiltration in rate in cm) cm/hr.) 0 0 0 0 0 0 5 0.083 1.67 0.84 0.83 10.0 10 0.167 3.33 1.76 1.57 9.41 15 0.250 5.00 2.76 2.24 8.96 20 0.333 6.67 3.77 2.90 8.71 25 0.417 8.33 4.85 3.48 8.35 30 0.500 10.00 5.93 4.07 8.14 60 1.000 20.00 13.27 6.73 6.73 90 1.500 30.00 21.15 8.85 5.90 120 2.000 40.00 29.33 10.67 5.34 150 2.500 50.00 37.87 12.13 4.85 4  CE 118 HYDROLOGY III. ESTIMATING INFLITRATION RATE 1) Horton infiltration In general, for a given constant storm, infiltration rates tend to decrease with time. The initial infiltration rate is the rate prevailing at the beginning of the storm and is maximum. Infiltration rates gradually decrease in time and reach a constant value. Horton observed the above facts and concluded that infiltration begins at some rate f o and exponentially decreases until it reaches a constant fc. He proposed the following infiltration equation where rainfall intensity i greater than fp at all times. where: fp = infiltration capacity in mm/hr at any time t fo = initial infiltration capacity in mm/hr fc = final constant infiltration capacity mm/hr at saturation, dependent on soil type and vegetation t = time in hour from the beginning of rainfall k = an exponential decay constant dependent on soil type and vegetation. Note that infiltration takes place at capacity rates only when the intensity of rainfall i equals or exceeds fp; that is f =fp when i  fp, but when i < fp, f < fp and f = i. 5  CE 118 HYDROLOGY The cumulative infiltration equation F(t) for the Horton method is found from the relationship d(F(t)/dt = f(t) = fp and is given by Indicative values for fo, fc, and k are given in Table below: Sample Problem No. 2: The initial infiltration capacity fo of a watershed is estimated as 1.5 mm/hr, and the time constant is taken to be 0.35 hr-1. The equilibrium capacity fc is 0.2 mm/hr. Use Horton’s equation to find a) the values at t= 10mins, 30mins, 1hr, 2hrs, and 6hrs. b) the total volume of infiltration over the 6-hr period. 6  CE 118 HYDROLOGY 2) The -Index Method The -index is the simplest method and is calculated by finding infiltration as a difference between gross rainfall and observed surface runoff. The -index method assumes that the loss is uniformly distributed across the rainfall pattern. Sample Problem No. 3: For the given rainfall data, compute the runoff volume if the  index is 1.0 in/hr. The watershed area is 0.875 mi2. Time (hr) i (in/hr) 0-2 1.40 2-5 2.30 5-7 1.10 7-10 0.70 10-12 0.30 7