Lecture 2 Hydrology PDF

Aquatic Ecosystem Hydrology Introduction to Hydrology Lecture 02 - Hydrology 1 Groundwater Concepts Lecture 02 - Hydrology 2 Groundwater Concepts Lecture 02 - Hydrology 3 Groundwater Concepts Lecture 02 - Hydrology 4 Groundwater Concepts...

Aquatic Ecosystem Hydrology Introduction to Hydrology Lecture 02 - Hydrology 1 Groundwater Concepts Lecture 02 - Hydrology 2 Groundwater Concepts Lecture 02 - Hydrology 3 Groundwater Concepts Lecture 02 - Hydrology 4 Groundwater Concepts Lecture 02 - Hydrology 5 Groundwater Concepts Lecture 02 - Hydrology 6 Groundwater Concepts ;Wet season: water table and rivers are high ;Dry season: water table and rivers are low springs and wells flow readily some springs and wells dry up Lecture 02 - Hydrology 7 Groundwater Concepts Any aquifer may be defined as a formation that contains sufficient saturated permeable material to yield significant quantity of water to wells Aquifers are usually of large areal extent and essentially underground storage reservoirs. the may be overlain or underlain by confined bed is relatively impermeable material adjacent to aquifer. Lecture 02 - Hydrology 8 Groundwater Concepts Aquifers are classified as unconfined or confined depending upon the presence or absence of a water table. An unconfined aquifer is one in which a water table serves as upper surface of zone of saturation, also known as a free, phreatic, or non- artesian aquifer. Lecture 02 - Hydrology 9 Groundwater Concepts Confined aquifer is one in which the groundwater is confined under pressure greater than atmospheric by overlying, relatively impermeable strata. Confined aquifers are also known as artesian or pressure aquifers Lecture 02 - Hydrology 10 Groundwater Recharge Water that replenishes aquifers Usually from surface water or precipitation that infiltrates, and then percolates through the vadose zone Recharge happens when percolating water finally reaches the water table, which is the top of the saturated zone. Above the water table is the unsaturated zone where water is held by capillary forces The root zone may capture some water that infiltrates and lift it back to the atmosphere. Lecture 02 - Hydrology 11  Scope of Hydrology Water is one the most valuable natural resources essential for human and animal life, industry and agriculture. It is also used for Power generation, navigation and fisheries Tremendous importance is given to the hydrology all over the world in the development and management of water resources for irrigation, water supply, flood control, and salinity control, Hydro power and navigation. Lecture 02 - Hydrology 12  USES OF ENGINEERING HYDROLOGY Hydrology is used to find out maximum probable flood at proposed sites e.g.. Dams. The variation of water production from catchments can be calculated and described by hydrology. Engineering hydrology enables us to find out the relationship between a catchment’s surface water and groundwater resources It helps us to know the required reservoir capacity to assure adequate water for irrigation or municipal water supply in droughts condition. Lecture 02 - Hydrology 13  Hydrological Processes Precipitation Evaporation Transpiration Infiltration Surface Runoff Groundwater outflow Lecture 02 - Hydrology 14 Hydrological equations In hydrology, a water balance equation (hydrologic equation) can be used to describe the flow of water in and out of a system. The law of water balance states that the inflows to any water system or area is equal to its outflows plus change in storage during a time interval. In hydrology , a water balance equation can be used to describe the flow of water in and out of a system. Lecture 02 - Hydrology 15 Hydrological equations A general water balance equation is: P = R + ET + ΔS where P is precipitation R is streamflow ET is evapotranspiration ΔS is the change in storage Lecture 02 - Hydrology 16 Hydrological equations Lecture 02 - Hydrology 17 Hydrological equations  EXAMPLE 1: For a given month, a 121 ha lake has 0.43 m3 /s of inflow, 0.37 m3 /s of outflow, and the total storage increase of 1.97 ha-m. A USGS gauge next to the lake recorded a total of 3.3 cm precipitation for the lake for the month. Assuming that infiltration loss is insignificant for the lake, determine the evaporation loss, in cm over the lake for the month. Lecture 02 - Hydrology 18 Hydrological equations  EXAMPLE 2:At a particular time, the storage in a river reach is 60 x 103 m 3. At that time, the inflow into the reach is 10 m3 /s and the outflow is 16 m3 /s. after two hours, the inflow and the outflow are 18 m 3 /s and 20 m3 /s respectively. Determine the change in storage during two hours period and the storage volume after two hours. Lecture 02 - Hydrology 19  Example 3 A lake had a water surface elevation of 103.200 m above datum at the beginning of a certain month. In that month, the lake received an average inflow of 6.0 m3/s from surface runoff sources. In the same period, the outflow from the lake had an average value of 6.5 m/s. Further, in that month, the lake received a rainfall of 145 mm and the evaporation from the lake surface was estimated as 6.10 cm. Lecture 02 - Hydrology 20  Example 3 cont. Write the water-budget equation for the lake and calculate the water surface elevation of the lake at the end of the month. The average lake-surface area can be taken as 5000 ha. Assume that there is no contribution to or from the groundwater storage. Lecture 02 - Hydrology 21  Example 4 In a given year, a catchment with an area of 2500 km2 received 1.3 m of precipitation. The average rate of flow measured in a river draining the catchment was 30 m3s-1 How much total river runoff occurred in the year (in m3)? What is the runoff coefficient? How much water is lost due to the combined effects of evaporation, transpiration, and infiltration? (Expressed in m).. Lecture 02 - Hydrology 22  Example 5  A small catchment of 150 ha area received a rainfall of 10.5 cm in 90 minutes due to a storm. At the outlet of the catchment, the stream draining the catchment was dry before the storm and experienced a runoff lasting for 10 hours with an average discharge of 1.5 m3/s. The stream was again dry after the runoff event. (a) What is the amount of water which was not available to runoff due to combined effect of infiltration, evaporation and transpiration? (b) What is the ratio of runoff to precipitation? Lecture 02 - Hydrology 23  Example 6 A catchment area of 140 km2 received 120 cm of rainfall in a year. At the outlet of the catchment, the flow in the stream draining the catchment was found to have an average rate of (i) 1.5 m3/s for the first 3 months, (ii) 2.0 m3/s for 6 months and (iii) 3.5 m3/s for the remaining 3 months Lecture 02 - Hydrology 24  (a) What is the runoff coefficient of the catchment? (ii) If the afforestation of the catchment reduces the runoff coefficient to 0.35, what is the increase in the abstraction from precipitation due to infiltration, evaporation and transpiration for the same annual rainfall of 120 cm? Lecture 02 - Hydrology 25 Global water budgets Lecture 02 - Hydrology 26  2 Precipitation The lifting of moist air masses in the atmosphere leads to the cooling and condensation which results in precipitation of water vapor from the atmosphere in the form of rain, snow, hail, and sleet. Raindrops falling to the ground are typically in the size range of 0.5–3 mm, while rain with droplet sizes less than 0.5 mm is called drizzle. Lecture 02 - Hydrology 27 Conditions for Occurrence of Precipitation . Cooling of air masses.  Formation of clouds due to condensation  Growth of water droplets. Accumulation of moisture Lecture 02 - Hydrology 28 Types of Precipitation Convective precipitation Orographic precipitation Cyclonic precipitation Convective precipitation :results from the lifting or upward movement of air that is warmer and lighter than its colder denser surrounding Lecture 02 - Hydrology 29  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. The typical thunderstorms resulting from heating of the atmosphere in the afternoon hours is the best example of convective rainfall. Thunderstorms occur throughout the world, especially in the summer. Lecture 02 - Hydrology 30 Convective precipitation Lecture 02 - Hydrology 31  Cyclonic precipitation Cyclonic precipitation results from the lifting of air converging into a low pressure area, or cyclone (figure 2). A cyclone is defined as an area of low pressure with a counter-clockwise (Northern Hemisphere) circulation of the air around it, usually inward, towards the center Lecture 02 - Hydrology 32  Cyclonic precipitation There are two kinds of cyclonic precipitation Non-frontal precipitation and frontal precipitation Non-frontal precipitation this phenomenon occurs when the moving mass of cold air meets the stationary warm nature of air mass. As a result of which the lighter warm air rises up and reaches the saturation point. The saturation point of lighter warm air causes precipitation known as Non-Frontal Precipitation. Lecture 02 - Hydrology 33 Cyclonic precipitation Lecture 02 - Hydrology 34 Cyclonic precipitation Frontal precipitation results when one air mass is lifted over another. A front is defined as the boundary between two air masses of different temperatures and densities Lecture 02 - Hydrology 35 Cyclonic precipitation Lecture 02 - Hydrology 36 Orographic precipitation orographic precipitation, rain, snow, or other precipitation produced when moist air is lifted as it moves over a mountain range. As the air rises and cools, orographic clouds form and serve as the source of the precipitation, most of which falls upwind of the mountain ridge. Lecture 02 - Hydrology 37 Orographic precipitation Lecture 02 - Hydrology 38  Precipitation Precipitation denotes all kinds of rainfall on surface of earth from vapor in the atmosphere (most of humidity at a height of 8 km from the Earth's surface). Lecture 02 - Hydrology 39 Precipitation rates vary geographically and over time Precipitation does not fall in the same amounts throughout the world, in a country, or even in a city. Here in Georgia, USA, it rains fairly evenly all during the year, around 40- 50 inches (102-127 centimeters (cm)) per year. Lecture 02 - Hydrology 40 Precipitation size and speed Have you ever watched a raindrop hit the ground during a large rainstorm and wondered how big the drop is and how fast it is falling? Or maybe you've wondered how small fog particles are and how they manage to float in the air. The table below shows the size, velocity of fall, and the density of particles (number of drops per square foot/square meter of air) for various types of precipitation, from fog to a cloudburst. Lecture 02 - Hydrology 41  Intensity Precipitation Median size Velocity of and speed Drops per inches/ diameter fall second hour (millimeters feet/second per square (cm/ ) (meters/seco foot hour) nd) (square meter) Fog 0.005 0.01 0.01 6,264,000 (0.013) (0.003) (67,425,000 Mist.002.1.7 2,510 (0.005) (.21) (27,000) Drizzle.01.96 13.5 14 (151) (0.025) (4.1) Light rain.04 1.24 15.7 26 (0.10) (4.8) (280) Moderate rain.15 1.60 18.7 46 (0.38) (5.7) (495) Heavy rain.60 2.05 22.0 46 (1.52) (6.7) (495) Excessive rain 1.60 2.40 24.0 76 (4.06) (7.3) (818) Cloudburst 4.00 2.85 25.9 113 (10.2) (7.9) (1,220) Lecture 02 - Hydrology 42 World distribution of precipitation The yearly precipitation averaged over the whole Earth is about 100 cm (39 inches), but this is distributed very unevenly. The regions of highest rainfall are found in the equatorial zone and the monsoon area of Southeast Asia. Middle latitudes receive moderate amounts of precipitation, but little falls in the desert regions of the subtropics and around the poles Lecture 02 - Hydrology 43 Measurement of Precipitation Measurement of Precipitation  All forms of precipitation are expressed in terms of the vertical depth of water that would accumulate on a level surface if all the precipitation were collected on it.  The simplest method of measuring precipitation is by setting up Rain gauges with known area and collecting & measuring at regular intervals. Lecture 02 - Hydrology 44 Measurement of Precipitation The following standard settings have to be adopted while selecting a site for the rain gauge station and installing the rain gauge. The site should be in an open space should not be installed on the top or the side of the hill A fence should be erected around the rain gauge station The gauge must always be mounted firmly Lecture 02 - Hydrology 45 Measurement of Precipitation Two kinds of rain gauges; 1. Non – recording type. 2. Recording type rain gauges. 1. Non - recording rain gauge: - they are non-recording because they do not record the rain but only collect the rain. Lecture 02 - Hydrology 46