Infiltration, Surface Runoff PDF

INFILTRATION soil changes abruptly from its initial state to the newly wet soil, denotes the boundary of the wetting zone. The The process of water entering soil due to...

INFILTRATION soil changes abruptly from its initial state to the newly wet soil, denotes the boundary of the wetting zone. The The process of water entering soil due to wetting front can range in length from a few centimeters irrigation or rainfall is known as infiltration. to meters, depending on the rate of infiltration and the Water moves through the soil from one place to physical characteristics of the soil. another through a process called percolation. The actual rate at which water percolates through the soil during a storm is known as the An easy-to-understand infiltration rate, and it needs to match either the analogy can help you rainfall rate or the infiltration capacities, understand the infiltration whichever is lower. The maximum rate at which process. Think about the a soil can absorb water under any given container in the figure that circumstance is known as its infiltration has wire gauze inside of it. capacity. When water is poured into The rate of soil water movement beneath the a container, some of it will surface largely determines the rate of fill it and some of it will overflow. Moreover, the container infiltration, and this movement of water persists has a limited capacity and cannot accommodate after an infiltration event as the infiltrated water additional flow once it is filled. This analogy, despite its is redistributed. excessive simplification, highlights two crucial points: (a) Surface runoff, evapotranspiration, soil erosion, the infiltration capacity, which is the maximum rate at groundwater recharge, and chemical transport which the ground can absorb water; and (b) the field in surface and subsurface waters are all capacity, which is the maximum volume of water that significantly influenced by infiltration and the ground can hold. percolation. The procedure can be schematically represented in the figure, where two scenarios— low intensity and high Zone 1 – A thin layer of saturated zone forms at the top. intensity rainfall—are taken into consideration. This is Zone 2 - There is a transition zone underneath zone 1. because the infiltrated water may increase soil moisture while also potentially contributing to ground water Zone 3 – The transmission zone, which is the next lower discharge. zone, is where the moisture moves downward. This zone has a moisture content that is higher than field capacity but not quite saturated. Its relatively uniform moisture content and unsaturated flow are further characteristics. Zone 4 – The wetting zone comes last. This zone's soil moisture content will be at or close to field capacity, and it will get progressively less as you go deeper. The wetting front, which is the line where the moisture content of the HYDROLOGY (HYDN02E) Gette 1  MEASUREMENT OF INFILTRATION in each case. Using water budget equation involving the volume of rainfall, infiltration and runoff, the infiltration FLOODINGTYPE INFILTROMETERS rate and its variation with time are estimated. Flooding type infiltrometers are experimental HYDROGRAPH ANALYSIS devices used to obtain data relating to variation of infiltration capacity with time. Two types of flooding Reasonable estimation of the infiltration infiltrometers are tube type and double ring infiltrometer capacity of a small watershed can be obtained by analyzing measured runoff hydrographs and SIMPLE (TUBETYPE) INFILTROMETER corresponding rainfall records. If sufficiently good rainfall This is a simple instrument consisting essentially records and runoff hydrographs corresponding to of a metal cylinder, 30 𝑐𝑚 diameter and 60 𝑐𝑚 long, isolated storms in a small watershed with fairly open at both ends. The cylinder is driven into the ground homogeneous soils are available, water budget to a depth of 50 𝑐𝑚. Water is poured into the top part to equations can be applied to estimate the abstraction by a depth of 5 𝑐𝑚 and a pointer is set to mark the water infiltration. level. As infiltration proceeds, the volume is made up by RUNOFF adding water a burette to keep the water level at the tip of the pointer. Knowing the volume of water added Runoff is the term used to describe how during different time intervals, the plot of the infiltration precipitation exits a catchment area and flows capacity vs time is obtained. The experiments are through a surface channel. As a result, it shows continued until a uniform rate of infiltration is obtained the catchment's output for a specific time and this may take 2 − 3 ℎ𝑜𝑢𝑟𝑠. period. Before runoff begins, the following conditions DOUBLE RING INFILTROMETER must be met for a given precipitation: This is the most commonly used infiltrometer is evapotranspiration, initial loss, infiltration, and designed to overcome the basic objection of the tube detention storage. The extra precipitation infiltrometer which is that the tube area is not the travels across the land surfaces to smaller representative of the infiltrating area. In this, two sets of channels once these are met. This part of the concentrating rings with diameter of 30 𝑐𝑚 and 60 𝑐𝑚 runoff is referred to as overland flow, and it and a minimum length of 25 𝑐𝑚. The two rings are entails the accumulation of a storage area inserted into the ground and water is applied into both above the surface and its subsequent drainage. rings to maintain constant depth of about 5 𝑐𝑚. The Overland flow typically occurs in the laminar outer rings provides water jackets to the infiltering water regime and has modest lengths and depths. from the inner ring and hence prevents the spreading Until the flow reaches the catchment outlet, out of the infiltering water of the inner ring. The water flows from several small channels join larger depth of the inner and outer rings are kept the same channels, and flows from these in turn combine during the observation period. The measurement of the to form a larger stream. Surface runoff is the water volume is done on the inner ring only flow that occurs in this mode, where it constantly moves across the surface as RAINFALL SIMULATOR overland flow, through the channels as open- In this a small plot of land, of about 2 𝑚 × 4 𝑚 channel flow, and out of the catchment outlet. size, is provided with a series of nozzles on the longer side with the arrangements to collects and measure surface runoff rate. The specially designed nozzles produce raindrops falling from a height of 2 𝑚 and are capable of producing various intensities of rainfall. Experiments are conducted under controlled conditions with various combination of intensities and durations and the surface runoff rates and volumes are measured HYDROLOGY (HYDN02E) Gette 2 Based on the time delay between precipitation and SCS-CN METHOD runoff, the runoff is classified into two categories: as Concepts for SCS-CN Method DIRECT RUNOFF The ratio of actual amount of direct runoff (𝑄) to It is the portion of runoff that gets into the maximum potential runoff (= 𝑃 − 𝐼𝑎) is equal to the ratio stream right away following a downpour. Surface runoff, of actual infiltration 𝐹 to the potential maximum immediate interflow, and precipitation on the stream's retention (𝑆) surface are all included. Direct runoff is the flow that 𝑸 𝑭 enters the stream as a result of snowmelt. Direct runoff = 𝑷 − 𝑰𝒂 𝑺 is occasionally referred to by terms like storm runoff and The amount of initial abstraction (𝐼𝑎) is some direct storm runoff. fraction of the potential maximum retention (𝑆) BASE FLOW 𝐼𝑎 = 𝜆𝑆 Base flow is the delayed flow that essentially (𝑃 − 𝐼𝑎 )2 (𝑃 − 𝜆𝑆)2 acts as groundwater flow and reaches a stream. This 𝑄= = 𝑓𝑜𝑟 𝑃 > 𝜆𝑆 𝑃 − 𝐼𝑎 + 𝑆 𝑃 + (1 − 𝜆)𝑆 category also includes many-time delayed interflow. The 𝑄 = 0 𝑓𝑜𝑟 𝑃 ≤ 𝜆𝑆 base flow of a perennial stream can be easily identified in its annual hydrograph as the stream's gradually Curve Number decreasing flow during a dry spell. The catchment's soil, vegetation, and land use complex, as well as the antecedent soil moisture condition in the catchment immediately before the start The response of a catchment to precipitation, or of the rainfall event, determine the parameter 𝑆, which runoff, is a reflection of the combined influence represents the potential maximum retention. of various rainfall, climate, and catchment characteristics. Thus, stream flow in its natural 25400 100 𝑆= − 254 = 254 ( − 1) state is what we refer to as natural flow or virgin 𝐶𝑁 𝐶𝑁 flow. Natural flow is defined as a stream's flow 25400 𝐶𝑁 = that is unaffected by human constructions, such 𝑆 + 254 as reservoirs and diversion structures. When a A CN value of 100 represents a condition of zero stream has storage or diversion structures, the potential retention and a value of 0 represents an downstream channel's flow is influenced by the infinitely abstracting catchment. hydraulic and operational properties of these Soil structures, which means it doesn't accurately reflect the actual runoff. The hydrological soil classification is used to The natural flow volume in time∆∆𝑡 at the determine CN. The infiltration and other features are terminal point of a catchment is expressed by used to categorize the soils into four classes: A, B, C, and water balance equation as D. The effective soil depth, average clay content, infiltration properties, and permeability are significant 𝑹𝒏 = (𝑹𝒐 − 𝑽𝒓 ) + 𝑽𝒅 + 𝑬 + 𝑬𝒙 + ∆𝑺 soil attributes that affect hydrological classification. ESTIMATING RUNOFF VOLUME Group A (Low Runoff Potential) High penetration SCS-CN method, developed by Soil rates in soils that are mostly composed of deep, Conservation Services of USA in 1969, is a simple, well-to-excessively-drained sands or gravels, predictable, and stable conceptual method for even when they are heavily saturated. There is a estimation of direct runoff depth based on storm rainfall high rate of water transmission in these soils. depth. This method is based on the water balance equation of the rainfall in a known interval of time, ∆𝑡. 𝑷 = 𝑰𝒂 + 𝑭 + 𝑸 HYDROLOGY (HYDN02E) Gette 3  Land Use Group B (Moderately Low Runoff Potential) Soils Runoff Curve Number [𝐶𝑁𝐼𝐼 ] for Hydrologic Soil that are mostly moderately deep to deeply, Cover Complexes [Under AMC-II Conditions] moderately well to well-drained, and have textures that range from moderately fine to moderately coarse, and that have moderate infiltration rates when fully wetted. Group C (Moderately High Runoff Potential) Soils that are mostly moderately deep to deeply, moderately well to well drained, and have textures that range from moderately fine to moderately coarse, but have low rates of infiltration when fully wet. Group D (High Runoff Potential) Soils with a permanent high water table, clay soils with a high swelling potential, shallow soils over nearly impermeable material, and soils with a clay pan or layer at or near the surface are the main 𝐶𝑁𝐼𝐼 Values for Sugarcane types of soils with extremely low infiltration rates when fully wetted. Antecedent Moisture Condition (AMC) The moisture content of the soil at the start of the rainfall-runoff event under consideration is referred to as the antecedent moisture condition. It is commonly 𝐶𝑁𝐼𝐼 Values for Suburban and Urban Land Use known that AMC controls both initial abstraction and infiltration. Total Rain in Previous 5 days AMC Type Dorman Season Growing Season I Less than 13 mm Less than 36 mm II 13 to 28 mm 36 to 53 mm III More than 28 mm More than 53 mm For AMC-I AMC-I Soils are dry but not to wilting point. Satisfactory 𝐶𝑁𝐼𝐼 𝐶𝑁𝐼 = cultivation has taken place. 2.281 − 0.01281𝐶𝑁𝐼𝐼 AMC-II Average Condition For AMC-III 𝐶𝑁𝐼𝐼 AMC-III Sufficient rainfall has occurred within the 𝐶𝑁𝐼𝐼𝐼 = 0.427 + 0.00573𝐶𝑁𝐼𝐼 immediate past 5 days. Saturated soil conditions prevail. Value of 𝜆 On the basis of extensive measurements in small size catchments SCS adopted 𝜆 = 0.2 as a standard value. (𝑃 − 0.2𝑆)2 𝑄= 𝑓𝑜𝑟 𝑃 > 0.2𝑆 𝑃 + 0.8𝑆 HYDROLOGY (HYDN02E) Gette 4  RAINFALL-RUNOFF CORRELATION RUNOFF CHARACTERISTIC OF STREAMS Many factors related to the climate and A study of annual hydrographs of streams enables one catchment affect the complex relationship between to classify streams into three classes namely, perennial, rainfall and corresponding runoff during a given period. intermittent and ephemeral stream. In addition, the lack of data necessitated the use of basic Perennial Stream correlations in order to estimate runoff accurately. One kind of watercourse that runs Correlating seasonal or annual measured runoff values continuously all year round is a perennial with corresponding rainfall values is one of the most stream. The typical sources of water for widely used techniques. perennial streams are springs, groundwater, and steady rainfall, which keeps the streams flowing even in the dry months. These streams are vital to ecosystems because they support a The coefficient of correlation can be computed as: variety of plants and animals and are frequently essential to the hydrological cycle, which supplies water to neighboring communities and HYDROGRAPH supports agriculture and recreation. Intermittent Stream Streams with baseflow on average once a year or more. The groundwater table is usually higher in the winter and spring, which increases the possibility that the groundwater level is higher than the stream channel's bed. As a result, an intermittent stream will typically have baseflow in the winter and spring and occasional baseflow in other seasons. Intermittent streams often have less developed physical, hydrological, and biological features A hydrograph is a plot of a stream's discharge measured than perennial streams due to their irregular chronologically against time. Depending on the time flow regimes. frame in question, there are Ephemeral Stream Streams that convey flow only during, Annual hydrographs showing variation of daily immediately following, and in response to a or weekly or 10 daily mean flows over a year. precipitation event that occurs above the Monthly hydrographs showing the variation of groundwater table. daily mean flows over a month. Seasonal hydrographs depicting the variation of the discharge in a particular season such as the monsoon season or dry season. Flood hydrographs or hydrographs due to a storm representing stream flow due to a storm over a catchment Studies of reservoirs, droughts, and streams' surface water potential can all benefit from the use of annual and seasonal hydrographs. A crucial tool for examining stream characteristics linked to floods is the flood hydrograph. HYDROLOGY (HYDN02E) Gette 5  ELEMENTS OF HYDROGRAPHS BASE FLOW SEPARATION – METHOD I (STRAIGHT-LINE METHOD) RISING LIMB A hydrograph's rising limb, sometimes referred to as the concentration curve, shows how the steady accumulation of storage in channels and across the catchmentsurface causes an increase in discharge. CREST SEGMENT By connecting the starting point of the surface Since the crest segment of a hydrograph runoff to a point on the recession limb that represents the end of the direct runoff, a straight line can be used in contains the peak flow, it is one of the most significant sections. When runoff from different catchment areas this method to separate the base flow. The start of the simultaneously contributes amounts to reach the direct runoff is shown by point A in the figure, and it is typically simple to locate due to the abrupt change in the maximum amount of flow at the basin outlet, the peak flow happens. runoff rate at that location. It is a little challenging to pinpoint Point B, which indicates the end of the direct RECESSION/FALLING LIMB runoff. 𝑁 = 0.83𝐴(𝑑𝑟𝑎𝑖𝑛𝑎𝑔𝑒 𝑎𝑟𝑒𝑎) 0.2 The withdrawal of water from the storage that To indicate the base flow and surface runoff, a straight was accumulated in the basing during the hydrograph's line connects points A and B. earlier phase isrepresented by the recession limb. BASE FLOW SEPARATION – METHOD II BASE FLOW SEPARATION This method involves extending the base flow Many hydrograph analyses aim to establish a curve that existed before the surface runoff I until it relationship between the effective rainfall and the intersects the ordinate drawn at the peak. A straight line surface-flow hydrograph. After separating the quick- connects this point to point B. Sections AC and CB draw response flow from the slow-response runoff, the boundaries between surface runoff and base flow. surface-flow hydrograph can be obtained from the overall storm hydrograph. Because of its rapid response, BASE FLOW SEPARATION – METHOD III the interflow is typically thought of as a component of the In this method the base flow recession curve surface flow. To obtain the surface flow hydrograph, only after the depletion of the flood water is extended the base flow needs to be subtracted from the total backwards until it intersects the ordinate at the point of storm hydrograph. There are three methods of base- inflection. Points A and F are joined by an arbitrary flow separation and the selection of within the three smooth curve. This method of base-flow separation is depends upon the local practice. The surface runoff realistic in situations where the groundwater hydrograph obtained after the base-flow separation is contributions are significant and reach the stream known as direct runoff hydrograph (DRH). quickly. HYDROLOGY (HYDN02E) Gette 6