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University of Toronto St. George
Romila Verma

Chapter 1: Introduction: 1.1 Water-facts and figures  Hydrology is the science dealing with the waters of the earth, their occurrences, distributions and circulation, their chemical and physical properties and their interaction with the environment.  about 97% occurs as saline water in the seas and oceans -> only the remaining 3% is fresh water (more that half is locked up in ice-sheets and glaciers and another substantial volume occurs as virtually immobile deep groundwater)  the really mobile fresh water, which contributes frequently and actively to rainfall, evaporation and streamflow, thus represents only about 0.3% of the global total  important to recognize that the small volume of mobile fresh water is itself distributed unevenly in both space and time  wetland and prairie, forest and scrub, snowfield and desert, each exhibits different regimes of precipitation, evaporation and streamflow, each offers different challenges of understanding for the hydrologist and of water management for the planner an engineer, and each poses different benefits and threats to human life and livelihood as between the developed and the developing world 1.2 The changing nature of hydrology  the greatest advances came largely through work of Perrault and Mariotte, whose work on the Seine drainage basin in northern France demonstrated that contrary to earlier assumption, rainfall was more than adequate to account for river flow  and Halley showed that the total flow of springs and rivers could be more than accounted for by evaporation from the oceans  challenge of modeling the global atmospheric and hydrosphere circulation in order to better predict the hydrological and other consequences of climate variability and climate change  another challenge is the need to respond to the ever increasing demands for water  the margin b/w global available stocks of fresh water and the amount used by human activities will diminish further in the future -> already 10% of the world’s population is affected by chronic water scarcity  there is an urgent need for a must clearer understanding of the physical, economic, social, and political consequences of large-scale water resource development and of major irrigation and flood defense schemes which are designed to reduce the impact of drought and floor disasters  the need for sustainable development (ie development that meets the needs of the present without compromising the ability of future generations to meet their own needs)  increasing recognition that water must be used more efficiently -> one way is to regard water as both a natural resource and an economic commodity incurring costs of abstraction, treatment and transportation to where it is required 1.3 The hydrological cycle and system  the interdependence and continuous movement of all phases of water (ie liquid, solid, and gas form the basis of the hydrology cycle)  water is indestructible so the total quantity of water in the hydrological cycle cannot be diminished as it changes from water vapor to liquid or solid and back again  instead the processes of streamflow, groundwater flow and evaporation ensure the never ending transfer of water b/w land, ocean and atmosphere, followed by its return as precipitation to the earth’s surface  water vapour in the atmosphere condenses and may give rise to precipitation  the precipitation that reaches the ground surface may: o be stored in the forms of pools, puddles and surface water which are usually evaporated into the atmosphere quite quickly o be stored as snow and ice before melting or sublimation occurs, possibly after the lapse of many years or even centuries o flow over the surface into streams and lakes, from where it will move either by evaporation into the atmosphere, or by seepage toward the groundwater, or by further surface flow into the oceans o infiltrate through the ground surface to join existing soil water o -> either by evaporation from the soil and vegetation cover or by throughflow towards stream channels or by downward percolation to the underlying groundwater where it may be held for periods ranging from weeks to millennia o -> the groundwater component will eventually be removed either by upward capillary movement to the soil surface or to the root zone of the vegetation cover, whence it will be returned by evaporation to the atmosphere or by groundwater seepage and flow into surface streams and into the oceans  a short burst of hydrological activity for a week or so may be followed by a long period of virtual inactivity, apart from a slow redistribution of groundwater at some depth below the surface  in cold climates the time delay b/w snowfall and the active involvement of the precipitated moisture, after melting, in the subsequent phases of the hydrological cycle, may range from months (seasonal snowpacks) through centuries (valley glaciers) to millennia (Antarctic icecap) -> also increasingly the cycle us interrupted and modified by human activities  Inflow = Outflow +/- Storage  each drainage basin can be regarded as an individual system receiving quantifiable inputs of precipitation and transforming these via various flows and storages, into quantifiable outputs of evaporation and stream flow  virtually every component of the drainage basin hydrological system may be modified by human activity. The most important of these modifications result from: o large scale modifications of channel flow and storage, for example by means of surface changes such as afforestation, deforestation, and urbanization, which affect surface runoff and the incidence or magnitude of flooding o the widespread development of irrigation and land drainage o the large scale abstraction of groundwater and surface water for domestic and industrial uses Chapter 2 (Sections 2.1-2.3, 2.7): PRECIPITATION 2.1: Introduction and Definitions p.14  Precipitation controls the hydrology of a region  Knowledge of precipitation patterns in space and time is essential to understand soil moisture, ground water recharge and river flows  Hydrologists are interested in the distribution itself... how much precipitation occurs and when and where it falls.  Hail: special case of precipitation since even though it falls to the ground a solid, it normally does so in temperature conditions which favour rapid melting. Therefore, hydrologically it acts like a heavy rain shower.  Vapour: gas that is below its critical temperature so it may be easily condensed or liquefied by small change in temperature or pressure  Vapour pressure: amount of water vapour in the air o the warmer the air the greater amount of water vapour it can hold. Once max amount is exceeded (by such factors of cooling) then condensation can occur.  Degree of saturation: max amount of water vapour that the air can hold before it is saturated  Mainsource of atmospheric moisture is oceans  At any given moment the total amount of water vapour in the atmosphere represents only 0.001% of the worlds total. o Yet this source serves as a continuing source of supply in the form or precipitation.  Environmental lapse rate: variation of air temp with height  Adiabatic: no exchange of heat  Clouds: mass of minute water droplets/ice crystals suspended in the atmosphere appearing white and gray. o at any given time ~1/2 of the earth’s surface is covered by clouds o have a very important effect on the radiation balance due to their high albedo o cloud appearance expresses air movement and different weather conditions o in warm clouds the main cause for droplet growth are collision and coalescence o precipitation happens when a body of moist air is cooled sufficiently for it to become saturated and if condensation nuclei are present for water droplets/ice crystals to form to a certain size o common for several cloud types to occur together  fog: low level clouds form due to cooling of lower air by cold ground surface 2.2: Precipitation mechanisms p. 18  Frontal and Cyclonic Precipitation o Precipitation often occurs as the result of large scale weather systems o In these systems, precipitation occurs along narrow boundaries, fronts, between airmasses o Cyclonic Systems: comprise air masses rotating anti clockwise in the northern hemisphere and clock wise in the southern hemisphere. Found in extra tropical areas. o Frontal Precipitation: warm air is forced to rise up and over a wedge of colder dense air.  Cold Fronts: normally have steep frontal surfaces. Rapid lifting and short heavy rains.  Warm Fronts: usually less steep, more gradual lifting and cooling less intense longer rains.  Convectional Precipitation o Higher intensity and shorter duration than rain from frontal systems o Results when heating of the ground surface causes warming of the air and locally strong vertical air motions occur o If air is thermally unstable, it continues to rise and the resulting cooling, condensation and cloud formation may result in short-intense precipitation o Can be very significant for flood hydrology  Orographic Precipitation: o Aka relief precipitation. Results from mechanical lifting of moist air over barriers such as mountain ranges or islands in oceans. o Similar to that of a cold front. o More rain on windward (upword) than lee ward ( downside)  Frontal and convective type storms reach maximum intensity near the beginning, cyclonic near the middle. 2.3: General Spatial patterns of precipitation p. 20  Average annual precipitation overland is ~720mm.  Water vapour content in atmosphere declines systematically from the equator to polar-regions. o Increase of rain in summer b/c of greater heating and evaporation o Greater rainfall occurs in equatorial regions with trade winds and monsoon climates, here annual precipitation may exceed 3000 mm  Lowest rain fall in: o High lat polar areas- b/c of descending air masses and low water content in cold areas o Subtrobical areas, home to largest deserts. –b/c of high pressured cells giving rise to descending dry air. 2.7 Hydrological Aspects of Snow p. 54  75% of the world’s freshwater is in the form of snow or ice  6% of the global precipitation falls as snow  Snow has a cooling effect on the climate by increasing albedo and modifying the surface radiation balance and near surface air temperature. o Also causes greater amount of energy to be expended on melting o Snow provides a store of water and provides protective insulting for soil and crops in the winter  More important to know the water equivalent of snow than the depth.  Prediction of snowmelt is important to estimate seasonal flood risks and in arid regions estimate the amount and time of water to melt for moutanin sources for irrigation.  Snow melt occurs when snow pack takes place  Snow pack: desnity of snow increases, snow crystals become large grained and albedo decreases. o Snowpack is ripe once equalized temperature at 0 degrees Celsius throughout the vertical profile of the snow pack. Chapter 2 Questions (2.1-2.2): 2.1 Explain the importance of the very short residence time of atmospheric water for hydrology: - air absorb moisture through evaporation, the mean residence time is nine days - Atmospheric water = water vapour, which always present in the air no matter how dry the air appear to be. - easily condensed or liquefied by a comparatively small change in temperature or pressure (2.5hPa out of 985hPa) - Once the amount of water vapour that the air can hold is exceeded, condensation occur, this is known as the dew point - The total amount of water vapour is less than 0.001% of the world’s total stock, but this small amount serves as a continuing source of supply in the form of precipitation over the whole world. - As air cools at a rate of 9.8 degree Celsius per 1 km (dry adiabatic lapse rate), water vapour condense and clouds form when the air parcel reaches at a point at which it has cooled sufficiently - Clouds is an important factor for radiation balance because of its high albedo nature and for precipitation - Precipitation is a major factor controlling the hydrology of a region  It is the main input of water to the Earth’s surface and is important to understand soil moisture, groundwater recharge and river flows  Occurs in a number of forms: liquid and solid form: rainfall and drizzle; snow, hail, sleet. 2.2 Discuss the principal uplift mechanisms resulting in precipitation - Precipitation takes place when a body of moist air is cooled sufficiently for it to become saturated and, if condensation nuclei are present, for water droplets or ice crystals to form. 1. Frontal and Cyclonic precipitation  Outside of the tropics, precipitation occurs along the fronts (narrow boundaries) between the air masses, together with associated low pressure, cyclonic, systems where there is convergence and uplift of air  Two air masses of different temperature, humidity and density meet. The warm moist air is forced to rise up and over a wedge of colder denser air  For example, a tropical maritime air masses meet a polar air mass; in Western Europe and parts of tropical area 2. Convectional precipitation  Results when heating of the ground surface causes warming of the air, thus locally strong vertical air motions occur  If the air is thermally unstable, it continues to rise. Thus resulting cooling condensation and cloud formation may lead to locally intense precipitation, but of limited duration.  Depends on heating and moisture in the air.  For example, Mostly occur in tropical regions, and other areas in summer; central US 3. Orographic precipitation  Results from the mechanical lifting of moist air over barrier such as mountain ranges or islands in oceans  Not as efficient in producing precipitation as a convective or cyclonic system  More rainfalls on windward than leeward slopes, because when air descends, it warms and the cloud and rain reduces  Intensity of orographic precipitation increase with the depth of the uplifted layer of moist air  Example, western India and western highland areas of the British isles Chapter 3 (Sections 3.1 and 3.3): INTERCEPTION 3.3  I = P – T – S  where I is Interception Loss, P is Precipitation, T is Throughfall and S is Stemflow  Due to difficulties of installing equipment underneath a vegetation canopy, this method has been used more for forest vegetation than for lower-order covers  Throughfall may be measured using funnel or through gauges placed beneath the forest canopy  Stemflow may be collected by small gutters sealed around the circumference of the trunk leading into a collecting container  Throughfall depends on canopy coverage and Leaf Area Index (LAI); whether the trees are evergreen or deciduous; and the leaf surface smoothness,  Stemflow may be influenced by branch orientation and by roughness of the bark  Leaf shapes and orientation can concentrate throughfall at drip points. Lots of issues with measuring throughfall due to this  Interception studies are much more difficult for grasses and other lower vegetation due to equipment difficulties Chapter 4 (Sections 4.1 4.2 4.4): EVAPORATION 4.1 Intro and Definitions p. 91  Evaporation-process by which liquid is changed to liquid is changed to gas (wet surface to atmosphere)  Water vapour- water in gaseous state  Transpiration- water taken up by plant roots from soil, which moves up plant and into atmosphere through leaves  Evapotranspiration- total evaporation (from soil and water surfaces) + transpiration (from plants)  Evaporation from seas and oceans causes large scale transfer of water vapour from oceans to continents, hence distribution of precipitation over land areas.  Evaporation controls 75% of net radiation reaching earth’s surface.  Sensible heat- radiation warms the atmosphere in contact with the ground by conduction and convection (measured by temperature change)  Latent heat- energy used or liberated in evaporation or condensation (change in state without change in temperature) 4.2 The process of Evaporation p. 94  Two primary aspects of evaporation: o (a) provision of sufficient energy at evaporating surface for latent heat of vapourization, o (b) operation of diffusion processes
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