Module 1 In Hydrometeorology PDF

A Module in Hydrometeorology Prepared By: NATHANIEL R. ALIBUYOG Mariano Marcos State University College of Engineering City of Batac, Ilocos Norte August 2020  Module 1 Introducti...

A Module in Hydrometeorology Prepared By: NATHANIEL R. ALIBUYOG Mariano Marcos State University College of Engineering City of Batac, Ilocos Norte August 2020  Module 1 Introduction to Hydrometeorology Learning Outcomes At the end of the lesson, you should be able to: define hydrology and discuss the importance of hydrology in engineering field enumerate the domains of hydrology Illustrate and discuss the hydrologic cycle apply the water balance equation Learning Input 1.1 Definition of Hydrology Hydrology is a branch of science that deals with the occurrence, distribution, movement and properties of water of the earth and earth’s atmosphere. It comes from two Greek words: "hydro" and "logos" meaning "water" and "science". Engineering hydrology is an applied earth science that uses hydrologic principles in the solution of engineering problems arising from human exploitation of the water resources. Engineering hydrology seeks to establish relations defining the spatial, temporal, seasonal, annual, regional, or geographical variability of water. Engineering hydrology utilizes scientific knowledge and mathematical principles to solve water-related problems in society. These include water resources management, calculation and prediction of runoff volumes, river-forecasting, controlling river flooding or soil erosion estimation of spillway and reservoir capacities, study soil-water-plant relationships in agriculture, estimate available water supply, establish risks in sizing hydraulic structures and systems, etc. 1.2 Domains of Hydrology Hydrology as defined above is very broad and thus it is subdivided into various domains of study. The domains of hydrology include the following: Hydrometeorology – define as the study of the transfer of water and energy between land - water surfaces and the lower atmosphere. Hydrogeology – defined as the study of the presence and movement of water in aquifers. Ecohydrology – defined as the study of interactions between organisms and the various processes of the hydrologic cycle. Chemical hydrology – defined as the study of the water's chemical characteristics.  Surface hydrology – defined as the study of hydrologic processes on the Earth's surface. Hydroinformatics – defined as the application of informatics to hydrologic applications. 1.3 Basic activities in hydrology The following are basic activities in hydrology: The measurement of basic variables characterizing the quantity and quality of water The assessment of other related characteristics describing the properties of basins, rivers and the inland water bodies The collection, storage and processing of hydrologic data The development of hydrologic models and related technology 1.4 The Hydrologic Cycle The hydrologic cycle is the central focus of hydrology. The cycle has no beginning or end, and its many processes occur continuously. As shown in Figure 1.1, water evaporates from oceans and land surface to become part of the atmosphere; water vapor is transported and lifted in the atmosphere until it condenses and precipitates on the land or the oceans; precipitated water be intercepted by vegetation, become overland flow over the ground surface, infiltrate into the ground, flow through the soil as subsurface flow, and discharges into streams as surface runoff. Much of the intercepted water and surface runoff returns to the atmosphere through evaporation. The infiltrated water may percolate deeper to recharge groundwater, later emerging in springs or seeping into streams to form surface runoff, and finally flowing out to the sea or evaporating into the atmosphere as the hydrologic cycle continuous. Figure 1.1 The Hydrologic cycle There are five basic processes that are very important in the hydrologic cycle: evapotranspiration, condensation, precipitation, infiltration, and runoff. These occur simultaneously and, except for precipitation, continuously. 1. Water vapor enters the atmosphere through evaporation and transpiration. Evaporation is the process of water changing into water vapor, and occurs when radiant energy from the sun heats water, causing the water molecules to become so active that some of them rise into the atmosphere as vapor. Transpiration is the discharging of water vapor into the atmosphere from living vegetation and occurs when plants take in water through the roots and release it through the leaves. The two processes combined are called evapotranspiration and can clean water by removing contaminants and pollution. Sublimation is the process of water changing directly from solid water to water vapor. 2. Once water vapor enters the atmosphere, it rises and cools. Water vapor in the air rises mostly by convection. This means that warm, humid air will rise, while cooler air will flow downward. As the warmer air rises, the water vapor will lose energy, causing its temperature to drop. As the water vapor cools, condensation (change from water vapor into liquid water) begins to form small drops of water (clouds). As these droplets bounce around and hit one another, they stick together and make larger drops. 3. When the drops of water become too heavy to be held up, they fall back to the earth and precipitation begins. Precipitation is water being released from clouds as rain, sleet, snow or hail, depending on the temperature 4. Once precipitation falls to the earth it begins to seep into the ground through the process called infiltration. The amount of water that infiltrates into the soil varies with land slope, the amount and type of vegetation, the soil and rock type, and whether the soil is already saturated with water. 5. Runoff includes the various ways by which water moves across the land. As it flows, the water may infiltrate into the ground, evaporate into the air, become stored in lakes, or be extracted for agricultural or other human uses. When the ground becomes saturated, the excess water drains into lakes, rivers, and oceans. This excess water is called runoff and the term describes the variety of ways by which water moves across the land. Runoff can also come from melted snow and ice. Surface water always travels towards the lowest point possible, usually the oceans. Along the way some water evaporates, percolates into the ground, or is used for agricultural, residential, or industrial purposes. Except the above five processes, there are some other processes that are important in the hydrologic cycle: Canopy interception is the precipitation that is intercepted by the plants and eventually evaporates back to the atmosphere.  Snowmelt refers to the runoff produced by melting snow. Subsurface Flow is the flow of water underground. Subsurface water may return to the surface or eventually seep into the oceans. Advection is the movement of water through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land. 1.5 Water Budget Equation Catchment Area is defined as the area of land draining into a stream or a water course at a given location. It is also known as drainage area or drainage basin. The areal extent of the catchment is obtained by tracing the ridge on a topographic map to delineate the catchment and measuring the area by a planimeter. Figure 1.2 Schematic sketch of catchment of River A at station M Water Budget Equation For a given problem area, say a catchment, in an interval of ∆t, the continuity equation for water in its various phases is written as Mass inflow – Mass outflow = change in mass storage If the density of the inflow, outflow and storage volumes are the same, then Vi – Vo = ∆S where: Vi is the inflow volume of water into the problem area during the time period Vo is the outflow volume of water from the problem area during the time period ∆S is the change in the storage of the volume over and under the given area during the given period. While realizing that all the terms in a hydrological water budget may not be known to the same degree of accuracy, an expression for the water budget of a catchment for a time interval of ∆t is written as: P – SR – G – E – T = ∆S where: P = precipitation SR = surface runoff G = net groundwater flow out of the catchment E = evaporation T = transpiration ∆S = change in storage The storage S consists of three components as S = Ss + Sm + Sg where: Ss = surface water storage Sm = water in storage as soil moisture Sg = water in storage as groundwater Thus, the change in storage can be written as: ∆S = ∆Ss + ∆Sm + ∆Sg Where all the terms will have the dimensions of volume. Note however that all these terms can be expressed as depth over the catchment area (e.g., centimeter). In terms of rainfall-runoff relationship, the water balance equation can also be written as SR = P – L where L = losses which include all water not available to runoff due to infiltration (causing addition to soil moisture and groundwater storage), evaporation, transpiration and surface storage. Example 1.1 A lake had a water surface elevation of 103.20 m above the 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 m3/s. Further, in that month, the lake received rainfall of 145 mm and the evaporation from the lake surface was estimated as 6.10 cm. 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. Given:  I = 6.0 m3/s Q = 6.5 m3/s P = 145 mm/month E = 6.10 cm/month A = 5000 ha Required: Water budget equation and the elevation of the surface of the lake Solution: In a time interval ∆t the water budget for the lake can be written as: Input volume – Output volume = change in storage of the lake (𝐼 ̅ Δ𝑡 + 𝑃𝐴) − (𝑄- Δ𝑡 + 𝐸𝐴) = Δ𝑆 where: 𝐼 =̅ average rate of inflow water into the lake 𝑄- = average rate of outflow from the lake P = precipitation E = evaporation A = average surface area of the lake ∆S = change in storage of volume of the lake Here, ∆t = 1 month = 30 days * 24 hrs/day * 60 min/hr * 60 s/min = 2.592 x 106 s = 2.592 Ms Inflow volume = 𝐼 ̅ Δ𝑡 = 6.0 m3/s * 2.592 Ms = 15. 552 M m3 Outlow volume = 𝑄- Δ𝑡 = 6.5 m3/s * 2.592 Ms = 16. 848 M m3 !"# %% ∗ #''' () ∗!',''' %! /() Input due to precipitation = 𝑃𝐴 = "" = 7.25 M m3 !''' " ,.!'.% ∗ #''' () ∗!',''' %! /() Outflow due to evaporation = 𝐸𝐴 = !''.%/% = 3.05 M m3 Hence, the change in storage is,  Change in storage, ∆S = (𝐼 ̅ Δ𝑡 + 𝑃𝐴) − (𝑄- Δ𝑡 + 𝐸𝐴) ∆S = (15.552 + 7.25) – (16.848 + 3.05) = 2.904 M m3 The change in elevation (∆z) is /0 2.3'" 4 %# ∆z =Δ𝑧 = 1 = #''' () ∗!',''' %! /() = 0.058 𝑚 The elevation of the lake at the end of the month is, Z = 103.2 m + 0.058 m Z = 103.258 m above the datum. Example 1.2 A small catchment of area 150 ha received a rainfall of 10.5 cm in 90 minutes due to storm. At the outlet of the catchment, the stream draining the catchment was dry before the storm and experience 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? What is the ratio of runoff to precipitation? Given: A = 150 ha P = 10.5 cm Q = 1.5 m3/s Required: a) Combined losses (infiltration, evaporation and transpiration) b) Ratio of runoff to precipitation Solution: The water budget equation for the catchment in a time ∆t is SR = P – L Where L = losses = water not available to runoff due to infiltration (causing addition to soil moisture and groundwater storage), evaporation, transpiration and surface storage. In the present case, ∆t = duration of the runoff = 10 hours Note that the rainfall occurred in the first 90 days (1.5 hours) and the rest 8.5 hours the precipitation was zero. a) Solving for the Losses: P = Input due to precipitation in 10 hours P = 150 ha * 10,000 m2/ha * 10.5 cm * 1m/100 cm = 157,000 m3 R = runoff volume = outflow at the catchment outlet in 10 hours R = 1.5 m3/s * 3600 s/hr * 10 hrs = 54,000 m3 Hence, Losses, L = 157,000 m3 – 54,000 m3 = 103,500 m3 b) Solving of the Ratio of runoff to precipitation 05 #",''' 6 = !#7''' = 0.343 This is the ratio is known as runoff coefficient which will be discussed in the succeeding module. 1.6 World Water Balance The following are the facts about the world water balance: The total quantity of water in the world is estimate to be about 1386 million cubic kilometers (M km3) About 96.5% of this water is contained in the ocean as saline water Some of the land amount to about 1% of the total water is also saline Only about 35 M km3 of fresh water is available Out of this, about 10.6 M km3 is both liquid and fresh and the remaining 24.4 M km3 is contained in frozen state as ice in polar regions and on mountain tops and glaciers The estimated quantities of global water distribution are presented in Table 1.1 and Table 1.2  Table 1.1 Estimated world water quantities Area Volume Total Water Fresh Water (106 km2) (M km3) (%) (%) 1. Oceans 3161.3 1338.0 96.5 - 2. Groundwater a. Fresh 134.8 10.530 0.76 30.1 b. Saline 134.8 12.870 0.93 - 3. Soil Moisture 82.0 0.0165 0.0012 0.05 4. Polar Ice 16.0 24.0235 1.7 68.6 5. Other ice and snow 0.3 0.3406 0.025 1.0 6. Lakes a. Fresh 1.2 0.0910 0.007 0.26 b. Saline 0.8 0.0854 0.006 - 7. Marshes 2.7 0.01147 0.008 0.03 8. Rivers 148.8 0.00212 0.0002 0.006 9. Biological Water 510.0 0.00112 0.0001 0.003 10. Atmospheric water 510.0 0.01290 0.001 0.04 Total: a) All kinds of water 510.0 1386.0 100.0 b) Fresh water 148.8 35.0 2.5 100.0 Source: Table from World water balance and water resources of the earth, UNESCO. 1975 Table 1.2 Global annual water balance Item Ocean Land 1. Area ( M km2) 361.30 148.8 2. Precipitation (km3/year) 458,000 119,000 (mm/year) 1270 800 3. Evaporation (km3/year) 505,000 72,000 (mm/year) 1400 484 4. Runoff to ocean 44,700 (i) Rivers (km3/year) 2,200 (ii) Groundwater (km3/year) 47,000 Total Runoff (km3/year) 316 Source: Table from World water balance and water resources of the earth, UNESCO. 1975 Table 1.3 shows the potential water resources across the different water resources region in the Philippines. It shows that water varies across region which could be attributed to several factors which we will discuss in the succeeding modules. Table 1.3 Potential water resources in the Philippines (Phil Environment Monitor 2003) Region Groundwater Surface Water Total Water Potential (MCM) Potential (MCM) Resources Potential (MCM) Northern Mindanao 2,116 29,000 31,116 Southern Mindanao 1,758 18,700 20,458 Western Visayas 1,144 14,200 15,344 Southeastern Mindanao 2,375 11,300 13,675 Western Mindanao 1,082 12,100 13,182 Eastern Visayas 2,557 9,350 11,907 Cagayan Valley 2,825 8,510 11,335 Central Luzon 1,721 7,890 9,611 Southern Tagalog 1,410 6,370 7,780 Ilocos 1,248 3,250 4,498 Bicol 1,085 3,060 4,145 Central Visayas 879 2,060 2,939 Total 20,200 125,790 145,990 Table 1.4 shows the units of the basic hydrologic variables. Each variable can be expressed in different units depending on the desired characteristics. Table 1.4 Hydrologic variables Variable Characteristics Units Precipitation Depth mm Intensity mm/hr Duration hr Evaporation Depth mm Intensity mm/hr Infiltration Depth mm Intensity mm/hr Runoff-Discharge Rate m3/s Volume m3 Equivalent height mm 1.7 Applications in Engineering Hydrology finds its greatest application in the design and operation of water resources engineering projects, such as: Irrigation Water supply Flood control Water power Navigation In all these projects, hydrological investigation for the proper assessment of the following factors are necessary: 1. The capacity of storage structures such as reservoirs 2. The magnitude of flood flows to enable safe disposal of the excess flow 3. The minimum flow and quantity of flow available at various seasons 4. The interaction of the flood wave and hydraulic structures, such as levees, reservoirs, barrages and bridges The hydrological study of a project should necessary precede structural and other detailed design studies. It involves the collection of relevant data and analysis of the data by applying the principle and theories of hydrology to seek solutions to practical problems. Many important projects in the past have failed due to improver assessment of the hydrological factors. Some typical failures of hydraulic structures are: Overtopping and consequent failure of an earthen dam due to an inadequate spillway capacity Failure of bridges and culverts due to excess flood flow Inability of a large reservoir to fill with water due to overestimation of the stream flow  Practice Task Answer the following questions 1. Describe the hydrologic cycle. Explain briefly the man’s interference in various parts of this cycle 2. What are significant features of global water balance studies 3. List the major activities in which hydrological studies are important. 4. 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 2.0 m3/s for 3 months, 3.0 m3/s for 6 months and 5.0 m3/s for 3 months. (i) What is the runoff coefficient of the catchment? (ii) If the afforestation of the catchment reduces the runoff coefficient to 0.50, what is the increase in the abstraction from precipitation due to infiltration, evaporation and transpiration, for the same annual rainfall of 120 cm? 5. A catchment has four sub-areas. The annual precipitation and evaporation from each of the sub-areas are given below. Assume that there is no change in the groundwater storage on an annual basis and calculate for the whole catchment the values of annual average (i) precipitation, and (ii) evaporation. What are the annual runoff coefficients for the sub-areas and for the total catchment taken as a whole?