Capiz State University CE 324 - Hydrology PDF

College of Engineering, Architecture and Technology CE 324 – HYDROLOGY A.Y. 2024 – 2025 PRECIPITATION Anggoling, Fahad O....

College of Engineering, Architecture and Technology CE 324 – HYDROLOGY A.Y. 2024 – 2025 PRECIPITATION Anggoling, Fahad O. Bernales, Geraldeen P. Castillo, Mariel U. Dasig, Gerick S. Delsocora, Angel P. Denosta, Clark James A. Dordas, Karren Grace I. Labto, Ryan A. Quistadio, Jorenn Hans P. Villareal, Michelle O. BSCE – 3B (GROUP 1) Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)  1. PRECIPITATION 1.1. Introduction 1.2. Formation of Precipitation 1.3. Different Forms of Precipitation 1.4. Different Types of Precipitation 1.5. Rainfall Characteristics  Depth  Duration  Intensity 1.6. Point Rainfall Measurements 1.7. Mass Rainfall Curve 1.8. Hyetograph 1.9. Different Types of Rain gauges 1.1. INTRODUCTION Precipitation is any liquid or frozen water that forms in the atmosphere and falls back to the earth. It is the primary input vector of the hydrologic cycle along with evaporation and infiltration. Its forms are rain, snow, and hail and variations of these such as drizzle and sleet. Precipitation is derived from atmospheric water, its form and quantity thus being influenced by the action of other climatic factors such as wind, temperature, and atmospheric pressure. Without precipitation, other hydrological processes such as infiltration, runoff, and groundwater recharge would cease to exist, leading to severe water scarcity. Its influence extends far beyond simply providing water for human consumption and agriculture. Understanding the characteristics and variability of precipitation is fundamental for a wide range of hydrological studies, including flood prediction, water resource management, climate modeling, and ecosystem assessment. The process of precipitation begins with the condensation of water vapor in the atmosphere, forming clouds as the air cools. When the droplets within these clouds coalesce and grow heavy enough, they fall to the ground due to gravitational forces. Factors such as temperature, humidity, and atmospheric pressure play vital roles in determining the type and amount of precipitation that a region receives. 1.2. FORMATION OF PRECIPITATION Two processes are considered to be capable of supporting the growth of droplets of sufficient mass (droplets from about 500 to 4000 μm in diameter) to overcome air resistance and consequently fall to the earth as precipitation. These are known as the ice crystal process and the collision- coalescence process. Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)  Ice Crystal Process The ice-crystal model, or Bergeron process, is the process of precipitation formation in the middle and high latitudes. Here, clouds form at altitudes where the temperatures are below the freezing point of water. In these clouds, water exists in its liquid form even though the temperatures are cold enough to freeze water. Water that has a temperature below freezing but is still in a liquid state is called "super-cooled water". Water in extremely small amounts such as cloud droplets can exist in such a state. Ice crystals are found co-existing with the super-cooled water in cold clouds. When this occurs, the ice crystals will grow at the expense of the water droplets. Why? Examine the saturation curve in Figure 1. It shows that at temperatures below freezing the saturation vapor pressure of ice is less than that over a droplet of water. This means that a water vapor gradient exists between the droplet and the ice. Water can evaporate off the droplet and deposit on the ice in response to the water vapor gradient. The droplet will dissipate in size while the ice crystal grows into a snow flake. Once the snow flake is large enough, it will fall to the surface. Thus, precipitation that falls in the middle and high latitudes starts out as snow. Whether it hits the surface as snow or rain depends on the temperature conditions through which the snowflake falls. Figure 1. Relationship between air temperature and vapor pressure at saturation Figure 2. Growth of ice crystals by deposition Collision-Coalescence Process Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)  The collision-coalescence model applies to warm clouds that form in the tropics. Warm clouds are those that form at altitudes where the air temperature is above freezing. For precipitation to form under this model, there needs to be a variety of different size condensation nuclei. Large condensation nuclei will create large water droplets while smaller condensation nuclei create small ones. In order for the droplets to make their way to the surface they have to be heavy enough to overcome the resistance imposed by upwardly rising air that is fueling the development of the cloud. The smaller, lighter droplets are easily suspended in the updrafts of air, while the larger heavy collector droplets fall and collide with the smaller ones. Upon collision, the droplets coalesce into a bigger droplet. As the droplet falls, resistance by the air flattens the droplet to the point where it becomes unstable and breaks apart. With enough collisions, the droplet achieves a size sufficient to fall all the way to the surface. Figure 3. Collision-coalescence of raindrops 1.3. DIFFERENT FORMS OF PRECIPITATION Precipitation is liquid or solid water falling from clouds to the Earth’s surface or formed on different bodies as a result of atmospheric water vapor condensation. Precipitation can be liquid, solid, or mixed. Liquid precipitation includes rain and drizzle. On the Earth’s surface or on different objects, liquid precipitation can be formed as dew or liquid film. Solid precipitation can be of forms that are more diverse. It falls as snow, hail, snow and ice pellets, ice needles, and ice crystals. At lower surface temperatures ice forming on solid objects are solid surface hydrometeors—frost, solid film, and ice. In free atmosphere, an analogue of such phenomena is airplane icing, when super- cooled cloud drops or precipitation freeze on the surface of an airplane. 1. Drizzle, sometimes called mist, consists of tiny liquid water droplets, usually with diameters between 0.1 and 0.5 mm, with such slow settling rates that they occasionally appear to float. Drizzle usually falls from low stratus and rarely exceeds 1 mm/hr. Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)  2. Rain consists of liquid water drops mostly larger than 0.5 mm in diameter. Rainfall usually refers to amounts of liquid precipitation. Light - rate of fall up to 2.5 mm/hr Moderate - from 2.8 to 7.6 mm/hr Heavy - over 7.6 mm/hr 3. Glaze is the ice coating, generally clear and smooth, formed on exposed surfaces by the freezing super cooled water deposited by rain or drizzle. 4. Rime is a white, opaque deposit of ice granules more or less separated by trapped air and formed by rapid freezing of super cooled water drops impinging on exposed objects. 5. Snow is composed of ice crystals, chiefly complex, branched hexagonal form, and often agglomerated into snowflakes, which may reach 100 mm in diameter. 6. Hail is precipitation in the form of balls of ice, produced in convective clouds, mostly cumulonimbus. Hailstones may be spherical, conical, or irregular shape, and range from about 5 to over 125 mm diameter. 7. Sleet consists of transparent, globular, solid grains of ice formed by the freezing of raindrops or refreezing of largely melted ice crystals falling through a layer of subfreezing air near the earth's surface. 1.4. DIFFERENT TYPES OF PRECIPITATION Precipitation is often typed according to the factor mainly responsible for the lifting which causes it. In nature, the effects of these various types of cooling are often interrelated, and the resulting precipitation cannot be identifies as being any one type. 1. Cyclonic precipitation results to from the lifting of air converging into low-pressure area, or cyclone. Cyclonic precipitation may be either frontal or non-frontal. Frontal precipitation results from the lifting of warm air on one side of a frontal surface over colder, denser air on the other side. Warm - front precipitation is formed in the warm air advancing upward over a cold air mass. Cold - front precipitation, on the other hand, is of showery nature and is formed in the warm air forced upward by an advancing mass of cold air, the leading edge of which is the surface cold front. 2. Convective precipitation is caused by the rising of warmer, lighter air in colder, denser surroundings. The difference in temperature may result from unequal heating at the surface, unequal cooling at the top of the air layer, or mechanical lifting when air is forced to pass over a denser, colder air mass or over a mountain barrier. Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)  3. Orographic precipitation results from a mechanical lifting over mountain barriers. In rugged terrain, the orographic influence is so marked that storm precipitation patters resemble that of mean annual precipitation. 1.5. RAINFALL CHARACTERISTICS Rainfall characteristics in hydrology include depth, duration, and intensity. These characteristics are important for understanding how rainfall contributes to the hydrologic cycle and soil water replenishment. DEPTH Rainfall depth is the total amount of rain that falls over a specific period of time, usually measured in millimeters or inches. The depth of rainfall in mm is measured as the distribution of rainfall per unit time in terms of a day, month, year, etc. The most common used measuring instruments are:  Standard rain gauge (non-recording)  Recording rain gauge (accumulated over a day)  Storage rain gauge (read at weekly or monthly intervals) Precipitation is expressed in terms of depth of total water obtained from the precipitation which would stand on an area. For example, 1 cm deep sheet of rainwater spread over an area of 1 sq km represents 104 m3 of volume of rainwater. When precipitation is less than 1 mm, it is recorded as trace. DURATION Rainfall duration is the amount of time that it rains, from the start to the end of a rainfall event. This includes any short breaks without rain that occur during the event. It plays a critical role in determining the hydrological response of a watershed to precipitation events. The storm duration must exceed the intensity duration, which defines the shortest time period of significant rainfall intensity within the storm event. Historically analyzed storms in a region can assist in setting appropriate durations. Different durations influence how quickly water is absorbed by the soil, as well as the timing of peak runoff in rivers and streams, which is crucial for flood risk assessment. INTENSITY The intensity of rainfall is the rate at which occurs and is measured in terms of depth per unit time. It is generally calculated from the data obtained from a weighing or siphon type of recording rain gauge. It is given by the slope of the curve of time versus cumulative depth obtained from a recording rain gauge. The common unit of measurement for intensity of rainfall is mm/h. The intensity is termed either light, moderate, or heavy depending on the respective rainfall intensities. Intensity may vary significantly within the same storm event, leading to differential impacts on hydrology and soil erosion. Higher intensity rainfall Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)  can lead to increased surface runoff and potential flooding, while lower intensity may promote infiltration and groundwater recharge. Additionally, rainfall intensity has a direct correlation with the size of raindrops, affecting erosion dynamics and vegetation damage rates in sensitive areas. 1.6. POINT RAINFALL MEASUREMENT POINT RAINFALL Rainfall recorded at a station is known as the point rainfall or the station rainfall. The data is represented in the forms of daily, weekly, monthly, seasonal or annual volumes (depth) of rainfall. These data can also be graphically represented in the form of bar diagrams. If the recorded data at a station is available for a period of more than 30 years, the normal monthly rainfall is calculated on a monthly basis or also on annual basis. The normal annual rainfall is the arithmetic average of the annual rainfall data collected at that station. The standard deviation of this data gives the variability of the recorded rainfall data. It is the general practice to revise this data on every 10 years basis by adding the new 10 years data and deleting the oldest 10 years of the data series. When the data on any year is less than the normal annual rainfall, it is called a dry year (deficient year) and when it is more, it is called a wet year (surplus year). For the analysis work, the rainfall data is generally presented in the forms of hyetograph (bar diagram), time series plot (chronological chart) or ordinate graphs by plotting the annual rainfall on the ordinate scale and time in years on the abscissa. To determine the trend pattern, present in the rainfall data series, or to smoothen out variations in the data series, moving average values are computed. The moving average values are the periodic averages of data lying in between the consecutive time intervals. Generally, the moving period (m) is taken as 3 or 5 years and the average value is plotted as the mid-value of the time interval. MEAN AVERAGE METHOD For hydrologic analysis, the average or the mean value of the total rainfall (precipitation) recorded at different stations over an area or a watershed is used, as each rain gauge represents only the point sampling of the area distribution of a storm. To obtain the average value of the data recorded at different stations in watershed or an area, the following three methods are generally followed: 1. Arithmetic mean method 2. Thiessen mean method 3. Isohyetal method The (mean) average values of rainfall depth obtained by these methods are also known as equivalent uniform depth (EUD). Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)  ARITHMETIC MEAN METHOD The arithmetic mean is the average of the precipitation data collected over a watershed (or an area) at different stations located within its boundary. It is calculated as Ʃ𝑃1 Pave = 𝑛 where Pave = average depth of rainfall over the area Ʃ𝑃1 = sum of rainfall amounts at individual rain-gauge stations n = number of rain-gauge stations in the area The arithmetic mean method is quick and easy to use, and can accommodate any number of stations reporting storm data. This method however ignores the spacing between stations as well as the orographical effects. Sometimes the method disregards the information available at nearby stations, and is used only when rainfall depths measured at various stations in a watershed show little variation. THIESSEN MEAN METHOD This method attempts to allow for non-uniform distribution of gauges by providing a weighting factor for each gauge. The stations are plotted on a base map and are connected by straight lines. Perpendicular bisectors are drawn to the straight lines, joining adjacent stations to form polygons, known as Thiessen polygons. The Thiessen mean method calculates the weighted average of the precipitation data based in the weightage given to the area closest to the gauging station. Each polygon area is assumed to be influenced by the rain gauge station inside it, i.e., if P1, P2, P3,.... are the rainfalls at the individual stations, and A1, A2, A3,.... are the areas of the polygons surrounding these stations, (influence areas) respectively, the average depth of rainfall for the entire basin is given by Ʃ(𝐴𝑟𝑒𝑎 ×𝑃𝑟𝑒𝑐𝑖𝑝𝑖𝑡𝑎𝑡𝑖𝑜𝑛) Pave = Ʃ𝐴𝑟𝑒𝑎 The results obtained are usually more accurate than those obtained by simple arithmetic averaging. The gauges should be properly located over the catchment to get regular shaped polygons. However, one of the serious limitations of the Thiessen method is its non-flexibility since a new Thiessen diagram has to be constructed every time if there is a change in the rain gauge network. The advantages of the Thiessen mean method are as follows: 1. The method also makes use of the data from nearby stations located outside the watershed. Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)  2. It allocates importance of measurement according to station spacing 3. It is easily adaptable on computers 4. Station weights remain constant when the same number of stations are used. The disadvantages of the Thiessen mean method are: 1. The method ignores the orographical effects 2. Whenever a new station is added, it leads to changes in the areas of polygons. Construction of Polygons 1. Join the adjacent rain gage Station A, B, C and D etc. of this area, thus, dividing the entire area in a series of triangle. 2. Draw the perpendicular bisector to each of these lines 3. The area enclosed by these perpendicular bisectors is served by respective rain gages. Thus, these perpendicular bisectors from a series of polygons around the rain gages stations and containing one and only one rain gage station in each polygon. 4. Find the value of rainfall un each station 5. Multiply the rainfall of each rain gage station by the respective areas of the polygon 6. Find the sum of all areas 7. Find the sum of all products 8. Compute the average precipitation using the formula ISOHYETAL METHOD The Isohyetal method calculates the weighted average of rainfall by considering the area between two consecutive lines joining points of equal rainfall magnitudes. These consecutive lines are called isohyets. The average rainfall between the successive isohyets taken as the average of the two isohyetal values are weighted with the area between the isohyets, added up and divided by the total area which gives the average depth of rainfall over the entire basin. Ʃ𝐴1−2 𝑃1−2 Pave = Ʃ𝐴1−2 where A1-2 = area between the two successive isohyets P1 and P2 𝑃1+ 𝑃2 P1-2 = 2 ƩA1-2 = A = total area of the basin The advantages of the Isohyetal mean method are: Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)  1. The method uses data from nearby stations located outside the watershed. 2. It allocates space according to station spacing as well as precipitation. 3. It is can show orographical effects. 4. It can take into account the storm morphology. 5. It is well adopted for pictorial presentation. 6. It yields information regarding aerial distribution between measurements. The disadvantages of the Isohyetal method are: 1. The method is tedious to apply. 2. It needs more time compared to other methods. 3. It is not very well suited for computer applications. 4. Whenever a new station is added, it leads to changes in the areas of polygons. 1.7. MASS RAINFALL CURVE The relationship between the accumulated precipitation and time is known as the mass rainfall curve. This relationship is drawn from the data obtained from a float type or a weighing bucket type rain gauge. For a large watershed, the average mass curve is drawn with the data of all recording rain gauges in the watershed. Mass rainfall curves are used for determination of (i) the duration and magnitude of a storm, and (ii) the intensities of rainfall at different time intervals during a storm by using the slope of the curve. Mass rainfall curves for the data of each recording rain gauge station in a watershed are plotted together on one graph sheet. From these curves, the average curve is constructed. Each curve is given an appropriate weightage depending on its location and nature. Stations located at greater distances from the watershed are given less weightage. 1.8. HYETOGRAPH A hyetograph is a graphical representation of the relationship between the rainfall intensity and time. It is a plot of the rainfall drawn on the ordinate axis against time on the abscissa axis. The hyetograph is a discrete representation of a rainfall hydrograph. Such representations of rainfall data are very convenient for hydrological analysis, especially for the estimation of design storms for prediction of floods. A hyetograph is constructed from an average mass curve of the watershed. To draw the hyetograph, a convenient time interval is chosen and for each of these time intervals, the corresponding reading of the accumulated rainfall is noted from the mass rainfall curve. From it, the rainfall intensity for Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)  that period is computed and these values of intensity are then plotted against the time interval to get the rainfall curve. 1.9. DIFFERENT TYPES OF RAIN GAUGES A rain gauge is a tool that measures how much rain falls in a certain area over a period of time. It works by collecting rainwater in a container and measuring how much water has accumulated. The measurement is usually given in millimeters. The primary purpose of a rain gauge is to collect and quantify the amount of rain that falls in a given area. It measures the liquid precipitation accurately by capturing rainwater in a specific container over a predetermined timeframe. This process enables meteorologists and hydrologists to monitor precipitation and analyze climate trends effective. Types of rain gauge: 1. Non-recording rain gauge 2. Recording rain gauge NON-RECORDING RAIN GAUGE It gives only total rainfall occurred during particular time period. Recording type rain- gauge gives hourly rainfall. Under non-recording type rain-gauges, one most commonly used in Symon’s rain-gauge. It gives the total rainfall that has occurred at a particular period. It essentially consists of a circular collecting area 127 mm in diameter connected to a funnel. The funnel discharges the rainfall into a receiving vessel. The funnel and the receiving vessel are housed in a metallic container. The components of this rain gauge are a shown in figure below. The water collected in the receiving bottle is measured by a graduated measuring jar with an accuracy of 0.1 ml. the rainfall is measured every day at 8:30 am IST and hence this rain gauge gives only Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)  depth of rainfall for previous 24 hours. During heavy rains, measurement is done 3 to 4 times a day. Thus Symons Rain gauge gives only the total depth of rainfall for previous 24 hours and doesn’t provide intensity and rainfall duration of the rainfall during different time interval of the day. RECORDING RAIN GAUGE These are rain gauges which can give a permanent, automatic rainfall record (without any bottle recording) in the form of a pen mounted on a clock driven chart. From the chart intensity or rate of rainfall in cm per hour or 6 hrs, 12 hrs…... besides the total amount of rainfall can be obtained. Advantages of recording rain gauges: 1. Necessity of an attendant does not arise 2. Intensity of rainfall at any time as well as total rainfall is obtained, whereas non-recording gauge gives only total rainfall. 3. Data from in accessible places (hilly regions) can be continuously obtained. Human errors are eliminated 4. Capacity of gauges is large 5. Time intervals are also recorded Disadvantages of recording rain gauges: 1. High initial investment cost 2. Recording is not reliable when faults in gauge arise (mechanical or electrical) till faults are corrected. Types of recording rain gauge: 1. Tipping bucket rain gauge 2. Weighing bucket rain gauge 3. Siphon or float type rain gauge (1) TIPPING BUCKET RAIN GAUGE This is the most common type of automatic rain gauge adopted by U S Meteorological Department. This consists of receiver draining into a funnel of 30 cm diameter. The catch (rainfall) from funnel falls into one of the pair of small buckets (tipping buckets). These buckets are so balanced that when 0.25 mm of rainfall collects in one bucket, it tips and brings the other bucket into position. Tipping of bucket completes an electric circuit causing the movement of pen to mark on clock driven receiving drum which carries a recorded sheet. Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)  These electric pulses generated are recorded at the control room far away from the rain gauge station. This instrument is further suited for digitalizing the output signal. The tipping bucket Rain gauge is quiet durable, simple to operate and convenient but it has following disadvantage:  It doesn’t give accurate result in case of intense rainfall, because some of rain which falls during the tipping of bucket is not measured.  Because of discontinuous nature of the record, the instrument is not satisfactory for using light drizzle or very light rain.  The time of beginning and ending of rainfall cannot be determined accurately.  This gauge is not suitable for measuring snow without heating the collector. (2) WEIGHING BUCKET RAIN GAUGE This is the most common type of recording or automatic rain gauge adopted by Indian Meteorological Department. The construction of this rain gauge is shown in figure below. It consists of a receiving bucket supported by a spring or lever. The receiving bucket is pushed down due to the increase in weight (due to accumulating rain fall). The pen attached to the arm continuously records the Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)  weight on a clock driven chart. The chart obtained from this rain gauge is a mass curve of rain fall. Mass curve of rainfall From the mass curve the average intensity of rainfall (cm/hr) can be obtained by calculating the slope of the curve at any instant of time. The patterns as well as total depth of rain fall at different instants can also be obtained. The advantages of this rain gauge are that it can record snow, hail and mixture of rain and snow. The disadvantages are:  The effect of temperature and friction on weighing mechanism may introduce error.  Failure of reverse mechanism results in loss of record.  Because of wind action on bucket, erotic traces may be recorded on the chart. (3) SIPHON OR FLOAT TYPE RAIN GAUGE This is also called integrating rain gauge as it depicts an integrated graph of rain fall with respect to time. The construction of this rain gauge is shown in figure below. A receiver and funnel arrangement drain the rainfall into a container, in which a float mechanism at the bottom is provided. As water accumulates, the Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)  float rises. A pen arm attached to the float mechanism continuously records the rainfall on a clock driven chart and also produces a mass curve of rain fall. When the water level rises above the crest of the siphon, the accumulated water in the container will be drained off by siphonic action. The rain gauge is ready to receive the new rainfall. OTHER TYPES OF RAIN GAUGE (A) U.S. STANDARD RAIN GAUGE The standard US National Weather Service rain gauge is a graduated cylinder with an 8 in funnel that fills a larger container. If rainwater overflows the inner cylinder, the larger outer container captures it. Measurements are taken, and excess water is poured into another cylinder to calculate total rainfall. A cone meter is sometimes used to prevent data leakage. The cylinder is usually marked in mm and can measure up to 250 mm (9.8 in) of rainfall in metric systems. Horizontal lines on the cylinder represent 0.5 mm (0.02 in) in Imperial units, while horizontal lines represent 0.01 in (0.25 mm) inches in metric systems). (B) PLUVIOMETER OF INTENSITIES The pluviometer of intensities, also known as Jardi's pluviometer, is a tool used to measure the average intensity of rainfall over a specific period. Initially designed for Catalonia, it has since spread worldwide. The instrument consists of a rotating drum, a graduated sheet of cardboard, and a pen driven by a buoy. As the rain falls, the water collected by the funnel falls into a container, raising the buoy and marking the cardboard. If the rainfall remains constant, the pen's mark is a horizontal line proportional to the amount of water that has fallen. When the pen reaches the top edge of the recording Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)  paper, the buoy is "up high in the tank," revealing the maximum flow that the apparatus can record. If the rain suddenly decreases, the container quickly lowers the buoy, resulting in a steep slope line that can reach the bottom of the recorded cardboard. The pluviometer of intensities has been used for over 95 years, particularly in Barcelona. (C) OPTICAL RAIN GAUGE This type of gauge has a row of collection funnels. In an enclosed space below each is a laser diode and a photo transistor detector. When enough water is collected to make a single drop, it drops from the bottom, falling into the laser beam path. The sensor is set at right angles to the laser so that enough light is scattered to be detected as a sudden flash of lights. The flashes from these photodetectors are then read and transmitted or recorded. Different type of optical range gauges have been used throughout the decades. The technology has also improved. (D) ACOUSTIC RAIN GAUGE Acoustic disdrometers, also referred to as hydrophones, are able to sense the sound signatures for each drop size as rain strikes a water surface within the gauge. Since each sound signature is unique, it is possible to invert the underwater sound field to estimate the drop-size distribution within the rain. Selected moments of the drop-size distribution yield rainfall rate, rainfall accumulation, and other rainfall properties. Accredited: Accrediting Agency of Chartered Colleges and Universities of the Philippines (AACCUP) Member: Philippine Association of State Universities and Colleges (PASUC) Agricultural Colleges Association of the Philippines (ACAP)