Hydrology and Water Processes PDF

A view, called “Earthrise,” that greeted the Apollo 8 astronauts as their spacecraft emerged from behind the Moon. Bill Anders - the Apollo 8 astronaut who took the “Earthrise” photo, “We came all this way to explore the Moon, and the most important thing is that we discovered the Earth.” “...

A view, called “Earthrise,” that greeted the Apollo 8 astronauts as their spacecraft emerged from behind the Moon. Bill Anders - the Apollo 8 astronaut who took the “Earthrise” photo, “We came all this way to explore the Moon, and the most important thing is that we discovered the Earth.” “The Blue Marble” - perhaps the most widely reproduced image of Earth and was taken in December 1972 by the crew of Apollo 17 during the last lunar mission. These early views profoundly altered our - Africa and Arabia are prominent in this classic image Qanat is a horizontal dug well used to extract groundwater. It is a gently sloping tunnel dug through alluvial material leads water by gravity flow beneath the water table. Lengths can extend up to 20 km, but most are less than 5km. Hydrology is concerned with the study of the occurrence, distribution, movement, and chemistry of all waters of the earth. Hydrogeology studies the interrelationship of geologic materials and processes with water. Geohydrology is more properly used in the engineering field. It is moving among the oceans, the atmosphere, the solid Earth, and the biosphere. This unending circulation of Earth’s water supply is called the hydrologic cycle. The cycle shows us many critical interrelationships among different parts of the Earth system.. Evaporation - It is the process by which water changes from a liquid to a gas or vapor state. - Evaporation is the primary pathway that water moves from a liquid state back into the water cycle as atmospheric water vapor. Absolute Humidity - is the number of grams of water per cubic meter Saturation Humidity - maximum amount of moisture the air can hold in given temperature - directly proportional to the temperature of air Relative Humidity - percent ratio of absolute humidity to the saturation humidity for the temperature of the air mass As R.H. approaches 100%, evaporation ceases Saturation humidity of air (grams per cubic meter) If the absolute humidity remains constant, the relative humidity will rise. When it reaches 100% any further cooling will result in condensation. Dew point - temperature at which condensation will begin Transpiration Growing plants are continuously pumping water from the ground into the atmosphere through the process of transpiration. Soil moisture (osmotic pressure) -roots-leaves Stomata - opening surface of leaves where air may pass through Amount of transpiration is a function of the density and size of drainage basin Phreatophytes -Plants with taproot system extending to the water table Xerophytes - desert plants Hydrophytes - aquatic plants Evapotranspiration Evaporation and Transpiration are combined into the single term evapotranspiration because both processes involve the transfer of water from the land to the atmosphere, and they often occur simultaneously in natural environments. Evapotranspiration Evapotranspiration can be measured directly using a lysimeter - a large container holding soil and plants. It is set outdoors and the initial soil-water content is determined. Changes in soil-moisture storage reveal how much of the added water is lost to evapotranspiration Condensation When an air mass with a relative humidity lower than 100% is cooled without losing moisture, the relative humidity will approach 100% as the air approaches the dew-point temperature. When the air mass is saturated, condensation may start to occur. Condensation generally requires a surface or nucleus on which to form. E.g. clay minerals, salts, combustion products Precipitation Precipitation is any form of water particle, whether liquid or solid, that falls from the atmosphere and reaches the ground. Precipitation puts water in the watershed. The following are the parameters needed to initiate precipitation: 1. Humid air mass must be cooled to the dew-point temperature. 2. Condensation of freezing nuclei must be present. 3. Droplets must coalesce to form raindrops. 4. Raindrops must be of sufficient size when they leave the clouds to ensure that they will not totally evaporate before they reach the ground. Precipitation measurements help determine water availability for evaporation and streamflow, and the risk of forest fires, landslides, and soil erosion. Precipitation Precipitation is any form of water particle, whether liquid or solid, that falls from the atmosphere and reaches the ground. Precipitation puts water in the watershed. The following are the parameters needed to initiate precipitation: 1. Humid air mass must be cooled to the dew-point temperature. 2. Condensation of freezing nuclei must be present. 3. Droplets must coalesce to form raindrops. 4. Raindrops must be of sufficient size when they leave the clouds to ensure that they will not totally evaporate before they reach the ground. Precipitation measurements help determine water availability for evaporation and streamflow, and the risk of forest fires, landslides, and soil erosion. Precipitation There are three major types of precipitation: Cyclonic Precipitation: It is caused by lifting associated with the horizontal convergence of inflowing atmosphere into an area of low pressure. Convective Precipitation: It results when air that is warmer than its surrounding rises and cools. The precipitation is of a shower type, varying from light showers to cloudbursts. Orographic Precipitation: It is caused when air masses are lifted as they move over mountain barriers. Precipitation is generally heavier on the windward slope than on the leeward slope Rainfall Measuring Instruments/Equipment Scientists can measure precipitation directly—using groundbased instruments such as rain gauges—or indirectly—using remote sensing techniques (e.g., from radar systems, aircraft, and Earth-observing satellites). A. Rain Gauges (also known as pluviometer, ombrometer,and hyetometer) - measures the amount of precipitation at a given location. - Experiments have shown that the size of the opening has little effect on the catch, except for very small (less than 3 cm in diameter. - Catches of precipitation gauges is affected by high winds - Location/placement of rain gauge is critical. - Gauge should be placed as close to the ground as possible to avoid wind. Open area, away from tress and buildings. - On steep slopes, it may be desirable to have the opening parallel to the slope. Tipping-Bucket Type Rain Gauge: How does it work? This is a 30.5 cm size rain gauge adopted for use by the US Weather Bureau. The catch from the funnel falls onto one of a pair of small buckets. These buckets are so balanced that when 0.25 mm of rainfall collects in one bucket, it tips and brings the other one in position. The water from the tipped bucket is collected in a storage can. The tipping actuates an electrically driven pen to trace a record on the clockwork-driven chart. The water collected in the storage can is measured at regular intervals to provide the total rainfall and also serve as a check. It may be noted that the record from the tipping bucket gives data on the intensity of rainfall. Further, the instrument is ideally suited for digitalizing of the output signal Earth-observing satellites Radar can be used to measure the intensity of precipitation on the area. Satellites carry instruments designed to observe specific atmospheric characteristics such as cloud temperatures and precipitation particles, or hydrometeors. These data are extremely useful for filling in data gaps that exist between rain gauge Effective depth of precipitation is the amount of water that infiltrates through the soil and reaches the water table, recharging aquifers. This may be determined for time periods ranging from the duration of part of a single storm to a year. The data are generally measurements of precipitation at a number of points throughout the drainage basin. Data that are missing at one or more stations as a result of equipment malfunction or operator absence creates a problem. But there are equations and ways to solve the missing data. To solve the problem, three close precipitation stations with full records that are evenly spaced around the station with a missing record are used. E.g. The ff. equation yields an estimate of the missing data (Actual Precipitation at Station Z) using the mean annual precipitation data from three index stations, A, B, C, and Z and actual precipitation of A, B, & C: Effective Depth of Precipitation When a weather station's equipment breaks down or someone forgets to record data, we end up with missing information. To fix this, we can use data from nearby stations that have full records. Imagine we have four stations: A, B, C, and Z. Station Z is missing data, but stations A, B, and C have complete records. Here's how we can estimate the missing data for station Z: Look for three nearby stations (A, B, and C) that have good data and are spaced out around station Z. The equation yields an estimate of the missing data at station Z. The mean annual precipitation 👎 at station Z and the three indx stations (A.B,C) and the actual precipitation (P) on all the stations and missing By using the data from these nearby stations, we can make a good guess about what the missing data at station Z should be. Calculating the Average Precipitation To convert the point rainfall values at various stations into an average value over a catchment, the following three methods are in use: (i) Arithmetical-mean method, (ii) Thiessen-polygon method, and (iii) Isohyetal method. Arithmetic Method When the rainfall measured at various stations in a catchment show little variation, the average precipitation over the catchment area is taken as the arithmetic mean of the station values. Isohyetal Method If the rain gauge network is not uniform, then some adjustment is necessary. The more accurate method is to draw a precipitation contour map with lines of equal rainfall (isohyetal lines). Isohyetal lines for the rain gauge network of the previous figure. The isohyets show contours of equal rainfall depth, with a contour interval of 0.5 cm. The contours are based on simple linear interpolation. Isohyetal Method Step1: Determine what contours of equal precipitation (called isohytes) you will use. This varies from situation to situation, but you want to have as many contours as necessary to get an accurate model, but not so many that your construction becomes cluttered. Step2: Draw a line between gauges that will be separated by isohyets. Step3: Plot points on those lines that correspond to the isohyets determined in Step 2. Step4: Now sketch the isohyets. Step5: Redraw the construction onto graph paper with the isohyetal lines. Then count the boxes between each of the isohyetal lines. Step6: Find the actual watershed area between each isohyet. These areas will be lettered starting with A at the top and moving alphabetically toward the bottom of the construction. Step7: Multiply the areas found in Step 6 by the average precipitation in the area. Step8: Divide the sum of the values found in Step 7 by the total area of the watershed to get the average rainfall in the area. Theissen Method The Thiessen method to adjust non uniform gauge distribution uses a weighing factor (based on the area within the drainage basin) for each rain gauge. Construction of Thiessen polygons on the rain gauge network of the previous figure. - The stations are connected with lines. - The perpendicular bisector of each line is found. - The bisectors are extended to form the polygons around each station. This method is also used when there are a few rain gauge stations compared to size. The polygons are formed as follows: 1) The stations are plotted on a map of the area drawn to a scale. 2) The adjoining stations are connected by the dashed lines. 3) Perpendicular bisectors are constructed on each of these dashed lines. 4) These bisectors form polygons around each station (effective area for the station within the polygon). For stations close to the boundary, the boundary lines form the closing limit of the polygons Stream Hydrographs A stream hydrograph shows the discharge of a river at a single location as a function of time.  Hypothetical storm hydrograph for a period of evenly distributed precipitation, separated into Horton overland flow, baseflow, direct precipitation, and interflow Cross-section of a gaining stream (also called as effluent stream), which is typical of humid regions, where groundwater recharges streams. B. Cross-section of a losing stream (influent stream), which is typical of arid regions, where streams can recharge groundwater A stream that is gaining during low-flow periods can temporarily become a losing stream during flood stage. Types of Streams Perennial– flow all year round Intermittent – seasonal Ephemeral – only during and immediately after heavy rain When a contour line crosses a stream or river, it forms a "V" shape.  The point of the "V" always points upstream (towards higher elevation) and away from the direction of water flow. Direction of the "V": If the "V" is pointing up, this indicates that you are moving upstream towards the source of the water. If the "V" is pointing down, it means you are moving downstream, following the water's flow towards lower elevations Strahler’s Stream Order - Method classifying of ordering hierarchy of channels - There is no increase in order when a segment of one order is joined by another of a lower order Shreve’s Stream Magnitude - defines the magnitude of a channel segment as the total number of tributaries that feed it. Stream Magnitude provides a good estimate of relative stream discharge for small river systems. TERMS Depression Storage -precipitation that falls on the land surface may be temporarily stored as ice and snow or water in puddles → Overland flow -water that flows over the land surface as either diffuse sheet flow (laminar or mixed laminar flow) or concentrated flow (turbulent flow) in rills and gullies → Infiltration -is the process of water entering/seeping into the subsurface. Vadose Zone (Zone of Aeration) -is the region below the land surface where soil pores contain both air and water. → Vadose Water -water stored in the vadose zone → Belt of Soil Water -the top of the vadose zone where the roots of plants can reach. → Interflow - refers to the lateral flow of water in the vadose zone. Gravity Drainage -is the process when excess vadose water is pulled downward by gravity. Capillary fringe - is the lowest portion of the vadose zone where the pores of the soil are filled with capillary water so that the saturation approaches 100%; however, the water is held in place by capillary forces. → Water Table - the top of the zone of saturation → Groundwater - water stored in the zone of saturation -flows through the rock and soil layers of the earth until it discharges as a spring or as a seepage into a pond, lake, stream, river, or ocean. → Baseflow - groundwater contribution to a stream